15 research outputs found
Metallicity dependence of turbulent pressure and macroturbulence in stellar envelopes
Macroturbulence, introduced as a fudge to reproduce the width and shape of
stellar absorption lines, reflects gas motions in stellar atmospheres. While in
cool stars, it is thought to be caused by convection zones immediately beneath
the stellar surface, the origin of macroturbulence in hot stars is still under
discussion. Recent works established a correlation between the
turbulent-to-total pressure ratio inside the envelope of stellar models and the
macroturbulent velocities observed in corresponding Galactic stars. To probe
this connection further, we evaluated the turbulent pressure that arises in the
envelope convective zones of stellar models in the mass range 1-125 Msun based
on the mixing-length theory and computed for metallicities of the Large and
Small Magellanic Cloud. We find that the turbulent pressure contributions in
models with these metallicities located in the hot high-luminosity part of the
Hertzsprung-Russel (HR) diagram is lower than in similar models with solar
metallicity, whereas the turbulent pressure in low-metallicity models
populating the cool part of the HR-diagram is not reduced. Based on our models,
we find that the currently available observations of hot massive stars in the
Magellanic Clouds appear to support a connection between macroturbulence and
the turbulent pressure in stellar envelopes. Multidimensional simulations of
sub-surface convection zones and a larger number of high-quality observations
are necessary to test this idea more rigorously.Comment: Accepted A&A, 8 p
Diagnostic of the unstable envelopes of Wolf-Rayet stars
The envelopes of stars near the Eddington limit are prone to various
instabilities. A high Eddington factor in connection with the Fe opacity peak
leads to convective instability, and a corresponding envelope inflation may
induce pulsational instability. Here, we investigate the occurrence and
consequences of both instabilities in models of Wolf-Rayet stars. We determine
the convective velocities in the sub-surface convective zones to estimate the
amplitude of the turbulent velocity at the base of the wind that potentially
leads to the formation of small-scale wind structures, as observed in several
WR stars. We also investigate the effect of mass loss on the pulsations of our
models. We approximated solar metallicity WR stars by models of mass-losing
helium stars, and we characterized the properties of convection in the envelope
adopting the standard MLT. Our results show the occurrence of sub-surface
convective regions in all studied models. Small surface velocity amplitudes are
predicted for models with masses below 10Msun. For models with M>10Msun, the
surface velocity amplitudes are of the order of 10km/s. Moreover we find the
occurrence of pulsations for stars in the mass range 9-14Msun, while mass loss
appears to stabilize the more massive WR stars. We confront our results with
observationally derived line variabilities of 17 WN stars. The data suggest
variability to occur for stars above 10Msun, which is increasing linearly with
mass above this value, in agreement with our results. We further find some of
our models to be unstable to radial pulsations, and predict local magnetic
fields of the order of hundreds of Gauss in WR stars more massive than 10Msun.
Our study relates the surface velocity fluctuations induced by sub-surface
convection to the formation of clumping in the inner part of the wind. From
this mechanism, we expect a stronger variability in more massive WR stars.Comment: A&A, accepte
Subsonic structure and optically thick winds from Wolf--Rayet stars
Wolf-Rayet star's winds can be so dense and so optically thick that the
photosphere appears in the highly supersonic part of the outflow, veiling the
underlying subsonic part of the star, and leaving the initial acceleration of
the wind inaccessible to observations. We investigate the conditions and the
structure of the subsonic part of the outflow of Galactic WR stars, in
particular of the WNE subclass; our focus is on the conditions at the sonic
point. We compute 1D hydrodynamic stellar structure models for massive helium
stars adopting outer boundaries at the sonic point. We find that the outflows
of our models are accelerated to supersonic velocities by the radiative force
from opacity bumps either at temperatures of the order of 200kK by the Fe
opacity bump or of the order of 50kK by the HeII opacity bump. For a given
mass-loss rate, the conditions in the subsonic part of the outflow are
independent from the detailed physical conditions in the supersonic part. The
close proximity to the Eddington limit at the sonic point allows us to
construct a Sonic HR diagram, relating the sonic point temperature to the L/M
ratio and the stellar mass-loss rate, thereby constraining the sonic point
conditions, the subsonic structure, and the stellar wind mass-loss rates from
observations. The minimum mass-loss rate necessary to have the flow accelerated
to supersonic velocities by the Fe opacity bump is derived. A comparison of the
observed parameters of Galactic WNE stars to this minimum mass-loss rate
indicates that their winds are launched to supersonic velocities by the
radiation pressure arising from the Fe-bump. Conversely, models which do not
show transonic flows from the Fe opacity bump form inflated envelopes. We
derive an analytic criterion for the appearance of envelope inflation in the
subphotospheric layers.Comment: A&A, Forthcoming article. 13 pages+
Inverse Compton cooling of thermal plasma in colliding-wind binaries
The inverse-Compton effect (IC) is a widely recognized cooling mechanism for
both relativistic and thermal electrons in various astrophysical environments,
including the intergalactic medium and X-ray emitting plasmas. Its effect on
thermal electrons is however frequently overlooked in theoretical and numerical
models of colliding-wind binaries (CWB). In this article, we provide a
comprehensive investigation of the impact of IC cooling in CWBs, presenting
general results for when the photon fields of the stars dominate the cooling of
the thermal plasma and when shocks at the stagnation point are expected to be
radiative. Our analysis shows that IC cooling is the primary cooling process
for the shocked-wind layer over a significant portion of the relevant parameter
space, particularly in eccentric systems with large wind-momentum ratios, e.g.,
those containing a Wolf-Rayet and O-type star. Using the binary system WR 140
as a case study, we demonstrate that IC cooling leads to a strongly radiative
shocked wind near periastron, which may otherwise remain adiabatic if only
collisional cooling was considered. Our results are further supported by 2D and
3D simulations of wind-wind collisions. Specifically, 3D magnetohydrodynamic
simulations of WR 140 show a significant decrease in hard-X-ray emission around
periastron, in agreement with observations but in contrast to equivalent
simulations that omit IC cooling. A novel method is proposed for constraining
mass-loss rates of both stars in eccentric binaries where the wind-collision
zone switches from adiabatic to radiative approaching periastron. IC scattering
is an important cooling process in the thermal plasma of CWBs.Comment: Accepted to MNRAS, 17 pages, 13 figure
Near-uniform internal rotation of the main-sequence γ Doradus pulsator KIC 7661054
We used Kepler photometry to determine the internal rotation rate of KIC 7661054, a chemically normal γ Dor star on the main sequence at spectral type F2.5 V. The core rotation period of 27.25 ±
0.06 d is obtained from the rotational splittings of a series of dipole g modes. The surface rotation period is calculated from a spectroscopic projected rotation velocity and a stellar radius computed from models. Literature data, obtained without inclusion of macroturbulence as a line-broadening mechanism, imply that the surface rotates much more quickly than the core, while our detailed analysis suggests that the surface may rotate slightly more quickly than the core and that the rotation profile is uniform within the 1-σ uncertainties. We discuss the pitfalls associated with the determination of surface rotation rates of slow rotators from spectroscopy in the absence of asteroseismic constraints. A broad signal is observed at low frequency, which we show cannot be attributed to rotation, contrary to previous suggestions concerning the origin of such signals
Near-uniform internal rotation of the main-sequence γ Doradus pulsator KIC 7661054
We used Kepler photometry to determine the internal rotation rate of KIC 7661054, a chemically normal γ Dor star on the main sequence at spectral type F2.5 V. The core rotation period of 27.25 ± 0.06 d is obtained from the rotational splittings of a series of dipole g modes. The surface rotation period is calculated from a spectroscopic projected rotation velocity and a stellar radius computed from models. Literature data, obtained without inclusion of macroturbulence as a line-broadening mechanism, imply that the surface rotates much more quickly than the core, while our detailed analysis suggests that the surface may rotate slightly more quickly than the core and that the rotation profile is uniform within the 1σ uncertainties. We discuss the pitfalls associated with the determination of surface rotation rates of slow rotators from spectroscopy in the absence of asteroseismic constraints. A broad signal is observed at low frequency, which we show cannot be attributed to rotation, contrary to previous suggestions concerning the origin of such signals
Wind-envelope interaction as the origin of the slow cyclic brightness variations of luminous blue variables
Luminous blue variables (LBVs) are hot, very luminous massive stars displaying large quasi-periodic variations in brightness, radius, and photospheric temperature on timescales of years to decades. The physical origin of this variability, called S Doradus cycle after its prototype, has remained elusive. We study the feedback of stellar wind mass-loss on the envelope structure in stars near the Eddington limit. We calculated a time-dependent hydrodynamic stellar evolution, applying a stellar wind mass-loss prescription with a temperature dependence inspired by the predicted systematic increase in mass-loss rates below 25 kK. We find that when the wind mass-loss rate crosses a well-defined threshold, a discontinuous change in the wind base conditions leads to a restructuring of the stellar envelope. The induced drastic radius and temperature changes, which occur on the thermal timescale of the inflated envelope, in turn impose mass-loss variations that reverse the initial changes, leading to a cycle that lacks a stationary equilibrium configuration. Our proof-of-concept model broadly reproduces the typical observational phenomenology of the S Doradus variability. We identify three key physical ingredients that are required to trigger the instability: inflated envelopes in close proximity to the Eddington limit, a temperature range where decreasing opacities do not lead to an accelerating outflow, and a mass-loss rate that increases with decreasing temperature, crossing a critical threshold value within this temperature range. Our scenario and model provide testable predictions, and open the door for a consistent theoretical treatment of the LBV phase in stellar evolution, with consequences for their further evolution as single stars or in binary systems
Bringing Stellar Evolution & Feedback Together: Summary of proposals from the Lorentz Center Workshop, 2022
Stars strongly impact their environment, and shape structures on all scales
throughout the universe, in a process known as ``feedback''. Due to the
complexity of both stellar evolution and the physics of larger astrophysical
structures, there remain many unanswered questions about how feedback operates,
and what we can learn about stars by studying their imprint on the wider
universe. In this white paper, we summarize discussions from the Lorentz Center
meeting `Bringing Stellar Evolution and Feedback Together' in April 2022, and
identify key areas where further dialogue can bring about radical changes in
how we view the relationship between stars and the universe they live in.Comment: Accepted to the Publications of the Astronomical Society of the
Pacifi