903 research outputs found
Magnetic field detection in the B2Vn star HR 7355
The B2Vn star HR 7355 is found to be a He-rich magnetic star.
Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal observatory
to measure the disk-averaged longitudinal magnetic field at various phases of
the presumed 0.52 d cycle. A variable magnetic field with strengths between B_z
= -2200 and +3200G was found, with confidence limits of 100 to 130G. The field
topology is that of an oblique dipole, while the star itself is seen about
equator-on. In the intensity spectra the HeI-lines show the typical equivalent
width variability of He-strong stars, usually attributed to surface abundance
spots. The amplitudes of the equivalent width variability of the HeI lines are
extraordinarily strong compared to other cases. These results not only put HR
7355 unambiguously among the early-type magnetic stars, but confirm its
outstanding nature: With v sin i = 320 km/s the parameter space in which
He-strong stars are known to exist has doubled in terms of rotational velocity.Comment: 6 pages, 5 figures, 1 Table. Accepted for publication in MNRAS
Letter
The (B0+?)+O6 system FN CMa: A case for tidal-pulsational interaction?
FN CMa is visually double with a separation of about 0.6arcsec. Sixty
high-cadence VLT/UVES spectra permit the A and B components to be disentangled,
as the relative contribution of each star to the total light entering the
spectrograph fluctuates between exposures due to changes in seeing. Component A
exhibits rapid line-profile variations, leading us to attribute the photometric
variability seen by HIPPARCOS (with a derived P=0.08866d) to this component.
From a total of 122 archival and new echelle spectra it is shown that component
A is an SB1 binary with an orbital period of 117.55 days. The eccentricity of
0.6 may result in tidal modulation of the pulsation(s) of component Aa.Comment: 2 pages, 1 figure, IAUS 272 - Active OB Stars: Structure, Evolution,
Mass Loss and Critical Limit
Differential interferometric phases at high spectral resolution as a sensitive physical diagnostic of circumstellar disks
Context. The circumstellar disks ejected by many rapidly rotating B stars
(so-called Be stars) offer the rare opportunity of studying the structure and
dynamics of gaseous disks at high spectral as well as angular resolution. Aims.
This paper explores a newly identified effect in spectro-interferometric phase
that can be used for probing the inner regions of gaseous edge-on disks on a
scale of a few stellar radii. Methods. The origin of this effect (dubbed
central quasi-emission phase signature, CQE-PS) lies in the velocity-dependent
line absorption of photospheric radiation by the circumstellar disk. At high
spectral and marginal interferometric resolution, photocenter displacements
between star and isovelocity regions in the Keplerian disk reveal themselves
through small interferometric phase shifts. To investigate the diagnostic
potential of this effect, a series of models are presented, based on detailed
radiative transfer calculations in a viscous decretion disk. Results. Amplitude
and detailed shape of the CQE-PS depend sensitively on disk density and size
and on the radial distribution of the material with characteristic shapes in
differential phase diagrams. In addition, useful lower limits to the angular
size of the central stars can be derived even when the system is almost
unresolved. Conclusions. The full power of this diagnostic tool can be expected
if it can be applied to observations over a full life-cycle of a disk from
first ejection through final dispersal, over a full cycle of disk oscillations,
or over a full orbital period in a binary system
Probing the distribution of dark matter in the Abell 901/902 supercluster with weak lensing
We present a weak shear analysis of the Abell 901/902 supercluster, composed
of three rich clusters at z=0.16. Using a deep R-band image from the 0.5 x 0.5
degree MPG/ESO Wide Field Imager together with supplementary B-band
observations, we build up a comprehensive picture of the light and mass
distributions in this region. We find that, on average, the light from the
early-type galaxies traces the dark matter fairly well, although one cluster is
a notable exception to this rule. The clusters themselves exhibit a range of
mass-to-light (M/L) ratios, X-ray properties, and galaxy populations. We
attempt to model the relation between the total mass and the light from the
early-type galaxies with a simple scale-independent linear biasing model. We
find M/L_B=130h for the early type galaxies with zero stochasticity, which, if
taken at face value, would imply Omega_m < 0.1. However, this linear relation
breaks down on small scales and on scales equivalent to the average cluster
separation (approximately 1 Mpc), demonstrating that a single M/L ratio is not
adequate to fully describe the mass-light relation in the supercluster. Rather,
the scatter in M/L ratios observed for the clusters supports a model
incorporating non-linear biasing or stochastic processes. Finally, there is a
clear detection of filamentary structure connecting two of the clusters, seen
in both the galaxy and dark matter distributions, and we discuss the effects of
cluster-cluster and cluster-filament interactions as a means to reconcile the
disparate descriptions of the supercluster.Comment: 23 pages, 19 figures. ApJ, accepte
Properties of mm galaxies: Constraints from K-band blank fields
We have used the IRAM Plateau de Bure mm interferometer to locate with
subarcsecond accuracy the dust emission of three of the brightest 1.2mm sources
in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM
30m telescope. We combine these results with deep B to K imaging and VLA
interferometry. Strikingly, none of the three accurately located mm galaxies
MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band
counterpart down to the faint limit of K>21.9. This implies that these three
galaxies are either extremely obscured and/or are at very high redshifts
(z>~4). We combine our results with literature data for 11 more (sub)mm
galaxies that are identified with similar reliability. In terms of their K-band
properties, the sample divides into three roughly equal groups: (i) undetected
to K~22, (ii) detected in the near-infrared but not the optical and (iii)
detected in the optical with the possibility of optical follow-up spectroscopy.
We find a trend in this sample between near-infrared to submm and submm to
radio spectral indices, which in comparison to spectral energy distributions
(SEDs) of low redshift infrared luminous galaxies suggests that the most
plausible primary factor causing the extreme near-infrared faintness of our
objects is their high redshift. We show that the near-infrared to radio SEDs of
the sample are inconsistent with SEDs that resemble local far-infrared cool
galaxies with moderate luminosities, which were proposed in some models of the
submm sky. We briefly discuss the implications of the results for our
understanding of galaxy formation.Comment: aastex, 5 figures. Accepted by Ap
- …