分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We assemble a large sample of 13,863 high-velocity stars (HiVels) with total velocity ${V}_{\rm{GSR}} \ge {\rm300} \rm{km\,s^{-1}}$, selected from the RAVE DR5, SDSS DR12, LAMOST DR8, APOGEE DR16, GALAH DR2, and $Gaia$ EDR3. In this HiVel sample, 43 sources are hypervelocity stars (HVSs) that have ${V}_{\rm{GSR}}$ exceeding their local escape velocities, $V_{\rm esc}$, 32 of which are discovered for the first time. Interestingly, all the HVSs are metal-poor and late-type, significantly different from the previous HVSs in the literature, which are largely massive, metal-rich early-type stars, identified originally by extreme radial velocity alone. This finding suggests that our newly found HVSs are ejected by different mechanisms from the previous population. To investigate their origins, for the 571 extreme HiVel stars with ${V}_{\rm{GSR}}\ge0.8V_{\rm{esc}}$ in our sample, we reconstruct their backward-integrated trajectories in the Galactic potential. According to the orbital analysis, no HVSs are found to be definitely ejected from the Galactic center (GC), while 8 late-type metal-poor HiVels are found to have a closest distance to the GC within 1 kpc. Intriguingly, 15 HiVels (including 2 HVSs) are found from their backward-integrated trajectories to have experienced a close encounter with the Sagittarius dwarf spheroidal galaxy (Sgr dSph), suggesting that they originated from this dSph. This hypothesis is supported by an analysis of the [$\alpha$/Fe]--[Fe/H] diagram. From a preliminary analysis of all the HiVels in our sample, we propose a general picture: Star ejection from Galactic subsystems such as dwarf galaxies and globular clusters, either via tidal stripping or even the Hills mechanism, can be an important channel to produce HiVels/HVSs, particularly the metal-poor late-type halo population.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Ba-enhanced stars are interesting probes of stellar astrophysics and Galactic formation history. In this work, we investigate the chemistry and kinematics for a large sample of Ba-enhanced ([Ba/Fe]$>$1.0) dwarf and subgiant stars with $5000 6000$\,K. They are likely consequences of stellar internal elemental transport processes, but they show very different elemental patterns to the hotter Am/Fm stars. Our results reveal a substantially lack of high-[$\alpha$/Fe] Ba-enhanced stars in the [Fe/H]--[$\alpha$/Fe] plane, which we dub as a {\em high-$\alpha$ desert}. We suggest it is due to a lower efficiency for producing Ba-enhanced stars by low-mass AGB progenitors in binary systems. Our results call for detailed modellings for these Ba-enhanced stellar peculiars, in the context of both stellar internal elemental transport and external mass accretion.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a sample of 135,873 RR Lyrae stars (RRLs) with precise photometric-metallicity and distance estimates from our newly calibrated $P-\phi_{31}-R_{21}-$[Fe/H]/$P-R_{21}-$[Fe/H] and $G$ band absolute magnitude-metallicity relations. The $P-\phi_{31}-R_{21}-$[Fe/H] and $P-R_{21}-$[Fe/H] relations for type RRab and type RRc stars are obtained from nearly 2700 Gaia-identified RRLs with precise $\phi_{31}$ and $R_{21}$ measurements from the light curves and metallicity estimates from spectroscopy. Using 236 nearby RRLs with accurate distances estimated from parallax measurements with Gaia EDR3, new $G$ band absolute magnitude-metallicity relations and near-infrared period-absolute magnitude-metallicity relations are constructed. External checks, using other high-resolution spectroscopic sample of field RRLs and RRL members of globular clusters (GCs), show that the typical uncertainties in our photometric-metallicity estimates are about 0.24 and 0.16 dex for type RRab/c stars, respectively, without significant systematic bias with respect to high-resolution spectroscopic metallicity measurements. The accuracies of these metallicity estimates are much improved, especially for RRab stars, when compared to those provided by the Gaia DR3 release. Validations of our distance estimates, again by using members of GCs, show that the typical distance errors are only 3-4%. The distance modulus $\mu_0=18.503\pm0.001(stat)\pm0.040(syst)$ mag for the LMC and $\mu_0=19.030\pm0.003(stat)\pm0.043(syst)$ mag for the SMC are estimated from our RRab sample, respectively, and are in excellent agreement with previous measurements. The mean metallicities of the LMC and SMC derived in this work are also consistent with the previous determinations. Using our sample, a steep metallicity gradient of $-0.024\pm0.001$ dex/kpc is found for the LMC, while a negligible metallicity gradient is obtained for the SMC.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： To understand the parameter degeneracy of M subdwarf spectra at low resolution, we assemble a large number of spectral features in the wavelength range of 0.6-2.5 {\mu}m with band strength quantified by narrowband indices. Based on the index trends of BT-Settl model sequences, we illustrate how the main atmospheric parameters (Teff, log g, [M/H], and [alpha/Fe]) affect each spectral feature differently. Furthermore, we propose a four-step process to determine the four parameters sequentially, which extends the basic idea proposed by Jao et al. Each step contains several spectral features that break the degeneracy effect when determining a specific stellar parameter. Finally, the feasibility of each spectroscopic diagnostic with different spectral qualities is investigated. The result is resolution-independent down to R~200.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Ba-enhanced stars are interesting probes of stellar astrophysics and Galactic formation history. In this work, we investigate the chemistry and kinematics for a large sample of Ba-enhanced ([Ba/Fe]$>$1.0) dwarf and subgiant stars with $5000 6000$\,K. They are likely consequences of stellar internal elemental transport processes, but they show very different elemental patterns to the hotter Am/Fm stars. Our results reveal a substantially lack of high-[$\alpha$/Fe] Ba-enhanced stars in the [Fe/H]--[$\alpha$/Fe] plane, which we dub as a {\em high-$\alpha$ desert}. We suggest it is due to a lower efficiency for producing Ba-enhanced stars by low-mass AGB progenitors in binary systems. Our results call for detailed modellings for these Ba-enhanced stellar peculiars, in the context of both stellar internal elemental transport and external mass accretion.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： To understand the parameter degeneracy of M subdwarf spectra at low resolution, we assemble a large number of spectral features in the wavelength range of 0.6-2.5 {\mu}m with band strength quantified by narrowband indices. Based on the index trends of BT-Settl model sequences, we illustrate how the main atmospheric parameters (Teff, log g, [M/H], and [alpha/Fe]) affect each spectral feature differently. Furthermore, we propose a four-step process to determine the four parameters sequentially, which extends the basic idea proposed by Jao et al. Each step contains several spectral features that break the degeneracy effect when determining a specific stellar parameter. Finally, the feasibility of each spectroscopic diagnostic with different spectral qualities is investigated. The result is resolution-independent down to R~200.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Meng Zhang
Mao-Sheng Xiang
Hua-Wei Zhang
Yuan-Sen Ting
Hans-Walter Rix
Ya-qian Wu
Yang Huang
Wei-Xiang Sun
Zhi-Jia Tian
Chun Wang
Xiao-Wei Liu
摘要： Recent observations have revealed a population of $\alpha$-element abundances enhanced giant stars with unexpected high masses ($\gtrsim$1 $M_\odot$) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages ($\tau<6$Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such $\alpha$-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-$\alpha$ old stellar population in the Galactic thick disk. The stellar abundances show that these high-$\alpha$ massive stars have $\alpha$- and iron-peak element abundances similar to those of the high-$\alpha$ old thick disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic "young" single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as, main-sequence (MS) + RGB, MS + AGB, RGB + RGB and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, $\alpha$-rich stars in the Galactic disk seem to be old.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a sample of 135,873 RR Lyrae stars (RRLs) with precise photometric-metallicity and distance estimates from our newly calibrated $P-\phi_{31}-R_{21}-$[Fe/H]/$P-R_{21}-$[Fe/H] and $G$ band absolute magnitude-metallicity relations. The $P-\phi_{31}-R_{21}-$[Fe/H] and $P-R_{21}-$[Fe/H] relations for type RRab and type RRc stars are obtained from nearly 2700 Gaia-identified RRLs with precise $\phi_{31}$ and $R_{21}$ measurements from the light curves and metallicity estimates from spectroscopy. Using 236 nearby RRLs with accurate distances estimated from parallax measurements with Gaia EDR3, new $G$ band absolute magnitude-metallicity relations and near-infrared period-absolute magnitude-metallicity relations are constructed. External checks, using other high-resolution spectroscopic sample of field RRLs and RRL members of globular clusters (GCs), show that the typical uncertainties in our photometric-metallicity estimates are about 0.24 and 0.16 dex for type RRab/c stars, respectively, without significant systematic bias with respect to high-resolution spectroscopic metallicity measurements. The accuracies of these metallicity estimates are much improved, especially for RRab stars, when compared to those provided by the Gaia DR3 release. Validations of our distance estimates, again by using members of GCs, show that the typical distance errors are only 3-4%. The distance modulus $\mu_0=18.503\pm0.001(stat)\pm0.040(syst)$ mag for the LMC and $\mu_0=19.030\pm0.003(stat)\pm0.043(syst)$ mag for the SMC are estimated from our RRab sample, respectively, and are in excellent agreement with previous measurements. The mean metallicities of the LMC and SMC derived in this work are also consistent with the previous determinations. Using our sample, a steep metallicity gradient of $-0.024\pm0.001$ dex/kpc is found for the LMC, while a negligible metallicity gradient is obtained for the SMC.
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