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At least 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

A LIKELY CLOSE-IN LOW-MASS STELLAR COMPANION TO THE TRANSITIONAL DISK STAR HD 142527

With the uniquely high contrast within 0.''1 ({Delta}mag(L') = 5-6.5 mag) available using Sparse Aperture Masking with NACO at Very Large Telescope, we detected asymmetry in the flux from the Herbig Fe star HD 142527 with a barycenter emission situated at a projected separation of 88 {+-} 5 mas (12.8 {+-} 1.5 AU at 145 pc) and flux ratios in H, K, and L' of 0.016 {+-} 0.007, 0.012 {+-} 0.008, and 0.0086 {+-} 0.0011, respectively (3{sigma} errors), relative to the primary star and disk. After extensive closure-phase modeling, we interpret this detection as a close-in, low-mass stellar companion with an estimated mass of {approx}0.1-0.4 M{sub Sun }. HD 142527 has a complex disk structure, with an inner gap imaged in both the near and mid-IR as well as a spiral feature in the outer disk in the near-IR. This newly detected low-mass stellar companion may provide a critical explanation of the observed disk structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE FORMATION OF HOT JUPITERS IN STELLAR BINARIES

We study the production of hot Jupiters (HJs) in stellar binaries. We show that the 'eccentric Kozai-Lidov' (EKL) mechanism can play a key role in the dynamical evolution of a star-planet-star triple system. We run a large set of Monte Carlo simulations including the secular evolution of the orbits, general relativistic precession, and tides, and we determine the semimajor axis, eccentricity, inclination, and spin-orbit angle distributions of the HJs that are produced. We explore the effect of different tidal friction parameters on the results. We find that the efficiency of forming HJs when taking the EKL mechanism into account is higher then previously estimated. Accounting for the frequency of stellar binaries, we find that this production mechanism can account for about 30% of the observed HJ population. Current observations of spin-orbit angles are consistent with this mechanism producing {approx}30% of all HJs, and up to 100% of the misaligned systems. Based on the properties of binaries without an HJ in our simulations, we predict the existence of many Jupiter-like planets with moderately eccentric and inclined orbits and semimajor axes of several AU.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE

Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than helium are expected to freeze out onto dust grains, and the ortho:para H{sub 2} ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5 Multiplication-Sign 10{sup -6} M{sub Sun }) can be maintained by far-UV photons locally produced by the impact of galactic cosmic rays with H{sub 2} molecules. Such FUV photons irradiate the icy mantles, liberating water vapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EVOLUTION OF THE HIGH-MASS END OF THE STELLAR INITIAL MASS FUNCTIONS IN STARBURST GALAXIES

We investigate the time evolution and spatial variation of the stellar initial mass function (IMF) in star-forming disk galaxies by using chemodynamical simulations with an IMF model depending both on local densities and metallicities ([Fe/H]) of the interstellar medium (ISM). We find that the slope ({alpha}) of a power-law IMF (N(m){proportional_to}m {sup -{alpha}}) for stellar masses larger than 1 M{sub Sun} evolves from the canonical Salpeter IMF ({alpha} Almost-Equal-To 2.35) to be moderately top-heavy one ({alpha} Almost-Equal-To 1.9) in the simulated disk galaxies with starbursts triggered by galaxy interaction. We also find that {alpha} in star-forming regions correlates with star formation rate densities ({Sigma}{sub SFR} in units of M{sub Sun} yr{sup -1} kpc{sup -2}). Feedback effects of Type Ia and II supernovae are found to prevent IMFs from being too top-heavy ({alpha} < 1.5). The simulation predicts {alpha} Almost-Equal-To 0.23log {Sigma}{sub SFR} + 1.7 for log {Sigma}{sub SFR} {>=} -2 (i.e., more top-heavy in higher {Sigma}{sub SFR}), which is reasonably consistent with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller {alpha} and thus are more top-heavy (d{alpha}/dR {approx} 0.07 kpc{sup -1} for R {<=} 5 kpc). The predicted radial {alpha} gradient can be tested against future observational studies of the {alpha} variation in star-forming galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ASTEROSEISMIC CLASSIFICATION OF STELLAR POPULATIONS AMONG 13,000 RED GIANTS OBSERVED BY KEPLER

Of the more than 150,000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves were made public in late 2011. More than 13,000 (over 85%) of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large-scale asteroseismic investigations. We automatically extracted individual frequencies and measured the period spacings of the dipole modes in nearly every red giant. These measurements naturally classify the stars into various populations, such as the red giant branch, the low-mass (M/M{sub Sun} {approx}< 1.8) helium-core-burning red clump, and the higher-mass (M/M{sub Sun} {approx}> 1.8) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars will undoubtedly provide an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

NITROGEN ISOTOPES IN ASYMPTOTIC GIANT BRANCH CARBON STARS AND PRESOLAR SiC GRAINS: A CHALLENGE FOR STELLAR NUCLEOSYNTHESIS

Isotopic ratios of C, N, Si, and trace heavy elements in presolar SiC grains from meteorites provide crucial constraints to nucleosynthesis. A long-debated issue is the origin of the so-called A+B grains, as of yet no stellar progenitor thus far has been clearly identified on observational grounds. We report the first spectroscopic measurements of {sup 14}N/{sup 15}N ratios in Galactic carbon stars of different spectral types and show that J- and some SC-type stars might produce A+B grains, even for {sup 15}N enrichments previously attributed to novae. We also show that most mainstream grains are compatible with the composition of N-type stars, but in some cases might also descend from SC stars. From a theoretical point of view, no astrophysical scenario can explain the C and N isotopic ratios of SC-, J-, and N-type carbon stars together, as well as those of many grains produced by them. This poses urgent questions to stellar physics.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

DIRECT DIAGNOSTICS OF FORMING MASSIVE STARS: STELLAR PULSATION AND PERIODIC VARIABILITY OF MASER SOURCES

The 6.7 GHz methanol maser emission, a tracer of forming massive stars, sometimes shows enigmatic periodic flux variations over several 10-100 days. In this Letter, we propose that these periodic variations could be explained by the pulsation of massive protostars growing under rapid mass accretion with rates of M-dot{sub *}{approx}>10{sup -3} M{sub Sun} yr{sup -1}. Our stellar evolution calculations predict that the massive protostars have very large radii exceeding 100 R{sub Sun} at maximum, and here we study the pulsational stability of such bloated protostars by way of the linear stability analysis. We show that the protostar becomes pulsationally unstable with various periods of several 10-100 days depending on different accretion rates. With the fact that the stellar luminosity when the star is pulsationally unstable also depends on the accretion rate, we derive the period-luminosity relation log (L/ L{sub Sun }) = 4.62 + 0.98log (P/100 days), which is testable with future observations. Our models further show that the radius and mass of the pulsating massive protostar should also depend on the period. It would be possible to infer such protostellar properties and the accretion rate with the observed period. Measuring the maser periods enables a direct diagnosis of the structure of accreting massive protostars, which are deeply embedded in dense gas and are inaccessible with other observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

STELLAR ROTATION PERIODS OF THE KEPLER OBJECTS OF INTEREST: A DEARTH OF CLOSE-IN PLANETS AROUND FAST ROTATORS

We present a large sample of stellar rotation periods for Kepler Objects of Interest, based on three years of public Kepler data. These were measured by detecting periodic photometric modulation caused by star spots, using an algorithm based on the autocorrelation function of the light curve, developed recently by McQuillan, Aigrain and Mazeh (2013). Of the 1919 main-sequence exoplanet hosts analyzed, robust rotation periods were detected for 737. Comparing the detected stellar periods to the orbital periods of the innermost planet in each system reveals a notable lack of close-in planets around rapid rotators. It appears that only slowly spinning stars with rotation periods longer than 5-10 days host planets on orbits shorter than 2 or 3 days, although the mechanism(s) that lead(s) to this is not clear.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EXPONENTIAL GALAXY DISKS FROM STELLAR SCATTERING

Stellar scattering off of orbiting or transient clumps is shown to lead to the formation of exponential profiles in both surface density and velocity dispersion in a two-dimensional non-self gravitating stellar disk with a fixed halo potential. The exponential forms for both nearly flat rotation curves and near-solid-body rotation curves. The exponential does not depend on initial conditions, spiral arms, bars, viscosity, star formation, or strong shear. After a rapid initial development, the exponential saturates to an approximately fixed scale length. The inner exponential in a two-component profile has a break radius comparable to the initial disk radius; the outer exponential is primarily scattered stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE INNER DYNAMICAL MASS ACROSS GALAXY MORPHOLOGY: A WEAK SCALING WITH TOTAL STELLAR MASS

We compile a sample of dispersion-supported and rotation-supported galaxies for which the dynamical mass enclosed at 500 pc can be measured. We find that this dynamical quantity increases only slowly with stellar mass (M{sub total}(<500 pc)∝M{sub stars}{sup 0.16}) over ∼9 decades in baryonic mass and ∼3 in length scale, with a sudden upturn at the highest masses (M {sub stars} ≳ 10{sup 10} M {sub ☉}). This upturn occurs for the earliest types (S0 and Sa) of disk galaxies, and is consistent with the ''extra'' mass within 500 pc being predominantly stellar. This sudden change may be indicative of different bulge formation mechanisms between early- and late-type disks. We also compare the data to the results of recent galaxy formation simulations. These broadly track the observed trend, with some tendency to put too much mass in the inner 500 pc, depending on the details of each simulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

INTERRUPTED STELLAR ENCOUNTERS IN STAR CLUSTERS

Strong encounters between single stars and binaries play a pivotal role in the evolution of star clusters. Such encounters can also dramatically modify the orbital parameters of binaries, exchange partners in and out of binaries, and are a primary contributor to the rate of physical stellar collisions in star clusters. Often, these encounters are studied under the approximation that they happen quickly enough and within a small enough volume to be considered isolated from the rest of the cluster. In this paper, we study the validity of this assumption through the analysis of a large grid of single–binary and binary–binary scattering experiments. For each encounter we evaluate the encounter duration, and compare this with the expected time until another single or binary star will join the encounter. We find that for lower-mass clusters, similar to typical open clusters in our Galaxy, the percent of encounters that will be “interrupted” by an interloping star or binary may be 20%–40% (or higher) in the core, though for typical globular clusters we expect ≲1% of encounters to be interrupted. Thus, the assumption that strong encounters occur in relative isolation breaks down for certain clusters. Instead, many strong encounters develop into more complex “mini-clusters,” which must be accounted for in studying, for example, the internal dynamics of star clusters, and the physical stellar collision rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Supernova host galaxies in the dark energy survey: I. Deep coadds, photometry, and stellar masses

ABSTRACT The 5-yr Dark Energy Survey Supernova Programme (DES-SN) is one of the largest and deepest transient surveys to date in terms of volume and number of supernovae. Identifying and characterizing the host galaxies of transients plays a key role in their classification, the study of their formation mechanisms, and the cosmological analyses. To derive accurate host galaxy properties, we create depth-optimized coadds using single-epoch DES-SN images that are selected based on sky and atmospheric conditions. For each of the five DES-SN seasons, a separate coadd is made from the other four seasons such that each SN has a corresponding deep coadd with no contaminating SN emission. The coadds reach limiting magnitudes of order ∼27 in g band, and have a much smaller magnitude uncertainty than the previous DES-SN host templates, particularly for faint objects. We present the resulting multiband photometry of host galaxies for samples of spectroscopically confirmed type Ia (SNe Ia), core-collapse (CCSNe), and superluminous (SLSNe) as well as rapidly evolving transients (RETs) discovered by DES-SN. We derive host galaxy stellar masses and probabilistically compare stellar-mass distributions to samples from other surveys. We find that the DES spectroscopically confirmed sample of SNe Ia selects preferentially fewer high-mass hosts at high-redshift compared to other surveys, while at low redshift the distributions are consistent. DES CCSNe and SLSNe hosts are similar to other samples, while RET hosts are unlike the hosts of any other transients, although these differences have not been disentangled from selection effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Galaxy properties as revealed by MaNGA – III. Kinematic profiles and stellar population gradients in S0s

ABSTRACT This is the third paper of a series where we study the stellar population gradients (SP; ages, metallicities, α-element abundance ratios, and stellar initial mass functions) of early-type galaxies (ETGs) at $z$ ≤ 0.08 from the Mapping Nearby Galaxies at APO Data Release 15 (MaNGA-DR15) survey. In this work, we focus on the S0 population and quantify how the SP varies across the population as well as with galactocentric distance. We do this by measuring Lick indices and comparing them to SP synthesis models. This requires spectra with high signal-to-noise ratio which we achieve by stacking in bins of luminosity (Lr) and central velocity dispersion (σ0). We find that: (1) there is a bimodality in the S0 population: S0s more massive than $3\times 10^{10}\, \mathrm{M}_\odot$ show stronger velocity dispersion and age gradients (age and σr decrease outwards) but little or no metallicity gradient, while the less massive ones present relatively flat age and velocity dispersion profiles, but a significant metallicity gradient (i.e. [M/H] decreases outwards). Above $2\times 10^{11}\, \mathrm{M}_\odot$, the number of S0s drops sharply. These two mass scales are also where global scaling relations of ETGs change slope. (2) S0s have steeper velocity dispersion profiles than fast-rotating elliptical galaxies (E-FRs) of the same luminosity and velocity dispersion. The kinematic profiles and SP gradients of E-FRs are both more similar to those of slow-rotating ellipticals (E-SRs) than to S0s, suggesting that E-FRs are not simply S0s viewed face-on. (3) At fixed σ0, more luminous S0s and E-FRs are younger, more metal rich and less α-enhanced. Evidently for these galaxies, the usual statement that ‘massive galaxies are older’ is not true if σ0 is held fixed.

Domínguez Sánchez, H.↗
DOE OSTI2020

Stellar mass as a galaxy cluster mass proxy: application to the Dark Energy Survey redMaPPer clusters

Abstract We introduce a galaxy cluster mass observable, μ⋆, based on the stellar masses of cluster members, and we present results for the Dark Energy Survey (DES) Year 1 (Y1) observations. Stellar masses are computed using a Bayesian model averaging method, and are validated for DES data using simulations and COSMOS data. We show that μ⋆ works as a promising mass proxy by comparing our predictions to X-ray measurements. We measure the X-ray temperature–μ⋆ relation for a total of 129 clusters matched between the wide-field DES Y1 redMaPPer catalogue and Chandra and XMM archival observations, spanning the redshift range 0.1 < $z$ < 0.7. For a scaling relation that is linear in logarithmic space, we find a slope of α = 0.488 ± 0.043 and a scatter in the X-ray temperature at fixed μ⋆ of $\sigma _{{\rm ln} T_\mathrm{ X}|\mu _\star }= 0.266^{+0.019}_{-0.020}$ for the joint sample. By using the halo mass scaling relations of the X-ray temperature from the Weighing the Giants program, we further derive the μ⋆-conditioned scatter in mass, finding $\sigma _{{\rm ln} M|\mu _\star }= 0.26^{+ 0.15}_{- 0.10}$. These results are competitive with well-established cluster mass proxies used for cosmological analyses, showing that μ⋆ can be used as a reliable and physically motivated mass proxy to derive cosmological constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Prospects for Galactic and stellar astrophysics with asteroseismology of giant stars in the TESS continuous viewing zones and beyond

ABSTRACT The NASA Transiting Exoplanet Survey Satellite (NASA-TESS) mission presents a treasure trove for understanding the stars it observes and the Milky Way, in which they reside. We present a first look at the prospects for Galactic and stellar astrophysics by performing initial asteroseismic analyses of bright (G < 11) red giant stars in the TESS southern continuous viewing zone (SCVZ). Using three independent pipelines, we detect νmax and Δν in 41 per cent of the 15 405 star parent sample (6388 stars), with consistency at a level of $\sim \! 2{{\ \rm per\ cent}}$ in νmax and $\sim \! 5{{\ \rm per\ cent}}$ in Δν. Based on this, we predict that seismology will be attainable for ∼3 × 105 giants across the whole sky and at least 104 giants with ≥1 yr of observations in the TESS-CVZs, subject to improvements in analysis and data reduction techniques. The best quality TESS-CVZ data, for 5574 stars where pipelines returned consistent results, provide high-quality power spectra across a number of stellar evolutionary states. This makes possible studies of, for example, the asymptotic giant branch bump. Furthermore, we demonstrate that mixed ℓ = 1 modes and rotational splitting are cleanly observed in the 1-yr data set. By combining TESS-CVZ data with TESS-HERMES, SkyMapper, APOGEE, and Gaia, we demonstrate its strong potential for Galactic archaeology studies, providing good age precision and accuracy that reproduces well the age of high [α/Fe] stars and relationships between mass and kinematics from previous studies based on e.g. Kepler. Better quality astrometry and simpler target selection than the Kepler sample makes this data ideal for studies of the local star formation history and evolution of the Galactic disc. These results provide a strong case for detailed spectroscopic follow-up in the CVZs to complement that which has been (or will be) collected by current surveys.

Mackereth, J. Ted↗
DOE OSTI2021

A machine learning approach to galaxy properties: joint redshift–stellar mass probability distributions with Random Forest

We demonstrate that highly accurate joint redshift–stellar mass probability distribution functions (PDFs) can be obtained using the Random Forest (RF) machine learning (ML) algorithm, even with few photometric bands available. As an example, we use the Dark Energy Survey (DES), combined with the COSMOS2015 catalogue for redshifts and stellar masses. We build two ML models: one containing deep photometry in the griz bands, and the second reflecting the photometric scatter present in the main DES survey, with carefully constructed representative training data in each case. We validate our joint PDFs for 10 699 test galaxies by utilizing the copula probability integral transform and the Kendall distribution function, and their univariate counterparts to validate the marginals. Benchmarked against a basic set-up of the template-fitting code bagpipes, our ML-based method outperforms template fitting on all of our predefined performance metrics. In addition to accuracy, the RF is extremely fast, able to compute joint PDFs for a million galaxies in just under 6 min with consumer computer hardware. Such speed enables PDFs to be derived in real time within analysis codes, solving potential storage issues. As part of this work we have developed galpro 1, a highly intuitive and efficient python package to rapidly generate multivariate PDFs on-the-fly. galpro is documented and available for researchers to use in their cosmology and galaxy evolution studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Deeper, Wider, Faster programme: exploring stellar flare activity with deep, fast cadenced DECam imaging via machine learning

ABSTRACT We present our 500 pc distance-limited study of stellar flares using the Dark Energy Camera as part of the Deeper, Wider, Faster programme. The data were collected via continuous 20-s cadence g-band imaging and we identify 19 914 sources with precise distances from Gaia DR2 within 12, ∼3 deg2, fields over a range of Galactic latitudes. An average of ∼74 min is spent on each field per visit. All light curves were accessed through a novel unsupervised machine learning techniques designed for anomaly detection. We identify 96 flare events occurring across 80 stars, the majority of which are M dwarfs. Integrated flare energies range from ∼1031–1037 erg, with a proportional relationship existing between increased flare energy with increased distance from the Galactic plane, representative of stellar age leading to declining yet more energetic flare events. In agreement with previous studies we observe an increase in flaring fraction from M0 to M6 spectral types. Furthermore, we find a decrease in the flaring fraction of stars as vertical distance from the galactic plane is increased, with a steep decline present around ∼100 pc. We find that $\sim 70{{\ \rm per\ cent}}$ of identified flares occur on short time-scales of <8 min. Finally, we present our associated flare rates, finding a volumetric rate of 2.9 ± 0.3 × 10−6 flares pc−3 h−1.

Webb, S.↗
DOE OSTI2021

Close substellar-mass companions in stellar wide binaries: discovery and characterization with APOGEE and Gaia DR2

We present a search for close, unresolved companions in a subset of spatially resolved Gaia wide binaries containing main-sequence stars within 200 pc of the Sun, utilizing the APOGEE–Gaia Wide Binary Catalog. A catalogue of 37 wide binaries was created by selecting pairs of stars with nearly identical Gaia positions, parallaxes, and proper motions, and then confirming candidates to be gravitationally bound pairs using APOGEE radial velocities. We identify close, unresolved stellar and substellar candidate companions in these multiple systems using (1) the Gaia binary main sequence and (2) observed periodic radial velocity variations in APOGEE measurements due to the influence of a close substellar-mass companion. The studied wide binary pairs reveal a total of four stellar-mass close companions in four different wide binaries, and four substellar-mass close companion candidates in two wide binaries. The latter are therefore quadruple systems, with one substellar-mass companion orbiting each wide binary component in an S-type orbit. Taken at face value, these candidate systems represent an enhancement of an order of magnitude over the expected occurrence rate of ~2 percent of stars having substellar companions >2 M Jup within ~100-d orbits; we discuss implications and possible explanations for this result. Finally, we compare chemical differences between the components of the wide binaries and the components of the candidate higher-order systems and find that any chemical influence or correlation due to the presence of close companions to wide binary stars is not discernible.

79 ASTRONOMY AND ASTROPHYSICS↗