分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Angular momentum is a key property regulating star formation and evolution. However, the physics driving the distribution of the stellar rotation rates of early-type main-sequence stars is as yet poorly understood. Using our catalog of 40,034 early-type stars with homogeneous $v\sin i$ parameters, we review the statistical properties of their stellar rotation rates. We discuss the importance of possible contaminants, including binaries and chemically peculiar stars. Upon correction for projection effects and rectification of the error distribution, we derive the distributions of our sample's equatorial rotation velocities, which show a clear dependence on stellar mass. Stars with masses less than $2.5\ {M_\odot}$ exhibit a unimodal distribution, with the peak velocity ratio increasing as stellar mass increases. A bimodal rotation distribution, composed of two branches of slowly and rapidly rotating stars, emerges for more massive stars ($M>2.5\ {M_\odot}$). For stars more massive than $3.0\ {M_\odot}$, the gap between the bifurcated branches becomes prominent. For the first time, we find that metal-poor ([M/H] $ 0.2$ dex) stars clearly show two branches. The difference could be attributed to unexpectedly high spin-down rates and/or in part strong magnetic fields in the metal-poor subsample.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We independently determine the zero-point offset of the Gaia early Data Release-3 (EDR3) parallaxes based on $\sim 110,000$ W Ursae Majoris (EW)-type eclipsing binary systems. EWs cover almost the entire sky and are characterized by a relatively complete coverage in magnitude and color. They are an excellent proxy for Galactic main-sequence stars. We derive a $W1$-band Period-Luminosity relation with a distance accuracy of $7.4\%$, which we use to anchor the Gaia parallax zero-point. The final, global parallax offsets are $-28.6\pm0.6$ $\mu$as and $-25.4\pm4.0$ $\mu$as (before correction) and $4.2\pm0.5$ $\mu$as and $4.6\pm3.7$ $\mu$as (after correction) for the five- and six-parameter solutions, respectively. The total systematic uncertainty is $1.8$ $\mu$as. The spatial distribution of the parallax offsets shows that the bias in the corrected Gaia EDR3 parallaxes is less than 10 $\mu$as across $40\%$ of the sky. Only $15\%$ of the sky is characterized by a parallax offset greater than 30 $\mu$as. Thus, we have provided independent evidence that the parallax zero-point correction provided by the Gaia team significantly reduces the prevailing bias. Combined with literature data, we find that the overall Gaia EDR3 parallax offsets for Galactic stars are $[-20, -30]$ $\mu$as and 4-10 $\mu$as, respectively, before and after correction. For specific regions, an additional deviation of about 10 $\mu$as is found.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We derive stellar parameters and abundances (`stellar labels') of 40,034 late-B and A-type main-sequence stars extracted from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Medium Resolution Survey (LAMOST--MRS). The primary selection of our early-type sample was obtained from LAMOST Data Release 7 based on spectral line indices. We employed the Stellar LAbel Machine (SLAM) to derive their spectroscopic stellar parameters, drawing on Kurucz spectral synthesis models with 6000 K $< T_\mathrm{eff} <$ 15,000 K and $-1$ dex $< \mathrm{[M/H]} <$ 1 dex. For a signal-to-noise ratio of $\sim 60$, the cross-validated scatter is $\sim$75 K, 0.06 dex, 0.05 dex, and $\sim 3.5\,\mathrm{km\,s^{-1}}$ for $T_\mathrm{eff}$, $\log g$, [M/H], and $v\sin i$, respectively. A comparison with objects with prior, known stellar labels shows great consistency for all stellar parameters, except for $\log g$. Although this is an intrinsic caveat that comes from the MRS's narrow wavelength coverage, it only has a minor effect on estimates of the stellar rotation rates because of the decent spectral resolution and the profile-fitting method employed. The masses and ages of our early-type sample stars were inferred from non-rotating stellar evolution models. This paves the way for reviewing the properties of stellar rotation distributions as a function of stellar mass and age.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Chenyu He
Weijia Sun
Chengyuan Li
Lu Li
Zhengyi Shao
Jing Zhong
Li Chen
Richard de Grijs
Baitian Tang
Songmei Qin
Zara Randriamanakoto
摘要： In addition to the extended main-sequence turnoffs widely found in young and intermediate-age (~ 600 Myr-2 Gyr-old) star clusters, some younger clusters even exhibit split main sequences (MSs). Different stellar rotation rates are proposed to account for the bifurcated MS pattern, with red and blue MSs (rMS and bMS) populated by fast and slowly rotating stars, respectively. Using photometry from Gaia Early Data Release 3, we report a Galactic open cluster with a bifurcated MS, NGC 2422 ( ~ 90 Myr). We exclude the possibilities that the bifurcated MS pattern is caused by photometric noise or differential reddening. We aim to examine if stellar rotation can account for the split MSs. We use spectra observed with the Canada-France-Hawaii Telescope and the Southern African Large Telescope, and directly measured v sin i, the projected rotational velocities, for stars populating the bMS and rMS. We find that their v sin i values are weakly correlated with their loci in the color-magnitude diagram because of contamination caused by a large fraction of rMS stars with low projected rotational velocities. Based on the spectral energy distribution fitting method, we suggest that these slowly rotating stars at the rMS may hide a binary companion, which breaks the expected v sin i-color correlation. Future time-domain studies focusing on whether these slowly rotating stars are radial velocity variables are crucial to test the roles of stellar rotation and binarity in generating the split MSs.
点击量
待评议
点击量
下载量
评论
