We combine a large database of population synthesis calculations, models for the star formation history of the universe, and a simple selection model for bursts to predict short GRB detection rates, ...redshift distributions, and host galaxy distributions. We compare our space of possible models with observations of short GRBs (rates and redshifts) and, when assuming short GRBs are produced from NS-NS binaries, the current estimates for NS-NS merger rates from close binary pulsars in the Milky Way. Whether short GRBs are assumed to arise from BH-NS or NS-NS mergers, we conclude that a fraction of models are in agreement with available short GRB and binary pulsar observations. We do not need to introduce artificial models with long delay times. Most commonly, models produce mergers preferentially in spiral galaxies if short GRBs arise from NS-NS mergers alone. On the other hand, typically BH-NS mergers can also occur in elliptical galaxies, in agreement with existing observations. We expect that a higher proportion of short GRBs should occur at moderate to high redshift (e.g., image) than has presently been observed, in agreement with recent observations which suggest a strong selection bias toward successful follow-up of low-redshift short GRBs. Finally, if we add plausible additional assumptions about what BH-NS mergers could produce short GRBs based on the work of Belczynski and coworkers, then we expect only a small fraction of BH-NS models could be consistent with all current available data.
The merger of close binary systems containing two neutron stars should produce a burst of gravitational waves, as predicted by the theory of general relativity. A reliable estimate of the ...double-neutron-star merger rate in the Galaxy is crucial in order to predict whether current gravity wave detectors will be successful in detecting such bursts. Present estimates of this rate are rather low, because we know of only a few double-neutron-star binaries with merger times less than the age of the Universe. Here we report the discovery of a 22-ms pulsar, PSR J0737-3039, which is a member of a highly relativistic double-neutron-star binary with an orbital period of 2.4 hours. This system will merge in about 85 Myr, a time much shorter than for any other known neutron-star binary. Together with the relatively low radio luminosity of PSR J0737-3039, this timescale implies an order-of-magnitude increase in the predicted merger rate for double-neutron-star systems in our Galaxy (and in the rest of the Universe).
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The observed samples of supernovae (SNe) and double compact objects (DCOs) provide several critical constraints on population synthesis models. The parameters of these models must be carefully chosen ...to reproduce, among other factors, (1) the formation rates of double neutron star (NS-NS) binaries and white dwarf-neutron star (WD-NS) binaries, estimated from binary samples, and (2) the Type II and Ib/c SN rates. Even allowing for extremely conservative accounting of the uncertainties in observational and theoretical predictions, we find that only a few plausible population synthesis models (roughly 9%) are consistent with DCO and SN rates empirically determined from observations. As a proof of concept, we describe the information that can be extracted about population synthesis models given these observational tests, including surprisingly good agreement with the neutron star kick distributions inferred from pulsar proper-motion measurements. In the present study, we find that the current observational constraints favor kicks described by a single Maxwellian with a characteristic velocity of about 350 km s super(-1)(i.e., at maximum likelihood; kick velocities between 100 and 700 km s super(-1) remain within the 90% confidence interval of unimodal distributions), mass-loss fractions during nonconservative but stable mass transfer episodes of about 90%, and common envelope parameters of about 0.15-0.5. Finally, we use the subset of astrophysically consistent models to predict the rates at which black hole-neutron star (BH-NS) and NS-NS binaries merge in the Milky Way and the nearby universe, assuming that Milky Way-like galaxies dominate. Inevitably, the resulting probability distributions for merger rates depend on our assumed priors for the population model input parameters. In this study we adopt relatively conservative priors (flat) for all model parameters covering a rather wide range of values. However, as we gain confidence in our knowledge of these inputs, the range of merger rates consistent with our knowledge should shift and narrow.
We examine the exchange of angular momentum between the component spins and the orbit in semi-detached double white dwarf binaries undergoing mass transfer through direct impact of the transfer ...stream. This work improves upon similar earlier studies in a number of ways. First, we self-consistently calculate the total angular momentum of the orbit at all times. Second, we calculate the particle's ballistic trajectory for each system, which allows us to determine the precise position and velocity of the particle upon accretion. Finally, we ensure that the total angular momentum is conserved, which requires the donor star spin to vary self-consistently. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum of the orbit at the expense of the spin angular momentum of the donor.
Formation of double compact objects Kalogera, V.; Belczynski, K.; Kim, C. ...
Physics reports,
04/2007, Letnik:
442, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Current observations of double neutron stars provide us with a wealth of information that we can use to investigate their evolutionary history and the physical conditions of neutron star formation. ...Understanding this history and formation conditions further allow us to make theoretical predictions for the formation of other double compact objects with one or two black hole (BH) components and assess the detectability of such systems by ground-based gravitational-wave (GW) interferometers. In this paper we summarize our group's body of work in the past few years and we place our conclusions and current understanding in the framework of other work in this area of astrophysical research.
We investigate the secular evolution of the orbital semimajor axis and eccentricity due to mass transfer in eccentric binaries, assuming conservation of total system mass and orbital angular ...momentum. Assuming a delta function mass transfer rate centered at periastron, we find rates of secular change of the orbital semimajor axis and eccentricity which are linearly proportional to the magnitude of the mass transfer rate at periastron. The rates can be positive as well as negative, so that the semimajor axis and eccentricity can increase as well as decrease in time. Adopting a delta function mass-transfer rate of 10 super(-9)M unk at periastron yields orbital evolution timescales ranging from a few Myr to a Hubble time or more, depending on the binary mass ratio and orbital eccentricity. Comparison with orbital evolution timescales due to disslpatlve tides furthermore shows that tides cannot, in all cases, circularize the orbit rapidly enough to justify the often-adopted assumption of instantaneous circularization at the onset of mass transfer. The formalism presented can be incorporated in binary evolution and population synthesis codes to create a self-consistent treatment of mass transfer in eccentric binaries.
We investigate the existence and properties of equipotential surfaces and Lagrangian points in nonsynchronous, eccentric binary star and planetary systems under the assumption of quasi-static ...equilibrium. We adopt a binary potential that accounts for nonsynchronous rotation and eccentric orbits and calculate the positions of the Lagrangian points as functions of the mass ratio, the degree of asynchronism, the orbital eccentricity, and the position of the stars or planets in their relative orbit. We find that the geometry of the equipotential surfaces may facilitate nonconservative mass transfer in nonsynchronous, eccentric binary star and planetary systems, especially if the component stars or planets are rotating supersynchronously at the periastron of their relative orbit. We also calculate the volume-equivalent radius of the Roche lobe as a function of the four parameters mentioned above. Contrary to common practice, we find that replacing the radius of a circular orbit in the fitting formula of Eggleton with the instantaneous distance between the components of eccentric binary or planetary systems does not always lead to a good approximation to the volume-equivalent radius of the Roche lobe. We therefore provide generalized analytic fitting formulae for the volume-equivalent Roche lobe radius appropriate for nonsynchronous, eccentric binary star and planetary systems. These formulae are accurate to better than 1% throughout the relevant two-dimensional parameter space that covers a dynamic range of 16 and 6 orders of magnitude in the two dimensions.
ABSTRACT Massive stars are usually found in binaries, and binaries with periods less than 10 days may have a preference for near equal component masses ("twins"). In this paper we investigate the ...evolution of massive twin binaries all the way to contact and the possibility that these systems can be progenitors of double neutron star binaries. The small orbital separations of observed double neutron star binaries suggest that the progenitor systems underwent a common envelope phase at least once during their evolution. Bethe & Brown proposed that massive binary twins will undergo a common envelope evolution while both components are ascending the red giant branch (RGB) or asymptotic giant branch (AGB) simultaneously, also known as double-core evolution. Using models generated from the stellar evolution code EZ (evolve zero-age main sequence), we determine the range of mass ratios resulting in a contact binary with both components simultaneously ascending the RGB or AGB as a function of the difference in birth times, Δτ. We find that, even for a generous Δτ = 5 Myr, the minimum mass ratio for an primary and increases for larger mass primaries. We use a smoothed particle hydrodynamics code, StarSmasher, to study specifically the evolution of q = 1 common envelope systems as a function of initial component mass, age, and orbital separation. We also consider a q = 0.997 system to test the effect of relaxing the constraint of strictly identical components. We find the dynamical stability limit, the largest orbital separation where the binary becomes dynamically unstable, as a function of the component mass and age. Finally, we calculate the efficiency of ejecting matter during the inspiral phase to extrapolate the properties of the remnant binary from our numerical results, assuming the common envelope is completely ejected. We find that for the nominal core masses, there is a minimum orbital separation for a given component mass such that the helium cores survive common envelope evolution in a tightly bound binary and are viable progenitors for double neutron stars.
We present direct constraints on how the formation of low-mass X-ray binary (LMXB) populations in galactic fields depends on stellar age. In this pilot study, we utilize Chandra and Hubble Space ...Telescope (HST) data to detect and characterize the X-ray point source populations of three nearby early-type galaxies: NGC 3115, 3379, and 3384. The luminosity-weighted stellar ages of our sample span approximately 3-10 Gyr. X-ray binary population synthesis models predict that the field LMXBs associated with younger stellar populations should be more numerous and luminous per unit stellar mass than older populations due to the evolution of LMXB donor star masses. Crucially, the combination of deep Chandra and HST observations allows us to test directly this prediction by identifying and removing counterparts to X-ray point sources that are unrelated to the field LMXB populations, including LMXBs that are formed dynamically in globular clusters, Galactic stars, and background active galactic nuclei/galaxies. We find that the "young" early-type galaxy NGC 3384 ( approximately 2-5 Gyr) has an excess of luminous field LMXBs (L sub(X) Asymptotically = to (5-10) x 10 super(37) erg s super(-1)) per unit K-band luminosity (L sub(K); a proxy for stellar mass) than the "old" early-type galaxies NGC 3115 and 3379 ( approximately 8-10 Gyr), which results in a factor of approximately 2-3 excess of L sub(X)/L sub(K) for NGC 3384. This result is consistent with the X-ray binary population synthesis model predictions; however, our small galaxy sample size does not allow us to draw definitive conclusions on the evolution field LMXBs in general. We discuss how future surveys of larger galaxy samples that combine deep Chandra and HST data could provide a powerful new benchmark for calibrating X-ray binary population synthesis models.
In recent years proper-motion measurements have been added to the set of observational constraints on the current properties of Galactic X-ray binaries. We develop an analysis that allows us to ...consider all this available information and reconstruct the full evolutionary history of X-ray binaries back to the time of core collapse and compact object formation. This analysis accounts for five evolutionary phases: mass transfer through the ongoing X-ray phase, tidal circularization before the onset of Roche lobe overflow, motion through the Galactic potential after the formation of the compact object, binary orbital dynamics at the time of core collapse, and hydrodynamic modeling of the core collapse that connects the compact object to its progenitor and any nucleosynthetic constraints available. In this first paper we present this analysis in a comprehensive manner and apply it to the soft X-ray transient GRO J1655-40. This is the first analysis that incorporates all observational constraints on the current system properties and uses the full three-dimensional peculiar velocity constraints right after core collapse instead of lower limits on the current space velocity given by the present-day radial velocity. We find that the system has remained within 200 pc from the Galactic plane throughout its entire lifetime and that the mass loss and a kick possibly associated with the black hole formation imparted a kick velocity of 45-115 km s super(-1) to the binary's center of mass. Right after black hole formation, the system consists of a 3.5-6.3 M sub( )black hole and a 2.3-4 M sub( )main-sequence star. At the onset of the X-ray phase the donor is still on the main sequence. We find that a symmetric black hole formation event cannot be formally excluded but that the associated system parameters are only marginally consistent with the currently observed binary properties. Black hole formation mechanisms involving an asymmetric supernova explosion with associated black hole kick velocities of a few tens of km s super(-1), on the other hand, satisfy the constraints much more comfortably. We also derive an upper limit on the black hole kick magnitude of 210 km s super(-1).
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