5,388 research outputs found
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
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
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
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