245 research outputs found
A Substellar Common Proper Motion Companion to the Pleiad HII 1348
We announce the identification of a proper motion companion to the star HII
1348, a K5V member of the Pleiades open cluster. The existence of a faint point
source 1.1arcsec away from HII 1348 was previously known from adaptive optics
imaging by Bouvier et al. However, because of a high likelihood of background
star contamination and in the absence of follow-up astrometry, Bouvier et al.
tentatively concluded that the candidate companion was not physically
associated with HII 1348. We establish the proper motion association of the
pair from adaptive optics imaging with the Palomar 5m telescope. Adaptive
optics spectroscopy with the integral field spectrograph OSIRIS on the Keck 10m
telescope reveals that the companion has a spectral type of M8\pm1. According
to substellar evolution models, the M8 spectral type resides within the
substellar mass regime at the age of the Pleiades. The primary itself is a
known double-lined spectroscopic binary, which makes the resolved companion,
HII 1348B, the least massive and widest component of this hierarchical triple
system and the first substellar companion to a stellar primary in the Pleiades.Comment: accepted by Ap
First Results from the CHARA Array. II. A Description of the Instrument
The CHARA Array is a six 1-m telescope optical/IR interferometric array
located on Mount Wilson California, designed and built by the Center for High
Angular Resolution Astronomy of Georgia State University. In this paper we
describe the main elements of the Array hardware and software control systems
as well as the data reduction methods currently being used. Our plans for
upgrades in the near future are also described
Orbital parameters, masses and distance to Beta Centauri determined with the Sydney University Stellar Interferometer and high resolution spectroscopy
The bright southern binary star beta Centauri (HR 5267) has been observed
with the Sydney University Stellar Interferometer (SUSI) and spectroscopically
with the ESO CAT and Swiss Euler telescopes at La Silla. The interferometric
observations have confirmed the binary nature of the primary component and have
enabled the determination of the orbital parameters of the system. At the
observing wavelength of 442 nm the two components of the binary system have a
magnitude difference of 0.15. The combination of interferometric and
spectroscopic data gives the following results: orbital period 357 days,
semi-major axis 25.30 mas, inclination 67.4 degrees, eccentricity 0.821,
distance 102.3 pc, primary and secondary masses M1 = M2 = 9.1 solar masses and
absolute visual magnitudes of the primary and secondary M1V = -3.85 and M2V =
-3.70. The high accuracy of the results offers a fruitful starting point for
future asteroseismic modelling of the pulsating binary components.Comment: 10 pages, 4 figures. Accepted for publication in MNRA
Abundances of metal-weak thick-disc candidates
High resolution spectra of 5 candidate metal-weak thick-disc stars suggested
by Beers & Sommer-Larsen (1995) are analyzed to determine their chemical
abundances. The low abundance of all the objects has been confirmed with
metallicity reaching [Fe/H]=-2.9. However, for three objects, the astrometric
data from the Hipparcos catalogue suggests they are true halo members. The
remaining two, for which proper-motion data are not available, may have
disc-like kinematics. It is therefore clear that it is useful to address
properties of putative metal-weak thick-disc stars only if they possess full
kinematic data. For CS 22894-19 the abundance pattern similar to those of
typical halo stars is found, suggesting that chemical composition is not a
useful discriminant between thick-disc and halo stars. CS 29529-12 is found to
be C enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s
process elements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr]
considerably larger than what is found in more metal-rich carbon-rich stars,
but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a).
Hipparcos data have been used to calculate the space velocities of 25 candidate
metal-weak thick-disc stars, thus allowing us to identify 3 bona fide members,
which support the existence of a metal-poor tail of the thick-disc, at variance
with a claim to the contrary by Ryan & Lambert (1995).Comment: to be published in MNRA
Abundance analysis of prime B-type targets for asteroseismology II. B6--B9.5 stars in the field of view of the CoRoT
The CoRoT satellite is collecting precise time-resolved photometry for tens
of asteroseismology targets. To ensure a correct interpretation of the CoRoT
data, the atmospheric parameters, chemical compositions, and rotational
velocities of the stars must be determined. The main goal of the ground-based
seismology support program for the CoRoT mission was to obtain photometric and
spectroscopic data for stars in the fields monitored by the satellite. These
ground-based observations were collected in the GAUDI archive. High-resolution
spectra of more than 200 B-type stars are available in this database, and about
45% of them is analysed here. To derive the effective temperature of the stars,
we used photometric indices. Surface gravities were obtained by comparing
observed and theoretical Balmer line profiles. To determine the chemical
abundances and rotational velocities, we used a spectrum synthesis method,
which consisted of comparing the observed spectrum with theoretical ones based
on the assumption of LTE. Atmospheric parameters, chemical abundances, and
rotational velocities were determined for 89 late-B stars. The dominant species
in their spectra are iron-peak elements. The average Fe abundance is
7.24+/-0.45 dex. The average rotational velocity is 126 km/sec, but there are
13 and 20 stars with low and moderate Vsin i values, respectively. The analysis
of this sample of 89 late B-type stars reveals many chemically peculiar (CP)
stars. Some of them were previously known, but at least 9 new CP candidates,
among which at least two HgMn stars, are identified in our study. These CP
stars as a group exhibit Vsin i values lower than the stars with normal surface
chemical composition.Comment: 21 pages, 13 figures, accepted to Astronomy and Astrophysic
The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems
We present B and R band spectroastrometry of a sample of 45 Herbig Ae/Be
stars in order to study their binary properties. All but one of the targets
known to be binary systems with a separation of ~0.1-2.0 arcsec are detected by
a distinctive spectroastrometric signature. Some objects in the sample exhibit
spectroastrometric features that do not appear attributable to a binary system.
We find that these may be due to light reflected from dusty halos or material
entrained in winds. We present 8 new binary detections and 4 detections of an
unknown component in previously discovered binary systems. The data confirm
previous reports that Herbig Ae/Be stars have a high binary fraction, 74+/-6
per cent in the sample presented here. We use a spectroastrometric
deconvolution technique to separate the spatially unresolved binary spectra
into the individual constituent spectra. The separated spectra allow us to
ascertain the spectral type of the individual binary components, which in turn
allows the mass ratio of these systems to be determined. In addition, we
appraise the method used and the effects of contaminant sources of flux. We
find that the distribution of system mass ratios is inconsistent with random
pairing from the Initial Mass Function, and that this appears robust despite a
detection bias. Instead, the mass ratio distribution is broadly consistent with
the scenario of binary formation via disk fragmentation.Comment: Accepted for publication in MNRAS, minor changes made in proof stag
The Multiplicity of Massive Stars: A High Angular Resolution Survey with the HST Fine Guidance Sensor
We present the results of an all-sky survey made with the Fine Guidance
Sensor on Hubble Space Telescope to search for angularly resolved binary
systems among the massive stars. The sample of 224 stars is comprised mainly of
Galactic O- and B-type stars and Luminous Blue Variables, plus a few luminous
stars in the Large Magellanic Cloud. The FGS TRANS mode observations are
sensitive to detection of companions with an angular separation between 0."01
and 1."0 and brighter than . The FGS observations resolved 52
binary and 6 triple star systems and detected partially resolved binaries in 7
additional targets (43 of these are new detections). These numbers yield a
companion detection frequency of 29% for the FGS survey. We also gathered
literature results on the numbers of close spectroscopic binaries and wider
astrometric binaries among the sample, and we present estimates of the
frequency of multiple systems and the companion frequency for subsets of stars
residing in clusters and associations, field stars, and runaway stars. These
results confirm the high multiplicity fraction, especially among massive stars
in clusters and associations. We show that the period distribution is
approximately flat in increments of log P. We identify a number of systems of
potential interest for long term orbital determinations, and we note the
importance of some of these companions for the interpretation of the radial
velocities and light curves of close binaries that have third companions.Comment: 37 pages, 4 figures, 5 tables; AJ, in press; Full version with
extended tables and large figure set can be found:
http://www.astro.umontreal.ca/~emily/fgs.pd
Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars?
We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT,
of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A.
Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be
(HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe
star. Remarkably, evidence of surface abundance spots is detected in the
spectra of HD 72106A. The magnetic fields of these objects display intensities
of order 1 kG at the photospheric level, are ordered on global scales, and
appear in approximately 10% of the stars studied. Based on these properties,
the detected stars may well represent pre-main sequence progenitors of the
magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and
2.4 solar masses) represent additional contradictions to the hypothesis of
Hubrig et al. (2000), who claim that magnetic fields appear in
intermediate-mass stars only after 30% of their main sequence evolution is
complete. Finally, we fail to confirm claims by Hubrig et al. (2004) of
magnetic fields in the Herbig Ae star HD 139614.Comment: 4 pages, 7 figure
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