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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.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Searching for Active Low-mass Stars in the CMa Star-forming Region: Multi-band Photometry with T80S

An exotic environment surrounds the young stellar groups associated with the Canis Major (CMa) OB1/R1 region, which probably was formed under feedback from at least three supernova events having occurred a few million years ago. We use astrometric data from the Gaia-DR2 to confirm the membership of the stars in CMa R1, based on proper motion and parallax, which revealed 514 new members and candidates. The mean age of 5 Myr estimated from the color–magnitude diagram characterizes the sources as likely pre-main-sequence candidates. In total, a sample of 694 stars detected with the T80-South telescope was analyzed according to different color–color diagrams, which were compared with theoretical colors from evolutionary models, aiming to reveal the objects that exhibit color excess due to accretion processes. Accretion and magnetic activity were also explored on the basis of empirical flux–flux relation, such as F {sub 660} and F {sub 861} that are related to Hα and Ca ii triplet emission, respectively. A low fraction (∼3 percent) of the sample have Hα excess and other colors expected for stars exhibiting chromospheric activity. The number of Class I and Class II objects, identified by the infrared (Wide-field Infrared Survey Explorer) colors, indicates a disk fraction of ∼6 percent, which is lower than that expected for stellar clusters with a similar age. Such a large sample of objects associated with CMa R1 without evidences of circumstellar accretion can be interpreted as a lack of disk-bearing stars, unusual for young star-forming regions. However, this may be explained as the result of supernova events.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Quasi-periodic X-ray eruptions years after a nearby tidal disruption event

Abstract Quasi-periodic eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks 1–5 . The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs) undergoing instabilities 6–8 or interacting with a stellar object in a close orbit 9–11 . It has been suggested that this disk could be created when the SMBH disrupts a passing star 8,11 , implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs 4,12 and two observed TDEs have exhibited X-ray flares consistent with individual eruptions 13,14 . TDEs and QPEs also occur preferentially in similar galaxies 15 . However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 h from AT2019qiz, a nearby and extensively studied optically selected TDE 16 . We detect and model the X-ray, ultraviolet (UV) and optical emission from the accretion disk and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs.

Science & Technology - Other Topics↗
DOE OSTI2023

The Metal-weak Milky Way Stellar Disk Hidden in the Gaia–Sausage–Enceladus Debris: The APOGEE DR17 View

We have for the first time identified the early stellar disk in the Milky Way by using a combination of elemental abundances and kinematics. Using data from APOGEE DR17 and Gaia we select stars in the Mg–Mn–Al–Fe plane with elemental abundances indicative of an accreted origin and find stars with both halo-like and disk-like kinematics. The stars with halo-like kinematics lie along a lower sequence in [Mg/Fe], while the stars with disk-like kinematics lie along a higher sequence. Combined with astroseismic observations, we determine that the stars with halo-like kinematics are old, 9–11 Gyr, and that the more evolved stellar disk is about 1–2 Gyr younger. We show that the in situ fraction of stars on deeply bound orbits is not small, in fact the inner Galaxy likely harbors a genuine in situ population together with an accreted one. In addition, we show that the selection of the Gaia–Sausage–Enceladus in the E n -L z plane is not very robust. In fact, radically different selection criteria give almost identical elemental abundance signatures for the accreted stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Pre-main-sequence Brackett Emitters in the APOGEE DR17 Catalog: Line Strengths and Physical Properties of Accretion Columns

Very young (t ≲ 10 Myr) stars possess strong magnetic fields that channel ionized gas from the interiors of their circumstellar disks to the surface of the star. Upon impacting the stellar surface, the shocked gas recombines and emits hydrogen spectral lines. To characterize the density and temperature of the gas within these accretion streams, we measure equivalent widths of Brackett (Br) 11–20 emission lines detected in 1101 APOGEE spectra of 326 likely pre-main-sequence accretors. For sources with multiple observations, we measure median epoch-to-epoch line strength variations of 10% in Br11 and 20% in Br20. We also fit the measured line ratios to predictions of radiative transfer models by Kwan & Fischer. We find characteristic best-fit electron densities of n e = 10 11 –10 12 cm -3 , and excitation temperatures that are inversely correlated with electron density (from T ~ 5000 K for n e ~ 10 12 cm -3 to T ~ 12,500 K at n e ~ 10 11 cm -3 ). These physical parameters are in good agreement with predictions from modeling of accretion streams that account for the hydrodynamics and radiative transfer within the accretion stream. We also present a supplementary catalog of line measurements from 9733 spectra of 4255 Brackett emission-line sources in the APOGEE Data Release 17 data set.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Seeking Self-regulating Simulations of Idealized Milky Way–like Galaxies

Abstract Precipitation is potentially a mechanism through which the circumgalactic medium (CGM) can regulate a galaxy’s star formation. Here, we present idealized simulations of isolated Milky Way–like galaxies intended to examine the ability of galaxies to self-regulate their star formation, in particular via precipitation. We also examine the impact of rotation in the CGM. Using six simulations, we explore variations in the initial CGM t cool / t ff ratio and rotation profile. Those variations affect the amount of gas accretion and star formation within the galactic disk. To encourage this accretion and better study its dependence on CGM structure, we gradually increase the efficiency of stellar feedback during the first half of our simulations. Yet despite this gradual increase, the resulting outflows quickly evacuate large, hot cavities within the CGM and even beyond r 200 . Some of the CGM gas avoids interacting with the cavities and is able to feed the disk along its midplane, but the cooling of feedback-heated gas far from the midplane is too slow to supply the disk with additional gas. Our simulations illustrate the importance of physical mechanisms in the outer CGM and IGM for star formation regulation in Milky Way–scale halos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The distribution of [α/Fe] in the Milky Way disc

ABSTRACT Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $p([\alpha /{\rm Fe}]\, |\, [{\rm Fe}/{\rm H}])$ in the Milky Way disk for the α-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius R, we model p([α/Fe]) as a sum of two Gaussians, representing ‘low-α’ and ‘high-α’ populations with scale heights $z_1=0.45\, {\rm kpc}$ and $z_2=0.95\, {\rm kpc}$, respectively. By accounting for age-dependent and z-dependent selection effects in APOGEE, we infer the [α/Fe] distributions that would be found for a fair sample of long-lived stars covering all z. Near the Solar circle, this distribution is bimodal at sub-solar [Fe/H], with the low-α and high-α peaks clearly separated by a minimum at intermediate [α/Fe]. In agreement with previous results, we find that the high-α population is more prominent at smaller R, lower [Fe/H], and larger |z|, and that the sequence separation is smaller for Si and Ca than for Mg, O, and S. We find significant intrinsic scatter in [α/Fe] at fixed [Fe/H] for both the low-α and high-α populations, typically ∼0.04-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on R, [Fe/H], and α-element, provide a quantitative target for chemical evolution models and a test for hydrodynamic simulations of disk galaxy formation. We argue that explaining the observed bimodality will probably require one or more sharp transitions in the disk’s gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A giant umbrella-like stellar stream around the tidal ring galaxy NGC 922

Tidal ring galaxies (TRGs) are rarely observed in the local universe due to their intrinsically transient nature. The tidal ring structures are the result of strong interactions between gas-rich stellar disks and smaller galactic systems, and do not last longer than ~500 Myr. Therefore, these are perfect scenarios in which to look for the debris of recently accreted dwarf galactic systems. Our goal is to study the low surface brightness stellar structures around the TRG NGC 922 and to revise the hypothesis of its formation in light of these new data. We present new deep images of the TRG NGC 922 and its surroundings from the DESI Legacy survey data and from our observations with an amateur telescope. These observations are compared with results from high-resolution N-body simulations that were designed to reproduce an alternative formation scenario for this peculiar galaxy. Our new observations unveil that the low surface brightness stellar tidal structures around NGC 922 are much more complex than reported in previous works. In particular, the formerly detected tidal spike-like structure at the north-east of the central galaxy disk is not connected with the dwarf companion galaxy PGC 3080368, which has been suggested as the intruder triggering the ring formation of NGC 922. The deep images reveal that this tidal structure mainly has a fainter giant umbrella-like shape, and thus it was formed from the tidal disruption of a different satellite. Using the broad-band g, r, and z DESI LS images, we measured the photometric properties of this stellar stream, estimating a total absolute magnitude in the r band of M r = -17.0 ± 0.03 mag and a total stellar mass for the stream of between 6.9 and 8.5×10 8 M ⊙ . We performed a set of N-body simulations to reproduce the observed NGC922-intruder interaction, suggesting a new scenario for the formation of its tidal ring from the infall of a gas-rich satellite around 150 Myr ago. Finally, our deep images also reveal a tidal shell around the dwarf galaxy PGC 3080368, a possible fossil of a recent merger with a smaller satellite, which may suggest it is in its first infall towards NGC 922.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Radiation GRMHD Models of Accretion onto Stellar-mass Black Holes. I. Survey of Eddington Ratios

We summarize results from a survey of radiation-dominated black hole accretion flows across a wide range of mass accretion rates, as well as two values of black hole spin and initial magnetic field geometry. These models apply an algorithm targeting direct solutions to the radiation transport equation in full general relativity and have been enabled by access to modern exascale computing systems. Super-Eddington accretion flows form geometrically thick radiation-pressure-supported disks that drive powerful equatorial outflows. A narrow funnel-shaped photosphere in the inner region results in very low radiative efficiencies in this regime. The structure of near- and sub-Eddington accretion depends on whether there is net vertical magnetic flux at the midplane of the disk. With net flux, the disk forms a thin, dense layer at the midplane surrounded by a magnetically dominated corona, whereas without net flux, the disk remains magnetically dominated everywhere. Although none of our models achieve the magnetically arrested disk regime, those with net vertical flux and a rapidly spinning black hole still produce powerful relativistic jets. Our calculations adopt simple opacity models (with scalings appropriate to stellar-mass black hole accretion). We discuss the application of our results to observations of X-ray binaries and ultraluminous X-ray sources such as Cyg X-3 and SS433. We also speculate on the application of our super-Eddington models to the interpretation of little red dots recently discovered by JWST.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way

We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 10$^{6}$ M $_{⊙}$, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 10$^{3}$–10$^{5}$ gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Lithium in T Coronae Borealis

T Coronae Borealis is a recurrent, symbiotic nova system currently in quiescence between its periodic ≈80 yr cycle of eruptions. Observations during inter-outburst epochs provide an opportunity to study properties of the accretion disk and the M red giant. Here we present new irradiated (blackbody veiling) models, incorporating modern molecular opacities and line lists, of spectra derived from high-resolution (22,000 ≲ R ≲ 120,000) optical echelle observations obtained at two epochs, one prior to and one post the 2015 rebrightening event at similar spectroscopic system phase. We find a lithium abundance in the secondary at both epochs to be comparable. The non-irradiated (classical) model atmospheres yield a lithium abundance, A(Li) = 1.3 ± 0.1. The irradiated model (veiled) atmospheres, which are likely a better representation of the system in which the white dwarf and accretion disk illuminate the red giant, give A(Li) = 2.4 ± 0.1.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Stratified wind from a super-Eddington X-ray binary is slower than expected

Accretion disks in strong gravity ubiquitously produce winds, seen as blueshifted absorption lines in the X-ray band of both stellar mass X-ray binaries (black holes and neutron stars) and supermassive black holes. Some of the most powerful winds (termed Eddington winds) are expected to arise from systems in which radiation pressure is sufficient to unbind material from the inner disk (L ≳ L Edd ). These winds should be extremely fast and carry a large amount of kinetic power, which, when associated with supermassive black holes, would make them a prime contender for the feedback mechanism linking the growth of those black holes with their host galaxies. Here we show the XRISM Resolve spectrum of the galactic neutron star X-ray binary, GX 13+1, which reveals one of the densest winds ever seen in absorption lines. This Compton-thick wind significantly attenuates the flux, making it appear faint, although it is intrinsically more luminous than usual (L ≳ L Edd ). However, the wind is extremely slow, more consistent with the predictions of thermal-radiative winds launched by X-ray irradiation of the outer disk than with the expected Eddington wind driven by radiation pressure from the inner disk. This puts new constraints on the origin of winds from bright accretion flows in binaries, but also highlights the very different origin required for the ultrafast (v ~ 0.3c) winds seen in recent Resolve observations of a supermassive black hole at a similarly high Eddington ratio.

Audard, Marc [Univ. of Geneva, Versoix (Switzerlan↗
DOE OSTI2020

The Stellar Velocity Distribution Function in the Milky Way Galaxy

The stellar velocity distribution function in the solar vicinity is reexamined using data from the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey’s DR16 and Gaia DR2. By exploiting APOGEE’s ability to chemically discriminate with great reliability the thin-disk, thick-disk, and (accreted) halo populations, we can, for the first time, derive the three-dimensional velocity distribution functions (DFs) for these chemically separated populations. We employ this smaller but more data-rich APOGEE+Gaia sample to build a data-driven model of the local stellar population velocity DFs and use these as basis vectors for assessing the relative density proportions of these populations over the 5 < R < 12 kpc and −1.5 < z < 2.5 kpc range as derived from the larger, more complete (i.e., all-sky, magnitude-limited) Gaia database. We find that 81.9% ± 3.1% of the objects in the selected Gaia data set are thin-disk stars, 16.6% ± 3.2% are thick-disk stars, and 1.5% ± 0.1% belong to the Milky Way stellar halo. We also find the local thick-to-thin-disk density normalization to be ρ {sub T}(R {sub ⊙})/ρ {sub t}(R {sub ⊙}) = 2.1% ± 0.2%, a result consistent with, but determined in a completely different way from, typical star-count/density analyses. Using the same methodology, the local halo-to-disk-density normalization is found to be ρ {sub H}(R {sub ⊙})/(ρ {sub T}(R {sub ⊙}) + ρ {sub t}(R {sub ⊙})) = 1.2% ± 0.6%, a value that may be inflated due to the chemical overlap of halo and metal-weak thick-disk stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

How do magnetized HED flows transition to collimated plasma jets? (Final Report)

We present results from a pulsed-power experiment designed to generate accretion-like flows in the laboratory that may be used to study the dynamics of astrophysical accretion-driven outflows. To emulate systems like Young Stellar Object (YSO) outflows, we create radially converging plasma flows in a short disk that transition into much longer bipolar outflows. Our experiment uses a ~1 MA pulsed-power driver to generate plasma from an array of thin aluminum wires mounted to a 3D-printed current path that controls the angular momentum and magnetic field topology. In this paper, we show that experimental data scale favorably to observed properties of YSO jets. This set of experiments tests the simplest version of our possible plasma dynamics, where outflows are generated without rotation or axial magnetic field present. The temperature, velocity and density of the disk and outflows are characterized using interferometry, gated optical and ultraviolet imaging, and Thomson scattering diagnostics. We discuss the Reynolds number, magnetic Reynolds number and Mach numbers calculated in three different stages of the experiment, and compare our results to 3D numerical simulations in the PERSEUS magnetohydrodynamics code. This report is a summary of the work done by the graduate students who were supported by the project.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

MaNGA 8313-1901: Gas Accretion Observed in a Blue Compact Dwarf Galaxy?

Gas accretion is an important process in the evolution of galaxies, but it has limited direct observational evidences. In this paper, we report the detection of a possible ongoing gas accretion event in a blue compact dwarf (BCD) galaxy, MaNGA 8313-1901, observed by the Mapping Nearby Galaxies and Apache Point Observatory (MaNGA) program. This galaxy has a distinct off-centered blue clump to the northeast (the NE clump) that shows low metallicity and enhanced star formation. The kinematics of the gas in the NE clump also seems to be detached from the host BCD galaxy. Together with the metallicity drop of the NE clump, it suggests that the NE clump likely has an external origin, such as gas accretion or galaxy interaction, rather than an internal origin, such as an H II complex in the disk. After removing the underlying host component, we find that the spectrum of the "pure" clump can match very well with a modeled spectrum containing a stellar population of the young stars (≤7 Myr) only. This may imply that the galaxy is experiencing an accretion of cold gas, instead of a merger event involving galaxies with significant preexisting old stars. We also find signs of another clump (the SW clump) at the southwest corner of the host galaxy, and the two clumps may share the same origin of gas accretion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Understanding the Angular Momentum Evolution of T Tauri and Herbig Ae/Be Stars

We investigate a sample of six Herbig Ae/Be stars belonging to the Orion OB1 association, as well as 73 low-mass objects, members of the σ Orionis cluster, in order to explore the angular momentum evolution at early stages of evolution, and its possible connection with main-sequence Ap/Bp magnetic stars. Using FIES and HECTOCHELLE spectra, we obtain projected rotational velocities through two independent methods. Individual masses, radii, and ages are computed using evolutionary models, distance, and cluster extinction. Under the assumption that similar physical processes operate in both T Tauri and Herbig Ae/Be stars, we construct snapshots of the protostar’s rotation against mass during the first 10 Myr with the aid of a rotational model that includes a variable disk lifetime, changes in the stellar moment of inertia, a dipolar magnetic field with variable strength, and angular momentum loss through stellar winds powered by accretion. We use these snapshots, as well as the rotational data, to infer a plausible scenario for the angular momentum evolution. We find that magnetic field strengths of a few kilo-Gauss at 3 Myr are required to match the rotational velocities of both groups of stars. Models with masses between 2 M {sub ⊙} and 3 M {sub ⊙} display larger angular momentum values by a factor of ~3, in comparison to stars of similar spectral types on the main sequence. Even though some quantitative estimates on this dramatic decrease with age for Ap/Bp magnetic main-sequence stars are presented, the results obtained for the angular momentum evolution do not explain their low rotation rates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Quantifying Chemical and Kinematical Properties of Galactic Disks

We aim to quantify the chemical and kinematical properties of Galactic disks with a sample of 119,558 giant stars having abundances and 3D velocities taken or derived from the APOGEE DR17 and Gaia EDR3 catalogs. A Gaussian mixture model is employed to distinguish the high-α and low-α sequences along the metallicity by simultaneously using chemical and kinematical data. Four disk components are identified and quantified; they are named the hαmp, hαmr, lαmp, and lαmr disks and correspond to the high-α or low-α, and metal-poor or metal-rich properties. Combined with the spatial and stellar-age information, we confirm that they are well interpreted by the two-infall formation model. The first infall of turbulent gas quickly forms the hot and thick hαmp disk with consequent thinner hαmr and lαmr disks. Then the second gas accretion forms a thinner and outermost lαmp disk. We find that the inside-out and upside-down scenario does not only satisfy the overall Galactic disk formation of these two major episodes but is also presented in the formation sequence of the three inner disks. Importantly, we reveal the inverse age–[M/H] trend of the lαmr disk, which means its younger stars are more metal-poor, indicating that the rejuvenated gas from the second accretion gradually dominates later star formation. Meanwhile, the recently formed stars converge to [M/H] ~ –0.1 dex, demonstrating a sufficient mixture of gas from two infalls.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Fragmentation in Gravitationally Unstable Collapsar Disks and Subsolar Neutron Star Mergers

Abstract Although stable neutron stars (NSs) can in principle exist down to massesM ns ≈ 0.1M ⊙ , standard models of stellar core-collapse predict a robust lower limitM ns ≳ 1.2M ⊙ , roughly commensurate with the Chandrasekhar massM Ch of the progenitor’s iron core (electron fractionY e ≈ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Y e < 0.5) for which M Ch ∝ Y e 2 is reduced to ≲1M ⊙ . Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly rotating stars (“collapsars”), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form subsolar-mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNSs, their subsequent coalescence offers a channel for precipitating subsolar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ∼Hz frequency Doppler modulation of the ssNS-merger gravitational-wave signals due to the binary’s orbital motion in the disk; (2) at least one additional gravitational-wave event (coincident within ≲hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched withr-process elements from the NS merger(s) embedded within the exploding stellar envelope (“kilonovae inside a supernova”).

Astronomy & Astrophysics↗