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All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Where massive stars form - Associated radio H II regions and CO clouds in the northern Milky Way

The sites of massive star formation in molecular clouds are investigated by comparing high-resolution radio surveys of molecular and ionized gas emission in the Milky Way. CO emission maps from the Massachusetts-Stony Brook survey of the first Galactic quadrant are used to locate, in l, b, and v, the molecular clouds associated with radio recombination-line H II regions. It is found that the radio H II regions are typically associated with giant molecular clouds (GMCs) with diameters of 20-60 pc and virial masses of 100,000 to a million solar masses. The radio H II regions appear preferentially concentrated toward the centers of the GMCs, contrary to the 'blister' picture of massive star formation on cloud surfaces.

Waller, W. H.↗
DOE OSTI2011

THE COSMIC CORE-COLLAPSE SUPERNOVA RATE DOES NOT MATCH THE MASSIVE-STAR FORMATION RATE

We identify a 'supernova rate problem': the measured cosmic core-collapse supernova rate is a factor of {approx}2 smaller (with significance {approx}2{sigma}) than that predicted from the measured cosmic massive-star formation rate. The comparison is critical for topics from galaxy evolution and enrichment to the abundance of neutron stars and black holes. We systematically explore possible resolutions. The accuracy and precision of the star formation rate data and conversion to the supernova rate are well supported, and proposed changes would have far-reaching consequences. The dominant effect is likely that many supernovae are missed because they are either optically dim (low-luminosity) or dark, whether intrinsically or due to obscuration. We investigate supernovae too dim to have been discovered in cosmic surveys by a detailed study of all supernova discoveries in the local volume. If possible supernova impostors are included, then dim supernovae are common enough by fraction to solve the supernova rate problem. If they are not included, then the rate of dark core collapses is likely substantial. Other alternatives are that there are surprising changes in our understanding of star formation or supernova rates, including that supernovae form differently in small galaxies than in normal galaxies. These possibilities can be distinguished by upcoming supernova surveys, star formation measurements, searches for disappearing massive stars, and measurements of supernova neutrinos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Evolution and Properties of Rotating Massive Star Populations

We investigate the integrated properties of massive (>10 M{sub ⊙}) rotating single-star stellar populations for a variety of initial rotation rates (v/v{sub crit}=0.0, 0.2, 0.4, 0.5, and 0.6). We couple the new MESA Isochrone and Stellar Tracks (MIST) models to the Flexible Stellar Population Synthesis (FSPS) package, extending the stellar population synthesis models to include the contributions from very massive stars (>100 M{sub ⊙}), which can be significant in the first ∼4 Myr after a starburst. These models predict ionizing luminosities that are consistent with recent observations of young nuclear star clusters. We also construct composite stellar populations assuming a distribution of initial rotation rates. Even in low-metallicity environments where rotation has a significant effect on the evolution of massive stars, we find that stellar population models require a significant contribution from fast-rotating (v/v{sub crit}>0.4) stars in order to sustain the production of ionizing photons beyond a few Myr following a starburst. These results have potentially important implications for cosmic reionization by massive stars and the interpretation of nebular emission lines in high-redshift star-forming galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Young Massive Star Cluster Westerlund 2 Observed with MUSE. III. A Cluster in Motion—The Complex Internal Dynamics

Analyzing the dynamical state of nearby young massive star clusters is essential for understanding star cluster formation and evolution during their earliest stages. In this work we analyze the stellar and gas kinematics of the young massive star cluster Westerlund 2 (Wd2) using data from the integral field unit Multi Unit Spectroscopic Explorer (MUSE) and complement them with proper motions from the Gaia DR2. The mean gas radial velocity of 15.9 km s{sup −1} agrees with the assumption that Wd2 is the result of a cloud–cloud collision. The gas motions show the expansion of the H ii region, driven by the radiation from the many OB stars in the cluster center. The velocity profile of the cluster member stars reveals an increasing velocity dispersion with decreasing stellar mass and that the low-mass stars show five distinct velocity groups. Based on their spatial correlation with the cluster’s two clumps, we concluded that this is the imprint of the initial cloud collapse that formed Wd2. A thorough analysis of the dynamical state of Wd2, which determines a dynamical mass range of M {sub dyn,Wd2} = (7.5 ± 1.9) × 10{sup 4} − (4.4 ± 1.1) × 10{sup 5} M {sub ⊙} and exceeds the photometric mass by at least a factor of two, leads to the conclusion that Wd2 is not massive enough to remain gravitationally bound. Additionally we also identify 22 runaway candidates with peculiar velocities between 30 and 546 km s{sup −1}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

THE THREE-DIMENSIONAL EVOLUTION TO CORE COLLAPSE OF A MASSIVE STAR

We present the first three-dimensional (3D) simulation of the final minutes of iron core growth in a massive star, up to and including the point of core gravitational instability and collapse. We capture the development of strong convection driven by violent Si burning in the shell surrounding the iron core. This convective burning builds the iron core to its critical mass and collapse ensues, driven by electron capture and photodisintegration. The non-spherical structure and motion generated by 3D convection is substantial at the point of collapse, with convective speeds of several hundreds of km s{sup −1}. We examine the impact of such physically realistic 3D initial conditions on the core-collapse supernova mechanism using 3D simulations including multispecies neutrino leakage and find that the enhanced post-shock turbulence resulting from 3D progenitor structure aids successful explosions. We conclude that non-spherical progenitor structure should not be ignored, and should have a significant and favorable impact on the likelihood for neutrino-driven explosions. In order to make simulating the 3D collapse of an iron core feasible, we were forced to make approximations to the nuclear network making this effort only a first step toward accurate, self-consistent 3D stellar evolution models of the end states of massive stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Radiation-Hydrodynamic Simulations of Massive Star Formation with Protostellar Outflows

We report the results of a series of AMR radiation-hydrodynamic simulations of the collapse of massive star forming clouds using the ORION code. These simulations are the first to include the feedback effects protostellar outflows, as well as protostellar radiative heating and radiation pressure exerted on the infalling, dusty gas. We find that that outflows evacuate polar cavities of reduced optical depth through the ambient core. These enhance the radiative flux in the poleward direction so that it is 1.7 to 15 times larger than that in the midplane. As a result the radiative heating and outward radiation force exerted on the protostellar disk and infalling cloud gas in the equatorial direction are greatly diminished. The simultaneously reduces the Eddington radiation pressure barrier to high-mass star formation and increases the minimum threshold surface density for radiative heating to suppress fragmentation compared to models that do not include outflows. The strength of both these effects depends on the initial core surface density. Lower surface density cores have longer free-fall times and thus massive stars formed within them undergo more Kelvin contraction as the core collapses, leading to more powerful outflows. Furthermore, in lower surface density clouds the ratio of the time required for the outflow to break out of the core to the core free-fall time is smaller, so that these clouds are consequently influenced by outflows at earlier stages of collapse. As a result, outflow effects are strongest in low surface density cores and weakest in high surface density one. We also find that radiation focusing in the direction of outflow cavities is sufficient to prevent the formation of radiation pressure-supported circumstellar gas bubbles, in contrast to models which neglect protostellar outflow feedback.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

X-RAY EMISSION LINE PROFILES FROM WIND CLUMP BOW SHOCKS IN MASSIVE STARS

The consequences of structured flows continue to be a pressing topic in relating spectral data to physical processes occurring in massive star winds. In a preceding paper, our group reported on hydrodynamic simulations of hypersonic flow past a rigid spherical clump to explore the structure of bow shocks that can form around wind clumps. Here we report on profiles of emission lines that arise from such bow shock morphologies. To compute emission line profiles, we adopt a two-component flow structure of wind and clumps using two 'beta' velocity laws. While individual bow shocks tend to generate double-horned emission line profiles, a group of bow shocks can lead to line profiles with a range of shapes with blueshifted peak emission that depends on the degree of X-ray photoabsorption by the interclump wind medium, the number of clump structures in the flow, and the radial distribution of the clumps. Using the two beta law prescription, the theoretical emission measure and temperature distribution throughout the wind can be derived. The emission measure tends to be power law, and the temperature distribution is broad in terms of wind velocity. Although restricted to the case of adiabatic cooling, our models highlight the influence of bow shock effects for hot plasma temperature and emission measure distributions in stellar winds and their impact on X-ray line profile shapes. Previous models have focused on geometrical considerations of the clumps and their distribution in the wind. Our results represent the first time that the temperature distribution of wind clump structures are explicitly and self-consistently accounted for in modeling X-ray line profile shapes for massive stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SELF-REGULATED SHOCKS IN MASSIVE STAR BINARY SYSTEMS

In an early-type, massive star binary system, X-ray bright shocks result from the powerful collision of stellar winds driven by radiation pressure on spectral line transitions. We examine the influence of the X-rays from the wind-wind collision shocks on the radiative driving of the stellar winds using steady-state models that include a parameterized line force with X-ray ionization dependence. Our primary result is that X-ray radiation from the shocks inhibits wind acceleration and can lead to a lower pre-shock velocity, and a correspondingly lower shocked plasma temperature, yet the intrinsic X-ray luminosity of the shocks, L{sub X}, remains largely unaltered, with the exception of a modest increase at small binary separations. Due to the feedback loop between the ionizing X-rays from the shocks and the wind driving, we term this scenario as self-regulated shocks. This effect is found to greatly increase the range of binary separations at which a wind-photosphere collision is likely to occur in systems where the momenta of the two winds are significantly different. Furthermore, the excessive levels of X-ray ionization close to the shocks completely suppress the line force, and we suggest that this may render radiative braking less effective. Comparisons of model results against observations reveal reasonable agreement in terms of log (L{sub X}/L{sub bol}). The inclusion of self-regulated shocks improves the match for kT values in roughly equal wind momenta systems, but there is a systematic offset for systems with unequal wind momenta (if considered to be a wind-photosphere collision).

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Tables and intercomparisons of evolutionary sequences of models for massive stars

Tables of evolutionary sequences of models for massive stars have been prepared for a variety of physical input parameters that are normally treated as free. These parameters include the interior convective mixing scheme, the mixing length in the outer convective envelope, the rate of stellar-wind mass loss, the initial stellar mass, and the initial chemical composition. Ranges of specified initial mass and initial chemical composition are M = 10-120 solar masses, Xe = 0.602-0.739, and Ze = 0.021-0.044. The tables cover evolution of the star from the ZAMS to either the end of core H burning or the end of core He burning. Differences among the evolutionary tracks are illustrated primarily in terms of the interior mixing scheme, since the amount and timing of stellar wind mass loss are still very uncertain for initial masses above about 30 solar masses.

Chin, Chao-Wen↗
NASA NTRS1999

Overshooting in Massive Stars: Conceptual Problems and Suggested Solution

Overshooting in massive stars faces a basic difficulty: what theoretical models offer is not what stellar structure studies need. The former use Delta(sub mu) = 0 and define the OV where the negative convective flux J vanishes while the latter need to know where the concentration flux J(sub C) vanishes. We suggest that J may be dynamically irrelevant and derive the new dynamic equations for J(sub C). A new feature emerges: for large concentration gradients VC, J(sub C) is no longer proportional to VC, as always assumed.

Canuto, V. M.↗
DOE OSTI2020

Massive Stars in Molecular Clouds Rich in High-energy Sources: The Bridge of G332.809–0.132 and CS 78 in NGC 6334

Detections of massive stars in the direction of the H ii region CS 78 in NGC 6334 and of G332.809−0.132 are here presented. The region covered by the G332.809−0.132 complex coincides with the RCW 103 stellar association. In its core (40′ in radius), approximately 110 OB candidate stars (K {sub s} < 10 mag and 0.4 < A{sub K{sub s}} < 1.6 mag) were identified using 2MASS, DENIS, and GLIMPSE data. This number of OB stars accounts for more than 50% of the observed number of Lyman continuum photons from this region. Medium-resolution K-band spectra were obtained for seven early types, including one WN 8 star and one Ofpe/WN 9 star; the latter is located near the RCW 103 remnant and its luminosity is consistent with a distance of ≈3 kpc. The area analyzed encloses 9 of the 34 OB stars previously known in RCW 103, as well as IRAS 16115−5044, which we reclassify as a candidate luminous blue variable. The line of sight is particularly interesting, crossing three spiral arms; a molecular cloud at −50 (with RCW 103 in the Scutum-Crux arm) and another at −90 km s{sup −1} (in the Norma arm) are detected, both rich in massive stars and supernova remnants. We also report the detection of a B supergiant as the main ionizing source of CS 78, 2MASS J17213513−3532415. Medium-resolution H- and K-band spectra display H i and He i lines, as well as Fe ii lines. By assuming a distance of 1.35 kpc, we estimate a bolometric magnitude of −6.16, which is typical of supergiants.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

INTERNAL GRAVITY WAVES IN MASSIVE STARS: ANGULAR MOMENTUM TRANSPORT

We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation, and consequent angular momentum transport by such waves. We find that these waves are generated efficiently and transport angular momentum on short timescales over large distances. We show that, as in Earth's atmosphere, IGW drive equatorial flows which change magnitude and direction on short timescales. These results have profound consequences for the observational inferences of massive stars, as well as their long term angular momentum evolution. We suggest IGW angular momentum transport may explain many observational mysteries, such as: the misalignment of hot Jupiters around hot stars, the Be class of stars, Ni enrichment anomalies in massive stars, and the non-synchronous orbits of interacting binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Strong Coupling of Hydrodynamics and Reactions in Nuclear Statistical Equilibrium for Modeling Convection in Massive Stars

We build on the simplified spectral deferred corrections (SDC) coupling of hydrodynamics and reactions to handle the case of nuclear statistical equilibrium (NSE) and electron/positron captures/decays in the cores of massive stars. Our approach blends a traditional reaction network on the grid with a tabulated NSE state from a very large, ${\mathcal O }(100)$ nuclei network. We demonstrate how to achieve second-order accuracy in the simplified-SDC framework when coupling NSE to hydrodynamics, with the ability to evolve the star on the hydrodynamics time step. We discuss the application of this method to convection in massive stars leading up to core collapse. We also show how to initialize the initial convective state from a 1D model in a self-consistent fashion. All of these developments are done in the publicly available Castro simulation code and the entire simulation methodology is fully GPU-accelerated.

Explosive nucleosynthesis↗
DOE OSTI2015

The multiplicity of massive stars: A high angular resolution survey with the HST fine guidance sensor

We present the results of an all-sky survey made with the Fine Guidance Sensor on the Hubble Space Telescope to search for angularly resolved binary systems among massive stars. The sample of 224 stars is comprised mainly of Galactic O- and B-type stars and luminous blue variables, plus a few luminous stars in the Large Magellanic Cloud. The FGS TRANS mode observations are sensitive to the detection of companions with an angular separation between 0.″01 and 1.″0 and brighter than △m=5. The FGS observations resolved 52 binary and 6 triple star systems and detected partially resolved binaries in 7 additional targets (43 of these are new detections). These numbers yield a companion detection frequency of 29% for the FGS survey. We also gathered literature results on the numbers of close spectroscopic binaries and wider astrometric binaries among the sample, and we present estimates of the frequency of multiple systems and the companion frequency for subsets of stars residing in clusters and associations, field stars, and runaway stars. These results confirm the high multiplicity fraction, especially among massive stars in clusters and associations. We show that the period distribution is approximately flat in increments of logP. We identify a number of systems of potential interest for long-term orbital determinations, and we note the importance of some of these companions for the interpretation of the radial velocities and light curves of close binaries that have third companions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

On the Weak-Wind Problem in Massive Stars: X-Ray Spectra Reveal a Massive Hot Wind in mu Columbae

Mu Columbae is a prototypical weak-wind O star for which we have obtained a high-resolution X-ray spectrum with the Chandra LETG/ACIS instrument and a low-resolution spectrum with Suzaku. This allows us, for the first time, to investigate the role of X-rays on the wind structure in a bona fide weak-wind system and to determine whether there actually is a massive hot wind. The X-ray emission measure indicates that the outflow is an order of magnitude greater than that derived from UV lines and is commensurate with the nominal wind-luminosity relationship for O stars. Therefore, the "weak-wind problem"--identified from cool wind UV/optical spectra--is largely resolved by accounting for the hot wind seen in X-rays. From X-ray line profiles, Doppler shifts, and relative strengths, we find that this weak-wind star is typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically confined plasma-the spectrum is soft and lines are broadened; Suzaku spectra confirm the lack of emission above 2 keV. Nor do the relative line shifts and widths suggest any wind decoupling by ions. The He-like triplets indicate that the bulk of the X-ray emission is formed rather close to the star, within five stellar radii. Our results challenge the idea that some OB stars are "weak-wind" stars that deviate from the standard wind-luminosity relationship. The wind is not weak, but it is hot and its bulk is only detectable in X-rays.

Weak-wind↗
DOE OSTI2013

THE {sup 12}C + {sup 12}C REACTION AND THE IMPACT ON NUCLEOSYNTHESIS IN MASSIVE STARS

Despite much effort in the past decades, the C-burning reaction rate is uncertain by several orders of magnitude, and the relative strength between the different channels {sup 12}C({sup 12}C, {alpha}){sup 20}Ne, {sup 12}C({sup 12}C, p){sup 23}Na, and {sup 12}C({sup 12}C, n){sup 23}Mg is poorly determined. Additionally, in C-burning conditions a high {sup 12}C+{sup 12}C rate may lead to lower central C-burning temperatures and to {sup 13}C({alpha}, n){sup 16}O emerging as a more dominant neutron source than {sup 22}Ne({alpha}, n){sup 25}Mg, increasing significantly the s-process production. This is due to the chain {sup 12}C(p, {gamma}){sup 13}N followed by {sup 13}N({beta} +){sup 13}C, where the photodisintegration reverse channel {sup 13}N({gamma}, p){sup 12}C is strongly decreasing with increasing temperature. Presented here is the impact of the {sup 12}C+{sup 12}C reaction uncertainties on the s-process and on explosive p-process nucleosynthesis in massive stars, including also fast rotating massive stars at low metallicity. Using various {sup 12}C+{sup 12}C rates, in particular an upper and lower rate limit of {approx}50,000 higher and {approx}20 lower than the standard rate at 5 Multiplication-Sign 10{sup 8} K, five 25 M {sub Sun} stellar models are calculated. The enhanced s-process signature due to {sup 13}C({alpha}, n){sup 16}O activation is considered, taking into account the impact of the uncertainty of all three C-burning reaction branches. Consequently, we show that the p-process abundances have an average production factor increased up to about a factor of eight compared with the standard case, efficiently producing the elusive Mo and Ru proton-rich isotopes. We also show that an s-process being driven by {sup 13}C({alpha}, n){sup 16}O is a secondary process, even though the abundance of {sup 13}C does not depend on the initial metal content. Finally, implications for the Sr-peak elements inventory in the solar system and at low metallicity are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1986

The massive star content of blue irregular galaxies

Three 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 2 of the 3 regions and is best fit by a synthesized spectrum of a burst of massive stars 2.5 to 3.0 million yr old.

Lamb, S. A.↗
DOE OSTI2010

FROM THE CONVERGENCE OF FILAMENTS TO DISK-OUTFLOW ACCRETION: MASSIVE STAR FORMATION IN W33A

Interferometric observations of the W33A massive star formation region, performed with the Submillimeter Array and the Very Large Array at resolutions from 5'' (0.1 pc) to 0.''5 (0.01 pc), are presented. Our three main findings are: (1) parsec-scale, filamentary structures of cold molecular gas are detected. Two filaments at different velocities intersect in the zone where the star formation is occurring. This is consistent with triggering of the star formation activity by the convergence of such filaments, as predicted by numerical simulations of star formation initiated by converging flows. (2) The two dusty cores (MM1 and MM2) at the intersection of the filaments are found to be at different evolutionary stages, and each of them is resolved into multiple condensations. MM1 and MM2 have markedly different temperatures, continuum spectral indices, molecular-line spectra, and masses of both stars and gas. (3) The dynamics of the 'hot-core' MM1 indicates the presence of a rotating disk in its center (MM1-Main) around a faint free-free source. The stellar mass is estimated to be {approx}10 M{sub sun}. A massive molecular outflow is observed along the rotation axis of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗