Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “STARS”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

THE MASSIVE STAR POPULATION IN M101. III. SPECTRA AND PHOTOMETRY OF THE LUMINOUS AND VARIABLE STARS

We discuss moderate-resolution spectra, multicolor photometry, and light curves of 31 of the most luminous stars and variables in the giant spiral M101. The majority are intermediate A- to F-type supergiants. We present new photometry and light curves for three known “irregular blue variables,” V2, V4, and V9, and identify a new candidate. Their spectra and variability confirm that they are luminous blue variable (LBV) candidates and V9 may be in an LBV-like maximum light state or eruption.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances

We study the spectrophotometric properties of a highly magnified (μ≃40--70) pair of stellar systems identified at z = 3.2222 behind the Hubble Frontier Field galaxy cluster MACS J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint (m{sub UV}≃30.6), young (≲100 Myr), low-mass (<10{sup 7} M{sub ⊙}), low-metallicity (12 + Log(O/H) ≃ 7.7, or 1/10 solar), and compact (30 pc effective radius) stellar systems separated by ≃300 pc after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (σ{sub v}≲25 km s{sup −1}) high-ionization lines. These features may be the signature of very hot (T>50,000 K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Lyα emission (e.g., C iv/Lyα > 15), despite the fact that the Lyα line flux is expected to be 150 times brighter (inferred from the Hβ flux). A spatially offset, strongly magnified (μ>50) Lyα emission with a spatial extent ≲7.6 kpc{sup 2} is instead identified 2 kpc away from the system. The origin of such a faint emission could be the result of fluorescent Lyα induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or may be associated with an underlying and barely detected object (with m{sub UV}>34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lyα emission—suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lyα detection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

The Milky Way Revealed by Variable Stars. I. Sample Selection of RR Lyrae Stars and Evidence for Merger History

In order to study the Milky Way, RR Lyrae (RRL) variable stars identified by Gaia, ASAS-SN, and ZTF sky survey projects have been analyzed as tracers in this work. Photometric and spectroscopic information of 3417 RRLs including proper motions, radial velocity, and metallcity are obtained from observational data of Gaia, LAMOST, GALAH, APOGEE, and RAVE. Precise distances of RRLs with typical uncertainties less than 3% are derived by using a recent comprehensive period–luminosity–metallicity relation. Our results from kinematical and chemical analysis provide important clues for the assembly history of the Milky Way, especially for the Gaia–Sausage ancient merger. The kinematical and chemical trends found in this work are consistent with those of recent simulations that indicated that the Gaia–Sausage merger had a dual origin in the Galactic thick disk and halo. As recent similar works have found, the halo RRL sample in this work contains a subset of radially biased orbits besides a more isotropic component. This higher orbital anisotropy component amounts to β ≃ 0.8, and it contributes between 42% and 83% of the halo RRLs at 4 < R( kpc) < 20.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

β equilibrium in neutron-star mergers [Beta equilibrium in neutron star mergers]

Here, we show that the commonly used criterion for β equilibrium in neutrino-transparent dense nuclear matter becomes invalid as temperatures rise above 1 MeV. Such temperatures are attained in neutron-star mergers. By numerically computing the relevant weak-interaction rates we find that the correct criterion for β equilibrium requires an isospin chemical potential that can be as large as 10–20 MeV, depending on the temperature at which neutrinos become trapped.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Connecting GRBs from binary neutron star mergers to nuclear properties of neutron stars

The fate of the binary neutron star (NS) merger remnants hinges sensitively upon the NS equation of state and the threshold mass, M ls , that separates a long-lived from a short-lived NS remnant. The nature of the electromagnetic counterparts is also influenced by the remnant type, particularly in determining whether a gamma-ray burst from a compact binary merger (cbGRB) is of short or long duration. We propose a novel approach to probe Mls by linking it to the estimated observed ratio of long to short cbGRBs. We find that current observations broadly favor a relatively high value for this transition, M ls ≃ 1.3M TOV , for which M TOV ≲ 2.6M ⊙ , consistent with numerical simulations, as also shown here. Furthermore, our results disfavor nuclear physics scenarios that would lead to catastrophic pressure loss at a few times nuclear density and temperatures of tens of MeV, leading to a rapid gravitational collapse of binaries with total mass M ≲ 1.3M TOV . Future individual gravitational wave events with on-axis cbGRBs can further bound Mls.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Three-dimensional Boltzmann-Hydro Code for Core-collapse in Massive Stars. II. The Implementation of Moving-mesh for Neutron Star Kicks

We present a newly developed moving-mesh technique for the multi-dimensional Boltzmann-Hydro code for the simulation of core-collapse supernovae (CCSNe). What makes this technique different from others is the fact that it treats not only hydrodynamics but also neutrino transfer in the language of the 3 + 1 formalism of general relativity (GR), making use of the shift vector to specify the time evolution of the coordinate system. This means that the transport part of our code is essentially general relativistic, although in this paper it is applied only to the moving curvilinear coordinates in the flat Minknowski spacetime, since the gravity part is still Newtonian. The numerical aspect of the implementation is also described in detail. Employing the axisymmetric two-dimensional version of the code, we conduct two test computations: oscillations and runaways of proto-neutron star (PNS). We show that our new method works fine, tracking the motions of PNS correctly. We believe that this is a major advancement toward the realistic simulation of CCSNe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Directed flow of quarks from the RHIC Beam Energy Scan measured by STAR

Directed flow (ν 1 ) is a good probe of the early-stage dynamics of collision systems, and the ν 1 slope at midrapidity ( dv 1 /dy| y=0 ) is sensitive to the system's equation of state. STAR has published ν 1 (y) measurements for ten particle species (π ± ,$p, \bar{p},Λ,\bar{Λ},&straightphi;,K^± and K_S^0$) in Au+Au collisions at eight beam energies from $ \sqrt{s_{NN}}$ = 7.7 GeV to 200 GeV. In this study, we employ a simple coalescence idea to decompose ν 1 of hadrons into ν 1 of constituent quarks. The dv 1 /dy values of $\bar{p},\bar{K},$ and $\bar{Λ}$ are used to test the coalescence sum rule for produced quarks. Data involving produced quarks support the coalescence picture at $ \sqrt{s_{NN}}$ =11.5 GeV to 200 GeV, and a sharp deviation from this picture is observed at 7.7 GeV. The dv 1 /dy of transported quarks is studied via net particles (net p and net Λ). In these proceedings, we further extract the ν 1 slopes of produced and transported quarks, assuming that the coalescence sum rule is valid.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Long-range collectivity in small collision systems with two- and four-particle correlations at STAR

New STAR measurements of two- and four-particle (c 2 {4}) azimuthal angle correlations are reported for p+Au collisions at 200 GeV and d+Au collisions from 19.6 to 200 GeV. A negative c 2 {4}, consistent with collective anisotropic flow, is observed in high multiplicity d+Au events at 62.4 and 200 GeV where the statistical significance of the data allows an analysis. A template fitting method, employed to subtract non-flow contributions to the two-particle correlations, allows the extraction of the anisotropy coefficients (ν 2 ) for these systems at their respective energies. The ν 2 results for different energies show a common dependence on the charged particle multiplicity density ( < dN ch /dη > ), which provide important insight on the nature of collective behavior in small collision systems.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Delineating effects of nuclear symmetry energy on the radii and tidal polarizabilities of neutron stars

Here, what can we learn about the density dependence of nuclear symmetry energy ${E}_{\mathrm{sym}}(\rho)$ from precise measurements of the radius (R 1.4 ) and/or tidal polarizability (Λ 1.4 ) of canonical neutron stars with a mass of 1.4 ${{\rm{M}}}_{\odot }$? With the ${E}_{\mathrm{sym}}(\rho)$ parameterized using three parameters L, K sym, and J sym which have the asymptotic meaning of being respectively the slope, curvature, and skewness of symmetry energy at saturation density, we found that, while both the R 1.4 and Λ 1.4 depend strongly on the slope L, the K sym and J sym parameters characterizing the high-density behavior of ${E}_{\mathrm{sym}}(\rho)$ also play appreciable roles. Thus, there is not a simple relation between the Λ 1.4 /R 1.4 and L alone. Precise measurements of just the Λ 1.4 and R 1.4 cannot completely determine the ${E}_{\mathrm{sym}}(\rho)$ but limit combinations of its parameters. In particular, stringent constraints approximately independent of the J sym on the L–K sym correlations can be obtained. However, infinite combinations of the larger (smaller) L and smaller (larger) K sym can lead to the same Λ 1.4 and R 1.4 . Additional observables including those from terrestrial nuclear experiments are thus necessary to break this degeneracy in order to completely determine the density dependence of nuclear symmetry energy ${E}_{\mathrm{sym}}(\rho)$.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

w22_stars: Turbulent Convection in a 19M ⊙ Star [Slide]

We find large scale turbulent convection within the O-shell region in our massive star model. Within this region we also observe a redistribution of angular momentum that differs from 1D stellar evolution models.

79 ASTRONOMY AND ASTROPHYSICS↗