Search

IW And-type dwarf novae are anomalous Z Cam stars featured with outbursts happening during standstill states, which are not expected in the standard disk instability model. The physical mechanisms for these variations remain unclear. In this study, we report the discovery of a new candidate IW And-type dwarf nova J0652+2436, identified with its frequent outbursts from the slowly rising standstill states. Luckily, the TESS observations during a long standstill state and the earlier K2 observations give a chance to find the orbital and negative superhump period in the light curve of J0652+2436, allowing the measurement of its mass ratio of 0.366. This mass ratio is marginally possible for the tidal instability to set in according to previous SPH simulations. Thus, we propose that the outbursts in J0652+2436 are likely to be caused by the growing accretion disk during standstills, in favor of the previous hypothesis of the mechanisms lying in all IW And stars. We conclude that J0652+2436 might be the first IW And star with both a precessing tilted disk and tidal instability, which will be an important laboratory for studying the accretion disk dynamics and help understand IW And phenomenon., Comment: 12 pages, 12 figures, accepted to MNRAS

Carbon stars are excellent kinematic tracers of galaxies and play important roles in understanding the evolution of the Galaxy. Therefore, it is worthwhile to search for them in a large amount of spectra. In this work, we build a new carbon star catalog based on the LAMOST DR7 spectra. The catalog contains 4542 spectra of 3546 carbon stars, identified through line index and near-infrared color-color diagrams. Through visual inspection of the spectra, we further subclassify them into 925 C--H, 384 C--R, 608 C--N, and 1292 Ba stars. However, 437 stars could not be sub-classified due to their low signal-to-noise. Moreover, by comparing with LAMOST DR7 pipeline we find 567 more carbon stars and visually sub-classify them. We find that on the $J-H$ vs. $H-K_{\rm s}$ two-color diagram, C--N stars can be reliably distinguished from the other three sub-types. Additionally, by utilizing the Gaia distance, we study the distribution of carbon stars in the H-R diagram and identify 258 dwarf carbon stars by the criterion $M_{\rm G}>$5.0\,mag. Finally, we present the spatial distribution in Galactic coordinates of the 3546 carbon stars. The majority of C-N, C-R, and Ba stars are distributed at low Galactic latitudes, while most C--H and dC stars distribute at high Galactic latitudes.

Modern large-scale photometric surveys have provided us with multi-band photometries of billions of stars. Determining the stellar atmospheric parameters, such as the effective temperature (\teff) and metallicities (\feh), absolute magnitudes ($M_{G}$), distances ($d$) and reddening values (\ebr) is fundamental to study the stellar populations, structure, kinematics and chemistry of the Galaxy. This work constructed an empirical stellar library which maps the stellar parameters to multi-band photometries from a dataset with Gaia parallaxes, LAMOST atmospheric parameters, and optical to near-infrared photometry from several photometric surveys. Based on the stellar library, we developed a new algorithm, SPar (\textbf{S}tellar \textbf{P}arameters from multib\textbf{a}nd photomet\textbf{r}y), which fits the multi-band stellar photometries to derive the stellar parameters (\teff, \feh, $M_G$, $d$ and \ebr) of the individual stars. The algorithm is applied to the multi-band photometric measurements of a sample of stars selected from the SMSS survey, which have stellar parameters derived from the spectroscopic surveys. The stellar parameters derived from multi-band photometries by our algorithm are in good agreement with those from the spectroscopic surveys. The typical differences between our results and the literature values are 170\,K for \teff, 0.23\,dex for \feh, 0.13\,mag for $M_G$ and 0.05\,mag for \ebr. The algorithm proved to be robust and effective and will be applied to the data of future large-scale photometric surveys such as the Mephisto and CSST surveys., Comment: 16 pages, 10 figures, Accepted by The Astronomical Journal on 7/8/2023

In this work, we present a catalog of cataclysmic variables (CVs) identified from the Sixth Data Release (DR6) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). To single out the CV spectra, we introduce a novel machine-learning algorithm called UMAP to screen out a total of 169,509 H$\alpha$-emission spectra, and obtain a classification accuracy of the algorithm of over 99.6$\%$ from the cross-validation set. We then apply the template matching program PyHammer v2.0 to the LAMOST spectra to obtain the optimal spectral type with metallicity, which helps us identify the chromospherically active stars and potential binary stars from the 169,509 spectra. After visually inspecting all the spectra, we identify 323 CV candidates from the LAMOST database, among them 52 objects are new. We further discuss the new CV candidates in subtypes based on their spectral features, including five DN subtype during outbursts, five NL subtype and four magnetic CVs (three AM Her type and one IP type). We also find two CVs that have been previously identified by photometry, and confirm their previous classification by the LAMOST spectra.

Since September 2018, LAMOST starts a new 5-year medium-resolution spectroscopic survey (MRS) using bright/gray nights. We present the scientific goals of LAMOST-MRS and propose a near optimistic strategy of the survey. A complete footprint is also provided. Not only the regular medium-resolution survey, but also a time-domain spectroscopic survey is being conducted since 2018 and will be end in 2023. According to the detailed survey plan, we expect that LAMOST-MRS can observe about 2 million stellar spectra with ~7500 and limiting magnitude of around G=15 mag. Moreover, it will also provide about 200 thousand stars with averagely 60-epoch observations and limiting magnitude of G~14 mag. These high quality spectra will give around 20 elemental abundances, rotational velocities, emission line profiles as well as precise radial velocity with uncertainty less than 1 km/s. With these data, we expect that LAMOST can effectively leverage sciences on stellar physics, e.g. exotic binary stars, detailed observation of many types of variable stars etc., planet host stars, emission nebulae, open clusters, young pre-main-sequence stars etc., Comment: 25 pages, 10 figues. Submitted to Reseach in Astronomy and Astrophysics

We present 22\,901 OB spectra of 16\,032 stars identified from LAMOST DR5 dataset. A larger sample of OB candidates are firstly selected from the distributions in the spectral line indices space. Then all 22\,901 OB spectra are identified by manual inspection. Based on a sub-sample validation, we find that the completeness of the OB spectra reaches about $89\pm22$\% for the stars with spectral type earlier than B7, while around $57\pm16$\% B8--B9 stars are identified. The smaller completeness for late B stars is lead to the difficulty to discriminate them from A0--A1 type stars. The sub-classes of the OB samples are determined using the software package MKCLASS. With a careful validation using 646 sub-samples, we find that MKCLASS can give fairly reliable sub-types and luminosity class for most of the OB stars. The uncertainty of the spectral sub-type is around 1 sub-type and the uncertainty of the luminosity class is around 1 level. However, about 40\% of the OB stars are failed to be assigned to any class by MKCLASS and a few spectra are significantly misclassified by MKCLASS. This is likely because that the template spectra of MKCLASS are selected from nearby stars in the solar neighborhood, while the OB stars in this work are mostly located in the outer disk and may have lower metallicity. The rotation of the OB stars may also be responsible for the mis-classifications. Moreover, we find that the spectral and luminosity classes of the OB stars located in the Galactic latitude larger than 20$^\circ$ are substantially different with those located in latitude smaller than 20$^\circ$, which may either due to the observational selection effect or hint a different origin of the high Galactic latitude OB stars., Comment: 17 pages, 18 figures, Accepted for publication in ApJS

With about 12 000 OB type stars selected from the LAMOST and Gaia survey, we study their 3 dimensional velocity distribution over the range of galactocentric radius from 6 to 15 kpc in the Galactic disk plane. A clear ripple pattern in the radial velocity ($V_R$) map is shown. The median $V_R$ reaches $-8$ km s$^{-1}$ at $R\sim9$ kpc, then increases to $\sim0$ km s$^{-1}$ at $R\sim12$ kpc, and later declines to below $-10$ km s$^{-1}$ beyond $R\sim13$ kpc. The median azimuthal velocity ($V_\phi$) map shows a similar pattern but has roughly $1/4$ phase difference with the radial velocity. Although the ripple of negative $V_R$ at $\sim9$ kpc extends to about 40$^\circ$ in the azimuth angle, it does not align with either the Local or the Perseus spiral arm. Moreover, the farther ripple beyond 13 kpc does not match the Outer spiral arm either. This indicates that the non-axisymmetric kinematic features are not induced by perturbations of known spiral structures. The central rotating bar can not lead to such patterns in the outer disk either. External perturbation of a dwarf galaxy or a dark matter sub-halo can induce such patterns but requires more evidence from both observations and simulations. The $V_\phi$ map in the $Z$--$V_Z$ plane of the OB stars is also investigated. Despite asymmetry to some degree, no spiral pattern is found. This is reasonable since most of the OB stars have ages much younger than 100 Myrs, which is smaller than one orbital period around the Galactic center., Comment: 8 pages, 5 figures; accepted for publication by ApJL on Jan 26, 2019

The abundance scatter of heavier r-process elements (Z > 56) relative to Fe ([r/Fe]) in metal-poor stars preserves excellent information of the star formation history and provides important insights into the various situations of the Galactic chemical enrichment. In this respect, the upper and lower boundaries of [r/Fe] could present useful clues for investigating the extreme situations of the star formation history and the early Galactic chemical evolution. In this paper, we investigate the formation of the upper and lower boundaries of [r/Fe] for the gas clouds. We find that, for a cloud from which metal-poor stars formed, the formation of the upper limits of [r/Fe] is mainly due to the pollution from a single main r-process event. For a cloud from which metal-poor stars formed, the formation of the lower limits of [r/Fe] is mainly due to the pollution from a single SN II event that ejects primary Fe., Comment: 12 pages, 3 figures, accepted by PASP

We determined the \fodd\ values, $0.46\pm0.08$, $0.51\pm0.09$, $0.50\pm0.13$, $0.48\pm0.12$, which correspond to the r-contribution 100\% for four r-II stars, \cs, \hen, \hes\, and \het, respectively. Our results suggest that almost all of the heavy elements (in the range from Ba to Pb) in r-II stars have a common origin, that is, from a single r-process (the main r-process). We found that the \fodd\ has a intrinsic nature, and should keep constant value of about 0.46 in the main r-process yields, which is responsible for the heavy element enhancement of r-II stars and of our Galaxy chemical enhancement. In addition, except the abundance ratio [Ba/Eu] the \fodd\ is also an important indicator, which can be used to study the relative contributions of the r- and s-process during the chemical evolution history of the Milky Way and the enhancement mechanism in stars with peculiar abundance of heavy elements., Comment: 13 pages, 2 figures, Accepted to ApJ

Historically, the weak s-process contribution to metal-poor stars is thought to be extremely small, due to the effect of the secondary-like nature of the neutron source 22Ne(a;n)25Mg in massive stars, which means that metal-poor weak s-process stars could not be found. ET0097 is the first observed carbon-enhanced metal-poor (CEMP) star in the Sculptor dwarf spheroidal galaxy. Because C is enriched and the elements heavier than Ba are not overabundant, ET0097 can be classified as a CEMP-no star. However, this star shows overabundances of lighter n-capture elements (i.e., Sr, Y and Zr). In this work, having adopted the abundance decomposition approach, we investigate the astrophysical origins of the elements in ET0097. We find that the light elements and iron-peak elements (from O to Zn) of the star mainly originate from the primary process of massive stars and the heavier n-capture elements (heavier than Ba) mainly come from the main r-process. However, the lighter n-capture elements such as Sr, Y and Zr should mainly come from the primary weak s-process. The contributed fractions of the primary weak s-process to the Sr, Y and Zr abundances of ET0097 are about 82%, 84% and 58% respectively, suggesting that the CEMP star ET0097 should have the footprints of the weak s-process. The derived result should be a significant evidence that the weak s-process elements can be produced in metal-poor massive stars., Comment: 8 pages, 3 figures, accepted for publication in Astrophysics and Space Science

We report the first measurement of the odd-isotope fractions for barium, \fodd\, in two extremely metal-poor stars: a CEMP-r/s star \he\ (\feh\,$=-2.42\pm0.11$) and an r-II star \cs\ (\feh\,$=-2.90\pm0.13$). The measured \fodd\ values are $0.23\pm0.12$ corresponding to $34.3\pm34.3$\% of the r-process contributions for \he\ and $0.43\pm0.09$ corresponding to $91.4\pm25.7$\% of the r-process contribution to Ba production for \cs. The high r-process signature of barium in \cs\ ($91.4\pm25.7\%$) suggests that the majority of the heavy elements in this star were synthesised via an r-process path, while the lower r-process value ($34.3\pm34.3\%$) found in \he\ indicates that the heavy elements in this star formed through a mix of s-process and r-process synthesis. These conclusions are consistent with studies based on AGB model calculations to fit their abundance distributions., Comment: 8 pages, 5 figures, accepted for publication in A&A

In this work, we present the new catalog of carbon stars from the LAMOST DR2 catalog. In total, 894 carbon stars are identified from multiple line indices measured from the stellar spectra. Combining the CN bands in the red end with \ctwo\ and other lines, we are able to identify the carbon stars. Moreover, we also classify the carbon stars into spectral sub-types of \ch, \CR, and \cn. These sub-types approximately show distinct features in the multi-dimensional line indices, implying that in the future we can use them to identify carbon stars from larger spectroscopic datasets. Meanwhile, from the line indices space, while the \cn\ stars are clearly separated from the others, we find no clear separation between \CR\ and \ch\ sub-types. The \CR\ and \ch\ stars seem to smoothly transition from one to another. This may hint that the \CR\ and \ch\ stars may not be different in their origins but look different in their spectra because of different metallicity. Due to the relatively low spectral resolution and lower signal-to-noise ratio, the ratio of $^{12}$C/$^{13}$C is not measured and thus the \cj\ stars are not identified., Comment: 34 pages, 19 figures, accepted for publication in ApJS

We study the abundances of {\alpha} elements, Fe-peak elements, and neutron-capture elements in Pal 1. We found that the abundances of the SNe Ia and main s-process components of Pal 1 are larger than those of the disk stars and the abundances of the primary component of Pal 1 are smaller than those of the disk stars with similar metallicity. The Fe abundances of Pal 1 and the disk stars mainly originate from the SNe Ia and the primary component, respectively. Although the {\alpha} abundances dominantly produced by the primary process for the disk stars and Pal 1, the contributions of the primary component to Pal 1 are smaller than the corresponding contributions to the disk stars. The Fe-peak elements V and Co mainly originate from the primary and secondary components for the disk stars and Pal 1, but the contributions of the massive stars to Pal 1 are lower than those of the massive stars to the disk stars. The Yabundances mainly originate from the weak r-component for the disk stars. However, the contributions of the main s-components and main r-components to Y are close to those of the weak r-component for Pal 1. The Ba abundances of Pal 1 and the disk stars mainly originate from the main s-component and the main r-component, respectively. Our calculated results imply that the unusual abundances of Pal could be explained by the top-light IMF for Pal 1 progenitor-system.

Many works have attempted to investigate the astrophysical origin of the neutron-capture elements in the metalpoor star HD 140283. However, no definite conclusions have been drawn. In this work, using the abundancedecomposed approach, we find that the metal-poor star HD 140283 is a weak r-process star. Although this star is a weak r-process star, its Ba abundance mainly originates from the main r-process. This is the reason that the ratio [Ba/Eu]= -0.58+- 0.15 for HD 140283 is close to the ratio of the main r-process. Based on the comparison of the abundances in the six-weak r-process stars, we find that their element abundances possess a robust nature. On the other hand, we find that the robust nature of the abundance of the extreme main r-process stars ([r/Fe]>= 1.5) can be extended to the lighter neutron-capture elements. Furthermore, the abundance characteristics of the weak r-process and main r-process are investigated. The abundance robustness of the two category r-process stars could be used as the constraint of the r-process theory and could be used to investigate the astrophysical origins of the elements in the metal-poor stars and population I stars., Comment: 9 pages, 11 figures

We present a sample of about 120,000 red clump candidates selected from the LAMOST DR2 catalog based on the empirical distribution model in the effective temperature vs. surface gravity plane. Although, in general, red clump stars are considered as the standard candle, they do not exactly stay in a narrow range of absolute magnitude, but may extend to more than 1 magnitude depending on their initial mass. Consequently, conventional oversimplified distance estimations with assumption of fixed luminosity may lead to systematic bias related to the initial mass or the age, which may potentially affect the study of the evolution of the Galaxy with red clump stars. We therefore employ an isochrone-based method to estimate the absolute magnitude of red clump stars from their observed surface gravities, effective temperatures, and metallicities. We verify that the estimation well removes the systematics and provide an initial mass/age independent distance estimates with accuracy less than 10%., Comment: 19 pages, 11 figures; accepted for publication for RAA

Historically,Ge has been considered to be a neutron-capture element. In this case, the r-process abundance of Ge is derived by subtracting the s-process abundance from the total abundance in the Solar system. However, the Ge abundance of the metal-poor star HD 108317 is lower than that of the scaled residual r-process abundance in the Solar system, about 1.2 dex. In this paper, based on a comparison of the Ge abundances of metal-poor stars and stellar yields, we find that the Ge abundances are not the result of the primary-like yields in massive stars and come mainly from the r-process. Based on the observed abundances of metal-poor stars, we derived the Ge abundances of the weak r-process and main r-process. The contributed percentage of the neutron-capture process to Ge in the Solar system is about 59 per cent, which means that the contributed percentage of the Ge residual abundance in the Solar system is about 41 per cent. We find that the Ge residual abundance is produced as secondary-like yields in massive stars. This implies that the element Ge in the Solar system is not produced solely by the neutron-capture process., Comment: 12 pages, 8 figures

The chemical abundances of metal-poor stars provide important clues to explore stellar formation history and set significant constraints on models of the r-process. In this work, we find that the abundance patterns of the light and iron group elements of the main r-process stars are very close to those of the weak r-process stars. Based on a detailed abundance comparison, we find that the weak r-process occurs in supernovae with a progenitor mass range of $\sim11-26M_{\odot}$. Using the SN yields given by Heger & Woosley and the abundances of the weak r-process stars, the weak r-process yields are derived. The SNe with a progenitor mass range of $15M_{\odot}

Many works have found unusual characteristics of elemental abundances in nearby dwarf galaxies. This implies that there is a key factor of galactic evolution that is different from that of the Milky Way (MW). The chemical abundances of the stars in the Fornax dwarf spheroidal galaxy (Fornax dSph) provide excellent information for setting constraints on the models of the galactic chemical evolution. In this work, adopting the five-component approach, we fit the abundances of the Fornax dSph stars, including $\alpha$ elements, iron group elements and neutron-capture elements. For most sample stars, the relative contributions from the various processes to the elemental abundances are not usually in the MW proportions. We find that the contributions from massive stars to the primary $\alpha$ elements and iron group elements increase monotonously with increasing [Fe/H]. This means that the effect of the galactic wind is not strong enough to halt star formation and the contributions from massive stars to $\alpha$ elements did not halted for [Fe/H]$\lesssim$-0.5. The average contributed ratios of various processes between the dSph stars and the MW stars monotonously decrease with increasing progenitor mass. This is important evidence of a bottom-heavy initial mass function (IMF) for the Fonax dSph, compared to the MW. Considering a bottom-heavy IMF for the dSph, the observed relations of [$\alpha$/Fe] versus [Fe/H], [iron group/Fe] versus [Fe/H] and [neutron-capture/Fe] versus [Fe/H] for the dSph stars can be explained., Comment: 38 pages, 11 figures, 2 tables. Accepted for publication in ApJ

In order to get a broader view of the s-process nucleosynthesis we study the abundance distribution of heavy elements of 35 barium stars and 24 CEMP-stars, including nine CEMP-s stars and 15 CEMP-r/s stars. The similar distribution of [Pb/hs] between CEMP-s and CEMP-r/s stars indicate that the s-process material of both CEMP-s and CEMP-r/s stars should have a uniform origin, i.e. mass transfer from their predominant AGB companions. For the CEMP-r/s stars, we found that the r-process should provide similar proportional contributes to the second s-peak and the third s-peak elements, and also be responsible for the higher overabundance of heavy elements than those in CEMP-s stars. Which hints that the r-process origin of CEMP-r/s stars should be closely linked to the main r-process. The fact that some small $r$ values exist for both barium and CEMP-s stars, implies that the single exposure event of the s-process nucleosynthesis should be general in a wide metallicity range of our Galaxy. Based on the relation between $C_{r}$ and $C_{s}$, we suggest that the origin of r-elements for CEMP-r/s stars have more sources. A common scenario is that the formation of the binary system was triggered by only one or a few supernova. In addition, accretion-induced collapse(AIC) or SN 1.5 should be the supplementary scenario, especially for these whose pre-AGB companion with higher mass and smaller orbit radius, which support the higher values of both $C_{r}$ and $C_{s}$., Comment: 21 pages, 12 figures, accepted publication in APSS

This work describes a study of elemental abundances for 30 metal-poor stars whose chemical abundances provide excellent information for setting constraints on models of neutron-capture processes. Based on the abundances of main r- and weak r-process stars, the abundance patterns of main r-process and weak r-process are obtained. The two r-process component coefficients are defined to determine the relative contributions from individual neutron-capture process to abundances of metal-poor stars. Based on the component coefficients, we find that metal-poor stars BD+4 2621 and HD 4306 are also weak r-process stars, which means that the abundance pattern produced by weak r-process is stable. All metal-poor star abundances contain the contributions of both main r-process and weak r-process. The elements produced by weak r-process have increased along with Fe over the polluted history. Most of the metal-poor star abundances do not follow the pattern observed in solar system, but there is a small fraction that do. For the low-[Sr/Fe] star BD-18 5550 ([Sr/Fe]$\lesssim-1$), neutron-capture element abundances can be explained by the mixture of two r-process components. Since lighter elements in this star cannot be fitted by the two components, the abundance pattern of P-component is estimated from those abundances., Comment: 40 pages, 10 figures, accepted for publication in PASP

The chemical abundances of the metal-poor stars in the stellar stream provide important information for setting constraints on models of neutron-capture processes. The study of these stars could give us a better understanding of r-process nucleosynthesis and chemical composition of the early Galaxy. Using the updated main r-process and weak r-process patterns, we fit abundances in the stellar stream stars. The weak r-process component coefficients are almost constant for the sample stars, including r-rich stars, which means that both weak r-process and Fe are produced as primary elements from SNeII and their yields have nearly a constant mass fraction. The difference between the stream stars and r-rich stars is obvious. For the stream stars, that the increase trend in the main r-process component coefficients as metallicity increases means the gradual increase in the production of main r-process elements relative to iron. This behavior implies that the masses of progenitors for the main r-process are smaller than those of the weak r-process. Furthermore, we find metal-poor stream star HD 237846 is a weak r-process star., Comment: 12 pages, 7 figures, accepted for publication in PASA

The chemical abundances of metal-poor stars are excellent sources of information for setting new constraints on models of Galactic chemical evolution at low metallicities. In this paper we present an attempt to fit the elemental abundances observed in the bright, metal-poor giant HD 175305, and derive isotopic fractions using a parametric model. The observed abundances can be wellmatched by the combined contributions froms- and r-processmaterial. The component coefficients of the r- and s-processes are C1 = 3.220 and C3 = 1.134, respectively. The Smisotopic fraction in this star where the observed neutron-capture elements are produced is predicted to be f 152+154 =0.582,which suggests that, even though the r-process is predominantly responsible for the synthesis of the neutron-capture elements in the early Galaxy, the onset of the s-process had already occurred at this metallicity of [Fe/H] = -1.6.

The chemical abundances of metal-poor stars are an excellent test bed by which to set new constraints on models of neutron-capture processes at low metallicity. Some r-process-rich (hereafter r-rich) metal-poor stars, such as HD221170, show an overabundance of the heavier neutron-capture elements and excesses of lighter neutron-capture elements. The study of these r-rich stars could give us a better understanding of weak and main r-process nucleosynthesis at low metallicity. Based on conclusions from the observation of metal-poor stars and neutron-capture element nucleosynthesis theory, we set up a model to determine the relative contributions from weak and main r-processes to the heavy-element abundances in metal-poor stars. Using this model, we find that the abundance patterns of light elements for most sample stars are close to the pattern of weak r-process stars, and those of heavier neutron-capture elements very similar to the pattern of main r-process stars, while the lighter neutron-capture elements can be fitted by the mixing of weak and main r-process material. The production of weak r-process elements appears to be associated with the light elements, while the production of main r-process elements is almost decoupled from that of the light elements. We compare our results with the observed data at low metallicities, showing that the predicted trends are in good agreement with the observed trends, at least for the metallicity range [Fe/H] < -2.1. For most sample stars, the abundance patterns of both neutron-capture elements and light elements could be best explained by a star formed in a molecular cloud that has been polluted by both weak and main r-process material., Comment: 9 pages, 5 figures

The very metal-poor star HE 0338-3945 shows a double-enhanced pattern of the neutron-capture elements. The study to this sample could make people gain a better understanding of s- and r-process nucleosynthesis at low metallicity. Using a parametric model,we find that the abundance pattern of the neutron-capture elements could be best explained by a binary system formed in a molecular cloud, which had been polluted by r-process material. The observed abundance pattern of C and N can be explained by an AGB model(Karakas & Lattanzio 2007), . Combing with the parameters obtained from Cui & Zhang (2006), we suggest that the initial mass of the AGB companion is most likely to be about 2.5Msun, which excludes the possibility of forming a type-1.5 supernova. By comparing with the observational abundance pattern of CS 22892-052, we find that the dominating production of O should accompany with the production of the heavy r-process elements of r+s stars. Similar to r-II stars, the heavy r-process elements are not produced in conjunction with all the light elements from Na to Fe group. The abundance pattern of the light and r-process elements for HE 0338-3945 is very close to the pattern of the r-II star CS 22892-052. So, we suggest that this star HE 0338-3945 should be a special r-II star., Comment: 18 pages, 6 figures, published by ApJ

The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [Ba/Mg]-[Mg/H] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass $M_{ms}\leq18M_{\odot}$ are infertile sources and the SNe II with 20$M_{\odot}\leq M_{ms}\leq 40M_{\odot}$are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20$M_{\odot}\leq M_{ms}\leq40M_{\odot}$ with compared to the all massive stars is about $\sim$18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced, Comment: 7 pages, 6 figures, Accepted for publication in Astrophysics and Space Science

Several stars at the low-metallicity extreme of the Galactic halo show large spreads of [Pb/hs]. Theoretically, a s-process pattern should be obtained from an AGB star with fixed metallicity and initial mass. For the third dredge-up and the s-process model, several important properties depend primarily on the core mass of AGB stars. Zijlstra (2004) reported that the initial-final-mass relation steepens at low metallicity, due to low mass-loss efficiency. This perhaps affects the model parameters of the AGB stars, e.g. the overlap factor and the neutron irradiation time, in particular at low metallicity. The calculated results show indeed that the overlap factor and the neutron irradiation time are significantly small at low metallicities, especially for 3.0Msun AGB stars. The scatter of [Pb/hs] found in low metallicities can therefore be explained naturally when varying the initial mass of the low-mass AGB stars., Comment: 5 pages, 7 EPS figures. Accepted for publication in MNRAS