Xiaoyu Kang

The evolution and star-formation history of M33

We construct a parametrized model to explore the main properties of the star-formation history of M33. We assume that the disc originates and grows by primordial gas infall and adopt a simple form of gas accretion rate with one free parameter, the infall time-scale. We also include the contribution of the gas outflow process. A major update of the model is that we adopt a molecular-hydrogen-correlated star-formation law and calculate the evolution of the atomic and molecular gas separately. Comparisons between the model predictions and observational data show that the model predictions are very sensitive to the adopted infall time-scale, while the gas-outflow process mainly influences the metallicity profile. A model adopting a moderate outflow rate and an inside-out formation scenario can be in good agreement with most of the observed constraints of the M33 disc. We also compare model predictions based on a molecular-hydrogen-correlated star-formation law and that based on the Kennicutt star-formation law. Our results imply that the molecular-hydrogen-correlated star-formation law should be preferred to describe the evolution of the M33 disc, especially the radial distributions of both the cold gas and the stellar population.

Testing three derivative methods of stellar population synthesis models

The number of methods used to study the properties of galaxies is increasing, and it is very important to test these methods. Galactic globular clusters (GCs) provide an excellent medium for such tests, because they can be considered as simple stellar populations. We present ages and metallicities for 40 Galactic GCs as determined from three publicly available techniques, namely the colour, Lick-index and spectrum-fitting methods, based on Bruzual & Charlot evolutionary population synthesis (EPS) models. By comparisons with the ages obtained from colour-magnitude diagrams (CMDs) and metallicities obtained from the spectra of stars, we are able to estimate the ability of these methods to determine GC parameters, which is essential. We find that: (i) for the metallicity, our derived metallicities agree with those derived from the spectra of stars - the Lick-index method is suitable for studying metallicity for stellar population systems in the range - 1.5 less than or similar to [Fe/H] less than or similar to -0.7 and the spectrum-fitting method is suitable for studying metallicity for stellar population systems in the range -2.3 less than or similar to [Fe/H] less than or similar to -1.5; (ii) for the age, these three methods have difficulties in age determination - our derived ages are smaller (by about 2.0 Gyr, on average) than the results of CMDs for all three methods. We use Vazdekis and Maraston models to analyse whether our results are dependent on EPS models, and find that the tendency of these two models is the same as that of Bruzual & Charlot models. Our results are independent of the EPS models. In addition, our test is based on old GCs and our conclusions may hold for old stellar population systems. Besides the age-metallicity degeneracy, we examine the possible effects of other factors (horizontal branch morphology, blue straggler stars, binary interactions and alpha-enhancement) and give a quantitative analysis of the influences of these factors on age determinations (except for alpha-enhancement). For the colour and spectrum-fitting methods, the age can be underestimated by about 0.0-3.0, 0.0-2.0 and 0.0-3.0Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively. For the Lick-index method, the lower limit of maximal change of age is 6.0, 5.0 and 3.0 Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively.

Binary interactions on the calibrations of star formation rate

Using the Yunnan evolutionary population synthesis (EPS) models with and without binary interactions, we present the luminosity of H alpha recombination line (L-H alpha), the luminosity of [O II]lambda 3727 forbidden line doublet (L-[O II]), the ultraviolet (UV) fluxes at 1500 and 2800 angstrom (L-i,L-UV) and far-infrared flux (L-FIR) for burst, S0, Sa-Sd and Irr galaxies, and present the calibrations of star formation rate (SFR) in terms of these diagnostics. By comparison, we find that binary interactions lower the SFR versus L-H alpha and SFR versus L-[O II] conversion factors by similar to 0.2 dex. The main reason is that binary interactions raise the UV flux (shortwards of the Lyman limit) of the stellar population (SP) in the age range 6.7 < log t/yr < 8.4 and thus more ionizing photons are present in the nebula. Moreover, binary interactions do not significantly vary the calibrations of SFR in terms of L-i,L-UV. This is because binary interactions raise the flux at 1500 angstrom of the SP in the range 8.75 < log t/yr < 9.2 and the maximal difference is about 1 dex. In addition, binary interactions have little effect on the flux at 2800 angstrom. At last, the calibration of SFR from L-FIR is almost unaffected by binary interactions. This is caused by the fact that binary interactions almost do not affect the bolometric magnitudes of SPs. We also discuss the effects of initial mass function (IMF), gas-recycle assumption and EPS models [including GISSEL98 (Galaxy Isochrone Synthesis Spectral Evolution Library), BC03, STARBURST99, POPSTAR and PEGASE models] on these SFR calibrations. Comparing the results obtained by using the Salpeter (hereafter S55) IMF with those by using the Miller & Scalo (hereafterMS79) IMF, we find that the SFR versus L-H alpha and SFR versus L-[O II] conversion factors obtained by using the S55 IMF are greater by 0.4 and 0.2 dex than those by using the MS79 IMF for the Yunnan models with and without binary interactions, respectively. The SFR versus L-i,L-UV and SFR versus L-FIR conversion factors by using the S55 IMF are larger by an amount of 0.2 dex than the corresponding ones by using the MS79 IMF. The inclusion of gas-recycle assumption only lowers these SFR calibrations at faint SFR. Moreover, comparing the results when using different EPS models, we find that the differences in the SFR versus L-H alpha and SFR versus L-[O II] conversion factors reach similar to 0.7 and 0.9 dex, the difference in the SFR versus L-FIR conversion factor reaches 0.4 and 0.8 dex, and the differences in the SFR versus L-i,L-UV conversion factors reach 0.3 and 0.2 dex when using the S55 and non-S55 IMFs (including Chabrier, Kroupa-Aarseth-Hurley, Kroupa-Tout-Gilmore and Miller-Scalo IMFs, partly caused by the difference in the IMF), respectively. At last, we give the conversion coefficients between SFR and these diagnostics for all models.

The star formation history of low-mass disk galaxies: A case study of NGC 300

Context. Since NGC300 is a bulgeless, isolated low-mass galaxy and it has not experienced radial migration during its evolution history, it can be treated as an ideal laboratory to test the simple galactic chemical evolution model.

Yunnan-III models for Evolutionary population synthesis

We build the Yunnan-III evolutionary population synthesis (EPS) models by using the stellar evolution code, BaSeL stellar spectra library and the initial mass functions (IMFs) of Kroupa and Salpeter, and present colours and integrated spectral energy distributions (ISEDs) of solar-metallicity stellar populations (SPs) in the range of 1Myr-15Gyr. The main characteristic of the Yunnan-III EPS models is the usage of a set of self-consistent solar-metallicity stellar evolutionary tracks (the masses of stars are from 0.1 to 100M⊙). This set of tracks is obtained by using the state-of-the-art code. code can evolve stellar models through thermally pulsing asymptotic giant branch (TP-AGB) phase for low- and intermediate-mass stars. By comparisons, we confirm that the inclusion of TP-AGB stars make the V K, V J and V R colours of SPs redder and the infrared flux larger at ages log(t/yr) > 7.6 (the differences reach the maximum at log(t/yr)�8.6, �0.5-0.2mag for colours, � 2 times for K band flux). We also find that the colour-evolution trends of Model with-TPAGB at intermediate and large ages are similar to those from the code, which employs the Padova-AGB stellar library, BaSeL spectral library and the Kroupa IMF. At last, we compare the colours with the other EPS models comprising TP-AGB stars (such as CB07, M05, V10 and POPSTAR), and find that the B V colour agrees with each other but the V K colour exists larger discrepancy among these EPS models (�1mag when 8< log(t/yr)< 9). The stellar evolutionary tracks, isochrones, colours and ISEDs can be obtained on request from the first author or from our website (http://www1.ynao.ac.cn/�zhangfh/). Using the isochrones, you can build your EPS models. Now the format of stellar evolutionary tracks is the same as that in the code, you can put them into the code and get the SP’s results. Moreover, the colours involving other passbands or on other systems (for example, HST F439W F555W colour on AB system) can also be obtained on request.

Effect of binary fraction on horizontal branch morphology under tidally enhanced stellar wind

Tidally enhanced stellar wind may affect horizontal branch (HB) morphology in globular clusters (GCs) by enhancing the mass loss of primary star during binary evolution. Lei et al. (2013a, AA 2013b, A&A, 554, 130) studied the effect of this kind of wind on HB morphology in detail, and their results indicated that a binary is a possible second-parameter (2P) candidate in GCs. Binary fraction is a very important fact in the tidally-enhanced-stellar-wind model. In this paper, we studied the effect of binary fraction on HB morphology by removing the effects of metallicity and age. Five different binary fractions (i.e., 10%, 15%, 20%, 30%, and 50%) are adopted in our model calculations. The synthetic HB morphologies with different binary fractions are obtained at different metallicities and ages. We found that, due to the great influence of metallicity and age, the effect of binary fraction on HB morphology may be masked by these two parameters. However, when the effects of metallicity and age are removed, the tendency of HB morphologies becoming bluer with increasing binary fractions is clearly presented. Furthermore, we compared our results with the observation by Milone et al. (2012, A&A, 540, 16). Our results are consistent with the observation at metal-rich and metal-poor GCs. For the GCs with intermediate metallicity, when the effect of age on HB morphology is removed, a weak tendency of HB morphologies becoming bluer with increasing binary fractions is presented in all regions of GCs, which is consistent with our results obtained in this metallicity range.

Uncertainties in the calibrations of star formation rate

Calibrations of star formation rate (SFR) are affected by many factors, for example the metallicity, the initial mass function (IMF) and the evolutionary population synthesis (EPS) model used. In this paper we will discuss the effects of binary interactions, metallicity, EPS model and IMF on several widely used SFR calibrations based on the EPS models of Yunnan with and without binary interactions, BC03, SB99, PEGASE and POPSTAR. The inclusion of binary interactions makes these SFR conversion coefficients smaller (by less than 0.2 dex), and these differences increase with metallicity. The differences in the calibration coefficient between the SFR and the luminosity of the H alpha recombination line (C-H alpha) and that between the SFR and the ultraviolet (UV) fluxes at 1500 and 2800 angstrom (C-i,C- UV), caused by the IMF, are independent of metallicity (0.03-0.33 dex), except for Delta C-H alpha,C- IMF when using the POPSTAR and for Delta C-i,C- UV,C- IMF when using the PEGASE models. Moreover, we find that L-2800 is not suitable for the linear calibration of SFR at low metallicities. Finally, we compare the effects of these factors on the SFR calibrations considered in this paper. The effects of metallicity/IMF and EPS model on C-H alpha and C-FIR (the conversion coefficient between SFR and the far-infrared flux) are the largest among these factors, respectively. For the calibration between SFR and C-i,C- UV, the effects of these factors are comparable.

The role of environment on the star formation history of disc galaxies

NGC 2403, NGC 300 and M33 are three nearby pure-disc galaxies with similar stellar mass in different environments; they are benchmarks for understanding late-type spiral galaxies in different environments. The chemical evolution and growth of their discs are investigated by using the simple chemical evolution model, in which their discs are assumed to originate and grow through the accretion of the primordial gas, and the gas outflow process is also taken into account. Through a comparative study of the best-fitting model-predicted star formation histories for them, we hope to derive a picture of the local environment on the evolution and star formation histories of galaxies and whether or not the isolated galaxies follow similar evolution history. Our results show that these three galaxies accumulated more than 50 per cent of their stellar mass at z