Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “MASSIVE STARS”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Reaction rate for carbon burning in massive stars

Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for 12 C+ 12 C fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of 12 C+ 12 C fusion cross sections where these backgrounds have been minimized. In conclusion, it is found that the astrophysical S factor exhibits a maximum around E cm =3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

On the Geometry of the Near-core Magnetic Field in Massive Stars

It is well-known that the cores of massive stars sustain a stellar dynamo with a complex magnetic field configuration. However, the same cannot be said for the field's strength and geometry at the convective–radiative boundary, which are crucial when performing asteroseismic inference. In this Letter, we present 3D magnetohydrodynamic (MHD) simulations of a 7 M ⊙ mid-main-sequence star, with particular attention given to the convective–radiative boundary in the near-core region. Our simulations reveal that the toroidal magnetic field is significantly stronger than the poloidal field in this region, contrary to recent assumptions. Moreover, the rotational shear layer, also important for asteroseismic inference, is specifically confined within the extent of the Brunt–Väisälä frequency peak. These results, which are based on the inferred properties of HD 43317, have widespread implications for asteroseismic studies of rotation, mixing, and magnetism in stars. While we expect our results to be broadly applicable across stars with similar Brunt–Väisälä frequency profiles and stellar masses, we also expect the MHD parameters (e.g., Re m ) and the initial stellar rotation rate to impact the geometry of the field and differential rotation at the convective–radiative interface.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

Axial rotation, tangled magnetic fields, and theoretical models of very massive stars

A simple method of computing theoretical models of very massive stars endowed with fast axial rotation and tangled magnetic fields is described and used in the present paper. Both of the two perturbing (nongravitational) forces induce changes in the luminosity and radius that are studied as functions of zero-age chemical composition, opacity, and evolutionary state of the interior. The central condensation of the star is found to have a significant influence on shifts of the upper main-sequence band in the H-R diagram if the perturbing force is concentrated in the stellar envelope (but not if the perturbing force is distributed so as to be approximately proportional to gravity everywhere); the layers of the envelope that contribute most heavily to the central condensation lie approximately at a radius fraction of r/R = 0.5. It is shown that fast uniform rotation and intense envelope magnetic fields lead to probably the largest possible shifts of the main-sequence band in the H-R diagram that rotation and magnetic fields can induce. These displacements are, however, too small to account for the total width of the observed main-sequence band at luminosities brighter than log (L/solar luminosity) = 4.5.

Stothers, R.↗
NASA NTRS1984

Explosive nucleosynthesis in massive stars - Comparison with the Cassiopeia A fast-moving knots

If the ejecta of a Type II supernova do not undergo extensive mixing, then, based on the explosion of current presupernova models, only a small fraction approximately equal to or less than 0.1 solar mass of the mantle of a massive star can yield abundances similar to those observed in the fast-moving knots of Cas A. This is shown to be independent of the detailed structure of the mantle and the supernova energy. Lack of mixing in Cas A is indicated by strong upper limits on the abundance ratios Ne/O, and Fe/O. If this is confirmed by further observations, then either Cas A is not the result of a standard progenitor of approximately equal to or less than 25 solar masses disrupted by a Type II supernova, or the picture of the last stages of stellar evolution in massive stars needs to be modified substantially.

Johnston, M. D.↗
DOE OSTI2012

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 {sup w}eak-wind problem{sup -}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.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The range of variation of the mass of the most massive star in stellar clusters derived from 35 million Monte Carlo simulations

A growing fraction of simple stellar population models, in an aim to create more realistic simulations capable of including stochastic variation in their outputs, begin their simulations with a distribution of discrete stars following a power-law function of masses. Careful attention is needed to create a correctly sampled initial mass function (IMF), and here we provide a solid mathematical method, called MASSCLEAN IMF Sampling, for doing so. We use our method to perform 10 million MASSCLEAN Monte Carlo stellar cluster simulations to determine the most massive star in a mass distribution as a function of the total mass of the cluster. We find that a maximum mass range is predicted, not a single maximum mass. This range is (1) dependent on the total mass of the cluster and (2) independent of an upper stellar mass limit, M{sub limit} , for unsaturated clusters and emerges naturally from our IMF sampling method. We then turn our analysis around, starting with our new database of 25 million simulated clusters, to constrain the highest mass star from the observed integrated colors of a sample of 40 low-mass Large Magellanic Cloud stellar clusters of known age and mass. Finally, we present an analytical description of the maximum mass range of the most massive star as a function of the cluster's total mass and present a new M{sub max} -M{sub cluster} relation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

EARLY PHASE OF MASSIVE STAR FORMATION: A CASE STUDY OF THE INFRARED DARK CLOUD G084.81-01.09

We mapped the Mid-course Space Experiment dark cloud G084.81-01.09 in the NH{sub 3}(1,1)-(4,4) lines and in the J = 1-0 transitions of {sup 12}CO, {sup 13}CO, C{sup 18}O, and HCO{sup +} in order to study the physical properties of infrared dark clouds, and to better understand the initial conditions for massive star formation. Six ammonia cores are identified with masses ranging from 60 to 250 M {sub sun}, a kinetic temperature of 12 K, and a molecular hydrogen number density n(H{sub 2}) {approx} 10{sup 5} cm{sup -3}. In our high-mass cores, the ammonia line width of 1 km s{sup -1} is larger than those found in lower mass cores but narrower than the more evolved massive ones. We detected self-reversed profiles in HCO{sup +} across the northern part of our cloud and velocity gradients in different molecules. These indicate an expanding motion in the outer layer and more complex motions of the clumps more inside our cloud. We also discuss the millimeter wave continuum from the dust. These properties indicate that our cloud is a potential site of massive star formation but is still in a very early evolutionary stage.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Role of thermal and photobeta decays in processes of nucleosynthesis of problematic p nuclei of {sup 113}In, {sup 115}Sn, {sup 92},{sup 94}Mo, and {sup 96},{sup 98}Ru in massive stars

The temperature dependence of the rates of {sup 113}Cd {yields}{sup 113}In, {sup 115}In {yields}{sup 115}Sn, {sup 92}Zr{yields}{sup 92}Nb {yields}{sup 92}Mo, {sup 94}Zr{yields}{sup 94}Nb {yields}{sup 94}Mo, {sup 96}Mo {yields}{sup 96}Tc{yields}{sup 96}Ru, and {sup 98}Mo{yields}{sup 98}Tc{yields}{sup 98}Ru thermal beta transitions was studied at temperatures of massive-star matter in the range of 1 Multiplication-Sign 10{sup 8}-6 Multiplication-Sign 10{sup 9} K. These decays are the possible channel of synthesis of the p nuclei of {sup 113}In, {sup 115}Sn, {sup 92},{sup 94}Mo, and {sup 96},{sup 98}Ru. The abundances of these nuclei present a challenge for models that study the explosivemechanism of synthesis. The contribution of photobeta decay to the synthesis of the aforementioned p nuclei was estimated. It was shown that the channel of thermal beta decay for {sup 113}In, {sup 115}Sn, {sup 94}Mo, and {sup 98}Ru p nuclei and the channel of photobeta decay for the {sup 96}Mo p nucleus could be efficient at the high-temperature quasiequilibrium stage of massive-star evolution.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1990

Global and radial variations in the efficiency of massive star formation among galaxies

In order to determine the regions within galaxies which give rise to the most efficient star formation and to test the hypothesis that galaxies with high infrared luminosities per unit molecular mass are efficiently producing high mass stars, researchers have undertaken an H alpha imaging survey in galaxies whose CO distributions have been measured as part of the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey. From these images researchers have derived global H alpha fluxes and distributions for comparison with far infrared radiation (FIR) fluxes and CO fluxes and distributions. Here, researchers present results on the global massive star formation efficiency (SFE = L sub H sub alpha/M(H2)) as a function of morphological type and environment, and on the radial distribution of the SFE within both peculiar and isolated galaxies. On the basis of comparison of the global L sub H sub alpha/M(H2) and L sub FIR/M(H2) for 111 galaxies, researchers conclude that environment rather than morphological type has the strongest effect on the global efficiency of massive star formation. Based on their study of a small sample, they find that the largest radial gradients are observed in the interacting/peculiar galaxies, indicating that environment affects the star formation efficiency within galaxies as well.

Allen, Lori E.↗
DOE OSTI2011

THE DIVERSITY OF MASSIVE STAR OUTBURSTS. I. OBSERVATIONS OF SN2009ip, UGC 2773 OT2009-1, AND THEIR PROGENITORS

Despite both being outbursts of luminous blue variables (LBVs), SN 2009ip and UGC 2773 OT2009-1 have very different progenitors, spectra, circumstellar environments, and possibly physical mechanisms that generated the outbursts. From pre-eruption Hubble Space Telescope images, we determine that SN 2009ip and UGC 2773 OT2009-1 have initial masses of {approx}> 60 and {approx}> 25 M{sub sun}, respectively. Optical spectroscopy shows that at peak, SN 2009ip had a 10,000 K photosphere and its spectrum was dominated by narrow H Balmer emission, similar to classical LBV giant outbursts, also known as 'supernova impostors'. The spectra of UGC 2773 OT2009-1, which also have narrow H{alpha} emission, are dominated by a forest of absorption lines, similar to an F-type supergiant. Blueshifted absorption lines corresponding to ejecta at a velocity of 2000-7000 km s{sup -1} are present in later spectra of SN 2009ip-an unprecedented observation for LBV outbursts, indicating that the event was the result of a supersonic explosion rather than a subsonic outburst. The velocity of the absorption lines increases between two epochs, suggesting that there were two explosions in rapid succession. A rapid fading and rebrightening event concurrent with the onset of the high-velocity absorption lines is consistent with the double-explosion model. A near-infrared excess is present in the spectra and photometry of UGC 2773 OT2009-1 that is consistent with {approx}2100 K dust emission. We compare the properties of these two events and place them in the context of other known massive star outbursts such as {eta} Car, NGC 300 OT2008-1, and SN 2008S. This qualitative analysis suggests that massive star outbursts have many physical differences that can manifest as the different observables seen in these two interesting objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PROTOSTELLAR OUTFLOWS AND RADIATIVE FEEDBACK FROM MASSIVE STARS

We carry out radiation hydrodynamical simulations of the formation of massive stars in the super-Eddington regime including both their radiative feedback and protostellar outflows. The calculations start from a prestellar core of dusty gas and continue until the star stops growing. The accretion ends when the remnants of the core are ejected, mostly by the force of the direct stellar radiation in the polar direction and elsewhere by the reradiated thermal infrared radiation. How long the accretion persists depends on whether the protostellar outflows are present. We set the mass outflow rate to 1% of the stellar sink particle's accretion rate. The outflows open a bipolar cavity extending to the core's outer edge, through which the thermal radiation readily escapes. The radiative flux is funneled into the polar directions while the core's collapse proceeds near the equator. The outflow thus extends the ''flashlight effect'', or anisotropic radiation field, found in previous studies from the few hundred AU scale of the circumstellar disk up to the 0.1 parsec scale of the core. The core's flashlight effect allows core gas to accrete on the disk for longer, in the same way that the disk's flashlight effect allows disk gas to accrete on the star for longer. Thus although the protostellar outflows remove material near the core's poles, causing slower stellar growth over the first few free-fall times, they also enable accretion to go on longer in our calculations. The outflows ultimately lead to stars of somewhat higher mass.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1965

Massive stars in quasi-static equilibrium.

Quasi-static equilibrium in massive stars during gravitational contraction including discussion of general relativistic and classical hydrodynamic results

GRAVITATIONAL COLLAPSE↗
NASA NTRS1969

Massive stars

Normal O, blue supergiants and brightest yellow and red supergiant massive stars, considering evolution, luminosity, lifetime mass loss, pulsational instability and supernovae

Stothers, R.↗
NASA NTRS1993

The evolution of massive stars including mass loss - Presupernova models and explosion

The evolution of massive stars of 35, 40, 60, and 85 solar masses is followed through all stages of nuclear burning to the point of Fe core collapse. Critical nuclear reaction and mass-loss rates are varied. Efficient mass loss during the Wolf-Rayet (WR) stage is likely to lead to final masses as small as 4 solar masses. For a reasonable parameterization of the mass loss, there may be convergence of all WR stars, both single and in binaries, to a narrow band of small final masses. Our representative model, a 4.25 solar-mass WR presupernova derived from a 60 solar mass star, is followed through a simulated explosion, and its explosive nucleosynthesis and light curve are determined. Its properties are similar to those observed in Type Ib supernovae. The effects of the initial mass and mass loss on the presupernova structure of small mass WR models is also explored. Important properties of the presupernova star and its explosion can only be obtained by following the complete evolution starting on the main sequence.

Woosley, S. E.↗
DOE OSTI2014

Big Fish in Small Ponds: massive stars in the low-mass clusters of M83

We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages ≤ 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the ∼<10{sup 3} M {sub ☉} clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Winds of Massive Stars: High-resolution X-Ray Spectra of Stars in NGC 3603

The cluster NGC 3603 hosts some of the most massive stars in the Galaxy. With a modest 50 ks exposure with the Chandra High Energy Grating Spectrometer, we have resolved emission lines in spectra of several of the brightest cluster members, which are of WNh and O spectral types. This observation provides our first definitive high-resolution spectra of such stars in this nearby starburst region. The stars studied have broadened X-ray emission lines, some with blueshifted centroids, and are characteristic of massive stellar winds with terminal velocities around 2000–3000 km s{sup −1}. X-ray luminosities and plasma temperatures are very high for both the WNh and O-stars studied. We conclude that their X-rays are likely the result of colliding winds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

The possible disappearance of a massive star in the low-metallicity galaxy PHL 293B

ABSTRACT We investigate a suspected very massive star in one of the most metal-poor dwarf galaxies, PHL 293B. Excitingly, we find the sudden disappearance of the stellar signatures from our 2019 spectra, in particular the broad H lines with P Cygni profiles that have been associated with a massive luminous blue variable (LBV) star. Such features are absent from our spectra obtained in 2019 with the Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observation and X-shooter instruments of the European Southern Observatory’s Very Large Telescope. We compute radiative transfer models using cmfgen, which fit the observed spectrum of the LBV and are consistent with ground-based and archival Hubble Space Telescope photometry. Our models show that during 2001–2011, the LBV had a luminosity L* = 2.5–3.5 × 106 L⊙, a mass-loss rate $\dot{M} = 0.005{-}0.020 ~{\rm M}_{\odot }$ yr−1, a wind velocity of 1000 km s−1, and effective and stellar temperatures of Teff = 6000–6800 and T* = 9500–15 000 K. These stellar properties indicate an eruptive state. We consider two main hypotheses for the absence of the broad emission components from the spectra obtained since 2011. One possibility is that we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration. Alternatively, the LBV could have collapsed to a massive black hole without the production of a bright supernova.

Allan, Andrew P.↗