22 research outputs found
On the H Behaviour of Blue Supergiants: Rise and Fall over the Bi-stability Jump
The evolutionary state of blue supergiants is still unknown. Stellar wind
mass loss is one of the dominant processes determining the evolution of massive
stars, and it may provide clues on the evolutionary properties of blue
supergiants. As the H line is the most oft-used mass-loss tracer in the
OB-star regime, we provide a detailed analysis of the H line for OB
supergiant models over an range between 30000 and 12500K. We find
a maximum in the H equivalent width at 22500 K - at the location of the
bi-stability jump. The H line-profile behaviour is characterised by two
branches of : (i) a "hot" branch between 30000 and 22500 K, where
H emission becomes stronger with decreasing , and (ii) a
"cool" branch between 22500 and 12500 K, where the line becomes weaker. Our
models show that this non-monotonic H behaviour is related to the
optical depth of Ly, finding that at the "cool" branch the population
of the 2nd level of hydrogen is enhanced in comparison to the 3rd level. This
is expected to increase line absorption, leading to weaker H flux when
drops from 22500 K downwards. We also show that for late B
supergiants (at below ~15000 K), the differences in the H
line between homogeneous and clumpy winds becomes insignificant. Moreover, we
show that at the bi-stability jump H changes its character completely,
from an optically thin to an optically thick line, implying that macro-clumping
should play an important role at temperatures below the bi-stability jump. This
would not only have consequences for the character of observed H line
profiles, but also for the reported discrepancies between theoretical and
empirical mass-loss rates.Comment: Accepted for publication in A&
Type IIP supernova light curves affected by the acceleration of red supergiant winds
We introduce the first synthetic light-curve model set of Type IIP supernovae
exploded within circumstellar media in which the acceleration of the red
supergiant winds is taken into account. Because wind acceleration makes the
wind velocities near the progenitors low, the density of the immediate vicinity
of the red supergiant supernova progenitors can be higher than that
extrapolated by using a constant terminal wind velocity. Therefore, even if the
mass-loss rate of the progenitor is relatively low, it can have a dense
circumstellar medium at the immediate stellar vicinity and the early light
curves of Type IIP supernovae are significantly affected by it. We adopt a
simple beta velocity law to formulate the wind acceleration. We provide
bolometric and multicolor light curves of Type IIP supernovae exploding within
such accelerated winds from the combinations of three progenitors, 12 - 16
Msun; five beta, 1-5; seven mass-loss rates, 1e-5 - 1e-2 Msun/yr; and four
explosion energies, 0.5e51 - 2e51 erg. All the light curve models are available
at https://goo.gl/o5phYb. When the circumstellar density is sufficiently high,
our models do not show a classical shock breakout as a consequence of the
interaction with the dense and optically-thick circumstellar media. Instead,
they show a delayed 'wind breakout', substantially affecting early light curves
of Type IIP supernovae. We find that the mass-loss rates of the progenitors
need to be 1e-3 - 1e-2 Msun/yr to explain typical rise times of 5 - 10 days in
Type IIP supernovae assuming a dense circumstellar radius of 1e15 cm.Comment: 12 pages, 9 figures, 2 tables, accepted by Monthly Notices of the
Royal Astronomical Societ
The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
The evolution and fate of very massive stars (VMS) is tightly connected to
their mass-loss properties. Their initial and final masses differ significantly
as a result of mass loss. VMS have strong stellar winds and extremely high
ionising fluxes, which are thought to be critical sources of both mechanical
and radiative feedback in giant Hii regions. However, how VMS mass-loss
properties change during stellar evolution is poorly understood. In the
framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss
transition region from optically thin O to denser WNh star winds, thereby
testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and
WNh stars, an unprecedented sample of stars with the highest masses and
luminosities known. We perform a spectral analysis of optical VFTS as well as
near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN
to obtain stellar and wind parameters. For the first time, we observationally
resolve the transition between optically thin O and optically thick WNh star
winds. Our results suggest the existence of a kink between both mass-loss
regimes, in agreement with recent MC simulations. For the optically thick
regime, we confirm the steep dependence on the Eddington factor from previous
theoretical and observational studies. The transition occurs on the MS near a
luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find
that - even when accounting for moderate wind clumping (with f = 0.1) - wind
mass-loss rates are enhanced with respect to standard prescriptions currently
adopted in stellar evolution calculations. We also show that this results in
substantial helium surface enrichment. Based on our spectroscopic analyses, we
are able to provide the most accurate ionising fluxes for VMS known to date,
confirming the pivotal role of VMS in ionising and shaping their environments.Comment: Accepted for publication in A&A, 19 pages, 14 figures, 6 tables, (74
pages appendix, 68 figures, 4 tables
Physics and evolution of the most massive stars in 30 Doradus
Context. The identification of stellar-mass black-hole mergers with up to 80 M⊙ as powerful sources of gravitational wave radiation led to increased interest in the physics of the most massive stars. The largest sample of possible progenitors of such objects, very massive stars (VMS) with masses up to 300 M⊙, have been identified in the 30 Dor star-forming region in the Large Magellanic Cloud (LMC). In this young starburst analogue, VMS were found to dominate stellar feedback. Despite their importance, the physics and evolution of VMS is highly uncertain, mainly due to their proximity to the Eddington limit.
Aims. In this work, we investigate the two most important effects that are thought to occur near the Eddington limit: enhanced mass loss through optically thick winds and the formation of radially inflated stellar envelopes.
Methods. We compute evolutionary models for VMS at LMC metallicity and perform a population synthesis of the young stellar population in 30 Dor. We adjust the input physics of our models to match the empirical properties of the single-star population in 30 Dor as derived in the framework of the VLT-Flames Tarantula Survey.
Results. Enhanced mass loss and envelope inflation near the Eddington limit have a dominant effect on the evolution of the most massive stars. While the observed mass-loss properties and the associated surface He-enrichment are well described by our new models, the observed O-star mass-loss rates are found to cover a much larger range than theoretically predicted, with particularly low mass-loss rates for the youngest objects. Also, the (rotational) surface enrichment in the O-star regime appears to not be well understood. The positions of the most massive stars in the Hertzsprung-Russell diagram (HRD) are affected by mass loss and envelope inflation. For instance, the majority of luminous B supergiants in 30 Dor, and the lack thereof at the highest luminosities, can be explained through the combination of envelope inflation and mass loss. Finally, we find that the upper limit for the inferred initial stellar masses in the greater 30 Dor region is significantly lower than in its central cluster, R 136, implying a variable upper limit for the masses of stars.
Conclusions. The implementation of mass-loss and envelope physics in stellar evolution models turns out to be essential for the modelling of the observable properties of young stellar populations. While the properties of the most massive stars (≳100 M⊙) are well described by our new models, the slightly less massive O stars investigated in this work show a much more diverse behaviour than previously thought, which has potential implications for rotational mixing and angular momentum transport. While the present models are a big step forward in the understanding of stellar evolution in the upper HRD, more work is needed to understand the mechanisms that regulate the mass-loss rates of OB stars and the physics of fast-rotating stars