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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Identifications of RR Lyrae Stars and Quasars from the Simulated Data of Mephisto-W Survey

We have investigated the feasibilities and accuracies of the identifications of RR Lyrae stars and quasars from the simulated data of the Multi-channel Photometric Survey Telescope (Mephisto) W Survey. Based on the variable sources light curve libraries from the Sloan Digital Sky Survey (SDSS) Stripe 82 data and the observation history simulation from the Mephisto-W Survey Scheduler, we have simulated the uvgriz multi-band light curves of RR Lyrae stars, quasars and other variable sources for the first-year observation of Mephisto W Survey. We have applied the ensemble machine learning algorithm Random Forest Classifier (RFC) to identify RR Lyrae stars and quasars, respectively. We build training and test samples and extract ~150 features from the simulated light curves and train two RFCs respectively for the RR Lyrae star and quasar classification. We find that, our RFCs are able to select the RR Lyrae stars and quasars with remarkably high precision and completeness, with purity = 95.4% and completeness = 96.9% for the RR Lyrae RFC and purity = 91.4% and completeness = 90.2% for the quasar RFC. In conclusion, we have also derived relative importances of the extracted features utilized to classify RR Lyrae stars and quasars.

(galaxies:) quasars: general↗
DOE OSTI2022

Trumpler 16-26: a new centrifugal magnetosphere star discovered via SDSS/APOGEE H -band spectroscopy

ABSTRACT We report the discovery of a new example of the rare class of highly magnetized, rapidly rotating, helium enhanced, early B stars that produce anomalously wide hydrogen emission due to a centrifugal magnetosphere (CM). The star is Trumpler 16-26, a B1.5 V member of the Trumpler 16 open cluster. A CM was initially suspected based on hydrogen Brackett series emission observed in SDSS/APOGEE H-band spectra. Similar to the other stars of this type, the emission was highly variable and at all times remarkable due to the extreme velocity separations of the double peaks (up to 1300 km s−1.) Another clue lay in the TESS light curve, which shows two irregular eclipses per cycle when phased with the likely 0.971 8115-d rotation period, similar to the behaviour of the well-known CM host star σ Ori E. To confirm a strong magnetic field and rotation-phase-locked variability, we initiated a follow-up campaign consisting of optical spectropolarimetry and spectroscopy. The associated data revealed a longitudinal magnetic field varying between −3.1 and +1.6 kG with the period found from photometry. The optical spectra confirmed rapid rotation (v sin i = 195 km s−1), surface helium enhancement, and wide, variable hydrogen emission. Tr16-26 is thus confirmed as the 20th known, the fourth most rapidly rotating, and the faintest CM host star yet discovered. With a projected dipole magnetic field strength of Bd > 11 kG, Tr16-26 is also among the most magnetic CM stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Star formation efficiency and AGN feedback in narrow-line Seyfert 1 galaxies with fast X-ray nuclear winds

ABSTRACT We present the first systematic study of the molecular gas and star formation efficiency in a sample of ten narrow-line Seyfert 1 galaxies selected to have X-ray Ultra Fast Outflows and, therefore, to potentially show AGN feedback effects. CO observations were obtained with the IRAM 30-m telescope in six galaxies and from the literature for four galaxies. We derived the stellar mass, star formation rate, AGN, and FIR dust luminosities by fitting the multi-band spectral energy distributions with the CIGALE code. Most of the galaxies in our sample lie above the main sequence (MS), and the molecular depletion time is one to two orders of magnitude shorter than the one typically measured in local star-forming galaxies. Moreover, we found a promising correlation between the star formation efficiency and the Eddington ratio, as well as a tentative correlation with the AGN luminosity. The role played by the AGN activity in the regulation of star formation within the host galaxies of our sample remains uncertain (little or no effect? positive feedback?). Nevertheless, we can conclude that quenching by the AGN activity is minor and that star formation will likely stop in a short time due to gas exhaustion by the current starburst episode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Distinguishing prompt-collapse binary neutron star mergers from binary black Holes: Tidal effects and remnant properties

We study the properties of remnants formed in prompt-collapse binary neutron star mergers. We consider nonspinning neutron star binaries over a range of total masses and mass ratios across a set of 22 equations of state, totaling 107 numerical relativity simulations. We report the final mass and spin of the systems (including the accretion disk and ejecta) to be constrained in a narrow range—0.98 ≲ 𝑀 𝑓 /𝑀 ≲ 0.99 for the mass and 0.85 ≲ 𝑎 𝑓 ≲ 0.95 for the dimensionless spin—regardless of the binary configuration and matter effects. This sets them apart from binary black hole merger remnants. We assess the detectability of the postmerger signal in a future 40 km Cosmic Explorer observatory and find that the signal-to-noise ratio in the postmerger of an optimally located and oriented binary at a distance of 100 Mpc can range from <1 to 8, depending on the binary configuration and equation of state, with a majority of them greater than 4 in the set of simulations that we consider. We also consider the distinguishability between prompt-collapse binary neutron star and binary black hole mergers with the same masses and spins. We find that Cosmic Explorer will be able to distinguish such systems primarily via the measurement of tidal effects in the late inspiral. Neutron star binaries with reduced tidal deformability $\tilde{Λ}$ as small as ∼ 3.5 can be identified up to a distance of 100 Mpc, while neutron star binaries with $\tilde{Λ}$ ∼ 22 can be identified to distances greater than 250 Mpc. This is larger than the distance up to which the postmerger will be visible. Finally, we discuss the possible implications of our findings for the equation of state of neutron stars from the gravitational wave event GW230529.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Assessing the impact of uniform rotation on the structure of neutron stars

Driven by recent laboratory experiments and astronomical observations, significant advances have deepened our understanding of neutron-star physics. NICER's Pulse Profile Modeling has refined our knowledge of neutron star masses and radii, while gravitational-wave detections have revealed key insights into the structure of neutron stars. Particularly relevant is the extraction of the tidal deformability by the LIGO-Virgo collaboration and the most recent determination of stellar radii by NICER, both suggesting a relatively soft equation of state (EOS) at intermediate densities. Additionally, measurements from the PREX collaboration and from pulsar timing suggest instead that the EOS is stiff in the vicinity of saturation density and at the highest densities accessible to date. But how stiff can the EOS be at these very high densities? Recent events featuring compact objects near the “lower mass gap” have raised questions about the existence of very massive neutron stars. Motivated by this finding and in light of new refinements to theoretical models, we explore the possibility that these massive objects may indeed be rapidly rotating neutron stars. Here, we explore how rotation affects both the maximum neutron star mass and their associated radii, and discuss the implications they may have on the equation of state.

Equations of state of nuclear matter↗
DOE OSTI2020

Total energy in supernova neutrinos and the tidal deformability and binding energy of neutron stars

The energy radiated in supernova neutrinos is a fundamental quantity that is closely related to the gravitational binding energy of a neutron star. Recently the tidal deformability of neutron stars was constrained by gravitational wave observations. By considering several equations of state, we find a strong correlation between the tidal deformability and neutron star binding energy. We use this correlation to sharpen predictions of the binding energy of neutron stars and the total neutrino energy in supernovae. We find a minimum binding energy for a neutron star formed in a supernova of ~ 1.5 × 10 53 ergs . Should the neutrino energy in a supernova be significantly below this value, it would strongly suggest new unobserved particles are carrying away some of the supernova energy. Alternatively, if the neutrino energy is observed above ~ 6 × 10 53 ergs , it would strongly imply the formation of a (perhaps surprisingly) massive neutron star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Filling the black hole mass gap: Avoiding pair instability in massive stars through addition of nonnuclear energy

In standard stellar evolution, stars with masses ranging from approximately 150 to 240M ⊙ are expected to evolve to a pair instability supernova with no black hole (BH) remnant. This evolutionary behavior leads to a predicted gap in the black hole mass function from approximately 50 to 140M ⊙ . Yet the LIGO and Virgo Collaborations recently discovered black holes of masses 66M ⊙ and 85M ⊙ in the gravitational wave event GW190521. Here, we propose a new method to populate the BH mass gap. If an energy source is added throughout the star in addition to nuclear fusion, it is possible for the altered evolution to avoid the complete destruction of a pair instability supernova, and instead a BH remnant is left behind. An example of an extra energy source is dark matter annihilation within the star, but our results hold more generally. We show this phenomenon by exploring the effect of adding an energy source independent of temperature and density to a 180M ⊙ star, using the MESA one-dimensional stellar evolution software. If ~50% of the star’s energy is due to this new source, the star is capable of avoiding the pair instability entirely and evolving towards a core-collapse supernova and ultimately a BH remnant with mass ~ 120M ⊙ .

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Voltage-Current Behavior of a Superconducting STAR ® Wire in a 6-Around-1 Cable Configuration

A 6-around-1 transposed cable using superconducting STAR® wires can be useful for future circular collider applications. We made three cable samples using single STAR® wires with a diameter of 1.3 mm. The first two samples, made with a cabling machine, used STAR® wires consisting of a 0.7 mm diameter Nb-Ti core. The third cable sample was manually wound and used a STAR® wire made with a 0.81 mm diameter Cu core. The first sample showed severe degradation after the cable was bent to a 75 mm radius. The current-carrying capability of the innermost and outermost REBCO tapes in the STAR® wire degraded by 42% to 98% and the middle REBCO tapes remained intact. This was also the case for the second and straight cable sample. After fabrication of the third cable sample, we observed only about 5% reduction in the current along the wire, measured at different locations inside the terminations. Finally, the results indicate that the differences in architecture or fabrication of the STAR® wires could have caused differences in critical current retention after cabling.

6-around-1 cable↗
DOE OSTI2022

Star Formation Rates for Elliptical Galaxies Derived from the Wise 12- and 22-µm Emission

We investigated the star formation rates (SFRs) of selected elliptical galaxies using the infrared data from the Wide-field Infrared Survey Explorer (WISE). The elliptical galaxies were selected from the Sloan Digital Sky Survey (SDSS) MPA-JHU catalog and morphology classified by Galaxy Zoo catalog. We used the 3.4-µm emission (W1) of WISE to represent the stellar mass of the galaxies and the 12- and 22-µm emission (W3 and W4) of WISE to represent the star formation activity of the galaxies. The W3- and W4-based star formation rates are usually overestimated due to the star emission in low star-forming galaxies. Stellar continuum emission could contribute to MIR fluxes of elliptical galaxies with an average overestimation of ~ 13.95 and ~ 0.77% from the observed fluxes of WISE W3 and W4-bands, which determined by using the estimated WISE W3 and W4 flux correction factors. Better SFRs can be obtained by subtracting the stellar contribution of the sources. Our result highlights the possibility of the existence of unknown mechanisms triggering the continuous star formation activities in elliptical galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

A Random Forest Approach to Identifying Young Stellar Object Candidates in the Lupus Star-forming Region

The identification and characterization of stellar members within a star-forming region are critical to many aspects of star formation, including formalization of the initial mass function, circumstellar disk evolution, and star formation history. Previous surveys of the Lupus star-forming region have identified members through infrared excess and accretion signatures. We use machine learning to identify new candidate members of Lupus based on surveys from two space-based observatories: ESA’s Gaia and NASA’s Spitzer. Astrometric measurements from Gaia's Data Release 2 and astrometric and photometric data from the Infrared Array Camera on the Spitzer Space Telescope, as well as from other surveys, are compiled into a catalog for the random forest (RF) classifier. The RF classifiers are tested to find the best features, membership list, non-membership identification scheme, imputation method, training set class weighting, and method of dealing with class imbalance within the data. We list 27 candidate members of the Lupus star-forming region for spectroscopic follow-up. Most of the candidates lie in Clouds V and VI, where only one confirmed member of Lupus was previously known. These clouds likely represent a slightly older population of star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Forward Modeling the Orbits of Companions to Pulsating Stars from Their Light Travel Time Variations

Mutual gravitation between a pulsating star and an orbital companion leads to a time-dependent variation in path length for starlight traveling to Earth. These variations can be used for coherently pulsating stars, such as the δ Scuti variables, to constrain the masses and orbits of their companions. Observing these variations for δ Scuti stars has previously relied on subdividing the light curve and measuring the average pulsation phase in equally sized subdivisions, which leads to undersampling near periapsis. We introduce a new approach that simultaneously forward models each sample in the light curve and show that this method improves upon current sensitivity limits—especially in the case of highly eccentric and short-period binaries. We find that this approach is sensitive enough to observe Jupiter mass planets around δ Scuti stars under ideal conditions, and use gravity-mode pulsations in the subdwarf B star KIC 7668647 to detect its companion without radial velocity data. We further provide robust detection limits as a function of the signal-to-noise ratio of the pulsation mode and determine that the minimum detectable light travel time amplitude for a typical Kepler δ Scuti is around 2 s. This new method significantly enhances the application of light travel time variations to detecting short-period binaries with pulsating components, and pulsating A-type exoplanet host stars, especially as a tool for eliminating false positives.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Estimating the Ages of FGK Dwarf Stars through the Use of GALEX FUV Magnitudes

Stellar age cannot be directly measured, yet age determinations are fundamental to understanding the evolution of stars, planets, and galaxies. The work presented here builds upon the idea of a stellar-activity age. We utilized far-ultraviolet (FUV) photometry acquired by the Galaxy Evolution Explorer (GALEX) space telescope as an indicator of chromospheric activity to infer ages of late-F, G, and K type dwarf stars. We derived a purely empirical correlation between FUV magnitudes and stellar age in conjunction with (B − V) color. Our attention is restricted to Sun-like stars with color range 0.55⩽(B−V)⩽0.71 and absolute magnitude range 4.3 ≤ M {sub V} ≤ 5.3. The correlation is defined in terms of a FUV-excess parameter Q(FUV−B,B−V). We related stellar age, τ, to Q through the relation log{sub e}(τ)=log{sub e}(a)+bQ, where a and b are fit parameters and functions of (B − V). This correlation is functional up to 6 Gyr for FGK dwarfs. With such a correlation, one only needs Johnson (B − V) and FUV measurements to estimate the stellar age for Population i dwarf stars of solar-like temperature and metallicity. Such a calibration has utility in population studies of FGK dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application, we have constructed activity and FUV–age distributions for a sample of thin and thick disk stars, as distinguished by their chemical abundances. Considerable overlap is found between the activity distribution and age range of the two populations. We discuss the possibility that some high-[α/Fe] thick disk stars were formed as a result of the accretion of dwarf galaxies as recently as 4 Gyr ago.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

BD+14°3061: A Luminous Yellow Post-AGB Star in the Galactic Halo

I report the discovery that the ninth-mag Galactic-halo star BD+14°3061 is a member of the rare class of luminous metal-poor “yellow post-AGB” stars. Its Gaia DR2 parallax implies an absolute magnitude of M V = -3.44 ± 0.27, and it is a very high-velocity star moving in a retrograde Galactic orbit. BD+14°3061 is a field analog of the half-dozen yellow PAGB stars known in Galactic globular clusters, which have closely similar absolute magnitudes. These objects are the visually brightest members of old stellar populations; their apparently narrow luminosity function makes them potentially useful as Population II standard candles. The spectral-energy distribution of BD+14°3061 out to 22 μm shows no evidence for circumstellar dust. The star is a low-amplitude semiregular pulsating variable, with typical periods of 30–32 days. A radial-velocity study suggests that it is a spectroscopic binary with a period of 429.6 days, making it similar to known binary yellow PAGB stars such as HD 46703 and BD+39°4926.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Measuring Young Stars in Space and Time. I. The Photometric Catalog and Extinction Properties of N44

In order to better understand the role of high-mass stellar feedback in regulating star formation in giant molecular clouds, we carried out a Hubble Space Telescope (HST) Treasury Program Measuring Young Stars in Space and Time (MYSST) targeting the star-forming complex N44 in the Large Magellanic Cloud (LMC). Using the F555W and F814W broadband filters of both the ACS and WFC3/UVIS, we built a photometric catalog of 461,684 stars down to m {sub F555W} ≃ 29 mag and m {sub F814W} ≃ 28 mag, corresponding to the magnitude of an unreddened 1 Myr pre-main-sequence star of ≈ 0.09 M {sub ☉} at the LMC distance. In this first paper we describe the observing strategy of MYSST and the data reduction procedure and present the photometric catalog. We identify multiple young stellar populations tracing the gaseous rim of N44's superbubble, together with various contaminants belonging to the LMC field population. We also determine the reddening properties from the slope of the elongated red clump (RC) feature by applying the machine-learning algorithm RANSAC, and we select a set of upper-main-sequence stars as primary probes to build an extinction map, deriving a relatively modest median extinction A {sub F555W} ≃ 0.77 mag. The same procedure applied to the RC provides A {sub F555W} ≃ 0.68 mag.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Large Adaptive Optics Survey for Substellar Objects around Young, Nearby, Low-mass Stars with Robo-AO

We present results from the Large Adaptive optics Survey for Substellar Objects, where the goal is to directly image new substellar companions (<70 M {sub Jup}) at wide orbital separations (≳50 au) around young (≲300 Myr), nearby (<100 pc), low-mass (≈0.1–0.8 M{sub ⊙}) stars. We report on 427 young stars imaged in the visible (i') and near-infrared (J or H ) simultaneously with Robo-AO on the Kitt Peak 2.1 m telescope and later the Maunakea University of Hawaii 2.2 m telescope. To undertake the observations, we commissioned a new infrared camera for Robo-AO that uses a low-noise high-speed SAPHIRA avalanche photodiode detector. We detected 121 companion candidates around 111 stars, of which 62 companions are physically associated based on Gaia DR2 parallaxes and proper motions, another 45 require follow-up observations to confirm physical association, and 14 are background objects. The companion separations range from 2 to 1101 au and reach contrast ratios of 7.7 mag in the near-infrared compared to the primary. The majority of confirmed and pending candidates are stellar companions, with ~5 being potentially substellar and requiring follow-up observations for confirmation. We also detected a 43 ± 9 M {sub Jup} and an 81 ± 5 M {sub Jup} companion that were previously reported. We found 34 of our targets have acceleration measurements detected using Hipparcos–Gaia proper motions. Of those, 58{sub -14}{sup +12}% of the 12 stars with imaged companion candidates have significant accelerations (χ{sup 2}>11.8), while only 23{sub -6}{sup +11}% of the remaining 22 stars with no detected companion have significant accelerations. The significance of the acceleration decreases with increasing companion separation. These young accelerating low-mass stars with companions will eventually yield dynamical masses with future orbit monitoring.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Formulating the r-mode Problem for Slowly Rotating Neutron Stars

We revisit the problem of inertial r-modes in stratified stars, drawing on a more precise description of the composition stratification in a mature neutron star. The results highlight issues with the traditional approach to the problem, leading us to rethink the computational strategy for the r-modes of nonbarotropic neutron stars. We outline two strategies for dealing with the problem. For moderate to slowly rotating neutron stars the only viable alternative may be to approach the problem numerically from the outset, while a meaningful slow-rotation calculation can be carried out for the fastest known spinning stars (which may be close to being driven unstable by the emission of gravitational waves). We demonstrate that the latter approach leads to a problem close, but not identical, to that for barotropic inertial modes. We also suggest that these reformulations of the problem likely resolve the long-standing problem of singular behavior associated with a corotation point in rotating relativistic neutron stars. This issue needs to be resolved in order to guide future gravitational-wave searches.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

60 Candidate High-velocity Stars Originating from the Sagittarius Dwarf Spheroidal Galaxy in Gaia EDR3

Using proper motions from Gaia Early Data Release 3 (Gaia EDR3) and radial velocities from several surveys, we identify 60 candidate high-velocity stars with a total velocity greater than 75% of the escape velocity that probably originated from the Sagittarius dwarf spheroidal galaxy (Sgr) by orbital analysis. Sgr’s gravity has little effect on the results and the Large Magellanic Cloud’s gravity has a nonnegligible effect on only a few stars. The closest approach of these stars to the Sgr occurred when the Sgr passed its pericenter (~38.2 Myr ago), which suggests they were tidally stripped from the Sgr. The positions of these stars in the Hertzsprung–Russell diagram and the chemical properties of 19 of them with available [Fe/H] are similar to the Sgr stream member stars. This is consistent with the assumption of their accretion origin. Two of the 60 are hypervelocity stars, which may also be produced by the Hills mechanism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Spectacular Nucleosynthesis from Early Massive Stars

Abstract Stars that formed with an initial mass of over 50 M ⊙ are very rare today, but they are thought to be more common in the early Universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early follow-up of the SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early Universe. J0931+0038 has a relatively high metallicity ([Fe/H] = −1.76 ± 0.13) but an extreme odd–even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass >50 M ⊙ , making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However, the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates the study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys.

79 ASTRONOMY AND ASTROPHYSICS↗