Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not ...well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the α-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)-main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.
Recent observational and theoretical studies indicate that the stellar initial mass function (IMF) varies systematically with the environment (star formation rate – SFR, metallicity). Although the ...exact dependence of the IMF on those properties is likely to change with improving observational constraints, the reported trend in the shape of the IMF appears robust. We present the first study aiming to evaluate the effect of the IMF variations on the measured cosmic SFR density (SFRD) as a function of metallicity and redshift,
f
SFR
(
Z
,
z
). We also study the expected number and metallicity of white dwarf, neutron star, and black hole progenitors under different IMF assumptions. Applying the empirically driven IMF variations described by the integrated galactic IMF (IGIMF) theory, we revise
f
SFR
(
Z
,
z
) obtained in our previous study that assumed a universal IMF. We find a lower SFRD at high redshifts and a higher fraction of metal-poor stars being formed than previously determined. In the local Universe, our calculation applying the IGIMF theory suggests more white dwarf and neutron star progenitors in comparison with the universal IMF scenario, while the number of black hole progenitors remains unaffected.
Context.
As the number of detected gravitational wave sources increases, the better we can understand the mass distribution of binary black holes (BBHs). This “stellar graveyard” shows several ...features, including an apparent mass gap that makes the distribution bimodal. In turn, the observed chirp mass distribution appears to be trimodal.
Aims.
We aim to investigate the extent to which we can explain the observed mass distribution based on stellar evolution, specifically with the hypothesis that the mass gap is caused by the difference between successful and failed supernovae (SNe).
Methods.
We posed a hypothetical remnant function, based on the literature of stellar evolution simulations, which relates initial mass to remnant mass, while including a “black hole island” and producing a bimodal remnant distribution. Moreover, we looked at observed type II SN rates in an attempt to detect the effect of failed SNe. Finally, using a simplified estimation of binary evolution, we determined the remnant distribution resulting from our remnant function and compared it with observations.
Results.
We find that failed SNe lower type II SN rates by approximately 25%, but the inferred rate from SN surveys is not accurate enough to confirm this. Furthermore, our estimation based on the remnant function produces a mass distribution that matches the general shape of the observed distributions of individual as well as chirp masses.
Conclusions.
Based on our research, we conclude that the failed SN mechanism and the presence of the black hole island are a natural hypothesis for explaining the individual BBH mass distribution and chirp mass distribution. However, to obtain a firmer conclusion, more detailed simulations are needed.
Abstract
We use very long baseline interferometry to measure the proper motions of three black hole X-ray binaries (BHXBs). Using these results together with data from the literature and Gaia DR2 to ...collate the best available constraints on proper motion, parallax, distance, and systemic radial velocity of 16 BHXBs, we determined their three-dimensional Galactocentric orbits. We extended this analysis to estimate the probability distribution for the potential kick velocity (PKV) a BHXB system could have received on formation. Constraining the kicks imparted to BHXBs provides insight into the birth mechanism of black holes (BHs). Kicks also have a significant effect on BH–BH merger rates, merger sites, and binary evolution, and can be responsible for spin–orbit misalignment in BH binary systems. 75 per cent of our systems have potential kicks $\gt 70\, \rm {km\,s^{-1}}$. This suggests that strong kicks and hence spin–orbit misalignment might be common among BHXBs, in agreement with the observed quasi-periodic X-ray variability in their power density spectra. We used a Bayesian hierarchical methodology to analyse the PKV distribution of the BHXB population, and suggest that a unimodal Gaussian model with a mean of 107 $\pm \,\,16\, \rm {km\,s^{-1}}$ is a statistically favourable fit. Such relatively high PKVs would also reduce the number of BHs likely to be retained in globular clusters. We found no significant correlation between the BH mass and PKV, suggesting a lack of correlation between BH mass and the BH birth mechanism. Our python code allows the estimation of the PKV for any system with sufficient observational constraints.
Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large ...radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY – SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA white dwarfs (displaying hydrogen-rich atmospheres), of which 46 were found to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. Twenty are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (≤0.45 M⊙) are detected as DDs, while just 23 of the remaining 567 targets with multiple spectra and mass > 0.45 M⊙ are double. The detected fraction amongst the low mass objects (36.4 ± 7.3%) is significantly higher than for the higher-mass, carbon-oxygen core dominated part of the sample (3.9 ± 0.8%), but it is much lower than expected from the detection efficiency for companion masses of 0.05 M⊙ or higher and a 100% binary fraction. This suggests either companion stars of with a mass below 0.05 M⊙ or some of the low mass white dwarfs are single.
I briefly review the method of population synthesis of binary stars and discuss the preliminary results of a study of the Galactic population of subdwarf B stars. In particular I focus on the ...formation of (apparently) single sdB stars and their relation to (apparently) single helium-core white dwarfs. I discuss the merits of mergers of two helium white dwarfs and interactions with sub-stellar companions for explaining these single objects. A preliminary conclusion is that the current observations suggest both mechanisms may contribute, but that the helium white dwarfs are likely formed in majority from interactions with sub-stellar companions.
AM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5–65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data ...Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars has yet to be discovered. As this value sets the background to the gravitational wave signal of LISA this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority has a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected.
ABSTRACT The theoretical and observed populations of pre-cataclysmic variables are dominated by systems with low-mass white dwarfs (WDs), while the WD masses in cataclysmic variables (CVs) are ...typically high. In addition, the space density of CVs is found to be significantly lower than in the theoretical models. We investigate the influence of nova outbursts on the formation and initial evolution of CVs. In particular, we calculate the stability of the mass transfer in the case where all of the material accreted on the WD is lost in classical novae and part of the energy to eject the material comes from a common-envelope-like interaction with the companion. In addition, we study the effect of an asymmetry in the mass ejection that may lead to small eccentricities in the orbit. We find that a common-envelope-like ejection significantly decreases the stability of the mass transfer, particularly for low-mass WDs. Similarly, the influence of asymmetric mass loss can be important for short-period systems and even more so for low-mass WDs; however, this influence likely disappears long before the next nova outburst due to orbital circularization. In both cases the mass-transfer rates increase, which may lead to observable (and perhaps already observed) consequences for systems that do survive to become CVs. However, a more detailed investigation of the interaction between nova ejecta and the companion and the evolution of slightly eccentric CVs is needed before definite conclusions can be drawn.
PDF
