In this article, we suggest a Sitter model in favor of compact stars in low-mass X-ray binaries.Here, we have considered the presence of a cosmological constant(on a small scale) to investigate the stellar structure. ...In this article, we suggest a Sitter model in favor of compact stars in low-mass X-ray binaries.Here, we have considered the presence of a cosmological constant(on a small scale) to investigate the stellar structure. We conclude that this approach is very suitable for the familiar physical model of compact stars in low-mass X-ray binaries. We calculate the probable radius, compactness(u) and surface redshift(Z_s) of six compact stars in low-mass X-ray binaries, namely Cyg X-2, V395 Carinae/2 S 0921–630, XTE J2123–058,X1822–371(V691 CrA), 4 U 1820–30 and GR Mus(XB 1254–690). We also offer a possible equation of state(EOS) for the stellar objects.展开更多
Based on the photographic and CCD observations of the relative motion of the A and B components of the binary system ADS 9346 obtained with the 26-inch refractor of Pulkovo Observatory during 1979–2019,we discover an...Based on the photographic and CCD observations of the relative motion of the A and B components of the binary system ADS 9346 obtained with the 26-inch refractor of Pulkovo Observatory during 1979–2019,we discover an invisible companion associated with star A.Comparison of the ephemerides with the positional and spectroscopic observations allowed us to calculate the preliminary orbit of the photocenter(P=15 yr).The minimal mass of the companion is approximately 0.13 M.The existence of the invisible low-mass companion is implied by the IR-excess based on IRAS data.To confirm this,additional observations of the radial velocity near the periastron need to be carried out.展开更多
A catalog of M stars has been published from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope data release 7(LAMOST DR7).We cross-matched the LAMOST M catalog with Kepler,Kepler 2(K2)and Transiting Exopla...A catalog of M stars has been published from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope data release 7(LAMOST DR7).We cross-matched the LAMOST M catalog with Kepler,Kepler 2(K2)and Transiting Exoplanet Survey Satellite(TESS)surveys.We obtained the light curves from the Kepler and TESS surveys.We detected 20,047 flare events from 4053 M stars and calculated their durations,amplitudes,and energies.We analyzed the distribution of the flare durations and energies.The distributions of flare duration for Kepler,K2 and TESS peak are at 3–3.5 hr,4–6 hr and 1–1.5 hr,respectively.This may be the result of the different cadences for the three data sets.The highest regions of the flare energies of Kepler,K2 and TESS are 32–32.5,32–32.5 and31–31.5 erg in Log format,respectively.A linear relationship between flare duration and energy emerges from our analysis.The ratio of flare duration to total observational time is approximately 0.2%–0.3%for the Kepler,K2 and TESS surveys.The occurrence rate of a star with a flare event increases from the M0 to M4 subtypes.We also determined the spatial distribution of the flare rate of M stars in the Milky Way.It seems that the flare rate decreases as the vertical height increases.The power index of the flare energies is in the region of 1.53–2.32,which is similar to previous result for solar type star(2.0).Further,we examined the relationship between the flare amplitude and chromospheric intensity.The flare activity increases rapidly with the increase in the HαEW at the lower values(less approximately 2?)and it increases slowly at the higher values.展开更多
Element diffusion has small but significant effects on the structure of the stellar interior.It is interesting to investigate the effects of element diffusion using asteroseismology.We have constructed two grids of st...Element diffusion has small but significant effects on the structure of the stellar interior.It is interesting to investigate the effects of element diffusion using asteroseismology.We have constructed two grids of stellar models,one with diffusion and one without,for solar-like stars with masses between 0.9 and 1.4 solar masses,and varied initial helium abundance and metallicity.The oscillation frequencies of all stellar models have also been calculated.Piecewise Hermite cubic polynomials are adopted to interpolate stellar p-mode frequencies at an arbitrary age on a stellar evolutionary track.We have investigated 16 Kepler solar-like stars by comparing the model frequencies with observations.The suggested ranges of stellar parameters and some global variables are obtained.For all stars,the best model reproduces the observational frequencies with aχ^(2)of the order of unity.It has been found that element diffusion is important in modeling solar-like stars.Without diffusion,the best value of the initial helium abundance is below the primordial helium abundance from Big Bang nucleosynthesis.When diffusion is taken into account,the required initial helium abundance increases to be higher than the primordial abundance.Diffusion also generally improves the frequency fitting results by reducing the minimum ofχ^(2).Investigation of the second difference of the oscillation frequencies on KIC 8694723 and KIC 10454113 indicates that the current model of element diffusion may underestimate the strength of settling.展开更多
Taking into consideration the effects of rotation and interior magnetic field during the lifetime of the star, we reconstruct the model of α Cen B to satisfy the latest nonasteroseismic and asteroseismic observationa...Taking into consideration the effects of rotation and interior magnetic field during the lifetime of the star, we reconstruct the model of α Cen B to satisfy the latest nonasteroseismic and asteroseismic observational constraints. We find that the effects can induce a change of about 0.3 μHz in the large frequency spacings and can speed up the star's evolution. The model of a Cen B has thereby been improved.展开更多
Within the framework of a non-local time-dependent stellar convection theory, we study in detail the effect of turbulent anisotropy on stellar pulsation stability. The results show that anisotropy has no substantial i...Within the framework of a non-local time-dependent stellar convection theory, we study in detail the effect of turbulent anisotropy on stellar pulsation stability. The results show that anisotropy has no substantial influence on pulsation stability of g modes and low-order (radial order nr 〈 5) p modes. The effect of turbulent anisotropy increases as the radial order increases. When turbulent anisotropy is neglected, most high-order (nr 〉 5) p modes of all low-temperature stars become unstable. Fortunately, within a wide range of the anisotropic parameter c3, stellar pulsation stability is not sensitive to the specific value of ca. Therefore it is safe to say that calibration errors of the convective parameter ca do not cause any uncertainty in the calculation of stellar pulsation stability.展开更多
Using stellar evolutionary models, we investigate the effects of convective overshooting on naked helium stars. We find that a larger value of overshooting parameter δov results in a larger convective core, which pro...Using stellar evolutionary models, we investigate the effects of convective overshooting on naked helium stars. We find that a larger value of overshooting parameter δov results in a larger convective core, which prolongs the lifetimes of naked helium stars on the helium main sequence and leads to higher effective temperatures and luminosities. For naked helium stars with masses lower than about 0.8 Mo, they hardly become giant stars as a result of a weak burning shell. However, naked helium stars with masses between about 0.8 M⊙ and 1.1 M⊙ can evolve into giant branch phases, and finally become carbon oxygen white dwarfs.展开更多
Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars, focusing on its influence on the formation and ex...Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when the stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of opacity in the region just above the H-burning shell and a decrease of opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity LH when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters (Cx and αTCM) of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of Cx and αTCM are all increased and the length of the blue loop increases with the values of parameters Cx and αTCM; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7M⊙ star has a less prominent difference between the classical and diffusive mixing model; 3) both the observations referring to the location of the Cepheid instability strip and the number ratio NB/NR of blue to red evolved stars in the Galactic open clusters can confine the two parameters in a range of 0.5 ≤ αLTCM ≤ 0.9 and 10-5 ≤ Cx ≤ 10-4 for the model of 5M⊙. However, for the case of the 7M⊙ star, there seems to be no such definite range to even only account for the observed number ratio NB/NR. In any case, our results based on the diffusive mixing model are on the whole in accordance with not only other theoretical ones but also the observations.展开更多
Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between(binary) stars and the central supermassive black hole(s). In this paper, we report 19 low mass F/G...Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between(binary) stars and the central supermassive black hole(s). In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a MonteCarlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected,and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.展开更多
Observations by the Atacama Large Millimetre/sub-millimetre Array of the dust continuum and ^13 CO(3–2) millimetre emissions of the triple stellar system GG Tau A are analysed,giving evidence for a rotating gas dis...Observations by the Atacama Large Millimetre/sub-millimetre Array of the dust continuum and ^13 CO(3–2) millimetre emissions of the triple stellar system GG Tau A are analysed,giving evidence for a rotating gas disc and a concentric and coplanar dust ring. The present work complements an earlier analysis(Tang et al.) by exploring detailed properties of the gas disc. A 95% confidence level upper limit of 0.24"(34 au) is placed on the disc scale height at a distance of 1"(140 au) from the central stars. Evidence for Keplerian rotation of the gas disc is presented,with the rotation velocity reaching-3.1 km s^-1 at 1" from the central stars,and a 99% confidence level upper limit of 9% is placed on relative contribution from a possible in-fall velocity. Variations of the intensity across the disc area are studied in detail and confirm the presence of a hot spot in the south-eastern quadrant. However several other significant intensity variations,in particular a depression in the northern direction,are also revealed. Variations of the intensity are found to be positively correlated to variations of the line width. Possible contributions to the measured line width are reviewed,suggesting an increase of the disc temperature and opacity with decreasing distance from the stars.展开更多
We present an exact analytical solution of the gravitational field equations describing a static spherically symmetric anisotropic quark matter distribution. The radial pressure inside the star is assumed to obey a li...We present an exact analytical solution of the gravitational field equations describing a static spherically symmetric anisotropic quark matter distribution. The radial pressure inside the star is assumed to obey a linear equation of state, while the tangential pressure is a complicated function of the radial coordinate. In order to obtain the general solution of the field equations a particular density profile inside the star is also assumed. The anisotropic pressure distribution leads to an increase in the maximum radius and mass of the quark star, which in the present model is around three solar masses.展开更多
We report on the first investigation into kinematics and chromospheric activity of M dwarfs from the Guo Shou Jing Telescope (also called the Large Sky Area Multi-Object Fiber Spectroscopic Telescope - LAMOST) data ...We report on the first investigation into kinematics and chromospheric activity of M dwarfs from the Guo Shou Jing Telescope (also called the Large Sky Area Multi-Object Fiber Spectroscopic Telescope - LAMOST) data release one (DR1). The sample comprises 71 304 M dwarfs. Their fundamental parameters such as spectral types, radial velocities, important molecular band indices and magnetic activities are measured. Their distances are determined by a spectroscopic parallax relation. Space motion (U, V, W) and Galactocentric cylindrical coordinates (R, θ, Z) for the M dwarfs are also computed. We examine velocity dispersion as a function of height from the Galactic plane and find that all three components of velocity dispersion in- crease with height as measured with respect to the Galactic plane. The investigation into chromospheric activities along the height from the Galactic plane confirms that M dwarfs closer to the Galactic plane are more likely to be active. We take a pure kinematical approach to select thin disk stars and thick disk stars from our sample, then to investigate the differences in properties between these two populations. Our analysis is in excellent agreement with previous studies and leads to a better understanding of the structure of the Galactic disk.展开更多
Two Li-rich candidates,TYC 1338-1410-1 and TYC 2825-596-1,were observed by the new high-resolution echelle spectrograph,LAMOST/HRS.Based on their high-resolution and high-signal-tonoise ratio(SNR)spectra,we derived st...Two Li-rich candidates,TYC 1338-1410-1 and TYC 2825-596-1,were observed by the new high-resolution echelle spectrograph,LAMOST/HRS.Based on their high-resolution and high-signal-tonoise ratio(SNR)spectra,we derived stellar parameters and abundances of 14 important elements for the two candidates.The stellar parameters and lithium abundances indicate that they are Li-rich K-type giants,and having A(Li)NLTE and 2.91 dex,respectively.Our analysis suggests that TYC 1338-1410-1 is probably a red giant branch(RGB)star at the bump stage,while TYC 2825-596-1 is most likely to be a core helium-burning red clump(RC)star.The line profiles of both spectra indicate that the two Li-rich giants are slow rotators and do not show infrared(IR)excess.We conclude that engulfment is not the lithium enrichment mechanism for either star.The enriched lithium of TYC 1338-1410-1 could be created via the Cameron-Fowler mechanism,while the lithium excess in TYC 2825-596-1 could be associated with either non-canonical mixing processes or He-flash.展开更多
Contact binaries consist of two strongly interacting component stars where they are filling their critical Roche lobes and sharing a common envelope.Most of them are main-sequence stars,but some of them are post main-...Contact binaries consist of two strongly interacting component stars where they are filling their critical Roche lobes and sharing a common envelope.Most of them are main-sequence stars,but some of them are post main-sequence systems.They are good astrophysical laboratories for studying several problems such as the merging of binary stars,evolution of the common envelope,the origin of luminous red nova outbursts and the formation of rapidly rotating single stars with possible planetary systems.A large number of contact binary candidates were detected by several photometric surveys around the world and many of them were observed by the LAMOST spectroscopic survey.Based on follow-up observations,the evolutionary states and geometrical structures of some systems were understood well.In this review,we will introduce and catalog new stellar atmospheric parameters(i.e.,the effective temperature(Teff),the gravitational acceleration(log(g)),metallicity([Fe/H])and radial velocity(Vr))for 9149 EW-type contact binaries that were obtained based on low-and medium-resolution spectroscopic surveys of LAMOST.Then we will focus on several groups of contact binary stars,i.e.,marginal contact binary systems,deep and low-mass ratio contact binary stars,binary systems below the short-period limit of contact binaries and evolved contact binaries.Marginal contact binaries are at the beginning of the contact stage,while deep and low-mass ratio contact binary stars are at the final evolutionary stage of tidally locked binaries.Several statistical relations including the period-temperature relation are determined well by applying LAMOST data and their formation and evolutionary states are reviewed.The period-color relation of M-type binaries reveals that there are contact binaries below the short-period limit.Searching for and investigating contact binaries near and below this limit will help us to understand the formation of contact binary systems and a new prediction for the short-period limit is about 0.15 d.Some evolved contact binaries were detected by the LAMOST survey where both components are sub-giants or giants.They provide a good opportunity to investigate evolution of the common envelope and are the progenitors of luminous red novae like V1309 Sco.展开更多
We present photometric analysis of three bright red nova progenitor contact binary systems:ASAS J082151-0612.6,TYC 7281-269-1 and TYC 7275-1968-1.The primary components in all three systems are solar-type low mass sta...We present photometric analysis of three bright red nova progenitor contact binary systems:ASAS J082151-0612.6,TYC 7281-269-1 and TYC 7275-1968-1.The primary components in all three systems are solar-type low mass stars with radii somewhat larger than their zero age main sequence counterparts.The secondaries,as in most contact binary systems,have radii and luminosities well above their main sequence counterparts.All three have extremely low mass ratios ranging from 0.075 to 0.097 and two have high degrees of contact,in excess of 75%.All three have mass ratios and separations below the theoretical values for orbital stability.Chromospheric activity,a hallmark of magnetic activity and magnetic braking,considered important in mediating angular momentum loss,is also explored.All three systems demonstrate the O’Connell effect,and all systems require the introduction of star spots for a better light curve solution.In addition,we show that ASAS J082151-0612.6 and TYC 7281-269-1 have a UV color excess in the range indicating high chromospheric activity.Another measure of potential significant magnetic activity is X-ray luminosity;TYC 7275-1968-1 and probably also TYC 7281-269-1 have X-ray luminosity well above other contact binary systems.We conclude that it is likely that all three are unstable and hence are potential merger candidates.展开更多
文摘In this article, we suggest a Sitter model in favor of compact stars in low-mass X-ray binaries.Here, we have considered the presence of a cosmological constant(on a small scale) to investigate the stellar structure. We conclude that this approach is very suitable for the familiar physical model of compact stars in low-mass X-ray binaries. We calculate the probable radius, compactness(u) and surface redshift(Z_s) of six compact stars in low-mass X-ray binaries, namely Cyg X-2, V395 Carinae/2 S 0921–630, XTE J2123–058,X1822–371(V691 CrA), 4 U 1820–30 and GR Mus(XB 1254–690). We also offer a possible equation of state(EOS) for the stellar objects.
基金RFBR according to the research projects 19-02-00843 A and 20-02-00563 A and with partial support of the grant 075-15-2020-780 from the Government of the Russian Federation and the Ministry of Higher Education and Science。
文摘Based on the photographic and CCD observations of the relative motion of the A and B components of the binary system ADS 9346 obtained with the 26-inch refractor of Pulkovo Observatory during 1979–2019,we discover an invisible companion associated with star A.Comparison of the ephemerides with the positional and spectroscopic observations allowed us to calculate the preliminary orbit of the photocenter(P=15 yr).The minimal mass of the companion is approximately 0.13 M.The existence of the invisible low-mass companion is implied by the IR-excess based on IRAS data.To confirm this,additional observations of the radial velocity near the periastron need to be carried out.
基金LAMOST is a National Major Scientific Project built by the Chinese Academy of Sciencesprovided by the National Development and Reform Commission+3 种基金the financial supports obtained from the NSFC No.11963002the fostering project of Guizhou University with No.201911the science research grants from the China Manned Space Project with No.CMS-CSST-2021-B07the Natural Science Foundation of Guizhou Province with No.2022164。
文摘A catalog of M stars has been published from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope data release 7(LAMOST DR7).We cross-matched the LAMOST M catalog with Kepler,Kepler 2(K2)and Transiting Exoplanet Survey Satellite(TESS)surveys.We obtained the light curves from the Kepler and TESS surveys.We detected 20,047 flare events from 4053 M stars and calculated their durations,amplitudes,and energies.We analyzed the distribution of the flare durations and energies.The distributions of flare duration for Kepler,K2 and TESS peak are at 3–3.5 hr,4–6 hr and 1–1.5 hr,respectively.This may be the result of the different cadences for the three data sets.The highest regions of the flare energies of Kepler,K2 and TESS are 32–32.5,32–32.5 and31–31.5 erg in Log format,respectively.A linear relationship between flare duration and energy emerges from our analysis.The ratio of flare duration to total observational time is approximately 0.2%–0.3%for the Kepler,K2 and TESS surveys.The occurrence rate of a star with a flare event increases from the M0 to M4 subtypes.We also determined the spatial distribution of the flare rate of M stars in the Milky Way.It seems that the flare rate decreases as the vertical height increases.The power index of the flare energies is in the region of 1.53–2.32,which is similar to previous result for solar type star(2.0).Further,we examined the relationship between the flare amplitude and chromospheric intensity.The flare activity increases rapidly with the increase in the HαEW at the lower values(less approximately 2?)and it increases slowly at the higher values.
基金Funding for Yunnan Observatories is co-sponsored by the National Key R&D Program of China(Grant No.2021YFA1600400/2021YFA1600402)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB 41000000)+3 种基金the National Natural Science Foundation of China(Grant Nos.1177064 and 12133011)the Foundation of the Chinese Academy of Sciences(Light of West China Program and Youth Innovation Promotion Association)the Yunnan Ten Thousand Talents Plan Young and Elite Talents ProjectInternational Centre of Supernovae,Yunnan Key Laboratory(No.202302AN360001)。
文摘Element diffusion has small but significant effects on the structure of the stellar interior.It is interesting to investigate the effects of element diffusion using asteroseismology.We have constructed two grids of stellar models,one with diffusion and one without,for solar-like stars with masses between 0.9 and 1.4 solar masses,and varied initial helium abundance and metallicity.The oscillation frequencies of all stellar models have also been calculated.Piecewise Hermite cubic polynomials are adopted to interpolate stellar p-mode frequencies at an arbitrary age on a stellar evolutionary track.We have investigated 16 Kepler solar-like stars by comparing the model frequencies with observations.The suggested ranges of stellar parameters and some global variables are obtained.For all stars,the best model reproduces the observational frequencies with aχ^(2)of the order of unity.It has been found that element diffusion is important in modeling solar-like stars.Without diffusion,the best value of the initial helium abundance is below the primordial helium abundance from Big Bang nucleosynthesis.When diffusion is taken into account,the required initial helium abundance increases to be higher than the primordial abundance.Diffusion also generally improves the frequency fitting results by reducing the minimum ofχ^(2).Investigation of the second difference of the oscillation frequencies on KIC 8694723 and KIC 10454113 indicates that the current model of element diffusion may underestimate the strength of settling.
基金The Ministry of Science and Technology of the People's Republic of China through Grant 2007CB815406the NSFC through Grants 10173021,10433030,10773003 and 10778601
文摘Taking into consideration the effects of rotation and interior magnetic field during the lifetime of the star, we reconstruct the model of α Cen B to satisfy the latest nonasteroseismic and asteroseismic observational constraints. We find that the effects can induce a change of about 0.3 μHz in the large frequency spacings and can speed up the star's evolution. The model of a Cen B has thereby been improved.
基金supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11403039, 11473037 and 11373069)
文摘Within the framework of a non-local time-dependent stellar convection theory, we study in detail the effect of turbulent anisotropy on stellar pulsation stability. The results show that anisotropy has no substantial influence on pulsation stability of g modes and low-order (radial order nr 〈 5) p modes. The effect of turbulent anisotropy increases as the radial order increases. When turbulent anisotropy is neglected, most high-order (nr 〉 5) p modes of all low-temperature stars become unstable. Fortunately, within a wide range of the anisotropic parameter c3, stellar pulsation stability is not sensitive to the specific value of ca. Therefore it is safe to say that calibration errors of the convective parameter ca do not cause any uncertainty in the calculation of stellar pulsation stability.
基金supported by the Xinjiang Science Fund for Distinguished Young Scholars under Grant Nos. 2013721014 and 2014721015the National Natural Science Foundation of China under Grant Nos. 11473024, 11363005 and 11163005
文摘Using stellar evolutionary models, we investigate the effects of convective overshooting on naked helium stars. We find that a larger value of overshooting parameter δov results in a larger convective core, which prolongs the lifetimes of naked helium stars on the helium main sequence and leads to higher effective temperatures and luminosities. For naked helium stars with masses lower than about 0.8 Mo, they hardly become giant stars as a result of a weak burning shell. However, naked helium stars with masses between about 0.8 M⊙ and 1.1 M⊙ can evolve into giant branch phases, and finally become carbon oxygen white dwarfs.
基金supported by the National Natural Science Foundation of China(Grant Nos.10973035 and 10673030)
文摘Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when the stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of opacity in the region just above the H-burning shell and a decrease of opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity LH when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters (Cx and αTCM) of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of Cx and αTCM are all increased and the length of the blue loop increases with the values of parameters Cx and αTCM; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7M⊙ star has a less prominent difference between the classical and diffusive mixing model; 3) both the observations referring to the location of the Cepheid instability strip and the number ratio NB/NR of blue to red evolved stars in the Galactic open clusters can confine the two parameters in a range of 0.5 ≤ αLTCM ≤ 0.9 and 10-5 ≤ Cx ≤ 10-4 for the model of 5M⊙. However, for the case of the 7M⊙ star, there seems to be no such definite range to even only account for the observed number ratio NB/NR. In any case, our results based on the diffusive mixing model are on the whole in accordance with not only other theoretical ones but also the observations.
基金supported by the National Natural Science Foundation of China (Grant Nos. 11303036, 11390371/4 and 11233004)
文摘Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between(binary) stars and the central supermassive black hole(s). In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a MonteCarlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected,and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.
基金funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (No.103.99–2016.50)support from the World Laboratory,Rencontres du Viet Nam+2 种基金the Odon Vallet fellowships,Vietnam National Space CenterGraduate University of Science and Technologythe French CNRS programs PNP,PNPS and PCMI
文摘Observations by the Atacama Large Millimetre/sub-millimetre Array of the dust continuum and ^13 CO(3–2) millimetre emissions of the triple stellar system GG Tau A are analysed,giving evidence for a rotating gas disc and a concentric and coplanar dust ring. The present work complements an earlier analysis(Tang et al.) by exploring detailed properties of the gas disc. A 95% confidence level upper limit of 0.24"(34 au) is placed on the disc scale height at a distance of 1"(140 au) from the central stars. Evidence for Keplerian rotation of the gas disc is presented,with the rotation velocity reaching-3.1 km s^-1 at 1" from the central stars,and a 99% confidence level upper limit of 9% is placed on relative contribution from a possible in-fall velocity. Variations of the intensity across the disc area are studied in detail and confirm the presence of a hot spot in the south-eastern quadrant. However several other significant intensity variations,in particular a depression in the northern direction,are also revealed. Variations of the intensity are found to be positively correlated to variations of the line width. Possible contributions to the measured line width are reviewed,suggesting an increase of the disc temperature and opacity with decreasing distance from the stars.
文摘We present an exact analytical solution of the gravitational field equations describing a static spherically symmetric anisotropic quark matter distribution. The radial pressure inside the star is assumed to obey a linear equation of state, while the tangential pressure is a complicated function of the radial coordinate. In order to obtain the general solution of the field equations a particular density profile inside the star is also assumed. The anisotropic pressure distribution leads to an increase in the maximum radius and mass of the quark star, which in the present model is around three solar masses.
基金Supported by the National Natural Science Foundation of China
文摘We report on the first investigation into kinematics and chromospheric activity of M dwarfs from the Guo Shou Jing Telescope (also called the Large Sky Area Multi-Object Fiber Spectroscopic Telescope - LAMOST) data release one (DR1). The sample comprises 71 304 M dwarfs. Their fundamental parameters such as spectral types, radial velocities, important molecular band indices and magnetic activities are measured. Their distances are determined by a spectroscopic parallax relation. Space motion (U, V, W) and Galactocentric cylindrical coordinates (R, θ, Z) for the M dwarfs are also computed. We examine velocity dispersion as a function of height from the Galactic plane and find that all three components of velocity dispersion in- crease with height as measured with respect to the Galactic plane. The investigation into chromospheric activities along the height from the Galactic plane confirms that M dwarfs closer to the Galactic plane are more likely to be active. We take a pure kinematical approach to select thin disk stars and thick disk stars from our sample, then to investigate the differences in properties between these two populations. Our analysis is in excellent agreement with previous studies and leads to a better understanding of the structure of the Galactic disk.
基金the National Key R&D Program of China(No.2019YFA0405502)the Key Research Program of the Chinese Academy of Sciences under grant No.XDPB09-02+8 种基金the National Natural Science Foundation of China(Grant Nos.11973052,11833006,11473033 and 11603037)the International partnership program’s Key foreign cooperation project,Bureau of International Cooperation,Chinese Academy of Sciences(No.114A32KYSB20160049)supported by the Astronomical Big Data Joint Research Center,co-founded by the National Astronomical Observatories,Chinese Academy of Sciences and Alibaba Cloudpartially supported by the Open Project Program of the Key Laboratory of Optical Astronomy,National Astronomical Observatories,Chinese Academy of Sciencesthe financial support from China Scholarship Council(CSC,No.201604910642)for his study at New Mexico State University in the United States of Americathe supports from Youth Innovation Promotion Association,Chinese Academy of SciencesGuo Shou Jing Telescope(the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST)is a National Major Scientific Project built by the Chinese Academy of Sciences.Funding for the project has been provided by the National Development and Reform Commissionthe NASA/IPAC Infrared Science Archive,which is funded by the National Aeronautics and Space Administrationthe Wide-field Infrared Survey Explorer,which is a joint project of the University of California,Los Angeles,and the Jet Propulsion Laboratory/California Institute of Technology,funded by the National Aeronautics and Space Administration。
文摘Two Li-rich candidates,TYC 1338-1410-1 and TYC 2825-596-1,were observed by the new high-resolution echelle spectrograph,LAMOST/HRS.Based on their high-resolution and high-signal-tonoise ratio(SNR)spectra,we derived stellar parameters and abundances of 14 important elements for the two candidates.The stellar parameters and lithium abundances indicate that they are Li-rich K-type giants,and having A(Li)NLTE and 2.91 dex,respectively.Our analysis suggests that TYC 1338-1410-1 is probably a red giant branch(RGB)star at the bump stage,while TYC 2825-596-1 is most likely to be a core helium-burning red clump(RC)star.The line profiles of both spectra indicate that the two Li-rich giants are slow rotators and do not show infrared(IR)excess.We conclude that engulfment is not the lithium enrichment mechanism for either star.The enriched lithium of TYC 1338-1410-1 could be created via the Cameron-Fowler mechanism,while the lithium excess in TYC 2825-596-1 could be associated with either non-canonical mixing processes or He-flash.
基金supported by the National Natural Science Foundation of China(Nos.11933008,11922306,11773066 and 11903076)the Open Project Program of the Key Laboratory of Optical Astronomy,National Astronomical Observatories,Chinese Academy of Sciences。
文摘Contact binaries consist of two strongly interacting component stars where they are filling their critical Roche lobes and sharing a common envelope.Most of them are main-sequence stars,but some of them are post main-sequence systems.They are good astrophysical laboratories for studying several problems such as the merging of binary stars,evolution of the common envelope,the origin of luminous red nova outbursts and the formation of rapidly rotating single stars with possible planetary systems.A large number of contact binary candidates were detected by several photometric surveys around the world and many of them were observed by the LAMOST spectroscopic survey.Based on follow-up observations,the evolutionary states and geometrical structures of some systems were understood well.In this review,we will introduce and catalog new stellar atmospheric parameters(i.e.,the effective temperature(Teff),the gravitational acceleration(log(g)),metallicity([Fe/H])and radial velocity(Vr))for 9149 EW-type contact binaries that were obtained based on low-and medium-resolution spectroscopic surveys of LAMOST.Then we will focus on several groups of contact binary stars,i.e.,marginal contact binary systems,deep and low-mass ratio contact binary stars,binary systems below the short-period limit of contact binaries and evolved contact binaries.Marginal contact binaries are at the beginning of the contact stage,while deep and low-mass ratio contact binary stars are at the final evolutionary stage of tidally locked binaries.Several statistical relations including the period-temperature relation are determined well by applying LAMOST data and their formation and evolutionary states are reviewed.The period-color relation of M-type binaries reveals that there are contact binaries below the short-period limit.Searching for and investigating contact binaries near and below this limit will help us to understand the formation of contact binary systems and a new prediction for the short-period limit is about 0.15 d.Some evolved contact binaries were detected by the LAMOST survey where both components are sub-giants or giants.They provide a good opportunity to investigate evolution of the common envelope and are the progenitors of luminous red novae like V1309 Sco.
基金financial support of the Ministry of Education,Science and Technological Development of the Republic of Serbia through contract No.451-03-68/2022-14/200104financial support of the Ministry of Education,Science and Technological Development of the Republic of Serbia through contract No.451-03-9/2021-14/200002。
文摘We present photometric analysis of three bright red nova progenitor contact binary systems:ASAS J082151-0612.6,TYC 7281-269-1 and TYC 7275-1968-1.The primary components in all three systems are solar-type low mass stars with radii somewhat larger than their zero age main sequence counterparts.The secondaries,as in most contact binary systems,have radii and luminosities well above their main sequence counterparts.All three have extremely low mass ratios ranging from 0.075 to 0.097 and two have high degrees of contact,in excess of 75%.All three have mass ratios and separations below the theoretical values for orbital stability.Chromospheric activity,a hallmark of magnetic activity and magnetic braking,considered important in mediating angular momentum loss,is also explored.All three systems demonstrate the O’Connell effect,and all systems require the introduction of star spots for a better light curve solution.In addition,we show that ASAS J082151-0612.6 and TYC 7281-269-1 have a UV color excess in the range indicating high chromospheric activity.Another measure of potential significant magnetic activity is X-ray luminosity;TYC 7275-1968-1 and probably also TYC 7281-269-1 have X-ray luminosity well above other contact binary systems.We conclude that it is likely that all three are unstable and hence are potential merger candidates.