124 research outputs found
Stability and evolution of super-massive stars (SMS)
Highly condensed gaseous objects with masses larger than 5x10^4 M_sun are
called super-massive stars. In the quasistationary contraction phase, the
hydrostatic equilibrium is determined by radiation pressure and gravitation.
The global structure is that of an n=3 polytrope at the stability limit. Small
relativistic corrections for example can initiate a free fall collapse due to
the 'post Newtonian' instability. Since the outcome of the final collapse -A
super-massive black hole or hypernova- depends sensitively on the structure and
the size of the object, when the instability sets in, it is important to
investigate in more detail the contraction phase of the SMS. If the gaseous
object is embedded in a dense stellar system, the central star cluster, the
interaction and coupling of both components due to dynamical friction changes
the energy balance and evolution of the SMS dramatically. Dynamical friction
between stars and gas, which can be estimated semi-analytically (see Just et
al. 1986), has three different effects on the two-component system. We discuss
in which evolutionary stages and parameter range these interaction processes
are relevant and how they can influence the stability and evolution of the SMS.Comment: 6 pages, 1 figure, needs eas.cls (included). EAS Publ. Series, Vol.
10 EDP, Paris in pres
The loss-cone problem in dense nuclei
We address the classical problem of star accretion onto a supermassive
central gaseous object in a galactic nucleus. The resulting supermassive
central gas-star object is assumed to be located at the centre of a dense
stellar system for which we use a simplified model consisting of a Plummer
model with an embedded density cusp using stellar point masses. From the number
of stars belonging to the loss-cone, which plunge onto the central object on
elongated orbits from outside, we estimate the accretion rate taking into
account a possible anisotropy of the surrounding stellar distribution. The
total heating rate in the supermassive star due to the loss-cone stars plunging
onto it is estimated. This semi-analytical study, revisiting and expanding
classical paper's work, is a starting point of future work on a more detailed
study of early evolutionary phases of galactic nuclei.
It merits closer examination, because it is one of the key features for the
link between cosmology and galaxy formation.Comment: 9 pages, 6 figures, MNRAS in pres
Colliding red giants in galactic nuclei: Shocks, jets, impact on the ISM, X- and gamma-rays, neutrinos, fusion ignition and afterglow
In galactic nuclei, stellar densities are so high that stars can physically
collide with each other. In this work we focus on the collision of red giants
and in particular on the formation of non-thermal processes through collisions
and their properties. We analytically address these points by evaluating
head-on collisions but also take into account scenarios with a deviation from
the radial orbit, which we treat in a perturbative fashion. The collisions
produce internal shocks with supersonic Mach numbers. Almost immediately,
jet-like structures with important Lorentz factors form. The debris from the
collision produces another shock wave which, when interacting with the
interstellar medium of a galactic nucleus, leads to particle acceleration. We
estimate the background flux in X- and gamma rays created by the background of
these collisions by deriving the spectral index within a radius of 100 Mpc and
find that they are high. Additionally, we make an estimate of the neutrino
production and find about neutrinos per square meter per second for a
collision at 100 Mpc from Earth. Also, we derive that there is a non-negligible
chance to ignite fusion during the collision, due to the squeezing of the
material. We investigate the possibility that the degenerate cores collide with
each other, leading to a high afterglow luminosity, and find that it is
non-negligible, although this should be addressed with dedicated numerical
simulations. Colliding red giants in galactic nuclei trigger a plethora of
high-energy phenomena, and have a particular gravitational wave emission
associated, as shown by us, so that their detection will allow us to rule out
alternatives.Comment: 30 pages, no figures, submitted. Abstract abridge
Transient stellar collisions as multimessenger probes: Non-thermal-, gravitational wave emission and the cosmic ladder argument
In dense stellar clusters like galactic nuclei and globular clusters stellar
densities are so high that stars might physically collide with each other. In
galactic nuclei the energy and power output can be close, and even exceed, to
those from supernovae events. We address the event rate and the electromagnetic
characteristics of collisions of main sequence stars (MS) and red giants (RG).
We also investigate the case in which the cores form a binary and emit
gravitational waves. In the case of RGs this is particularly interesting
because the cores are degenerate. We find that MS event rate can be as high as
tens per year, and that of RGs one order of magnitude larger. The collisions
are powerful enough to mimic supernovae- or tidal disruptions events. We find
Zwicky Transient Facility observational data which seem to exhibit the features
we describe. The cores embedded in the gaseous debris experience a friction
force which has an impact on the chirping mass of the gravitational wave. As a
consequence, the two small cores in principle mimic two supermassive black
holes merging. However, their evolution in frequency along with the precedent
electromagnetic burst and the ulterior afterglow are efficient tools to reveal
the impostors. In the particular case of RGs, we derive the properties of the
degenerate He cores and their H-burning shells to analyse the formation of the
binaries. The merger is such that it can be misclassified with SN Ia events.
Because the masses and densities of the cores are so dissimilar in values
depending on their evolutionary stage, the argument about standard candles and
cosmic ladder should be re-evaluated.Comment: 32 pages. Accepted for publication ApJ, minor change
Underluminous tidal disruptions
We have evidence of X-ray flares in several galaxies consistent with a a star
being tidally disrupted by a supermassive black hole (MBH). If the star starts
on a nearly parabolic orbit relative to the MBH, one can derive that the
fallback rate follows a decay in the bolometric luminosity. We have
modified the standard version of the smoothed-particle hydrodynamics (SPH) code
{\sc Gadget} to include a relativistic treatment of the gravitational forces.
We include non-spinning post-Newtonian corrections to incorpore the periapsis
shift and the spin-orbit coupling up to next-to-lowest order. We run a set of
simulations for different penetration factors in both the Newtonian- and the
relativistic regime. We find that tidal disruptions around MBHs in the
relativistic cases are underluminous for values starting at ; i.e. the fallback curves produced in the relativistic cases are
progressively lower compared to the Newtonian simulations as the penetration
parameter increases. This is due to the fact that, contrary to the Newtonian
cases, we find that all relativistic counterparts feature a survival core for
penetration factors going to values as high as . We derive a
relativistic calculation which shows that geodesics of the elements in the star
converge as compared to the Newtonian case, allowing for a core to survive the
tidal disruption. A survival core should consistently emerge from any TDE with
. The higher the value, the lower the colour
temperatures than derived from standard accretion models.Comment: 17 pages, submitte
Super-massive stars: Radiative transfer
The concept of central super-massive stars (, where is the mass of the super-massive star) embedded in
dense stellar systems was suggested as a possible explanation for high- energy
emissions phenomena occurring in active galactic nuclei and quasars (Vilkoviski
1976, Hara 1978), such as X-ray emissions (Bahcall and Ostriker, 1975). SMSs
and super-massive black holes are two possibilities to explain the nature of
super-massive central objects, and super-massive stars may be an intermediate
step towards the formation of super-massive black holes (Rees 1984). Therefore
it is important to study such a dense gas-star system in detail. We address
here the implementation of radiative transfer in a model which was presented in
former work (Amaro-Seoane and Spurzem 2001, Amaro-Seoane et al. 2002). In this
sense, we extend here and improve the work done by Langbein et al. (1990) by
describing the radiative transfer in super-massive stars using previous work on
this subject (Castor 1972).Comment: 2 pages, to appear in "Galatic Dynamics", eds. C. Boily, P. Patsis,
C. Theis, S. Portegies Zwart, R. Spurzem, EDP Sciences 2003 (JENAM 2002
Conference in Porto, September 2-7, Workshop "Galactic Dynamics"). Needs
eas.cls (also included
Gravitational waves from eccentric intermediate-mass black hole binaries
If binary intermediate-mass black holes (IMBHs; with masses between 100 and
10^4 \Msun) form in dense stellar clusters, their inspiral will be detectable
with the planned Laser Interferometer Space Antenna (LISA) out to several Gpc.
Here we present a study of the dynamical evolution of such binaries using a
combination of direct -body techniques (when the binaries are well
separated) and three-body relativistic scattering experiments (when the
binaries are tight enough that interactions with stars occur one at a time). We
find that for reasonable IMBH masses there is only a mild effect on the
structure of the surrounding cluster even though the binary binding energy can
exceed the binding energy of the cluster. We demonstrate that, contrary to
standard assumptions, the eccentricity in the LISA band can be in {\em some}
cases as large as and that it induces a measurable phase
difference from circular binaries in the last year before merger. We also show
that, even though energy input from the binary decreases the density of the
core and slows down interactions, the total time to coalescence is short enough
(typically less than a hundred million years) that such mergers will be unique
snapshots of clustered star formation.Comment: Accepted for publication by ApJ Lett
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