arXiv subjects

Fenghui Zhang

Publications and source records attributed to Fenghui Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2015

The effects of binary interactions on parameter determinations for early-type galaxies

Based on stellar population models without (SSP) and with (BSP) binary interactions, we investigate the effects of binary interactions on parameter determinations for early-type galaxies (ETGs). We present photometric redshift (photo-z), age and spectral type for photometric data sample by fitting observed magnitudes with the SSP and BSP models. Our results show that binary interactions have no effect on photo-z estimation. Once we neglect binary interactions, the age of ETGs will be underestimated, by contrast, the effects on the age estimations can be negligible for other type of galaxies. For ETG sample, we derive their properties by fitting their spectra with the SSP and BSP models. When comparing these galaxy properties, we find no variation of the overall metallicities for ETGs among the SSP and BSP models. Moreover, the inclusion of binary interactions can affect age estimations. Our results show that the BSP-fitted ages in ~33.3% of ETG sample are around 0.5-1.0 Gyr larger than the SSP-fitted ages; ~44.2\% are only 0.1-0.5 Gyr larger; the rest ~22.5% are approximately equal. By comparisons, we find the difference of the star formation rate between the SSP and BSP models is large at the late evolution stage.

astro-ph.GA↗

arXiv2015

The Evolution of Interacting Spiral Galaxy NGC 5194

NGC 5194 (M51a) is a grand-design spiral galaxy and undergoing interactions with its companion. Here we focus on investigating main properties of its star-formation history (SFH) by constructing a simple evolution model, which assumes that the disc builds up gradually by cold gas infall and the gas infall rate can be parameterizedly described by a Gaussian form. By comparing model predictions with the observed data, we discuss the probable range for free parameter in the model and then know more about the main properties of the evolution and SFH of M51a. We find that the model predictions are very sensitive to the free parameter and the model adopting a constant infall-peak time $t_{\rm p}\,=\,7.0{\rm Gyr}$ can reproduce most of the observed constraints of M51a. Although our model does not assume the gas infall time-scale of the inner disc is shorter than that of the outer disc, our model predictions still show that the disc of M51a forms inside-out. We find that the mean stellar age of M51a is younger than that of the Milky Way, but older than that of the gas-rich disc galaxy UGC 8802. In this paper, we also introduce a 'toy' model to allow an additional cold gas infall occurred recently to imitate the influence of the interaction between M51a and its companion. Our results show that the current molecular gas surface density, the SFR and the UV-band surface brightness are important quantities to trace the effects of recent interaction on galactic SF process.

astro-ph.GA↗

arXiv2014

White-dwarf+main-sequence binaries identified from the tenth data release of Solan Digital Sky Survey

We have presented 309 new white-dwarf (WD) + main-sequence (MS) star binaries identified from the Sloan Digital Sky Survey (SDSS) Tenth Data Release (DR10). The majority of them consist of a white dwarf and a low-mass secondary (typically M dwarf) companion. The SDSS spectra of the newly found WDMS binaries with a DA/DB white dwarf and an M/late-K dwarf companion are analyzed based on a spectral decomposition/fitting method. White dwarf effective temperatures, surface gravities and masses together with the secondary star spectral types are obtained, and the stellar parameters of DA WDs with $T_{\rm eff}\la 14,000$ K are revised to the results in the case of 3D model atmosphere. Two independent distance estimates are derived from the flux-scaling factors between the WDMS SDSS spectra and the white dwarf and M-dwarf model spectra. It is found that about more than 20 per cent of the newly found WDMS binaries show a significant discrepancy between the two distance estimates. This might be caused by the effects of M-dwarf stellar activity or irradiation of the M dwarf companions by the white dwarf. The stellar parameter distributions are used to investigate the global properties of newly found WDMS binaries, the results shown in this work are consistent with those derived by previous investigators. Some WDMS binaries have been observed more than one ($2-4$) times by SDSS, it is found that four of them exhibit not only Hydrogen emission in all observable Balmer series lines in addition to He I line Balmer lines but also the rapid variation in the radial velocities of the components in these binaries. This suggests that they should be the post common envelope binaries (PCEBs) with a short orbital period. These young PCEBs are very important for limiting the results of common envelope phase of the binary evolution.

astro-ph.SR↗

arXiv2014

Effect of dynamical interactions on integrated properties of globular clusters

Globular Clusters (GCs) are generally treated as natural validators of simple stellar population (SSP) models. However, there are still some differences between real GCs and SSPs. In this work we use a direct $N$-body simulation code {\hf Nbody6} to study the influences of dynamical interactions, metallicity and primordial binaries on Milky Way globular clusters' integrated properties. Our models start with $N=100,000$ stars, covering a metallicity range $Z=0.0001-0.02$, a subset of our models contain primordial binaries, resulting in a binary fraction as currently observed at a model age of GCs.Stellar evolution and external tidal field representative for an average Milky Way GC are taken into consideration. The integrated colours and Lick indices are calculated using BaSeL and Bluered stellar spectral libraries separately. By including dynamical interactions, our model clusters show integrated features (colours up to 0.01mag bluer, H$β$ up to 0.1Å$ $ greater and [MgFe]$'$ 0.05Å$ $ smaller) making the clusters appear slightly younger than the model clusters without dynamical interactions. This effect is caused mainly by the preferential loss of low-mass stars which have a stronger contribution to redder passbands as well as different spectral features compared to higher-mass stars. In addition, this effect is larger at lower metallicities. On the contrary, the incorporation of primordial binaries reduces this effect.

astro-ph.GA↗

arXiv2014

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, 2013b) studied the effect of this kind of wind on HB morphology in details, and their results indicated that binary is a possible secondparameter (2P) candidate in GCs. Binary fraction is an 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 that HB morphologies become bluer with increasing of binary fractions is clearly presented. Furthermore, we compared our results with the observation by Milone et al. (2012). Our results are consistent well with the observation at metalrich and metal-poor GCs. For the GCs with intermediate metallicity, when the effect of age on HB morphology is removed, a weak tendency that HB morphologies become bluer with increasing of binary fractions is presented in all regions of GCs, which is consistent with our results obtained in this metallicity range.

astro-ph.SR↗

arXiv2013

Uncertainties in the calibrations of star formation rate

The calibrations of star formation rate (SFR) are prone to be affected by many factors, such as metallicity, initial mass function (IMF), evolutionary population synthesis (EPS) models and so on. In this paper we will discuss the effects of binary interactions, metallicity, EPS models 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 (less than 0.2dex), and these differences increase with metallicity. The differences in the calibration coefficient between SFR and the luminosity of $\rm Hα$ recombination line (C$_{\rm Hα}$) and that between SFR and the ultraviolet (UV) fluxes at 1500 and 2800\,$\rm Å$ (C$_{i, {\rm UV}}$), caused by IMF, are independent of metallicity (0.03-0.33\,dex) except $Δ$C$_{\rm Hα, IMF}$ when using the POPSTAR and $Δ$C$_{i, {\rm UV, IMF}}$ when using the PEGASE models. Moreover, we find that $L_{\rm 2800}$ is not suitable to the linear calibration of SFR at low metallicities. At last, we compare the effects of these several factors on the SFR calibrations considered in this paper. The effects of metallicity/IMF and EPS models on the C$_{\rm Hα}$ and C$_{\rm FIR}$ (the conversion coefficient between SFR and the far-infrared flux) are the largest among these factors, respectively. For the calibration between SFR and C$_{i, {\rm UV}}$, the effects of these several factors are comparable.

astro-ph.SR↗

arXiv2013

Algorithms for Cut Problems on Trees

We study the {\sc multicut on trees} and the {\sc generalized multiway Cut on trees} problems. For the {\sc multicut on trees} problem, we present a parameterized algorithm that runs in time $O^{*}(ρ^k)$, where $ρ= \sqrt{\sqrt{2} + 1} \approx 1.555$ is the positive root of the polynomial $x^4-2x^2-1$. This improves the current-best algorithm of Chen et al. that runs in time $O^{*}(1.619^k)$. For the {\sc generalized multiway cut on trees} problem, we show that this problem is solvable in polynomial time if the number of terminal sets is fixed; this answers an open question posed in a recent paper by Liu and Zhang. By reducing the {\sc generalized multiway cut on trees} problem to the {\sc multicut on trees} problem, our results give a parameterized algorithm that solves the {\sc generalized multiway cut on trees} problem in time $O^{*}(ρ^k)$, where $ρ= \sqrt{\sqrt{2} + 1} \approx 1.555$ time.

cs.DS↗

arXiv2013

Features of amplification of dipole magnetic field with linear ferromagnetic concentrator

Amplification of a low frequency magnetic field from a dipole source by means of linear ferromagnetic concentrator of different dimensions (permalloy micro wire) has been investigated experimentally. The results obtained strengthen the validity of the analytical relation for amplification which allows for nonlinear dependence of the magnetic permeability upon the value and distribution of the dipole field over concentrator length.

physics.ins-det↗

arXiv2012

The Evolution and Star Formation History of M33

We construct a parameterized model to explore the main properties of the star formation history of M33. We assume that the disk originates and grows by the primordial gas infall and adopt the simple form of gas accretion rate with one free parameter, the infall time-scale. We also include the contribution of 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 the 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. The model adopting a moderate outflow rate and an inside-out formation scenario can be in good agreement with most of observed constraints of M33 disk. We also compare the model predictions based on the 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 disk, especially the radial distributions of both the cold gas and the stellar population.

astro-ph.CO↗

arXiv2012

Evolution of luminosity function and obscuration of AGN: connecting X-ray and Infrared

We present a detailed comparison between the 2-10 keV hard X-ray and infrared (IR) luminosity function (LF) of active galactic nuclei (AGN). The composite X-ray to IR spectral energy distributions (SEDs) of AGN used for connecting the hard X-ray LF (HXLF) and IR LF (IRLF) are modeled with a simple but well tested torus model based on the radiative transfer and photoionization code CLOUDY. Four observational determinations of the evolution of 2-10 keV HXLF and six evolution models of the obscured type-2 AGN fraction ($f_2$) have been considered. The 8.0 and 15 \micron LFs for the total, unobscured type-1 and obscured type-2 AGN are predicted from the HXLFs, and then compared with the measurements currently available. We find that the IRLFs predicted from HXLFs tend to underestimate the number of the most IR-luminous AGN. This is independent of the choices of HXLF and $f_2$, and even more obvious for the HXLFs recently measured. We show that the discrepancy between the HXLFs and IRLFs can be largely resolved when the anticorrelation between the UV to X-ray slope $α_{\mathrm{ox}}$ and UV luminosity $L_{\rm UV}$ is appropriately considered. We also discuss other possible explanations for the discrepancy, such as the missing population of Compton-thick AGN and possible contribution of star-formation in the host to the mid-IR. Meanwhile, we find that the HXLFs and IRLFs of AGN can be more consistent with each other if the obscuration mechanisms of quasars and Seyferts are assumed to be different, corresponding to their different triggering and fueling mechanisms. More accurate measurements of the IRLFs of AGN, especially that determined at smaller redshift bins and more accurately separated to that for type-1 and type-2, are very helpful for clarifying these interesting issues.

astro-ph.GA↗

arXiv2011

Testing three derivative methods of stellar population synthesis models

The number of methods used to study the properties of galaxies is increased, and testing these methods is very important. Galactic globular clusters (GCs) provide an excellent medium for such test, 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, including colour, Lick-index and spectrum-fitting methods, based on Bruzual & Charlot evolutionary population synthesis (EPS) models. By comparing with the ages obtained from colour-magnitude diagrams (CMDs) and metallicities obtained from spectra of stars, we are able to estimate the ability of 'these methods on determination of GCs parameters, which is absolutely necessary. As a result, we find that: (i) for the metallicity, our derived metallicities agree with those derived from the spectra of stars, Lick-index method is suitable to study metallicity for the stellar population systems in the range of -1.5=<[Fe/H]=<-0.7 and spectrum- fitting method is suitable to study metallicity for the stellar population systems in the range of -2.3=<[Fe/H]=<-1.5; (ii) for the age, these three methods have difficulties in age determination, our derived ages are smaller (about 2.0 Gyr, on average) than the results of CMDs for all these three methods. We use Vazdekis and Maraston models to analyze 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 the old GCs and our conclusions may hold for old stellar population systems. The whole abstract can be found in my PDF version.

astro-ph.GA↗

arXiv2010

Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars

With more and more exoplanets being detected, it is paid closer attention to whether there are lives outside solar system. We try to obtain habitable zones and the probability distribution of terrestrial planets in habitable zones around host stars. Using Eggleton's code, we calculate the evolution of stars with masses less than 4.00 \mo. We also use the fitting formulae of stellar luminosity and radius, the boundary flux of habitable zones, the distribution of semimajor axis and mass of planets and the initial mass function of stars. We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 \mo, and calculate the habitable zones of host stars, affected by stellar effective temperature. We achieve the probability distribution of terrestrial planets in habitable zones around host stars. We also calculate that the number of terrestrial planets in habitable zones of host stars is 45.5 billion, and the number of terrestrial planets in habitable zones around K type stars is the most, in the Milky Way.

astro-ph.SR↗

arXiv2010

Habitable Zones and UV Habitable Zones around Host Stars

Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds can not go along. We try to obtain the continuous ultraviolet habitable zones, and compare the ultraviolet habitable zones with the habitable zones of host stars. Using the boundary ultraviolet radiation of ultraviolet habitable zone, we calculate the ultraviolet habitable zones of host stars with masses from 0.08 to 4.00 \mo. For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones. For hot subdwarf as a host star, the distance of the ultraviolet habitable zone is about ten times more than that of the habitable zone, which is not suitable for life existence.

astro-ph.SR↗

arXiv2010

Habitable Zones of Host Stars During the Post-MS Phase

A star will become brighter and brighter with stellar evolution, and the distance of its habitable zone will become farther and farther. Some planets outside the habitable zone of a host star during the main sequence phase may enter the habitable zone of the host star during other evolutionary phases. A terrestrial planet within the habitable zone of its host star is generally thought to be suited to life existence. Furthermore, a rocky moon around a giant planet may be also suited to life survive, provided that the planet-moon system is within the habitable zone of its host star. Using Eggleton's code and the boundary flux of habitable zone, we calculate the habitable zone of our Solar after the main sequence phase. It is found that Mars' orbit and Jupiter's orbit will enter the habitable zone of Solar during the subgiant branch phase and the red giant branch phase, respectively. And the orbit of Saturn will enter the habitable zone of Solar during the He-burning phase for about 137 million years. Life is unlikely at any time on Saturn, as it is a giant gaseous planet. However, Titan, the rocky moon of Saturn, may be suitable for biological evolution and become another Earth during that time. For low-mass stars, there are similar habitable zones during the He-burning phase as our Solar, because there are similar core masses and luminosities for these stars during that phase.

astro-ph.SR↗

arXiv2008

The dynamical stability of W Ursae Majoris-type systems

Theoretical study indicates that a contact binary system would merge into a rapidly rotating single star due to tidal instability when the spin angular momentum of the system is more than a third of its orbital angular momentum. Assuming that W UMa contact binary systems rigorously comply with the Roche geometry and the dynamical stability limit is at a contact degree of about 70%, we obtain that W UMa systems might suffer Darwin's instability when their mass ratios are in a region of about 0.076--0.078 and merge into the fast-rotating stars. This suggests that the W UMa systems with mass ratio $q\leq0.076$ can not be observed. Meanwhile, we find that the observed W UMa systems with a mass ratio of about 0.077, corresponding to a contact degree of about 86% would suffer tidal instability and merge into the single fast-rotating stars. This suggests that the dynamical stability limit for the observed W UMa systems is higher than the theoretical value, implying that the observed systems have probably suffered the loss of angular momentum due to gravitational wave radiation (GR) or magnetic stellar wind (MSW).

astro-ph↗

arXiv2008

The evolutionary status of W Ursae Majoris-type systems

Well-determined physical parameters of 130 W UMa systems have been collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in $M-J$ diagram which is consistent with the results derived from the analysis of observed spectral type, $M-R$ and $M-L$ diagrams of W UMa systems. $M-R$ and $M-L$ diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component so that they are not in thermal equilibrium and undergo thermal relaxation oscillation (TRO). Meanwhile, the distribution of angular momentum, together with the distribution of mass ratio, suggests that the mass ratio of the observed W UMa systems is decreased with the decrease of their total mass. This could be the result of the dynamical evolution of W UMa systems which suffer angular momentum loss (AML) and mass loss due to magnetic stellar wind (MSW). Consequently, the tidal instability forces these systems towards the lower q values and finally to fast rotating single stars.

astro-ph↗

arXiv2007

Potential of colors for determining age and metallicity of stellar populations

Colors are usually not used for constraining stellar populations because they are thought to have the well-known agemetallicity degeneracy, but some recent works show that colors can also be used. A simple stellar population synthesis model is widely used, but there is no analysis for its colors. We try to find colors that can potentially be used to determine the age and metallicity of stellar populations by the standard model. Principal component analysis and relative sensitive parameter techniques are used in this work. As a result, U-K, U-H, U-J, B-K, B-H, U-I, B-J, and V-K are found to be more important for studying populations than others. Pairs of colors such as B-K and B-V are found to be able to disentangle the stellar age-metallicity degeneracy via the high-resolution model, while pairs such as U-K and R-I may be used instead when the low-resolution model is used. Furthermore, the u-g and r-i colors of the low-resolution model seem to have the same potential, but there are no such colors for the high-resolution one. In conclusion, some colors have been shown to have the potential to determine the age and metallicity of stellar populations, but relative metallicity and age sensitivities of colors in different stellar population synthesis models are usually different. In addition, minor star formations will make star systems look younger and more metal rich than their dominating populations.

astro-ph↗

arXiv2006

Binary Stellar Population Synthesis Study 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 $Λ$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$β$ 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.

astro-ph↗