398 research outputs found
The Puzzling Frequencies of CEMP and NEMP Stars
We present the results of binary population simulations of carbon- and
nitrogen-enhanced metal-poor (CEMP and NEMP) stars. We show that the observed
paucity of very nitrogen-rich stars puts strong constraints on possible
modifications of the initial mass function at low metallicity.Comment: 3 pages, contribution to "The Origin of the Elements Heavier than
Iron" in honor of the 70th birthday of Roberto Gallino, Torino, Italy,
September 200
Third Dredge-up in Low Mass Stars: Solving the LMC Carbon Star Mystery
A long standing problem with asymptotic giant branch (AGB) star models has
been their inability to produce the low-luminosity carbon stars in the Large
and Small Magellanic Clouds. Dredge-up must begin earlier and extend deeper. We
find this for the first time in our models of LMC metallicity. Such features
are not found in our models of SMC metallicity. The fully implicit and
simultaneous stellar evolution code STARS has been used to calculate the
evolution of AGB stars with metallicities of Z=0.008 and Z=0.004, corresponding
to the observed metallicities of the Large and Small Magellanic Clouds,
respecitively. Third dredge-up occurs in stars of 1Msol and above and carbon
stars were found for models between 1Msol and 3Msol. We use the detailed models
as input physics for a population synthesis code and generate carbon star
luminosity functions. We now find that we are able to reproduce the carbon star
luminosity function of the LMC without any manipulation of our models. The SMC
carbon star luminosity function still cannot be produced from our detailed
models unless the minimum core mass for third dredge-up is reduced by 0.06Msol.Comment: 6 pages, 5 figures. Accepted for publication in MNRA
The interaction of core-collapse supernova ejecta with a stellar companion
The progenitors of many core-collapse supernovae (CCSNe) are expected to be
in binary systems. By performing a series of three-dimensional hydrodynamical
simulations, we investigate how CCSN explosions affect their binary companion.
We find that the amount of removed stellar mass, the resulting impact velocity,
and the chemical contamination of the companion that results from the impact of
the SN ejecta, strongly increases with decreasing binary separation and
increasing explosion energy. Also, it is foud that the impact effects of CCSN
ejecta on the structure of main-sequence (MS) companions, and thus their long
term post-explosion evolution, is in general not be dramatic.Comment: 5 pages, 3 figures, poster contribution: IAU Symposium 346 "High Mass
X-ray Binaries: illuminating the passage from massive binaries to merging
compact objects", Vienna, Austria, 27-31 August 2018. arXiv admin note:
substantial text overlap with arXiv:1509.0363
The interaction of core-collapse supernova ejecta with a companion star
The progenitors of many CCSNe are expected to be in binary systems. After the
SN explosion, the companion may suffer from mass stripping and be shock heated
as a result of the impact of the SN ejecta. If the binary system is disrupted,
the companion is ejected as a runaway and hypervelocity star. By performing a
series of 3D hydrodynamical simulations of the collision of SN ejecta with the
companion star, we investigate how CCSN explosions affect their companions. We
use the BEC code to construct the detailed companion structure at the time of
SN explosion. The impact of the SN blast wave on the companion is followed by
means of 3D SPH simulations using the Stellar GADGET code. For main-sequence
(MS) companions, we find that the amount of removed mass, impact velocity, and
chemical contamination of the companion that results from the impact of the SN
ejecta, strongly increases with decreasing binary separation and increasing
explosion energy. Their relationship can be approximately fitted by power laws,
which is consistent with the results obtained from impact simulations of
SNe~Ia. However, we find that the impact velocity is sensitive to the momentum
profile of the outer SN ejecta and, in fact, may decrease with increasing
ejecta mass, depending on the modeling of the ejecta. Because most companions
to Ib/c CCSNe are in their MS phase at the moment of the explosion, combined
with the strongly decaying impact effects with increasing binary separation, we
argue that the majority of these SNe lead to inefficient mass stripping and
shock heating of the companion star following the impact of the ejecta. Our
simulations show that the impact effects of Ib/c SN ejecta on the structure of
MS companions, and thus their long-term post-explosion evolution, is in general
not dramatic. We find that at most 10% of their mass is lost, and their
resulting impact velocities are less than 100 km/s.Comment: Accepted for publication in Astronomy and Astrophysics, some minor
typographical errors are fixed, the affiliation of second author is correcte
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
BINSTAR: a new binary stellar evolution code
We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code. We also computed a large grid of models for various masses (2 ≤ M/M⊙ ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E2, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E2 during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E2 weakly depends on core overshooting but is substantially increased when the metallicity becomes lower
- …