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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Gas fraction and depletion time of massive star-forming galaxies at z ∼ 3.2: no change in global star formation process out to z > 3

The observed evolution of the gas fraction and its associated depletion time in main-sequence (MS) galaxies provides insights on how star formation proceeds over cosmic time. We report ALMA detections of the rest-frame ∼300 μm continuum observed at 240 GHz for 45 massive (〈log(M{sub ⋆}(M{sub ⊙}))〉=10.7), normal star-forming (〈log(sSFR(yr{sup −1}))〉=−8.6), i.e., MS, galaxies at z≈3.2 in the COSMOS field. From an empirical calibration between cold neutral, i.e., molecular and atomic, gas mass M{sub gas} and monochromatic (rest-frame) infrared luminosity, the gas mass for this sample is derived. Combined with stellar mass M{sub ⋆} and star formation rate (SFR) estimates (from MagPhys fits) we obtain a median gas fraction of μ{sub gas}=M{sub gas}/M{sub ⋆}=1.65{sub −0.19}{sup +0.18} and a median gas depletion time t{sub depl.}(Gyr)=M{sub gas/SFR}=0.68{sub −0.08}{sup +0.07}; correction for the location on the MS will only slightly change the values. The reported uncertainties are the 1σ error on the median. Our results are fully consistent with the expected flattening of the redshift evolution from the 2-SFM (2 star formation mode) framework which empirically prescribes the evolution assuming a universal, log-linear relation between SFR and gas mass coupled to the redshift evolution of the specific star formation rate (sSFR) of MS galaxies. While t{sub depl.} shows only a mild dependence on location within the MS, a clear trend of increasing μ{sub gas} across the MS is observed (as known from previous studies). Further, we comment on trends within the MS and (in)consistencies with other studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

[C II] LINE EMISSION IN MASSIVE STAR-FORMING GALAXIES AT z = 4.7

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II] 157.7 {mu}m fine structure line and thermal dust continuum emission from a pair of gas-rich galaxies at z = 4.7, BR1202-0725. This system consists of a luminous quasar host galaxy and a bright submillimeter galaxy (SMG), while a fainter star-forming galaxy is also spatially coincident within a 4'' (25 kpc) region. All three galaxies are detected in the submillimeter continuum, indicating FIR luminosities in excess of 10{sup 13} L{sub Sun} for the two most luminous objects. The SMG and the quasar host galaxy are both detected in [C II] line emission with luminosities L{sub [CII]} = (10.0 {+-} 1.5) Multiplication-Sign 10{sup 9} L{sub Sun} and L{sub [CII]} = (6.5 {+-} 1.0) Multiplication-Sign 10{sup 9} L{sub Sun }, respectively. We estimate a luminosity ratio L{sub [CII]}/L{sub FIR} = (8.3 {+-} 1.2) Multiplication-Sign 10{sup -4} for the starburst SMG to the north and L{sub [CII]}/L{sub FIR} = (2.5 {+-} 0.4) Multiplication-Sign 10{sup -4} for the quasar host galaxy, in agreement with previous high-redshift studies that suggest lower [C II]-to-FIR luminosity ratios in quasars than in starburst galaxies. The third fainter object with a flux density S{sub 340GHz} = 1.9 {+-} 0.3 mJy is coincident with a Ly{alpha} emitter and is detected in HST ACS F775W and F814W images but has no clear counterpart in the H band. Even if this third companion does not lie at a redshift similar to BR1202-0725, the quasar and the SMG represent an overdensity of massive, infrared luminous star-forming galaxies within 1.3 Gyr of the big bang.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Atomic Absorption Line Diagnostics for the Physical Properties of Red Supergiants

Red supergiants (RSGs) are evolved massive stars that represent extremes, in both their physical sizes and their cool temperatures, of the massive star population. The effective temperature (T {sub eff}) is the most critical physical property needed to place an RSG on the Hertzsprung–Russell Diagram, due to the stars’ cool temperatures and resulting large bolometric corrections. Several recent papers have examined the potential utility of atomic line equivalent widths (EWs) in cool supergiant (CSG) spectra for determining T {sub eff} and other physical properties and found strong correlations between Ti i and Fe i spectral features and T {sub eff} in earlier-type CSGs (G and early K) but poor correlations in M-type stars, a spectral subtype that makes up a significant fraction of RSGs. We have extended this work by measuring the EWs of Ti, Fe, and Ca lines in late K- and M-type RSGs in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud, and compared these results to the predictions of the theoretical stellar LTE atmosphere models (MARCS) stellar atmosphere models. Our analyses show a poor correlation between T {sub eff} and the Fe i and Ti i lines in our observations (at odds with strong correlations predicted by stellar atmosphere models), but do find statistically significant correlations between T {sub eff} and the Ca ii triplet (CaT) features of Milky Way RSGs, suggesting that this could be a potential diagnostic tool for determining T {sub eff} in M-type supergiants. We also examine correlations between these spectral features and other physical properties of RSGs (including metallicity, surface gravity, and bolometric magnitude), and consider the underlying physics driving the evolution of atomic line spectra in RSGs.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2022

Understanding Structure in Line-Driven Stellar Winds Using Ultraviolet Spectropolarimetry in the Time Domain

The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further evolution and eventual supernova, and preconditions the surrounding medium for all following events, with consequences for ISM energization, chemical enrichment, and dust formation. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind driving. In the past, mass-loss rates have been characterized via collisional emission processes such as optical Hα and free-free radio emission, but these so-called “density squared” diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds and their stochastic instabilities. Furthermore, large-scale structures initiating in surface anisotropies and propagating throughout the wind can also affect wind driving and alter mass-loss diagnostics. Time series spectroscopy of high resonance-line opacity in the UV, capable of high resolution and high signal-to-noise, are required to better understand these complex dynamics, and more accurately determine mass-loss rates. The proposed Polstar mission (Scowen et al. 2022, this volume) provides the necessary resolution at the Sobolev (∼10 km s −1 ) or sound-speed (∼20 km s −1 ) scale, for over three dozen bright galactic massive stars with signal-to noise an order of magnitude above that of the celebrated MEGA campaign (Massa et al. 1995) of the International Ultraviolet Explorer (IUE), via continuous observations that track propagating structures through the winds in real time. Supporting geometric constraints are provided by the polarimetric capabilities present in all the datasets of such a mission.

Polstar – NASA MIDEX↗
DOE OSTI2013

PRE-SUPERNOVA EVOLUTION OF ROTATING SOLAR METALLICITY STARS IN THE MASS RANGE 13-120 M {sub Sun} AND THEIR EXPLOSIVE YIELDS

We present the first set of a new generation of models of massive stars with a solar composition extending between 13 and 120 M {sub Sun }, computed with and without the effects of rotation. We included two instabilities induced by rotation: the meridional circulation and the shear instability. We implemented two alternative schemes to treat the transport of the angular momentum: the advection-diffusion formalism and the simpler purely diffusive one. The full evolution from the pre-main sequence up to the pre-supernova stage is followed in detail with a very extended nuclear network. The explosive yields are provided for a variety of possible mass cuts and are available at the Web site http://www.iasf-roma.inaf.it/orfeo/public{sub h}tml. We find that both the He and the CO core masses are larger than those of their non-rotating counterparts. Also the C abundance left by the He burning is lower than in the non-rotating case, especially for stars with an initial mass of 13-25 M {sub Sun }, and this affects the final mass-radius relation, basically the final binding energy, at the pre-supernova stage. The elemental yields produced by a generation of stars rotating initially at 300 km s{sup -1} do not change substantially with respect to those produced by a generation of non-rotating massive stars, the main differences being a slight overproduction of the weak s-component and a larger production of F. Since rotation also affects the mass-loss rate, either directly or indirectly, we find substantial differences in the lifetimes as O-type and Wolf-Rayet subtypes between the rotating and non-rotating models. The maximum mass exploding as Type IIP supernova ranges between 15 and 20 M {sub Sun} in both sets of models (this value depends basically on the larger mass-loss rates in the red supergiant phase due to the inclusion of the dust-driven wind). This limiting value is in remarkably good agreement with current estimates.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1987

IUE observations of luminous blue star associations in irregular galaxies

Two regions of recent star formation in blue irregular galaxies were observed with the IUE in the short wavelength, low dispersion mode. The spectra indicate that the massive star content is similar in these regions and is best fit by massive stars formed in a burst and now approximately 2.5 to 3.0 million years old.

Lamb, S. A.↗
NASA NTRS1999

Photodissociation Regions in the Interstellar Medium of Galaxies

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

Hollenbach, David J.↗
NASA NTRS1989

Shell nebulae around luminous evolved stars

Shell nebulae around luminous Population I Wolf-Rayet, Of, and P-Cygni stars are astrophysically interesting since they are indicators of pre-supernova mass loss and how such massive stars prepare their surrounding interstellar medium prior to explosion. Some twenty-odd such nebulae are known, for which detailed study of their morphological and spectroscopic characteristics have only begun in this decade. In this paper, some of these characteristics are reviewed in general, and new observations are reported. Emphasis has been placed on several 'prototype 'objects (NGC 7635, NGC 2359, NGC 6888, and the Eta Carinae condensations) to illustrate the varied massive-star mass-loss, the physics of their winds and shell ejecta, and related nucleosynthesis effects in the compositions of the winds and shells.

Dufour, Reginald J.↗
NASA NTRS2013

Direct Imaging Discovery of A Super-Jupiter Around the Late B-Type Star K And*

We present the direct imaging discovery of an extrasolar planet, or possible low-mass brown dwarf, at a projected separation of 55 plus or minus 2 AU (1."058 plus or minus 0."007) from the B9-type star K And. The planet was detected with Subaru/HiCIAO (Subaru/High Contrast Instrument for the Subaru Next Generation Adaptive Optics) during the SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) survey and confirmed as a bound companion via common proper motion measurements. Observed near-infrared magnitudes of J equals 16.3 plus or minus 0.3, H equals 15.2 plus or minus 0.2, K (sub s) = 14.6 plus or minus 0.4, and L prime equals 13.12 plus or minus 0.09 indicate a temperature of approximately 1700 degrees Kelvin. The galactic kinematics of the host star are consistent with membership in the Columba Association, implying a corresponding age of 30 (exp from plus 20 to minus10) Myr. The systems age, combined with the companion photometry, points to a model-dependent companion mass approximately 12.8 times the mass of Jupiter. The host stars estimated mass of 2.4 to 2.5 times the mass of the sun places it among the most massive stars ever known to harbor an extrasolar planet or low-mass brown dwarf. While the mass of the companion is close to the deuterium burning limit, its mass ratio, orbital separation, and likely planet-like formation scenario imply that it may be best defined as a super-Jupiter with properties similar to other recently discovered companions to massive stars.

HiCIAO↗
DOE OSTI2014

Neutrinos and nucleosynthesis in core-collapse supernovae

Massive stars (M > 8-10 M{sub ⊙}) undergo core collapse at the end of their life and explode as supernova with ~ 10⁵¹ erg of kinetic energy. While the detailed supernova explosion mechanism is still under investigation, reliable nucleosynthesis calculations based on successful explosions are needed to explain the observed abundances in metal-poor stars and to predict supernova yields for galactic chemical evolution studies. To predict nucleosynthesis yields for a large number of progenitor stars, computationally efficient explosion models are required. We model the core collapse, bounce and subsequent explosion of massive stars assuming spherical symmetry and using detailed microphysics and neutrino physics combined with a novel method to artificially trigger the explosion (PUSH). We discuss the role of neutrinos, the conditions in the ejecta, and the resulting nucleosynthesis.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

THE KELVIN-HELMHOLTZ INSTABILITY IN ORION: A SOURCE OF TURBULENCE AND CHEMICAL MIXING

Hydrodynamical instabilities are believed to power some of the small scale (0.1-10 pc) turbulence and chemical mixing in the interstellar medium. Identifying such instabilities has always been difficult, but recent observations of a wavelike structure (the Ripples) in the Orion nebula have been interpreted as a signature of the Kelvin-Helmholtz instability (KHI), occurring at the interface between the H II region and the molecular cloud. However, this has not been verified theoretically. In this Letter, we investigate theoretically the stability of this interface using observational constraints for the local physical conditions. A linear analysis shows that the H II/molecular cloud interface is indeed KH unstable for a certain range of magnetic field orientation. We find that the maximal growth rates correspond to typical timescales of a few 10{sup 4} years and instability wavelengths of 0.06-0.6 pc. We predict that after 2 Multiplication-Sign 10{sup 5} years the KHI saturates and forms a turbulent layer of about 0.5 pc. The KHI can remain in linear phase over a maximum distance of 0.75 pc. These spatial and timescales are compatible with the Ripples representing the linear phase of the KHI. These results suggest that the KHI may be crucial to generate turbulence and to bring heavy elements injected by the winds of massive stars in H II regions to colder regions where planetary systems around low-mass stars are being formed. This could apply to the transport of {sup 26}Al injected by a massive star in an H II region to the nascent solar system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Radio-loud versus Radio-quiet Gamma-Ray Bursts: The Role of Binary Progenitors

We explore the possibility that radio-loud gamma-ray bursts (GRBs) result from the collapse of massive stars in interacting binary systems, while radio-quiet GRBs are produced by the collapse of single massive stars. A binary collapsar system can have the necessary angular momentum and energy budget to explain the longer prompt gamma-ray durations and higher isotropic energies seen in the the radio-loud subsample of long GRBs. Additionally, binary systems can lead to rich and extended circumstellar environments that allow for the presence of the long-lived radio afterglows seen in the radio-loud systems. Finally, the relative fraction of stars in binary systems versus single-star systems appears consistent with the fraction of radio-loud versus radio-quiet GRBs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The formation and gravitational-wave detection of massive stellar black hole binaries

If binaries consisting of two ∼100 M{sub ☉} black holes exist, they would serve as extraordinarily powerful gravitational-wave sources, detectable to redshifts of z ∼ 2 with the advanced LIGO/Virgo ground-based detectors. Large uncertainties about the evolution of massive stars preclude definitive rate predictions for mergers of these massive black holes. We show that rates as high as hundreds of detections per year, or as low as no detections whatsoever, are both possible. It was thought that the only way to produce these massive binaries was via dynamical interactions in dense stellar systems. This view has been challenged by the recent discovery of several ≳ 150 M{sub ☉} stars in the R136 region of the Large Magellanic Cloud. Current models predict that when stars of this mass leave the main sequence, their expansion is insufficient to allow common envelope evolution to efficiently reduce the orbital separation. The resulting black hole-black hole binary remains too wide to be able to coalesce within a Hubble time. If this assessment is correct, isolated very massive binaries do not evolve to be gravitational-wave sources. However, other formation channels exist. For example, the high multiplicity of massive stars, and their common formation in relatively dense stellar associations, opens up dynamical channels for massive black hole mergers (e.g., via Kozai cycles or repeated binary-single interactions). We identify key physical factors that shape the population of very massive black hole-black hole binaries. Advanced gravitational-wave detectors will provide important constraints on the formation and evolution of very massive stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Probing final stages of stellar evolution with X-ray observations of SN 2013ej

Massive stars shape their surroundings with mass loss from winds during their lifetimes. Fast ejecta from supernovae (SNe), from these massive stars, shock this circumstellar medium. Emission generated by this interaction provides a window into the final stages of stellar evolution, by probing the history of mass loss from the progenitor. Here we use Chandra and Swift X-ray observations of the type II-P/L SN 2013ej to probe the history of mass loss from its progenitor. We model the observed X-rays as emission from both heated circumstellar matter and SN ejecta. The circumstellar density profile probed by the SN shock reveals a history of steady mass loss during the final 400 years. The inferred mass loss rate of 3×10{sup −6} M{sub ⊙} yr{sup −1} points back to a 14 M{sub ⊙} progenitor. Soon after the explosion we find significant absorption of reverse shock emission by a cooling shell. The column depth of this shell observed in absorption provides an independent and consistent measurement of the circumstellar density seen in emission. We also determine the efficiency of cosmic ray acceleration from X-rays produced by Inverse Compton scattering of optical photons by relativistic electrons. Only about 1% of the thermal energy is used to accelerate electrons. Our X-ray observations and modeling provide stringent tests for models of massive stellar evolution and micro-physics of shocks.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

Unsolved problems

The level of understanding of the physics of single, isolated B stars is assessed and unresolved problems are defined. The significant observational results concerning the effective temperatures, radii, masses and mantles are summarized. The results of the theory of the evolution of massive stars are confronted with the observed luminosities and effective temperatures of B stars. In addition the implications of stellar spectra theory are compared with observed spectra and a heuristic model for a mantle is developed. The chief unresolved problems for B stars concern developing detailed models for (1) the internal structure of massive stars which are beginning to evolve rapidly as they complete burning hydrogen in their cores; (2) mantles; and (3) the transfer of radiation in high temperature inhomogeneous moving bodies of gas.

Source record↗
DOE OSTI2016

STAR FORMATION IN W3—AFGL 333: YOUNG STELLAR CONTENT, PROPERTIES, AND ROLES OF EXTERNAL FEEDBACK

One of the key questions in the field of star formation is the role of stellar feedback on the subsequent star formation process. The W3 giant molecular cloud complex at the western border of the W4 super bubble is thought to be influenced by the massive stars in W4. This paper presents a study of the star formation activity within AFGL 333, a ∼10{sup 4} M {sub ⊙} cloud within W3, using deep JHK {sub s} photometry obtained from the NOAO Extremely Wide Field Infrared Imager combined with Spitzer IRAC and MIPS photometry. Based on the infrared excess, we identify 812 candidate young stellar objects (YSOs) in the complex, of which 99 are Class I and 713 are Class II sources. The stellar density analysis of YSOs reveals three major stellar aggregates within AFGL 333, namely AFGL 333 Main, AFGL 333 NW1 and AFGL 333 NW2. The disk fraction within AFGL 333 is estimated to be ∼50%–60%. We use the extinction map made from the H−K{sub s} colors of the background stars and CO data to understand the cloud structure and to estimate the cloud mass. From the stellar and cloud mass associated with AFGL 333, we infer that the region is currently forming stars with an efficiency of ∼4.5% and at a rate of ∼2–3 M {sub ⊙} Myr{sup −1} pc{sup −2}. In general, the star formation activity within AFGL 333 is comparable to that of nearby low mass star-forming regions. We do not find any strong evidence to suggest that the stellar feedback from the massive stars of nearby W4 super bubble has affected the global star formation properties of the AFGL 333 region.

79 ASTRONOMY AND ASTROPHYSICS↗