47 research outputs found
The formation and evolution of very massive stars in dense stellar systems
The early evolution of dense stellar systems is governed by massive single
star and binary evolution. Core collapse of dense massive star clusters can
lead to the formation of very massive objects through stellar collisions
( 1000 \msun). Stellar wind mass loss determines the evolution and final
fate of these objects, and decides upon whether they form black holes (with
stellar or intermediate mass) or explode as pair instability supernovae,
leaving no remnant. We present a computationaly inexpensive evolutionary scheme
for very massive stars that can readily be implemented in an N-body code. Using
our new N-body code 'Youngbody' which includes a detailed treatment of massive
stars as well as this new scheme for very massive stars, we discuss the
formation of intermediate mass and stellar mass black holes in young starburst
regions. A more detailed account of these results can be found in Belkus et al.
2007.Comment: 2 pages, 2 figures. To appear in conference proceedings for IAUS246,
200
Binary populations and stellar dynamics in young clusters
We first summarize work that has been done on the effects of binaries on
theoretical population synthesis of stars and stellar phenomena. Next, we
highlight the influence of stellar dynamics in young clusters by discussing a
few candidate UFOs (unconventionally formed objects) like intermediate mass
black holes, Eta Carinae, Zeta Puppis, Gamma Velorum and WR 140.Comment: Contributed paper IAU 250: Massive Stars as Cosmic Engine
The evolution of very massive stars
Publisher's version/PDFCore collapse of dense massive star clusters is unavoidable, and this leads to the formation of massive objects, with masses of up to 1000 M[circled dot] and even larger. When these objects become stars, stellar wind mass loss determines their evolution and final fate, and decides on whether they form black holes (with normal mass or with intermediate mass) or explode as a pair-instability supernova. In this paper we discuss the evolution of very massive stars and present a convenient evolution recipe that can be implemented in a gravitational N-body code to study the dynamics of dense massive clusters
Detection of IMBHs with ground-based gravitational wave observatories: A biography of a binary of black holes, from birth to death
Even though the existence of intermediate-mass black holes (IMBHs, black
holes with masses ranging between ) has not yet been
corroborated observationally, these objects are of high interest for
astrophysics. Our understanding of the formation and evolution of supermassive
black holes (SMBHs), as well as galaxy evolution modeling and cosmography would
dramatically change if an IMBH were to be observed. From a point of view of
traditional photon-based astronomy, {which relies on the monitoring of
innermost stellar kinematics}, the {\em direct} detection of an IMBH seems to
be rather far in the future. However, the prospect of the detection and
characterization of an IMBH has good chances in lower-frequency
gravitational-wave (GW) astrophysics using ground-based detectors such as LIGO,
Virgo and the future Einstein Telescope (ET). We present an analysis of the
signal of a system of a binary of IMBHs (BBH from now onwards) based on a
waveform model obtained with numerical relativity simulations coupled with
post-Newtonian calculations at the highest available order. IMBH binaries with
total masses between would produce significant
signal-to-noise ratios (SNRs) in advanced LIGO and Virgo and the ET. We have
computed the expected event rate of IMBH binary coalescences for different
configurations of the binary, finding interesting values that depend on the
spin of the IMBHs. The prospects for IMBH detection and characterization with
ground-based GW observatories would not only provide us with a robust test of
general relativity, but would also corroborate the existence of these systems.
Such detections should allow astrophysicists to probe the stellar environments
of IMBHs and their formation processes.Comment: 30 pp. Accepted for publication ApJ. Event rates calculated from
scratc
The WR population predicted by massive single star and by massive binary evolution
We discuss differences between massive single star and massive close binary
population number synthesis predictions of WR stars. We show that the WC/WN
number ratio as function of metallicity depends significantly on whether or not
binaries are included. Furthermore, the observed WC(+OB)/WN(+OB) number ratio
in the Solar neighborhood seems to indicate that the WR mass loss rates are
lower by another factor two compared to recently proposed clumping corrected
formalisms. We then demonstrate that the observed lower luminosity distribution
of single WN stars can be explained in a satisfactory way by massive single
star evolutionary computations where the red supergiant phase is calculated
using a stellar wind mass loss rate formalism that is based on recent
observations.Comment: 13 pages, 4 figures; comments and criticisms on this preprint are
very welcom
A multiphysics and multiscale software environment for modeling astrophysical systems
We present MUSE, a software framework for combining existing computational
tools for different astrophysical domains into a single multiphysics,
multiscale application. MUSE facilitates the coupling of existing codes written
in different languages by providing inter-language tools and by specifying an
interface between each module and the framework that represents a balance
between generality and computational efficiency. This approach allows
scientists to use combinations of codes to solve highly-coupled problems
without the need to write new codes for other domains or significantly alter
their existing codes. MUSE currently incorporates the domains of stellar
dynamics, stellar evolution and stellar hydrodynamics for studying generalized
stellar systems. We have now reached a "Noah's Ark" milestone, with (at least)
two available numerical solvers for each domain. MUSE can treat multi-scale and
multi-physics systems in which the time- and size-scales are well separated,
like simulating the evolution of planetary systems, small stellar associations,
dense stellar clusters, galaxies and galactic nuclei.
In this paper we describe three examples calculated using MUSE: the merger of
two galaxies, the merger of two evolving stars, and a hybrid N-body simulation.
In addition, we demonstrate an implementation of MUSE on a distributed computer
which may also include special-purpose hardware, such as GRAPEs or GPUs, to
accelerate computations. The current MUSE code base is publicly available as
open source at http://muse.liComment: 24 pages, To appear in New Astronomy Source code available at
http://muse.l
Integrated spectral energy distributions of binary star composite stellar populations
This paper presents theoretical integrated spectral energy distributions
(SEDs) of binary star composite stellar populations (bsCSPs) in early-type
galaxies, and how the bsCSP model can be used for spectral studies of galaxies.
All bsCSPs are built basing on three adjustable inputs (metallicity, ages of
old and young components). The effects of binary interactions and stellar
population mixture are taken into account. The results show some UV-upturn SEDs
naturally for bsCSPs. The SEDs of bsCSPs are affected obviously by all of three
stellar population parameters, and the effects of three parameters are
degenerate. This suggests that the effects of metallicity, and the ages of the
old (major in stellar mass) and young (minor) components of stellar populations
should be taken into account in SED studies of early-type galaxies. The
sensitivities of SEDs at different wavelengths to the inputs of a stellar
population model are also investigated. It is shown that UV SEDs are sensitive
to all of three stellar population parameters, rather than to only stellar age.
Special wavelength ranges according to some SED features that are relatively
sensitive to stellar metallicity, young-component age, and old-component age of
bsCSPs are found by this work. For example, the shapes of SEDs with wavelength
ranges of 5110-5250AA, 5250--5310AA, 5310--5350AA, 5830--5970AA, 20950--23550AA
are relatively sensitive to the stellar metallicity of bsCSPs. The shapes of
SEDs within 965-985AA, 1005--1055AA, 1205--1245AA are sensitive to
old-component age, while SED features within the wavelength ranges of
2185--2245AA, 2455--2505AA, 2505--2555AA, 2775--2825AA, 2825--2875AA to
young-component age.Comment: 10 pages, 12 figures, Accepted to publish in MNRA
The evolution of massive and very massive stars in clusters
The present paper reviews massive star (initial mass smaller than 120 M0) and
very massive star (initial mass larger than 120 M0) evolution. I will focus on
evolutionary facts and questions that may critically affect predictions of
population and spectral synthesis of starburst regions. We discuss the
ever-lasting factor 2 or more uncertainty in the stellar wind mass loss rates.
We may ask ourselves if stellar rotation is one of the keys to understand the
universe, why so many massive stars are binary components and why binaries are
ignored or are considered as the poor cousins by some people? And finally, do
ultra luminous X-ray sources harbor an intermediate mass black hole with a mass
of the order of 1000 M0?Comment: 16 pages, 7 figures, Review talk presented at the conference From
Taurus to the Antennae, Sheffield 4-8th August 200
Wind modelling of very massive stars up to 300 solar masses
Some studies have claimed a universal stellar upper-mass limit of 150 Msun. A
factor that is often overlooked is that there might be a difference between the
current and initial masses of the most massive stars, as a result of mass loss.
We present Monte Carlo mass-loss predictions for very massive stars in the
range 40-300 Msun, with large luminosities and Eddington factors Gamma. Using
our new dynamical approach, we find an upturn in the mass-loss vs. Gamma
dependence, at the point where the winds become optically thick. This coincides
with the location where wind efficiency numbers surpass the single-scattering
limit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests a
transition from common O-type winds to Wolf-Rayet characteristics at the point
where the winds become optically thick. This transitional behaviour is also
revealed with respect to the wind acceleration parameter beta, which starts at
values below 1 for the optically thin O-stars, and naturally reaches values as
high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding
concerns the transition in spectral morphology of the Of and WN characteristic
He II line at 4686 Angstrom. When we express our mass-loss predictions as a
function of the electron scattering Gamma_e (=L/M) only, we obtain a mass-loss
Gamma dependence that is consistent with a previously reported power-law Mdot
propto Gamma^5 (Vink 2006) that was based on our semi-empirical modelling
approach. When we express Mdot in terms of both Gamma and stellar mass, we find
Mdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm that
the Gamma-effect on the mass-loss predictions is much stronger than that of an
increased helium abundance, calling for a fundamental revision in the way mass
loss is incorporated in evolutionary models of the most massive stars.Comment: minor language changes (Astronomy & Astrophysics in press - 11 pages,
10 figures