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The Institute of Physics (IOP) is a leading scientific society promoting physics and bringing physicists together for the benefit of all. It has a worldwide membership of around 50 000 comprising physicists from all sectors, as well as those with an interest in physics. It works to advance physics research, application and education; and engages with policy makers and the public to develop awareness and understanding of physics. Its publishing company, IOP Publishing, is a world leader in professional scientific communications.

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Table of contents

Volume 686

Number 2, 2008 October 20

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749

Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets

M. Kowalski, D. Rubin, G. Aldering, R. J. Agostinho, A. Amadon, R. Amanullah, C. Balland, K. Barbary, G. Blanc, P. J. Challis, A. Conley, N. V. Connolly, R. Covarrubias, K. S. Dawson, S. E. Deustua, R. Ellis, S. Fabbro, V. Fadeyev, X. Fan, B. Farris, G. Folatelli, B. L. Frye, G. Garavini, E. L. Gates, L. Germany, G. Goldhaber, B. Goldman, A. Goobar, D. E. Groom, J. Haissinski, D. Hardin, I. Hook, S. Kent, A. G. Kim, R. A. Knop, C. Lidman, E. V. Linder, J. Mendez, J. Meyers, G. J. Miller, M. Moniez, A. M. Mourão, H. Newberg, S. Nobili, P. E. Nugent, R. Pain, O. Perdereau, S. Perlmutter, M. M. Phillips, V. Prasad, R. Quimby, N. Regnault, J. Rich, E. P. Rubenstein, P. Ruiz-Lapuente, F. D. Santos, B. E. Schaefer, R. A. Schommer, R. C. Smith, A. M. Soderberg, A. L. Spadafora, L.-G. Strolger, M. Strovink, N. B. Suntzeff, N. Suzuki, R. C. Thomas, N. A. Walton, L. Wang, W. M. Wood-Vasey, and J. L. Yun

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Abstract

We present a new compilation of Type Ia supernovae (SNe Ia), a new data set of low-redshift nearby-Hubble-flow SNe, and new analysis procedures to work with these heterogeneous compilations. This "Union" compilation of 414 SNe Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older data sets, as well as the recently extended data set of distant supernovae observed with the Hubble Space Telescope (HST). A single, consistent, and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is Ω_Λ = 0.713^{+ 0.027}_−0.029(stat)^{+ 0.036}_−0.039(sys) , for a flat, ΛCDM universe. Assuming a constant equation of state parameter, w, the combined constraints from SNe, BAO, and CMB give w = − 0.969^{+ 0.059}_−0.063(stat)^{+ 0.063}_−0.066(sys) . While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a w that varies with redshift. In particular, the current SN data do not yet significantly constrain w at z > 1. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.

https://doi.org/10.1086/589937 Cited by References

779

Temperature Differences in the Cepheid Instability Strip Require Differences in the Period-Luminosity Relation in Slope and Zero Point

Allan Sandage and G. A. Tammann

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Abstract

A graphical and an algebraic demonstration is made to show why the slope and zero point of the Cepheid period-luminosity (P-L) relation is rigidly coupled with the slope and zero point of the Cepheid instability strip in the HR diagram. In this way it is shown why it is logically inconsistent to adopt a fixed P-L slope for all galaxies if the intrinsic color-period relations differ in slope for some of them. The graphical demonstration of this inconsistency uses an arbitrary (toy) ridgeline in the instability strip, while the algebraic demonstration uses the pulsation equation into which the observed P-L relations for the Galaxy and the LMC are put to predict the temperature zero points and slopes of the instability strips. Agreement between the predicted and the observed slopes in the instability strips argue that the observed P-L differences between the Galaxy and LMC are real. The direct evidence for different P-L slopes in different galaxies is displayed by comparing the Cepheid data in the Galaxy, the combined data in NGC 3351 and NGC 4321, in M31, LMC, SMC, IC 1613, NGC 3109, and in Sextans A+B. The P-L slopes for the Galaxy, NGC 3351, NGC 4321, and M31 are nearly identical and are the steepest in the sample. The P-L slopes decrease monotonically with metallicity in the order listed, showing that the P-L relation is not the same in different galaxies, complicating their use in calibrating the extragalactic distance scale.

https://doi.org/10.1086/591306 Cited by References

785

Beyond Caustic Crossings: Properties of Binary Microlensing Light Curves

Christopher Night, Rosanne Di Stefano, and Megan Schwamb

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Abstract

Binary microlensing light curves have a variety of morphologies. Many are indistinguishable from point-lens light curves. Of those that deviate from the point-lens form, caustic crossing light curves have tended to dominate identified binary-lens events. Other distinctive signatures of binary lenses include significant asymmetry, multiple peaks, and repeating events. We have quantified, using high-resolution simulations, the theoretically expected relative numbers of each type of binary-lens event, based on its measurable characteristics. We find that a microlensing survey with current levels of photometric uncertainty and sampling should contain at least as many non-caustic crossing binary-lens events as caustic crossing events; in future surveys with more sensitive photometry, the contribution of distinctive non-caustic crossing events will be even greater. To show that this result is robust, we investigate the influence of several physical effects, including blending, sampling rate, and various binary populations.

https://doi.org/10.1086/590320 Cited by References

801

Can Supermassive Black Holes Form in Metal-enriched High-Redshift Protogalaxies?

K. Omukai, R. Schneider, and Z. Haiman

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Abstract

Primordial gas in protogalactic DM halos with virial temperatures T_vir gsim

10⁴ K begins to cool and condense via atomic hydrogen. Provided that this gas is irradiated by a strong UV flux and remains free of H₂ and other molecules, it has been proposed that the halo with T_vir ~ 10⁴ K may avoid fragmentation and lead to the rapid formation of an SMBH as massive as M ≈ 10⁵–10⁶ M_☉. This "head start" would help explain the presence of SMBHs with inferred masses of several times 10⁹ M_☉, powering the bright quasars discovered in the SDSS at redshift z gsim

6. However, high-redshift DM halos with T_vir ~ 10⁴ K are likely already enriched with at least trace amounts of metals and dust produced by prior star formation in their progenitors. Here we study the thermal and chemical evolution of low-metallicity gas exposed to extremely strong UV radiation fields. Our results, obtained in one-zone models, suggest that gas fragmentation is inevitable above a critical metallicity, whose value is between Z_cr ≈ 3 × 10⁻⁴ Z_☉ (in the absence of dust) and as low as Z_cr ≈ 5 × 10⁻⁶ Z_☉ (with a dust-to-gas mass ratio of about 0.01Z/Z_☉). We propose that when the metallicity exceeds these critical values, dense clusters of low-mass stars may form at the halo nucleus. Relatively massive stars in such a cluster can then rapidly coalesce into a single more massive object, which may produce an intermediate-mass BH remnant with a mass up to M lesssim

10²–10³ M_☉.

https://doi.org/10.1086/591636 Cited by References

815

The Self-Regulated Growth of Supermassive Black Holes

Joshua D. Younger, Philip F. Hopkins, T. J. Cox, and Lars Hernquist

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Abstract

We present a series of simulations of the self-regulated growth of supermassive black holes (SMBHs) in galaxies via three different fueling mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in all three scenarios follow the same black hole fundamental plane (BHFP) and correlation with bulge binding energy seen in simulations of major mergers, and observed locally. Furthermore, provided that the total gas supply is significantly larger than the mass of the SMBH, its limiting mass is not influenced by the amount of gas available or the efficiency of black hole growth. This supports the assertion that SMBHs accrete until they reach a critical mass at which feedback is sufficient to unbind the gas locally, terminating the inflow and stalling further growth. At the same time, while minor and major mergers follow the same projected correlations (e.g., the M_BH-σ and Magorrian relations), SMBHs grown via disk instabilities do not, owing to structural differences between the host bulges. This finding is supported by recent observations of SMBHs in pseudobulges and bulges in barred systems, as compared to those hosted by classical bulges. Taken together, this provides support for the BHFP and binding energy correlations as being more "fundamental" than other proposed correlations in that they reflect the physical mechanism driving the coevolution of SMBHs and spheroids.

https://doi.org/10.1086/591639 Cited by References

829

Gravitational Wave Recoil and the Retention of Intermediate-Mass Black Holes

Kelly Holley-Bockelmann, Kayhan Gültekin, Deirdre Shoemaker, and Nicolas Yunes

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Abstract

During the inspiral and merger of a binary black hole, gravitational radiation is emitted anisotropically due to asymmetries in the merger configuration. This anisotropic radiation leads to a gravitational wave kick, or recoil velocity, as large as ~4000 km s⁻¹. We investigate the effect gravitational recoil has on the retention of intermediate-mass black holes (IMBHs) within the population of Galactic globular clusters by simulating the response of IMBHs to black hole mergers. Assuming that our current understanding of IMBH formation is correct and yields an IMBH seed in every globular cluster, we find a significant problem in retaining low-mass IMBHs ( lesssim

1000 M_☉) in the typical merger-rich globular cluster environment. Given a uniform black hole spin distribution and orientation and a stellar-mass black hole mass function generated in a low-metallicity system, we find that only three of the Milky Way globular clusters can retain an IMBH with an initial mass of 200 M_☉. Even if IMBHs have an initial mass of 1000 M_☉, only 60 would remain within Milky Way globular clusters, and each would reside only in the most massive clusters. Our calculations show that if there are black holes of mass M > 50 M_☉ in a cluster, repeated IMBH-black hole encounters will eventually eject a M = 1000 M_☉ IMBH with greater than 30% probability. As a consequence, a large population of rogue black holes may exist in our Milky Way halo. We briefly discuss the dynamical implications of this process and its possible connection to ultraluminous X-ray sources (ULXs).

https://doi.org/10.1086/591218 Cited by References

838

Steep-Spectrum Radio Emission from the Low-Mass Active Galactic Nucleus GH 10

J. M. Wrobel, J. E. Greene, L. C. Ho, and J. S. Ulvestad

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Abstract

GH 10 is a broad-lined active galactic nucleus (AGN) energized by a black hole of mass 800,000 M_☉. It was the only object detected by Greene et al. in their Very Large Array (VLA) survey of 19 low-mass AGNs discovered by Greene & Ho. New VLA imaging at 1.4, 4.9, and 8.5 GHz reveals that GH 10's emission has an extent of less than 320 pc, has an optically thin synchrotron spectrum with a spectral index α = − 0.76 ± 0.05 (S_ν ∝ ν^{+ α}), is less than 11% linearly polarized, and is steady—although poorly sampled—on timescales of weeks and years. Circumnuclear star formation cannot dominate the radio emission, because the high inferred star formation rate, 18 M_☉ yr⁻¹, is inconsistent with the rate of less than 2 M_☉ yr⁻¹ derived from narrow Hα and [O II] λ3727 emission. Instead, the radio emission must be mainly energized by the low-mass black hole. GH 10's radio properties match those of the steep-spectrum cores of Palomar Seyfert galaxies, suggesting that, like those galaxies, the emission is outflow-driven. Because GH 10 is radiating close to its Eddington limit, it may be a local analog of the starting conditions, or seeds, for supermassive black holes. Future imaging of GH 10 at higher linear resolution thus offers an opportunity to study the relative roles of radiative versus kinetic feedback during black hole growth.

https://doi.org/10.1086/592041 Cited by References

843

A Numerical Model of Hercules A by Magnetic Tower: Jet/Lobe Transition, Wiggling, and the Magnetic Field Distribution

Masanori Nakamura, Ian L. Tregillis, Hui Li, and Shengtai Li

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Abstract

We apply magnetohydrodynamic (MHD) modeling to the radio galaxy Hercules A to investigate the jet-driven shock, jet/lobe transition, wiggling, and magnetic field distribution associated with this source. The model consists of magnetic tower jets in a galaxy cluster environment, which has been discussed in a series of our papers. The profile of the underlying ambient gas plays an important role in the jet/lobe morphology. The balance between the magnetic pressure generated by the axial current and the ambient gas pressure can determine the lobe radius. The jet body is confined jointly by the external pressure and gravity inside the cluster core radius R_c, while outside R_c it expands radially to form fat lobes in a steeply decreasing ambient thermal pressure gradient. The current-carrying jets are responsible for generating a strong, tightly wound helical magnetic field. This magnetic configuration will be unstable against the current-driven kink mode, which visibly grows beyond R_c, where a separation between the jet forward and return currents occurs. The reversed pinch profile of the global magnetic field associated with the jet and lobes produces projected $\boldsymbol{B}$ -vector distributions aligned with the jet flow and the lobe edge. An AGN-driven shock powered by the expanding magnetic tower jet surrounds the jet/lobe structure and heats the ambient ICM. The lobes expand subsonically; no obvious hot spots are produced at the heads of lobes. Several key features in our MHD modeling may be qualitatively supported by observations of Hercules A.

https://doi.org/10.1086/591222 Cited by References

851

A Molecular Einstein Ring at z = 4.12: Imaging the Dynamics of a Quasar Host Galaxy Through a Cosmic Lens

Dominik A. Riechers, Fabian Walter, Brendon J. Brewer, Christopher L. Carilli, Geraint F. Lewis, Frank Bertoldi, and Pierre Cox

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Abstract

We present high-resolution (0.3'') Very Large Array imaging of the molecular gas in the host galaxy of the high-redshift quasar PSS J2322+1944 (z = 4.12). These observations confirm that the molecular gas (CO) in the host galaxy of this quasar is lensed into a full Einstein ring and reveal the internal gas dynamics in this system. The ring has a diameter of ~1.5^'' and thus is sampled over ~20 resolution elements by our observations. Through a model-based lens inversion, we recover the velocity gradient of the molecular reservoir in the quasar host galaxy of PSS J2322+1944. The Einstein ring lens configuration enables us to zoom in on the emission and to resolve scales down to lesssim

1 kpc. From the model-reconstructed source, we find that the molecular gas is distributed on a scale of 5 kpc and has a total mass of M(H₂) = 1.7 × 10¹⁰ M_☉. A basic estimate of the dynamical mass gives M_dyn = 4.4 × 10¹⁰ sin⁻² i M_☉, that is, only ~2.5 times the molecular gas mass and ~30 times the black hole mass (assuming that the dynamical structure is highly inclined). The lens configuration also allows us to tie the optical emission to the molecular gas emission, which suggests that the active galactic nucleus does reside within, but not close to the center of, the molecular reservoir. Together with the (at least partially) disturbed structure of the CO, this suggests that the system is interacting. Such interaction, possibly caused by a major "wet" merger, may be responsible for both feeding the quasar and fueling the massive starburst of 680 M_☉ yr⁻¹ in this system, in agreement with recently suggested scenarios of quasar activity and galaxy assembly in the early universe.

https://doi.org/10.1086/591434 Cited by References

859

Radiative Efficiency and Content of Extragalactic Radio Sources: Toward a Universal Scaling Relation between Jet Power and Radio Power

L. Bîrzan, B. R. McNamara, P. E. J. Nulsen, C. L. Carilli, and M. W. Wise

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Abstract

We present an analysis of the energetics and particle content of the lobes of 24 radio galaxies at the cores of cooling clusters. The radio lobes in these systems have created visible cavities in the surrounding hot, X-ray-emitting gas, which allow direct measurement of the mechanical jet power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. We find that jet (cavity) power increases with radio synchrotron power approximately as P_jet ~ L^β_radio, where 0.35 ≤ β ≤ 0.70 depending on the bandpass of measurement and state of the source. However, the scatter about these relations caused by variations in radiative efficiency spans more than 4 orders of magnitude. A number of factors contribute to this scatter, including aging, entrainment, variations in magnetic field strengths, and the partitioning of energy between electrons and nonradiating heavy particles. After accounting for variations in synchrotron break frequency (age), the scatter is reduced by ≈50% , yielding the most accurate scaling relation available between the lobe radio power and the jet (cavity) power. Furthermore, we place limits on the magnetic field strengths and particle content of the radio lobes using a variety of X-ray constraints. We find that the lobe magnetic field strengths vary between a few to several tens of microgauss depending on the age and dynamical state of the lobes. If the cavities are maintained in pressure balance with their surroundings and are supported by internal fields and particles in equipartition, the ratio of energy in electrons to heavy particles (k) must vary widely from approximately unity to 4000, consistent with heavy (hadronic) jets.

https://doi.org/10.1086/591416 Cited by References

881

Spatially Resolved Far-Ultraviolet Spectroscopy of the Nuclear Region of NGC 1068

Wei Zheng, Jun-Xian Wang, Gerard A. Kriss, David Sahnow, Mark Allen, Michael Dopita, Zlatan Tsvetanov, and Geoffrey Bicknell

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Abstract

We carry out high-resolution FUSE spectroscopy of the nuclear region of NGC 1068. The first set of spectra was obtained with a 30'' square aperture that collected all emission from the narrow-line region. The data reveal a strong broad O VI component of FWHM ~3500 km s⁻¹ and two narrow O VI λλ1031, 1037 components of ~350 km s⁻¹. The C III λ977 and N III λ991 emission lines in this spectrum can be fitted with a narrow component of FWHM ~1000 km s⁻¹ and a broad one of ~2500 km s⁻¹. Another set of seven spatially resolved spectra was made using a long slit of 1.25'' × 20'' at steps of ~1'' along the axis of the emission-line cone. We find the following: (1) Major emission lines in the FUSE wavelength range consist of a broad and a narrow component. (2) There is a gradient in the velocity field for the narrow O VI component of ~200 km s⁻¹ from ~2^'' southwest of the nucleus to ~4'' northeast. A similar pattern is also observed with the broad O VI component, with a gradient of ~3000 km s⁻¹. These are consistent with the HST STIS findings and suggest a biconical structure in which the velocity field is mainly radial outflow. (3) A major portion of the C III and N III line flux is produced in the compact core. They are therefore not effective temperature diagnostics for the conical region. (4) The best-fit UV continuum suggests virtually no reddening, and the He II I(λ1640)/I(λ1085) ratio suggests a consistently low extinction factor across the cone. At ~2'' northeast of the nucleus there is a region characterized by (a) a strong Lyα flux but normal C IV flux, (b) a broad O VI line, and (c) a significantly enhanced C III flux.

https://doi.org/10.1086/591547 Cited by References

892

The Host Galaxy and Central Engine of the Dwarf Active Galactic Nucleus POX 52

Carol E. Thornton, Aaron J. Barth, Luis C. Ho, Robert E. Rutledge, and Jenny E. Greene

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Abstract

We present new multiwavelength observations of the dwarf Seyfert 1 galaxy POX 52 in order to investigate the properties of the host galaxy and the active nucleus and to examine the mass of its black hole, previously estimated to be ~10⁵ M_☉. HST ACS HRC images show that the host galaxy has a dwarf elliptical morphology (M_I = − 18.4 mag, Sérsic index n = 4.3) with no detected disk component or spiral structure, confirming previous results from ground-based imaging. X-ray observations from both Chandra and XMM-Newton show strong (factor of 2) variability over timescales as short as 500 s, as well as a dramatic decrease in the absorbing column density over a 9 month period. We attribute this change to a partial covering absorber, with a 94% covering fraction and N_H = 58^{+ 8.4}_−9.2 × 10²¹ cm ⁻², that moved out of the line of sight in between the XMM-Newton and Chandra observations. Combining these data with observations from the VLA, Spitzer, and archival data from 2MASS and GALEX, we examine the SED of the active nucleus. Its shape is broadly similar to typical radio-quiet quasar SEDs, despite the very low bolometric luminosity of L_bol = 1.3 × 10⁴³ ergs s⁻¹. Finally, we compare black hole mass estimators, including methods based on X-ray variability, and optical scaling relations using the broad Hβ line width and AGN continuum luminosity, finding a range of black hole mass from all methods to be M_BH = (2.2–4.2) × 10⁵ M_☉, with an Eddington ratio of L_bol/L_Edd ≈ 0.2–0.5.

https://doi.org/10.1086/591519 Cited by References

911

In-Depth Chandra Study of the AGN Feedback in Virgo Elliptical Galaxy M84

A. Finoguenov, M. Ruszkowski, C. Jones, M. Brüggen, A. Vikhlinin, and E. Mandel

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Abstract

Using deep Chandra observations of M84, we study the energetics of the interaction between the black hole and the interstellar medium of this early-type galaxy. We perform a detailed two-dimensional reconstruction of the properties of the X-ray-emitting gas using a constrained Voronoi tessellation method, identifying the mean trends and carrying out the fluctuation analysis of the thermodynamical properties of the hot ISM. In addition to the PV work associated with the bubble expansion, we identify and measure the wave energy associated with the mildly supersonic bubble expansion. We show that, depending on the age of the cavity and the associated wave, the waves can make a substantial contribution to the total energy release from the AGN. The energy dissipated in the waves tends to be concentrated near the center of M84 and in the direction perpendicular to the bubble outflow, possibly due to the interference of the waves generated by the expansion of northern and southern bubbles. We also find direct evidence for the escape of radio plasma from the ISM of the host galaxy into the intergalactic medium.

https://doi.org/10.1086/591662 Cited by References

918

Galaxy Clusters in the IRAC Dark Field. I. Growth of the Red Sequence

J. E. Krick, J. A. Surace, D. Thompson, M. L. N. Ashby, J. L. Hora, V. Gorjian, and L. Yan

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Abstract

Using three newly identified galaxy clusters at z ~ 1 (photometric redshift) we measure the evolution of the galaxies within clusters from high redshift to the present day by studying the growth of the red cluster sequence. The clusters are located in the Spitzer Infrared Array Camera (IRAC) Dark Field, an extremely deep mid-infrared survey near the north ecliptic pole with photometry in 18 total bands from X-ray through far-IR. Two of the candidate clusters are additionally detected as extended emission in matching Chandra data in the survey area, allowing us to measure their masses to be M₅₀₀ = (6.2 ± 1.0) × 10¹³ and (3.6 ± 1.1) × 10¹³ M_☉. For all three clusters we create a composite color-magnitude diagram in rest-frame B − K using our deep HST and Spitzer imaging. By comparing the fraction of low-luminosity member galaxies on the composite red sequence with the corresponding population in local clusters at z = 0.1 taken from COSMOS, we examine the effect of a galaxy's mass on its evolution. We find a deficit of faint galaxies on the red sequence in our z ~ 1 clusters, which implies that more massive galaxies have evolved in clusters faster than less massive galaxies, and that the less massive galaxies are still forming stars in clusters such that they have not yet settled onto the red sequence.

https://doi.org/10.1086/591788 Cited by References

927

Subgrid Modeling of AGN-driven Turbulence in Galaxy Clusters

Evan Scannapieco and Marcus Brüggen

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Abstract

Hot, underdense bubbles powered by active galactic nuclei (AGNs) are likely to play a key role in halting catastrophic cooling in the centers of cool-core galaxy clusters. We present three-dimensional simulations that capture the evolution of such bubbles, using an adaptive mesh hydrodynamic code, FLASH3, to which we have added a subgrid model of turbulence and mixing. While pure hydro simulations indicate that AGN bubbles are disrupted into resolution-dependent pockets of underdense gas, proper modeling of subgrid turbulence indicates that this is a poor approximation to a turbulent cascade that continues far beyond the resolution limit. Instead, Rayleigh-Taylor instabilities act to effectively mix the heated region with its surroundings, while at the same time preserving it as a coherent structure, consistent with observations. Thus, bubbles are transformed into hot clouds of mixed material as they move outward in the hydrostatic intracluster medium (ICM), much as large airbursts lead to a distinctive "mushroom cloud" structure as they rise in the hydrostatic atmosphere of Earth. Properly capturing the evolution of such clouds has important implications for many ICM properties. In particular, it significantly changes the impact of AGNs on the distribution of entropy and metals in cool-core clusters such as Perseus.

https://doi.org/10.1086/591228 Cited by References

948

The Resolved Properties of Extragalactic Giant Molecular Clouds

Alberto D. Bolatto, Adam K. Leroy, Erik Rosolowsky, Fabian Walter, and Leo Blitz

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Abstract

We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H₂ factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H₂ conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H₂ envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.

https://doi.org/10.1086/591513 Cited by References

966

The Evolution of Dusty Star Formation and Stellar Mass Assembly in Clusters: Results from the IRAC 3.6, 4.5, 5.8, and 8.0 μm Cluster Luminosity Functions

Adam Muzzin, Gillian Wilson, Mark Lacy, H. K. C. Yee, and S. A. Stanford

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Abstract

We present a catalog of 99 candidate clusters and groups of galaxies in the redshift range 0.1 < z_phot < 1.3 discovered in the Spitzer FLS. The clusters are selected by their R_c − 3.6 μ m galaxy color-magnitude relation using the cluster red-sequence algorithm. Using this cluster sample, we compute the 3.6, 4.5, 5.8, and 8.0 μ m cluster LFs. Similar to previous studies, we find that for the bands that trace stellar mass at these redshifts (3.6 and 4.5 μ m ) the evolution in M^* is consistent with a passively evolving population of galaxies with a high formation redshift (z_f > 1.5). Using the 3.6 μ m LF as a proxy for stellar luminosity, we remove this component from the MIR (5.8 and 8.0 μ m ) cluster LFs and measure the LF of dusty star formation/AGNs in clusters. We find that at z < 0.4 the bright end of the cluster 8.0 μ m LF is well described by a composite population of quiescent galaxies and regular star-forming galaxies with a mix consistent with typical cluster blue fractions; however, at z > 0.4, an additional population of dusty starburst galaxies is required to properly model the 8.0 μ m LFs. Comparison to field studies at similar redshifts shows a strong differential evolution in the field and cluster 8.0 μ m LFs with redshift. At z ~ 0.65 8.0 μ m -detected galaxies are more abundant in clusters compared to the field, but thereafter the number of 8.0 μ m sources in clusters declines with decreasing redshift, and by z ~ 0.15, clusters are underdense relative to the field by a factor of ~5. The rapid differential evolution between the cluster and field LFs is qualitatively consistent with recent field galaxy studies that show that the star formation rates of galaxies in high-density environments are larger than those in low-density environments at higher redshift.

https://doi.org/10.1086/591542 Cited by References

995

High-Resolution X-Ray Imaging of the Center of IC 342

Daisy S. Y. Mak, Chun S. J. Pun, and Albert K. H. Kong

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Abstract

We present the results of a high-resolution (θ_FWHM ~ 0.5'') 12 ks Chandra HRC-I observation of the starburst galaxy IC 342 taken on 2006 April 2. We identify 23 X-ray sources within the central 30' × 30' region of IC 342 and resolve the historical ultraluminous X-ray source (ULX), X3, near the nucleus into two sources, namely, C12 and C13. The brighter source C12, with L_{0.08–10 keV} = (6.66 ± 0.45) × 10³⁸ erg s⁻¹, is spatially extended (≈82 pc × 127 pc ). From the astrometric registration of the X-ray image, C12 is at R.A. = 03^h46^m48.43^s, decl. = +68°05'47.45'', and is closer to the nucleus than C13. Thus, we conclude that source is not an ULX and must instead be associated with the nucleus. The fainter source C13, with L_{0.08–10 keV} = (5.1 ± 1.4) × 10³⁷ erg s⁻¹, is consistent with a point source located at 6.51'' (position angle P.A. ≈ 240°) of C12. We also analyzed astrometrically corrected optical Hubble Space Telescope (HST) and radio Very Large Array images; a comparison with the X-ray image showed similarities in their morphologies. Regions of star formation within the central region of IC 342 are clearly visible in the HST Hα image, which contains three optical star clusters and our detected X-ray source C12. We find that the observed X-ray luminosity of C12 is very close to the predicted X-ray emission from a starburst, suggesting that the nuclear X-ray emission of IC 342 is dominated by a starburst. Furthermore, we discuss the possibility of active galactic nucleus (AGN) emission in the nucleus of IC 342, which we can neither prove nor discard with our data.

https://doi.org/10.1086/591023 Cited by References

1007

AGB Connection and Ultraviolet Luminosity Excess in Elliptical Galaxies

Alberto Buzzoni and Rosa A. González-Lópezlira

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Abstract

Relying on infrared surface brightness fluctuactions to trace AGB properties in a sample of elliptical galaxies in the Virgo and Fornax Clusters, we assess the puzzling origin of the ``UV upturn'' phenomenon, recently traced to the presence of a hot horizontal branch (HB) stellar component. The UV upturn actually signals a profound change in the galaxy stellar populations, involving both the hot stellar component and red giant evolution. In particular, the strengthening of the UV rising branch is always seen to correspond to a shortening in AGB deployment; this trend can be readily interpreted as an age effect, perhaps mildly modulated by metal abundance. Brightest stars in ellipticals are all found to be genuine AGB members, all the way, and with the AGB tip exceeding the RGB tip by some 0.5-1.5 mag. The inferred core mass of these stars is found to be lesssim

0.57 M_☉ among giant ellipticals. This value accounts for the recognized deficiency of planetary nebulae in these galaxies, as a result of a lengthy transition time for the post-AGB stellar core to become a hard UV emitter and eventually ``fire up'' the nebula. The combined study of galaxy (1550 − V)₀ color and integrated Hβ index points to a a bimodal temperature distribution for the HB with both a red clump and an extremely blue component, in a relative proportion [N(RHB) : N(BHB)] ~ [80 : 20]. For the BHB stellar population, [Fe/H] values of either sime

–0.7 or

+0.5 dex may provide the optimum ranges to feed the needed low-mass stars (M_*

0.58 M_☉) that at some stage begin to join the standard red clump stars.

https://doi.org/10.1086/589984 Cited by References

1019

Galaxy Bulges as Tests of CDM versus MOND in Strong Gravity

HongSheng Zhao, Bing-Xiao Xu, and Clare Dobbs

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Abstract

The tight correlation between galaxy bulges and their central black hole masses likely emerges in a phase of rapid collapse and starburst at high redshift, due to the balance of gravity on gas with the feedback force from starbursts and the wind from the black hole; the average gravity per unit mass of gas is ~2 × 10⁻¹⁰ m s⁻² during the starburst phase. This level of gravity could come from the real r⁻¹ cusps of cold dark matter (CDM) halos, but the predicted gravity would have a large scatter due to dependence on cosmological parameters and formation histories. Better agreement is found with the gravity from the scalar field in some covariant versions of MOND, which can create the mirage of a Newtonian effective dark halo of density Π r⁻¹ near the center, where the characteristic surface density Π = 130α⁻¹ M_☉ pc ⁻² and α is a fundamental constant of order unity fixed by the Lagrangian of the covariant theory if neglecting environmental effects. We show with a toy analytical model and a hydrodynamical simulation that a constant background gravity due to MOND/TeVeS scalar field implies a critical pressure synchronizing starbursts and the formation of galaxy bulges and ellipticals. A universal threshold for the formation of the brightest regions of galaxies in a MONDian universe suggests that the central black holes, bulges, and ellipticals would respect tight correlations like the M_bul − M_BH − σ relations. In general, MOND tends to produce tight correlations in galaxy properties, because its effective halo has less freedom and scatter than CDM halos.

https://doi.org/10.1086/591490 Cited by References

1030

Delving Deeper into the Tumultuous Lives of Galactic Dwarfs: Modeling Star Formation Histories

Chris Orban, Oleg Y. Gnedin, Daniel R. Weisz, Evan D. Skillman, Andrew E. Dolphin, and Jon A. Holtzman

View abstract View article PDF

Abstract

The paucity of observed dwarf galaxies in the Local Group relative to the abundance of predicted dark matter halos remains one of the greatest puzzles of the ΛCDM paradigm. Solving this puzzle now requires not only matching the numbers of objects but also understanding the details of their star formation histories. We present a summary of such histories derived from the HST data using the color-magnitude diagram fitting method. To reduce observational uncertainties, we condense the data into five cumulative parameters: the fractions of stellar mass formed in the last 1, 2, 5, and 10 Gyr, and the mean stellar age. We interpret the new data with a phenomenological model based on the mass assembly histories of dark matter halos and the Schmidt law of star formation. The model correctly predicts the radial distribution of the dwarfs and the fractions of stars formed in the last 5 and 10 Gyr. However, in order to be consistent with the observations, the model requires a significant amount of recent star formation in the last 2 Gyr. Within the framework of our model, this prolonged star formation can be achieved by adding a stochastic variation in the density threshold of the star formation law. The model results are not sensitive to late gas accretion, the slope of the Schmidt law, or the details of cosmic reionization. A few discrepancies still remain: our model typically predicts too large stellar masses, only a modest population of ultrafaint dwarfs, and a small number of dwarfs with anomalously young stellar populations. Nevertheless, the observed star formation histories of Local Group dwarfs are generally consistent the expected star formation in cold dark matter halos.

https://doi.org/10.1086/591496 Cited by References

1045

Searching for Dark Matter with X-Ray Observations of Local Dwarf Galaxies

T. E. Jeltema and S. Profumo

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Abstract

A generic feature of weakly interacting massive particle (WIMP) dark matter models is the emission of photons over a broad energy band resulting from the stable yields of dark matter pair annihilation. Inverse Compton scattering off cosmic microwave background photons of energetic electrons and positrons produced in dark matter annihilation is expected to produce significant diffuse X-ray emission. Dwarf galaxies are ideal targets for this type of dark matter search technique, being nearby, dark matter dominated systems free of any astrophysical diffuse X-ray background. In this paper, we present the first systematic study of X-ray observations of local dwarf galaxies aimed at the search for WIMP dark matter. We outline the optimal energy and angular ranges for current telescopes and analyze the systematic uncertainties connected to electron/positron diffusion. We do not observe any significant X-ray excess, and we translate this null result into limits on the mass and pair annihilation cross section for particle dark matter. Our results indicate that X-ray observations of dwarf galaxies currently constrain dark matter models at the same level as or even more strongly than gamma-ray observations of the same systems, although at the expenses of introducing additional assumptions and related uncertainties in the modeling of diffusion and energy loss processes. The limits we find constrain portions of the supersymmetric parameter space, particularly if the effect of dark matter substructures is included. Finally, we comment on the role of future X-ray satellites (e.g., Constellation-X, XEUS) and on their complementarity with GLAST and other gamma-ray telescopes in the quest for particle dark matter.

https://doi.org/10.1086/591495 Cited by References

1056

The Magellanic Zoo: Mid-Infrared Spitzer Spectroscopy of Evolved Stars and Circumstellar Dust in the Magellanic Clouds

G. C. Sloan, K. E. Kraemer, P. R. Wood, A. A. Zijlstra, J. Bernard-Salas, D. Devost, and J. R. Houck

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Abstract

We observed a sample of evolved stars in the Large and Small Magellanic Clouds (LMC and SMC) with the Infrared Spectrograph on the Spitzer Space Telescope. Comparing samples from the SMC, LMC, and the Galaxy reveals that the dust production rate depends on metallicity for oxygen-rich stars, but carbon stars with similar pulsation properties produce similar quantities of dust, regardless of their initial metallicity. Other properties of the oxygen-rich stars also depend on metallicity. As the metallicity decreases, the fraction of naked (i.e., dust-free) stars increases, and among the naked stars, the strength of the 8 μm absorption band from SiO decreases. Our sample includes several massive stars in the LMC with long pulsation periods that produce significant amounts of dust, probably because they are young and relatively metal-rich. Little alumina dust is seen in circumstellar shells in the SMC and LMC, unlike in Galactic samples. Three oxygen-rich sources also show emission from magnesium-rich crystalline silicates. Many also show an emission feature at 14 μm. The one S star in our sample shows a newly detected emission feature centered at 13.5 μm. At lower metallicity, carbon stars with similar amounts of amorphous carbon in their shells have stronger absorption from molecular acetylene (C₂H₂) and weaker emission from SiC and MgS dust, as discovered in previous studies.

https://doi.org/10.1086/591437 Cited by References

1082

Evolution of Star Clusters near the Galactic Center: Fully Self-Consistent N-Body Simulations

M. Fujii, M. Iwasawa, Y. Funato, and J. Makino

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Abstract

We have performed fully self-consistent N-body simulations of star clusters near the Galactic center (GC). Such simulations have not been previously performed, because it is difficult to generate fast and accurate simulations of these systems using conventional methods. We used the Bridge code, which integrates the parent galaxy using a tree algorithm and the star cluster using a fourth-order Hermite scheme with individual time steps. The interaction between the parent galaxy and the star cluster is calculate with a tree algorithm. Therefore, the Bridge code can handle both the orbital and internal evolution of star clusters correctly at the same time. We here investigate the evolution of star clusters using the Bridge code, and compare the results with previous studies. We find that (1) the inspiral timescale of the star clusters is shorter than that obtained with "traditional" simulations, in which the orbital evolution of star clusters is calculated analytically using the dynamical friction formula, and (2) core collapse of the star cluster increases the core density and helps the cluster to survive. The initial conditions of star clusters are not so severe as previously suggested.

https://doi.org/10.1086/591483 Cited by References

1094

Characterizing the Nonthermal Emission of Cassiopeia A

E. A. Helder and J. Vink

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Abstract

We report on our analysis of the 1 Ms Chandra observation of the supernova remnant Cas A in order to localize, characterize, and quantify the nonthermal X-ray emission. More specifically, we investigated whether the X-ray synchrotron emission from the inside of the remnant is from the outward shock, but projected toward the inner ring, or from the inner shell. We tackled this problem by employing a Lucy-Richardson deconvolution technique and measuring spectral indices in the 4.2-6 keV band. We show that most of the continuum emission is coming from an inner ring that coincides with the previously reported location of the reverse shock. This inner ring also includes filaments whose X-ray emission has been found to be dominated by X-ray synchrotron emission. The X-ray emission from these filaments, both at the forward shock and from the inner ring, have relatively hard spectra with spectral index >–3.1. The regions emitting hard X-ray continuum contribute about 54% of the total X-ray emission in the 4.2-6 keV. This is lower than that suggested by extrapolating the hard X-ray spectrum as measured by BeppoSAX PDS and INTEGRAL. This can be reconciled by assuming a gradual steepening of the spectrum toward higher energies. We argue that the X-ray synchrotron emission is mainly coming from the western part of the reverse shock. The reverse shock in the west is almost at rest in our observation frame, corresponding to a relatively high reverse shock velocity of ~6000 km s⁻¹ in the frame of the freely expanding ejecta.

https://doi.org/10.1086/591242 Cited by References

1103

Evolution of Perturbed Accelerating Relativistic Shock Waves

Giuseppe Palma, Andrea Mignone, Mario Vietri, and Luca Del Zanna

View abstract View article PDF

Abstract

We study the evolution of an accelerating hyperrelativistic shock under the presence of upstream inhomogeneities wrinkling the discontinuity surface. The investigation is conducted by means of numerical simulations using the PLUTO code for astrophysical fluid dynamics. The reliability and robustness of the code are demonstrated against well-known results coming from the linear perturbation theory. We then follow the nonlinear evolution of two classes of perturbing upstream atmospheres and conclude that no lasting wrinkle can be preserved indefinitely by the flow. Finally, we derive analytically a description of the geometrical effects of a turbulent upstream ambient on the discontinuity surface.

https://doi.org/10.1086/591263 Cited by References

1125

Sensitivity of PDR Calculations to Microphysical Details

N. P. Abel, P. A. M. van Hoof, G. Shaw, G. J. Ferland, and T. Elwert

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Abstract

Our understanding of physical processes in photodissociation regions or photon-dominated regions (PDRs) largely depends on the ability of spectral synthesis codes to reproduce the observed infrared emission-line spectrum. In this paper, we explore the sensitivity of a single PDR model to microphysical details. Our calculations use the Cloudy spectral synthesis code, recently modified to include a wealth of PDR physical processes. We show how the chemical/thermal structure of a PDR, along with the calculated spectrum, changes when the treatment of physical processes such as grain physics and atomic/molecular rates are varied. We find a significant variation in the intensities of PDR emission lines, depending on different treatments of the grain physics. We also show how different combinations of the cosmic-ray ionization rate, inclusion of grain-atom/ion charge transfer, and the grain size distribution can lead to very similar results for the chemical structure. In addition, our results show the utility of Cloudy for the spectral modeling of molecular environments.

https://doi.org/10.1086/591505 Cited by References

1137

Three-Dimensional Modeling of Compressible Magnetohydrodynamic Turbulence in the Local Interstellar Medium

A. A. Chernyshov, K. V. Karelsky, and A. S. Petrosyan

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Abstract

We apply the large eddy simulation technique to carry out a three-dimensional numerical simulation of compressible magnetohydrodynamic turbulence in conditions relevant to the local interstellar medium. In accord with the large eddy simulation method, the large-scale part of the flow is computed directly and only small-scale structures of turbulence are modeled. The small-scale motion is eliminated from the initial system of equations of motion by filtering procedures, and its effect is taken into account by special closures referred to as the subgrid-scale models. Establishment of the weakly compressible limit with a Kolmogorov-like density fluctuation spectrum is shown in the present work. We use our computation results to study the dynamics of the turbulent plasma beta and anisotropic properties of the magnetoplasma fluctuations in the local interstellar medium.

https://doi.org/10.1086/591642 Cited by References

1145

Blazing Trails: Microquasars as Head-Tail Sources and the Seeding of Magnetized Plasma into the ISM

S. Heinz, H. J. Grimm, R. A. Sunyaev, and R. P. Fender

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Abstract

We discuss the dynamics of microquasar jets in the interstellar medium, with specific focus on the effects of the X-ray binaries' space velocity with respect to the local Galactic standard of rest. We argue that, during late stages in the evolution of large scale radio nebulae around microquasars, the ram pressure of the interstellar medium due to the microquasar's space velocity becomes important, and that microquasars with high velocities form the Galactic equivalent of extragalactic head-tail sources, i.e., that they leave behind trails of stripped radio plasma. Because of their higher space velocities, low-mass X-ray binaries are more likely to leave trails than high-mass X-ray binaries. We show that the volume of radio plasma released by microquasars over the history of the Galaxy is comparable to the disk volume, and argue that a fraction of a few percent of the radio plasma left behind by the X-ray binary is likely mixed with the neutral phases of the ISM before the plasma is removed from the disk by buoyancy. Because the formation microquasars is an unavoidable by-product of star formation, and because they can travel far from their birthplaces, their activity likely has important consequences for the evolution of magnetic fields in forming galaxies. We show that radio emission from the plasma inside the trail should be detectable at low frequencies. We suggest that LMXBs with high detected proper motions, such as XTE J1118+480, will be the best candidates for such a search.

https://doi.org/10.1086/591435 Cited by References

1155

Far-Ultraviolet Observations of the Ophiuchus Region with SPEAR

D.-H. Lee, K.-I. Seon, K. W. Min, Y. S. Park, I. S. Yuk, J. Edelstein, E. J. Korpela, R. Sankrit, S. J. Park, and K. S. Ryu

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Abstract

We present the first far-ultraviolet (FUV; 1370-1670 Å) image of the Ophiuchus molecular cloud region, observed with the SPEAR imaging spectrograph. The flux levels of the diffuse FUV continuum are in reasonable agreement with those of the Voyager observations in the shorter FUV wavelengths (912-1216 Å), provided that the diffuse FUV emission is dominated by the spectra from late O- and early B-type stars. The observed region of the present study was divided into five subregions according to their FUV intensities, and the spectrum was obtained for each subregion with prominent H₂ fluorescent emission lines. A synthetic model of the H₂ fluorescent emission indicates that the molecular cloud has more or less uniform physical parameters over the Ophiuchus region, with a hydrogen density n_H of 500 cm⁻³ and a H₂ column density N(H₂) of 2 × 10²⁰ cm⁻². It is notable that the observed diffuse FUV continuum is well reproduced by a single-scattering model with scattered starlight from the dust cloud located at ~120-130 pc, except at a couple of regions with high optical depth. The model also gives reasonable properties of the dust grains of the cloud with an albedo a of 0.36 ± 0.20 and a phase function asymmetry factor g of 0.52 ± 0.22.

https://doi.org/10.1086/591778 Cited by References

1162

Recent FUSE Observations of Diffuse O VI Emission from the Interstellar Medium

W. Van Dyke Dixon and Ravi Sankrit

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Abstract

We present new results from our survey of diffuse O VI-emitting gas in the interstellar medium with the Far Ultraviolet Spectroscopic Explorer (FUSE). Background observations obtained since 2005 have yielded 11 new O VI detections of 3 σ significance, and archival searches have revealed two more. An additional 15 sight lines yield interesting upper limits. Combined with previous results, these observations reveal the large-scale structure of the O VI-bearing gas in the quadrant of the sky centered on the Magellanic Clouds. The most prominent feature is a layer of low-velocity O VI emission extending more than 70° from the Galactic plane. At low latitudes (|b| < 30°), the emission comes from narrow, high-density interfaces in the local ISM. At high latitudes, the emission is from extended, low-density regions in the Galactic halo. We also detect O VI emission from the interface region of the Magellanic system, a structure recently identified from H I observations. These are the first detections of emission from high-ionization species in the Magellanic system outside of the Clouds themselves.

https://doi.org/10.1086/591780 Cited by References

1174

Driven and Decaying Turbulence Simulations of Low-Mass Star Formation: From Clumps to Cores to Protostars

Stella S. R. Offner, Richard I. Klein, and Christopher F. McKee

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Abstract

Molecular clouds are observed to be turbulent, but the origin of this turbulence is not well understood. As a result, there are two different approaches to simulating molecular clouds, one in which the turbulence is allowed to decay after it is initialized, and one in which it is driven. We use the adaptive mesh refinement (AMR) code, Orion, to perform high-resolution simulations of molecular cloud cores and protostars in environments with both driven and decaying turbulence. We include self-gravity, use a barotropic equation of state, and represent regions exceeding the maximum grid resolution with sink particles. We analyze the properties of bound cores such as size, shape, line width, and rotational energy, and we find reasonable agreement with observation. At high resolution the different rates of core accretion in the two cases have a significant effect on protostellar system development. Clumps forming in a decaying turbulence environment produce high-multiplicity protostellar systems with Toomre Q unstable disks that exhibit characteristics of the competitive accretion model for star formation. In contrast, cores forming in the context of continuously driven turbulence and virial equilibrium form smaller protostellar systems with fewer low-mass members. Our simulations of driven and decaying turbulence show some statistically significant differences, particularly in the production of brown dwarfs and core rotation, but the uncertainties are large enough that we are not able to conclude whether observations favor one or the other.

https://doi.org/10.1086/590238 Cited by References

1195

A Spitzer View of Protoplanetary Disks in the γ Velorum Cluster

Jesús Hernández, Lee Hartmann, Nuria Calvet, R. D. Jeffries, R. Gutermuth, J. Muzerolle, and J. Stauffer

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Abstract

We present new Spitzer Space Telescope observations of stars in the young (~5 Myr) γ Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified five debris disks around A-type stars and five to six debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the infrared flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that γ Velorum stellar cluster is slightly older than reported ages and the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr.

https://doi.org/10.1086/591224 Cited by References

1209

The Early-Time Optical Properties of Gamma-Ray Burst Afterglows

A. Melandri, C. G. Mundell, S. Kobayashi, C. Guidorzi, A. Gomboc, I. A. Steele, R. J. Smith, D. Bersier, C. J. Mottram, D. Carter, M. F. Bode, P. T. O'Brien, N. R. Tanvir, E. Rol, and R. Chapman

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Abstract

We present a multiwavelength analysis of 63 gamma-ray bursts observed with the world's three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to ~10⁶ seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 14/24 GRBs can be modeled with the forward-shock model, explaining the remaining 10 is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power law, may be due to additional emission from the central engine. Thirty-nine GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only 10 were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of ~50%. Additional emission in the early-time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.

https://doi.org/10.1086/591243 Cited by References

1231

Perpendicular Transport of Energetic Charged Particles in Nonaxisymmetric Two-Component Magnetic Turbulence

D. Ruffolo, P. Chuychai, P. Wongpan, J. Minnie, J. W. Bieber, and W. H. Matthaeus

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Abstract

We examine energetic charged particle diffusion perpendicular to a mean magnetic field B₀ due to turbulent fluctuations in a plasma, relaxing the common assumption of axisymmetry around B₀ and varying the ratio of two fluctuation components, a slab component with parallel wavenumbers and a two-dimensional (2D) component with perpendicular wavenumbers. We perform computer simulations mostly for 80% 2D and 20% slab energy and a fluctuation amplitude on the order of B₀. The nonlinear guiding center (NLGC) theory provides a reasonable description of asymptotic perpendicular diffusion as a function of the nonaxisymmetry and particle energy. These values are roughly proportional to the particle speed times the field line diffusion coefficient, with a prefactor that is much lower than in the classical field line random walk model of particle diffusion. NLGC predicts a prefactor in closer agreement with simulations. Next we consider extreme fluctuation anisotropy and the approach to reduced dimensionality. For 99% slab fluctuation energy, field line trajectories are diffusive, but the particle motion is subdiffusive. For 99% 2D fluctuation energy, both field lines and particle motions are initially subdiffusive and then diffusive, but NLGC gives unreliable results. The time dependence of the running particle diffusion coefficient shows that in all cases asymptotic diffusion is preceded by free streaming and subdiffusion, but the latter differs from standard compound subdiffusion. We can model the time profiles in terms of a decaying negative correlation of the perpendicular velocity due to the possibility of backtracking along magnetic field lines.

https://doi.org/10.1086/591493 Cited by References

1245

Resonant Cyclotron Scattering in Magnetars' Emission

N. Rea, S. Zane, R. Turolla, M. Lyutikov, and D. Götz

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Abstract

We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of 10 magnetars, including canonical and transient anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). In this scenario, nonthermal magnetar spectra in the soft X-rays (i.e., below ~10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters as in the commonly used empirical blackbody plus power-law model. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below 10 keV require the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). Although this model in its present form does not explain the hard X-ray emission (i.e., above ~20 keV) of a few of these sources, we took this further component into account in our modeling not to overlook its contribution in the ~4-10 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitude higher than the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities are also in better agreement with more recent estimates. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars shows that the RCS model is capable of catching the main features of the spectra observed below ~10 keV.

https://doi.org/10.1086/591264 Cited by References

1261

The Rate and Spatial Distribution of Novae in M101 (NGC 5457)

E. A. Coelho, A. W. Shafter, and K. A. Misselt

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Abstract

A new multiepoch Hα imaging study of M101 (NGC 5457) has been carried out as part of a larger campaign to study the rate and stellar population of extragalactic novae. The survey yielded a total of 13 nova detections from 10 epochs of M101 observations spanning a 3 yr period. After correcting for the temporal coverage and survey completeness, a global nova rate of 11.7^{+ 1.9}_−1.5 yr⁻¹ is found. This value corresponds to a luminosity-specific nova rate of 1.23 ± 0.27 novae per year per 10¹⁰ L_{☉ ,K} when the K luminosity is derived from the B − K color, or 1.94 ± 0.42 novae per year per 10¹⁰ L_{☉ ,K} when the K magnitude from the Two Micron All Sky Survey is used. These values are consistent with previous estimates by Shafter et al. that were based on more limited data. The spatial distribution of the combined nova sample from the present survey and from the earlier Shafter et al. survey shows that the specific frequency of novae closely follows the integrated background light of the galaxy.

https://doi.org/10.1086/591517 Cited by References

1269

Detection of Magnetic Massive Stars in the Open Cluster NGC 3766

M. Virginia McSwain

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Abstract

A growing number of observations indicate that magnetic fields are present among a small fraction of massive O- and B-type stars, yet the origin of these fields remains unclear. Here we present the results of a VLT/FORS1 spectropolarimetric survey of 15 B-type members of the open cluster NGC 3766. We have detected two magnetic B stars in the cluster, including one with a large field of nearly 2 kG, and we find marginal detections of two additional stars. There is no correlation between the observed longitudinal field strengths and the projected rotational velocity, suggesting that a dynamo origin for the fields is unlikely. We also use the oblique dipole rotator model to simulate populations of magnetic stars with uniform or slightly varying magnetic flux on the ZAMS. None of the models successfully reproduces our observed range in B_l and the expected number of field detections, and we rule out a purely fossil origin for the observed fields.

https://doi.org/10.1086/591784 Cited by References

1275

Analytic Criteria for the Mechanical and Thermal Stability of Magnetic Stars with Finite Electrical Conductivity

Richard B. Stothers

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Abstract

The usual assumption of infinite electrical conductivity has been relaxed in a new analytic treatment of the magnetohydrodynamics of a variable magnetic star in the simple one-zone approximation. For a not too low electrical conductivity, the magnetic pressure changes with the mass density as roughly ρ^4/3, and the Joule heating rate goes as roughly ρ. The magnetic effective adiabatic exponent of about 4/3 proves to have only a small influence on the criteria for dynamical, secular, and pulsational stability, but the Joule heating rate directly affects the secular and pulsational stability criteria. Thus, a finite electrical conductivity tends to stabilize a magnetic star secularly and to destabilize it pulsationally. These specific results apply, however, only to purely radial perturbations of the star's upper radiative layers.

https://doi.org/10.1086/591546 References

1280

Detection of a Hot Subdwarf Companion to the Be Star FY Canis Majoris

Geraldine J. Peters, Douglas R. Gies, Erika D. Grundstrom, and M. Virginia McSwain

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Abstract

The rapid rotation of Be stars may be caused in some cases by past mass and angular momentum accretion in an interacting binary in which the mass donor is currently viewed as a small, hot subdwarf stripped of its outer envelope. Here we report on the spectroscopic detection of such a subdwarf in the Be binary system FY Canis Majoris from the analysis of data acquired by the IUE spacecraft and KPNO Coudé Feed Telescope over the course of 16 and 21 yr, respectively. We present a double-lined spectroscopic orbit for the binary based on radial velocities from the IUE spectra and use the orbital solutions with a Doppler tomography algorithm to reconstruct the components' UV spectra. The subdwarf is hot (T_eff = 45 ± 5 kK) and has a mass of about 1.3 M_☉ and a radius of about 0.6 R_☉. It contributes about 4% as much flux as the Be star does in the FUV. We also present observations of the Hα and He I λ6678 emission features that are formed in the circumstellar disk of the Be star. Orbital flux and velocity variations in the He I λ6678 profile indicate that much of the emission forms along the disk rim facing the hot subdwarf where the disk is probably heated by the incident radiation from the subdwarf. A study of the FUV infall shell lines discovered in the 1980s confirms their episodic presence but reveals that they tend to be found around both quadrature phases, unlike the pattern in Algol binaries. Phase-dependent variations in the UV N V doublet suggest the presence of a N-enhanced wind from the subdwarf and a possible shock-interaction region between the stars where the subdwarf's wind collides with the disk of the Be star.

https://doi.org/10.1086/591145 Cited by References

1292

Accretion and Destruction of Planetesimals in Turbulent Disks

Shigeru Ida, Tristan Guillot, and Alessandro Morbidelli

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Abstract

We study the conditions for collisions between planetesimals to be accretional or disruptive in turbulent disks, through analytical arguments based on fluid dynamical simulations and orbital integrations. In turbulent disks, the velocity dispersion of planetesimals is pumped up by random gravitational perturbations from density fluctuations of the disk gas. When the velocity dispersion is larger than the planetesimals' surface escape velocity, collisions between planetesimals do not result in accretion and may even lead to their destruction. In disks with a surface density equal to that of the "minimum-mass solar nebula" and with nominal magnetorotational instability (MRI) turbulence, we find that accretion proceeds only for planetesimals with sizes above ~300 km at 1 AU and ~1000 km at 5 AU. We find that accretion is facilitated in disks with smaller masses. However, at 5 AU and for nominal turbulence strength, km-sized planetesimals are in a highly erosive regime even for a disk mass as small as a fraction of the mass of Jupiter. The existence of giant planets implies that either turbulence was weaker than calculated by standard MRI models or some mechanism was capable of producing Ceres-mass planetesimals on very short timescales. In any case, our results show that in the presence of turbulence planetesimal accretion is most difficult in massive disks and at large orbital distances.

https://doi.org/10.1086/591903 Cited by References

1302

Predicting the Yields of Photometric Surveys for Transiting Extrasolar Planets

Thomas G. Beatty and B. Scott Gaudi

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Abstract

We develop a method for predicting the yield of transiting planets from a photometric survey given the parameters of the survey (nights observed, bandpass, exposure time, telescope aperture, locations of the target fields, observational conditions, and detector characteristics), as well as the underlying planet properties (frequency, period and radius distributions). Using our updated understanding of transit surveys provided by the experiences of the survey teams, we account for those factors that have proven to have the greatest effect on the survey yields. Specifically, we include the effects of the surveys' window functions, adopt revised estimates of the giant planet frequency, account for the number and distribution of main-sequence stars in the survey fields, and include the effects of Galactic structure and interstellar extinction. We approximate the detectability of a planetary transit using a signal-to-noise ratio (S/N) formulation. We argue that our choice of detection criterion is the most uncertain input to our predictions, and has the largest effect on the resulting planet yield. Thus, drawing robust inferences about the frequency of planets from transit surveys will require that the survey teams impose and report objective, systematic, and quantifiable detection criteria. Nevertheless, with reasonable choices for the minimum S/N, we calculate yields that are generally lower, more accurate, and more realistic than previous predictions. As examples, we apply our method to the Trans-Atlantic Exoplanet Survey, the XO survey, and the Kepler mission. We discuss red noise and its possible effects on planetary detections. We conclude with estimates of the expected detection rates for future wide-angle synoptic surveys.

https://doi.org/10.1086/591441 Cited by References

1331

XO-5b: A Transiting Jupiter-sized Planet with a 4 Day Period

Christopher J. Burke, P. R. McCullough, Jeff A. Valenti, Doug Long, Christopher M. Johns-Krull, P. Machalek, Kenneth A. Janes, B. Taylor, Michael L. Fleenor, C. N. Foote, Bruce L. Gary, Enrique García-Melendo, J. Gregorio, and T. Vanmunster

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Abstract

The star XO-5 (GSC 02959–00729, V = 12.1, G8 V) hosts a Jupiter-sized, R_p = 1.15 ± 0.12 R_J, transiting extrasolar planet, XO-5b, with an orbital period of 4.2 days. The planet's mass, M_p = 1.15 ± 0.08 M_J, and surface gravity, g_p = 22 ± 5 m s⁻², are large for its orbital period compared to most other transiting planets. However, the deviation from the M_p-P relationship for XO-5b is not as large as for GJ 436b, HAT-P-2b, and XO-3b. By coincidence, XO-5 overlies the extreme H I plume that emanates from the interacting galaxy pair NGC 2444/NGC 2445 (Arp 143).

https://doi.org/10.1086/591497 Cited by References

1341

The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b

David Charbonneau, Heather A. Knutson, Travis Barman, Lori E. Allen, Michel Mayor, S. Thomas Megeath, Didier Queloz, and Stéphane Udry

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Abstract

We present Spitzer Space Telescope time series photometry of the exoplanet system HD 189733 spanning two times of secondary eclipse, when the planet passes out of view behind the parent star. We estimate the relative eclipse depth in five distinct bands and find the planet-to-star flux ratio to be 0.256% ± 0.014% (3.6 μm), 0.214% ± 0.020% (4.5 μm), 0.310% ± 0.034% (5.8 μm), 0.391% ± 0.022% (8.0 μm), and 0.598% ± 0.038% (24 μm). For consistency, we reanalyze a previously published time series to deduce a contrast ratio in an additional band, 0.519% ± 0.020% (16 μm). Our data are strongly inconsistent with a Planck spectrum, and we clearly detect emission near 4 μm as predicted by published theoretical models in which this feature arises from a corresponding opacity window. Unlike recent results for the exoplanet HD 209458b, we find that the emergent spectrum from HD 189733b is best matched by models that do not include an atmospheric temperature inversion. Taken together, these two studies provide initial observational support for the idea that hot Jupiter atmospheres diverge into two classes, in which a thermal inversion layer is present for the more strongly irradiated objects.

https://doi.org/10.1086/591635 Cited by References

1349

Solar Cycle Related Changes at the Base of the Convection Zone

Charles S. Baldner and Sarbani Basu

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Abstract

The frequencies of solar oscillations are known to change with solar activity. We use principal component analysis to examine these changes with high precision. In addition to the well-documented changes in solar normal mode oscillations with activity as a function of frequency, which originate in the surface layers of the Sun, we find a small but statistically significant change in frequencies with an origin at and below the base of the convection zone. We find that at r = (0.712^{+ 0.0097}_−0.0029) R_☉, the change in sound speed is δ c²/c² = (7.23 ± 2.08) × 10⁻⁵ between high and low activity. This change is very tightly correlated with solar activity. In addition, we use the splitting coefficients to examine the latitudinal structure of these changes. We find changes in sound speed correlated with surface activity for r gsim

0.9 R_☉.

https://doi.org/10.1086/591514 Cited by References

1362

Flows and Nonthermal Velocities in Solar Active Regions Observed with the EUV Imaging Spectrometer on Hinode: A Tracer of Active Region Sources of Heliospheric Magnetic Fields?

G. A. Doschek, H. P. Warren, J. T. Mariska, K. Muglach, J. L. Culhane, H. Hara, and T. Watanabe

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Abstract

From Doppler velocity maps of active regions constructed from spectra obtained by the EUV Imaging Spectrometer (EIS) on the Hinode spacecraft we observe large areas of outflow (20-50 km s⁻¹) that can persist for at least a day. These outflows occur in areas of active regions that are faint in coronal spectral lines formed at typical quiet-Sun and active region temperatures. The outflows are positively correlated with nonthermal velocities in coronal plasmas. The bulk mass motions and nonthermal velocities are derived from spectral line centroids and line widths, mostly from a strong line of Fe XII at 195.12 Å. The electron temperature of the outflow regions estimated from an Fe XIII to Fe XII line intensity ratio is about (1.2–1.4) × 10⁶ K. The electron density of the outflow regions derived from a density-sensitive intensity ratio of Fe XII lines is rather low for an active region. Most regions average around 7 × 10⁸ cm⁻³, but there are variations on pixel spatial scales of about a factor of 4. We discuss results in detail for two active regions observed by EIS. Images of active regions in line intensity, line width, and line centroid are obtained by rastering the regions. We also discuss data from the active regions obtained from other orbiting spacecraft that support the conclusions obtained from analysis of the EIS spectra. The locations of the flows in the active regions with respect to the longitudinal photospheric magnetic fields suggest that these regions might be tracers of long loops and/or open magnetic fields that extend into the heliosphere, and thus the flows could possibly contribute significantly to the solar wind.

https://doi.org/10.1086/591724 Cited by References

1372

The Current Sheet Associated with the 2003 November 4 Coronal Mass Ejection: Density, Temperature, Thickness, and Line Width

A. Ciaravella and J. C. Raymond

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Abstract

In the wake of the 2003 November 4 coronal mass ejection associated with the largest solar flare of the last sunspot cycle, a current sheet (CS) was observed by the Ultraviolet Coronagraph Spectrometer (UVCS) as a narrow bright feature in the [Fe XVIII] (10^6.8 K) line. This is the first UV observation in which the CS evolution is followed from its onset. UV spectra provide diagnostics of electron temperature, emission measure, Doppler shift, line width, and size of the CS as function of time. Since the UVCS slit was inside the Mark IV K-coronameter (MK4) field of view, the combination of UV spectra and MK4 white light data provides estimates of the electron density and depth along the line of sight of the CS. The thickness of the CS in the [Fe XVIII] line is far larger than classical or anomalous resistivity would predict, and it might indicate an effective resistivity much larger than anomalous resistivity, such as that due to hyperdiffusion. The broad [Fe XVIII] line profiles in the CS cannot be explained as thermal widths. They result from a combination of bulk motions and turbulence. The Petschek reconnection mechanism and turbulent reconnection may be consistent with the observations.

https://doi.org/10.1086/590655 Cited by References

1383

Hinode, TRACE, SOHO, and Ground-based Observations of a Quiescent Prominence

P. Heinzel, B. Schmieder, F. Fárník, P. Schwartz, N. Labrosse, P. Kotrč, U. Anzer, G. Molodij, A. Berlicki, E. E. DeLuca, L. Golub, T. Watanabe, and T. Berger

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Abstract

A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by the Hinode SOT, in X-rays by the Hinode XRT, and in the 195 Å channel by TRACE. Moreover, ground-based observatories (GBOs) provided calibrated Hα intensities. Simultaneous extreme-UV (EUV) data were also taken by the Hinode EIS and SOHO SUMER and CDS instruments. Here we have selected the SOT Hα image taken at 13:19 UT, which nicely shows the prominence fine structure. We compare this image with cotemporaneous ones taken by the XRT and TRACE and show the intensity variations along several cuts parallel to the solar limb. EIS spectra were obtained about half an hour later. Dark prominence structure clearly seen in the TRACE and EIS 195 Å images is due to the prominence absorption in H I, He I, and He II resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in the XRT images is entirely due to X-ray emissivity blocking. We use TRACE, EIS, and XRT data to estimate the amount of absorption and blocking. The Hα integrated intensities independently provide us with an estimate of the Hα opacity, which is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. However, spatial averaging of the Hα and EUV data have quite different natures, which must be taken into account when evaluating the true opacities. We demonstrate this important effect here for the first time. Finally, based on this multiwavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence.

https://doi.org/10.1086/591018 Cited by References

1397

The Variation of Relative Magnetic Helicity around Major Flares

Sung-Hong Park, Jeongwoo Lee, G. S. Choe, Jongchul Chae, Hyewon Jeong, Guo Yang, Ju Jing, and Haimin Wang

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Abstract

We have investigated the variation of magnetic helicity over a span of several days around the times of 11 X-class flares which occurred in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696, and 10720) using the magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). As a major result we found that each of these major flares was preceded by a significant helicity accumulation, (1.8–16) × 10⁴² Mx² over a long period (0.5 to a few days). Another finding is that the helicity accumulates at a nearly constant rate, (4.5–48) × 10⁴⁰ Mx² hr⁻¹, and then becomes nearly constant before the flares. This led us to distinguish the helicity variation into two phases: a phase of monotonically increasing helicity and the following phase of relatively constant helicity. As expected, the amount of helicity accumulated shows a modest correlation with time-integrated soft X-ray flux during flares. However, the average helicity change rate in the first phase shows even stronger correlation with the time-integrated soft X-ray flux. We discuss the physical implications of this result and the possibility that this characteristic helicity variation pattern can be used as an early warning sign for solar eruptions.

https://doi.org/10.1086/591117 Cited by References

1404

On the Penumbral Jetlike Features and Chromospheric Bow Shocks

M. Ryutova, T. Berger, Z. Frank, and A. Title

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Abstract

We present observations of sunspot penumbrae obtained during the disk passage of AR 10923 (2006 November 10-20) with the SOT instrument on Hinode in 4305 Å G band and Ca II λ3968 H line. Along with recently discovered jetlike features (Katsukawa et al. 2007), we find other kinds of bright elongated transients abundantly pervading the entire penumbra and drifting as a whole in a direction almost perpendicular to their long axes. Their measured velocities strongly depend on their orientation with respect to the line of sight and range from sime

1 to

20 km s⁻¹. We present quantitative analysis of these features and interpret them relative to our recent penumbral model (Ryutova et al. 2008) to show that they are produced by shocks resulting from a slingshot effect associated with the ongoing reconnection processes in neighboring penumbral filaments. Due to sharp stratification of the low atmosphere, postreconnection flux tubes moving upward quickly accelerate. At transonic velocities a bow (detached) shock is formed in front of the flux tube, as usually occurs in cases of blunt bodies moving with supersonic velocities. Observed parameters of transients are in good agreement with calculated parameters of bow shocks. On some, much more rare occasions compared to "drifting" bow-shock-type transients, there appear compact bright transients moving in the radial direction, along their long axis, and having velocities of 20-50 km s⁻¹. We relate these features to a category of true microjets.

https://doi.org/10.1086/591498 Cited by References

1420

A Low-Order Solar Dynamo Model: Inferred Meridional Circulation Variations Since 1750

Dário Passos and Ilídio Lopes

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Abstract

In this work we present the possible variations that the meridional circulation of the Sun might have undergone during the last 250 years. In order to do this, we reduce an α − Ω dynamo to a low-order system that focuses on the time evolution of one of the solar magnetic field components. Afterward we used a method based on the analysis of phase space of the superficial toroidal magnetic field to infer changes in the superficial meridional circulation. We used sunspot numbers to build a time series that approximately represents the magnetic field behavior. After reconstructing the time series' phase space we assume equilibrium solutions for each solar cycle and we fit them to our model. The resulting fit parameters are shown to depend on background quantities of the theoretical model, such as magnetic diffusivity, differential rotation, meridional circulation, etc. The methodology presented here allows one to extract information about the meridional circulation average behavior, and possibly other parameters, from more that 250 years of sunspot number observations.

https://doi.org/10.1086/591511 Cited by References

1426

Measurement of the Magnetic Properties of FeH in Its X⁴Δ and F⁴Δ States from Sunspot Spectra

Jeremy J. Harrison and John M. Brown

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Abstract

The spin-rotational levels of FeH in its F⁴Δ and X⁴Δ states are significantly perturbed by nearby electronic states. As a result, it is not possible to make reliable calculations of its magnetic dipole moments from a simple effective Hamiltonian. In this paper, we report experimental measurements of the magnetic dipole moments of FeH in several rotational levels of the two lowest spin components of the v = 0 levels of both the F⁴Δ and X⁴Δ states. The observational data are taken from the Sunspot Spectral Atlas of Wallace et al.; the local magnetic flux densities are determined from the splittings of atomic lines present in the spectrum. These results considerably extend the range of known magnetic dipole moments for FeH and provide many lines in the F⁴Δ–X⁴Δ transition at 990 nm (the Wing-Ford system) for astronomers to use for the measurement of the local magnetic fields of cool stars where suitable atomic lines are not available for this purpose.

https://doi.org/10.1086/591541 Cited by References

1432

Development of Flux Imbalances in Solar Activity Nests and the Evolution of Filament Channels

V. Gaizauskas

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Abstract

Bipolar active regions tend to emerge in tight clusters that persist at the same location in so-called activity nests. This study examines how flux evolves inside three different arrangements of interacting nests. Each contains ~2 × 10²³ Mx, and each develops local flux imbalances that interact to form filaments and filament channels. They include: a pair of isolated closely packed nests; a pair of widely spaced nests with neighboring nests on their outer flanks; and a chain of three closely packed nests. These cases result in flux imbalances that are, respectively: large and concentrated on the outer edges of the nests; large and concentrated between the nests; and weak and concentrated on the outer edges of the nests. An amount of flux equivalent to a single large sunspot pair, but composed entirely of weaker flux densities (< mid

50

G), is representative of the net fluxes measured for all three examples of multiple activity nests. In the majority of cases, the pools of net flux form filament channels, i.e., configurations with a clear horizontal component of the magnetic field directed along a polarity inversion line (PIL). This study proposes that large quiescent filaments and their channels are natural storehouses of magnetic energy constructed by surface flows out of slowly reconnecting pools of "orphaned" magnetic flux that originate at outer boundaries of decaying activity nests.

https://doi.org/10.1086/591633 References

1447

Magnetic Flux Loss and Flux Transport in a Decaying Active Region

M. Kubo, B. W. Lites, T. Shimizu, and K. Ichimoto

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Abstract

We estimate the temporal change of magnetic flux normal to the solar surface in a decaying active region by using a time series of the spatial distribution of vector magnetic fields in the photosphere. The vector magnetic fields are derived from full spectropolarimetric measurements with the Solar Optical Telescope aboard Hinode. We compare a magnetic flux loss rate to a flux transport rate in a decaying sunspot and its surrounding moat region. The amount of magnetic flux that decreases in the sunspot and moat region is very similar to magnetic flux transported to the outer boundary of the moat region. The flux loss rates [(dF/dt)_loss] of magnetic elements with positive and negative polarities balance each other around the outer boundary of the moat region. These results suggest that most of the magnetic flux in the sunspot is transported to the outer boundary of the moat region as moving magnetic features, and then removed from the photosphere by flux cancellation around the outer boundary of the moat region.

https://doi.org/10.1086/592064 Cited by References

1454

Flux Pumping and Magnetic Fields in the Outer Penumbra of a Sunspot

Nicholas H. Brummell, Steven M. Tobias, John H. Thomas, and Nigel O. Weiss

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Abstract

The filamentary structure of a sunspot penumbra is believed to be magnetoconvective in origin. In the outer penumbra there is a difference in inclination of up to 30°-40° between the magnetic fields associated with bright and dark filaments, and the latter fields plunge downward below the surface toward the edge of the spot. We have proposed that these fields are dragged downward by magnetic pumping caused by the external granular convection. In this paper we model this process in a more elaborate idealized configuration that includes the curvature force exerted by an arched magnetic field in addition to magnetic buoyancy, and demonstrate that magnetic pumping remains an efficient mechanism for holding flux submerged. We discuss the implications of these results for the magnetic structure of the outer penumbra.

https://doi.org/10.1086/591640 Cited by References

1466

Acceleration of Suprathermal Tails in the Solar Wind

L. A. Fisk and G. Gloeckler

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Abstract

Suprathermal particles are ubiquitously present in the solar wind, with energies typically from ~1 keV nucleon⁻¹ to ~a few MeV nucleon⁻¹. Remarkably, the suprathermal particles exhibit a common spectral shape in many different circumstances; the distribution function is a power law in particle speed, with spectral index of –5. The observations cannot be explained by traditional stochastic acceleration, which yields spectra that depend on, e.g., the momentum diffusion coefficient, and thus the spectra are expected to be different in different circumstances. A theory is presented in which the particles are accelerated in thermally isolated compressional turbulence. Thermal isolation is valid in spatially homogeneous conditions in the solar wind, and when properly applied, yields suprathermal tails that always have the required spectral shape. The theory describes the time evolution of the spectrum to its equilibrium form and predicts the high-speed cutoff on the acceleration. The high-speed cutoff is shown to be in good agreement with observations of quiet-time spectra at Earth and is consistent with observations throughout the outer heliosphere.

https://doi.org/10.1086/591543 Cited by References

1474

Laboratory Evidence for Efficient Water Formation in Interstellar Ices

S. Ioppolo, H. M. Cuppen, C. Romanzin, E. F. van Dishoeck, and H. Linnartz

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Abstract

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O₂, or O₃ on icy grains, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O₂ ice is bombarded by H or D atoms under ultrahigh vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H₂O (D₂O) with a rate that is surprisingly independent of temperature. This formation route converts O₂ into H₂O via H₂O₂ and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.

https://doi.org/10.1086/591506 Cited by References

1480

Integrated Near-Infrared Band Strengths of Solid CH₄ and Its Mixtures with N₂

R. Brunetto, G. Caniglia, G. A. Baratta, and M. E. Palumbo

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Abstract

We studied icy CH₄ and its mixtures with N₂ (temperature 16-40 K), using near-IR transmittance spectroscopy (1.0-3.6 μm), and monitoring the film growth using interference patterns of two lasers. We measured peak position, full width at half-maximum, and strengths of the methane bands, and density and real refractive index of the icy films. Results confirm and extend but also partially contradict previous studies on similar mixtures. Experimental data can be applied to interpret observations of solar system (trans-Neptunian objects) and interstellar ices, where methane and nitrogen are believed to be present. We predict the optical depths of two methane NIR bands in the line of sight of some dense molecular clouds.

https://doi.org/10.1086/591509 Cited by References

1486

Gas Phase Study of C⁺ Reactions of Interstellar Relevance

Oscar Martinez, Jr., Nicholas B. Betts, Stephanie M. Villano, Nicole Eyet, Theodore P. Snow, and Veronica M. Bierbaum

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Abstract

The current uncertainty in many reaction rate constants causes difficulties in providing satisfactory models of interstellar chemistry. Here we present new measurements of the rate constants and product branching ratios for the gas phase reactions of C⁺ with NH₃, CH₄, O₂, H₂O, and C₂H₂, using the flowing afterglow-selected ion flow tube (FASIFT) technique. Results were obtained using two instruments that were separately calibrated and optimized; in addition, low ionization energies were used to ensure formation of ground-state C⁺, the purities of the neutral reactants were verified, and mass discrimination was minimized.

https://doi.org/10.1086/591548 Cited by References

1493

Pathways to Oxygen-Bearing Molecules in the Interstellar Medium and in Planetary Atmospheres: Cyclopropenone (c-C₃H₂O) and Propynal (HCCCHO)

Li Zhou, Ralf I. Kaiser, Li Gyun Gao, Agnes H. H. Chang, Mao-Chang Liang, and Yuk L. Yung

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Abstract

We investigated the formation of two C₃H₂O isomers, i.e., cyclopropenone (c-C₃H₂O) and propynal (HCCCHO), in binary ice mixtures of carbon monoxide (CO) and acetylene (C₂H₂) at 10 K in an ultrahigh vacuum machine on high-energy electron irradiation. The chemical evolution of the ice samples was followed online and in situ via a Fourier transform infrared spectrometer and a quadrupole mass spectrometer. The temporal profiles of the cyclopropenone and propynal isomers suggest (pseudo-) first-order kinetics. The cyclic structure (c-C₃H₂O) is formed via an addition of triplet carbon monoxide to ground-state acetylene (or vice versa); propynal (HCCCHO) can be synthesized from a carbon monoxide-acetylene complex via a [HCO...CCH] radical pair inside the matrix cage. These laboratory studies showed for the first time that both C₃H₂O isomers can be formed in low-temperature ices via nonequilibrium chemistry initiated by energetic electrons as formed in the track of Galactic cosmic ray particles penetrating interstellar icy grains in cold molecular clouds. Our results can explain the hitherto unresolved gas phase abundances of cyclopropenone in star-forming regions via sublimation of c-C₃H₂O as formed on icy grains in the cold molecular cloud stage. Implications for the heterogeneous oxygen chemistry of Titan and icy terrestrial planets and satellites suggest that the production of oxygen-bearing molecules such as C₃H₂O may dominate on aerosol particles compared to pure gas phase chemistry.

https://doi.org/10.1086/591072 Cited by References

1503

EAZY: A Fast, Public Photometric Redshift Code

Gabriel B. Brammer, Pieter G. van Dokkum, and Paolo Coppi

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Abstract

We describe a new program for determining photometric redshifts, dubbed EAZY. The program is optimized for cases where spectroscopic redshifts are not available, or are only available for a biased subset of the galaxies. The code combines features from various existing codes: it can fit linear combinations of templates, it includes optional flux- and redshift-based priors, and its user interface is modeled on the popular HYPERZ code. A novel feature is that the default template set, as well as the default functional forms of the priors, are not based on (usually highly biased) spectroscopic samples, but on semianalytical models. Furthermore, template mismatch is addressed by a novel rest-frame template error function. This function gives different wavelength regions different weights, and ensures that the formal redshift uncertainties are realistic. We introduce a redshift quality parameter, Q_z, which provides a robust estimate of the reliability of the photometric redshift estimate. Despite the fact that EAZY is not "trained" on spectroscopic samples, the code (with default parameters) performs very well on existing public data sets. For K-selected samples in CDF-South and other deep fields, we find a 1 σ scatter in Δ z/(1 + z) of 0.034, and we provide updated photometric redshift catalogs for the FIRES, MUSYC, and FIREWORKS surveys.

https://doi.org/10.1086/591786 Cited by References

1514

An Alternative Approach for Imaging in Optical Interferometry

J.-L. Menut, B. Valat, B. Lopez, F.-X. Schmider, F. Vakili, S. Jankov, Y. Bresson, S. Lagarde, R. G. Petrov, A. Domiciano, L. Mosoni, and W. C. Danchi

View abstract View article PDF

Abstract

We describe a mathematical formalism for the teaching optical interferometer concept developed by P. Lawson. In this experiment, the co-addition of several interferometric fringe patterns obtained for different baselines between individual telescopes is the image of the source, for the simple reason that the fringe patterns build up the image of the observed source through a convolution product. This basic principle is of interest since it allows one to tackle the image reconstruction for optical long-baseline interferometry through an approach which is complementary to the use of the Fourier plane. Thus, image reconstruction can be thought of in the fringe plane. It allows a better understanding of the fundamental limits of the image dynamical range.

https://doi.org/10.1086/590418 References

1523

Erratum: "Our Peculiar Motion Away from the Local Void" (ApJ, 676, 184 [2008])

R. Brent Tully, Edward J. Shaya, Igor D. Karachentsev, Hélène M. Courtois, Dale D. Kocevski, Luca Rizzi, and Alan Peel

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https://doi.org/10.1086/591904 Cited by

1524

Erratum: "Aluminum-, Calcium- and Titanium-rich Oxide Stardust in Ordinary Chondrite Meteorites" (ApJ, 682, 1450 [2008])

Larry R. Nittler, Conel M. O'D. Alexander, Roberto Gallino, Peter Hoppe, Ann N. Nguyen, Frank J. Stadermann, and Ernst K. Zinner

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https://doi.org/10.1086/592290 Cited by