Table of contents

The photon spectral energy distribution of the powerful transient Sw J1644+57 resembles those of the brightest ultra-luminous X-ray sources (ULXs). The transient nature of Sw J1644+57 is likely the result of a tidal disruption of a star by a supermassive black hole. The stellar disk generates accretion power at super-Eddington rates and the observational properties of Sw J1644+57 indicate—in analogy with ULXs—that the accretion flow maintains a high level of radiative efficiency with a corresponding super-Eddington luminosity. Due to its similarity to ULXs, this powerful transient may be thought of as an ultra-luminous X-ray event (ULX-E). Observational tests for this ULX-E model are proposed as well.

We present compelling evidence for the complexity of the Fornax dwarf spheroidal. By disentangling three different stellar subpopulations in its red giant branch, we are able to study in detail the dependence between kinematics and metallicity. A well-defined ordering in velocity dispersion, spatial concentration, and metallicity is evident in the subpopulations. We also present evidence for a significant misalignment between the angular momentum vectors of the old- and intermediate-age populations. According to the Hubble Space Telescope measurement of Fornax's proper motion, this corresponds to counter-rotation. These ingredients are used to construct a novel evolutionary history of the Fornax dwarf spheroidal, characterized as a late merger of a bound pair.

The interstellar medium (ISM) is a dynamical system, in which the plasma is naturally driven out of ionization equilibrium due to atomic and dynamic processes operating on different timescales. We report on the first 0.5 pc resolution three-dimensional hydrodynamical simulation of the ISM to date, including the disk–halo interaction, and featuring a detailed time-dependent calculation of the non-equilibrium ionization structure. In particular, we study the effects of the history of the plasma on the cooling functions and determine the associated X-ray emission at low temperatures. The main results of this work are: (1) in a dynamical ISM, the time-dependent ionization structure, and therefore the cooling function, varies in space and time depending on the initial conditions and its history; (2) the cooling paths can be quite different for gases with the same initial temperature, but having different evolution histories; and (3) due to delayed recombination in a dynamic plasma, X-ray emission can occur at low temperatures becoming, eventually, stronger than the corresponding emission from a plasma in collisional ionization equilibrium at 10^6.2 K. This has far-reaching consequences for the interpretation of EUV/X-ray spectra. We emphasize that these results can be generalized and applied to any astrophysical system, in which radiative cooling is relevant.

Taking into account the rotation of mass-accreting white dwarfs (WDs) whose masses exceed the Chandrasekhar mass, we extend our new single degenerate model for the progenitors of Type Ia supernovae (SNe Ia), accounting for two types of binary systems: those with a main-sequence companion and those with a red-giant (RG) companion. We present a mass distribution of WDs exploding as SNe Ia, where the WD mass ranges from 1.38 to 2.3 M_☉. These progenitor models are assigned to various types of SNe Ia. A lower mass range of WDs (1.38 M_☉ < M_WD ≲ 1.5 M_☉), which are supported by rigid rotation, corresponds to normal SNe Ia. A variety of spin-down time may lead to a variation of brightness. A higher mass range of WDs (M_WD ≳ 1.5 M_☉), which are supported by differential rotation, corresponds to brighter SNe Ia such as SN 1991T. In this case, a variety of the WD mass may lead to a variation of brightness. We also show the evolutionary states of the companion stars at SN Ia explosions and pose constraints on the unseen companions. In the WD+RG systems, in particular, most of the RG companions have evolved to helium/carbon–oxygen WDs in the spin-down phase before the SN Ia explosions. In such a case, we do not expect any prominent signature of the companion immediately before and after the explosion. We also compare our new models with the recent stringent constraints on the unseen progenitors of SNe Ia such as SN 2011fe.

New spectroscopic observations show that the double-degenerate system NLTT 16249 is in a close orbit (a = 5.6 ± 0.3 R_☉) with a period of 1.17 days. The total mass of the system is estimated between 1.47 and 2.04 M_☉ but it is not expected to merge within a Hubble timescale (t_merge ≈ 10¹¹ yr). Vennes & Kawka originally identified the system because of the peculiar composite hydrogen (DA class) and molecular (C₂–DQ class and CN) spectra and the new observations establish this system as the first DA plus DQ close double degenerate. Also, the DQ component was the first of its class to show nitrogen dredged up from the core in its atmosphere. The star may be viewed as the first known DQ descendant of the born-again PG1159 stars. Alternatively, the presence of nitrogen may be the result of past interactions and truncated evolution in a close binary system.

As part of a program to study translucent interstellar clouds, NGC 2024 IRS 1 was observed by the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope. IRS 1 is a heavily reddened B0.5 V star (E(B − V) = 1.69, R_V = 4.5, and A_V = 7.61) lying outside the core of the NGC 2024 cluster and not considered to be the ionizing source of the nebula. At wavelengths below about 1300 Å, the observed spectrum deviates from that expected of a reddened B star, showing an increase in flux to shorter wavelengths and the presence of P-Cygni profiles in both the C iii] λ1176 and N v λ1240 lines. Because of the presence of these lines, the absence of any P-Cygni signature in the Si iv λ1394/1403 lines, and the measurement of extended emission on the detector focal plane, we believe this portion of the spectrum to be scattered light from an O6 to B0 supergiant. The COS entrance aperture is 2.5 arcsec in diameter and can let in substantial scattered light from the nebula. NGC 2024 IRS 2b is an O8 star and believed to be the ionizing source of the nebula. It is currently identified as a main-sequence star, but should it be a supergiant the presence of such an evolved star implies an age for the cluster of the order of 3 Myr, in conflict with age determinations based on infrared spectroscopy, and may have implications for the energetics and density structure of the nebula.

On 2012 May 17.2 UT, only 1.5 ± 0.2 days after explosion, we discovered SN 2012cg, a Type Ia supernova (SN Ia) in NGC 4424 (d ≈ 15 Mpc). As a result of the newly modified strategy employed by the Lick Observatory Supernova Search, a sequence of filtered images was obtained starting 161 s after discovery. Utilizing recent models describing the interaction of supernova (SN) ejecta with a companion star, we rule out a ∼1 M_☉ companion for half of all viewing angles and a red-giant companion for nearly all orientations. SN 2012cg reached a B-band maximum of 12.09 ± 0.02 mag on 2012 June 2.0 and took ∼17.3 days from explosion to reach this, typical for SNe Ia. Our pre-maximum-brightness photometry shows a narrower-than-average B-band light curve for SN 2012cg, though slightly overluminous at maximum brightness and with normal color evolution (including some of the earliest SN Ia filtered photometry ever obtained). Spectral fits to SN 2012cg reveal ions typically found in SNe Ia at early times, with expansion velocities ≳14,000 km s⁻¹ at 2.5 days past explosion. Absorption from C ii is detected early, as well as high-velocity components of both Si ii λ6355 and Ca ii. Our last spectrum (13.5 days past explosion) resembles that of the somewhat peculiar SN Ia 1999aa. This suggests that SN 2012cg will have a slower-than-average declining light curve, which may be surprising given the faster-than-average rising light curve.

Most of the baryons from galaxies have been "missing" and several studies have attempted to map the circumgalactic medium (CGM) of galaxies in their quest. We report on X-ray observations made with the Chandra X-Ray Observatory probing the warm-hot phase of the CGM of our Milky Way at about 10⁶ K. We detect O vii and O viii absorption lines at z = 0 in extragalactic sight lines and measure accurate column densities using both Kα and Kβ lines of O vii. We then combine these measurements with the emission measure of the Galactic halo from literature to derive the density and the path length of the CGM. We show that the warm-hot phase of the CGM is massive, extending over a large region around the Milky Way, with a radius of over 100 kpc. The mass content of this phase is over 10 billion solar masses, many times more than that in cooler gas phases and comparable to the total baryonic mass in the disk of the Galaxy. The missing mass of the Galaxy appears to be in this warm-hot gas phase.

A supernova is a likely source of short-lived radioisotopes (SLRIs) that were present during the formation of the earliest solar system solids. A suitably thin and dense supernova shock wave may be capable of triggering the self-gravitational collapse of a molecular cloud core while simultaneously injecting SLRIs. Axisymmetric hydrodynamics models have shown that this injection occurs through a number of Rayleigh–Taylor (RT) rings. Here we use the FLASH adaptive mesh refinement hydrodynamics code to calculate the first fully three-dimensional (3D) models of the triggering and injection process. The axisymmetric RT rings become RT fingers in 3D. While ∼100 RT fingers appear early in the 3D models, only a few RT fingers are likely to impact the densest portion of the collapsing cloud core. These few RT fingers must then be the source of any SLRI spatial heterogeneity in the solar nebula inferred from isotopic analyses of chondritic meteorites. The models show that SLRI injection efficiencies from a supernova several parsecs away fall at the lower end of the range estimated for matching SLRI abundances, perhaps putting them more into agreement with recent reassessments of the level of ⁶⁰Fe present in the solar nebula.

We report the existence of spiral arms in the recently formed gaseous and dusty disk of the closest giant elliptical, NGC 5128 (Centaurus A), using high-resolution ¹²CO(2–1) observations of the central 3' (3 kpc) obtained with the Submillimeter Array. This provides evidence that spiral-like features can develop within ellipticals if enough cold gas exists. We elucidate the distribution and kinematics of the molecular gas in this region with a resolution of 44 × 19 (80 pc × 40 pc). The spiral arms extend from the circumnuclear gas at a radius of 200 pc to at least 1 kpc. The general properties of the arms are similar to those in spiral galaxies: they are trailing, the width is ∼500 ± 200 pc, and the pitch angle is 20°. From independent estimates of the time when the H i-rich galaxy merger occurred, we infer that the formation of spiral arms happened on a timescale of less than ∼10⁸ yr. The formation of spiral arms increases the gas density and thus the star formation efficiency in the early stages of the formation of a disk.

Planetary systems discovered by the Kepler space telescope exhibit an intriguing feature. While the period ratios of adjacent low-mass planets appear largely random, there is a significant excess of pairs that lie just wide of resonances and a deficit on the near side. We demonstrate that this feature naturally arises when two near-resonant planets interact in the presence of weak dissipation that damps eccentricities. The two planets repel each other as orbital energy is lost to heat. This moves near-resonant pairs just beyond resonance, by a distance that reflects the integrated dissipation they experienced over their lifetimes. We find that the observed distances may be explained by tides if tidal dissipation is unexpectedly efficient (tidal quality factor ∼10). Once the effect of resonant repulsion is accounted for, the initial orbits of these low-mass planets show little preference for resonances. This could constrain their origin.

For the first time, we study the evolution of the stellar mass–size relation for star-forming galaxies from z ∼ 4 to z ∼ 7 from Hubble-WFC3/IR camera observations of the HUDF and Early Release Science field. The sizes are measured by determining the best-fit model to galaxy images in the rest-frame 2100 Å with the stellar masses estimated from spectral energy distribution fitting to rest-frame optical (from Spitzer/IRAC) and UV fluxes. We show that the stellar mass–size relation of Lyman break galaxies (LBGs) persists, at least to z ∼ 5, and the median size of LBGs at a given stellar mass increases toward lower redshifts. For galaxies with stellar masses of 9.5 < log (M_*/M_☉) < 10.4, sizes evolve as (1 + z)^{−1.20 ± 0.11}. This evolution is very similar for galaxies with lower stellar masses of 8.6 < log (M_*/M_☉) < 9.5 which is r_e∝(1 + z)^{−1.18 ± 0.10}, in agreement with simple theoretical galaxy formation models at high z. Our results are consistent with previous measurements of the LBGs mass–size relation at lower redshifts (z ∼ 1–3).

The initial conditions of massive star and star cluster formation are expected to be cold, dense, and high column density regions of the interstellar medium, which can reveal themselves via near-, mid-, and even far-infrared absorption as infrared dark clouds (IRDCs). Elucidating the dynamical state of IRDCs thus constrains theoretical models of these complex processes. In particular, it is important to assess whether IRDCs have reached virial equilibrium, where the internal pressure balances that due to the self-gravitating weight of the cloud plus the pressure of the external environmental. We study this question for the filamentary IRDC G035.39–00.33 by deriving mass from combined NIR and MIR extinction maps and velocity dispersion from C¹⁸O (1–0) and (2–1) line emission. In contrast to our previous moderately super-virial results based on ¹³CO emission and MIR-only extinction mapping, with improved mass measurements we now find that the filament is consistent with being in virial equilibrium, at least in its central parsec-wide region where ∼1000 M_☉ snakes along several parsecs. This equilibrium state does not require large-scale net support or confinement by magnetic fields.

Using a sample of 28 H ii regions from the literature with measured temperature inhomogeneity parameter, t², we present a statistical correction to the chemical abundances determined with the T_e(4363/5007) method. We used the t² values to correct the oxygen gaseous abundances and consider the oxygen depletion into dust to calculate the total abundances for these objects. This correction is used to obtain a new calibration of Pagel's strong-line method, R₂₃, to determine oxygen abundances in H ii regions. Our new calibration simultaneously considers the temperature structure, the ionization structure, and the fraction of oxygen depleted into dust grains. Previous calibrations in the literature have included one or two of these factors; this is the first time all three are taken into account. This recalibration reconciles the systematic differences among the temperatures found from different methods. We find that the total correction due to thermal inhomogeneities and dust depletion amounts to an increase in the O/H ratio of H ii regions by factors of 1.7–2.2 (or 0.22–0.35 dex). This result has important implications in various areas of astrophysics such as the study of the higher end of the initial mass function, the star formation rate, and the mass–metallicity relation of galaxies, among others.

We present an efficient numerical self-consistent field method for calculating a gravitational model of a rotating liquid planet to spherical harmonic degree ∼30 and a precision ∼10⁻¹² in the external gravity field. The method's accuracy is validated by comparing results, for Jupiter rotation parameters, with the exact Maclaurin constant-density solution. The method can be generalized to non-constant density.

The intergalactic medium was reionized before redshift z ∼ 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H₂) cooled gas inside the smallest galaxies, minihalos (MHs) of mass between 10⁵ and 10⁸ M_☉. Although the very first stars began forming inside these MHs before redshift z ∼ 40, their contribution has, to date, been ignored in large-scale simulations of this cosmic reionization. Here we report results from the first reionization simulations to include these first stars and the radiative feedback that limited their formation, in a volume large enough to follow the crucial spatial variations that influenced the process and its observability. We show that, while MH stars stopped far short of fully ionizing the universe, reionization began much earlier with MH sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. This boost should be readily detectable by Planck, although within current Wilkinson Microwave Anisotropy Probe uncertainties. If reionization ended as late as z_ov ≲ 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by other probes.

Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a carbon–oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such "tidal novae" may occur in all WD binaries containing a CO WD, at orbital periods between 5 minutes and 20 minutes, and precede the final merger by 10⁵–10⁶ years.

We present results from the first numerical analysis to support the hypothesis, first proposed in Coleman et al., that the Fornax dwarf galaxy was formed from the minor merging of two dwarfs about 2 Gyr ago. Using orbits for the Fornax dwarf that are consistent with the latest proper motion measurements, our dynamical evolution models show that the observed asymmetric shell-like substructures can be formed from the remnant of a smaller dwarf during minor merging. These models also predict the formation of diffuse stellar streams. We discuss how these stellar substructures depend on model parameters of dwarf–dwarf merging, and how the intermediate-age subpopulations found in the vicinity of these substructures may be formed from gas accretion in past merger events. We also suggest that one of Fornax's globular clusters originates from a merged dwarf companion, and demonstrate where as yet undetected tidal streams or H i gas formed from the dwarf merging may be found in the outer halo of the Galaxy.

The existence of 10⁹M_☉ black holes (BHs) in massive galaxies by z ∼ 7 is one of the great unsolved mysteries in cosmological structure formation. One theory argues that they originate from the BHs of Pop III stars at z ∼ 20 and then accrete at the Eddington limit down to the epoch of reionization, which requires that they have constant access to rich supplies of fuel. Because early numerical simulations suggested that Pop III stars were ≳100 M_☉, the supermassive black hole (SMBH) seeds considered up to now were 100–300 M_☉. However, there is a growing numerical and observational consensus that some Pop III stars were tens of solar masses, not hundreds, and that 20–40 M_☉ BHs may have been much more plentiful at high redshift. However, we find that natal kicks imparted to 20–40 M_☉ Pop III BHs during formation eject them from their halos and hence their fuel supply, precluding them from Eddington-limit growth. Consequently, SMBHs are far less likely to form from low-mass Pop III stars than from very massive ones.

We report the discovery of a one-sided 36 (24 kpc, projected) long jet in the high-redshift, z = 4.72, quasar GB 1428+4217 in new Chandra X-ray and Very Large Array (VLA) radio observations. This is the highest redshift kiloparsec-scale X-ray/radio jet known. Analysis of archival very long baseline interferometry 2.3 and 8.6 GHz data reveal a faint one-sided jet extending out to ∼200 pc and aligned to within ∼30° of the Chandra/VLA emission. The 36 distant knot is not detected in an archival Hubble Space Telescope image, and its broadband spectral energy distribution is consistent with an origin from inverse Compton scattering of cosmic microwave background photons for the X-rays. Assuming also equipartition between the radiating particles and magnetic field, the implied jet Lorentz factor is ≈5. This is similar to the other two known z ∼ 4 kpc scale X-ray jet cases and smaller than typically inferred in lower-redshift cases. Although there are still but a few such very high redshift quasar X-ray jets known, for an inverse Compton origin, the present data suggest that they are less relativistic on large scales than their lower-redshift counterparts.

We report on the discovery of large-amplitude flickering from V648 Car (= SS73-17), a poorly studied object listed among the very few hard X-ray-emitting symbiotic stars. We performed millimagnitude precision optical photometry with the Swope Telescope at the Las Campanas Observatory, Chile, and found that V648 Car shows large U-band variability over timescales of minutes. To our knowledge, it exhibits some of the largest flickering of a symbiotic star ever reported. Our finding supports the hypothesis that symbiotic white dwarfs producing hard X-rays are predominantly powered by accretion, rather than quasi-steady nuclear burning, and have masses close to the Chandrasekhar limit. No significant periodicity is evident from the flickering light curve. The All Sky Automated Survey long-term V light curve suggests the presence of a tidally distorted giant accreting via Roche lobe overflow, and a binary period of ∼520 days. On the basis of the outstanding physical properties of V648 Car as hinted at by its fast and long-term optical variability, as well as by its nature as a hard X-ray emitter, we therefore call for simultaneous follow-up observations in different bands, ideally combined with time-resolved optical spectroscopy.

The origin of superluminous supernovae (SLSNe), especially the source of their huge luminosities, has not been clarified yet. While a strong interaction between SN ejecta and dense circumstellar media (CSM) is a leading scenario, alternative models have been proposed. In this Letter, we suggest new diagnostics to discriminate the strong SN–CSM interaction scenario from the others: a decline in the luminosity ("dip") before the main peak of the light curve (LC). This dip is an unavoidable consequence of having a dense CSM within which the shock breakout occurs. If a dense CSM shell is located far at large radii from the progenitor inside, it takes time for the SN ejecta to reach it and the early LC can be powered by the SN ejecta before the collision. Once the SN ejecta collides with the dense CSM, the electron density and thus the Thomson scattering opacity suddenly increase. Photons are unable to go out of the shock even if there is a source of emission inside, which results in the dip in the LC. This dip is a solid prediction from the strong interaction scenario irrespective of power source for the early emission. Eventually, the forward shock breaks out from within the dense CSM, and the luminosity increases through continuous strong SN–CSM interaction, resulting in an SLSN. The possible dip observed in the hydrogen-poor SLSN, 2006oz, could be the first example of this signature and give support to the SN–CSM interaction as the power source of SLSN 2006oz.

Far-ultraviolet (FUV) radiation plays an important role in determining chemical abundances in protoplanetary disks. H i Lyman α (Lyα) is suspected to be the dominant component of the FUV emission from Classical T Tauri Stars (CTTSs), but is difficult to measure directly due to circumstellar and interstellar H i absorption. To better characterize the intrinsic Lyα radiation, we present FUV spectra of 14 CTTSs taken with the Hubble Space Telescope Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph instruments. H₂ fluorescence, commonly seen in the spectra of CTTSs, is excited by Lyα photons, providing an indirect measure of the Lyα flux incident upon the warm disk surface. We use observed H₂ progression fluxes to reconstruct the CTTS Lyα profiles. The Lyα flux correlates with total measured FUV flux, in agreement with an accretion-related source of FUV emission. With a geometry-independent analysis, we confirm that in accreting T Tauri systems Lyα radiation dominates the FUV flux (∼1150 Å –1700 Å). In the systems surveyed this one line comprises 70%–90% of the total FUV flux.

Understanding the evolution of organic molecules in ice grains in the interstellar medium (ISM) under cosmic rays, stellar radiation, and local electrons and ions is critical to our understanding of the connection between ISM and solar systems. Our study is aimed at reaching this goal of looking directly into radiation-induced processing in these ice grains. We developed a two-color laser-desorption laser-ionization time-of-flight mass spectroscopic method (2C-MALDI-TOF), similar to matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectroscopy. Results presented here with polycyclic aromatic hydrocarbon (PAH) probe molecules embedded in water-ice at 5 K show for the first time that hydrogenation and oxygenation are the primary chemical reactions that occur in astrophysical ice analogs when subjected to Lyα radiation. We found that hydrogenation can occur over several unsaturated bonds and the product distribution corresponds to their stabilities. Multiple hydrogenation efficiency is found to be higher at higher temperatures (100 K) compared to 5 K—close to the interstellar ice temperatures. Hydroxylation is shown to have similar efficiencies at 5 K or 100 K, indicating that addition of O atoms or OH radicals to pre-ionized PAHs is a barrierless process. These studies—the first glimpses into interstellar ice chemistry through analog studies—show that once accreted onto ice grains PAHs lose their PAH spectroscopic signatures through radiation chemistry, which could be one of the reason for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.