Jorick S. Vink

THE MISSING LUMINOUS BLUE VARIABLES AND THE BISTABILITY JUMP

We discuss an interesting feature of the distribution of luminous blue variables (LBVs) on the H-R diagram, and we propose a connection with the bistability jump seen in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Doradus instability strip at luminosities between log(L/Lsolar)~=5.6 and 5.8. The upper boundary, interestingly, is also where the temperature-dependent S Doradus instability strip intersects the bistability jump at about Teff~=21,000 K. Because of increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log(L/Lsolar)=5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses, the order-of-magnitude increase in the wind density at the bistability jump leads to the formation of a modest to strong pseudophotosphere that might alter a stars apparent position on the H-R diagram. The effect is strongest for LBVs approaching 10 Msolar, where the pseudophotospheres are sufficiently extended to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants [curiously, the upper boundary at log(L/Lsolar)~=5.8 coincides with the upper luminosity limit for red supergiants]. Further work is obviously needed, especially with regard to a possible evolutionary connection between the ``missing LBVs and the most luminous red supergiants and yellow hypergiants. Specifically, yellow hypergiants such as IRC +10420 and rho Cas occupy the same luminosity range as the missing LBVs and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the H-R diagram are concerned.

Probing the circumstellar structure of Herbig Ae/Be stars

We present Hspectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarisation across His 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 Hin 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 Hemission 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 Hemission.

Predictions of variable mass loss for luminous blue variables

We present radiation-driven wind models for Luminous Blue Variables (LBVs) and predict their mass-loss rates. We study the eects of lower masses and modified abundances in comparison to the normal OB supergiants, and we find that the main dierence in mass loss is due to the lower masses of LBVs. In addition, we find that the increase in helium abundance changes the mass-loss properties by small amounts (up to about 0.2 dex in log ˙ M), while CNO processing is relatively unimportant for the mass-loss rate. A comparison between our mass loss predictions and the observations is performed for four relatively well-studied LBVs. The comparison shows that (i) the winds of LBVs are driven by radiation pressure on spectral lines, (ii) the variable mass loss behaviour of LBVs during their S Doradus-type variation cycles is explained by changes in the line driving eciency, notably due to the recombination/ionisation of Feiv/iii and Feiii/ii, and finally, (iii) the winds of LBVs can be used to derive their masses, as exemplified by the case of AG Car, for which we derive a present-day mass of 35 M.

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.

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 Hα 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 Hα emission and recent (last ∼10 years) strong variability.

Hot horizontal branch stars: Predictions for mass loss Winds, rotation, and the low gravity problem

We predict mass-loss rates for the late evolutionary phases of low-mass stars, with special emphasis on the con- sequences for the morphology of the Horizontal Branch (HB). We show that the computed rates, as predicted by the most plausible mechanism of radiation pressure on spectral lines, are too low to produce EHB/sdB stars. This invalidates the scenario recently outlined by Yong et al. (2000) to create these objects by mass loss on the HB. We argue, however, that mass loss plays a role in the distribution of rotational velocities of hot HB stars, and may - together with the enhancement of heavy element abundances due to radiative levitation - provide an explanation for the so-called low gravity problem. The mass loss recipe derived for hot HB (and extreme HB, sdB, sdOB) stars may also be applied to post-HB (AGB-manque, UV-bright) stars over a range in eective temperatures between 12 500-40 000 K.

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 Hα ,H β and Hγ . Changes in the linear polarization 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. disc-like). Furthermore, all objects with detections show similar characteristics at the three spectral lines, despite differences in transition probability and optical depth going from Hα to Hγ . A large fraction of early Herbig Be stars (B0‐B3) observed show line-depolarization effects. However, the early Herbig Ae stars (A0‐A2), observed for comparison, show intrinsic line-polarization 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.

Polarimetric line profiles for scattering off rotating disks

We predict polarimetric line profiles for scattering off rotating disks using a Monte Carlo technique. We have dis- covered that there is a marked difference between scattering of line emission by a disk that reaches the stellar surface, and a disk with an inner hole. For the case with an inner hole, we find single position-angle rotations, similar to those predicted by analytic models. For the case of an undisrupted disk, we find double rotations in the position angle - an effect not reported before. We show that this new effect is due to the finite-sized star interacting with the disks rotational velocity field. Since a gradual increase of the hole size transforms the double rotations smoothly back into single ones - as the line emission object approaches that of a point source - our models demonstrate the diagnostic potential of line polarimetry in determining not only the disk inclination, but also the size of the disk inner hole. Our models are generic, and relevant to any line emitting object that is embedded in a rotating disk. Examples are: Herbig Ae stars, T Tauri stars, other young stars, early-type stars with disks, post-AGB stars, cataclysmic variables and other binary systems, as well as extra-galactic objects, such as the disks around super-massive black holes.

Modelling the clumping-induced polarimetric variability of hot star winds

Context. Clumping in the winds of massive stars may significantly reduce empirical mass-loss rates, and which in turn may have a large impact on our understanding of massive star evolution. Aims. Here, we investigate wind-clumping through the linear polarization induced by light scattering off the clumps. Methods. Through the use of an analytic wind clumping model, we predict the time evolution of the linear polarimetry over a large parameter space. We concentrate on the Luminous Blue Variables, which display the greatest amount of polarimetric variability and for which we recently conducted a spectropolarimetric survey. Results. Our model results indicate that the observed level of polarimetric variability can be reproduced for two regimes of parameter space: one of a small number of massive, optically-thick clumps; and one of a very large number of low-mass clumps. Conclusions. Although a systematic time-resolved monitoring campaign is required to distinguish between the two scenarios, we currently favour the latter, given the short timescale of the observed polarization variability. As the polarization is predicted to scale linearly with mass-loss rate, we anticipate that all hot stars with very large mass-loss rates should display polarimetric variability. This is consistent with recent findings that intrinsic polarization is more common in stars with strong Hα emission.

IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

We report on the discovery of over 50 strong H alpha emitting objects towards the large OB association Cyg OB2 and the H II region DR 15 on its southern periphery. This was achieved using the INT Photometric H alpha Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r, i and H alpha photometry from IPHAS, and near-infrared J, H and K photometry from Two Micron All Sky Survey. The position of the objects in the (J - H) versus (H - K) diagram strongly suggests most of them are young. Many show Ca II infrared triplet emission indicating that they are in a pre-main-sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the H II region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable. The emission-line objects towards Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main-sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.