420 research outputs found
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.Comment: Accepted to ApJ; small corrections to Table 5 and associated figure
A Stellar Rotation Census of B Stars: from ZAMS to TAMS
Two recent observing campaigns provide us with moderate dispersion spectra of
more than 230 cluster and 370 field B stars. Combining them and the spectra of
the B stars from our previous investigations (430 cluster and 100
field B stars) yields a large, homogeneous sample for studying the rotational
properties of B stars. We derive the projected rotational velocity ,
effective temperature, gravity, mass, and critical rotation speed for each star. We find that the average is significantly lower
among field stars because they are systematically more evolved and spun down
than their cluster counterparts. The rotational distribution functions of
for the least evolved B stars show that lower mass B
stars are born with a larger proportion of rapid rotators than higher mass B
stars. However, the upper limit of that may separate
normal B stars from emission line Be stars (where rotation promotes mass loss
into a circumstellar disk) is smaller among the higher mass B stars. We compare
the evolutionary trends of rotation (measured according to the polar gravity of
the star) with recent models that treat internal mixing. The spin-down rates
observed in the high mass subset () agree with predictions, but
the rates are larger for the low mass group (). The faster spin
down in the low mass B stars matches well with the predictions based on
conservation of angular momentum in individual spherical shells. Our results
suggest the fastest rotators (that probably correspond to the emission line Be
stars) are probably formed by evolutionary spin up (for the more massive stars)
and by mass transfer in binaries (for the full range of B star masses).Comment: 44 pages, 10 figures, accepted for publication in Ap
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.Comment: Accepted to A
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.Comment: 4 pages, to appear in the proceedings of IAU Symposium 266: Star
Cluster
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.Comment: 4 pages, to appear in the proceedings of IAU Symposium 266: Star
Cluster
Multiwavelength Observations of Gamma-ray Binary Candidates
A rare group of high mass X-ray binaries (HMXBs) are known that also exhibit
MeV, GeV, and/or TeV emission ("gamma-ray binaries"). Expanding the sample of
gamma-ray binaries and identifying unknown Fermi sources are currently of great
interest to the community. Based upon their positional coincidence with the
unidentified Fermi sources 1FGL J1127.7-6244c and 1FGL J1808.5-1954c, the Be
stars HD 99771 and HD 165783 have been proposed as gamma-ray binary candidates.
During Fermi Cycle 4, we have performed multiwavelength observations of these
sources using XMM-Newton and the CTIO 1.5m telescope. We do not confirm high
energy emission from the Be stars. Here we examine other X-ray sources in the
field of view that are potential counterparts to the Fermi sources.Comment: 2012 Fermi Symposium proceedings - eConf C12102
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