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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,747 records · Page 97
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DOE OSTI2014

The close binary frequency of Wolf-Rayet stars as a function of metallicity in M31 and M33

Massive star evolutionary models generally predict the correct ratio of WC-type and WN-type Wolf-Rayet stars at low metallicities, but underestimate the ratio at higher (solar and above) metallicities. One possible explanation for this failure is perhaps single-star models are not sufficient and Roche-lobe overflow in close binaries is necessary to produce the 'extra' WC stars at higher metallicities. However, this would require the frequency of close massive binaries to be metallicity dependent. Here we test this hypothesis by searching for close Wolf-Rayet binaries in the high metallicity environments of M31 and the center of M33 as well as in the lower metallicity environments of the middle and outer regions of M33. After identifying ∼100 Wolf-Rayet binaries based on radial velocity variations, we conclude that the close binary frequency of Wolf-Rayets is not metallicity dependent and thus other factors must be responsible for the overabundance of WC stars at high metallicities. However, our initial identifications and observations of these close binaries have already been put to good use as we are currently observing additional epochs for eventual orbit and mass determinations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Chemical and kinematical properties of galactic bulge stars surrounding the stellar system Terzan 5

As part of a study aimed at determining the kinematical and chemical properties of Terzan 5, we present the first characterization of the bulge stars surrounding this puzzling stellar system. We observed 615 targets located well beyond the tidal radius of Terzan 5 and found that their radial velocity distribution is well described by a Gaussian function peaked at (v {sub rad}) = +21.0 ± 4.6 km s{sup –1} with dispersion σ {sub v} = 113.0 ± 2.7 km s{sup –1}. This is one of the few high-precision spectroscopic surveys of radial velocities for a large sample of bulge stars in such a low and positive latitude environment (b = +1.°7). We found no evidence of the peak at (v {sub rad}) ∼ +200 km s{sup –1} found in Nidever et al. Strong contamination of many observed spectra by TiO bands prevented us from deriving the iron abundance for the entire spectroscopic sample, introducing a selection bias. The metallicity distribution was finally derived for a subsample of 112 stars in a magnitude range where the effect of the selection bias is negligible. The distribution is quite broad and roughly peaked at solar metallicity ([Fe/H] ≅ +0.05 dex) with a similar number of stars in the super-solar and in the sub-solar ranges. The population number ratios in different metallicity ranges agree well with those observed in other low-latitude bulge fields, suggesting (1) the possible presence of a plateau for |b| < 4° in the ratio between stars in the super-solar (0 < [Fe/H] <0.5 dex) and sub-solar (–0.5 < [Fe/H] <0 dex) metallicity ranges; (2) a severe drop in the metal-poor component ([Fe/H] <–0.5) as a function of Galactic latitude.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Red giant stars from Sloan Digital Sky Survey. I. The general field

We have obtained a sample of ∼22,000 red giant branch (RGB) stars based on stellar parameters, provided by the ninth data release of the Sloan Digital Sky Survey, and the CH(G)/MgH indices, measured from the included spectra. The Galactic rest-frame velocity of V {sub gsr} versus longitude for the sample shows the existence of several groups of stars from globular clusters and known streams. Excluding these substructures, a sample of ∼16,000 RGB stars from the general field is used to investigate the properties of the thick disk, the inner halo, and the outer halo of our Galaxy. The metallicity and rotational velocity distributions are investigated for stars at 0 kpc < |Z| < 10 kpc. It is found that the canonical thick disk dominates at 0 kpc < |Z| < 2 kpc and its contribution becomes negligible at |Z| > 3 kpc. The MWTD is present and overlaps with the inner halo at 1 kpc < |Z| < 3 kpc. The inner halo starts at 2 kpc < |Z| < 3 kpc and becomes the dominated population for 4 kpc < |Z| < 10 kpc. For halo stars with |Z| > 5 kpc, bimodal metallicity distributions are found for 20 kpc < |Z| < 25 kpc and 35 kpc < RR < 45 kpc, which suggests a dual halo, the inner and the outer halo, as reported in Carollo et al. at low |Z| values. The peak of metallicity for the inner halo is at [Fe/H] ∼ –1.6 and appears to be at [Fe/H] ∼ –2.3 for the outer halo. The transition point from the inner to the outer halo is located at |Z| ∼ 20 kpc and RR ∼ 35 kpc.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Images of gravitational and magnetic phenomena derived from two-dimensional back-projection Doppler tomography of interacting binary stars

We have used two-dimensional back-projection Doppler tomography as a tool to examine the influence of gravitational and magnetic phenomena in interacting binaries that undergo mass transfer from a magnetically active star onto a non-magnetic main-sequence star. This multitiered study of over 1300 time-resolved spectra of 13 Algol binaries involved calculations of the predicted dynamical behavior of the gravitational flow and the dynamics at the impact site, analysis of the velocity images constructed from tomography, and the influence on the tomograms of orbital inclination, systemic velocity, orbital coverage, and shadowing. The Hα tomograms revealed eight sources: chromospheric emission, a gas stream along the gravitational trajectory, a star-stream impact region, a bulge of absorption or emission around the mass-gaining star, a Keplerian accretion disk, an absorption zone associated with hotter gas, a disk-stream impact region, and a hot spot where the stream strikes the edge of a disk. We described several methods used to extract the physical properties of the emission sources directly from the velocity images, including S-wave analysis, the creation of simulated velocity tomograms from hydrodynamic simulations, and the use of synthetic spectra with tomography to sequentially extract the separate sources of emission from the velocity image. In summary, the tomography images have revealed results that cannot be explained solely by gravitational effects: chromospheric emission moving with the mass-losing star, a gas stream deflected from the gravitational trajectory, and alternating behavior between stream state and disk state. Our results demonstrate that magnetic effects cannot be ignored in these interacting binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Cosmic web and star formation activity in galaxies at z ∼ 1

We investigate the role of the delineated cosmic web/filaments on star formation activity by exploring a sample of 425 narrow-band selected Hα emitters, as well as 2846 color-color selected underlying star-forming galaxies for a large-scale structure at z = 0.84 in the COSMOS field from the HiZELS survey. Using the scale-independent Multi-scale Morphology Filter algorithm, we are able to quantitatively describe the density field and disentangle it into its major components: fields, filaments, and clusters. We show that the observed median star formation rate (SFR), stellar mass, specific SFR, the mean SFR-mass relation, and its scatter for both Hα emitters and underlying star-forming galaxies do not strongly depend on different classes of environment, in agreement with previous studies. However, the fraction of Hα emitters varies with environment and is enhanced in filamentary structures at z ∼ 1. We propose mild galaxy-galaxy interactions as the possible physical agent for the elevation of the fraction of Hα star-forming galaxies in filaments. Our results show that filaments are the likely physical environments that are often classed as the 'intermediate' densities and that the cosmic web likely plays a major role in galaxy formation and evolution which has so far been poorly investigated.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Early-type galaxies at intermediate redshift observed with Hubble space telescope WFC3: perspectives on recent star formation

We present an analysis of the stellar populations of 102 visually selected early-type galaxies (ETGs) with spectroscopic redshifts (0.35 ≲ z ≲ 1.5) from observations in the Early Release Science program with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We fit one- and two-component synthetic stellar models to the ETGs UV-optical-near-IR spectral energy distributions and find that a large fraction (∼40%) are likely to have experienced a minor (f{sub YC} ≲ 10% of stellar mass) burst of recent (t{sub YC} ≲ 1 Gyr) star formation. The measured age and mass fraction of the young stellar populations do not strongly trend with measurements of galaxy morphology. We note that massive (M > 10{sup 10.5} M {sub ☉}) recent star-forming ETGs appear to have larger sizes. Furthermore, high-mass, quiescent ETGs identified with likely companions populate a distinct region in the size-mass parameter space, in comparison with the distribution of massive ETGs with evidence of recent star formation (RSF). We conclude that both mechanisms of quenching star formation in disk-like ETGs and (gas-rich, minor) merger activity contribute to the formation of young stars and the size-mass evolution of intermediate redshift ETGs. The number of ETGs for which we have both HST WFC3 panchromatic (especially UV) imaging and spectroscopically confirmed redshifts is relatively small, therefore, a conclusion about the relative roles of both of these mechanisms remains an open question.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Stellar signatures of AGN-jet-triggered star formation

To investigate feedback between relativistic jets emanating from active galactic nuclei and the stellar population of the host galaxy, we analyze the long-term evolution of the orbits of the stars formed in the galaxy-scale simulations by Gaibler et al. of jets in massive, gas-rich galaxies at z ∼ 2-3. We find strong, jet-induced differences in the resulting stellar populations of galaxies that host relativistic jets and galaxies that do not, including correlations in stellar locations, velocities, and ages. Jets are found to generate distributions of increased radial and vertical velocities that persist long enough to effectively augment the stellar structure of the host. The jets cause the formation of bow shocks that move out through the disk, generating rings of star formation within the disk. The bow shock often accelerates pockets of gas in which stars form, yielding populations of stars with significant radial and vertical velocities, some of which have large enough velocities to escape the galaxy. These stellar population signatures can serve to identify past jet activity as well as jet-induced star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

NON-LOCAL THERMODYNAMICAL EQUILIBRIUM EFFECTS ON THE IRON ABUNDANCE OF ASYMPTOTIC GIANT BRANCH STARS IN 47 TUCANAE

We present the iron abundance of 24 asymptotic giant branch (AGB) stars, members of the globular cluster 47 Tucanae, obtained with high-resolution spectra collected with the FEROS spectrograph at the MPG/ESO 2.2 m Telescope. We find that the iron abundances derived from neutral lines (with a mean value [Fe I/H] =–0.94 ± 0.01, σ = 0.08 dex) are systematically lower than those derived from single ionized lines ([Fe II/H] =–0.83 ± 0.01, σ = 0.05 dex). Only the latter are in agreement with those obtained for a sample of red giant branch (RGB) cluster stars, for which the Fe I and Fe II lines provide the same iron abundance. This finding suggests that non-local thermodynamical equilibrium (NLTE) effects driven by overionization mechanisms are present in the atmosphere of AGB stars and significantly affect the Fe I lines while leaving Fe II features unaltered. On the other hand, the very good ionization equilibrium found for RGB stars indicates that these NLTE effects may depend on the evolutionary stage. We discuss the impact of this finding on both the chemical analysis of AGB stars and on the search for evolved blue stragglers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A FEEDBACK-DRIVEN BUBBLE G24.136+00.436: A POSSIBLE SITE OF TRIGGERED STAR FORMATION

We present a multi-wavelength study of the IR bubble G24.136+00.436. The J = 1-0 observations of {sup 12}CO, {sup 13}CO, and C{sup 18}O were carried out with the Purple Mountain Observatory 13.7 m telescope. Molecular gas with a velocity of 94.8 km s{sup –1} is found prominently in the southeast of the bubble, shaped as a shell with a total mass of ∼2 × 10{sup 4} M {sub ☉}. It was likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores, whose dense (a few of 10{sup 3} cm{sup –3}) and massive (a few of 10{sup 3} M {sub ☉}) characteristics coupled with the broad linewidths (>2.5 km s{sup –1}) suggest that they are promising sites for forming high-mass stars or clusters. This could be further consolidated by the detection of compact H II regions in Cores A and E. We tentatively identified and classified 63 candidate young stellar objects (YSOs) based on the Spitzer and UKIDSS data. They are found to be dominantly distributed in regions with strong molecular gas emission, indicative of active star formation, especially in the shell. The H II region inside the bubble is mainly ionized by a ∼O8V star(s), of the dynamical age of ∼1.6 Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as the timescales involved, indicate a possible scenario for triggering star formation, signified by the ''collect and collapse'' process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SUPPRESSION OF STAR FORMATION IN NGC 1266

NGC 1266 is a nearby lenticular galaxy that harbors a massive outflow of molecular gas powered by the mechanical energy of an active galactic nucleus (AGN). It has been speculated that such outflows hinder star formation (SF) in their host galaxies, providing a form of feedback to the process of galaxy formation. Previous studies, however, indicated that only jets from extremely rare, high-power quasars or radio galaxies could impart significant feedback on their hosts. Here we present detailed observations of the gas and dust continuum of NGC 1266 at millimeter wavelengths. Our observations show that molecular gas is being driven out of the nuclear region at M-dot {sub out}≈110 M{sub ⊙} yr{sup –1}, of which the vast majority cannot escape the nucleus. Only 2 M {sub ☉} yr{sup –1} is actually capable of escaping the galaxy. Most of the molecular gas that remains is very inefficient at forming stars. The far-infrared emission is dominated by an ultra-compact (≲ 50 pc) source that could either be powered by an AGN or by an ultra-compact starburst. The ratio of the SF surface density (Σ{sub SFR}) to the gas surface density (Σ{sub H{sub 2}}) indicates that SF is suppressed by a factor of ≈50 compared to normal star-forming galaxies if all gas is forming stars, and ≈150 for the outskirt (98%) dense molecular gas if the central region is powered by an ultra-compact starburst. The AGN-driven bulk outflow could account for this extreme suppression by hindering the fragmentation and gravitational collapse necessary to form stars through a process of turbulent injection. This result suggests that even relatively common, low-power AGNs are able to alter the evolution of their host galaxies as their black holes grow onto the M-σ relation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A HERO'S LITTLE HORSE: DISCOVERY OF A DISSOLVING STAR CLUSTER IN PEGASUS

We report the discovery of an ultra-faint stellar system in the constellation of Pegasus. This concentration of stars was detected by applying our overdensity detection algorithm to the Sloan Digital Sky Survey Data Release 10 and confirmed with deeper photometry from the Dark Energy Camera (DECam) at the 4 m Blanco telescope. The best-fitting model isochrone indicates that this stellar system, Kim 1, features an old (12 Gyr) and metal-poor ([Fe/H] ∼ -1.7) stellar population at a heliocentric distance of 19.8 ± 0.9 kpc. We measure a half-light radius of 6.9 ± 0.6 pc using a Plummer profile. The small physical size and the extremely low luminosity are comparable to the faintest known star clusters Segue 3, Koposov 1 and 2, and Muñoz 1. However, Kim 1 exhibits a lower star concentration and is lacking a well-defined center. It also has an unusually high ellipticity and irregular outer isophotes, which suggests that we are seeing an intermediate mass star cluster being stripped by the Galactic tidal field. An extended search for evidence of an associated stellar stream within the 3 deg{sup 2} DECam field remains inconclusive. The finding of Kim 1 is consistent with current overdensity detection limits and supports the hypothesis that there are still a substantial number of extreme low-luminosity star clusters undetected in the wider Milky Way halo.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ONE PLANE FOR ALL: MASSIVE STAR-FORMING AND QUIESCENT GALAXIES LIE ON THE SAME MASS FUNDAMENTAL PLANE AT z ∼ 0 AND z ∼ 0.7

Scaling relations between galaxy structures and dynamics have been studied extensively for early- and late-type galaxies, both in the local universe and at high redshifts. The abundant differences between the properties of disky and elliptical, or star-forming and quiescent, galaxies seem to be characteristic of the local universe; such clear distinctions begin to disintegrate as observations of massive galaxies probe higher redshifts. In this paper we investigate the existence of the mass fundamental plane of all massive galaxies (σ ≳ 100 km s{sup –1}). This work includes local galaxies (0.05 < z < 0.07) from the Sloan Digital Sky Survey, in addition to 31 star-forming and 72 quiescent massive galaxies at intermediate redshift (z ∼ 0.7) with absorption-line kinematics from deep Keck-DEIMOS spectra and structural parameters from Hubble Space Telescope imaging. In two-parameter scaling relations, star-forming and quiescent galaxies differ structurally and dynamically. However, we show that massive star-forming and quiescent galaxies lie on nearly the same mass fundamental plane, or the relationship between stellar mass surface density, stellar velocity dispersion, and effective radius. The scatter in this relation (measured about log σ) is low: 0.072 dex (0.055 dex intrinsic) at z ∼ 0 and 0.10 dex (0.08 dex intrinsic) at z ∼ 0.7. This 3D surface is not unique: virial relations, with or without a dependence on luminosity profile shapes, can connect galaxy structures and stellar dynamics with similar scatter. This result builds on the recent finding that mass fundamental plane has been stable for early-type galaxies since z ∼ 2. As we now find that this also holds for star-forming galaxies to z ∼ 0.7, this implies that these scaling relations of galaxies will be minimally susceptible to progenitor biases owing to the evolving stellar populations, structures, and dynamics of galaxies through cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A STATISTICAL RECONSTRUCTION OF THE PLANET POPULATION AROUND KEPLER SOLAR-TYPE STARS

Using the cumulative catalog of planets detected by the NASA Kepler mission, we reconstruct the intrinsic occurrence of Earth- to Neptune-size (1-4 R {sub ⊕}) planets and their distributions with radius and orbital period. We analyze 76,711 solar-type (0.8 < R {sub *}/R {sub ☉} < 1.2) stars with 430 planets on 20-200 day orbits, excluding close-in planets that may have been affected by the proximity to the host star. Our analysis considers errors in planet radii and includes an ''iterative simulation'' technique that does not bin the data. We find a radius distribution that peaks at 2-2.8 Earth radii, with lower numbers of smaller and larger planets. These planets are uniformly distributed with logarithmic period, and the mean number of such planets per star is 0.46 ± 0.03. The occurrence is ∼0.66 if planets interior to 20 days are included. We estimate the occurrence of Earth-size planets in the ''habitable zone'' (defined as 1-2 R {sub ⊕}, 0.99-1.7 AU for solar-twin stars) as 6.4{sub −1.1}{sup +3.4}%. Our results largely agree with those of Petigura et al., although we find a higher occurrence of 2.8-4 Earth-radii planets. The reasons for this excess are the inclusion of errors in planet radius, updated Huber et al. stellar parameters, and also the exclusion of planets that may have been affected by proximity to the host star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Old neutron stars as probes of isospin-violating dark matter

Isospin-violating dark matter (IVDM), which couples differently with protons and neutrons, provides a promising mechanism to ameliorate the tension among recent direct detection experiments. Assuming dark matter (DM) is non-interacting bosonic asymmetric IVDM, we investigate how the existence of old neutron stars limits the DM–proton scattering cross-section σ{sub p}, especially the effects of the isospin-violating DM–nucleon interactions and the symmetry energy in the equation of state (EOS) of isospin asymmetric nuclear matter. Our calculations are completely based on general relativity and the structure of neutron stars is obtained by solving the Tolman–Oppenheimer–Volkoff equations with nuclear matter EOS constrained by terrestrial experiments. We find that, by considering the more realistic neutron star model rather than a simple uniform neutron sphere as usual, the σ{sub p} bounds from old neutron stars can be varied by more than an order of magnitude depending on the specific values of the DM neutron-to-proton coupling ratio f{sub n}/f{sub p}, and they can be further varied by more than a factor of two depending on the density dependence of the symmetry energy. In particular, we demonstrate that the observed nearby isolated old neutron star PSR B1257+12 can set a very strong limit on σ{sub p} for low-mass DM particles (⩽20 GeV) that reaches a sensitivity beyond the current best limits from direct detection experiments and disfavors the DM interpretation of previously reported positive experimental results, including the IVDM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STAR FORMATION IN DISK GALAXIES. III. DOES STELLAR FEEDBACK RESULT IN CLOUD DEATH?

Stellar feedback, star formation, and gravitational interactions are major controlling forces in the evolution of giant molecular clouds (GMCs). To explore their relative roles, we examine the properties and evolution of GMCs forming in an isolated galactic disk simulation that includes both localized thermal feedback and photoelectric heating. The results are compared with the three previous simulations in this series, which consists of a model with no star formation, star formation but no form of feedback, and star formation with photoelectric heating in a set with steadily increasing physical effects. We find that the addition of localized thermal feedback greatly suppresses star formation but does not destroy the surrounding GMC, giving cloud properties closely resembling the run in which no stellar physics is included. The outflows from the feedback reduce the mass of the cloud but do not destroy it, allowing the cloud to survive its stellar children. This suggests that weak thermal feedback such as the lower bound expected for a supernova may play a relatively minor role in the galactic structure of quiescent Milky-Way-type galaxies, compared to gravitational interactions and disk shear.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Can very massive population III stars produce a super-collapsar?

A fraction of the first generation of stars in the early universe may be very massive (≳300 M{sub ⊙}) as they form in metal-free environments. Formation of black holes from these stars can be accompanied by supermassive collapsars to produce long gamma-ray bursts of a unique type having a very high total energy (∼10{sup 54} erg) as recently suggested by several authors. We present new stellar evolution models of very massive Population III stars including the effect of rotation to provide theoretical constraints on super-collapsar progenitors. We find that the angular momentum condition for a super-collapsar can be fulfilled if magnetic torques are ignored, in which case Eddington–Sweet circulations play the dominant role for the transport of angular momentum. We further find that the initial mass range for super-collapsar progenitors would be limited to 300 M{sub ⊙}≲M≲700 M{sub ⊙}. However, all of our very massive star models of this mass range end their lives as red supergiants rather than blue supergiants, in good agreement with most of the previous studies. The predicted final fate of these stars is either a jet-powered type IIP supernova or an ultra-long, relatively faint gamma-ray transient, depending on the initial amount of angular momentum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

P-MaNGA: GRADIENTS IN RECENT STAR FORMATION HISTORIES AS DIAGNOSTICS FOR GALAXY GROWTH AND DEATH

We present an analysis of the data produced by the MaNGA prototype run (P-MaNGA), aiming to test how the radial gradients in recent star formation histories, as indicated by the 4000 Å break (D{sub n}(4000)), Hδ absorption (EW(Hδ{sub A})), and Hα emission (EW(Hα)) indices, can be useful for understanding disk growth and star formation cessation in local galaxies. We classify 12 galaxies observed on two P-MaNGA plates as either centrally quiescent (CQ) or centrally star-forming (CSF), according to whether D{sub n}(4000) measured in the central spaxel of each datacube exceeds 1.6. For each spaxel we generate both 2D maps and radial profiles of D{sub n}(4000), EW(Hδ{sub A}), and EW(Hα). We find that CSF galaxies generally show very weak or no radial variation in these diagnostics. In contrast, CQ galaxies present significant radial gradients, in the sense that D{sub n}(4000) decreases, while both EW(Hδ{sub A}) and EW(Hα) increase from the galactic center outward. The outer regions of the galaxies show greater scatter on diagrams relating the three parameters than their central parts. In particular, the clear separation between centrally measured quiescent and star-forming galaxies in these diagnostic planes is largely filled in by the outer parts of galaxies whose global colors place them in the green valley, supporting the idea that the green valley represents a transition between blue-cloud and red-sequence phases, at least in our small sample. These results are consistent with a picture in which the cessation of star formation propagates from the center of a galaxy outward as it moves to the red sequence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Massive stars in the W33 giant molecular complex

In order to assess the long-term star formation history, we performed an infrared spectroscopic search for massive stars, detecting for the first time 14 early-type stars, including one WN6 star and four O4–7 stars.

79 ASTRONOMY AND ASTROPHYSICS↗