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At least 271 records · Page 15
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NASA NTRS2016

On the Nature of the Enigmatic Object IRAS 19312+1950: A Rare Phase of Massive Star Formation?

IRAS19312+1950 is a peculiar object that has eluded firm characterization since its discovery, with combined maser properties similar to an evolved star and a young stellar object (YSO). To help determine its true nature, we obtained infrared spectra of IRAS19312+1950 in the range 5-550 microns using the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a compact, slightly asymmetric continuum source at 170 microns, indicative of a large, dusty circumstellar envelope. The far-IR CO emission line spectrum reveals two gas temperature components: approx. = 0.22 Stellar Mass of material at 280+/-18 K, and ≈1.6 Me of material at 157+/-3 K. The OI 63 micron line is detected on-source but no significant emission from atomic ions was found. The HIFI observations display shocked, high-velocity gas with outflow speeds up to 90 km/s along the line of sight. From Spitzer spectroscopy, we identify ice absorption bands due to H2O at 5.8 microns and CO2 at 15 microns. The spectral energy distribution is consistent with a massive, luminous (approx. 2 × 10(exp 4) Stellar Luminosity) central source surrounded by a dense, warm circumstellar disk and envelope of total mass approx. 500-700 Stellar Mass with large bipolar outflow cavities. The combination of distinctive far-IR spectral features suggest that IRAS19312+1950 should be classified as an accreting, high-mass YSO rather than an evolved star. In light of this reclassification, IRAS19312+1950 becomes only the fifth high-mass protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH maser line ratios may not be reliable observational discriminators between evolved stars and YSOs.

ISM: molecules aEuro" masers aEuro" stars↗
DOE OSTI2016

ON THE NATURE OF THE ENIGMATIC OBJECT IRAS 19312+1950: A RARE PHASE OF MASSIVE STAR FORMATION?

IRAS 19312+1950 is a peculiar object that has eluded firm characterization since its discovery, with combined maser properties similar to an evolved star and a young stellar object (YSO). To help determine its true nature, we obtained infrared spectra of IRAS 19312+1950 in the range 5–550 μ m using the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a compact, slightly asymmetric continuum source at 170 μ m, indicative of a large, dusty circumstellar envelope. The far-IR CO emission line spectrum reveals two gas temperature components: ≈0.22 M {sub ⊙} of material at 280 ± 18 K, and ≈1.6 M {sub ⊙} of material at 157 ± 3 K. The O i 63 μ m line is detected on-source but no significant emission from atomic ions was found. The HIFI observations display shocked, high-velocity gas with outflow speeds up to 90 km s{sup −1} along the line of sight. From Spitzer spectroscopy, we identify ice absorption bands due to H{sub 2}O at 5.8 μ m and CO{sub 2} at 15 μ m. The spectral energy distribution is consistent with a massive, luminous (∼2 × 10{sup 4} L {sub ⊙}) central source surrounded by a dense, warm circumstellar disk and envelope of total mass ∼500–700 M {sub ⊙}, with large bipolar outflow cavities. The combination of distinctive far-IR spectral features suggest that IRAS 19312+1950 should be classified as an accreting, high-mass YSO rather than an evolved star. In light of this reclassification, IRAS 19312+1950 becomes only the fifth high-mass protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH maser line ratios may not be reliable observational discriminators between evolved stars and YSOs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MASSIVE STAR FORMATION TOWARD G28.87+0.07 (IRAS 18411-0338) INVESTIGATED BY MEANS OF MASER KINEMATICS AND RADIO TO INFRARED CONTINUUM OBSERVATIONS

We used the Very Long Baseline Array and the European VLBI Network to perform phase-referenced very long baseline interferometry observations of the three most powerful maser transitions associated with the high-mass star-forming region G28.87+0.07: the 22.2 GHz H{sub 2}O, 6.7 GHz CH{sub 3}OH, and 1.665 GHz OH lines. We also performed Very Large Array (VLA) observations of the radio continuum emission at 1.3 and 3.6 cm and Subaru observations of the continuum emission at 24.5 {mu}m. Two centimeter-continuum sources are detected and one of them (named hot molecular core (HMC)) is compact and placed at the center of the observed distribution of H{sub 2}O, CH{sub 3}OH, and OH masers. The bipolar distribution of line-of-sight velocities and the pattern of the proper motions suggest that the water masers are driven by a (proto)stellar jet interacting with the dense circumstellar gas. The same jet could both excite the centimeter-continuum source named HMC (interpreted as free-free emission from shocked gas) and power the molecular outflow observed at larger scales-although one cannot exclude that the free-free continuum is rather originating from a hypercompact H II region. At 24.5 {mu}m, we identify two objects separated along the north-south direction, whose absolute positions agree with those of the two VLA continuum sources. We establish that {approx}90% of the luminosity of the region ({approx}2 Multiplication-Sign 10{sup 5} L{sub Sun }) is coming from the radio source HMC, which confirms the existence of an embedded massive young stellar object exciting the masers and possibly still undergoing heavy accretion from the surrounding envelope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Multidimensional low-Mach number time-implicit hydrodynamic simulations of convective helium shell burning in a massive star

A realistic parametrization of convection and convective boundary mixing in conventional stellar evolution codes is still the subject of ongoing research. Furthermore, to improve the current situation, multidimensional hydrodynamic simulations are used to study convection in stellar interiors. Such simulations are numerically challenging, especially for flows at low Mach numbers which are typical for convection during early evolutionary stages. We explore the benefits of using a low-Mach hydrodynamic flux solver and demonstrate its usability for simulations in the astrophysical context. Simulations of convection for a realistic stellar profile are analyzed regarding the properties of convective boundary mixing. The time-implicit Seven-League Hydro (SLH) code was used to perform multidimensional simulations of convective helium shell burning based on a 25 M ⊙ star model. The results obtained with the low-Mach AUSM + -up solver were compared to results when using its non low-Mach variant AUSM B + -up. We applied well-balancing of the gravitational source term to maintain the initial hydrostatic background stratification. The computational grids have resolutions ranging from 180 × 90 2 to 810 × 540 2 cells and the nuclear energy release was boosted by factors of 3 × 10 3 , 1 × 10 4 , and 3 × 10 4 to study the dependence of the results on these parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Energy Loss of Neutrinos in 20 M{sub {center_dot}} and 40 M{sub {center_dot}} Massive Stars

The progress from the theoretical and experimental aspects in neutrino physics shines light into neutrino physics as well as in astrophysics. It is known that when neutrinos pass through a medium, the neutrinos interact with electrons before emerging from the stars with an effective potential energy produced due to coherent forward scatterings that occurred in the medium. This potential engenders significant changes in the neutrino masses and their mixing in the medium. Thus electron neutrinos would oscillate into different mass eigenstate and this is dependent on the energy of the neutrinos. Some of the energy will be lost in the coherent scattering with the electrons by the charged current interaction. We have calculated the energy loss of the neutrinos by using a stopping power equation for both rotating and non-rotating 20 M{sub {center_dot}} and 40 M{sub {center_dot}} stars. The total energy loss of neutrinos depends on the electron density in the stars and on the survival probability of the electron neutrino. For high electron density profile and the survival probability, the energy loss will be significant.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Nucleosynthesis in neutrino-driven, aspherical supernova explosion of a massive star

We examine explosive nucleosynthesis of p-nuclei during a delayed neutrino-driven, aspherical supernova explosion aided by standing accretion shock instability, based on two-dimensional hydrodynamic simulations of the explosion of a 15M{sub {center_dot}} star. We find that p-nuclei are mainly produced through {gamma}-processes, and that the nuclei lighter than {sup 92}Mo are abundantly synthesized in slightly neutron-rich bubbles with electron fractions of Y{sub e}{<=}0.48. {sup 94}Mo, {sup 96}Ru, and {sup 98}Ru, are underproduced compared with the solar system, as in the spherical model.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

The massive star population in M101. II. Spatial variations in the recent star formation history

We investigate star formation history (SFH) as a function of radius in M101 using archival Hubble Space Telescope Advanced Camera for Surveys photometry. We derive the SFH from the resolved stellar populations in five 2' wide annuli. Binning the SFH into time frames corresponding to stellar populations traced by Hα, far-ultraviolet, and near-ultraviolet emission, we find that the fraction of stellar populations young enough to contribute in Hα is 15%-35% in the inner regions, compared to less than 5% in the outer regions. This provides a sufficient explanation for the lack of Hα emission at large radii. We also model the blue to red supergiant ratio in our five annuli, examine the effects that a metallicity gradient and variable SFH have on the predicted ratios, and compare to the observed values. We find that the radial behavior of our modeled blue to red supergiant ratios is highly sensitive to both spatial variations in the SFH and metallicity. Incorporating the derived SFH into modeled ratios, we find that we are able to reproduce the observed values at large radii (low metallicity), but at small radii (high metallicity) the modeled and observed ratios are discrepant.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE MASSIVE STAR POPULATION IN M101. III. SPECTRA AND PHOTOMETRY OF THE LUMINOUS AND VARIABLE STARS

We discuss moderate-resolution spectra, multicolor photometry, and light curves of 31 of the most luminous stars and variables in the giant spiral M101. The majority are intermediate A- to F-type supergiants. We present new photometry and light curves for three known “irregular blue variables,” V2, V4, and V9, and identify a new candidate. Their spectra and variability confirm that they are luminous blue variable (LBV) candidates and V9 may be in an LBV-like maximum light state or eruption.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

RADIATION PRESSURE FROM MASSIVE STAR CLUSTERS AS A LAUNCHING MECHANISM FOR SUPER-GALACTIC WINDS

Galactic outflows of cool ({approx}10{sup 4} K) gas are ubiquitous in local starburst galaxies and in most high-redshift galaxies. Hot gas from supernovae has long been suspected as the primary driver, but this mechanism suffers from its tendency to destroy the cool gas. We propose a modification of the supernova scenario that overcomes this difficulty. Star formation is observed to take place in clusters. We show that, for L* galaxies, the radiation pressure from clusters with M{sub cl} {approx}> 10{sup 6} M{sub sun} is able to expel the surrounding gas at velocities in excess of the circular velocity v{sub c} of the disk galaxy. This cool gas travels above the galactic disk before supernovae erupt in the driving cluster. Once above the disk, the cool outflowing gas is exposed to radiation and hot gas outflows from the galactic disk, which in combination drive it to distances of {approx}50 kpc. Because the radiatively driven clouds grow in size as they travel, and because the hot gas is more dilute at large distance, the clouds are less subject to destruction. Therefore, unlike wind-driven clouds, radiatively driven clouds can give rise to the metal absorbers seen in quasar spectra. We identify these cluster-driven winds with large-scale galactic outflows. The maximum cluster mass in a galaxy is an increasing function of the galaxy's gas surface density, so only starburst galaxies are able to drive cold outflows. We find the critical star formation rate for launching large-scale cool outflows to be {Sigma}-dot{sub *}{sup crit} approx. 0.05 M{sub sun} yr{sup -1} kpc{sup -2}, in good agreement with observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

ON THE NATURE OF THE PROTOTYPE LUMINOUS BLUE VARIABLE AG CARINAE. II. WITNESSING A MASSIVE STAR EVOLVING CLOSE TO THE EDDINGTON AND BISTABILITY LIMITS

We show that the significantly different effective temperatures (T{sub eff}) achieved by the luminous blue variable AG Carinae during the consecutive visual minima of 1985-1990 (T{sub eff} {approx_equal} 22,800 K) and 2000-2001 (T{sub eff} {approx_equal} 17,000 K) place the star on different sides of the bistability limit, which occurs in line-driven stellar winds around T{sub eff} {approx} 21,000 K. Decisive evidence is provided by huge changes in the optical depth of the Lyman continuum in the inner wind as T{sub eff} changes during the S Dor cycle. These changes cause different Fe ionization structures in the inner wind. The bistability mechanism is also related to the different wind parameters during visual minima: the wind terminal velocity was 2-3 times higher and the mass-loss rate roughly two times smaller in 1985-1990 than in 2000-2003. We obtain a projected rotational velocity of 220 {+-} 50 km s{sup -1} during 1985-1990 which, combined with the high luminosity (L{sub *} = 1.5 x 10{sup 6} L{sub sun}), puts AG Car extremely close to the Eddington limit modified by rotation ({Omega}{Gamma} limit): for an inclination angle of 90{sup 0}, {Gamma}{sub {Omega}} {approx}> 1.0 for M{sub sun} {approx}< 60. Based on evolutionary models and mass budget, we obtain an initial mass of {approx}100 M{sub sun} and a current mass of {approx}60-70 M{sub sun} for AG Car. Therefore, AG Car is close to, if not at, the {Omega}{Gamma} limit during visual minimum. Assuming M = 70 M{sub sun}, we find that {Gamma}{sub {Omega}} decreases from 0.93 to 0.72 as AG Car expands toward visual maximum, suggesting that the star is not above the Eddington limit during maximum phases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE ASTROPHYSICAL IMPLICATIONS OF DUST FORMATION DURING THE ERUPTIONS OF HOT, MASSIVE STARS

Dust formation in the winds of hot stars is inextricably linked to the classic eruptive state of luminous blue variables because it requires very high mass-loss rates, M-dot {approx}>10{sup -2.5} M{sub sun} year{sup -1}, for grains to grow and for the non-dust optical depth of the wind to shield the dust formation region from the true stellar photosphere. Thus, dusty shells around hot stars trace the history of 'great' eruptions, and the statistics of such shells in the Galaxy indicate that these eruptions are likely the dominant mass-loss mechanism for evolved, M{sub ZAMS} {approx}> 40 M{sub Sun} stars. Dust formation at such high M-dot also explains why very large grains (a{sub max} {approx}> 1 {mu}m) are frequently found in these shells, since a{sub max}{proportional_to} M-dot . The statistics of these shells (numbers, ages, masses, and grain properties such as a{sub max}) provide an archaeological record of this mass-loss process. In particular, the velocities v{sub shell}, transient durations (where known), and ejected masses M{sub shell} of the Galactic shells and the supernova (SN) 'impostors' proposed as their extragalactic counterparts are very different. While much of the difference is a selection effect created by shell lifetimes {proportional_to}(v{sub shell}{radical}(M{sub shell})){sup -1}, more complete Galactic and extragalactic surveys are needed to demonstrate that the two phenomena share a common origin given that their observed properties are essentially disjoint. If even small fractions (1%) of SNe show interactions with such dense shells of ejecta, as is currently believed, then the driving mechanism of the eruptions must be associated with the very final phases of stellar evolution, suggestive of some underlying nuclear burning instability.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

G048.66-0.29: PHYSICAL STATE OF AN ISOLATED SITE OF MASSIVE STAR FORMATION

We present continuum observations of the infrared dark cloud (IRDC) G48.66-0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to several molecular line observations. The Herschel maps are used to derive temperature and column density maps of G48 using a model based on a modified blackbody. We find that G48 has a relatively simple structure and is relatively isolated; thus, this IRDC provides an excellent target to study the collapse and fragmentation of a filamentary structure in the absence of complicating factors such as strong external feedback. The derived temperature structure of G48 is clearly non-isothermal from cloud to core scale. The column density peaks are spatially coincident with the lowest temperatures ({approx}17.5 K) in G48. A total cloud mass of {approx}390 M{sub Sun} is derived from the column density maps. By comparing the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS bands to evolutionary models, we find that two cores are likely to evolve into high-mass stars (M{sub *} {>=} 8 M{sub Sun }). The derived mean projected separation of point sources is smaller than in other IRDCs but in good agreement with theoretical predications for cylindrical collapse. We detect several molecular species such as CO, HCO{sup +}, HCN, HNC, and N{sub 2}H{sup +}. CO is depleted by a factor of {approx}3.5 compared to the expected interstellar abundance, from which we conclude that CO freezes out in the central region. Furthermore, the molecular clumps, associated with the submillimeter peaks in G48, appear to be gravitationally unbound or just pressure confined. The analysis of critical line masses in G48 shows that the entire filament is collapsing, overcoming any internal support.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

LOW MACH NUMBER MODELING OF CORE CONVECTION IN MASSIVE STARS

This work presents three-dimensional simulations of core convection in a 15 M{sub Sun} star halfway through its main sequence lifetime. To perform the necessary long-time calculations, we use the low Mach number code MAESTRO, with initial conditions taken from a one-dimensional stellar model. We first identify several key factors that the one-dimensional initial model must satisfy to ensure efficient simulation of the convection process. We then use the three-dimensional simulations to examine the effects of two common modeling choices on the resulting convective flow: using a fixed composition approximation and using a reduced domain size. We find that using a fixed composition model actually increases the computational cost relative to using the full multi-species model because the fixed composition system takes longer to reach convection that is in a quasi-static state. Using a reduced (octant rather than full sphere) simulation domain yields flow with statistical properties that are within a factor of two of the full sphere simulation values. Both the octant and full sphere simulations show similar mixing across the convection zone boundary that is consistent with the turbulent entrainment model. However, the global character of the flow is distinctly different in the octant simulation, showing more rapid changes in the large-scale structure of the flow and thus a more isotropic flow on average.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Unveiling the secrets of metallicity and massive star formation using DLAS along gamma-ray bursts

We present the largest, publicly available sample of damped Lyα systems (DLAs) along the lines of sight of Swift-discovered gamma-ray bursts (GRBs) in order to investigate the environmental properties of long GRB hosts in the z = 1.8–6 redshift range. Compared with the most recent quasar DLA sample (QSO-DLA), our analysis shows that GRB-DLAs probe a more metal-enriched environment at z≳3, up to [X/H]∼−0.5. In the z = 2–3 redshift range, despite the large number of lower limits, there are hints that the two populations may be more similar (only at a 90% significance level) than at higher redshifts. Also, at high-z, the GRB-DLA average metallicity seems to decline at a lower rate than the QSO-DLAs: GRB-DLA hosts may be polluted with metals at least as far as ∼2 kpc from the GRB explosion site, probably due to previous star formation episodes and/or supernova explosions. This shallow metallicity trend, now extended up to z∼5, confirms previous results that GRB hosts are star-forming and have, on average, higher metallicities than the general QSO-DLA population. Finally, our host metallicity measurements are broadly consistent with the predictions derived from the hypothesis of two channels of GRB progenitors, one of which is mildly affected by a metallicity bias, although more data are needed to constrain the models at z≳4.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

PRESUPERNOVA EVOLUTION AND EXPLOSIVE NUCLEOSYNTHESIS OF ZERO METAL MASSIVE STARS

We present a new set of zero metallicity models in the range 13-80 M{sub Sun} together to the associated explosive nucleosynthesis. These models are fully homogeneous with the solar metallicity set we published in Limongi and Chieffi and will be freely available at the Online Repository for the FRANEC Evolutionary Output Web site. A comparison between these yields and an average star that represents the average behavior of most of the very metal-poor stars in the range -5.0 < [Fe/H] < -2.5 confirms previous findings that only a fraction of the elemental [X/Fe] may be fitted by the ejecta of standard core collapse supernovae.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THREE-DIMENSIONAL BOLTZMANN HYDRO CODE FOR CORE COLLAPSE IN MASSIVE STARS. I. SPECIAL RELATIVISTIC TREATMENTS

We propose a novel numerical method for solving multi-dimensional, special relativistic Boltzmann equations for neutrinos coupled with hydrodynamics equations. This method is meant to be applied to simulations of core-collapse supernovae. We handle special relativity in a non-conventional way, taking account of all orders of v/c. Consistent treatment of the advection and collision terms in the Boltzmann equations has been a challenge, which we overcome by employing two different energy grids: Lagrangian remapped and laboratory fixed grids. We conduct a series of basic tests and perform a one-dimensional simulation of core-collapse, bounce, and shock-stall for a 15 M {sub ☉} progenitor model with a minimum but essential set of microphysics. We demonstrate in the latter simulation that our new code is capable of handling all phases in core-collapse supernova. For comparison, a non-relativistic simulation is also conducted with the same code, and we show that they produce qualitatively wrong results in neutrino transfer. Finally, we discuss a possible incorporation of general relativistic effects into our method.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

X-ray emission from massive stars in CYG OB2

We report on the analysis of the Chandra-ACIS data of O, B, and WR stars in the young association Cyg OB2. X-ray spectra of 49 O-stars, 54 B-stars, and 3 WR-stars are analyzed and for the brighter sources, the epoch dependence of the X-ray fluxes is investigated. The O-stars in Cyg OB2 follow a well-defined scaling relation between their X-ray and bolometric luminosities: log ((L{sub X})/(L{sub bol}))=−7.2±0.2. This relation is in excellent agreement with the one previously derived for the Carina OB1 association. Except for the brightest O-star binaries, there is no general X-ray overluminosity due to colliding winds in O-star binaries. Roughly half of the known B-stars in the surveyed field are detected, but they fail to display a clear relationship between L{sub X} and L{sub bol}. Out of the three WR stars in Cyg OB2, probably only WR 144 is itself responsible for the observed level of X-ray emission, at a very low log ((L{sub X})/(L{sub bol}))=−8.8±0.2. The X-ray emission of the other two WR-stars (WR 145 and 146) is most probably due to their O-type companion along with a moderate contribution from a wind–wind interaction zone.

79 ASTRONOMY AND ASTROPHYSICS↗