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NASA NTRS1988

The Algol-like binary TT Hydrae - The stars, circumstellar matter, and superionized plasma

This paper reports on superionized UV emission lines discovered in TT Hydrae (HD 97528), a semidetached eclipsing binary system in the Southern-Hemisphere sky. The list of emission lines observed is typical for interacting nondegenerate binaries of the Algol type, but with system-specific relative-intensity characteristics. The primary component of the system is a B9.5 V main-sequence star with effective temperature of 9800 K. Its mass equals 2.25 solar masses; the radius is 1.9 solar radii; and surface gravity log g equals 4.23. The secondary star has a mass of 0.41 solar mass and fills its critical Roche lobe. Evidence obtained on mass interaction supports the conclusion that HD 97528 is a normal semidetached system.

Plavec, Mirek J.↗

NASA NTRS1994

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis stars. 18: Coordinated VLA, ROSAT, and IUE observations of RS CVn binaries

As part of a coordinated program of multi-wavelength observations of RS CVn close binary systems, we observed 15 systems with the Very Large Array (VLA) and 10 systems with IUE, simultaneously or nearly simultaneously with the ROSAT All Sky Survey observations of these stars. Of the 22 systems observed with ROSAT, three were observed both by IUE and the VLA. Radio observations were made at 3.6, 6 and 20 cm. Of the 15 observed RS CVn systems, we detected 11 with greater than 4 sigma confidence at one or more wavelengths. The IUE observations were made within the RIASS (ROSAT-IUE All Sky Survey) program. We present the results of the VLA observations, along with the corresponding subsets of the ROSAT Position Sensitive Proportional Counter (PSPC) X-ray and Wide Field Camera (WFC) XUV survey, and RIASS IUE observations. We obtained an extended VLA/IUE/ROSAT simultaneous coverage of one system, TY Pyx, covering more than one orbital period. These observations reveal that the quiescent radio flux of TY Pyx is relatively constant over time scales of up to 7 hours, but that it did change by a factor of 3 over 24 hours, probably due to a flare on 1990 Nov. 12. The UV, XUV and X-ray fluxes do not show large day-to-day or phase-related variability. The observation of the decay phase of a radio flare on EI Eri, with no accompanying X-ray or XUV flare, suggests that the lack of a strong correlation between X-ray and radio flares previously noted for dMe flare stars holds for RS CVn systems as well. We suggest that the radio flare may have been due to a coherent emission process such as electron cyclotron emission. The simultaneous measurements presented here provide a unique test of the general correlation between radio and soft X-ray luminosities, L(sub radio) approximately L(sub x exp m) (Drake et al. 1989) with a power-law slope close to unity, which was previously derived using data obtained years apart. Our derived slopes are consistent with and thus support the general correlations between coronal and chromospheric/transition region emissions previously derived from nonsimultaneous measurements of a much larger sample of these variable sources. However, the importance of simultaneous measurements for accurate energy balance calculations is stressed.

Fox, David C.↗

NASA NTRS1984

The evolution of highly compact binary stellar systems in globular clusters

A highly compact binary represents a system which is composed of a collapsed object (degenerate dwarf, neutron star, or black hole) in orbit with a low-mass (equal to or less than 0.5 solar mass) secondary star. Matter may be transferred from the secondary to the collapsed star due to the decay of the orbit resulting from the emission of gravitational radiation. The present investigation has the objective to study quantitatively the evolution of highly compact binaries in globular cluster cores, subject to the interplay of gravitational radiation and collisions with field stars. The investigation is exploratory in nature. The numerical methods employed are based on the techniques developed by Rappaport et al. (1982). It is found that occasional close encounters with field stars strongly dominate the evolution of highly compact binaries in dense globular cluster cores. Attention is given to the applicability of the findings to observations of X-ray sources and cataclysmic variables.

Krolik, J. H.↗

NASA NTRS2017

Stability of Multi-Planet Systems in the Alpha Centauri System

We evaluate the extent of the regions within the alpha Centauri AB star system where small planets are able to orbit for billion-year timescales (Quarles & Lissauer 2016, Astron. J. 151, 111), as well as how closely-spaced planetary orbits can be within those regions in which individual planets can survive. Although individual planets on low inclination, low eccentricity, orbits can survive throughout the habitable zones of both stars, perturbations from the companion star imply that the spacing of planets in multi-planet systems within the habitable zones of each star must be significantly larger than the spacing of similar multi-planet systems orbiting single stars in order to be long-lived. Because the binary companion induces a forced eccentricity upon the orbits of planets in orbit around either star, appropriately-aligned circumstellar orbits with small initial eccentricities are stable to slightly larger initial semimajor axes than are initially circular orbits. Initial eccentricities close to forced eccentricities can have a much larger affect on how closely planetary orbits can be spaced, and therefore on how many planets may remain in the habitable zones, although the required spacing remains significantly higher than for planets orbiting single stars.

Alpha Centauri AB star system↗

NASA NTRS1974

Galactic X-ray sources

Data on galactic X-ray sources from rocket and balloon experiments and the UHURU satellite are reported. The outline emphasizes general discrete source models and begins with some a priori considerations about X-ray emission mechanisms and their applicability, after which contemporary data are interpreted according to these models. Topics covered include the production of X-rays by various mechanisms, source regimes, observational parameters, stellar distributions, individual stars, flare stars, supernovae and pulsars, the Crab nebula, old and young remnants, binary systems, and others.

Holt, S. S.↗

NASA NTRS1982

A statistical model of the photomultiplier gain process with applications to optical pulse detection

A Markov diffusion model was used to determine an approximate probability density for the random gain. This approximate density preserves the correct second-order statistics and appears to be in reasonably good agreement with experimental data. The receiver operating curve for a pulse counter detector of PMT cathode emission events was analyzed using this density. The error performance of a simple binary direct detection optical communication system was also derived. Previously announced in STAR as N82-25100

Tan, H. H.↗

NASA NTRS1989

A compact planetary nebula around the hot white dwarf EGB 6/PG 0950 + 139

The remarkable central star (0950 + 139), a very hot DA/DAO white dwarf, of the planetary nebula EGB 6 is described. Follow-up observations relevant to the analyses of both the nebula and the stellar photosphere are presented. Three kinds of scenarios are discussed to account for the existence of this peculiar nebula, but none appears very promising. The first consideration is that the nebula was ejected from the white dwarf as a discret event. This hypothesis is heavily constrained by the nebular size, density, and expansion rate; by the low luminosity and radius of the star; and by the absence of evidence for variation in density-sensitive forbidden lines from 1978 to 1987. No plausible mechanism can cause the observed amount of mass to be lost directly from a white dwarf in a steady or sporadic wind, at outflow velocities orders of magnitude below the escape velocity. Final consideration is given to the possibility that the gas is lost from a close companion star, but there is no evidence that this is a close binary system.

Liebert, James↗

NASA NTRS1991

Frontiers of stellar evolution

The present conference discusses theoretical and observational views of star formation, spectroscopic constraints on the evolution of massive stars, very low mass stars and brown dwarfs, asteroseismology, globular clusters as tests of stellar evolution, observational tests of stellar evolution, and mass loss from cool evolved giant stars. Also discussed are white dwarfs and hot subdwarfs, neutron stars and black holes, supernovae from single stars, close binaries with evolved components, accretion disks in interacting binaries, supernovae in binary systems, stellar evolution and galactic chemical evolution, and interacting binaries containing compact components.

Lambert, David L.↗

NASA NTRS1995

A potential cyclotron line signature in low-luminosity X-ray sources

Estimates indicate there may be greater than or approximately equal to 10(exp 3) low-luminosity X-ray pulsars (L less than or approximately equal to 10(exp 34) ergs/s) in the Galaxy undergoing 'low-state' wind accretion in Be/X-ray binary systems, and approximately 10(exp 8)-10(exp 9) isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures (kT(sub e) less than or approximately equal to 300 eV), low-luminosity accreting neutron stars with magnetic fields B approximately (0.7-7) x 10(exp 12) G could emit a substantial fraction (0.5%-5%) of their total luminosity in a moderately broadened (Epsilon/delta Epsilon approximately 2-4) cyclotron emission line which peaks in the energy range approximately 5-20 keV. The bulk of the thermal emission from these stars will be in the extreme ultraviolet/soft X-ray regime. In sharp contrast, the nonthermal cyclotron component predicted here will not be strongly absorbed, and consequently it may be the only distinguishing signature for the bulk of these low-luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations of the newly discovered candidate isolated neutron star MS 0317.7-6477, and the Be/X-ray transient pulsar 4U 0115+63 in its quiescent state. We note that an emission-like feature similar to the one we predict here has been reported in the energy spectrum of the unusual X-ray pulsar 1E 2259+586.

Nelson, Robert W.↗

NASA NTRS2014

The HEROES Balloon-Borne Hard X-Ray Telescope

The High Energy Replicated Optics to Explore the Sun (HEROES) payload flew on a balloon from Ft. Sumner, NM, September 21-22, 2013. HEROES is sensitive from about 20-75 keV and comprises 8 optics modules (HPD approximately 33" as flown), each consisting of 13-14 nickel replicated optics shells and 8 matching Xenon-filled position-sensitive proportional counter detectors (dE/E=0.05 @ 60 keV). Our targets included the Sun, the Crab Nebula and pulsar and the black hole binary GRS 1915+105. HEROES was pointed using a day/night star camera system for astrophysical observations and a newly developed Solar Aspect System for solar observations (with a shutter protecting the star camera.) We have successfully detected the Crab Nebula. Analyses for GRS 1915+105 and the Sun are ongoing. In this presentation, I will describe the HEROES mission, the data analysis pipeline and calibrations, preliminary results, and plans for follow-on missions.

Wilson-Hodge, C.↗

NASA NTRS2020

NuSTAR Observation of GROJ1744−28 at Low Mass Accretion Rate

Context.Neutron stars in low-mass X-ray binaries (LMXBs) are important systems to study the physics of accretion onto compact ob-jects. The system GRO J1744−28 is particularly interesting, as it usually shows clear pulsations as well as X-ray bursts. Additionally,there are claims for a magnetic field of 5×1011G through the detection of a Cyclotron Resonant Scattering Feature (CRSF).Aims.We present the spectral analysis of GRO J1744−28 using∼29 ks of NuSTAR data taken in 2017 February at a low luminosityof 3.2×1036erg s−1(3–50 keV). Our goal is to study the variability of the source spectrum with pulse phase and to search for the claimed CRSF. Methods.The continuum spectrum is modeled with an absorbed power-law with exponential cut-off, and an additional iron line component. We find no obvious indications for a CRSF and therefore perform a detailed cyclotron line search using statistical methods.We perform this search on pulse phase-averaged as well as phase-resolved spectra. Results.GRO J1744−28 was observed at one of the lowest luminosities studied so far. The previously detected Type II X-ray bursts re absent. Clear pulsations at a period of 2.141124(9) Hz are detected. The pulse profile shows an indication of a secondary peak,which was not seen at higher flux. The 4σupper limit for the strength of a CRSF in the 3–20 keV band is 0.07 keV, lower than the strength of the line found at higher luminosity. Conclusions.The detection of pulsations shows that the source did not enter the “propeller” regime, even though the source flux of4.15×10−10erg cm−2s−1was almost one order of magnitude below the threshold for the propeller regime claimed in previous studies on this source. The transition into the propeller regime in GRO J1744−28 must therefore be below a luminosity of 3.2×1036erg s−1(3–50 keV), which implies a surface magnetic field.2.9×1011G and mass accretion rate.1.7×1016g s−1. A change of the CRSF depth as function of luminosity is not unexpected and has been observed in other sources. This result possibly implies a change in emission geometry as function of mass accretion rate to reduce the depth of the line below our detection limit.

Ole König↗

NASA NTRS2022

An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing

We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled light curves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant >1 mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6–4.4 M ⨀ . This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact-object lens is relatively nearby (0.70–1.92 kpc) and has a slow transverse motion of <30 kms -1 . OB110462 shows significant tension between models well fit to photometry versus astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources, are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are <2M ⨀ , and they are unlikely to be black holes; two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (∼10 8 ) and find reasonable agreement given the small sample size.

Astrophysical black holes↗

NASA NTRS2001

Stellar and Binary Evolution in Star Clusters

This paper presents a final report on research activities covered on Stellar and Binary Evolution in Star Clusters. Substantial progress was made in the development and dissemination of the "Starlab" software environment. Significant improvements were made to "kira," an N-body simulation program tailored to the study of dense stellar systems such as star clusters and galactic nuclei. Key advances include (1) the inclusion of stellar and binary evolution in a self-consistent manner, (2) proper treatment of the anisotropic Galactic tidal field, (3) numerous technical enhancements in the treatment of binary dynamics and interactions, and (4) full support for the special-purpose GRAPE-4 hardware, boosting the program's performance by a factor of 10-100 over the accelerated version. The data-reduction and analysis tools in Starlab were also substantially expanded. A Starlab Web site (http://www.sns.ias.edu/-starlab) was created and developed. The site contains detailed information on the structure and function of the various tools that comprise the package, as well as download information, "how to" tips and examples of common operations, demonstration programs, animations, etc. All versions of the software are freely distributed to all interested users, along with detailed installation instructions.

McMillan, Stephen L. W.↗

NASA NTRS1983

Contact binary stars

Ultraviolet and X-ray surveys of the W Ursae Majoris type stars are reviewed. These systems exhibit extended coronas and transition regions that are confined close to the optically determined surfaces. Correlations of X-ray activity with period or rotational velocity indicate a turn-over or saturation of emission at the short periods or high velocities found in the W UMa-type systems. For a number of systems, ultraviolet emission appears to be anti-correlated with the strength of X-ray emission. These observations are discussed in terms of solar structures, activity, and evolution.

Dupree, A. K.↗

NASA NTRS2002

Studying X-Ray Binaries with High Energy Frequency Quasi-Periodic Oscillations

The goal of this investigation is to further our understanding of the dynamics of secreting neutron stars and black holes in the hope of using these systems as probes of the physics of strong gravitational fetus. The main focus of this work has been a multi-year program of simultaneous millisecond X-ray timing and spectral observations carried out with the Rossi X-Ray Timing Explorer (RXTE) to perform the X-ray timing and one of the satellites Asca, BeppoSAX, or Chandra to perform X-ray spectral measurements. With the advent of Chandra, we have extended our work to incLude extragalactic X-ray binaries. We conducted a comprehensive study of the X-ray and radio behavior of the Black Hole Candidate (BHC) X-ray transient XTE J1550-564 using RXTE, Chandra, and the Australian Telescope Compact Array (ATCA). We showed that strong radio emission is associated with major X-ray outbursts involving an X-ray state transition, while a compact radio jet is seen in the low/hard X-ray state found in the outburst decay. Interesting, the total energy required to produce the compact jet may be a substantial fraction of the total accretion energy of the system in that state. We also performed a detailed study of the spectral and timing properties of the decay. In joint RXTE/BeppoSAX observations of the neutron-star X-ray binary Cyg X-2, we discovered a correlation between the timing properties (the frequency of the horizontal branch oscillations) and the properties of a soft, thermal component of the X-ray spectrum. d e showed that more det<iiled measurements of this source may lead to constraints on models of the accretion flow and the emission of X- ray from accreting neutron stars. We have completed analysis of RXTE observations of the X-ray transient SAX J1750.8-2900 made after detection of X-ray bursts from the source with the BeppoSAX Wide-Field Camera. We discovered millisecond oscillations in both the persistent emission and in the X-ray bursts.

Kaaret, P.↗

NASA NTRS1993

VLBI astrometric identification of the radio emitting region in Algol and determination of the orientation of the close binary

A minute displacement of the radio source in Algol was measured by VLBI during two consecutive orbital revolutions of the close binary. The magnitude of the displacement unambiguously indicates that the less massive star of the close binary, a K subgiant, is the star responsible for the nonthermal radio emission of the system. This is consistent with the idea that the radio emission in Algol is related to the strong magnetic activity of the subgiant. The orientation and sense of the displacement on the sky that are directly deduced from our astrometric VLBI observations imply: (1) that the orbital plane of the close binary is at P.A. = +52 deg +/- 5 deg; and (2) that the sense of circulation of the close binary is clockwise, as seen on the sky. Thus, the long-period and close binary orbital motions are almost orthogonal and counterrevolving and this is relevant for evolution and dynamical studies.

Lestrade, Jean-Francois↗

NASA NTRS1995

The 2RE J0241-525: A Nearby Post T-Tauri Visual Binary System

We present high spatial resolution X-ray observations, photometry and spectroscopy of the two low mass, active stars proposed as optical counterparts to the extreme ultraviolet source 2RE J0241-525 (equal to EUVE J0241-530). It is confirmed that both stars, which are of types dK7e and dM3e and separated by 22 arcsecs, are sources of soft X-ray emission and exhibit substantial chromospheric activity. Radial velocity measurements indicate that the two components are physically associated and most probably single. The projected equatorial velocities are measured as (75 +/- 3) km/s and (11.7 +/- 0.7) km/s for the hotter and cooler components, respectively, and whilst the hotter component has a relatively high photospheric lithium abundance, log N(Li) equal to 1.5 +/- 0.2, we are unable to detect any lithium in the cooler star. Isochrone fitting to this 'mini-cluster' yields an age of (3-70) Myr and a distance of (19-60) pc. An empirical comparison of the lithium abundances with those for similar stars in young clusters and associations narrows this age range to (5-30) Myr and a corresponding distance of (26-50) pc. We conclude that this object is a nearby post T-Tauri system, but we cannot locate any possible birth site. It appears unlikely that the system can have been ejected from a nearby open cluster in a two or three body encounter.

Jeffries, R. D.↗

NASA NTRS1994

Astronomical observations of phenomena in disks

Astronomical constraints on disks around young stellar objects are reviewed in this abstract. Since disks around the youngest stars cannot generally be resolved spatially, most results are based on analyses of the infrared and radio emission from circumstellar dust and gas. At least half of all young stars seem to have substantial circumstellar disks at an age of approximately 10(exp 6) yr. The young objects without disks tend to be binary stellar systems, though not exclusively, and some close binaries may have outer disks that are not tidally disrupted. Probably most, if not all, stars formed originally with circumstellar disks, but evolutionary timescales may differ dramatically. All the results, disk disappearance, disk mass around young stars, and long-wavelength dust opacities, are subject to the additional uncertainty of the effects of infalling dusty envelopes, which produce the disks in the first place.

Hartmann, L.↗