26 research outputs found
Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of Luminous Blue
Variables (LBVs) in the Galaxy and the Magellanic Clouds, in order to
investigate the geometries of their winds. We find that at least half of our
sample show changes in polarization across the strong H emission line,
indicating that the light from the stars is intrinsically polarized and
therefore that asphericity already exists at the base of the wind. Multi-epoch
spectropolarimetry on four targets reveals variability in their intrinsic
polarization. Three of these, AG Car, HR Car and P Cyg, show a position angle
(PA) of polarization which appears random with time. Such behaviour can be
explained by the presence of strong wind-inhomogeneities, or `clumps' within
the wind. Only one star, R 127, shows variability at a constant PA, and hence
evidence for axi-symmetry as well as clumpiness. However, if viewed at low
inclination, and at limited temporal sampling, such a wind would produce a
seemingly random polarization of the type observed in the other three stars.
Time-resolved spectropolarimetric monitoring of LBVs is therefore required to
determine if LBV winds are axi-symmetric in general.
The high fraction of LBVs ( 50%) showing intrinsic polarization is to be
compared with the lower 20-25 % for similar studies of their
evolutionary neighbours, O supergiants and Wolf-Rayet stars. We anticipate that
this higher incidence is due to the lower effective gravities of the LBVs,
coupled with their variable temperatures within the bi-stability jump regime.
This is also consistent with the higher incidence of wind asphericity that we
find in LBVs with strong H emission and recent (last 10 years)
strong variability.Comment: 20 pages, 12 figures, accepted by A&
Dusty Blue Supergiants: News from High-Angular Resolution Observations
An overview is presented of the recent advances in understanding the B[e]
phenomenon among blue supergiant stars in light of high-angular resolution
observations and with an emphasis on the results obtained by means of long
baseline optical stellar interferometry. The focus of the review is on the
circumstellar material and evolutionary phase of B[e] supergiants, but recent
results on dust production in regular blue supergiants are also highlighted.Comment: 8 pages, 2 figures. Published in "Advances in Astronomy" by Hindawi
Publishing Corporatio
The Lutz-Kelker bias in trigonometric parallaxes
The theoretical prediction that trigonometric parallaxes suffer from a
statistical effect, has become topical again now that the results of the
Hipparcos satellite have become available. This statistical effect, the
so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This
has the implication that inferred distances, and hence inferred luminosities
are too small. Published analytic calculations of the Lutz-Kelker bias indicate
that the inferred luminosity of an object is, on average, 30% too small when
the error in the parallax is only 17.5%. Yet, this bias has never been
determined empirically. In this paper we investigate whether there is such a
bias by comparing the best Hipparcos parallaxes which ground-based
measurements. We find that there is indeed a large bias affecting parallaxes,
with an average and scatter comparable to predictions. We propose a simple
method to correct for the LK bias, and apply it successfully to a sub-sample of
our stars. We then analyze the sample of 26 `best' Cepheids used by Feast &
Catchpole (1997) to derive the zero-point of the fundamental mode pulsators and
leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid
parallaxes- of 18.56 +/- 0.08, consistent with previous estimates.Comment: MNRAS Letters in press; 6 pages LaTeX, 6 ps figure
Linear spectropolarimetry of young and other emission line stars
The aim of this article is to demonstrate the useful role that can be played
by spectropolarimetric observations of young and evolved emission line stars
that analyse the linearly polarized component in their spectra. At the time of
writing, this demonstration has to be made on the basis of optical data since
there is no common-user infrared facility, in operation, that offers the
desired combination of spectral resolution and sensitivity. Here we focus on
what can be learned from linear spectropolarimetry alone at reasonably high
spectral resolution and at S/N. And we remind that the near
infrared (1--2 micron) has the potential to out-perform the optical as a domain
to work in because of the greatly reduced interstellar obscuration at these
wavelengths. This point has been reached at a time when theory, exploiting
flexible Monte Carlo methods, is fast becoming a powerful tool. In short we
have the complex phenomena, and the rise of the modelling capability to match
-- good data are the missing link.Comment: 11 pages, ESO Conference on High Resolution Infrared Spectroscop