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： The behavior of the shock wave in the atmosphere of the non-fundamental mode RR Lyrae pulsator remains a mystery. In this work, we firstly report a blueshifted Mg triplet emission in continuous spectroscopic observations for a non-Blazhko RRc pulsator (Catalina-1104058050978) with LAMOST medium resolution spectra. We analyse the photometric observations from Catalina Sky Survey of this RRc pulsator with pre-whitening sequence method and provide the ephemeris and phases. An additional frequency signal with $P_1/P_x = 0.69841$ is detected and discussed. The redshift and radial velocity of the spectra are provided by fitting process with $S\acute{e}rsic$ functions and cross-correlation method. Moreover, we plot the variation of H$\alpha$ and Mg lines in a system comoving with the pulsation. Clear evolution of comoving blueshifted hydrogen and Mg emission is observed, which further confirms the existence of shock waves in RRc pulsators. The shock-triggered emission lasts over $15\%$ of the pulsation cycle, which is much longer than the previous observations.
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
摘要： Hydrogen emissions of RR Lyrae variables are the imprints of shock waves traveling through their atmospheres. We develop a pattern recognition algorithm, which is then applied to single-epoch spectra of SDSS and LAMOST. These two spectroscopic surveys covered $\sim$ 10,000 photometrically confirmed RR Lyrae stars. We discovered in total 127 RR Lyrae stars with blueshifted Balmer emission feature, including 103 fundamental mode (RRab), 20 first-overtone (RRc), 3 double-mode (RRd), and 1 Blazhko type (temporary classification for RR Lyrae stars with strong Blazhko modulation in Catalina sky survey that cannot be characterized) RR Lyrae variable. This forms the largest database to date of the properties of hydrogen emission in RR Lyrae variables. Based on ZTF DR5, we carried out a detailed light-curve analysis for the Blazhko type RR Lyrae star with hydrogen emission of long-term modulations. We characterize the Blazhko type RR Lyrae star as an RRab and point out a possible Blazhko period. Finally, we set up simulations on mock spectra to test the performance of our algorithm and on the real observational strategy to investigate the occurrence of the "first apparition".
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Aiming at exploring the nature of dark energy (DE), we use forty-three observational Hubble parameter data (OHD) in the redshift range $0 < z \leqslant 2.36$ to make a cosmological model-independent test of the $\Lambda$CDM model with two-point $Omh^2(z_{2};z_{1})$ diagnostic. In $\Lambda$CDM model, with equation of state (EoS) $w=-1$, two-point diagnostic relation $Omh^2 \equiv \Omega_m h^2$ is tenable, where $\Omega_m$ is the present matter density parameter, and $h$ is the Hubble parameter divided by 100 $\rm km s^{-1} Mpc^{-1}$. We utilize two methods: the weighted mean and median statistics to bin the OHD to increase the signal-to-noise ratio of the measurements. The binning methods turn out to be promising and considered to be robust. By applying the two-point diagnostic to the binned data, we find that although the best-fit values of $Omh^2$ fluctuate as the continuous redshift intervals change, on average, they are continuous with being constant within 1 $\sigma$ confidence interval. Therefore, we conclude that the $\Lambda$CDM model cannot be ruled out.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xianyong Bai
Hui Tian
Yuanyong Deng
Zhanshan Wang
Jianfeng Yang
Xiaofeng Zhang
Yonghe Zhang
Runze Qi
Nange Wang
Yang Gao
Jun Yu
Chunling He
Zhengxiang Shen
Lun Shen
Song Guo
Zhenyong Hou
Kaifan Ji
Xingzi Bi
Wei Duan
Xiao Yang
摘要： The Solar Upper Transition Region Imager (SUTRI) onboard the Space Advanced Technology demonstration satellite (SATech-01), which was launched to a sun-synchronous orbit at a height of 500 km in July 2022, aims to test the on-orbit performance of our newly developed Sc-Si multi-layer reflecting mirror and the 2kx2k EUV CMOS imaging camera and to take full-disk solar images at the Ne VII 46.5 nm spectral line with a filter width of 3 nm. SUTRI employs a Ritchey-Chretien optical system with an aperture of 18 cm. The on-orbit observations show that SUTRI images have a field of view of 41.6'x41.6' and a moderate spatial resolution of 8" without an image stabilization system. The normal cadence of SUTRI images is 30 s and the solar observation time is about 16 hours each day because the earth eclipse time accounts for about 1/3 of SATech-01's orbit period. Approximately 15 GB data is acquired each day and made available online after processing. SUTRI images are valuable as the Ne VII 46.5 nm line is formed at a temperature regime of 0.5 MK in the solar atmosphere, which has rarely been sampled by existing solar imagers. SUTRI observations will establish connections between structures in the lower solar atmosphere and corona, and advance our understanding of various types of solar activity such as flares, filament eruptions, coronal jets and coronal mass ejections.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xianyong Bai
Hui Tian
Yuanyong Deng
Zhanshan Wang
Jianfeng Yang
Xiaofeng Zhang
Yonghe Zhang
Runze Qi
Nange Wang
Yang Gao
Jun Yu
Chunling He
Zhengxiang Shen
Lun Shen
Song Guo
Zhenyong Hou
Kaifan Ji
Xingzi Bi
Wei Duan
Xiao Yang
摘要： The Solar Upper Transition Region Imager (SUTRI) onboard the Space Advanced Technology demonstration satellite (SATech-01), which was launched to a sun-synchronous orbit at a height of 500 km in July 2022, aims to test the on-orbit performance of our newly developed Sc-Si multi-layer reflecting mirror and the 2kx2k EUV CMOS imaging camera and to take full-disk solar images at the Ne VII 46.5 nm spectral line with a filter width of 3 nm. SUTRI employs a Ritchey-Chretien optical system with an aperture of 18 cm. The on-orbit observations show that SUTRI images have a field of view of 41.6'x41.6' and a moderate spatial resolution of 8" without an image stabilization system. The normal cadence of SUTRI images is 30 s and the solar observation time is about 16 hours each day because the earth eclipse time accounts for about 1/3 of SATech-01's orbit period. Approximately 15 GB data is acquired each day and made available online after processing. SUTRI images are valuable as the Ne VII 46.5 nm line is formed at a temperature regime of 0.5 MK in the solar atmosphere, which has rarely been sampled by existing solar imagers. SUTRI observations will establish connections between structures in the lower solar atmosphere and corona, and advance our understanding of various types of solar activity such as flares, filament eruptions, coronal jets and coronal mass ejections.
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