22 research outputs found
Nature of the Extreme Ultraluminous X-ray Sources
In this proof-of-concept study we demonstrate that in a binary system mass
can be transferred toward an accreting compact object at extremely high rate.
If the transferred mass is efficiently converted to X-ray luminosity (with
disregard of the classical Eddington limit) or if the X-rays are focused into a
narrow beam then binaries can form extreme ULX sources with the X-ray
luminosity of Lx>10^42 erg/s. For example, Lasota & King argued that the
brightest known ULX (HLX-1) is a regular binary system with a rather low-mass
compact object (a stellar-origin black hole or a neutron star). The predicted
formation efficiencies and lifetimes of binaries with the very high mass
transfer rates are large enough to explain all observed systems with extreme
X-ray luminosities. These systems are not only limited to binaries with
stellar-origin black hole accretors. Noteworthy, we have also identified such
objects with neutron stars. Typically, a 10 Msun black hole is fed by a massive
(10 Msun) Hertzsprung gap donor with Roche lobe overflow rate of 10^-3 Msun/yr
(2600 MEdd). For neutron star systems the typical donors are evolved low-mass
(2 Msun) helium stars with Roche lobe overflow rate of 10^-2 Msun/yr. Our study
does not prove that any particular extreme ULX is a regular binary system, but
it demonstrates that any ULX, including the most luminous ones, may potentially
be a short-lived phase in the life of a binary star.Comment: ApJ - accepted (significant changes
Strange quark stars in binaries: formation rates, mergers and explosive phenomena
The existence of strange quark stars has been proposed many years ago. More
recently, the possible co-existence of a first family composed of "normal"
neutron stars with a second family of strange quark stars has been proposed as
a solution of problems related to the maximum mass and to the minimal radius of
these compact stellar objects. In this paper we study the mass distribution of
compact objects formed in binary systems and the relative fractions of quark
and neutron stars in different subpopulations. We incorporate the strange quark
star formation model provided by the two-families scenario and we perform a
large-scale population synthesis study in order to obtain the population
characteristics. In our model, below a critical gravitational mass
only normal (hadron) neutron stars exist. Then in
the mass range strange quark stars and neutron stars coexist. Finally, above
all compact objects are strange quark stars. We argue that
is in the range . According to our
results, the main channel for strange quark star formation in binary systems is
accretion from a secondary companion on a neutron star.This opens the
possibility of having explosive GRB-like phenomena not related to supernovae
and not due to the merger of two neutron stars. The enhancement in the number
of compact objects in the co-existence mass range is not very pronounced. The number of
double strange quark star's systems is rather small with only a tiny fraction
which merge within a Hubble time. This drastically limits the flux of
strangelets produced by the merger, which turns out to be compatible with all
limits stemming from Earth and lunar experiments.Comment: 11 pages, 10 figures, minor typos corrected, ApJ, 846, 16
Merger of compact stars in the two-families scenario
We analyse the phenomenological implications of the two-families scenario on
the merger of compact stars. That scenario is based on the coexistence of both
hadronic stars and strange quark stars. After discussing the classification of
the possible mergers, we turn to detailed numerical simulations of the merger
of two hadronic stars, i.e., "first family" stars in which delta resonances and
hyperons are present, and we show results for the threshold mass of such
binaries, for the mass dynamically ejected and the mass of the disk surrounding
the post-merger object. We compare these results with those obtained within the
one-family scenario and we conclude that relevant signatures of the
two-families scenario can be suggested, in particular: the possibility of a
rapid collapse to a black hole for masses even smaller than the ones associated
to GW170817; during the first milliseconds, oscillations of the postmerger
remnant at frequencies higher than the ones obtained in the one-family
scenario; a large value of the mass dynamically ejected and a small mass of the
disk, for binaries of low total mass. Finally, based on a population synthesis
analysis, we present estimates of the number of mergers for: two hadronic
stars; hadronic star - strange quark star; two strange quark stars. We show
that for unequal mass systems and intermediate values of the total mass, the
merger of a hadronic star and a strange quark star is very likely (GW170817 has
a possible interpretation into this category of mergers). On the other hand,
mergers of two strange quark stars are strongly suppressed.Comment: 18 pages, 16 figure
Synthetic Population of Binary Cepheids. II. The effect of companion light on the extragalactic distance scale
Because of their period-luminosity relation (PLR), classical Cepheids play a
key role in the calibration of the extragalactic distance scale and the
determination of the Hubble-Lema\^{i}tre constant . Recent findings show
that the majority of classical Cepheids should be in binary or multiple
systems, which might undermine their accuracy, as the extra -- and unaccounted
for -- light from the companions of Cepheids causes a shift in the PLR. We
quantify this shift using synthetic populations of binary Cepheids that we
developed for this purpose, as described in Paper I of this series. We find
that while all PLRs are shifted toward brighter values due to the excess light
from the companions, the bias in the relative distance modulus between two
galaxies hosting binary Cepheids can be either positive or negative, depending
on the percentage of binary Cepheids in them. If the binarity percentage in the
two galaxies is similar, the effect of binarity is canceled. Otherwise, it
introduces a shift in the distance modulus of the order of millimags in the
near-infrared passbands and Wesenheit indices, and tens of millimags in the
visual domain; its exact value depends on the variant of the synthetic
population (a unique combination of metallicity, star formation history, shape
and location of the instability strip, and initial parameter distributions).
Such shifts in distance moduli to type Ia supernova host galaxies introduce an
additional statistical error on , which however does not prevent measuring
with a precision of 1%.Comment: 16 pages, 11 figures, accepted for publication in The Astrophysical
Journa
Synthetic population of binary Cepheids. I. The effect of metallicity and initial parameter distribution on characteristics of Cepheids' companions
The majority of classical Cepheids are binary stars, yet the contribution of
companions' light to the total brightness of the system has been assumed
negligible and lacked a thorough, quantitative evaluation. We present an
extensive study of synthetic populations of binary Cepheids, that aims to
characterize Cepheids' companions (e.g. masses, evolutionary and spectral
types), quantify their contribution to the brightness and color of Cepheid
binaries, and assess the relevance of input parameters on the results. We
introduce a collection of synthetic populations, which vary in metal content,
initial parameter distribution, location of the instability strip edges, and
star formation history. Our synthetic populations are free from the selection
bias, while the percentage of Cepheid binaries is controlled by the binarity
parameter. We successfully reproduce recent theoretical and empirical results:
the percentage of binary Cepheids with main sequence (MS) companions, the
contrast-mass ratio relation for binary Cepheids with MS companions, the
appearance of binary Cepheids with giant evolved companions as outlier data
points above the period-luminosity relation. Moreover, we present the first
estimation of the percentage of binary Cepheids in the Large Magellanic Cloud
and announce the quantification of the effect of binarity on the slope and
zero-point of multiband period-luminosity relations, which will be reported in
the next paper of this series.Comment: 29 pages, 19 figures (5 in the Appendix), accepted for Ap
Missing Black Holes Unveil The Supernova Explosion Mechanism
It is firmly established that the stellar mass distribution is smooth,
covering the range 0.1-100 Msun. It is to be expected that the masses of the
ensuing compact remnants correlate with the masses of their progenitor stars,
and thus it is generally thought that the remnant masses should be smoothly
distributed from the lightest white dwarfs to the heaviest black holes.
However, this intuitive prediction is not borne out by observed data. In the
rapidly growing population of remnants with observationally determined masses,
a striking mass gap has emerged at the boundary between neutron stars and black
holes. The heaviest neutron stars reach a maximum of two solar masses, while
the lightest black holes are at least five solar masses. Over a decade after
the discovery, the gap has become a significant challenge to our understanding
of compact object formation. We offer new insights into the physical processes
that bifurcate the formation of remnants into lower mass neutron stars and
heavier black holes. Combining the results of stellar modeling with
hydrodynamic simulations of supernovae, we both explain the existence of the
gap, and also put stringent constraints on the inner workings of the supernova
explosion mechanism. In particular, we show that core-collapse supernovae are
launched within 100-200 milliseconds of the initial stellar collapse, implying
that the explosions are driven by instabilities with a rapid (10-20 ms) growth
time. Alternatively, if future observations fill in the gap, this will be an
indication that these instabilities develop over a longer (>200 milliseconds)
timescale.Comment: ApJ, accepted: comments added on recent Ugliano et al. and Kreidberg
et al. studie
Compact Remnant Mass Function: Dependence on the Explosion Mechanism and Metallicity
The mass distribution of neutron stars and stellar-mass black holes provides
vital clues into the nature of stellar core collapse and the physical engine
responsible for supernova explosions. Using recent advances in our
understanding of supernova engines, we derive mass distributions of stellar
compact remnants. We provide analytical prescriptions for compact object masses
for major population synthesis codes. In an accompanying paper, Belczynski et
al., we demonstrate that these qualitatively new results for compact objects
can explain the observed gap in the remnant mass distribution between ~2-5
solar masses and that they place strong constraints on the nature of the
supernova engine. Here, we show that advanced gravitational radiation detectors
(like LIGO/VIRGO or the Einstein Telescope) will be able to further test the
supernova explosion engine models once double black hole inspirals are
detected.Comment: 37 pages with 16 figures, submitted to Ap