arXiv subjects

M. Virginia McSwain

Publications and source records attributed to M. Virginia McSwain.

35 records · Page 2Linked to original sources

arXiv2010

The Membership and Distance of the Open Cluster Collinder 419

The young open cluster Collinder 419 surrounds the massive O star, HD 193322, that is itself a remarkable multiple star system containing at least four components. Here we present a discussion of the cluster distance based upon new spectral classifications of the brighter members, UBV photometry, and an analysis of astrometric and photometric data from the UCAC3 and 2MASS catalogs. We determine an average cluster reddening of E(B-V)=0.37 +- 0.05 mag and a cluster distance of 741 +- 36 pc. The cluster probably contains some very young stars that may include a reddened M3 III star, IRAS~20161+4035.

astro-ph.GA↗

arXiv2009

Multiwavelength Observations of the Runaway Binary HD 15137

HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations.

astro-ph.SR↗

arXiv2009

Analysis of B and Be Star Populations of the Double Cluster h and chi Persei

We present blue optical spectra of 92 members of h and chi Per obtained with the WIYN telescope at Kitt Peak National Observatory. From these spectra, several stellar parameters were measured for the B-type stars, including V sin i, T_eff, log g_polar, M_star, and R_star. Stromgren photometry was used to measure T_eff and log g_polar for the Be stars. We also analyze photometric data of cluster members and discuss the near-to-mid IR excesses of Be stars.

astro-ph.SR↗

arXiv2009

Non-radial Pulsations in the Open Cluster NGC 3766

Non-radial pulsations (NRPs) are a proposed mechanism for the formation of decretion disks around Be stars and are important tools to study the internal structure of stars. NGC 3766 has an unusually large fraction of transient Be stars, so it is an excellent location to study the formation mechanism of Be star disks. High resolution spectroscopy can reveal line profile variations from NRPs, allowing measurements of both the degree, l, and azimuthal order, m. However, spectroscopic studies require large amounts of time with large telescopes to achieve the necessary high S/N and time domain coverage. On the other hand, multi-color photometry can be performed more easily with small telescopes to measure l only. Here, we present representative light curves of Be stars and non-emitting B stars in NGC 3766 from the CTIO 0.9m telescope in an effort to study NRPs in this cluster.

astro-ph.SR↗

arXiv2009

HD 69686: A Mysterious High Velocity B Star

We report on the discovery of a high velocity B star, HD 69686. We estimate its space velocity, distance, surface temperature, gravity, and age. With these data, we are able to reconstruct the trajectory of the star and to trace it back to its birthplace. We use evolutionary tracks for single stars to estimate that HD 69686 was born 73 Myr ago in the outer part of our Galaxy ($r \sim 12$ kpc) at a position well below the Galactic plane ($z \sim -1.8$ kpc), a very unusual birthplace for a B star. Along the star's projected path in the sky, we also find about 12 other stars having similar proper motions, and their photometry data suggest that they are located at the same distance as HD 69686 and probably have the same age. We speculate on the origin of this group by star formation in a high velocity cloud or as a Galactic merger fragment.

astro-ph.SR↗

arXiv2009

Variability of Be Stars in Southern Open Clusters

We recently discovered a large number of highly active Be stars in the open cluster NGC 3766, making it an excellent location to study the formation mechanism of Be star disks. To explore whether similar disk appearances and/or disappearances are common among the Be stars in other open clusters, we present here multiple epochs of H-alpha spectroscopy for 296 stars in eight open clusters. We identify 12 new transient Be stars and confirm 17 additional Be stars with relatively stable disks. By comparing the H-alpha equivalent widths to the photometric y - H-alpha colors, we present a method to estimate the strength of the H-alpha emission when spectroscopy is not available. For a subset of 128 stars in four open clusters, we also use blue optical spectroscopy and available Stromgren photometry to measure their projected rotational velocities, effective temperatures, and polar surface gravities. We combine our Be star detections from these four clusters to investigate physical differences between the transient Be stars, stable Be stars, and normal B-type stars with no line emission. Both types of Be stars are faster rotating populations than normal B-type stars, and we find no significant physical differences between the transient and stable Be stars in our sample.

astro-ph.SR↗

arXiv2009

The Orbits of the Gamma-ray Binaries LS I +61 303 and LS 5039

LS I +61 303 and LS 5039 are two of only a handful of known high mass X-ray binaries (HMXBs) that exhibit very high energy emission in the MeV-TeV range, and these "gamma-ray binaries" are of renewed interest due to the recent launch of the Fermi Gamma-ray Space Telescope. Here we present new radial velocities of both systems based on recent red and blue optical spectra. Both systems have somewhat discrepant orbital solutions available in the literature, and our new measurements result in improved orbital elements and resolve the disagreements. The improved geometry of each orbit will aid in studies of the high energy emission region near each source.

astro-ph.HE↗

arXiv2009

Research Science and Education: The NSF's Astronomy and Astrophysics Postdoctoral Fellowship

The NSF's Astronomy and Astrophysics Postdoctoral Fellowship (AAPF) is exceptional among the available postdoctoral awards in Astronomy and Astrophysics. The fellowship is one of the few that allows postdoctoral researchers to pursue an original research program, of their own design, at the U.S. institution of their choice. However, what makes this fellowship truly unique is the ability of Fellows to lead an equally challenging, original educational program simultaneously. The legacy of this singular fellowship has been to encourage and advance leaders in the field who are equally as passionate about their own research as they are about sharing that research and their passion for astronomy with students and the public. In this positional paper we address the importance of fellowships like the AAPF to the astronomical profession by identifying the science and educational contributions that Fellows have made to the community. Further, we recommend that fellowships that encourage leading postdoctoral researchers to also become leaders in Astronomy education be continued and expanded.

astro-ph.IM↗

arXiv2008

Detection of Magnetic Massive Stars in the Open Cluster NGC 3766

A growing number of observations indicate that magnetic fields are present among a small fraction of massive O- and B-type stars, yet the origin of these fields remains unclear. Here we present the results of a VLT/FORS1 spectropolarimetric survey of 15 B-type members of the open cluster NGC 3766. We have detected two magnetic B stars in the cluster, including one with a large field of nearly 2 kG, and we find marginal detections of two additional stars. There is no correlation between the observed longitudinal field strengths and the projected rotational velocity, suggesting that a dynamo origin for the fields is unlikely. We also use the Oblique Dipole Rotator model to simulate populations of magnetic stars with uniform or slightly varying magnetic flux on the ZAMS. None of the models successfully reproduces our observed range in B_l and the expected number of field detections, and we rule out a purely fossil origin for the observed fields.

astro-ph↗

arXiv2008

Detection of a Hot Subdwarf Companion to the Be Star FY Canis Majoris

The rapid rotation of Be stars may be caused in some cases by past mass and angular momentum accretion in an interacting binary in which the mass donor is currently viewed as a small, hot subdwarf stripped of its outer envelope. Here we report on the spectroscopic detection of such a subdwarf in the Be binary system FY Canis Majoris from the analysis of data acquired by the IUE spacecraft and KPNO Coude Feed Telescope over the course of 16 and 21 years, respectively. We present a double-lined spectroscopic orbit for the binary based upon radial velocities from the IUE spectra and use the orbital solutions with a Doppler tomography algorithm to reconstruct the components' UV spectra. The subdwarf is hot (T_eff = 45+/-5 kK) and has a mass of about 1.3 M_sun and a radius of about 0.6 R_sun. It contributes about 4% as much flux as the Be star does in the FUV. We also present observations of the H-alpha and He I 6678 emission features that are formed in the circumstellar disk of the Be star. Orbital flux and velocity variations in the He I 6678 profile indicate that much of the emission forms along the disk rim facing the hot subdwarf where the disk is probably heated by the incident radiation from the subdwarf. A study of the FUV infall shell lines discovered in the 1980s confirms their episodic presence but reveals that they tend to be found around both quadrature phases, unlike the pattern in Algol binaries. Phase-dependent variations in the UV N V doublet suggest the presence of a N-enhanced wind from the subdwarf and a possible shock-interaction region between the stars where the subdwarf's wind collides with the disk of the Be star.

astro-ph↗

arXiv2007

The B and Be Star Population of NGC 3766

We present multiple epochs of H-alpha spectroscopy for 47 members of the open cluster NGC 3766 to investigate the long term variability of its Be stars. Sixteen of the stars in this sample are Be stars, including one new discovery. Of these, we observe an unprecedented 11 Be stars that undergo disk appearances and/or near disappearances in our H-alpha spectra, making this the most variable population of Be stars known to date. NGC 3766 is therefore an excellent location to study the formation mechanism of Be star disks. From blue optical spectra of 38 cluster members and existing Stromgren photometry of the cluster, we also measure rotational velocities, effective temperatures, and polar surface gravities to investigate the physical and evolutionary factors that may contribute to the Be phenomenon. Our analysis also provides improvements to the reddening and distance of NGC 3766, and we find E(B-V) = 0.22 +/- 0.03 and (V-M_V)_0 = 11.6 +/- 0.2, respectively. The Be stars are not associated with a particular stage of main-sequence evolution, but they are a population of rapidly rotating stars with a velocity distribution generally consistent with rotation at 70-80% of the critical velocity, although systematic effects probably underestimate the true rotational velocities so that the rotation is much closer to critical. Our measurements of the changing disk sizes are consistent with the idea that transitory, nonradial pulsations contribute to the formation of these highly variable disks.

astro-ph↗

arXiv2006

Runaway Massive Binaries and Cluster Ejection Scenarios

The production of runaway massive binaries offers key insights into the evolution of close binary stars and open clusters. The stars HD 14633 and HD 15137 are rare examples of such runaway systems, and in this work we investigate the mechanism by which they were ejected from their parent open cluster, NGC 654. We discuss observational characteristics that can be used to distinguish supernova ejected systems from those ejected by dynamical interactions, and we present the results of a new radio pulsar search of these systems as well as estimates of their predicted X-ray flux assuming that each binary contains a compact object. Since neither pulsars nor X-ray emission are observed in these systems, we cannot conclude that these binaries contain compact companions. We also consider whether they may have been ejected by dynamical interactions in the dense environment where they formed, and our simulations of four-body interactions suggest that a dynamical origin is possible but unlikely. We recommend further X-ray observations that will conclusively identify whether HD 14633 or HD 15137 contain neutron stars.

astro-ph↗

arXiv2006

A Spectroscopic Study of Field and Runaway OB Stars

Identifying binaries among runaway O- and B-type stars offers valuable insight into the evolution of open clusters and close binary stars. Here we present a spectroscopic investigation of 12 known or suspected binaries among field and runaway OB stars. We find new orbital solutions for five single-lined spectroscopic binaries (HD 1976, HD 14633, HD 15137, HD 37737, and HD 52533), and we classify two stars thought to be binaries (HD 30614 and HD 188001) as single stars. In addition, we reinvestigate their runaway status using our new radial velocity data with the UCAC2 proper motion catalogs. Seven stars in our study appear to have been ejected from their birthplaces, and at least three of these runaways are spectroscopic binaries and are of great interest for future study.

astro-ph↗

arXiv2005

NGC 6791: an exotic open cluster or the nucleus of a tidally disrupted galaxy?

We report on high resolution Echelle spectroscopy of 20 giant stars in the Galactic old open clusters NGC 6791 obtained with Hydra at the WIYN telescope. High precision radial velocity allow us to isolate 15 {\it bona fide} cluster members. From 10 of them we derive a global [M/H]=+0.39$\pm$0.05. We therefore confirm that NGC 6791 is extremely metal rich, exhibits a few marginally sub-solar abundance ratios, and within the resolution of our spectra does not show evidences of spread in metal abundance. With these new data we re-derive the cluster fundamental parameters suggesting that it is about 8 Gyr old and 4.3 kpc far from the Sun. The combination of its chemical properties, age, position, and Galactic orbit hardly makes NGC 6791 a genuine Population I open cluster. We discuss possible interpretations of the cluster peculiarities suggesting that the cluster might be what remains of a much larger system, whose initial potential well could have been sufficient to produce high metallicity stars, and which has been depopulated by the tidal field of the Galaxy. Alternatively, its current properties may be explained by the perturbation of the Galactic bar on an object originated well inside the solar ring, where the metal enrichment had been very fast.

astro-ph↗

arXiv2005

Binary and Multiple O-Type Stars in the Cas OB6 Association

We present the results of time-resolved spectroscopy of 13 O-type stars in the Cas OB6 stellar association. We conducted a survey for radial velocity variability in search of binary systems, which are expected to be plentiful in young OB associations. Here we report the discovery of two new single-lined binaries, and we present new orbital elements for three double-lined binaries (including one in the multiple star system HD 17505). One of the double-lined systems is the eclipsing binary system DN Cas, and we present a preliminary light curve analysis that yields the system inclination, masses, and radii. We compare the spectra of the single stars and the individual components of the binary stars with model synthetic spectra to estimate the stellar effective temperatures, gravities, and projected rotational velocities. We also make fits of the spectral energy distributions to derive E(B-V), R=A_V/E(B-V), and angular diameter. A distance of 1.9 kpc yields radii that are consistent with evolutionary models. We find that 7 of 14 systems with spectroscopic data are probable binaries, consistent with the high binary frequency found for other massive stars in clusters and associations.

astro-ph↗

arXiv2005

The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters

Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. Of these, five clusters are probably not physically associated groups and our results for two other clusters are not reliable, but we identify 52 definite Be stars and an additional 129 Be candidates in the remaining clusters. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age MS (ZAMS). We show that a spin-up phase at the terminal-age MS (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation. Our results indicate a possible increase in the fraction of Be stars with increasing cluster distance from the Galactic center (in environments of decreasing metallicity). However, the trend is not significant and could be ruled out due to the intrinsic scatter in our data. We also find no relationship between the fraction of Be stars and cluster density.

astro-ph↗

arXiv2004

A Photometric Technique to Search for Be Stars in Open Clusters

We describe a technique to identify Be stars in open clusters using Stromgren b, y, and narrow-band Halpha photometry. We first identify the B-type stars of the cluster using a theoretical isochrone fit to the (b-y, y) color-magnitude diagram. The strongest Be stars are easily identified in a (b-y, y-Halpha) color-color diagram, but those with weaker Halpha emission (classified as possible Be star detections) may be confused with evolved or foreground stars. Here we present such photometry plus Halpha spectroscopy of members of the cluster NGC 3766 to demonstrate the accuracy of our technique. Statistical results on the relative numbers of Be and B-type stars in additional clusters will be presented in a future paper.

astro-ph↗