Table of contents

Accurate mass determination of clusters of galaxies is crucial if they are to be used as cosmological probes. However, there are some discrepancies between cluster masses determined based on gravitational lensing and X-ray observations assuming strict hydrostatic equilibrium (i.e., the equilibrium gas pressure is provided entirely by thermal pressure). Cosmological simulations suggest that turbulent gas motions remaining from hierarchical structure formation may provide a significant contribution to the equilibrium pressure in clusters. We analyze a sample of massive clusters of galaxies drawn from high-resolution cosmological simulations and find a significant contribution (20%–45%) from non-thermal pressure near the center of relaxed clusters, and, in accord with previous studies, a minimum contribution at about 0.1 R_vir, growing to about 30%–45% at the virial radius, R_vir. Our results strongly suggest that relaxed clusters should have significant non-thermal support in their core region. As an example, we test the validity of strict hydrostatic equilibrium in the well-studied massive galaxy cluster A1689 using the latest high-resolution gravitational lensing and X-ray observations. We find a contribution of about 40% from non-thermal pressure within the core region of A1689, suggesting an alternate explanation for the mass discrepancy: the strict hydrostatic equilibrium is not valid in this region.

We present high spectral resolution, optical spectra of the Herbig Be star MWC 147, in which we spectrally resolve several emission lines, including the [O i] lines at 6300 and 6363 Å. Their highly symmetric, double-peaked line profiles indicate that the emission originates in a rotating circumstellar disk. We deconvolve the Doppler-broadened [O i] emission lines to obtain a measure of emission as a function of distance from the central star. The resulting radial surface brightness profiles are in agreement with a disk structure consisting of a flat, inner, gaseous disk and a flared, outer, dust disk. The transition between these components at 2–3 AU corresponds to the estimated dust sublimation radius. The width of the double-peaked Mg ii line at 4481 Å suggests that the inner disk extends to at least 0.10 AU, close to the corotation radius.

We have investigated the nature of flare emission from Sgr A* during multi-wavelength observations of this source that took place in 2004, 2005, and 2006. We present evidence for dimming of submillimeter and radio flux during the peak of near-IR flares. This suggests that the variability of Sgr A* across its wavelength spectrum is phenomenologically related. The model explaining this new behavior of flare activity could be consistent with adiabatically cooling plasma blobs that are expanding but also partially eclipsing the background quiescent emission from Sgr A*. When a flare is launched, the plasma blob is most compact and is brightest in the optically thin regime whereas the emission in radio/submillimeter wavelengths has a higher opacity. Absorption in the observed light curve of Sgr A* at radio/submillimeter flux is due to the combined effects of lower brightness temperature of plasma blobs with respect to the quiescent brightness temperature and high opacity of plasma blobs. This implies that plasma blobs are mainly placed in the magnetosphere of a disk-like flow or further out in the flow. The depth of the absorption being larger in submillimeter than in radio wavelengths implies that the intrinsic size of the quiescent emission increases with increasing wavelength which is consistent with previous size measurements of Sgr A*. Lastly, we believe that occultation of the quiescent emission of Sgr A* at radio/submillimeter by IR flares can be used as a powerful tool to identify flare activity at its earliest phase of its evolution.

Recent searches by unbiased, wide-field surveys have uncovered a group of extremely luminous optical transients. The initial discoveries of SN 2005ap by the Texas Supernova Search and SCP-06F6 in a deep Hubble pencil beam survey were followed by the Palomar Transient Factory confirmation of host redshifts for other similar transients. The transients share the common properties of high optical luminosities (peak magnitudes ∼−21 to −23), blue colors, and a lack of H or He spectral features. The physical mechanism that produces the luminosity is uncertain, with suggestions ranging from jet-driven explosion to pulsational pair instability. Here, we report the most detailed photometric and spectral coverage of an ultra-bright transient (SN 2010gx) detected in the Pan-STARRS 1 sky survey. In common with other transients in this family, early-time spectra show a blue continuum and prominent broad absorption lines of O ii. However, about 25 days after discovery, the spectra developed type Ic supernova features, showing the characteristic broad Fe ii and Si ii absorption lines. Detailed, post-maximum follow-up may show that all SN 2005ap and SCP-06F6 type transients are linked to supernovae Ic. This poses problems in understanding the physics of the explosions: there is no indication from late-time photometry that the luminosity is powered by ⁵⁶Ni, the broad light curves suggest very large ejected masses, and the slow spectral evolution is quite different from typical Ic timescales. The nature of the progenitor stars and the origin of the luminosity are intriguing and open questions.

Bernstein et al. found that the population of faint (R> 26) trans-Neptunian objects (TNOs) known at that time was dominated by "Classical" objects, which have low inclinations (i < 5°) and distances of 40–45 AU. Since those observations, the number of faint TNOs whose orbits are sufficiently well known to be classified as "Classical" or "Excited" has grown from 7 to 39. We analyze the dynamical classifications of faint TNOs known today and find that this population is dominated by Excited objects. We discuss some implications of this result.

We present an analysis of the large-scale galaxy distribution around two possible warm-hot intergalactic medium (WHIM) absorption systems reported along the Markarian 421 sight line. Using the Sloan Digital Sky Survey (SDSS), we find a prominent galaxy filament at the redshift of the z = 0.027 X-ray absorption line system. The filament exhibits a width of 3.2 Mpc and a length of at least 20 Mpc, comparable to the size of WHIM filaments seen in cosmological simulations. No individual galaxies fall within 350 projected kpc so it is unlikely that the absorption is associated with gas in a galaxy halo or outflow. Another, lower-significance X-ray absorption system was reported in the same Chandra spectrum at z = 0.011, but the large-scale structure in its vicinity is far weaker and may be a spurious alignment. By searching for similar galaxy structures in 140 random smoothed SDSS fields, we estimate an ∼5%–10% probability of the z = 0.027 absorber-filament alignment occurring by chance. If these two systems are indeed physically associated, this would represent the first known coincidence between a large-scale galaxy structure and a blind X-ray WHIM detection.

Galaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, however, unknown. We present the first measurement of the shutdown timescale for an individual quasar using X-ray observations of the nearby galaxy IC 2497, which hosted a luminous quasar no more than 70,000 years ago that is still seen as a light echo in "Hanny's Voorwerp," but whose present-day radiative output is lower by at least two, and more likely by over four, orders of magnitude. This extremely rapid shutdown provides new insight into the physics of accretion in supermassive black holes and may signal a transition of the accretion disk to a radiatively inefficient state.

The existence of stable magnetic configurations in white dwarfs, neutron stars, and various non-convective stellar regions is now well recognized. It has recently been shown numerically that various families of equilibria, including axisymmetric mixed poloidal–toroidal configurations, are stable. Here we test the stability of an analytically derived non-force-free magnetic equilibrium resulting from an initial relaxation (self-organization) process, using three-dimensional magnetohydrodynamic simulations: the obtained mixed configuration is compared with the dynamical evolution of its purely poloidal and purely toroidal components, both known to be unstable. The mixed equilibrium shows no sign of instability under white noise perturbations. This configuration therefore provides a good description of magnetic equilibrium topology inside non-convective stellar objects and will be useful to initialize magneto-rotational transport in stellar evolution codes and in multi-dimensional magnetohydrodynamic simulations.

Hydrogen depleted environments are considered an essential requirement for the formation of fullerenes. The recent detection of C₆₀ and C₇₀ fullerenes in what was interpreted as the hydrogen-poor inner region of a post-final helium shell flash planetary nebula (PN) seemed to confirm this picture. Here, we present strong evidence that challenges the current paradigm regarding fullerene formation, showing that it can take place in circumstellar environments containing hydrogen. We report the simultaneous detection of polycyclic aromatic hydrocarbons (PAHs) and fullerenes toward C-rich and H-containing PNe belonging to environments with very different chemical histories such as our own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon. These observations suggest that modifications may be needed to our current understanding of the chemistry of large organic molecules as well as the chemical processing in space.

The first Herschel Hi-Gal images of the Galactic plane unveil the far-infrared diffuse emission of the interstellar medium with an unprecedented angular resolution and sensitivity. In this Letter, we present the first analysis of these data in combination with those of Spitzer GLIMPSE and MIPSGAL. We selected a relatively diffuse and low excitation region of the l ∼ 59° Hi-Gal Science Demonstration Phase field to perform a pixel-by-pixel fitting of the 8 to 500 μm spectral energy distribution (SED) using the DustEM dust emission model. We derived maps of the very small grain (VSG) and polycyclic aromatic hydrocarbon (PAH) abundances from the model. Our analysis allows us to illustrate that the aromatic infrared band intensity does not necessarily trace the PAH abundance but rather the product of "abundance × column density × intensity of the exciting radiation field." We show that the spatial structure of PACS 70 μm maps resemble the shorter wavelengths (e.g., IRAC 8 μm) maps, because they trace both the intensity of exciting radiation field and column density. We also show that the modeled VSG contribution to PACS 70 μm (PACS 160 μm) band intensity can be up to 50% (7%). The interpretation of diffuse emission spectra at these wavelengths must take stochastically heated particles into account. Finally, this preliminary study emphasizes the potential of analyzing the full dust SED sampled by Herschel and Spitzer data, with a physical dust model (DustEM) to reach the properties of the dust at simultaneously large and small scales.

Rotation measures (RMs) of pulsars and extragalactic point sources have been known to reveal large-scale antisymmetries in the Faraday rotation sky with respect to the Galactic plane and halo that have been interpreted as signatures of the mean magnetic field in the Galactic halo. We describe Faraday rotation measurements of the diffuse Galactic polarized radio emission over a large region in the northern Galactic hemisphere. Through application of RM synthesis we achieve sensitive Faraday rotation maps with high angular resolution, capable of revealing fine-scale structures of ∼1° in the Faraday rotation sky. Our analysis suggests that the observed antisymmetry in the Faraday rotation sky at b>0° is dominated by the magnetic field around a local H i bubble at a distance of 100 pc, and not by the magnetic field of the Galactic halo. We derive physical properties of the magnetic field of this shell, which we find to be 20–34 μG strong. It is clear that the diffuse polarized radio emission contains important information about the local magneto-ionic medium, which cannot yet be derived from Faraday RMs of extragalactic sources or pulsars alone.

We simulate a Kepler-like observation of a theoretical exoplanet population and show that the observed orbital period distribution of the Kepler giant planet candidates is best matched by an average stellar specific dissipation function Q'_* in the interval 10⁶ ≲ Q'_* ≲ 10⁷. In that situation, the few super-Earths that are driven to orbital periods of P < 1 day by dynamical interactions in multiple-planet systems will survive tidal disruption for a significant fraction of the main-sequence lifetimes of their stellar hosts. Consequently, though these very hot super-Earths are not characteristic of the overall super-Earth population, their substantial transit probability implies that they should be significant contributors to the full super-Earth population uncovered by Kepler. As a result, the CoRoT-7 system may be the first representative of a population of very hot super-Earths that we suggest should be found in multiple-planet systems preferentially orbiting the least-dissipative stellar hosts in the Kepler sample.

We report a sizable class of type 1 active galactic nuclei (AGNs) with unusually weak near-infrared (1–3 μm) emission in the XMM-COSMOS type 1 AGN sample. The fraction of these "hot-dust-poor" AGNs increases with redshift from 6% at low redshift (z < 2) to 20% at moderate high redshift (2 < z < 3.5). There is no clear trend of the fraction with other parameters: bolometric luminosity, Eddington ratio, black hole mass, and X-ray luminosity. The 3 μm emission relative to the 1 μm emission is a factor of 2–4 smaller than the typical Elvis et al. AGN spectral energy distribution (SED), which indicates a "torus" covering factor of 2%–29%, a factor of 3–40 smaller than required by unified models. The weak hot dust emission seems to expose an extension of the accretion disk continuum in some of the source SEDs. We estimate the outer edge of their accretion disks to lie at (0.3–2.0) × 10⁴ Schwarzschild radii, ∼10–23 times the gravitational stability radii. Formation scenarios for these sources are discussed.

To address the question of whether the so-called ultracompact dwarf galaxies (UCDs) are the remnant nuclei of destroyed early-type dwarf galaxies (dEs), we analyze the stellar population parameters of the nuclei of 34 Virgo dEs, as well as 10 Virgo UCDs, including one that we discovered and which we report on here. Based on absorption-line strength (Lick index) measurements, we find that nuclei of Virgo dEs have younger stellar population ages than UCDs, with averages of 5 Gyr and >10 Gyr, respectively. In addition to this, the metallicity also differs: dE nuclei are on average more metal-rich than UCDs. On the other hand, comparing the stellar population parameters at the same local galaxy density, with UCDs being located in the high-density cluster regions, we do not find any difference in the stellar populations of dE nuclei and UCDs. In those regions, the dE nuclei are as old and as metal-poor as UCDs. This evidence suggests that the Virgo UCDs may have formed through the stripping of dE nuclei.

We present a new method for probing the physical conditions and metal enrichment of the intergalactic medium: the composite spectrum of Lyα forest absorbers. We apply this technique to a sample of 9480 Lyα absorbers with redshift 2 < z < 3.5 identified in the spectra of 13,279 high-redshift quasars from the Sloan Digital Sky Survey (SDSS) Fifth Data Release (DR5). Absorbers are selected as local minima in the spectra with 2.4 < τ_Lyα < 4.0; at SDSS resolution (≈150 km s⁻¹ FWHM), these absorbers are blends of systems that are individually weaker. In the stacked spectra, we detect seven Lyman series lines and metal lines of O vi, N v, C iv, C iii, Si iv, C ii, Al ii, Si ii, Fe ii, Mg ii, and O i. Many of these lines have peak optical depths of <0.02, but they are nonetheless detected at high statistical significance. Modeling the Lyman series measurements implies that our selected systems have total H i column densities $N_{{\rm H}\,{\mathsc{i}}} \approx 10^{15.4}\,{\rm cm}^{-2}$. Assuming typical physical conditions $\rho /\bar{\rho }=10$, T = 10⁴–10^4.5 K, and [Fe/H]=−2 yields reasonable agreement with the line strengths of high-ionization species, but it underpredicts the low-ionization species by two orders of magnitude or more. This discrepancy suggests that the low-ionization lines arise in dense, cool, metal-rich clumps, present in some absorption systems.

Recent neutron star observations suggest that the masses and radii of neutron stars may be smaller than previously considered, which would disfavor a purely nucleonic equation of state (EoS). In our model, we use a flavor SU(3) sigma model that includes Δ resonances and hyperons in the EoS. We find that if the coupling of the Δ resonances to the vector mesons is slightly smaller than that of the nucleons, we can reproduce both the measured mass–radius relationship and the extrapolated EoS.

We have spectroscopically discovered a pair of twin, nitrogen-type, hydrogen-rich, Wolf-Rayet stars (WN8–9h) that are both surrounded by circular, mid-infrared-bright nebulae detected with the Spitzer Space Telescope and MIPS instrument. The emission is probably dominated by a thermal continuum from cool dust, but also may contain contributions from atomic line emission. There is no counterpart at shorter Spitzer/IRAC wavelengths, indicating a lack of emission from warm dust. The two nebulae are probably wind-swept stellar ejecta released by the central stars during a prior evolutionary phase. The nebulae partially overlap on the sky and we speculate on the possibility that they are in the early stage of a collision. Two other evolved massive stars have also been identified within the area subtended by the nebulae, including a carbon-type Wolf-Rayet star (WC8) and an O7–8 III–I star, the latter of which appears to be embedded in one of the larger WN8–9h nebulae. The derived distances to these stars imply that they are coeval members of an association lying 4.9 ± 1.2 kpc from Earth, near the intersection of the Galaxy's Long Bar and the Scutum-Centaurus spiral arm. This new association represents an unprecedented display of complex interactions between multiple stellar winds, outflows, and the radiation fields of evolved massive stars.

We use a six-year data set (2004–2010) of mid-infrared spectra measured by Cassini's Composite InfraRed Spectrometer to search for seasonal variations in Titan's atmospheric temperature and composition. During most of Cassini's mission Titan's northern hemisphere has been in winter, with an intense stratospheric polar vortex highly enriched in trace gases, and a single south-to-north circulation cell. Following northern spring equinox in mid-2009, dramatic changes in atmospheric temperature and composition were expected, but until now the temporal coverage of polar latitudes has been too sparse to discern trends. Here, we show that during equinox and post-equinox periods, abundances of trace gases at both poles have begun to increase. We propose that increases in north polar trace gases are due to a seasonal reduction in gas depletion by horizontal mixing across the vortex boundary. A simultaneous south polar abundance increase suggests that Titan is now entering, or is about to enter, a transitional circulation regime with two branches, rather than the single branch circulation pattern previously observed.

In 1997 and 2008 we used the WFPC2 camera on board the Hubble Space Telescope to obtain two sets of narrow-band Hα images of the runaway Wolf-Rayet (WR) star WR 124 surrounded by its nebula M1–67. This two-epoch imaging provides an expansion parallax and thus a practically assumption-free geometric distance to the nebula, d = 3.35 ± 0.67 kpc. Combined with the global velocity distribution in the ejected nebula, this confirms the extreme runaway status of WR 124. WR stars embedded within such ejection nebulae at the point of core collapse would produce different supernova characteristics from those expected for stars surrounded by wind-filled cavities. In galaxies with extremely low ambient metallicity, Z ⩽ 10⁻³Z_☉, γ-ray bursts originating from fast-moving runaway WR stars may produce afterglows which appear to be coming from regions with a relatively homogeneous circumburst medium.

Quasi-biennial oscillations (QBOs) are frequently observed in solar activity indices. However, no clear physical mechanism for the observed variations has been suggested so far. Here, we study the stability of magnetic Rossby waves in the solar tachocline using the shallow water magnetohydrodynamic approximation. Our analysis shows that the combination of typical differential rotation and a toroidal magnetic field with a strength of ⩾10⁵ G triggers the instability of the m = 1 magnetic Rossby wave harmonic with a period of ∼2 years. This harmonic is antisymmetric with respect to the equator and its period (and growth rate) depends on the differential rotation parameters and magnetic field strength. The oscillations may cause a periodic magnetic flux emergence at the solar surface and consequently may lead to the observed QBO in solar activity features. The period of QBOs may change throughout a cycle, and from cycle to cycle, due to variations of the mean magnetic field and differential rotation in the tachocline.

We investigate structure and kinematics of the second generation of stars (SG) formed from gaseous ejecta of the first generation of stars (FG) in forming globular clusters (GCs). We consider that SG can be formed from gaseous ejecta from asymptotic giant branch stars of FG with the initial total mass of 10⁶ M_☉–10⁸ M_☉ to explain the present masses of the Galactic GCs. Our three-dimensional hydrodynamical simulations with star formation show that SG formed in the central regions of FG can have a significant amount of rotation (V/σ≈ 0.8–2.5). The rotational amplitude of SG can depend strongly on the initial kinematics of FG. We thus propose that some GCs composed of FG and SG had a significant amount of rotation when they were formed. We also suggest that although later long-term (∼10 Gyr) dynamical evolution of stars can smooth out the initial structural and kinematical differences between FG and SG to a large extent, initial flattened structures and rotational kinematics of SG can be imprinted on shapes and internal rotation of the present GCs. We discuss these results in terms of internal rotation observed in the Galactic GCs.

Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H] <−2 dex, and down to below −4 dex, can be efficiently identified within the Radial Velocity Experiment (RAVE) survey of bright stars, without requiring additional confirmatory observations. We determine a calibration of the equivalent width of the calcium triplet lines measured from the RAVE spectra onto true [Fe/H], using high spectral resolution data for a subset of the stars. These RAVE iron abundances are accurate enough to obviate the need for confirmatory higher-resolution spectroscopy. Our initial study has identified 631 stars with [Fe/H] ⩽−2, from a RAVE database containing approximately 200,000 stars. This RAVE-based sample is complete for stars with [Fe/H] ≲−2.5, allowing statistical sample analysis. We identify three stars with [Fe/H] ≲−4. Of these, one was already known to be "ultra metal-poor," one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H] = −4.0, rather than the published [Fe/H] = −3.3, and derive [C/Fe] = +0.9, and [N/Fe] = +3.2, and the third is at the limit of our signal-to-noise ratio. RAVE observations are ongoing and should prove to be a rich source of bright, easily studied, very metal-poor stars.

We model multiwavelength afterglow data from the short gamma-ray burst (GRB) 090510 using a combined leptonic–hadronic model of synchrotron radiation from an adiabatic blast wave. High-energy, ≳100 MeV, emission in our model is dominated by proton-synchrotron radiation, while electron-synchrotron radiation dominates in the X-ray and ultraviolet wavelengths. The collimation-corrected GRB energy, depending on the jet-break time, in this model could be as low as 3 × 10⁵¹ erg but two orders of magnitude larger than the absolute γ-ray energy. We also calculated the opacities for electron–positron pair production by γ-rays and found that TeV γ-rays from proton-synchrotron radiation can escape the blast wave at early time, and their detection can provide evidence of a hadronic emission component dominating at high energies.

To characterize the magnetic field structure of the outflow and core region within a prototypical high-mass star-forming region, we analyzed polarized CO(3–2)—for the first time observed with the Submillimeter Array—as well as 880 μm submillimeter continuum emission from the high-mass outflow/disk system IRAS 18089−1732. Both emission features with polarization degrees at a few percent level indicate that the magnetic field structure is largely aligned with the outflow/jet orientation from small core scales to larger outflow scales. Although quantitative estimates are crude, the analysis indicates that turbulent energy dominates over magnetic energy. The data also suggest a magnetic field strength increase from the lower-density envelope to the higher-density core.

We present optical images of 217P/LINEAR taken with the Kiso 105 cm Schmidt Telescope. The images revealed a day-by-day variation of the 217P/LINEAR shape, indicating an outburst occurred. Since our observation started before the outburst event, we examined the morphological evolution of the expanding dust cloud produced by the outburst. It was found that the dust cloud expanded at a velocity of 120–140 m s⁻¹ and that the comet became brighter by 1.7–2.3 mag. Using the observational result, we estimated that the onset time was 2009 October 13.4 and that the total mass released by the outburst was in the range of 10⁶–10⁹ kg. No fragments or split nuclei brighter than 18.5 mag (1.1 km in radius) were confirmed in our observations. Compared with other outbursts, we concluded that it was a relatively small event for an outburst.

We estimated the dynamical surface mass density (Σ) at the solar Galactocentric distance between 2 and 4 kpc from the Galactic plane, as inferred from the observed kinematics of the thick disk. We find Σ(z = 2 kpc) = 57.6 ± 5.8 M_☉ pc⁻², and it shows only a tiny increase in the z range considered by our investigation. We compared our results with the expectations for the visible mass, adopting the most recent estimates in the literature for contributions of the Galactic stellar disk and interstellar medium, and proposed models of the dark matter distribution. Our results match the expectation for the visible mass alone, never differing from it by more than 0.8 M_☉ pc⁻² at any z, and thus we find little evidence for any dark component. We assume that the dark halo could be undetectable with our method, but the dark disk, recently proposed as a natural expectation of the ΛCDM models, should be detected. Given the good agreement with the visible mass alone, models including a dark disk are less likely, but within errors its existence cannot be excluded. In any case, these results put constraints on its properties: thinner models (scale height lower than 4 kpc) reconcile better with our results and, for any scale height, the lower-density models are preferred. We believe that successfully predicting the stellar thick disk properties and a dark disk in agreement with our observations could be a challenging theoretical task.