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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

High-velocity CP2 stars in the Galactic halo

The second subclass of chemically peculiar stars, the CP2 stars, are early-type stars exhibiting anomalous abundances with variable line strengths possibly also accompanied by photometric variability that typically belong to the Galactic disk. However, a small fraction of these objects were recently found to be located far from the Galactic plane and are thought to belong to the Galactic halo, which is unexpected for this class of objects. Our study investigates the halo membership of the proposed halo CP2 star candidates based on their velocities and Galactic orbits, to determine their points of origin. In addition, we searched for further halo CP2 star candidates using an as-yet-unpublished sample of CP stars. Our analysis relied on the astrometry from the early third Gaia data release and radial velocities based on the spectroscopy from LAMOST and SDSS, which was also employed in spectral classification. The light variability of the confirmed CP2 stars in our sample was analyzed using data from the ZTF and ATLAS surveys. After filtering our initial sample using kinematic and spectroscopic criteria, we identified six CP2 stars with kinematical properties consistent with a halo membership. The orbits of these stars are in agreement with an origin in the Galactic disk where they were probably ejected through dynamical interactions or in the binary supernova scenario, making them the first runaway CP2 stars known.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

eROSITA study of the globular cluster 47 Tucanae

Here, we present the results of the analysis of five observations of the globular clutser 47 Tucanae (47 Tuc) with the extended Roentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spektrum-Roentgen-Gamma (Spektr-RG, SRG). We study the X-ray population in the field of one of the most massive globular clusters in our Milky Way. We focused on the classification of point-like sources in the field of 47 Tuc. The unresolved dense core of 47 Tuc (1.7 radius) and also sources that show extended emission are excluded from this study. We applied different methods of X-ray spectral and timing analysis together with multi-wavelength studies to classify the X-rays sources in the field of 47 Tuc. We detected 888 point-like sources in the energy range of 0.2–5.0 keV. We identified 126 background active galactic nuclei and 25 foreground stars. One of the foreground stars is classified as a variable M dwarf. We also classified 14 X-ray sources as members of 47 Tuc, including one symbiotic star, two quiescent low-mass X-ray binaries, and four cataclysmic variables. There are also five X-ray sources that can either be a cataclysmic variable or a contact binary, and also one X-ray source can be an active binary (type RS CVn). We identified one X-ray binary that belongs to the Small Magellanic Cloud. Moreover, we calculated the X-ray luminosity function of 47 Tuc. No significant population that seems to belong to the globular cluster is observed in the energy range of 0.5–2.0 keV using eROSITA observations.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

Interactions in Massive Colliding Wind Binaries

The most massive stars (M> 60 Solar Mass) play crucial roles in altering the chemical and thermodynamic properties of their host galaxies. Stellar mass is the fundamental stellar parameter that determines their ancillary properties and which ultimately determines the fate of these stars and their influence on their galactic environs. Unfortunately, stellar mass becomes observationally and theoretically less well constrained as it increases. Theory becomes uncertain mostly because very massive stars are prone to strong, variable mass loss which is difficult to model. Observational constraints are uncertain too. Massive stars are rare, and massive binary stars (needed for dynamical determination of mass) are rarer still: and of these systems only a fraction have suitably high orbital inclinations for direct photometric and spectroscopic radial-velocity analysis. Even in the small number of cases in which a high-inclination binary near the upper mass limit can be identified, rotational broadening and contamination of spectral line features from thick circumstellar material (either natal clouds or produced by strong stellar wind driven mass loss from one or both of he stellar components) biases the analysis. In the wilds of the upper HR diagram, we're often left with indirect and circumstantial means of determining mass, a rather unsatisfactory state of affairs.

Corcoran, M.↗
NASA NTRS1995

Simultaneous UV and optical study of O star winds and UV and optical covariability of O star winds

Simultaneous ultraviolet and optical observations of 10 bright O stars were organized in several observing campaigns lasting 3-6 days each. The observing campaigns included 12 observatories in the Northern hemisphere obtaining high resolution spectroscopy, photometry, and polarimetry, as well as 24-hour coverage with the IUE (International Ultraviolet Explorer) observatory. Over 600 high dispersion SWP spectra were acquired with IUE at both NASA and VILSPA for the completion of this work. The massive amount of data from these observing campaigns, both from IUE and the ground-based instruments, has been reduced and analyzed. The accompanying paper describes the data acquisition, analysis, and conclusions of the study performed. The most important results of this study are the strong confirmation of the ubiquitous variability of winds of O stars, and the critical correlation between rotation of the star and the wind variability as seen in the ultraviolet and optical spectral lines.

Nichols, Joy S.↗
NASA NTRS2000

Accretion Onto Magnetic Degenerate Stars

While the original objectives of this research program included the study of radiative processes in cataclysmic variables and the evolution of neutron star magnetic fields, the scope of the reported research expanded to other related topics as this project developed. This final report therefore describes the results of our research in the following areas: 1) Irradiation-driven mass transfer cycles in cataclysmic variables and low-mass X-ray binaries; 2) Propeller effect and magnetic field decay in isolated old neutron stars; 3) Decay of surface magnetic fields in accreting neutron stars and pulsars; 4) Finite-Difference Hydrodynamic simulations of mass transfer in binary stars.

Frank, Juhan↗
NASA NTRS1984

CPD -48 deg 1577 - The brightest known cataclysmic variable

The discovery is reported of a V = 9.4 mag star which has the properties of a cataclysmic variable. Its UBV colors are similar to those of UX UMa, and its brightness fickers with an amplitude of the order of 0.1 mag. Larger brightness variations (0.5 mag) are found on a time scale of years. Its spectrum is characterized by extremely weak absorption lines of He I and He II, as well as extremely broad, shallow lines of H. A broad K line is very weakly present in absorption. Emission cores with a very slow decrement are seen in the H lines. From a comparison with the known properties of other cataclysmic variables, the distance, reddening, and mass exchange rate are estimated. The space density of cataclysmic variables is also estimated, since the original spectroscopic survey is complete to magnitude B = 10.0.

Krzeminski, W.↗
NASA NTRS2001

Analysis and Quality Assurance of the SKYMAP 4.0 Guidance and Tracking Star Catalog: The NASA SKY2000 Spacecraft Attitude Determination Star Catalog

An updated and improved NASA spacecraft attitude determination catalog, now called SKY2000, Version 3, has been prepared and quality assured. The highest priority goals were to replace the astrometric (positions and motions) and photometric (brightnesses and colors) data with the most recent and accurate data available. Quality assurance has been performed in a fairly straightforward manner, i.e., without extensive data checking and analysis, and many errors and Inconsistencies were corrected. Additional work should eventually be done on the variability and multiple-star data In the catalog, while certain other data can be significantly Improved. The current version of the catalog can be found at the GSFC Flight Dynamics website: http://cheli.gsfc.nasa.gov/dist/attitude/skymap.html. Supporting information and reference materials (published papers, format and data descriptions, etc.) can also be found at the website.

Warren, Wayne H., Jr.↗
NASA NTRS1979

Stellar luminosity stability - Luminosity variations and light curve period changes in BY Draconis stars

The implications for convection in late type stars arising from the observations of BY Draconis type variability are examined. The possibility that the total luminosity of such a star is not constant is emphasized, and further observational tests to better define the variability are suggested. An alternative to standard spot models is explored in which the 'missing' flux from 'dark' spots is temporally redistributed; this model makes definite predictions about the correlation of mean light and color, and about the quiescent (i.e., unspotted) magnitudes of BY Draconis stars. The time scales of the long-period variability of these stars appear to require secular changes in convective energy transport. Consideration is given to the evidence for period changes in the optical light curves, and it is concluded that the reality of such changes is far less certain than previously claimed.

Hartmann, L.↗
NASA NTRS1986

The IRAS colors of normal galaxies

In the IRAS color-color diagram, galaxies powered only by star formation are distributed in a band such that as systems get cooler at 12/25 microns they get warmer at 60/100 microns. A simple phenomenology is proposed which describes the emission from each galaxy as a combination of two components: cool, relatively constant 'cirrus' emission from the neutral medium, and from 'active' regions warmer emission whose colors depend on mean physical parameters in these regions. The model accounts for the IRAS colors of all galaxies detected as point sources in all four bands. The model implies that a galaxy's far-infrared flux as measured by IRAS is not simply proportional to the recent star formation rate, because only a (variable) fraction of the infrared is due to massive young stars, and only part of these stars' luminosity is reemitted in the infrared.

Helou, George↗
NASA NTRS2005

AM CVn Stars: Structure and Evolution of Ultra-Short Period Interacting Binaries

This is the final report of a FUSE program to study the physics of accretion and outflows in ultra-compact, helium dominated, disk-accreting binaries. With FUSE, we observed the AM CVn binary V803 Cen, which is one of only two AM CVn systems observed by FUSE to date. V803 Cen is a short-period interacting binary in which a hydrogen-deficient white dwarf transfers mass to another white dwarf via a hot, steady-state accretion disk. Unlike other cataclysmic variables (CVs), AM CVn stars have undergone double common envelope evolution (one for each white dwarf in the binary) and so probe an alternate route of evolution in binary stars. Our goals in this project were to investigate how the structure of the accretion disk and the link between the disk and wind outflows are affected by the absence of hydrogen in the system and by the compact size of the binary and the accretion disk.

Froning, Cynthia↗
NASA NTRS1998

A Search for X-Ray Evidence of a Compact Companion to the Unusual Wolf-Rayet Star HD 50896 (EZ CMa)

We analyze results of a approx.25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays (greater than or equal to 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from less than or equal to 0.4 keV up to at least approx. 2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N(sub H) approx. 10(exp 22)/sq cm. The absorption-corrected X-ray luminosity L(sub x)(0.5 - 10 keV) = 10(exp 32.85) erg/s gives L(sub x)/ L(sub bol) approx. 10(exp -6), a value that is typical of WN stars. No X-ray variability was detected. Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near approx. 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion.

Skinner, Stephen L.↗
NASA NTRS1997

The Nature and Evolutionary History of GRO J1744-28

GRO J1744-28 is the first known X-ray source to display bursts, periodic pulsations, and quasi-periodic oscillations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability from various accreting neutron stars. The orbital period is 11.8 days, and the measured mass function of 1.31 x 10(exp -4) solar mass is one of the smallest among all known binaries. If we assume that the donor star is a low-mass giant transferring matter through the inner Lagrange point, then we can show that its mass is lower than approximately 0.7 solar mass and probably closer to 0.25 solar mass. Higher mass, but unevolved, donor stars are shown to be implausible. We also demonstrate that the current He core mass of the donor star lies in the range of 0.20-0.25 solar mass. Thus, this system is most likely in the final stages of losing its hydrogen-rich envelope, with only a small amount of mass remaining in the envelope. If this picture is correct, then GRO J1744-28 may well represent the closest observational link that we have between the low-mass X-ray binaries and recycled binary pulsars in wide orbits. We have carried out a series of binary evolution calculations and explored, both systematically and via a novel Monte Carlo approach, the range of initial system parameters and input physics that can lead to the binary parameters of the present-day GRO J1744-28 system. The input parameters include both the initial total mass and the core mass of the donor star, the neutron-star mass, the strength of the magnetic braking, the mass-capture fraction, and the specifics of the core mass/radius relation for giants. Through these evolution calculations, we compute probability distributions for the current binary system parameters (i.e., the total mass, core mass, radius, luminosity, and K-band magnitude of the donor star, the neutron star mass, the orbital inclination angle, and the semimajor axis of the binary). Our calculations yield the following values for the GRO J1744-28 system parameters (with 95% confidence limits in parentheses): donor star mass: 0.24 solar mass (0.2-0.7 solar mass); He core mass of the donor star: 0.22 solar mass (0.20-0.25 solar mass); neutron-star mass: 1.7 solar mass (1.39-1.96 solar mass); orbital inclination angle: 18deg (7deg-22deg); semi- major axis: 64 lt-s (60-67 lt-s); radius of the donor star: 6.2 solar radius(6-9 solar radius); luminosity of donor star: 23 solar luminosity (15-49 solar luminosity), and long-term mass transfer rate at the current epoch: 5 x 10(exp -10)solar mass/yr (2 x 10(exp -10) to 5 x 10(exp -9)solar mass/yr). We deduce that the magnetic field of the underlying neutron star lies in the range of approximately 1.8 x 10(exp 11)G to approximately 7 x 10(exp 11)G, with a most probable value of 2.7 x 10(exp 11)G. This is evidently sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to the type 1 X-ray bursts observed in most X-ray bursters. We present a simple paradigm for magnetic accreting neutron stars where X-ray pulsars, GRO J1744-28, the Rapid Burster, and the type 1 X-ray bursters may form a continuum of possible behaviors among accreting neutron stars, with the strength of the neutron-star magnetic field serving as a crucial parameter that determines the mode of X-ray variability from a given object.

Rappaport, S.↗
NASA NTRS1992

Variability of near-infrared emission lines in NGC 4151 - Implications for nuclear star formation

Multiaperture infrared spectra of the circumnuclear region of the Seyfert galaxy NGC 4151 taken over a 10 year period show that the N = 7-4 B-gamma hydrogen recombination line is variable. The H2 quadrupole line emission shows no sign of variability. It is shown that there is a substantial contribution to the nuclear B-gamma flux from the active nucleus, making it impossible to determine star formation rates from infrared hydrogen recombination lines in active galaxies. The lack of variability in the H2 line fluxes indicates that ionizing photons from the active nucleus is not important in exciting the molecular line emission.

Prestwich, Andrea H.↗
NASA NTRS2007

Variability of Disk Emission in Pre-Main Sequence and Related Stars. I. HD 31648 and HD 163296 - Isolated Herbig Ae Stars Driving Herbig-Haro Flows

Infrared photometry and spectroscopy covering a time span of a quarter century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are isolated Herbig Ae stars that exhibit signs of active accretion, including driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was found to be relatively quiescent photometrically in its inner disk region, with the exception of a major increase in emitted flux in a broad wavelength region centered near 3 pm in 2002. In contrast, HD 31648 has exhibited sporadic changes in the entire 3-13 pm region throughout this span of time. In both stars the changes in the 1-5 pm flux indicate structural changes in the region of the disk near the dust sublimation zone, possibly causing its distance from the star to vary with time. Repeated thermal cycling through this region will result in the preferential survival of large grains, and an increase in the degree of crystallinity. The variability observed in these objects has important consequences for the interpretation of other types of observations. For example, source variability will compromise models based on interferometry measurements unless the interferometry observations are accompanied by nearly-simultaneous photometric data.

Sitko, Michael L.↗
NASA NTRS2008

Variability of Disk Emission in Pre-main-sequence and Related Stars. I. HD 31648 and HD 163296: Isolated Herbig Ae Stars Driving Herbig-Haro Flows

Infrared photometry and spectroscopy covering a time span of a quarter-century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are isolated Herbig Ae stars that exhibit signs of active accretion, including driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was found to be relatively quiescent photometrically in its inner disk region, with the exception of a major increase in emitted flux in a broad wavelength region centered near 3 micron in 2002. In contrast, HD 31648 has exhibited sporadic changes in the entire 3-13 micron region throughout this span of time. In both stars, the changes in the 1-5 micron flux indicate structural changes in the region of the disk near the dust sublimation zone, possibly causing its distance from the star to vary with time. Repeated thermal cycling through this region will result in the preferential survival of large grains, and an increase in the degree of crystallinity. The variability observed in these objects has important consequences for the interpretation of other types of observations. For example, source variability will compromise models based on interferometry measurements unless the interferometry observations are accompanied by nearly simultaneous photometric data.

Sitko, Michael L.↗
DOE OSTI2022

Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs

Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyze atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts' extended pre-main-sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean-analog environments. We contrast the solar system scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and the desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars ≤0.4 M ⊙ experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals, and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favored around FGK stellar types relative to M dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗