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At least 685 records · Page 38
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NASA NTRS1992

Binaries in globular clusters

Recent observations have shown that globular clusters contain a substantial number of binaries most of which are believed to be primordial. We discuss different successful optical search techniques, based on radial-velocity variables, photometric variables, and the positions of stars in the color-magnitude diagram. In addition, we review searches in other wavelengths, which have turned up low-mass X-ray binaries and more recently a variety of radio pulsars. On the theoretical side, we give an overview of the different physical mechanisms through which individual binaries evolve. We discuss the various simulation techniques which recently have been employed to study the effects of a primordial binary population, and the fascinating interplay between stellar evolution and stellar dynamics which drives globular-cluster evolution.

Hut, Piet↗
NASA NTRS1993

Bright near-infrared sources within 1 deg of the Galactic center. I - Survey and 1-20 micron photometry

Results of a near-IR survey of 0.55 sq deg toward the Galactic center are reported. Additional IR photometry of 50 objects found in this survey was made in order to investigate the nature of luminous stars in the central region of the Milky Way including all sources with K less than 7.6 and H-K not less than 1.4. In addition to candidates for normal M-type stars and long-period variables, four objects whose energy spectra peak at about 5 microns have been detected in a small region around (l, b) = (0.15, 0.0 deg), near the crossing of the 6-cm radio arc with the Galactic plane. These four might be young stellar objects near the Galactic center. It is suggested that the relative depth change in the silicate absorption is localized to a fairly small region around the Galactic center.

Nagata, Tetsuya↗
NASA NTRS1995

X-ray emission from the PSR B1259-63 system near apastron

The PSR B1259-63 system contains a 47 ms radio pulsar in a highly eccentric binary with a Be-star companion. Strongly time-variable X-ray emission was reported from this system as the pulsar was near apastron in 1992- early 1993. The variability was primarily deduced from an apparent nondetection of the PSR B1259-63 system during a first preapastron ROSAT observation in 1992 February. We have reanalyzed the ROSAT observations of the PSR B1259-63 system. Contrary to the results of a previous analysis, we find that the PSR B1259-63 system was detected by ROSAT during the first off-axis 1992 February observation. The intensity of the soft X-ray emission of the PSR B1259-63 system before and after the 1992 apastron appears to vary at most by a factor of approx. 2. Our results sensibly constrain theoretical models of X-ray emission from the PSR B1259-63 system.

Greiner, J.↗
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↗
NASA NTRS1992

An astrometric study of the Mira variable R Leonis and the occultation binary 19 Leonis

The Multichannel Astrometric Photometer (MAP) was used to determine parallaxes of stars in the region centered on the Mira variable R Leonis. R Leonis, with a parallax of 8.3 +/- 1.0 mas (120 +/- 15 pc), is significantly closer than most recent estimates. At visual wavelengths, its mean physical diameter is 612 +/- 96 times that of the sun. The photocentric motion of R Leonis is not suggestive of either optical or orbital companions, excluding possible secondary components over a wide range of magnitudes as well as orbital periods of up to approximately one decade. Nineteen Leonis, an occultation-speckle binary with a well-determined orbit and a reference star in the R Leonis field, has a distance of 92.6 +/- 12 pc. The 15.2 yr period binary exhibits a slight perturbation during the 4 yr of MAP observations. The similar primary and secondary of this A7 Vn system are 2.41 +/- 0.96 and 2.31 +/- 0.92 solar masses, respectively.

Gatewood, George↗
NASA NTRS1992

Interrogation of duplicitous stars with an APT

Preliminary results from intensive spectroscopic and APT monitoring of two interacting binary systems are presented. Both V644 Mon (Be + K:) and HD 37453 (F5 II + B) show complex, composite, and variable spectral. APT observations extending over three years show both stars to vary by 0.1-0.2 mag in V. The photometric variability of V644 Mon appears to be irregular, though there is some evidence for periodic behavior in the 50-60 day range. HD 37453 has an orbital period of 66.75 days; the best-fit photometric period is not quite half this value, indicating the star is an ellipsoidal variable.

Bopp, Bernard W.↗
NASA NTRS2024

Short–Period Variables in TESS Full–Frame Image Light Curves Identified via Convolutional Neural Networks

The Transiting Exoplanet Survey Satellite (TESS) mission measured light from stars in ∼85% of the sky throughout its 2 yr primary mission, resulting in millions of TESS 30-minute-cadence light curves to analyze in the search for transiting exoplanets. To search this vast data set, we aim to provide an approach that is computationally efficient, produces accurate predictions, and minimizes the required human search effort. We present a convolutional neural network that we train to identify short-period variables. To make a prediction for a given light curve, our network requires no prior target parameters identified using other methods. Our network performs inference on a TESS 30-minute-cadence light curve in ∼5 ms on a single GPU, enabling large-scale archival searches. We present a collection of 14,156 short-period variables identified by our network. The majority of our identified variables fall into two prominent populations, one of close-orbit main-sequence binaries and another of δ Scuti stars. Our neural network model and related code are additionally provided as open-source code for public use and extension.

Convolutional neural networks↗
NASA NTRS1995

A deep PSPC obervation of the Cyg OB2 association

The Cyg OB2 Association contains six of the fifteen most luminous stars in the Galaxy as well as the most luminous and most heavily reddened Galactic star, Cyg OB2 #12. In addition, the association contains a number of Wolf-Rayet stars, at least one early-type binary (Cyg OB2 #5), and a trapezium-like system (Cyg OB2 #8). Though not physically associated with the association, the X-ray source Cyg X-3 lies within a half degree of the center of the association. The association is a known source of X-rays. The X-ray emission from this region includes a contribution from Cyg X-3, as well as coronal emission from OB stars and hot diffuse gas. The region was extensively sampled by the HEAO-2 IPC (136 ks, which includes a single pointing of 57 ks on the central region of the association). A 3.6 ks PSPC observation of this region was obtained in the AO1 pointing phase. In addition, the four brightest OB stars (Cyg OB2 #5, #8A, #9, and #12) have been extensively monitored with the VLA. These stars show evidence of strong radio variability, and nonthermal radio emission characteristics. Several of these radio observations were taken within about a month of an IPC observation, and the two PSPC observations. We obtained a 19.5 ks exposure of the Cyg OB2 Association using the ROSAT PSPC instrument. Scientific tasks included: (1) extracting the PSPC spectral distributions of the brightest OB stars; (2) comparing the 19.5 ks spectral data with the 3.6 ks spectral data; (3) investigating the long term X-ray variability (15 years); and (4) comparing the X-ray and radio variability.

Waldron, Wayne↗
DOE OSTI2021

Stellar migration and chemical enrichment in the milky way disc: a hybrid model

ABSTRACT We develop a hybrid model of galactic chemical evolution that combines a multiring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc’s abundance structure: (i) the dependence of the [O/Fe]–[Fe/H] distribution on radius Rgal and mid-plane distance |z|; (ii) the changing shapes of the [O/H] and [Fe/H] distributions with Rgal and |z|; (iii) a broad distribution of [O/Fe] at sub-solar metallicity and changes in the [O/Fe] distribution with Rgal, |z|, and [Fe/H]; (iv) a tight correlation between [O/Fe] and stellar age for [O/Fe] > 0.1; (v) a population of young and intermediate-age α-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age–[O/H] and age–[Fe/H] relations, with large scatter and a median age of ∼4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate ∼2 Gyr ago improve agreement with the observed age–[Fe/H] and age–[O/H] relations, but worsen agreement with the observed age–[O/Fe] relation. None of our models predict an [O/Fe] distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice).

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2005

Using Beta Cen to Probe the Photosphere-Wind Connection to B Stars

The goal of the project was to probe the connection between the photospheric pulsations and the X-ray production in the stellar winds of B stars. The B1 III star Beta Centauri is a Beta Cephei variable with a high X-ray count rate, and was observed by XMM. Analysis of XMM X-ray spectra showed characteristic plasma temperatures of 0.1, 0.2 and 0.6 keV. Analysis of He-like forbidden-to-intercombination lines was performed to determine the dominant locations of the X-ray emission. At Prism, work focused on developing and upgrading spectral models to calculate forbidden-to-intercombination line ratios using more detailed atomic models than the simple models typically used in the analysis of X-ray spectral lines, thereby allowing us to assess the effects of a broader variety of populating and depopulating channels for energy levels contributing to the observed X-ray line emission. Based on Ne IX lines, the source location was determined to be no farther than 4 stellar radii. Examination of the time variability of the XMM data for Beta Cen showed no significant correlation with the known optical pulsation period for the star, and also did not find any significant periods of variation at X-ray wavelengths.

MacFarlane, Joseph J.↗
NASA NTRS2021

Spectroscopic Signatures of the Vanishing Natural Coronagraph of eta Carinae

Eta Carinae is a massive interacting binary system shrouded in a complex circumstellar environment whose evolution is the source of the long-term brightening observed during the last 80 years. An occulter, acting as a natural coronagraph, impacts observations from our perspective, but not from most other directions. Other sight-lines are visible to us through studies of the Homunculus reflection nebula. The coronagraph appears to be vanishing, decreasing the extinction towards the central star, and causing the star's secular brightening. In contrast, the Homunculus remains at an almost constant brightness. The coronagraph primarily suppresses the stellar continuum, to a lesser extent the wind lines, and not the circumstellar emission lines. This explains why the absolute values of equivalent widths (EWs) of the emission lines in our direct view are larger than those seen in reflected by the Homunculus, why the direct view absolute EWs are decreasing with time, and why lower-excitation spectral wind lines formed at larger radii (e.g Fe ii 4585A) decrease in intensity at a faster pace than higher excitation lines that form closer to the star (e.g. Hdelta). Our main result is that the star, despite its 10-fold brightening over two decades, is relatively stable. A vanishing coronagraph that can explain both the large flux evolution and the much weaker spectral evolution. This is contrary to suggestions that the long-term variability is intrinsic to the primary star that is still recovering from the Great Eruption with a decreasing mass-loss rate and a polar wind that is evolving at a slower pace than at the equator.

A. Damineli↗
NASA NTRS1985

Evidence for supermassive stars in three Seyfert galaxy nuclei from IUE spectra

The variability in the ultraviolet continua and the C IV 1550 A emission profiles in the three Seyfert 1 galaxies NGC 4151, NGC 5548, and Fairall 9 are consistent with origin in supermassive stars undergoing explosive mass loss. Blackbody fits to continua of each imply a single thermal source whose area either remains constant or increases moderately as its temperature decreases, with the surface area consistent with stars having masses from 10 to the 6th to 10 to the 8th solar mass. Line emission profiles of C IV 1550 A in the same spectra are individually consistent with uniform expansion of shells of emitting gas expelled from the stars at radial velocities from 10,000 to 16,000 km/s. The kinetic energy removed by repetitive production of such massive shells approximately balances the nuclear energy production expected from supermassive stars of the indicated size. Finally, the observed nature and time scale of the variability of NGC 4151 is consistent with the relatively sudden appearance, and subsequent cooling, of the underlying superstellar surface as one such shell becomes transparent during its expansion.

Stoner, R.↗
NASA NTRS1984

Hot subdwarfs in detached binary systems and thick-disk cataclysmic variables from the Palomar-Green survey

A total of 19 stars from the Palomar Green (PG) survey which were found to show composite spectra, generally with a hot subdwarf primary and a cool main-sequence secondary star, are discussed. The primary components have Teff values between 27,000 and 38,000 K (Mv = +5 to +7) and log g = about 6.5, while the secondaries generally show spectral types G8 to K4.5; however, these ranges are undoubtedly limited by selection effects, and extremely helium-rich subdwarfs were excluded as well. Aproximately half of all the PG subdwarfs could be in undetected binaries. Only two thick-disk cataclysmic variables with predominantly absorption-line spectra were found in the PG survey, which provides strong evidence that this kind of cataclysmic variable is luminous and confined to the Galactic plane (Population I). On the other hand, a few of the detached binaries show evidence for spectroscopic variations. Those which are close enough for a significant reflection effect on the secondary star may be progenitors to cataclysmic variable systems.

Ferguson, D. H.↗
NASA NTRS1994

The ensemble variability properties of faint QSOs

A refined sample of 64 variable objects with stellar image structure has been identified in SA 57 to B approximately = 22.5, over a time baseline of 15 yr, sampled at 11 distinct epochs. The photometric data typically have a root-mean-square (rms) error at B = 22 of only 0.05 mag. Thirty-five quasars in this field have already been spectroscopically confirmed, 34 of which are among the sample of variables. Of the other variables, six are known spectroscopically to be stars, 10 additional objects are stars based on reliable detection of proper motion, and one is spectroscopically a narrow emission line galaxy. Of the 13 remaining variables, it is argued that they are a mixture of distant halo subdwarfs and quasars with starlike colors. We compute the ensemble average structure function and autocorrelation function from the light curves in the respective quasar rest frames, which are used to investigate the general dependences on apparent magnitude, absolute magnitude, and redshift.

Trevese, Dario↗
NASA NTRS1985

Stellar activity cycles

The variety of stellar chromospheric and coronal activity provides a framework for understanding solar and stellar magnetic activity. It is pointed out that the recognition of an 11-yr periodicity within the record of sunspot numbers is a relatively recent discovery of just over a century ago. The solar activity cycle is examined, taking into account the sunspot cycle, and solar dynamo models. Time-averaged stellar chromospheric activity levels are considered along with long-term activity fluctuations of cool stars. Attention is given to the direct measurement of stellar magnetic fields, solar luminosity variations, the RS Canum Venaticorum and BY Draconis variables, continuum variations in lower main sequence stars, chromospheric variations in lower main sequence stars, and chromospheric variations in evolved stars.

Baliunas, S. L.↗
NASA NTRS1995

Spectral and Temporal Characteristics of X-Ray-Bright Stars in the Pleiades

We follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond & Smith (1977) one and two-temperature models have been used to fit the position-sensitive proportional counter (PSPC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L(sub x)/L(sub bol). A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of approx. 10(exp 30) ergs/sec. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of approx. 18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.

Gagne, Marc↗
NASA NTRS1979

Einstein observations of the Orion Nebula

A single 3.5 hour observation on 1979 March 6 with the imaging proportional counter aboard the Einstein Observatory has revealed the presence of numerous low-luminosity (10 to the 31st ergs/s) X-ray sources in a region centered near the Trapezium in the Orion Nebula. In addition to the Trapezium, 22 discrete sources have been detected in a 1 x 1 deg field at or above the 4 sigma level. These sources are clearly associated with star formation, as indicated by the similarity of their spatial distribution with that of the nebular variables, a class of pre-main-sequence stars. Because the field is quite crowded, however, individual identifications of these sources are difficult. In addition to the discrete sources, there is evidence for diffuse or unresolved emission from two ridges extending 16 deg to the north and 15 deg to the west of the Trapezium. The nature of the discrete sources and of the diffuse emission is discussed.

Ku, W. H.-M.↗
NASA NTRS1990

Photometry of variable AFGL sources

Results of infrared photometric observations of 63 AFGL sources over the past 9 years are presented. Using these data together with previous measurements of these stars, pulsation periods and mean photometric characteristics are determined. These stars are found to lie midway between optically identified Mira variables and the radio-luminous OH/IR stars in their period distribution and photometric properties. For the sample as a whole, there is no evidence for sudden or transient behavior such as a switch in pulsation mode. Rather, these stars suggest rapid, but continuous, evolution from shorter period Miras with weak mass loss to longer periods and larger mass-loss rates. The carbon-rich stars in the sample have the same period distribution as the oxygen-rich stars. None of the carbon stars have periods as long as those of the very long period radio-luminous OH/IR stars.

Jones, Terry Jay↗