arXiv subjects

Rene D. Oudmaijer

Publications and source records attributed to Rene D. Oudmaijer.

25 records · Page 2Linked to original sources

arXiv2002

Probing the circumstellar structure of Herbig Ae/Be stars

We present Halpha spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarisation across Halpha is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc-like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarisation across Halpha in the Herbig Be stars, to line polarisations in the Ae group. The frequency of depolarisations detected in the Herbig Be stars (7/12) is particularly interesting as, by analogy to classical Be stars, it may be the best evidence to date that the higher mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, 9 out of 11 show a line polarisation effect that can be understood in terms of a compact Halpha emission that is itself polarised by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung-Russell Diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarised line emission apparent in the Ae stars may be masked in the Herbig Be stars due to their higher levels of Halpha emission.

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arXiv2002

Do Herbig Ae/Be Stars have Discs?

H_alpha spectropolarimetry on Herbig Ae/Be stars shows that the innermost regions of intermediate mass (2 -- 15 M_sun) Pre-Main Sequence stars are flattened. This may be the best evidence to date that the higher mass Herbig Be stars are embedded in circumstellar discs. A second outcome of our study is that the spectropolarimetric signatures for the lower mass Herbig Ae stars differ from those of the higher mass Herbig Be stars. Depolarisations across H_alpha are observed in the Herbig Be group, whereas line polarisations are common amongst the Herbig Ae stars in our sample. These line polarisation effects can be understood in terms of a compact H_alpha source that is polarised by a rotating disc-like configuration. The difference we detect between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung-Russell Diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. However, it is also possible that the compact polarised line component, present in the Herbig Ae stars, is masked in the Herbig Be stars due to their higher levels of H_alpha emission.

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arXiv2001

Are all young stars disc accretors?

We now understand how low mass stars evolve through cloud collapse and disc accretion, but whether higher mass stars are also disc accretors is as yet unknown. Spectropolarimetry observations can help in answering this basic question, as they probe the first few stellar radii around young stars.

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arXiv1999

Time-resolved spectroscopy of the peculiar Halpha variable Be star HD 76534

We present time-resolved spectroscopy of the Be star HD 76534, which was observed to have an Halpha outburst in 1995, when the line went from photospheric absorption to emission at a level of more than two times the continuum within 2.5 hours. To investigate the short-term behaviour of the spectrum of HD 76534 we have obtained 30 spectra within two hours real-time and searched for variations in the spectrum. Within the levels of statistical significance, no variability was found. Rather than periodic on short time scales, the Halpha behaviour seems to be commonly episodic on longer (>1 year) time scales, as an assessment of the existing data on the Halpha line and the Hipparcos photometry suggests. HD 76534 underwent only 1 photometric outburst in the 3 year span that the star was monitored by the Hipparcos satellite.

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arXiv1998

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.

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arXiv1997

Diffuse interstellar bands in the spectra of massive YSOs

We have compared the B-V colour excess, E(B-V), obtained for a sample of five optically visible massive YSOs both from diffuse interstellar bands (DIBs) in their spectra and from their optical continuum slopes. Our targets are HD 200775, BD+40 4124, MWC 1080, MWC 297 and MWC 349A. First, E(B-V) towards each of the targets is derived by dereddening the observed continua to match those of B-type standard stars. A survey of DIBs in the spectra of the massive YSOs, and a control field star, then reveals that the DIBs are significantly weaker in the former than would be expected based on the total E(B-V) values. This result is strengthened by the finding that the DIBs in the control field star, HD 154445, have on average the strength expected from its continuum E(B-V). A rough estimate of the foreground reddening of intervening diffuse interstellar medium shows it to be smaller than the DIB-E(B-V), implying that at least part of the DIB carriers are formed within the parental molecular clouds in which the YSOs are embedded. The formation efficiency of the DIBs varies strongly however from cloud to cloud. The DIB-E(B-V) compares favourably with the total E(B-V) towards BD+40 4124, but is almost negligible in the line of sight towards MWC 297. Despite this general, but not unexpected, deficit we provide evidence that the DIB at 5849 Angstrom is a good tracer of total extinction in these lines of sight.

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arXiv1995

Time resolved spectroscopy of the post-AGB star HD56126

We have investigated the report of Tamura and Takeuti that the Halpha line of the F-type post-AGB star HD56126 is variable on time scales of minutes. To this end, HD56126 was observed on two occasions with the William Herschel Telescope. Seventeen, respectively thirty spectra were taken within time span of 1.5 hours in order to detect any short term variations. We find that the Halpha line profile changed strongly over the two month interval, but no evidence is found for short term variability. The variability Tamura and Takeuti claim to find is probably due to the low signal-to-noise in their spectra.

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