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Results for “STAR EVOLUTION”

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 1,261 records · Page 70

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NASA NTRS1991

Time evolution of interstellar dust and far-infrared luminosity of disk galaxies

The evolution of interstellar dust in disk galaxies is modeled, assuming that dust forms predominantly in molecular clouds associated with star formation. Analytical solutions for the dust abundance in disk galaxies as a function of galaxy age are obtained for the prompt initial enrichment and accretion models of chemical evolution, consistent with observations of the heavy element abundance in the Galaxy. Star formation rates in the disks of galaxies are taken as either constant or decreasing exponentially with time. It is found that the total amount of dust in the early history of galaxies can be up to 4 times the value observed today. The total emission from dust in galaxies is calculated, using an average dust temperature derived from IRAS observations. In the strongly evolving models, the far-infrared luminosity from galaxies can be roughly two orders of magnitude larger than the current value.

Wang, Boqi↗

DOE OSTI2022

Is Jet Re-orientation the Elusive Trigger for Star Formation Suppression in Radio Galaxies?

Abstract Jet re-orientation associated with the time evolution of radio quasars explains the formation of X-shaped radio galaxies and their preference for isolated environments. But since X-shaped radio galaxies are generally not found in dense environments (e.g., groups/clusters), the jet re-orientation phenomenon for radio galaxies in groups and clusters has been ignored. We take a closer look at the re-orientation of FRI jets with respect to FRII jets, and find that it may constitute the as-yet unidentified trigger for star formation suppression in radio galaxies. We show how the recently explored radio “red geyser” galaxies can be interpreted in this context and ultimately reveal a deeper understanding of why FRII radio galaxies are on one side of the star formation enhancement/suppression divide compared to FRI radio galaxies.

Astronomy & Astrophysics↗

DOE OSTI2022

The COS Legacy Archive Spectroscopy Survey (CLASSY) Treasury Atlas

Far-ultraviolet (FUV; ~1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N 1500 Å ≳ 5/resel), high-resolution (R ~ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M * (M ⊙ ) < 10.1), star formation rate (–2.0 < log SFR (M ⊙ yr –1 ) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O 32 < 38.0), reddening (0.02 < E(B – V) < 0.67), and nebular density (10 < n e (cm –3 ) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ~ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2020

A NICER View of Spectral and Profile Evolution for Three X-ray-Emitting Millisecond Pulsars

We present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three energetic rotation-powered millisecond pulsars (MSPs): PSRs B1937+21, B1821−24, and J0218+4232. We fit Gaussians and Lorentzians to the pulse profiles for different energy sub-bands of the soft X-ray regime to measure the energy dependence of pulse separation and width. We find that the separation between pulse components of PSR J0218+4232 decreases with increasing energy at >3σ confidence. The 95% upper limit on pulse separation evolution for PSRs B1937+21 and B1821−24 is less than 2 milliperiods per keV. Our phase-resolved spectral results provide updated constraints on the non-thermal X-ray emission of these three pulsars. The photon indices of the modeled X-ray emission spectra for each pulse component of PSR B1937+21 are inconsistent with each other at the 90% confidence level, suggesting different emission origins for each pulse. We find that the PSR B1821−24 and PSR J0218+4232 emission spectra are invariant with phase at the 90% confidence level. We describe the implications of our profile and spectral results in the context of equatorial current sheet emission models for these three MSPs with non-thermal, magnetospheric X-ray emission.

Dominick M Rowan↗

NASA NTRS2014

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets

We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large-amplitude, high-frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restricted our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verified that these systems are stable for 108 years with N-body simulations and calculated the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We ran a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculated differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculated the outer edge of habitability for two conditions: (1) the full evolution of the planetary spin and orbit and (2) the eccentricity and obliquity fixed at their average values. We recovered previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but we also found that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes.

habitability of exoplanets↗

DOE OSTI2022

Star formation and AGN feedback in the local Universe: Combining LOFAR and MaNGA

The effect of active galactic nuclei (AGN) on their host galaxies – in particular their levels of star formation – remains one of the key outstanding questions of galaxy evolution. Successful cosmological models of galaxy evolution require a fraction of energy released by an AGN to be redistributed into the interstellar medium to reproduce the observed stellar mass and luminosity function and to prevent the formation of over-massive galaxies. Observations have confirmed that the radio-AGN population is energetically capable of heating and redistributing gas at all phases, however, direct evidence of AGN enhancing or quenching star formation remains rare. With modern, deep radio surveys and large integral field spectroscopy (IFS) surveys, we can detect fainter synchrotron emission from AGN jets and accurately probe the star-forming properties of galaxies, respectively. Here, in this paper, we combine data from the LOw Frequency ARray Two-meter Sky Survey (LoTSS) with data from one of the largest optical IFS surveys, Mapping Nearby Galaxies at Apache Point Observatory (MaNGA), to probe the star-forming properties of 307 local (z < 0.15) galaxies that host radio-detected AGN (RDAGN). We compare our results to a robust control sample of non-active galaxies that each match the stellar mass, redshift, visual morphology, and inclination of a RDAGN host. We find that RDAGN and control galaxies have broad star-formation rate (SFR) distributions, typically lie below the SFMS, and have negative stellar light-weighted age gradients. These results indicate that AGN selected based on their current activity are not responsible for suppressing their host galaxies’ star formation. Rather, our results support the maintenance mode role that radio AGN are expected to have in the local Universe.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

New Conductive Opacities for White Dwarf Envelopes

Thanks to their continuous cooling and relative simplicity, white dwarf stars are routinely used to measure the ages of stellar populations. The usefulness of white dwarfs as cosmochronometers depends on the availability of accurate cooling models. A key ingredient of those models is conductive opacities, which largely govern the cooling rate. In this work, we present improved conductive opacities for the regime of moderate coupling and moderate degeneracy that characterizes an important portion of the envelopes of DA and DB white dwarfs. Here, we find differences of up to a factor of 3 between our calculations and the commonly used opacities of Cassisi et al., which we attribute to an improved account of electron–electron scattering. The cooling models are strongly affected by those changes in the conductive opacities: the age of a 4000 K white dwarf can be reduced by as much as 2 Gyr. We provide analytical fits to our new opacities to facilitate the implementation of this important effect in white dwarf evolution codes.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1983

Stellar evolution in N-body simulations of disk galaxies. I

The Kalnajs (1972, 1976) Omega models of global mass and velocity distributions are employed in the present two-dimensional N-body simulation, which allows for a spectrum of particle masses, stellar explosions, explosion remnant interactions with an interstellar medium, and the creation of new stars from the gas. Two sequences of runs using the Omega values of 0.8 and 0.9 examine the separate and combined effects of particle mass distribution, the gravitational influence of an interstellar gas distribution on the N-body particles, and stellar evolution, allowing for stellar explosions and star formation from the gas. It is found that both Omega values' nonequilibrium results dramatically change when evolution is allowed to occur. These results call for more realistic coupled N-body and evolution simulations in order to improve the understanding of disk galaxy evolution.

Comins, N. F.↗

NASA NTRS1980

Thermonuclear-flash models for X-ray burst sources

Theoretical models for X-ray burst sources that invoke thermonuclear flashes in the surface layers of an accreting neutron star are discussed. Emphasis is placed on the studies by Joss (1978) and Joss and Li (1979) on the evolution of the helium-burning shell. Numerical calculations with regard to the mass accretion rate, core temperature of the neutron star and the sensitivity of the flash properties to the assumed mass and radius of the neutron star are considered. Attention is also given to the behavior of the surface luminosity following a thermonuclear flash, the decline from maximum X-ray luminosity, structure of the surface layers prior to and during the first helium-burning flash and the temporal evolution of the first X-ray burst.

Joss, P. C.↗

NASA NTRS2011

The Unresolved Star-Forming Galaxy Component of the Extragalactic Gamma Ray Background

We present new theoretical estimates of the contribution of unresolved star-forming galaxies to the extragalactic gamma-ray background (EGB) as measured by EGRET and the Fermi-LAT. We employ several methods for determining the star-forming galaxy contribution the the EGB, including a method positing a correlation between the gamma-ray luminosity of a galaxy and its rate of star formation as calculated from the total infrared luminosity, and a method that makes use of a model of the evolution of the galaxy gas mass with cosmic time. We find that depending on the model, unresolved star-forming galaxies could contribute significantly to the EGB as measured by the Fermi-LAT at energies between approx. 300 MeV and approx. few GeV. However, the overall spectrum of unresolved star-forming galaxies can explain neither the EGRET EGB spectrum at energies between 50 and 200 MeV nor the Fermi-LAT EGB spectrum at energies above approx. few GeV.

Venters, Tonia M.↗

NASA NTRS2011

New Theoretical Estimates of the Contribution of Unresolved Star-Forming Galaxies to the Extragalactic Gamma-Ray Background (EGB) as Measured by EGRET and the Fermi-LAT

Venters, Tonia M.↗

NASA NTRS1993

The occurrence and properties of disks around young stars

The paper discusses the occurrence and properties of disks around young stars, emphasizing in particular how these may relate to planet formation and the evolution of the solar system. The global properties of such disks often resemble those attributed to the primitive solar nebula, suggesting that conditions appropriate for planet formation commonly accompany the birth of low-mass stars. Disk masses, between 0.001 and 1 solar mass, are generally lower than those of the stars, and may represent only a fraction (less than about 10 percent) of the total system mass. From the paucity of near-IR radiation from some disks it is inferred that the inner regions there are gaps where the opacity from small particles becomes vanishingly small. Evidence is presented to the effect that gaps in the inner disks develop preferentially in the oldest objects, suggesting that, with time, matter is lost or accumulates into large particles such as planetesimals, which cannot yet be detected.

Beckwith, Steven V. W.↗

DOE OSTI2022

Is Terzan 5 the remnant of a building block of the Galactic bulge? Evidence from APOGEE

ABSTRACT It has been proposed that the globular cluster-like system Terzan 5 is the surviving remnant of a primordial building block of the Milky Way bulge, mainly due to the age/metallicity spread and the distribution of its stars in the α–Fe plane. We employ Sloan Digital Sky Survey data from the Apache Point Observatory Galactic Evolution Experiment to test this hypothesis. Adopting a random sampling technique, we contrast the abundances of 10 elements in Terzan 5 stars with those of their bulge field counterparts with comparable atmospheric parameters, finding that they differ at statistically significant levels. Abundances between the two groups differ by more than 1σ in Ca, Mn, C, O, and Al, and more than 2σ in Si and Mg. Terzan 5 stars have lower [α/Fe] and higher [Mn/Fe] than their bulge counterparts. Given those differences, we conclude that Terzan 5 is not the remnant of a major building block of the bulge. We also estimate the stellar mass of the Terzan 5 progenitor based on predictions by the Evolution and Assembly of GaLaxies and their Environments suite of cosmological numerical simulations, concluding that it may have been as low as ∼3 × 108 M⊙ so that it was likely unable to significantly influence the mean chemistry of the bulge/inner disc, which is significantly more massive (∼1010 M⊙). We briefly discuss existing scenarios for the nature of Terzan 5 and propose an observational test that may help elucidate its origin.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2021

Subaru Hyper Suprime-Cam Survey of Cygnus OB2 Complex – I. Introduction, photometry, and source catalogue

ABSTRACT Low-mass star formation inside massive clusters is crucial to understand the effect of cluster environment on processes like circumstellar disc evolution, planet, and brown dwarf formation. The young massive association of Cygnus OB2, with a strong feedback from massive stars, is an ideal target to study the effect of extreme environmental conditions on its extensive low-mass population. We aim to perform deep multiwavelength studies to understand the role of stellar feedback on the IMF, brown dwarf fraction and circumstellar disc properties in the region. We introduce here, the deepest and widest optical photometry of 1.5○ diameter region centred at Cygnus OB2 in r2, i2, z, and Y-filters, using Subaru Hyper Suprime-Cam (HSC). This work presents the data reduction, source catalogue generation, data quality checks, and preliminary results about the pre-main sequence sources. We obtain 713 529 sources in total, with detection down to ∼28, 27, 25.5, and 24.5 mag in r2, i2, z, and Y-band, respectively, which is ∼3 – 5 mag deeper than the existing Pan-STARRS and GTC/OSIRIS photometry. We confirm the presence of a distinct pre-main sequence branch by statistical field subtraction of the central 18 arcmin region. We find the median age of the region as ∼5 ± 2 Myr with an average disc fraction of ∼9 per cent. At this age, combined with A $_V\, \sim$ 6 – 8 mag, we detect sources down to a mass range of ∼0.01–0.17 M⊙. The deep HSC catalogue will serve as the groundwork for further studies on this prominent active young cluster.

Gupta, Saumya (ORCID:0000000161843958)↗

DOE OSTI2020

Subaru/HSC deep optical imaging of infrared sources in the AKARI North Ecliptic Pole-Wide field

In this work, we present a five-broad-band (grizy) photometric catalogue of Subaru/Hyper Suprime-Cam (HSC) optical imaging observations at around the North Ecliptic Pole (NEP) where the AKARI infrared (IR) satellite conducted a large survey (NEP-Wide survey). The observations cover almost all the NEP-Wide survey field down to the depth of 28.1, 26.8, 26.3, 25.5, and 25.0 mag (5σ) at grizy, respectively. The five-band HSC catalogue contains about 2.6 million objects, and 70 959 AKARI NEP-Wide counterpart sources are identified in the catalogue. We added existing supplementary catalogues from the u band to the far-IR band, and estimated photo-z for the AKARI-HSC sources. We achieved σΔz/(1 + zs) = 0.06 and an outlier rate of 13.4 per cent at z = 0.2–1.5. Using the spectral energy distribution (SED) template fitting, we classified the AKARI-HSC galaxies into four categories, namely quiescent, star-forming, Type1 active galactic nucleus (AGN), and Type2 AGN, in each redshift bin. Then, at z > 1, the mean SED of star-forming galaxies in mid-IR (3–10 μm) range is significantly different from that of spiral galaxies in the nearby Universe, indicating that many of star-forming galaxies at z > 1 contain a heat source capable of heating dust to temperatures that radiate thermal emission in the mid-IR range. Furthermore, we estimated the number fraction of AGNs (fAGN) in each bin of redshift and IR luminosity (L IR ), and examined the dependence of redshift and L IR . In log(L IR /L ⊙ ) = 11.0–14.0, the fAGN shows a significant increase with increasing redshift, regardless of the LIR bins. In contrast, the fAGN shows a slight increase against L IR at z < 1 and no increase with increasing L IR at z > 1.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Searching for Milky Way twins: Radial abundance distribution as a strict criterion

We search for Milky Way-like galaxies among a sample of approximately 500 galaxies. The characteristics we considered of the candidate galaxies are the following: stellar mass M $\star$ , optical radius R 25 , rotation velocity V rot , central oxygen abundance (O/H) 0 , and abundance at the optical radius (O/H) R 25 . If the values of R 25 and M $\star$ of the galaxy were close to that of the Milky Way, then the galaxy was referred to as a structural Milky Way analogue (sMWA). The oxygen abundance at a given radius of a galaxy is defined by the evolution of that region (astration level, that is, the fraction of gas converted into stars, as well as gas exchange with the surroundings), and we then assumed that the similarity of (O/H) 0 and (O/H) R 25 in two galaxies suggests a similarity in their (chemical) evolution. If the values of (O/H) 0 and (O/H) R 25 in the galaxy were close to that of the Milky Way, then the galaxy was referred to as an evolutionary Milky Way analogue (eMWA). If the galaxy was simultaneously an eMWA and sMWA, then the galaxy was considered a Milky Way twin. We find that the position of the Milky Way on the (O/H) 0 –(O/H) R 25 diagram shows a large deviation from the general trend in the sense that the (O/H) R 25 in the Milky Way is appreciably lower than in other galaxies of similar (O/H) 0 . This feature of the Milky Way evidences that its (chemical) evolution is not typical. We identify four galaxies (NGC 3521, NGC 4651, NGC 2903, and MaNGA galaxy M-8341-09101) that are simultaneously sMWA and eMWA and can therefore be considered as Milky Way twins. In previous studies, Milky Way-like galaxies were selected using structural and morphological characteristics, that is, sMWAs were selected. We find that the abundances at the centre and at the optical radius (evolutionary characteristics) provide a stricter criterion for selecting real Milky Way twins.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Impact of Pycnonuclear Fusion Uncertainties on the Cooling of Accreting Neutron Star Crusts

Abstract The observation of X-rays during quiescence from transiently accreting neutron stars provides unique clues about the nature of dense matter. This, however, requires extensive modeling of the crusts and matching the results to observations. The pycnonuclear fusion reaction rates implemented in these models are theoretically calculated by extending phenomenological expressions and have large uncertainties spanning many orders of magnitude. We present the first sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations. We also couple the reaction network with the thermal evolution code dStar to further study their impact on the neutron star cooling curves in quiescence. Varying the pycnonuclear fusion reaction rates alters the depth at which nuclear heat is deposited although the total heating remains constant. The enhancement of the pycnonuclear fusion reaction rates leads to an overall shallower deposition of nuclear heat. The impurity factors are also altered depending on the type of ashes deposited on the crust. These total changes correspond to a variation of up to 9 eV in the modeled cooling curves. While this is not sufficient to explain the shallow heat source, it is comparable to the observational uncertainties and can still be important for modeling the neutron star crust.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

A JWST Survey of the Supernova Remnant Cassiopeia A

We present initial results from a James Webb Space Telescope (JWST) survey of the youngest Galactic core-collapse supernova remnant, Cassiopeia A (Cas A), made up of NIRCam and MIRI imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). We also present four exploratory positions of MIRI Medium Resolution Spectrograph integral field unit spectroscopy that sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Surprising discoveries include (1) a weblike network of unshocked ejecta filaments resolved to ~0.01 pc scales exhibiting an overall morphology consistent with turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity; (2) a thick sheet of dust-dominated emission from shocked CSM seen in projection toward the remnant's interior pockmarked with small (~1'') round holes formed by ≲0$^"_•$1 knots of high-velocity ejecta that have pierced through the CSM and driven expanding tangential shocks; and (3) dozens of light echoes with angular sizes between ~0$^"_•$1 and 1' reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission (≲20 nJy at 3 μm) from the neutron star in Cas A's center and tightly constrain scenarios involving a possible fallback disk. These JWST survey data and initial findings help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, the metal and dust enrichment of galaxies, and the origin of compact remnant objects.

79 ASTRONOMY AND ASTROPHYSICS↗