Evolutionary Population Synthesis Research Articles

Carbon- and Oxygen-rich stars in MaStar: identification and classification

Abstract Carbon- and Oxygen-rich stars populating the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution are relevant contributors to the spectra of ∼1 Gyr old populations. Atmosphere models for these types are uncertain, due to complex molecules and mass-loss effects. Empirical spectra are then crucial, but samples are small due to the short (∼3 Myr) TP-AGB lifetime. Here we exploit the vastness of the MaNGA Stellar library MaStar (∼60, 000 spectra) to identify C,O-rich type stars. We define an optical colour selection with cuts of (g − r) > 2 and (g − i) < 1.55(g − r) − 0.07, calibrated with known C- and O- rich spectra. This identifies C-,O-rich stars along clean, separated sequences. An analogue selection is found in V, R, I bands. Our equation identifies C-rich and O-rich spectra with predictive performance metric F1-scores of 0.72 and 0.74 (over 1), respectively. We finally identify 41 C- and 87 O-rich type AGB stars in MaStar, 5 and 49 of which do not have a SIMBAD counterpart. We also detect a sample of non-AGB, dwarf C-stars. We further design a fitting procedure to classify the spectra into broad spectral types, by using as fitting templates empirical C and O-rich spectra. We find remarkably good fits for the majority of candidates and categorise them into C- and O-rich bins following existing classifications, which correlate to effective temperature. Our selection models can be applied to large photometric surveys (e.g. Euclid, Rubin). The classified spectra will facilitate future evolutionary population synthesis models.

Relation between spectral indices and binary fractions in globular clusters

Context. We study the relation between the known binary fraction and spectral absorption feature index to judge whether (and potentially which) spectral absorption feature indices are suitable for determining the binary fraction. Aims. The determination of the binary fraction is important in studies of binary star formation, evolutionary population synthesis models, and other works. The number of binary stars is difficult to determine for nearly all stellar systems because the individual stars are need to be resolved photometrically or spectroscopically. By comparison, their integrated spectra or spectral absorption feature indices are relatively easy to obtain. Methods. We used Galactic globular clusters (GCs) as our sample since they have relatively accurate binary fraction measurements and spectroscopic observations along the radial direction. When studying the relation between binary fractions and the spectral absorption feature index, we used three types of binary fractions: one with a mass ratio of q > 0.5 (f(q > 0.5)) and two with a total binary fraction (the methods of counting (f(tot)mf) and fitting (f(tot)mc)), calculated and obtained the equivalent widths or magnitudes of 46 spectral absorption feature indices at three spectral resolutions (FWHMLick/IDS, 5, and 15 Å). Since the regions for the binary fraction measurements (photometric) and the spectroscopic observations are different, we used the method of constructing the radial binary-fraction profile to get the binary fractions corresponding to the regions in the spectroscopic observations. The construction of the radial binary-fraction profile was obtained by using the python curve_fit module to fit the measured and analytic binary fraction values. The analytic value was expressed by taking advantage of the King surface-density profile and the assumed forms with respect to the radial binary-fraction profile (linear, quadratic, exponential, and Gaussian). Results. We find that the low-resolution (15 Å) spectrum is not suitable for this study and the binary fraction type would affect the results: f(q > 0.5) and f(tot)mc exhibit better correlations with the spectral absorption feature index than f(tot)mf and the difference in metallicity would significantly affect the above relationship. Finally, to eliminate the effects of metallicity, age, and dynamical evolution, we only used those GCs with multiple spectra observed among different regions. We find that OIII-1, OIII-2, HγF, HδF, HγA, HδA, Hβ, Ca4455, C24668, and TiO1 indices have strong correlations with binary fraction. The two OIII indices are the most sensitive to the binary fraction, followed by four Balmer indices – the two narrower central bandpass Balmer indices (∼20 Å, F-definition) are more sensitive than the wider two (∼40 Å, A-definition) and, lastly, the Ca4455, C24668, and TiO1 indices. Using the binary fraction-sensitive spectral absorption feature indices in combination with the age- and metallicity-sensitive spectral absorption feature indices, we can determine the ages or metallicities first. Then we can go on to obtain the binary fractions for those stellar systems in which the individual stars cannot be resolved (e.g., dense or distant stellar systems). Furthermore, we are then able to carry out further studies of binary star formation and improve evolutionary population synthesis models.

A resolved study of the inner regions of nearby galaxies with an excess of young massive stars: missing link in the AGN–starburst connection?

ABSTRACT We have selected galaxies with very high levels of H α emission (EQW(H α) >700 Å.) in their central regions from the final data release of the MaNGA survey. Our study focuses on 14 very well-resolved nearby galaxies with stellar masses in the range 9.5 < log M*/(M⊙) < 11.5. We investigate a variety of procedures for selecting galaxy regions that are likely to harbour excess populations of young massive stars, finding that selection in the 2D space of extinction-corrected H α EQW and [S iii]/[S ii] line ratio produces the best results. By comparing stacked spectra covering these regions with stacked spectra covering normal starburst regions with 100 Å<EQW(H α) <200 Å, we obtain the following main results: (1) Clear signatures of excess Wolf–Rayet stars are found in half of the H α excess regions, (2) galaxy regions containing excess Wolf–Rayet stars are more often associated with the presence of high-ionization emission lines characteristic of accreting black holes. Excess [Ne iii] is detected in 4 out of 8 of the WR regions and there are tentative [Fe x] detections in 2 galaxies. (3) Regions of the galaxy with excess Wolf–Rayet stars are located where the interstellar medium has larger ionized gas turbulent velocities and higher neutral gas overdensities. We make a first attempt to constrain changes in the high-mass end of the stellar initial mass function (IMF) using the HR-pyPopStar evolutionary population synthesis models that include high-wavelength-resolution theoretical atmosphere libraries for Wolf–Rayet stars.

Triple Evolution: An Important Channel in the Formation of Type Ia Supernovae

Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear explosions in white dwarfs (WDs). Commonly considered formation pathways include two merging WDs (the double-degenerate channel) and a single WD accreting material from a H or He donor (the single-degenerate channel). Since the predicted SN Ia rates from WDs in binaries are thought to be insufficient to explain the observed SN Ia rate, it is important to study similar interactions in higher-order multiple-star systems such as triple systems. We use the evolutionary population synthesis code Multiple Stellar Evolution (MSE) to study the stellar evolution, binary interactions, and gravitational dynamics of the triple-star systems. Also, unlike previous studies, prescriptions are included to simultaneously take into account the single- and double-degenerate channels, and we consider triples across the entire parameter space (including those with tight inner binaries). We explore the impact of typically ignored or uncertain physics such as flybys and common envelope prescription parameters on our results. The majority of systems undergo circular mergers to explode as SNe Ia, while eccentric collisions contribute to 0.4%−4% of SN Ia events. The time-integrated SN Ia rate from the triple channel is found to be , which is, surprisingly, similar to that of the isolated binary channel, where the SN Ia rate is . This implies that triples, when considering their entire parameter space, yield an important contribution to the overall SN Ia rate.

Population synthesis on ultra-luminous X-ray sources with an accreting neutron star: Wind Roche-lobe overflow cases

Very recently, wind Roche-lobe overflow (WRLOF) has been suggested as a possible mass transfer mechanism for ultra-luminous X-ray sources (ULXs) and, to date, two neutron-star (NS) ULXs (i.e., NGC 7793 P13 and NGC 300 ULX-1) are remarkable and hard to understand in the current, usual RLOF picture. In this work, we test if the two sources could fit into the WRLOF paradigm. By using an evolutionary population synthesis method, we modeled the population of NS ULXs with (super)giant donors, taking the WRLOF accretion mode into account. We find that the population of wind-fed NS ULXs in the WRLOF mode is distinct in numbers and binary parameters from that in the traditional Bondi-Hoyle-Lyttleton mode, and it is strongly metallicity dependent. The number of NS ULXs with (super)giant donors can be enhanced greatly, by one or two orders of magnitude, depending on the metallicity adopted. Sources with massive (∼15−40 M⊙) (super)giant donors dominate wind-fed NS ULXs in the very low metallicities, while sources in near solar cases are dominated by a red supergiant with a lower mass M2 < 10 M⊙ instead. Moreover, the two NS ULXs can be well reproduced in the WRLOF paradigm, which significantly enriches our understanding of the nature of ULXs and the population. We also present the current distributions of binary parameters of wind-fed NS ULXs, which may be further testified by future high-resolution optical and X-ray observations of these populations.

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.

Photoionisation modelling of the H II regions surrounding star-forming regions within the metallicity range Z=0.003-0.012

We present the new approach to the ionisation structure modelling for the high-metallicity H II regions. The method is based on the multicomponent photoionisation modelling (MPhM) of these objects that takes into account their complicate structure due to superwind from the central star-forming region. The complex structure of H II region has been divided into internal and external components. Internal components correspond to the region of free expanding superwind and the cavity of superwind, respectively, while the external ones — to a thick layer of gas compressed by a superwind shock, and hydrodynamically undisturbed outer part of H II region, where the most of observed strong emission lines are formed. The components of the model were calculated within the assumption of spherical symmetry. The gas photoinisation was caused by the ionising quanta of both direct and diffuse ionising radiation. The fluxes of this radiation were calculated during the simulation using the radiative transfer equations which account for all important processes in the H II region causing this transfer. The diffuse ionising radiation was calculated in the Outward Only approach. In the region of free expansion of the superwind the chemical abundances were determined using the evolutionary population synthesis models of a star-forming region. The distributions of the electron temperature and density in the external components were obtained in such modelling as the solutions of the photoionisation energy balance equation. The stop criterion for evolutionary modelling corresponding to the condition of equilibrium of pressure on the boundary between the third and fourth components was adopted. The evolutionary grid of multicomponent high-metallicity models of the H II regions was calculated. It was shown that the internal structure of a H II region under certain conditions can cause the lack of quanta in the spectrum of ionising radiation.

Evolutionary Population Synthesis model with binary stars – Yunnan-II model

AbstractBy considering a modified version of the evolutionary population synthesis (EPS) model for stellar populations (SPs) comprising binary stars, the retrieved galaxy and HII-region parameters/properties differ from the case of neglecting binary stars. The retrieved age, stellar metallicity and mass of galaxies increase (e.g. ∼ 0.2 dex when using spectral fitting algorithm), whilst the star formation rate decreases (∼0.2 dex). The radiation fields from intermediate-age SPs with binary stars could be potentially important ionizing sources in HII regions. Under this possibility, the theoretical division between star forming galaxy and AGN on the diagnostic diagrams would move towards the up-right corner and the retrieved gaseous metallicity would decrease.Our prediction for the birth rate of binary neutron stars in SPs ranges from 10−9 to 10−6${\M {^\minus 1_\odot}} $ yr−1 when the kick velocity is from 0 to 190 km s−1.

Globular Cluster — Bulge connection: Population synthesis models with multiple populations

AbstractRecent analyses of Lee et al. (2018, 2019) have confirmed that Galactic bulge consists of stellar populations originated from Milky Way globular clusters (MWGCs). Motivated by this, here we present the evolutionary population synthesis (EPS) for the Galactic bulge and early-type galaxies (ETGs) with the realistic treatment of individual variations in light elements observed in the MWGCs. We have utilized our model with GC-origin populations to explain the CN spread observed in ETGs, and the results show remarkable matches with the observations. We further employ our model to estimate the age of ETGs, which are considered as good analogs for the MW bulge. We find that, without the effect of our new treatments, EPS models will almost always underestimate the true age of ETGs. Our analysis indicates that the EPS with GC-origin populations is an essential constraint in determining the ETG formation epoch and is closely related to understanding the evolution of the Universe.

Effects of Binary Interactions on the Early-type Galaxy Chemical Evolution and Gas Cooling Functiontwo

In the galaxy parameter fitting by means of stellar population synthesis, it is found that compared with the evolutionary population synthesis (EPS) model without binary interactions, the stellar age and metallicity of a galaxy derived from the EPS model with binary interactions are larger. But, we are still unclear how the binary interactions affect the galaxy evolution. For the early-type galaxies with the UV-excess phenomenon, there are two main-stream explanations: recent star formation (RSF) and binary interactions. In this study, we obtain the mass return rate and chemical yield for the stellar populations with and without binary interactions. In combination with the galaxy chemical evolution and photoionization models, we study the effects of binary interactions on the chemical evolution and metallicity evolution for the early-type galaxies with the UV-excess phenomenon under the two formation mechanisms. We find that the inclusion of binary interactions can raise the ejected mass, metallicity, alpha element, and accelerate the gas cooling. These can reasonably explain the conclusions made by the EPS models. Moreover, we find that the gas cooling is more efficient under the UV-excess formation mechanism by the binary interactions rather than the RSF, and the ratio of element abundance is different for the two mechanisms, which can be further used to distinguish these two mechanisms.

High-mass X-ray binaries: Evolutionary population synthesis modeling

AbstractUsing an evolutionary population synthesis code, we modeled the universal, featureless X-ray luminosity function of high-mass X-ray binaries (HMXBs) in star-forming galaxies. We put constraints on the natal kicks, super-Eddington accretion factor, as well as common envelope prescriptions usually adopted (i.e., the αCE formalism and the γ algorithm), and presented the detailed properties of HMXBs under different models, which may be investigated further by future high-resolution X-ray and optical observations.

Constraints from luminosity-displacement correlation of high-mass X-ray binaries

AbstractWe have modeled the luminosity-displacement correlation of high-mass X-ray binaries (HMXBs) with an evolutionary population synthesis (EPS) code. Detailed properties including offsets of simulated HMXBs are presented under both common envelope prescriptions usually adopted (i.e., the αCE formalism and the γ algorithm), and several theoretical models describing the natal kicks. We suggest that the distinct observational properties may be used as potential evidence to discriminate between models.

A new stellar spectrum interpolation algorithm and its application to Yunnan-III evolutionary population synthesis models

In evolutionary population synthesis (EPS) models, we need to convert stellar evolutionary parameters into spectra via interpolation in a stellar spectral library. For theoretical stellar spectral libraries, the spectrum grid is homogeneous on the effective-temperature and gravity plane for a given metallicity. It is relatively easy to derive stellar spectra. For empirical stellar spectral libraries, stellar parameters are irregularly distributed and the interpolation algorithm is relatively complicated. In those EPS models that use empirical stellar spectral libraries, different algorithms are used and the codes are often not released. Moreover, these algorithms are often complicated. In this work, based on a radial basis function (RBF) network, we present a new spectrum interpolation algorithm and its code. Compared with the other interpolation algorithms that are used in EPS models, it can be easily understood and is highly efficient in terms of computation. The code is written in MATLAB scripts and can be used on any computer system. Using it, we can obtain the interpolated spectra from a library or a combination of libraries. We apply this algorithm to several stellar spectral libraries (such as MILES, ELODIE-3.1 and STELIB-3.2) and give the integrated spectral energy distributions (ISEDs) of stellar populations (with ages from 1 Myr to 14 Gyr) by combining them with Yunnan-III isochrones. Our results show that the differences caused by the adoption of different EPS model components are less than 0.2 dex. All data about the stellar population ISEDs in this work and the RBF spectrum interpolation code can be obtained by request from the first author or downloaded from http://www1.ynao.ac.cn/similar to zhangfh.

Probing evolutionary population synthesis models in the near infrared with early-type galaxies

We performed a near-infrared (NIR, $\sim$1.0$\mu$m-2.4$\mu$m) stellar population study in a sample of early type galaxies. The synthesis was performed using five different evolutionary population synthesis libraries of models. Our main results can be summarized as follows: low spectral resolution libraries are not able to produce reliable results when applied to the NIR alone, with each library finding a different dominant population. The two newest higher resolution models, on the other hand, perform considerably better, finding consistent results to each other and to literature values. We also found that optical results are consistent with each other even for lower resolution models. We also compared optical and NIR results, and found out that lower resolution models tend to disagree in the optical and in the NIR, with higher fraction of young populations in the NIR and dust extinction $\sim$1 magnitude higher than optical values. For higher resolution models, optical and NIR results tend do aggree much better, suggesting that a higher spectral resolution is fundamental to improve the quality of the results.

The primordial helium abundance determination using multicomponent photoionization modelling of low-metallicity H II regions

The method for the multicomponent photoionization modelling (MPhM) of low-metallicity H II regions surrounding the starburst region was developed. The internal structure of the H II region has been determined using the evolutionary modelling of the superwind bubble surrounding the star-forming region. Models of Chevalier and Clegg (1985) and Weaver et al. (1977) have been used to determine the radial distribution of the gas density, the velocity of gas layers, and the temperature within internal components (the region of the superwind free expansion and the cavity, respectively). The chemical abundances in region of the superwind free expansion were obtained from the evolutionary population synthesis with including of rotating stars. The chemical abundances within cavity were defined by averaging over mass the chemical compositions of mixture of the abundances of gas from superwind and ones within outer component, because of gas evaporation from external component into the cavity. External components of our models describe a high-density, thin shell of gas formed by superwind shock and a typical undisturbed hydrodynamically H II region, respectively. Evolutionary grids of multicomponent low-metallicity models are calculated. A comparative analysis of the results of their calculation with the observed data has been carried out. The ionic abundances averaged over modelling volume as well as chemical composition assumed in models were used to derive the new expressions for ionization-correction factors that were used to redetermine the chemical compositions of 88 H II regions in blue compact dwarf galaxies. It must be noticed that we used for this propose the ionic abundances obtained by Izotov et al. (2007). In result the primordial helium abundance and its enrichment during stellar chemical evolution of matter were determined.

Yonsei Evolutionary Population Synthesis (YEPS). II. Spectro-photometric Evolution of Helium-enhanced Stellar Populations

Abstract The discovery of multiple stellar populations in Milky Way globular clusters (GCs) has stimulated various follow-up studies on helium-enhanced stellar populations. Here we present the evolutionary population synthesis models for the spectro-photometric evolution of simple stellar populations (SSPs) with varying initial helium abundance (Y ini). We show that Y ini brings about dramatic changes in spectro-photometric properties of SSPs. Like the normal-helium SSPs, the integrated spectro-photometric evolution of helium-enhanced SSPs is also dependent on metallicity and age for a given Y ini. We discuss the implications and prospects for the helium-enhanced populations in relation to the second-generation populations found in the Milky Way GCs. All of the models are available at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm.

Modeling of H II region radiation surrounding the starburst knot taking into account the evolution of structures formed by the superwind

The method for the multicomponent photoionization modeling of the H II region radiation surrounding the starburst knot is presented. The internal structure of the H II region has been determined using the evolutionary model of the superwind bubble from the starburst center. Models of Chevalier and Clegg (1985) and Weaver et al. (1977) have been used to determine the radial distribution of the gas density, the velocity of gas layers, and the temperature in the region of the superwind free expansion and in the cavity, respectively. The chemical content of the internal components of the bubble has been set by the results of the modeling of the evolutionary population synthesis. External components of our models describe a high-density, thin layer of gas formed by the shock wave of stellar superwind from the surrounding gas and a typical H II region, respectively. Input model parameters have been taken from the precalculated evolutionary starburst models based on three types of evolutionary tracks. Evolutionary grids of multicomponent low-metallicity models are calculated. A comparative analysis of the results of their calculation with the observational data has been carried out.

Exploration of spin-down rate of neutron stars in high-mass X-ray binaries

We use the evolutionary population synthesis method to investigate the statistical properties of the wind-fed neutron star (NS) compact ($P_{\rm orb}<10$ days) high-mass X-ray binaries (HMXBs) in our Galaxy, based on different spin-down models. We find that the spin-down rate in the supersonic propeller phase given \textbf{by assuming that the surrounding material is treated as forming a quasi-static atmosphere} or \textbf{by assuming that the characteristic velocity of matter and the typical Alfv$\acute{e}$n velocity of material in the magnetospheric boundary layer are comparable to the sound speed in the external medium} is too low to produce the observed number of compact HMXBs. We also find that the models suggested \textbf{by assuming that the infalling material is ejected with the corotation velocity at the magnetospheric radius when the magnetospheric radius is larger than the corotation radius} and \textbf{by simple integration of the magnetic torque over the magnetosphere} with a larger spin-down rate than that given by \citet{dav81} or \citet{is75} can predict a reasonable number of observed wind-fed NS compact HMXBs. Our calculated results indicate that subsonic propeller phase may not exist at all by comparing with the observed particular distributions of wind-fed NS compact HMXBs in the $P_s-P_{orb}$ diagram. However, the spin-down rate suggested by \citet{wan85,dai06,jia05} and that given by \citet{dav73} both seem reasonable to produce the observed distribution of wind-fed NS compact HMXBs in the $P_s-P_{orb}$ diagram. We cannot find which spin-down rate seems more reasonable from our calculations.

Revisiting the role of the thermally pulsating asymptotic-giant-branch phase in high-redshift galaxies

We study the debated contribution from thermally pulsing asymptotic giant branch (TP-AGB) stars in evolutionary population synthesis models. We investigate the Spectral Energy Distributions (SEDs) of a sample of 51 spectroscopically confirmed, high-z ($1.3<z_{\rm spec}<2.7$), galaxies using three evolutionary population synthesis models with strong, mild and light TP-AGB. Our sample is the largest of spectroscopically confirmed galaxies on which such models are tested so far. Galaxies were selected as passive, but we model them using a variety of star formation histories in order not to be dependent on this pre-selection. We find that the observed SEDs are best fitted with a significant contribution of TP-AGB stars or with substantial dust attenuation. Without including reddening, TP-AGB-strong models perform better and deliver solutions consistent within $1\sigma$ from the best-fit ones in the vast majority of cases. Including reddening, all models perform similarly. Using independent constraints from observations in the mid- and far-IR, we show that low/negligible dust attenuation, i.e. $E(B-V)\lesssim 0.05$ , should be preferred for the SEDs of passively-selected galaxies. Given that TP-AGB-light models give systematically older ages for passive galaxies, we suggest number counts of passive galaxies at higher redshifts as a further test to discriminate among stellar population models.

Binary stars on the galaxy SFH

AbstractUsing the Yunnan-II evolutionary population synthesis models comprising binary stars, we find that the inclusion of binary stars can raise the derived stellar metallicity Z* and/or age t (degeneracy problem), raise the stellar mass M*, lower the gaseous metallicity Zgas and star formation rate (SFR) of galaxies. This means that a few stars form recently in galaxies, while more stars form during the entire evolution process when considering binary stars. If the degeneracy between t and Z* can be broken, its effect on the feedback process and star formation history can be determined.