arXiv subjects

J. S. Vink

Publications and source records attributed to J. S. Vink.

133 records · Page 8Linked to original sources

arXiv2008

Spectropolarimetry of the Massive Post-Red Supergiants IRC +10420 and HD 179821

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.

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arXiv2007

The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

[Abridged] We present a comprehensive study of the metallicity dependence of the mass-loss rates in stationary stellar winds of hot massive stars. Assuming a power-law dependence of mass loss on metallicity, Mdot \propto Z^{m}, and adopting a theoretical relation between the terminal velocity and metallicity, v_inf \propto Z^{0.13} (Leitherer et al.), we find m = 0.83 +/- 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 10^{5.2} Lsun. Within the errors, this result agrees with the prediction of m = 0.69 +/- 0.10 from Vink et al. For the high luminosity stars we find the mass loss rates to be greater than the predictions, implying wind clumping factors in their line-forming regions of ~4. For lower luminosity stars, the winds are so weak that their strengths cannot be reliably derived from optical lines, and one must rely on analysis of UV lines. In the low-luminosity domain the Galactic WLR is found to be much steeper than expected from theory, leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L ~ 10^{4.75} Lsun, the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars are overestimated as a result of wind clumping, the degree of clumping would likely to be independent of metallicity, so the scalings derived in this study are expected to remain correct.

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arXiv2007

The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC

[Abridged] We have studied the optical spectra of 28 O- and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star-forming region N11. Stellar parameters are determined using an automated fitting method, combining the stellar atmosphere code FASTWIND with the genetic-algorithm optimisation routine PIKAIA. Results for stars in the LH9 and LH10 associations of N11 are consistent with a sequential star formation scenario, in which activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2, of which the hottest is found to be ~49-54 kK (cf. ~45-46 kK for O3 stars). The masses of helium-enriched dwarfs and giants are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally-enhanced mixing, leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. For bright giants and supergiants no such mass-discrepancy is found, implying that these stars follow tracks of modestly (or non-)rotating objects. Stellar mass-loss properties were found to be intermediate to those found in massive stars in the Galaxy and the SMC, and comparisons with theoretical predictions at LMC metallicity yielded good agreement over the luminosity range of our targets, i.e. 5.0 < log L/L(sun) < 6.1.

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arXiv2007

Testing the predicted mass-loss bi-stability jump at radio wavelengths

In this study, we test the theoretically predicted mass-loss behaviour as a function of stellar effective temperature across the so-called `bi-stability' jump (BSJ). We gathered radio observations of 30 OB supergiants (O8-B3). We derived the radio mass-loss rates and wind efficiencies, and compared our results with Halpha mass-loss rates and predictions based on radiation-driven wind models. he wind efficiency shows the possible presence of a local maximum around an effective temperature of 21~000 K -- in qualitative agreement with predictions. We also find that the radio mass-loss rates show good agreement with empirical Halpha rates. However, the empirical mass-loss rates are larger than the predicted rates from radiation-driven wind theory for objects above the BSJ temperature, whilst they are smaller for the rest. A new wind momenta-luminosity relation for O8-B0 stars has been derived.

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arXiv2007

On the difference between Herbig Ae and Herbig Be stars

We present linear spectropolarimetric data for eight Herbig Be and four Herbig Ae stars at H alpha, H beta and H gamma. Changes in the linear polarisation are detected across all Balmer lines for a large fraction of the observed objects, confirming that the small-scale regions surrounding these objects are flattened (i.e. disk-like). Furthermore, all objects with detections show similar characteristics at the three spectral lines, despite differences in transition probability and optical depth going from H alpha to H gamma. A large fraction of early Herbig Be stars (B0-B3) observed show line depolarisation effects. However the early Herbig Ae stars (A0-A2), observed for comparison, show intrinsic line polarisation signatures. Our data suggest that the popular magnetic accretion scenario for T Tauri objects may be extended to Herbig Ae stars, but that it may not be extended to early Herbig Be stars, for which the available data are consistent with disc accretion.

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arXiv2006

Radio evidence on the mass loss bi-stability jump

We present the results of a first campaign of radio continuum observations toward supergiants with spectral types in the range O8 - B3. Three targets out of twelve were detected with the ATCA and the VLA. The measured fluxes and the upper limits yielded values of the stellar mass-loss rates dM/dt and the wind efficiencies. The comparison of predicted and derived values of dM/dt shows a good agreement.

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arXiv2005

Near-infrared line spectropolarimetry of hot massive stars

In order to study the inner parts of the circumstellar material around optically faint, infrared bright objects, we present the first medium-resolution spectropolarimetric data taken in the near-infrared. In this paper we discuss Pa beta line data of GL 490, a well-known embedded massive young stellar object, and of MWC 349A and MWC 342, two optically faint stars that are proposed to be in the pre-main sequence phase of evolution. As a check on the method, the classical Be star zeta Tau, known to display line polarization changes at optical wavelengths, was observed as well. Three of our targets show a ``line effect'' across Pa beta. For zeta Tau and MWC 349A this line effect is due to depolarisation by a circumstellar electron-scattering disk. In both cases, the position angle of the polarisation is consistent with that of the larger scale disks imaged at other wavelengths, validating infrared spectropolarimetry as a means to detect flattening on small scales. The tentative detection of a rotation in the polarization position angle at Pa beta in the embedded massive young stellar object GL 490 suggests the presence of a small scale rotating accretion disk with an inner hole - similar to those recently discovered at optical wavelengths in Herbig Ae and T Tauri stars.

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