Population Synthesis Models Research Articles

Pre-supernova evolution, compact-object masses, and explosion properties of stripped binary stars

The era of large transient surveys, gravitational-wave observatories, and multi-messenger astronomy has opened up new possibilities for our understanding of the evolution and final fate of massive stars. Most massive stars are born in binary or higher-order multiple systems and exchange mass with a companion star during their lives. In particular, the progenitors of a large fraction of compact-object mergers, and Galactic neutron stars (NSs) and black holes (BHs) have been stripped of their envelopes by a binary companion. Here, we study the evolution of single and stripped binary stars up to core collapse with the stellar evolution code MESA and their final fates with a parametric supernova (SN) model. We find that stripped binary stars can have systematically different pre-SN structures compared to genuine single stars and thus also different SN outcomes. These differences are already established by the end of core helium burning and are preserved up to core collapse. Consequently, we find that Case A and B stripped stars and single and Case C stripped stars develop qualitatively similar pre-SN core structures. We find a non-monotonic pattern of NS and BH formation as a function of CO core mass that is different in single and stripped binary stars. In terms of initial mass, single stars of ≳35 M⊙ all form BHs, while this transition is only at about 70 M⊙ in stripped stars. On average, stripped stars give rise to lower NS and BH masses, higher explosion energies, higher kick velocities, and higher nickel yields. Within a simplified population-synthesis model, we show that our results lead to a significant reduction in the rates of BH–NS and BH–BH mergers with respect to typical assumptions made on NS and BH formation. Therefore, our models predict lower detection rates of such merger events with for example the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) than is often considered. Further, we show how certain features in the NS–BH mass distribution of single and stripped stars relate to the chirp-mass distribution of compact object mergers. Further implications of our findings are discussed with respect to the missing red-supergiant problem, a possible mass gap between NSs and BHs, X-ray binaries, and observationally inferred nickel masses from Type Ib/c and IIP SNe.

Planetesimal formation at the gas pressure bump following a migrating planet

Context. Many scenarios have been proposed to avoid known difficulties in planetesimal formation such as drift or fragmentation barriers. However, in these scenarios planetesimals in general only form at some specific locations in protoplanetary discs. On the other hand, it is generally assumed in planet formation models and population synthesis models that planetesimals are broadly distributed in the protoplanetary disc. Aims. We propose a new scenario in which planetesimals can form in broad areas of these discs. Planetesimals form at the gas pressure bump formed by a first-generation planet (e.g. formed by pebble accretion) and the formation region spreads inward in the disc as the planet migrates. Methods. We used a simple 1D Lagrangian particle model to calculate the radial distribution of pebbles in the gas disc perturbed by a migrating embedded planet. We consider that planetesimals form by streaming instability at the points where the pebble-to-gas density ratio on the mid-plane becomes larger than unity. In this work, we fixed the Stokes number of pebbles and the mass of the planet to study the basic characteristics of this new scenario. We also studied the effect of some key parameters, such as the gas disc model, the pebble mass flux, the migration speed of the planet, and the strength of turbulence. Results. We find that planetesimals form in wide areas of protoplanetary discs provided the flux of pebbles is typical and the turbulence is not too strong. The planetesimal surface density depends on the pebble mass flux and the migration speed of the planet. The total mass of the planetesimals and the orbital position of the formation area strongly depend on the pebble mass flux. We also find that the profile of the planetesimal surface density and its slope can be estimated by very simple equations. Conclusions. We show that our new scenario can explain the formation of planetesimals in broad areas. The simple estimates we provide for the planetesimal surface density profile can be used as initial conditions for population synthesis models.

Role of the Fan Beam Model in Population Synthesis of Isolated Radio Pulsars

Abstract On the basis of the P sr P op P y software package, we developed an evolution model of population synthesis for isolated radio pulsars by incorporating the fan beam model, an alternative radio emission beam model to the widely used conal beam model. With proper choice of related models and parameters, we successfully reproduced the observational distributions of Galactic longitude (l) and latitude (b), spin period, period derivative, dispersion measure and 1.4 GHz flux density for the 1214 isolated pulsars discovered by the Parkes multibeam and Swinburne pulsar surveys. The size of the underlying population of radio-loud pulsars is found to be 2.27 × 106, which is larger than the underlying population of radio-loud pulsars simulated with the conal beam model. The model is used to estimate the number of isolated radio pulsars detected by the Galactic-plane pulsar survey with Five hundred-meter Aperture Spherical Telescope (FAST). Approximately 2700 and 240 new isolated pulsars are expected to be discovered in the inner galactic plane (20° < l < 90°, ∣b∣ < 10°) and the outer galactic plane (150° < l < 210°, ∣b∣ < 10°), respectively. These numbers are smaller than those estimated by the evolution models with conal beams and the snapshot models.

Numerical study of coorbital thermal torques on cold or hot satellites

ABSTRACT We evaluate the thermal torques exerted on low-mass planets embedded in gaseous protoplanetary discs with thermal diffusion, by means of high-resolution three-dimensional hydrodynamics simulations. We confirm that thermal torques essentially depend on the offset between the planet and its corotation, and find a good agreement with analytic estimates when this offset is small compared to the size of the thermal disturbance. For larger offsets that may be attained in discs with a large pressure gradient or a small thermal diffusivity, thermal torques tend towards an asymptotic value broadly compatible with results from a dynamical friction calculation in an unsheared medium. We perform a convergence study and find that the thermal disturbance must be resolved over typically 10 zones for a decent agreement with analytic predictions. We find that the luminosity at which the net thermal torque changes sign matches that predicted by linear theory within a few percents. Our study confirms that thermal torques usually supersede Lindblad and corotation torques by almost an order of magnitude for low-mass planets. As we increase the planetary mass, we find that the ratio of thermal torques to Lindblad and corotation torques is progressively reduced, and that the thermal disturbance is increasingly distorted by the horseshoe flow. Overall, we find that thermal torques are dominant for masses up to an order of magnitude larger than implemented in recent models of planetary population synthesis. We finally briefly discuss the case of stellar or intermediate-mass objects embedded in discs around active galactic nuclei.

Pairing function of visual binary stars

ABSTRACTAn all-sky sample of 1227 visual binaries based on Washington Double Star catalogue is constructed to infer the initial mass function (IMF), mass ratio, and projected distance distribution with a dedicated population synthesis model. Parallaxes from Gaia DR2 and Hipparcos are used to verify the distance distribution. The model is validated on the single-star Tycho-2 sample and successfully reproduces the observed magnitudes and angular separations. The projected separation distribution follows f(s) ∼ s−1.2 in 102–2 × 103 au range for 1–4.5 m⊙ primary stars. Several algorithms are explored as pairing functions. Random pairing is confidently rejected. Primary-constrained pairing (PCP) and split-core pairing (SCP), the scenarios adopting primary component’s or total system’s mass as fundamental, are considered. The preferred IMF slope is α ∼ 2.8 either way. A simple power-law mass ratio distribution is unlikely, but the introduction of a twin excess provides a favourable result. PCP with f(q) ∼ q−1 is preferred with a tiny twin fraction, models with f(q) ∼ q−1.5 are acceptable when a larger twin excess is allowed. SCP is similar to PCP when a larger slope of the power law is adopted: f(q) ∼ qβ + 0.7.

Comparisons of Different Fitting Methods for the Physical Parameters of a Star Cluster Sample of M33 with Spectroscopy and Photometry

Star clusters are good tracers for formation and evolution of galaxies. We compared different fitting methods by using spectra (or by combining photometry) to determine the physical parameters. We choose a sample of 17 star clusters in M33, which previously lacked spectroscopic observations. The low-resolution spectra were taken with the Xinglong 2.16-m reflector of NAOC. The photometry used in the fitting includes $\rm u_{SC}$ and $\rm v_{SAGE}$ bands from the SAGE survey, as well as the published $UBVRI$ and $ugriz$ photometry. We firstly derived ages and metallicities with the {\sc ULySS} (Vazdekis et al. and {\sc pegase-hr}) SSP model and the Bruzual \& Charlot (2003) (BC03) stellar population synthesis models for the full-spectrum fitting. The fitting results of both the BC03 and {\sc ULySS} models seem consistent with those of previous works as well. Then we add the SAGE $\rm u_{SC}$ and $\rm v_{SAGE}$ photometry in the spectroscopic fitting with the BC03 models. It seems the results become much better, especially for the Padova 2000+Chabrier IMF set. Finally we add more photometry data, $UBVRI$ and $ugriz$, in the fitting and we found that the results do not improve significantly. Therefore, we conclude that the photometry is useful for improving the fitting results, especially for the blue bands ($\lambda <4000$ {\AA}), e.g., $\rm u_{SC}$ and $\rm v_{SAGE}$ band. At last, we discuss the "UV-excess" for the star clusters and we find five star clusters have UV-excess, based on the $GALEX$ FUV, NUV photometry.

The origin and impact of Wolf-Rayet-type mass loss

AbstractClassical Wolf-Rayet (WR) stars mark an important stage in the late evolution of massive stars. As hydrogen-poor massive stars, these objects have lost their outer layers, while still losing further mass through strong winds indicated by their prominent emission line spectra. Wolf-Rayet stars have been detected in a variety of different galaxies. Their strong winds are a major ingredient of stellar evolution and population synthesis models. Yet, a coherent theoretical picture of their strong mass-loss is only starting to emerge. In particular, the occurrence of WR stars as a function of metallicity (Z) is still far from being understood.To uncover the nature of the complex and dense winds of Wolf-Rayet stars, we employ a new generation of model atmospheres including a consistent solution of the wind hydrodynamics in an expanding non-LTE situation. With this technique, we can dissect the ingredients driving the wind and predict the resulting mass-loss for hydrogen-depleted massive stars. Our modelling efforts reveal a complex picture with strong, non-linear dependencies on the luminosity-to-mass ratio and Z with a steep, but not totally abrupt onset for WR-type winds in helium stars. With our findings, we provide a theoretical motivation for a population of helium stars at low Z, which cannot be detected via WR-type spectral features. Our study of massive He-star atmosphere models yields the very first mass-loss recipe derived from first principles in this regime. Implementing our first findings in stellar evolution models, we demonstrate how traditional approaches tend to overpredict WR-type mass loss in the young Universe.

Modelling the Galactic very-high-energyγ-ray source population

Context.The High Energy Stereoscopic System Galactic plane survey (HGPS) is to date the most comprehensive census of Galacticγ-ray sources at very high energies (VHE; 100 GeV ≤ E ≤ 100 TeV). As a consequence of the limited sensitivity of this survey, the 78 detectedγ-ray sources comprise only a small and biased subsample of the overall population. The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount.Aims.We study the VHEγ-ray source population in the Milky Way. For this purpose population-synthesis models are derived based on the distributions of source positions, extents, and luminosities.Methods.Several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources.Results.The power-law indices of luminosity and radius functions are determined to range between −1.6 and −1.9 for luminosity and −1.1 and −1.6 for radius. A two-arm spiral structure with central bar is discarded as spatial distribution of VHE sources, while azimuth-symmetric distributions and a distribution following a four-arm spiral structure without bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of (1.6 − 6.3) × 1036ph s−1and (3 − 15) × 10−10ph cm−2s−1, respectively.Conclusions.Depending on the model, the HGPS sample accounts for (68 − 87)% of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 and 9.

Wolf–Rayet stars in the Antennae unveiled by MUSE

ABSTRACT We present the analysis of archival Very Large Telescope (VLT) Multi-Unit Spectroscopic Explorer (MUSE) observations of the interacting galaxies NGC 4038/39 (a.k.a. the Antennae) at a distance of 18.1 Mpc. Up to 38 young star-forming complexes with evident contribution from Wolf–Rayet (WR) stars are unveiled. We use publicly available templates of Galactic WR stars in conjunction with available photometric extinction measurements to quantify and classify the WR population in each star-forming region, on the basis of its nearly Solar oxygen abundance. The total estimated number of WR stars in the Antennae is 4053 ± 84, of which there are 2021 ± 60 WNL and 2032 ± 59 WC-types. Our analysis suggests a global WC to WN-type ratio of 1.01 ± 0.04, which is consistent with the predictions of the single star evolutionary scenario in the most recent bpass stellar population synthesis models.

Yonsei Evolutionary Population Synthesis (YEPS) Model. III. Surface Brightness Fluctuation of Normal and Helium-enhanced Simple Stellar Populations

We present an evolutionary population synthesis model of the surface brightness fluctuation (SBF) for normal and He-enriched simple stellar populations (SSPs). While our SBF model for the normal-He population agrees with other existing models, the He-rich population, containing hotter horizontal-branch stars and brighter red-clump stars than the normal-He population, entails a substantial change in the SBF of SSPs. We show that the SBF magnitudes are affected by He-rich populations at least $\sim$0.3~mag even in $I$- and near-IR bands at given colors, from which the SBF-based distances are often derived. Due to uncertainties both in observations and models, however, the SBFs of Galactic globular clusters and early-type galaxies do not allow verifying the He-enriched model. We propose that when combined with independent metallicity and age indicators such as ${\rm Mg}_2$ and ${\rm H}\beta$, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems at a distance out to $\gtrsim 20$\,Mpc. A full set of the spectro-photometric and SBF data for SSPs from the Yonsei Evolutionary Population Synthesis (YEPS) model is available for download at http://cosmic.yonsei.ac.kr/YEPS.htm.

Effect of pebble flux-regulated planetesimal formation on giant planet formation

Context. The formation of gas giant planets by the accretion of 100 km diameter planetesimals is often thought to be inefficient. A diameter of this size is typical for planetesimals and results from self-gravity. Many models therefore use small kilometer-sized planetesimals, or invoke the accretion of pebbles. Furthermore, models based on planetesimal accretion often use the ad hoc assumption of planetesimals that are distributed radially in a minimum-mass solar-nebula way. Aims. We use a dynamical model for planetesimal formation to investigate the effect of various initial radial density distributions on the resulting planet population. In doing so, we highlight the directive role of the early stages of dust evolution into pebbles and planetesimals in the circumstellar disk on the subsequent planet formation. Methods. We implemented a two-population model for solid evolution and a pebble flux-regulated model for planetesimal formation in our global model for planet population synthesis. This framework was used to study the global effect of planetesimal formation on planet formation. As reference, we compared our dynamically formed planetesimal surface densities with ad hoc set distributions of different radial density slopes of planetesimals. Results. Even though required, it is not the total planetesimal disk mass alone, but the planetesimal surface density slope and subsequently the formation mechanism of planetesimals that enables planetary growth through planetesimal accretion. Highly condensed regions of only 100 km sized planetesimals in the inner regions of circumstellar disks can lead to gas giant growth. Conclusions. Pebble flux-regulated planetesimal formation strongly boosts planet formation even when the planetesimals to be accreted are 100 km in size because it is a highly effective mechanism for creating a steep planetesimal density profile. We find that this leads to the formation of giant planets inside 1 au already by pure 100 km planetesimal accretion. Eventually, adding pebble accretion regulated by pebble flux and planetesimal-based embryo formation as well will further complement this picture.

The MOSDEF Survey: The First Direct Measurements of the Nebular Dust Attenuation Curve at High Redshift*

Abstract We use a sample of 532 star-forming galaxies at redshifts z ≃ 1.4–2.6 with deep rest-frame optical spectra from the MOSFIRE Deep Evolution Field (MOSDEF) survey to place the first constraints on the nebular attenuation curve at high redshift. Based on the first five low-order Balmer emission lines detected in the composite spectra of these galaxies (Hα through ), we derive a nebular attenuation curve that is similar in shape to that of the Galactic extinction curve, suggesting that the dust covering fraction and absorption/scattering properties along the lines of sight to massive stars at high redshift are similar to those of the average Milky Way sight line. The curve derived here implies nebular reddening values that are, on average, systematically larger than those derived for the stellar continuum. In the context of stellar population synthesis models that include the effects of stellar multiplicity, the difference in reddening of the nebular lines and stellar continuum may imply molecular cloud crossing timescales that are a factor of longer than those inferred for local molecular clouds, star formation rates that are constant or increasing with time such that newly formed and dustier OB associations always dominate the ionizing flux, and/or that the dust responsible for reddening the nebular emission may be associated with nonmolecular (i.e., ionized and neutral) phases of the interstellar medium. Our analysis points to a variety of investigations of the nebular attenuation curve that will be enabled with the next generation of ground- and space-based facilities.

Toward a Direct Measure of the Galactic Acceleration

Abstract High-precision spectrographs can enable not only the discovery of exoplanets, but can also provide a fundamental measurement in Galactic dynamics. Over about 10 year baselines, the expected change in the line-of-sight velocity due to the Galaxy’s gravitational field for stars at ∼kiloparsec scale distances above the Galactic midplane is ∼few tens of cm s−1, and may be detectable by the current generation of high-precision spectrographs. Here, we provide theoretical expectations for this measurement based on both static models of the Milky Way and isolated Milky Way simulations, as well from controlled dynamical simulations of the Milky Way interacting with dwarf galaxies. We simulate a population synthesis model to analyze the contribution of planets and binaries to the Galactic acceleration signal. We find that while low-mass, long-period planetary companions are a contaminant to the Galactic acceleration signal, their contribution is very small. Our analysis of ∼10 years of data from the Lick–Carnegie Exoplanet Survey HIRES/Keck precision radial-velocity (RV) survey shows that slopes of the RV curves of standard RV stars agree with expectations of the local Galactic acceleration near the Sun within the errors, and that the error in the slope scales inversely as the square root of the number of observations. Thus, we demonstrate that a survey of stars with low intrinsic stellar jitter at kiloparsec distances above the Galactic midplane for realistic sample sizes can enable a direct determination of the dark matter density.

Connecting planet formation and astrochemistry

The chemical composition of planetary atmospheres has long been thought to store information regarding where and when a planet accretes its material. Predicting this chemical composition theoretically is a crucial step in linking observational studies to the underlying physics that govern planet formation. As a follow-up to an earlier study of ours on hot Jupiters, we present a population of warm Jupiters (semi-major axis between 0.5 and 4 AU) extracted from the same planetesimal formation population synthesis model as used in that previous work. We compute the astrochemical evolution of the proto-planetary disks included in this population to predict the carbon-to-oxygen (C/O) and nitrogen-to-oxygen (N/O) ratio evolution of the disk gas, ice, and refractory sources, the accretion of which greatly impacts the resulting C/Os and N/Os in the atmosphere of giant planets. We confirm that the main sequence (between accreted solid mass and the atmospheric C/O) we found previously is largely reproduced by the presented population of synthetic warm Jupiters. As a result, the majority of the population falls along the empirically derived mass-metallicity relation when the natal disk has solar or lower metallicity. Planets forming from disks with high metallicity ([Fe/H] &gt; 0.1) results in more scatter in chemical properties, which could explain some of the scatter found in the mass-metallicity relation. Combining predicted C/Os and N/Os shows that Jupiter does not fall among our population of synthetic planets, suggesting that it likely did not form in the inner 5 AU of the Solar System before proceeding into a Grand Tack. This result is consistent with a recent analysis of the chemical composition of Jupiter’s atmosphere, which suggests that it accreted most of its heavy element abundance farther than tens of AU away from the Sun. Finally, we explore the impact of different carbon refractory erosion models, including the location of the carbon erosion front. Shifting the erosion front has a major impact on the resulting C/Os of Jupiter- and Neptune-like planets, but warm Saturns see a smaller shift in C/Os since their carbon and oxygen abundances are equally impacted by gas and refractory accretion.

Mass-loss along the red giant branch in 46 globular clusters and their multiple populations

ABSTRACT The location of Galactic globular clusters’ (GC) stars on the horizontal branch (HB) should mainly depend on GC metallicity, the ‘first parameter’, but it is actually the result of complex interactions between the red giant branch (RGB) mass-loss, the coexistence of multiple stellar populations with different helium content, and the presence of a ‘second parameter’ that produces dramatic differences in HB morphology of GCs of similar metallicity and ages (like the pair M3–M13). In this work, we combine the entire data set from the Hubble Space Telescope Treasury survey and stellar evolutionary models, to analyse the HBs of 46 GCs. For the first time in a large sample of GCs, we generate population synthesis models, where the helium abundances for the first and the ‘extreme’ second generations are constrained using independent measurements based on RGB stars. The main results are as follows: (1) The mass-loss of first-generation stars is tightly correlated to cluster metallicity. (2) The location of helium enriched stars on the HB is reproduced only by adopting a higher RGB mass-loss than for the first generation. The difference in mass-loss correlates with helium enhancement and cluster mass. (3) A model of ‘pre-main sequence disc early loss’, previously developed by the authors, explains such a mass-loss increase and is consistent with the findings of multiple-population formation models predicting that populations more enhanced in helium tend to form with higher stellar densities and concentrations. (4) Helium-enhancement and mass-loss both contribute to the second parameter.

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

ABSTRACT We present the escape fraction of hydrogen ionizing photons (fesc) from a sample of 34 high-resolution cosmological zoom-in simulations of galaxies at z ≥ 5 in the Feedback in Realistic Environments project, post-processed with a Monte Carlo radiative transfer code for ionizing radiation. Our sample consists of 8500 haloes in Mvir ∼ 108–$10^{12}\, M_{\odot }$ (M* ∼ 104–$10^{10}\, M_{\odot }$) at z = 5–12. We find the sample average 〈fesc〉increases with halo mass for Mvir ∼ 108–$10^{9.5}\, M_{\odot }$, becomes nearly constant for 109.5–$10^{11}\, M_{\odot }$, and decreases at ${\gtrsim}10^{11}\, M_{\odot }$. Equivalently, 〈fesc〉 increases with stellar mass up to $M_{\ast }\sim 10^8\, M_{\odot }$ and decreases at higher masses. Even applying single-star stellar population synthesis models, we find a moderate 〈fesc〉 ∼ 0.2 for galaxies at $M_{\ast }\sim 10^8\, M_{\odot }$. Nearly half of the escaped ionizing photons come from stars 1–3 Myr old and the rest from stars 3–10 Myr old. Binaries only have a modest effect, boosting 〈fesc〉 by ∼25–35 per cent and the number of escaped photons by 60–80 per cent. Most leaked ionizing photons are from vigorously star-forming regions that usually contain a feedback-driven kpc-scale superbubble surrounded by a dense shell. The shell is forming stars while accelerated, so new stars formed earlier in the shell are already inside the shell. Young stars in the bubble and near the edge of the shell can fully ionize some low-column-density paths pre-cleared by feedback, allowing a large fraction of their ionizing photons to escape. The decrease of 〈fesc〉 at the high-mass end is due to dust attenuation, while at the low-mass end, 〈fesc〉 decreases owing to inefficient star formation and hence feedback. At fixed mass, 〈fesc〉 tends to increase with redshift. Although the absolute 〈fesc〉does not fully converge with resolution in our simulations, the mass- and redshift-dependence of 〈fesc〉 is likely robust. Our simulations produce sufficient ionizing photons for cosmic reionization.

A closer look at non-interacting He stars as a channel for producing the old population of type Ia supernovae

The nature of the progenitors of type Ia supernovae (SNe Ia) remains a mystery. Binary systems consisting of a white dwarf (WD) and a main-sequence (MS) donor are potential progenitors of SNe Ia, in which a thermonuclear explosion of the WD may occur when its mass reaches the Chandrasekhar limit during accretion of material from a companion star. In the present work, we address theoretical rates and delay times of a specific MS donor channel to SNe Ia, in which a helium (He) star + MS binary produced from a common envelope event subsequently forms a WD + MS system without the He star undergoing mass transfer by Roche lobe overflow. By combining the results of self-consistent binary evolution calculations with population synthesis models, we find that the contribution of SNe Ia in this channel is around 2.0 × 10−4 yr−1. In addition, we find that delay times of SNe Ia in this channel cover a range of about 1.0–2.6 Gyr, and almost all SNe Ia produced in this way (about 97%) have a delay time of ≳1 Gyr. While the rate of SN Ia in this work is about 10% of the overall SN Ia rate, the channel represents a possible contribution to the old population (1–3 Gyr) of observed SNe Ia.

Delayed Photons from Binary Evolution Help Reionize the Universe

Abstract High-resolution numerical simulations including feedback and aimed at calculating the escape fraction (f esc) of hydrogen-ionizing photons often assume stellar radiation based on single-stellar population synthesis models. However, strong evidence suggests the binary fraction of massive stars is ≳70%. Moreover, simulations so far have yielded values of f esc falling only on the lower end of the ∼10%–20% range, the amount presumed necessary to reionize the universe. Analyzing a high-resolution (4 pc) cosmological radiation-hydrodynamic simulation, we study how f esc changes when we include two different products of binary stellar evolution—stars stripped of their hydrogen envelopes and massive blue stragglers. Both produce significant amounts of ionizing photons 10–200 Myr after each starburst. We find the relative importance of these photons to be amplified with respect to escaped ionizing photons, because peaks in star formation rates (SFRs) and f esc are often out of phase by this 10–200 Myr. Additionally, low-mass, bursty galaxies emit Lyman continuum radiation primarily from binary products when SFRs are low. Observations of these galaxies by the James Webb Space Telescope could provide crucial information on the evolution of binary stars as a function of redshift. Overall, including stripped stars and massive blue stragglers increases our photon-weighted mean escape fraction ( ) by ∼13% and ∼10%, respectively, resulting in . Our results emphasize that using updated stellar population synthesis models with binary stellar evolution provides a more sound physical basis for stellar reionization.

The Red Supergiant Binary Fraction of the Large Magellanic Cloud

Abstract The binary fraction of unevolved massive stars is thought to be 70%–100% but there are few observational constraints on the binary fraction of the evolved version of a subset of these stars, the red supergiants (RSGs). Here we identify a complete sample of RSGs in the Large Magellanic Cloud (LMC) using new spectroscopic observations and archival UV, IR, and broadband optical photometry. We find 4090 RSGs with , with 1820 of them having , which we believe is our completeness limit. We additionally spectroscopically confirmed 38 new RSG + B-star binaries in the LMC, bringing the total known up to 55. We then estimated the binary fraction using a k-nearest neighbors algorithm that classifies stars as single or binary based on photometry with a spectroscopic sample as a training set. We take into account observational biases such as line-of-sight stars and binaries in eclipse while also calculating model-dependent corrections for RSGs with companions that our observations were not designed to detect. Based on our data, we find an initial result of for RSGs with O- or B-type companions. Using the Binary Population and Spectral Synthesis models to correct for unobserved systems, this corresponds to a total RSG binary fraction of . This number is in broad agreement with what we would expect given an initial OB binary distribution of 70%, a predicted merger fraction of 20%–30%, and a binary interaction fraction of 40%–50%.

An upgraded interpolator of the radial basis function network for spectral calculation based on empirical stellar spectral library

Stellar population synthesis is an important method in galaxy and star-cluster studies. In stellar population synthesis models, a stellar spectral library is necessary for the integrated spectra of the stellar population. Usually, the stellar spectral library is utilized for the transformation between the stellar atmospheric parameters and stellar spectrum. The empirical stellar spectral library has distinct advantages over the theoretical library. However, for the empirical spectral library, the distribution of stars is irregular in the stellar atmospheric parameter space, which makes the traditional interpolator difficult to get accurate results. In this work, we will provide an improved radial basis function interpolator which is implemented to obtain the interpolated stellar spectra based on the empirical stellar spectral library. For this interpolator, we use the relation between the standard variance σ in the Gaussian radial basis function and the density distribution of stars in the stellar atmospheric parameter space to give the prior constraint on this σ. Moreover, we also consider the anisotropic radial basis function by the advantage of the local dispersion of stars in the stellar atmospheric parameter space. Furthermore, we use the empirical stellar spectral library MILES to test this interpolator. On the whole, the interpolator has a good performance except for the edge of the low-temperature region. At last, we compare this interpolator with our work in 2018, and the interpolation result shows an obvious improvement. Users can apply this interpolator to retrieve the interpolated spectra based on the stellar spectral library quickly and easily.