231 research outputs found
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 properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: beta CMi
While the presence of discs around classical Be stars is well established,
their origin is still uncertain. To understand what processes result in the
creation of these discs and how angular momentum is transported within them,
their physical properties must be constrained. This requires comparing high
spatial and spectral resolution data with detailed radiative transfer
modelling. We present a high spectral resolution, R~80,000, sub milli-arcsecond
precision, spectroastrometric study of the circumstellar disc around the Be
star beta CMi. The data are confronted with three-dimensional, NLTE radiative
transfer calculations to directly constrain the properties of the disc.
Furthermore, we compare the data to disc models featuring two velocity laws;
Keperian, the prediction of the viscous disc model, and angular momentum
conserving rotation. It is shown that the observations of beta CMi can only be
reproduced using Keplerian rotation. The agreement between the model and the
observed SED, polarisation and spectroastrometric signature of beta CMi
confirms that the discs around Be stars are well modelled as viscous decretion
discs.Comment: Accepted for publication in MNRA
Sub-milliarcsecond precision spectro-astrometry of Be stars
The origin of the disks around Be stars is still not known. Further progress
requires a proper parametrization of their structure, both spatially and
kinematically. This is challenging as the disks are very small. Here we assess
whether a novel method is capable of providing these data. We obtained spectro
astrometry around the Pa beta line of two bright Be stars, alpha Col and zeta
Tau, to search for disk signatures. The data, with a pixel to pixel precision
of the centroid position of 0.3..0.4 milliarcsecond is the most accurate such
data to date. Artefacts at the 0.85 mas level are present in the data, but
these are readily identified as they were non-repeatable in our redundant
datasets. This does illustrate the need of taking multiple data to avoid
spurious detections. The data are compared with simple model simulations of the
spectro astrometric signatures due to rotating disks around Be stars. The upper
limits we find for the disk radii correspond to disk sizes of a few dozen
stellar radii if they rotate Keplerian. This is very close to observationally
measured and theoretically expected disk sizes, and this paper therefore
demonstrates that spectro-astrometry, of which we present the first such
attempt, has the potential to resolve the disks around Be stars.Comment: 6 pages, A&A accepte
S 111 and the polarization of the B[e] supergiants in the Magellanic Clouds
We have obtained linear polarization measurements of the Large Magellanic Cloud B[e] supergiant S 111 using optical imaging polarimetry. The intrinsic polarization found is consistent with the presence of an axisymmetric circumstellar envelope. We have additionally estimated the electron density for S 111 using data from the literature and revisited the correlation between polarization and envelope parameters of the B[e] supergiant stars using more recent IR calibration color data. The data suggest that the polarization can be indeed explained by electron scattering. We have used Monte Carlo codes to model the continuum polarization of the Magellanic B[e] supergiants. The results indicate that the electron density distribution in their envelopes is closer to a homogeneous distribution rather than an r dependence. At the same time, the data are best fitted by a spherical distribution with density contrast than a cylindrical distribution. The data and the model results support the idea of the presence of an equatorial disk and of the two-component wind model for the envelopes of the B[e] supergiants. Spectropolarimetry would help further our knowledge of these envelopes
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