Publications (7)0 Total impact
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ABSTRACT: (abridged) As the favoured progenitors of long-duration gamma-ray bursts, massive stars may represent our best signposts of individual objects in the early Universe, but special conditions seem required to make these bursters, which might originate from the progenitor's rapid rotation and associated asymmetry. To obtain empirical constraints on the interplay between stellar rotation and wind asymmetry, we perform linear Halpha spectropolarimetry on a sample of 18 spectroscopically peculiar massive O stars, including OVz, Of?p, Oe, and Onfp stars, supplemented by an earlier sample of 20 O supergiants. Despite their rapid rotation (with vsin(i) up to 400 km/s) most O-type stars are found to be spherically symmetric, but with notable exceptions amongst specific object classes. We divide the peculiar O stars into four distinct categories: Groups III and IV include the Oe stars and Onfp stars, which are on the high-end tail of the O star rotation distribution and have in the past been claimed to be embedded in disks. Here we report the detection of a classical depolarization ``line effect'' in the Oe star HD 45314, but the overall incidence of line effects amongst Oe stars is significantly lower (1 out of 6) than amongst Be stars. The chance that the Oe and Be datasets are drawn from the same parent population is negligible (with 95% confidence). This implies there is as yet no evidence for a disk hypothesis in Oe stars, providing relevant constraints on the physical mechanism that is responsible for the Be phenomenon. Finally, we find that 3 out of 4 of the group IV Onfp stars show evidence for complex polarization effects, which are likely related to rapid rotation, and we speculate on the evolutionary links to B[e] stars. Comment: Astronomy & Astrophyics (in press) 12 pages
09/2009;
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ABSTRACT: We present medium resolution spectropolarimetry and long term photo-polarimetry of two massive post-red supergiants, IRC +10420 and HD 179821. The data provide new information on their circumstellar material as well as their evolution. In IRC +10420, the polarization of the Halpha line is different to that of the continuum, which indicates that the electron-scattering region is not spherically symmetric. The observed long term changes in the polarimetry can be associated with an axi-symmetric structure, along the short axis of the extended reflection nebulosity. Long term photometry reveals that the star increased in temperature until the mid-nineties, after which the photospheric flux in the optical levelled off. As the photometric changes are mostly probed in the red, they do not trace high stellar temperatures sensitively. And so, it is not obvious whether the star has halted its increase in temperature or not. For HD 179821 we find no polarization effects across any absorption or emission lines, but observe very large polarization changes of order 5% over 15 years. Unexpectedly, during the same period, the optical photometry displayed modest variability at the 0.2 magnitude level. Several explanations for this puzzling fact are discussed. Most of which, involving asymmetries in the circumstellar material, seem to fail as there is no evidence for the presence of hot, dusty material close to the star. Alternatively, the variations can be explained by the presence of a non-radially pulsating photosphere. Changes in the photometry hint at an increase in temperature corresponding to a change through two spectral subclasses over the past ten years. Comment: 13 pages, 6 figures, MNRAS accepted
01/2008;
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ABSTRACT: We present the results of Monte Carlo mass-loss predictions for massive stars covering a wide range of stellar parameters. We critically test our predictions against a range of observed mass-loss rates -- in light of the recent discussions on wind clumping. We also present a model to compute the clumping-induced polarimetric variability of hot stars and we compare this with observations of Luminous Blue Variables, for which polarimetric variability is larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an ideal testbed for studies of wind clumping and wind geometry, as well as for wind strength calculations, and we propose they may be direct supernova progenitors.
09/2007;
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ABSTRACT: We present the first systematic spectro-polarimetric study of Luminous
Blue Variables (LBVs), and find that at least half those objects studied
display evidence for intrinsic polarization -- a signature of
significant inhomogeneity at the base of the wind. Furthermore,
multi-epoch observations reveal that the polarization is variable in
both strength and position angle. This evidence points away from a
simple axi-symmetric wind structure à la the B[e] supergiants,
and instead suggests a wind consisting of localised density
enhancements, or `clumps'. We show with an analytical model that, in
order to produce the observed variability, the clumps must be large,
produced at or below the photosphere, and ejected on timescales of days.
More details of LBV wind-clumping will be determined through further
analysis of the model and a polarimetric monitoring campaign.
11/2006; 355:173.
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ABSTRACT: In this contribution we review the properties of the post-red supergiant
IRC +10420. Its A-type supergiant spectral classification, high wind
outflow velocities, strong infrared excess combined with its abundances
typical of massive evolved stars strongly indicate that IRC +10420 is a
post-Red Supergiant. Indeed, the object can be classed as a genuine
evolved ``A[e]'' supergiant, and as such may provide important clues to
the understanding of the B[e] phenomenon. We give an update on the
photometric evolution of the star and present new spectropolarimetric
data. These imply that the Hα line forming region deviates from
spherical symmetry.
11/2006; 355:181.
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ABSTRACT: We present new medium-resolution spectropolarimetric observations of HD 45677 in the B and R-bands. A change in polarisation is detected across Halpha, Hbeta and Hgamma confirming that the ionised region around the star is aspherical. (Q,U) points associated with these emission lines occur away from the continuum, defining a polarisation vector which points in the same direction for each of the lines at an average intrinsic polarisation angle of 164 +/- 3 degrees. These data were combined with past photometric and polarimetric data from the literature to investigate any variability. We find that HD 45677 is both photometrically and polarimetrically variable and that these changes are linked. We suggest that these variations may be caused by an aspherical blowout and by deriving a least-squares fit to the B-band polarimetric data in Q-U space, we show that the blowout occurs at an intrinsic polarisation angle of 175 +/- 1 degrees, along the same angle as the proposed geometry of the ionised region. Comment: 7 pages, 4 figures, accepted MNRAS
10/2006;
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ABSTRACT: Circumstellar disks play an important role in many stages of the
evolution of stars. However, it is only possible to directly image
circumstellar disks for a few of the nearest stars. For massive stars,
the situation is even more difficult, as they are on average further
away than the more numerous low mass stars. In order to shed light on
such disks we are pursuing a programme of spectropolarimetry. This
technique can reveal the presence of disks on a spatial scale of order
stellar radii, while it has the potential for further modeling. Here we
present our results on two groups of stars, on young, intermediate mass,
pre-main sequence Herbig Ae/Be stars and on a sample of massive evolved
objects, the Luminous Blue Variables. We also present initial results
obtained at near-infrared wavelengths.
11/2005; 343:227.
Institutions
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2008
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Rochester Institute of Technology
Rochester,
NY,
USA
