Very Massive Stars: a metallicity-dependent upper-mass limit, slow winds, and the self-enrichment of Globular Clusters

One of the key questions in Astrophysics concerns the issue of whether there exists an upper-mass limit to stars, and if so, what physical mechanism sets this limit, which might also determine if the upper-mass limit is metallicity (Z) dependent. We argue that mass loss by radiation-driven winds mediated by line opacity is one of the prime candidates setting the upper-mass limit. We present mass-loss predictions (dM/dt_wind) from Monte Carlo radiative transfer models for relatively cool (Teff = 15kK) inflated very massive stars (VMS) with large Eddington Gamma factors in the mass range 100-1000 Msun as a function of metallicity down to 1/100 Z/Zsun. We employ a hydrodynamic version of our Monte Carlo method, allowing us to predict the rate of mass loss (dM/dt_wind) and the terminal wind velocity (vinf) simultaneously. Interestingly, we find wind terminal velocities (vinf) that are low (100-500 km/s) over a wide Z-range, and we propose that the slow winds from VMS are an important source of self-enrichment in globular clusters. We also find mass-loss rates (dM/dt_wind), exceeding the typical mass-accretion rate (dM/dt_accr) of 0.001 Msun/yr during massive-star formation. We express our mass-loss predictions as a function of mass and Z, finding log dM/dt = -9.13 + 2.1 log(M/Msun) + 0.74 log(Z/Zsun) (Msun/yr). Even if stellar winds would not directly halt & reverse mass accretion during star formation, if the most massive stars form by stellar mergers stellar wind mass loss may dominate over the rate at which stellar growth takes place. We therefore argue that the upper-mass limit is effectively Z-dependent due to the nature of radiation-driven winds. This has dramatic consequences for the most luminous supernovae, gamma-ray bursts, and other black hole formation scenarios at different Cosmic epochs.Comment: 9 pages, 3 figures. Accepted by Astronomy & Astrophysics. Small textual change

Linear Spectropolarimetry and the Circumstellar Media of Young and Massive Stars

Linear spectropolarimetry is a powerful tool to probe circumstellar structures on spatial scales that cannot yet be achieved through direct imaging. In this review I discuss the role that emission-line polarimetry can play in constraining geometrical and physical properties of a wide range of circumstellar environments, varying from the accretion disks around pre-main sequence T Tauri and Herbig Ae/Be stars, to the issue of stellar wind clumping, and the aspherical outflows from the massive star progenitors of supernovae and long gamma-ray bursts at low metallicity.Comment: 12 pages, 12 figures, Invited Review in Stellar Polarimetry: From Birth to Deat

Winds from stripped low-mass Helium stars and Wolf-Rayet stars

We present mass-loss predictions from Monte Carlo radiative transfer models for helium (He) stars as a function of stellar mass, down to 2 Msun. Our study includes both massive Wolf-Rayet (WR) stars and low-mass He stars that have lost their envelope through interaction with a companion. For these low-mass He-stars we predict mass-loss rates that are an order of magnitude smaller than by extrapolation of empirical WR mass-loss rates. Our lower mass-loss rates make it harder for these elusive stripped stars to be discovered via line emission, and we should attempt to find them through alternative methods instead. Moreover, lower mass-loss rates will make it less likely that low-mass He stars provide stripped-envelope supernovae (SNe) of type Ibc. We express our mass-loss predictions as a function of L and Z, and not as a function of the He abundance, as we do not consider this physically astute given our earlier work. The exponent of the dM/dt vs. Z dependence is found to be 0.61, which is less steep than relationships derived from recent empirical atmospheric modelling. Our shallower exponent will make it more challenging to produce "heavy" black holes of order 40 Msun, as recently discovered in the gravitational wave event GW 150914, making low metallicity for these types of events even more necessary.Comment: A&A Letters - accepted - 5 pages - 1 figure. Minor text change

Fast & slow winds from supergiants and Luminous Blue Variables

We predict quantitative mass-loss rates and terminal wind velocities for early-type supergiants and luminous blue variables (LBVs) using a dynamical version of the Monte Carlo radiative transfer method. First, the observed drop in terminal wind velocity around spectral type B1 is confirmed by the Monte Carlo method -- at the correct effective temperature of about 21 000 K. This drop in wind velocity is much steeper than would be expected from the drop in escape speed for cooler stars. The results may be particularly relevant for slow winds inferred for some High-Mass X-ray binaries. Second, the strength of the mass-loss bi-stability jump is found to be significantly larger than previously assumed. Not only could this make bi-stability braking more efficient in massive star evolution, but a rotationally-induced version of the bi-stability mechanism may now be capable of producing the correct density of outflowing disks around B[e] supergiants, although multi-dimensional modelling including the disk velocity structure is still needed. For LBVs, we find the bi-stability jump to become larger at higher metallicities, but perhaps surprisingly also larger at lower Eddington parameters. This may have consequences for the role of LBVs in the evolution of massive stars at different metallicities and Cosmic Epochs. Finally, our predicted low wind velocities may be important for explaining the slow outflow speeds of supernova type IIb/IIn progenitors, for which the direct LBV-SN link was first introduced.Comment: 7 pages, 11 figure

Mass loss predictions for Subdwarf B stars

We present the results of Monte Carlo mass-loss predictions for hot low-mass stars, specifically for Subdwarf B (SdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create SdB stars. We argue, however, that the mass loss plays a role in the chemical abundance patterns observed both in field SdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manque, UV-bright) stars, over a range in effective temperatures between 10000 and 50000 Kelvin. Finally, we present preliminary spectral synthesis on the more luminous SdB stars for which emission cores in Halpha have been detected (Heber et al. 2002). We find that these line profiles can indeed be interpreted as the presence for a stellar wind with mass loss of the order of log(Mdot) = -11 (Msun/year).Comment: 8 pages, 1 figure, to appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer, ed. P. Maxte