32 research outputs found
H-alpha Emission Variability in the gamma-ray Binary LS I +61 303
LS I +61 303 is an exceptionally rare example of a high mass X-ray binary
(HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of
"gamma-ray binaries". Here we present H-alpha spectra that show strong
variability during the 26.5 day orbital period and over decadal time scales. We
detect evidence of a spiral density wave in the Be circumstellar disk over part
of the orbit. The H-alpha line profile also exhibits a dramatic emission burst
shortly before apastron, observed as a redshifted shoulder in the line profile,
as the compact source moves almost directly away from the observer. We
investigate several possible origins for this red shoulder, including an
accretion disk, mass transfer stream, and a compact pulsar wind nebula that
forms via a shock between the Be star's wind and the relativistic pulsar wind.Comment: Accepted to Ap
The B and Be star population of NGC 3766
We present multiple epochs of Hα 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 Hα 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 Strömgren 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 - MV )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
The ejection of runaway massive binaries
The runaway O-type stars HD 14633 and HD 15137 are both SB1 systems that were probably ejected from the open cluster NGC 654. Were these stars dynamically ejected by close gravitational encounters in the dense cluster, or did the binaries each receive a kick from a supernova in one member? We present new results from our investigation of the optical, X-ray, and radio properties of these binary systems to discuss the probable ejection scenarios. We argue that these binaries may have been ejected via dynamical interactions in the dense cluster environment. © 2007 International Astronomical Union
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
Estimating Be Star Disk Radii using H-alpha Emission Equivalent Widths
We present numerical models of the circumstellar disks of Be stars, and we
describe the resulting synthetic H-alpha emission lines and maps of the
wavelength-integrated emission flux projected onto the sky. We demonstrate that
there are monotonic relationships between the emission line equivalent width
and the ratio of the angular half-width at half maximum of the projected disk
major axis to the radius of the star. These relationships depend mainly upon
the temperatures of the disk and star, the inclination of the disk normal to
the line of sight, and the adopted outer boundary for the disk radius. We show
that the predicted H-alpha disk radii are consistent with those observed
directly through long baseline interferometry of nearby Be stars (especially
once allowance is made for disk truncation in binaries and for dilution of the
observed H-alpha equivalent width by continuum disk flux in the V-band).Comment: 12 pages, 2 figures, ApJL in pres
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.Comment: Accepted to ApJ, 13 pages, preprint2 styl
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.Comment: Accepted to ApJ, 11 page