Binary Population Synthesis Models Research Articles

Stellar mergers are common

The observed Galactic rate of stellar mergers or the initiation of common envelope phases brighter than M_V=-3 (M_I=-4) is of order 0.5 (0.3)/year with 90% confidence statistical uncertainties of 0.24-1.1 (0.14-0.65) and factor of 2 systematic uncertainties. The (peak) luminosity function is roughly dN/dL L^(-1.4+/-0.3), so the rates for events more luminous than V1309 Sco (M_V=-7 mag) or V838Mon (M_V=-10 mag) are lower at r~0.1/year and 0.03/year, respectively. The peak luminosity is a steep function of progenitor mass, L M^(2-3). This very roughly parallels the scaling of luminosity with mass on the main sequence, but the transients are ~2000-4000 times more luminous at peak. Combining these, the mass function of the progenitors, dN/dM M^(-2.0+/-0.8), is consistent with the initial mass function, albeit with broad uncertainties. These observational results are also broadly consistent with the estimates of binary population synthesis models. While extragalactic variability surveys can better define the rates and properties of the high luminosity events, systematic, moderate depth (I>16 mag) surveys of the Galactic plane are needed to characterize the low luminosity events. The existing Galactic samples are only ~20% complete and Galactic surveys are (at best) reaching a typical magnitude limit of <13 mag.

THE X-RAY LUMINOSITY FUNCTIONS OF FIELD LOW-MASS X-RAY BINARIES IN EARLY-TYPE GALAXIES: EVIDENCE FOR A STELLAR AGE DEPENDENCE

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 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 AGN/galaxies. We find that the "young" early-type galaxy NGC 3384 (~2-5 Gyr) has an excess of luminous field LMXBs (L_X > (5-10) x 10^37 erg/s) per unit K-band luminosity (L_K; a proxy for stellar mass) than the "old" early-type galaxies NGC 3115 and 3379 (~8-10 Gyr), which results in a factor of ~2-3 excess of LX/LK 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.

Producing Type Iax supernovae from a specific class of helium-ignited WD explosions

It has recently been proposed that one sub-class of type Ia supernovae (SNe Ia) is sufficiently both distinct and common to be classified separately from the bulk of SNe Ia, with a suggested class name of "type Iax supernovae" (SNe Iax), after SN 2002cx. However, their progenitors are still uncertain. We study whether the population properties of this class might be understood if the events originate from a subset of sub-Chandrasekhar mass explosions. In this potential progenitor population, a carbon--oxygen white dwarf (CO WD) accumulates a helium layer from a non-degenerate helium star; ignition of that helium layer then leads to ignition of the CO WD. We incorporated detailed binary evolution calculations for the progenitor systems into a binary population synthesis model to obtain rates and delay times for such events. The predicted Galactic event rate of these explosions is ~1.5\times10^{-3}{yr}^{-1} according to our standard model, in good agreement with the measured rates of SNe Iax. In addition, predicted delay times are ~70Myr-800Myr, consistent with the fact that most of SNe Iax have been discovered in late-type galaxies. If the explosions are assumed to be double-detonations -- following current model expectations -- then based on the CO WD masses at explosion we also estimate the distribution of resulting SN brightness (-13 \gtrsim M_{bol} \gtrsim -19mag), which can reproduce the empirical diversity of SNe Iax. We speculate on why binaries with non-degenerate donor stars might lead to SNe Iax if similar systems with degenerate donors do not. We suggest that the high mass of the helium layer necessary for ignition at the lower accretion rates typically delivered from non-degenerate donors might be necessary to produce SN 2002cx-like characteristics, perhaps even by changing the nature of the CO ignition.

EVIDENCE FOR ELEVATED X-RAY EMISSION IN LOCAL LYMAN BREAK GALAXY ANALOGS

In this paper, we study the relationship between the 2-10 keV X-ray luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We present Chandra observations for four new GALEX-selected LBAs. Including previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies, similar to the ratios found for stacked LBGs in the early Universe (z>2). We show that these LBAs are unlikely to harbor AGN, based on their optical and X-ray spectra and the spatial distribution of the X-rays in three spatially extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the X-ray emission in these galaxies, based on their high specific SFRs (sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar populations. Since both LBAs and LBGs have lower dust attenuations and metallicities compared to similar samples of more typical local galaxies, we investigate the effects of dust extinction and metallicity on the L_X/SFR for the broader population of galaxies with high sSFRs (>10^{-10}/yr). The estimated dust extinctions (corresponding to column densities of N_H<10^{22}/cm^2) are expected to have insignificant effects on observed L_X/SFR ratio for the majority of galaxy samples. We find that the observed relationship between L_X/SFR and metallicity appears consistent with theoretical expectations from X-ray binary population synthesis models. Therefore, we conclude that lower metallicities, related to more luminous HMXBs such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity than found in typical galaxies in the local Universe.

TWO NEW LONG-PERIOD HOT SUBDWARF BINARIES WITH DWARF COMPANIONS

Hot subdwarf stars with F-K main sequence binary companions have been known for decades, but the first orbital periods for such systems were published just recently. Current observations suggest that most have long periods, on the order of years, and that some are or once were hierarchical triple systems. As part of a survey with the Hobby-Eberly Telescope, we have been monitoring the radial velocities of several composite-spectra binaries since 2005 in order to determine their periods, velocities, and eccentricities. Here we present observations and orbital solutions for two of these systems, PG 1449+653 and PG 1701+359. Similar to the other sdB+F/G/K binaries with solved orbits, their periods are long, 909 d and 734 d, respectively, and pose a challenge to current binary population synthesis models of hot subdwarf stars. Intrigued by their relatively large systemic velocities, we also present a kinematical analysis of both targets and find that neither is likely a member of the Galactic thin disk.

Binary Star Stellar Population Synthesis Model For Astrophysical Studies

AbstractBinary stars possibly exists in most galaxies and star clusters. Their evolution can lead to significant change in stellar population studies. Binary star to fit group (BS2fit: ∞) has built up a binary star stellar population synthesis model and used it in a few works. This page is to introduce the model and its possible applications.

THE X-RAY STAR FORMATION STORY AS TOLD BY LYMAN BREAK GALAXIES IN THE 4 Ms CDF-S

We present results from deep X-ray stacking of >4000 high redshift galaxies from z~1 to 8 using the 4 Ms Chandra Deep Field South (CDF-S) data, the deepest X-ray survey of the extragalactic sky to date. The galaxy samples were selected using the Lyman break technique based primarily on recent HST ACS and WFC3 observations. Based on such high specific star formation rates (sSFRs): log SFR/M* > -8.7, we expect that the observed properties of these LBGs are dominated by young stellar populations. The X-ray emission in LBGs, eliminating individually detected X-ray sources (potential AGN), is expected to be powered by X-ray binaries and hot gas. We find, for the first time, evidence of evolution in the X-ray/SFR relation. Based on X-ray stacking analyses for z<4 LBGs (covering ~90% of the Universe's history), we find that the 2-10 keV X-ray luminosity evolves weakly with redshift (z) and SFR as log LX = 0.93 log (1+z) + 0.65 log SFR + 39.80. By comparing our observations with sophisticated X-ray binary population synthesis models, we interpret that the redshift evolution of LX/SFR is driven by metallicity evolution in HMXBs, likely the dominant population in these high sSFR galaxies. We also compare these models with our observations of X-ray luminosity density (total 2-10 keV luminosity per Mpc^3) and find excellent agreement. While there are no significant stacked detections at z>5, we use our upper limits from 5<z<8 LBGs to constrain the SMBH accretion history of the Universe around the epoch of reionization.

A RADIAL VELOCITY STUDY OF COMPOSITE-SPECTRA HOT SUBDWARF STARS WITH THE HOBBY–EBERLY TELESCOPE

Many hot subdwarf stars show composite spectral energy distributions indicative of cool main sequence companions. Binary population synthesis (BPS) models demonstrate such systems can be formed via Roche lobe overflow or common envelope evolution but disagree on whether the resulting orbital periods will be long (years) or short (days). Few studies have been carried out to assess the orbital parameters of these spectroscopic composite binaries; current observations suggest the periods are long. To help address this problem, we selected fifteen moderately-bright (V~13) hot subdwarfs with F-K dwarf companions and monitored their radial velocities (RVs) from January 2005 to July 2008 using the bench-mounted Medium Resolution Spectrograph on the Hobby-Eberly Telescope (HET). Here we describe the details of our observing, reduction, and analysis techniques and present preliminary results for all targets. By combining the HET data with recent observations from the Mercator telescope, we are able to calculate precise orbital solutions for three systems using more than 6 years of observations. We also present an up-to-date period histogram for all known hot subdwarf binaries, which suggests those with F-K main sequence companions tend to have orbital periods on the order of several years. Such long periods challenge the predictions of conventional BPS models, although a larger sample is needed for a thorough assessment of the models' predictive success. Lastly, one of our targets has an eccentric orbit, implying some composite-spectrum systems might have formerly been hierarchical triple systems, in which the inner binary merged to create the hot subdwarf.

POPULATION SYNTHESIS OF HOT SUBDWARFS: A PARAMETER STUDY

Binaries that contain a hot subdwarf (sdB) star and a main sequence companion may have interacted in the past. This binary population has historically helped determine our understanding of binary stellar evolution. We have computed a grid of binary population synthesis models using different assumptions about the minimum core mass for helium ignition, the envelope binding energy, the common envelope ejection efficiency, the amount of mass and angular momentum lost during stable mass transfer, and the criteria for stable mass transfer on the red giant branch and in the Hertzsprung gap. These parameters separately and together can significantly change the entire predicted population of sdBs. Nonetheless, several different parameter sets can reproduce the observed subpopulation of sdB + white dwarf and sdB + M dwarf binaries, which has been used to constrain these parameters in previous studies. The period distribution of sdB + early F dwarf binaries offers a better test of different mass transfer scenarios for stars that fill their Roche lobes on the red giant branch.

Estimation of compact binary coalescense rates from short gamma-ray burst redshift measurements

Short gamma-ray bursts are believed to originate from the merger of two compact objects. If this scenario is correct, these bursts will be accompanied by the emission of strong gravitational waves, detectable by current or planned GW detectors, such as LIGO and Virgo. No detection of a gravitational wave has been made up to date. In this paper I will use a set of observed redshift measurements of short gamma-ray bursts to fit a model in order to determine the rate of such merger events in the nearby universe. Various corrections will be included in that calculation, as the field-of-view of the satellite missions, the beaming factors of gamma-ray bursts and other parameters. The computed rate estimations will be compared to other rate estimations, based on observations on binary neutron stars and population synthesis models. Given the upper limit established by LIGO/Virgo measurements, it is possible to draw conclusions on the beaming angle of gamma-ray bursts.

A lack of close binaries among hot horizontal branch stars in globular clusters

Models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars, but the observations of their analogues in globular clusters has posed new problems, while the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars. We search for binaries of period P<200 days among a sample of blue horizontal branch stars (Teff=12000-22000 K) in NGC2808, a cluster known to host three distinct stellar populations and a multimodal horizontal branch. The final sample consists of 64 targets. The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of about 75 days. We detect no RV variable object among stars cooler than the photometric G1 gap at 17000 K, while two close (P<10 days) and two intermediate-period (P=10-50 days) systems are found among hotter targets. The close and intermediate-period binary fraction for stars cooler than the gap are fc<5% and fip<10%, respectively, with 95% confidence. The most probable values among hotter stars are fc~20% and fip~30%, but the 90%- confidence level intervals are large (6-42% and 11-72%, respectively). The G1 gap appears as a discontinuity in the binary faction, with a higher incidence of binaries among hotter stars, but a constant increase in f with temperature rather than a discontinuity cannot be excluded from our observations. We find that intermediate-period binaries, never investigated before among cluster HB stars, could play an important role, being more than ~15-20% of the hottest stars of our sample. Our results indicate that fc among hot HB stars is most probably higher for younger clusters, confirming the recently proposed age-fc relation. However, the large observed difference in binary fraction between clusters (e.g. NGC2808 and NGC6752) is still not reproduced by binary population synthesis models.

Binary Stellar Population Synthesis Model

Using Yunnan evolutionary population synthesis (EPS) models, we present integrated colours, integrated spectral energy distributions (ISEDs) and absorption-line indices defined by the Lick Observatory image dissector scanner (Lick/IDS) system, for an extensive set of instantaneous-burst binary stellar populations (BSPs) with interactions. By comparing the results for populations with and without interactions we show that the inclusion of binary interactions makes the appearance of the population substantially bluer. This effect raises the derived age and metallicity of the population.To be used in the studies of modern spectroscopic galaxy surveys at intermediate/high spectral resolution, we also present intermediate- (3 Å) and high-resolution (∼ 0.3 Å) ISEDs and Lick/IDS absorption-line indices for BSPs. To directly compare with observations the Lick/IDS absorption indices are also presented by measuring them directly from the ISEDs.

A Study of Binary Stellar Population Synthesis of Elliptical Galaxies

We determine relative stellar ages and metallicities mainly for about 80 elliptical galaxies in low and high density environments via the latest binary stellar population (BSP) synthesis model and test a latest hierarchical formation model of elliptical galaxies which adopted the new $\Lambda$CDM cosmology for the first time. The stellar ages and metallicities of galaxies are estimated from two high-quality published spectra line indices, i.e. H$\beta$ and [MgFe]. The results show that elliptical galaxies have stellar populations older than 3.9 Gyr and more metal rich than 0.02. Most of our results are in agreement with predictions of the model: First, elliptical galaxies in denser environment are redder and have older populations than field galaxies. Second, elliptical galaxies with more massive stellar components are redder while have older and more metal rich populations than less massive ones. Third, the most massive galaxies are shown to have the oldest and most metal rich stars. However, some of our results are found to be different with predictions of the galaxy formation model, i.e. the metallicity distributions of low- and high-density elliptical galaxies and the relations relating to cluster-centric distance.

The Brightest Point X‐Ray Sources in Elliptical Galaxies and the Mass Spectrum of Accreting Black Holes

We propose that the shape of the upper-end X-ray luminosity function (XLF) observed in elliptical galaxies for point sources carries valuable information about the black hole (BH) mass spectrum among old X-ray transients formed in the galaxies. Here we present the line of arguments and analysis that support this connection and the methodology for deriving the BH mass spectrum slope from the observed XLF slope. We show that this underlying BH mass spectrum is modified by a weighting factor that is related to the transient duty cycle, and it generally depends on the host galaxy age, the BH mass, and the X-ray binary (XRB) donor type (main-sequence, red giant, or white dwarf donors). We find that the observed XLF is dominated by transient BH systems in outburst (a prediction possibly testable by future observations) but that the assumption of a constant duty cycle for all systems leads to results inconsistent with current observations. We also find that the derived BH mass slope depends on the strength of the angular momentum loss due to magnetic braking for main-sequence donors. More specifically, we find that for standard magnetic braking, BH XRBs with red giant donors dominate the upper-end XLF but that for weaker magnetic braking prescriptions, main-sequence donors are found to be dominant. The methodology presented here can be used in the future as our understanding of the transient duty and its dependence on binary and mass transfer properties improves. Under certain assumptions for this dependence, we derive a differential BH mass spectrum slope of 2.5; an upper BH mass cutoff at 20 M☉ is needed to understand the very brightest of the BH XRBs in elliptical galaxies. We also show that our quantitative results are robust against expected variations by factors of a few of the outburst peak X-ray luminosities. We expect that our analysis will eventually help to constrain binary population synthesis models and the adopted relations between black holes and the masses of their progenitors.

Hot subdwarfs from the ESO Supernova Ia Progenitor Survey

We present the analysis of a high-resolution, high-quality sample of optical spectra for 76 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey (SPY, [CITE]). Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by fitting the profiles of hydrogen and helium lines using synthetic spectra calculated from LTE and NLTE model atmospheres. We perform a detailed comparison of our measurements with theoretical calculations, both for single star evolution and for binary population synthesis models of close binary evolution. The luminosity evolution given by the standard EHB evolutionary tracks from [CITE] shows an overall agreement in shape with our observations, although a constant offset in luminosity exists. The various simulation sets for binary formation channels of sdB stars calculated by [CITE] are compared individually to our data for testing our current understanding of sdB formation processes and the physical effects involved. The best-matching sets manage to reproduce the observed sdB distribution in the temperature-gravity-plane well. However, they do not match the observed cumulative luminosity function, indicating that theoretical improvement is necessary. We also investigate composite-spectrum objects showing clear signatures of a cool companion with optical and infrared photometry. These stars have cool main sequence companions of spectral types F to K. Typical helium abundances of composite and non-composite sdB stars do not differ.

Simulations of the evolution of the X-Ray properties of a young stellar population

We present an X-ray binary population synthesis model, and use it to simulate the evolution of X-ray binaries formed in a burst of star formation of duration 20 Myr and star-formation rate 10 M $_{\odot}$ yr −1 . We find that the X-ray luminous phase persists long after the star-formation episode has ended. The 2-10 keV X-ray luminosity can reach values of 10 40 −10 41 erg s −1 , comparable to those of some Seyfert galaxies. These models are directly applicable to starburst galaxies as well as LINERs powered by vigorous star formation. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Exploring the Nature of Weak Chandra Sources Near the Galactic Centre

We present early results from the first near-IR imaging of the weak X-ray sources discovered in the recent Chandra/ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC. Once the data analysis is complete we will discuss our results in the context of binary population synthesis models.

The origin of subdwarf B stars - II

We have carried out a detailed binary population synthesis (BPS) study of the formation of subdwarf B (sdB) stars and related objects (sdO, sdOB stars) using the latest version of the BPS code developed by Han and co-workers. We systematically investigate the importance of the five main evolutionary channels in which the sdB stars form after one or two common-envelope (CE) phases, one or two phases of stable Roche lobe overflow (RLOF) or as the result of the merger of two helium white dwarfs (WDs). Our best BPS model can satisfactorily explain the main observational characteristics of sdB stars, in particular their distributions in the orbital period-minimum companion mass (log P-M-comp) diagram and in the effective temperature-surface gravity (T-eff- log g) diagram, their distributions of orbital period, log (gtheta(4))(theta = 5040 K / T-eff) and mass function, their binary fraction and the fraction of sdB binaries with WD companions, their birth rates and their space density. We obtain a Galactic formation rate for sdB stars of 0.014- 0.063 yr(-1) with a best estimate of similar to0.05 yr(-1) and a total number in the Galaxy of 2.4-9.5 x 10(6) with a best estimate of similar to6 x 10(6); half of these may be missing in observational surveys owing to selection effects. The intrinsic binary fraction is 76-89 per cent, although the observed frequency may be substantially lower owing to the selection effects. The first CE ejection channel, the first stable RLOF channel and the merger channel are intrinsically the most important channels, although observational selection effects tend to increase the relative importance of the second CE ejection and merger channels. We also predict a distribution of masses for sdB stars that is wider than is commonly assumed and that some sdB stars have companions of spectral type as early as B. The percentage of A-type stars with sdB companions can in principle be used to constrain some of the important parameters in the binary evolution model. We conclude that (i) the first RLOF phase needs to be more stable than is commonly assumed, either because the critical mass ratio q(crit) for dynamical mass transfer is higher or because of tidally enhanced stellar wind mass loss; (ii) mass transfer in the first stable RLOF phase is non-conservative, and the mass lost from the system takes away a specific angular momentum similar to that of the system; and (iii) common-envelope ejection is very efficient.

The Progenitors of Type Ia Supernovae

Using binary population synthesis (BPS), we studied the birthrates of SNe Ia for two progenitor models – the single degenerate model and the double degenerate model. We find that the birthrates from both models are within a factor of a few comparable to those inferred observationally. For each model, we investigate different star-formation histories (single star burst or constant star formation), different metallicities (Z = 0.02, 0.004, 0.001), different parameters for the BPS model (mass transfer efficiency during stable mass transfer, common-envelope ejection parameters) and obtain the evolution of birthrates with time.