252 research outputs found
The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel
We report new radial velocity observations of GP Vel/HD77581, the optical
companion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more than
two complete orbits of the system, we detect evidence for tidally induced
non-radial oscillations on the surface of GP Vel, apparent as peaks in the
power spectrum of the residuals to the radial velocity curve fit. By removing
the effect of these oscillations (to first order) and binning the radial
velocities, we have determined the semi-amplitude of the radial velocity curve
of GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measured
semi-amplitude of the pulsar's orbit, the mass ratio of the system is
0.081+/-0.005. We are able to set upper and lower limits on the masses of the
component stars as follows. Assuming GP Vel fills its Roche lobe then the
inclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain the
masses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star and
M_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle is
i=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe is
beta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun and
M_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits is
also possible, corresponding to other combinations of i and beta. In addition,
we note that if the zero phase of the radial velocity curve is allowed as a
free parameter, rather than constrained by the X-ray ephemeris, a significantly
improved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shift
of 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of the
radial velocity curve may indicate the presence of an additional radial
velocity component at the orbital period.Comment: Accepted for publication in Astronomy & Astrophysic
The Worst Distortions of Astrometric Instruments and Orthonormal Models for Rectangular Fields of View
The non-orthogonality of algebraic polynomials of field coordinates
traditionally used to model field-dependent corrections to astrometric
measurements, gives rise to subtle adverse effects. In particular, certain
field dependent perturbations in the observational data propagate into the
adjusted coefficients with considerable magnification. We explain how the worst
perturbation, resulting in the largest solution error, can be computed for a
given non-orthogonal distortion model. An algebraic distortion model of full
rank can be converted into a fully orthonormal model based on the Zernike
polynomials for a circular field of view, or a basis of functions constructed
from the original model by a variant of the Gram-Schmidt orthogonalization
process for a rectangular field of view. The relative significance of
orthonormal distortion terms is assessed simply by the numerical values of the
corresponding coefficients. Orthonormal distortion models are easily extendable
when the distribution of residuals indicate the presence of higher order terms.Comment: 1 figure; submitted in PAS
The B-type variable HD 131120 modelled by rotational modulation
We present ground-based multi-colour Geneva photometry and high-resolution spectra of the variable B-type star HD 131120. All sets of data reveal a monoperiodic star with a period of 1.569 days. We compare the variability of the star with a non-radial g-mode pulsation model and with a rotational modulation model. The latter model appears to be the best explanation for the observed line-profile variations of the star. Moreover, contrary to the pulsational model, it is able to reproduce the behaviour of the first three moments of the Si and He lines as well as the large variability of their equivalent width. We therefore remove the star from the list of new Slowly Pulsating B stars found from Hipparcos data. Based on observations obtained with the Swiss photometric telescope and ESO's CAT/CES telescope, both situated at La Silla, Chile.Peer reviewe
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with the
Short Wavelength Spectrometer (SWS) during the Post-Helium program of the
Infrared Space Observatory (ISO). This program is aimed at extending the Morgan
& Keenan classification scheme into the near-infrared. Later type stars will be
discussed in a seperate publication. The observations consist of 57 SWS
Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra
acquired during the nominal mission with a similar observational setting. For
B-type stars, this sample provides ample spectral converage in terms of subtype
and luminosity class. For O-type stars,the ISO sample is coarse and therefore
is complemented with 8 UKIRT L'-band observations. In terms of the presence of
diagnostic lines, the L'-band is likely the most promising of the near-infrared
atmospheric windows for the study of the physical properties of B stars.
Specifically, this wavelength interval contains the Brackett alpha, Pfund
gamma, and other Pfund lines which are probes of spectral type, luminosity
class and mass loss. Here, we present simple empirical methods based on the
lines present in the 2.4 to 4.1 microns interval that allow the determination
of: the spectral type of B dwarfs and giants to within two subtypes; the
luminosity class of B stars to within two classes; the mass-loss rate of O
stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&
A Sino-German 6 cm polarization survey of the Galactic plane II. The region from 129 degree to 230 degree longitude
Linearly polarized Galactic synchrotron emission provides valuable
information about the properties of the Galactic magnetic field and the
interstellar magneto-ionic medium, when Faraday rotation along the line of
sight is properly taken into account. We aim to survey the Galactic plane at 6
cm including linear polarization. At such a short wavelength Faraday rotation
effects are in general small and the Galactic magnetic field properties can be
probed to larger distances than at long wavelengths. The Urumqi 25-m telescope
is used for a sensitive 6 cm survey in total and polarized intensities. WMAP
K-band (22.8 GHz) polarization data are used to restore the absolute zero-level
of the Urumqi U and Q maps by extrapolation. Total intensity and polarization
maps are presented for a Galactic plane region of 129 degree < l < 230 degree
and |b| < 5 degree in the anti-centre with an angular resolution of 9'5 and an
average sensitivity of 0.6 mK and 0.4 mK Tb in total and polarized intensity,
respectively. We briefly discuss the properties of some extended Faraday
Screens detected in the 6 cm polarization maps. The Sino-German 6 cm
polarization survey provides new information about the properties of the
magnetic ISM. The survey also adds valuable information for discrete Galactic
objects and is in particular suited to detect extended Faraday Screens with
large rotation measures hosting strong regular magnetic fields.Comment: 17 pages, 20 figures, accepted by A&A. Resolutions of the figures
have been significantly reduced. For version with full resolution, see
http://159.226.88.6/zmtt/6cm/papers/gao.paper2.pd
The Interstellar Environment of our Galaxy
We review the current knowledge and understanding of the interstellar medium
of our galaxy. We first present each of the three basic constituents - ordinary
matter, cosmic rays, and magnetic fields - of the interstellar medium, laying
emphasis on their physical and chemical properties inferred from a broad range
of observations. We then position the different interstellar constituents, both
with respect to each other and with respect to stars, within the general
galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts
3EG J2016+3657: Confirming an EGRET Blazar Behind the Galactic Plane
We recently identified the blazar-like radio source G74.87+1.22 (B2013+370)
as the counterpart of the high-energy gamma-ray source 3EG J2016+3657 in the
Galactic plane. However, since most blazar identifications of EGRET sources are
only probabilistic in quality even at high Galactic latitude, and since there
also exists a population of unidentified Galactic EGRET sources, we sought to
obtain additional evidence to support our assertion that 3EG J2016+3657 is a
blazar. These new observations consist of a complete set of classifications for
the 14 brightest ROSAT X-ray sources in the error circle, of which B2013+370
remains the most likely source of the gamma-rays. We also obtained further
optical photometry of B2013+370 itself which shows that it is variable,
providing additional evidence of its blazar nature. Interestingly, this field
contains, in addition to the blazar, the plerionic supernova remnant CTB 87,
which is too distant to be the EGRET source, and three newly discovered
cataclysmic variables, all five of these X-ray sources falling within 16' of
each other. This illustrates the daunting problem of obtaining complete
identifications of EGRET sources in the Galactic plane.Comment: 16 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
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
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