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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-phase Metallicity Gradients: The Effects of Ram Pressure Stripping

Recent observations of galaxies in a cluster at z = 0.35 show that their integrated gas-phase metallicities increase with decreasing cluster-centric distance. To test whether ram pressure stripping (RPS) is the underlying cause, we use a semianalytic model to quantify the “observational bias” that RPS introduces into the aperture-based metallicity measurements. We take integral field spectroscopy of local galaxies, remove gas from their outer galactic disks via RPS, and then conduct mock slit observations of cluster galaxies at z = 0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of −0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a mean metallicity enhancement of +0.02 dex at a fixed stellar mass. This gas removal and subsequent quenching of star formation preferentially removes low-mass cluster galaxies from the observed star-forming population. As only the more massive star-forming galaxies survive to reach the cluster core, RPS produces a cluster-scale stellar mass gradient of −0.05log(M{sub ∗}/M{sub ⊙})/Mpc. This mass segregation drives the predicted cluster-scale metallicity gradient of −0.03 dex/Mpc. However, the effects of RPS alone cannot explain the higher metallicities measured in cluster galaxies at z = 0.35. We hypothesize that additional mechanisms including steep internal metallicity gradients and self-enrichment due to gas strangulation are needed to reproduce our observations at z = 0.35.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

An optimist's guide to supernovae

There are some hints that a satisfactory agreement between observations and models is developing in several areas of supernova research. The light curves and spectra of Type I supernovae look so much like the models that they may well have their origin in the synthesis of Ni-56 in the explosion of a compact star. Type II supernovae are found where massive stars form, have light curves that correspond to 10 to the 51st ergs suddenly deposited inside a red supergiant, and may in fact be the explosions of massive stars. Some young supernova remnants, such as Cas A, show abundance patterns that bear a striking resemblance to those seen deep inside models of massive stars on the verge of destruction. Observations of old supernova remnants provide energy estimate of 10 to the 51st ergs - just the amount needed for the models of the outbursts.

Kirshner, R. P.↗
DOE OSTI2014

Trigonometric distance and proper motion of IRAS 20056+3350: a massive star-forming region on the solar circle

We report our measurement of the trigonometric distance and proper motion of IRAS 20056+3350, obtained from the annual parallax of H{sub 2}O masers. Our distance of D=4.69{sub −0.51}{sup +0.65} kpc, which is 2.8 times larger than the near kinematic distance adopted in the literature, places IRAS 20056+3350 at the leading tip of the Local arm and proximal to the solar circle. Using our distance, we reevaluate past observations to reveal IRAS 20056+3350 as a site of massive star formation at a young stage of evolution. This result is consistent with the spectral energy distribution of the source evaluated with published photometric data from UKIDSS, WISE, AKARI, IRAS, and the submillimeter continuum. Both analytical approaches reveal the luminosity of the region to be 2.4 × 10{sup 4} L {sub ☉}, and suggest that IRAS 20056+3350 is forming an embedded star of ≥16 M {sub ☉}. We estimated the proper motion of IRAS 20056+3350 to be (μ{sub α}cos δ, μ{sub δ}) = (–2.62 ± 0.33, –5.65 ± 0.52) mas yr{sup –1} from the group motion of H{sub 2}O masers, and use our results to estimate the angular velocity of Galactic rotation at the Galactocentric distance of the Sun, Ω{sub 0} = 29.75 ± 2.29 km s{sup –1} kpc{sup –1}, which is consistent with the values obtained for other tangent point and solar circle objects.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2011

Fermi-LAT Detection of Gamma-Ray Emission in the Vicinity of the Star Forming Regions W43 and Westerlund 2

Particle acceleration in massive star forming regions can proceed via a large variety of possible emission scenarios, including high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary (here WR 20a and WR I2Ia), collective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. In view of the recent Fermi-LAT detection of HESS JI023-575 (in the vicinity of Westerlund 2), we examine another very high energy (VHE) gamma-ray source, HESS JI848-0145 (in the vicinity ofW43), possibly associated with a massive star cluster. Considering multi-wavelength data, in particular TeV gamma-rays, we examine the available evidence that the gamma-ray emission coincident with Westerlund 2 and W43 could originate in particles accelerated by the above-mentioned mechanisms in massive star clusters.

Lemoine-Goumard, M.↗
DOE OSTI2011

IRDC G030.88+00.13: A TALE OF TWO MASSIVE CLUMPS

Massive stars (M {approx}>10 M{sub sun}) form from collapse of parsec-scale molecular clumps. How molecular clumps fragment to give rise to massive stars in a cluster with a distribution of masses is unclear. We search for cold cores that may lead to future formation of massive stars in a massive (>10{sup 3} M{sub sun}), low luminosity (4.6 x 10{sup 2} L{sub sun}) infrared dark cloud (IRDC) G030.88+00.13. The NH{sub 3} data from the Very Large Array (VLA) and Green Bank Telescope reveal that the extinction feature seen in the infrared consists of two distinctive clumps along the same line of sight. The C1 clump at 97 km s{sup -1} coincides with the extinction in the Spitzer 8 and 24 {mu}m. Therefore, it is responsible for the majority of the IRDC. The C2 clump at 107 km s{sup -1} is more compact and has a peak temperature of 45 K. Compact dust cores and H{sub 2}O masers revealed in the Submillimeter Array and VLA observations are mostly associated with C2, and none are within the IRDC in C1. The luminosity indicates that neither the C1 nor C2 clump has yet to form massive protostars. But C1 might be at a precluster forming stage. The simulated observations rule out 0.1 pc cold cores with masses above 8 M{sub sun} within the IRDC. The core masses in C1 and C2 and those in high-mass protostellar objects suggest an evolutionary trend that the mass of cold cores increases over time. Based on our findings, we propose an empirical picture of massive star formation that protostellar cores and the embedded protostars undergo simultaneous mass growth during the protostellar evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DECIPHERING THE IONIZED GAS CONTENT IN THE MASSIVE STAR-FORMING COMPLEX G75.78+0.34

We present subarcsecond observations toward the massive star-forming region G75.78+0.34. We used the Very Large Array to study the centimeter continuum and H{sub 2}O and CH{sub 3}OH maser emission, and the Owens Valley Radio Observatory and Submillimeter Array to study the millimeter continuum and recombination lines (H40{alpha} and H30{alpha}). We found radio continuum emission at all wavelengths, coming from three components: (1) a cometary ultracompact (UC) H II region with an electron density {approx}3.7 Multiplication-Sign 10{sup 4} cm{sup -3}, excited by a B0 type star, and with no associated dust emission; (2) an almost unresolved UCH II region (EAST), located {approx}6'' to the east of the cometary UCH II region, with an electron density {approx}1.3 Multiplication-Sign 10{sup 5} cm{sup -3}, and associated with a compact dust clump detected at millimeter and mid-infrared wavelengths; and (3) a compact source (CORE), located {approx}2'' to the southwest of the cometary arc, with a flux density increasing with frequency, and embedded in a dust condensation of 30 M{sub Sun }. The CORE source is resolved into two compact and unresolved sources which can be well fit by two homogeneous hypercompact H II regions each one photoionized by a B0.5 zero-age main sequence star, or by free-free radiation from shock-ionized gas resulting from the interaction of a jet/outflow system with the surrounding environment. The spatial distribution and kinematics of water masers close to the CORE-N and S sources, together with excess emission at 4.5 {mu}m and the detected dust emission, suggest that the CORE source is a massive protostar driving a jet/outflow.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

A HOT JUPITER ORBITING THE 1.7 M {sub sun} SUBGIANT HD 102956

We report the detection of a giant planet in a 6.4950 day orbit around the 1.68 M {sub sun} subgiant HD 102956. The planet has a semimajor axis a = 0.081 AU and a minimum mass M{sub P} sin i =0.96 M {sub Jup}. HD 102956 is the most massive star known to harbor a hot Jupiter, and its planet is only the third known to orbit within 0.6 AU of a star more massive than 1.5 M {sub sun}. Based on our sample of 137 subgiants with M {sub *}>1.45 M {sub sun}, we find that 0.5%-2.3% of A-type stars harbor a close-in planet (a < 0.1 AU) with M{sub P} sin i > 1 M {sub Jup}, consistent with hot-Jupiter occurrence for Sun-like stars. Thus, the paucity of planets with 0.1 AU < a < 1.0 AU around intermediate-mass stars may be an exaggerated version of the 'period valley' that is characteristic of planets around Sun-like stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Simulations of Fractal Star Cluster Formation. I. New Insights for Measuring Mass Segregation of Star Clusters with Substructure

We compare the existent methods, including the minimum spanning tree based method and the local stellar density based method, in measuring mass segregation of star clusters. We find that the minimum spanning tree method reflects more the compactness, which represents the global spatial distribution of massive stars, while the local stellar density method reflects more the crowdedness, which provides the local gravitational potential information. It is suggested to measure the local and the global mass segregation simultaneously. We also develop a hybrid method that takes both aspects into account. This hybrid method balances the local and the global mass segregation in the sense that the predominant one is either caused by dynamical evolution or purely accidental, especially when such information is unknown a priori. In addition, we test our prescriptions with numerical models and show the impact of binaries in estimating the mass segregation value. As an application, we use these methods on the Orion Nebula Cluster (ONC) observations and the Taurus cluster. We find that the ONC is significantly mass segregated down to the 20th most massive stars. In contrast, the massive stars of the Taurus cluster are sparsely distributed in many different subclusters, showing a low degree of compactness. The massive stars of Taurus are also found to be distributed in the high-density region of the subclusters, showing significant mass segregation at subcluster scales. Meanwhile, we also apply these methods to discuss the possible mechanisms of the dynamical evolution of the simulated substructured star clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2022

The Polstar High Resolution Spectropolarimetry MIDEX Mission

The Polstar mission will provide a space-borne 60 cm spectropolarimeter operating at ultraviolet (UV) wavelengths, capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter and inform massive star evolution, affect the stellar remnant population, and stir and enrich the interstellar medium (ISM). These will be achieved by investigating the dynamical geometries in the winds and disks of hot stars, the composition and magnetic alignment of interstellar dust, and the star-forming accretion disks of UV-bright stars at an important transition boundary. Together these areas map out a kind of two-way interface between massive stars and their effect on our galaxy, wherein the stellar winds enrich the ISM with metals and kinetic energy, preconditioning their environment and the stellar endpoints prior to undergoing supernova. The ISM dust in turn reveals the composition and magnetic environment leading to new star formation, and the accretion disks of Herbig Ae/Be stars reveal how the ISM gas returns to make new massive stars. Polstar will combine high-resolution spectroscopy in the time domain with high-precision UV polarimetry. Doppler-shifted UV resonance line opacity will provide information about circumstellar kinematics, while polarization gives complementary geometric information about unseen structures. The composition and magnetic alignment of the smallest interstellar dust grains provides a probe of the ISM utilizing radiative alignment theory (RAT). Polstar will operate in the far-UV (FUV) at 122–200 nm at high spectral resolution of around R ∼ 30k, and at FUV and near-UV (NUV) wavelengths of 122–320 nm at lower spectral resolutions of 0.1 - 1k. Detection of polarization levels as weak as 0.1% are expected, with a temporal cadence ranging from 5–10 minutes for most wind variability studies, to hours or days for sampling rotation, to days or weeks for sampling binary orbits, to months to a year for sampling substructure in the inner regions of protoplanetary disks. Sub-meter-class aperture is well suited to access this wide array of time domain science, made possible by restricting to a few hundred bright, massive stars, necessarily extincted by a small to moderate column of interstellar dust, informing both the attributes of the stars and the ISM through which they are seen. As such, the focus is on our own galaxy and its evolutionary drivers, but a few targets in the Magellanic clouds offer the potential to extend this understanding to low-metallicity environments.

Paul A Scowen↗
DOE OSTI2012

Gamma-ray follow-up studies on η Carinae

Observations of high-energy γ-rays recently revealed a persistent source in spatial coincidence with the binary system η Carinae. Since modulation of the observed γ-ray flux on orbital time scales has not been reported so far, an unambiguous identification was hitherto not possible. In particular, the observations made by the Fermi Large Area Telescope (LAT) posed additional questions regarding the actual emission scenario. Analyses show two energetically distinct components in the γ-ray spectrum, which are best described by an exponentially cutoff power-law function (CPL) at energies below 10 GeV and a power-law (PL) component dominant at higher energies. The increased exposure in conjunction with the improved instrumental response functions of the LAT now allow us to perform a more detailed investigation of location, spectral shape, and flux time history of the observed γ-ray emission. Furthermore, we detect a weak but regular flux decrease over time. This can be understood and interpreted in a colliding-wind binary scenario for orbital modulation of the γ-ray emission. We find that the spectral shape of the γ-ray signal agrees with a single emitting particle population in combination with significant absorption by γ-γ pair production. We are able to report on the first unambiguous detection of GeV γ-ray emission from a colliding-wind massive star binary. By studying the correlation of the flux decrease with the orbital separation of the binary components allows us to predict the behaviour up to the next periastron passage in 2014.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

The γ -process nucleosynthesis in core-collapse supernovae

The γ-process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average yields of γ-process nucleosynthesis from massive stars are still insufficient to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are a factor of ten lower than the yields of the other γ-process nuclei. We investigate the γ-process in five sets of core-collapse supernova models published in the literature with initial masses of 15, 20, and 25 M ⊙ at solar metallicity. We compared the γ-process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass relative to their solar values. Further, we investigated the contribution of C–O shell mergers in the supernova progenitors as an additional site of the γ-process. Our analysis shows that a large scatter among the different models exists for both the γ-process integrated yields and the isotopic ratios. We find only ten ratios that agree with their solar values, all the others differ by at least a factor of three from the solar values in all the considered sets of models. The γ-process within C–O shell mergers mostly influences the isotopic ratios that involve intermediate and heavy proton-rich isotopes with A > 100. We conclude that there are large discrepancies both among the different data sets and between the model predictions and the solar abundance distribution. More calculations are needed; particularly updating the nuclear network, because the majority of the models considered in this work do not use the latest reaction rates for the γ-process nucleosynthesis. Moreover, the role of C–O shell mergers requires further investigation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Rotational and radial velocities of 1.3-2.2 M {sub ☉} red giants in open clusters

This study presents the rotational distribution of red giant (RG) stars in 11 old to intermediate age open clusters. The masses of these stars are all above the Kraft break, so they lose negligible amounts of their birth angular momentum (AM) during the main-sequence (MS) evolution. However, they do span a mass range with quite different AM distributions imparted during formation, with the stars less massive than ∼1.6M {sub ☉} arriving on the MS with lower rotation rates than the more massive stars. The majority of RGs in this study are slow rotators across the entire red giant branch regardless of mass, supporting the picture that intermediate-mass stars rapidly spin down when they evolve off the MS and develop convection zones capable of driving a magnetic dynamo. Nevertheless, a small fraction of RGs in open clusters show some level of enhanced rotation, and faster rotators are as common in these clusters as in the field RG population. Most of these enhanced rotators appear to be red clump stars, which is also true of the underlying stellar sample, while others are clearly RGs that are above or below the clump. In addition to rotational velocities, the radial velocities (RVs) and membership probabilities of individual stars are also presented. Cluster heliocentric RVs for NGC 6005 and Pismis 18 are reported for the first time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AN INFRARED CENSUS OF DUST IN NEARBY GALAXIES WITH SPITZER (DUSTiNGS). II. DISCOVERY OF METAL-POOR DUSTY AGB STARS

The DUSTiNGS survey (DUST in Nearby Galaxies with Spitzer) is a 3.6 and 4.5 μm imaging survey of 50 nearby dwarf galaxies designed to identify dust-producing asymptotic giant branch (AGB) stars and massive stars. Using two epochs, spaced approximately six months apart, we identify a total of 526 dusty variable AGB stars (sometimes called ''extreme'' or x-AGB stars; [3.6]-[4.5] > 0.1 mag). Of these, 111 are in galaxies with [Fe/H] < –1.5 and 12 are in galaxies with [Fe/H] < –2.0, making them the most metal-poor dust-producing AGB stars known. We compare these identifications to those in the literature and find that most are newly discovered large-amplitude variables, with the exception of ≈30 stars in NGC 185 and NGC 147, 1 star in IC 1613, and 1 star in Phoenix. The chemical abundances of the x-AGB variables are unknown, but the low metallicities suggest that they are more likely to be carbon-rich than oxygen-rich and comparisons with existing optical and near-IR photometry confirm that 70 of the x-AGB variables are confirmed or likely carbon stars. We see an increase in the pulsation amplitude with increased dust production, supporting previous studies suggesting that dust production and pulsation are linked. We find no strong evidence linking dust production with metallicity, indicating that dust can form in very metal-poor environments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AN INFRARED CENSUS OF DUST IN NEARBY GALAXIES WITH SPITZER (DUSTINGS). I. OVERVIEW

Nearby resolved dwarf galaxies provide excellent opportunities for studying the dust-producing late stages of stellar evolution over a wide range of metallicity (–2.7 ≲ [Fe/H] ≲ –1.0). Here, we describe DUSTiNGS (DUST in Nearby Galaxies with Spitzer): a 3.6 and 4.5 μm post-cryogen Spitzer Space Telescope imaging survey of 50 dwarf galaxies within 1.5 Mpc that is designed to identify dust-producing asymptotic giant branch (AGB) stars and massive stars. The survey includes 37 dwarf spheroidal, 8 dwarf irregular, and 5 transition-type galaxies. This near-complete sample allows for the building of statistics on these rare phases of stellar evolution over the full metallicity range. The photometry is >75% complete at the tip of the red giant branch for all targeted galaxies, with the exception of the crowded inner regions of IC 10, NGC 185, and NGC 147. This photometric depth ensures that the majority of the dust-producing stars, including the thermally pulsing AGB stars, are detected in each galaxy. The images map each galaxy to at least twice the half-light radius to ensure that the entire evolved star population is included and to facilitate the statistical subtraction of background and foreground contamination, which is severe at these wavelengths. In this overview, we describe the survey, the data products, and preliminary results. We show evidence for the presence of dust-producing AGB stars in eight of the targeted galaxies, with metallicities as low as [Fe/H] = –1.9, suggesting that dust production occurs even at low metallicity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Optical photometry and spectroscopy of the low-luminosity, broad-lined Ic supernova iPTF15dld

Core-collapse stripped-envelope supernova (SN) explosions reflect the diversity of physical parameters and evolutionary paths of their massive star progenitors. We have observed the Type Ic SN iPTF15dld (z = 0.047), reported by the Palomar Transient Factory. Spectra were taken starting 20 rest-frame days after maximum luminosity and are affected by a young stellar population background. Broad spectral absorption lines associated with the SN are detected over the continuum, similar to those measured for broad-lined, highly energetic SNe Ic. The light curve and maximum luminosity are instead more similar to those of low luminosity, narrow-lined Ic SNe. This suggests a behaviour whereby certain highly stripped-envelope SNe do not produce a large amount of 56 Ni, but the explosion is sufficiently energetic that a large fraction of the ejecta is accelerated to higher than usual velocities. Here, we estimate SN iPTF15dld had a main-sequence progenitor of 20–25 M ⊙ , produced a 56 Ni mass of ~0.1–0.2 M ⊙ , had an ejecta mass of [2–10] M⊙, and a kinetic energy of [1–18] × 10 51 erg.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Lifetime measurements of excited states in 15 O

The CNO cycle is the main energy source in stars more massive than our Sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. One of the largest uncertainties in the CNO chain of reactions comes from the uncertainty in the 14 N(p,γ) 15 O reaction rate. This uncertainty arises predominantly from the uncertainty in the lifetime of the subthreshold state in 15 O at E x = 6792 keV. Previous measurements of this state's lifetime are significantly discrepant. Here, we report on a new lifetime measurement of this state, as well as the excited states in 15 O at E x = 5181 keV and E x = 6172 keV, populated via the 14 N(p,γ) 15 O reaction at proton energies of E p = 1020 keV and E p = 1570 keV. Furthermore, the lifetimes have been determined with the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets with Mo, Ta, and W backing. We obtained lifetimes from the weighted average of the three measurements, allowing us to account for systematic differences between the backing materials. For the 6792 keV state, we obtained a τ = 0.6 ± 0.4 fs. To provide cross-validation of our method, we measured the known lifetimes of the states at 5181 and 6172 keV to be τ = 7.5 ± 3.0 and τ = 0.7 ± 0.5 fs, respectively, in good agreement with previous measurements.

6 ≤ A ≤ 19↗
DOE OSTI2018

Neutrinos from Choked Jets Accompanied by Type-II Supernovae

The origin of the IceCube neutrinos is still an open inquiry. Upper limits from diffuse gamma-ray observations predict that the neutrino sources are either distant or hidden from gamma-ray observations. It is possible that the neutrinos are produced in jets that are formed in core-collapsing massive stars and fail to break out, the so-called choked jets. We report neutrinos from the jets choked in the hydrogen envelopes of red supergiant stars. Fast photo-meson cooling softens the neutrino spectrum, making it hard to explain the PeV neutrinos observed by IceCube in a one-component scenario, but a two-component model can explain the spectrum. Moreover, we predict that a newly born jet-driven type-II supernova may be observed to be associated with a neutrino burst detected by IceCube.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1990

Gas flows in elliptical galaxies

In preparation for the next generation of x ray telescopes, researchers have begun a program investigating the evolving x ray properties of elliptical galaxies. Their galaxy models consist of a modified King profile for the luminous portion of the galaxy and can include an isothermal dark halo comprising 90 percent of the total mass. The stellar population is assumed to form at a rate which decreases exponentially on a dynamical time scale with a Salpeter initial mass function. Stellar mass loss occurs instantaneously as stars evolve off the main sequence. All stars more massive than 8 solar mass produce type II supernovae, while less massive stars loss mass through a planetary nebulae. The evolving rate of type I supernovae is normalized to a fraction, gamma sub sn I, of Tammann's (1974) value. All of this information is then incorporated into a one-dimensional hydrodynamics code to determine the evolving dynamical state of the interstellar medium.

David, L. P.↗