分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shoucheng Wang
Bingqiu Chen
Jun Ma
Qian Long
Haibo Yuan
Dezi Liu
Zhimin Zhou
Wei Liu
Jiamin Chen
Zizhao He
摘要： Context.Identification of new star cluster candidates in M31 is fundamental for the study of the M31 stellar cluster system. The machine-learning method convolutional neural network (CNN) is an efficient algorithm for searching for new M31 star cluster candidates from tens of millions of images from wide-field photometric surveys. Aims.We search for new M31 cluster candidates from the high-quality $g$- and $i$-band images of 21,245,632 sources obtained from the Pan-Andromeda Archaeological Survey (PAndAS) through a CNN. Methods.We collected confirmed M31 clusters and noncluster objects from the literature as our training sample. Accurate double-channel CNNs were constructed and trained using the training samples. We applied the CNN classification models to the PAndAS $g$- and $i$-band images of over 21 million sources to search new M31 cluster candidates. The CNN predictions were finally checked by five experienced human inspectors to obtain high-confidence M31 star cluster candidates. Results.After the inspection, we identified a catalogue of 117 new M31 cluster candidates. Most of the new candidates are young clusters that are located in the M31 disk. Their morphology, colours, and magnitudes are similar to those of the confirmed young disk clusters. We also identified eight globular cluster candidates that are located in the M31 halo and exhibit features similar to those of confirmed halo globular clusters. The projected distances to the M31 centre for three of them are larger than 100\,kpc.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The dust extinction curves toward individual sight lines in M33 are derived for the first time with a sample of reddened O-type and B-type supergiants obtained from the LGGS. The observed photometric data are obtained from the LGGS, PS1 Survey, UKIRT, PHATTER Survey, GALEX, Swift/UVOT and XMM-SUSS. We combine the intrinsic spectral energy distributions (SEDs) obtained from the ATLAS9 and Tlusty stellar model atmosphere extinguished by the model extinction curves from the silicate-graphite dust model to construct model SEDs. The extinction traces are distributed along the arms in M33, and the derived extinction curves cover a wide range of shapes ($R_V \approx 2-6$), indicating the complexity of the interstellar environment and the inhomogeneous distribution of interstellar dust in M33. The average extinction curve with $R_V \approx 3.39$ and dust size distribution $dn/da \sim a^{-3.45}{\rm exp}(-a/0.25)$ is similar to that of the MW but with a weaker 2175 Ang bump and a slightly steeper rise in the far-UV band. The extinction in the $V$ band of M33 is up to 2 mag, with a median value of $ A_V \approx 0.43$ mag. The multiband extinction values from the UV to IR bands are also predicted for M33, which will provide extinction corrections for future works. The method adopted in this work is also applied to other star-resolved galaxies (NGC 6822 and WLM), but only a few extinction curves can be derived because of the limited observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zheng Yu
Ji Li
Bingqiu Chen
Yang Huang
Shuhua jia
Maosheng Xiang
Haibo Yuan
Jianrong Shi
Chun Wang
Xiaowei Liu
摘要： Using a sample of 96,201 primary red clump (RC) stars selected from the LAMOST and Gaia surveys, we investigate the stellar structure of the Galactic disk. The sample stars show two separated sequences of high-[{\alpha}/Fe] and low-[{\alpha}/Fe] in the [{\alpha}/Fe]-[Fe/H] plane. We divide the sample stars into five mono-abundance populations (MAPs) with different ranges of [{\alpha}/Fe] and [Fe/H], named as the high-[{\alpha}/Fe], high-[{\alpha}/Fe] & high-[Fe/H], low-[Fe/H], solar, high-[Fe/H] MAPs respectively. We present the stellar number density distributions in the R R Z plane, and the scale heights and scale lengths of the individual MAPs by fitting their vertical and radial density profiles. The vertical profiles, the variation trend of scale height with the Galactocentric radius, indicate that there is a clear disk flare in the outer disk both for the low-[{\alpha}/Fe] and the high-[{\alpha}/Fe] MAPs. While the radial surface density profiles show a peak radius of 7 kpc and 8 kpc for the high-[{\alpha}/Fe] and low-[{\alpha}/Fe] MAPs, respectively. We also investigate the correlation between the mean rotation velocity and metallicity of the individual MAPs, and find that the mean rotation velocities are well separated and show different trends between the high-[{\alpha}/Fe] and the low-[{\alpha}/Fe] MAPs. At last, we discuss the character of the high-[{\alpha}/Fe] & high-[Fe/H] MAP and find that it is more similar to the high-[{\alpha}/Fe] MAP either in the radial and vertical density profiles or in the rotation velocity.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： M dwarfs are the dominating type of stars in the solar neighbourhood. They serve as excellent tracers for the study of the distribution and properties of the nearby interstellar dust. In this work, we aim to obtain high accuracy reddening values of M dwarf stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 8 (DR8). Combining the LAMOST spectra with the high-quality optical photometry from the Gaia Early Data Release 3 (Gaia EDR3), we have estimated the reddening values $E(G_{\rm BP}-G_{\rm RP})$ of 641,426 M dwarfs with the machine-learning algorithm Random Forest regression. The typical reddening uncertainty is only 0.03 mag in $E(G_{\rm BP}-G_{\rm RP})$. We have obtained the reddening coefficient $R_{(G_{\rm BP}-G_{\rm RP})}$, which is a function of the stellar intrinsic colour $(G_{\rm BP}-G_{\rm RP})_0$ and reddening value $E(B-V)$. The values of $E(B-V)$ are also provided for the individual stars in our catalogue. Our resultant high accuracy reddening values of M dwarfs, combined with the Gaia parallaxes, will be very powerful to map the fine structures of the dust in the solar neighbourhood.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a large catalogue of molecular clouds with accurate distance estimates in the southern sky. Based on the three-dimensional (3D) dust extinction map and the best-fit extinction and distance information of over 17 million stars presented in Guo et al., we have identified 250 dust/molecular clouds in the southern sky using a hierarchical structure identification algorithm. Amongst them, 71 clouds locating at high Galactic latitudes ($b < -10^\circ$). We have estimated the distances to the clouds by fitting the extinction versus distance profiles of the lines of sight overlapping with the clouds using a simple Gaussian dust distribution model. The typical uncertainties of the distances are less than 7 per$\,$cent. We also provide the physical properties of the individual clouds, including the linear radius, mass and surface mass density.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 -- 1.2 mag in color g-i, -0.3 -- -2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u-g, g-r, r-i, and i-z colors simultaneously to an accuracy of 0.2 -- 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ~ 70 per cent and efficiency of ~ 80 per cent in selecting metal-poor red giant stars of [Fe/H] $\le$ -1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Precise correction of dust reddening is fundamental to obtain the intrinsic parameters of celestial objects. The Schlegel et al. (SFD) and the Planck 2D extinction maps are widely used for the reddening correction. In this work, using accurate reddening determinations of about two million stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) data release 5 (DR5) and Gaia DR2, we check and calibrate the SFD and Planck maps in the middle and high Galactic latitudes. The maps show similar precision in reddening correction. We find small yet significant spatially dependent biases for the four maps, which are similar between the SFD and Planck2014-R maps, and between the Planck2014-Tau and Planck2019-Tau maps. The biases show a clear dependence on the dust temperature and extinction for the SFD and Planck2014-R maps. While those of the Planck2014-Tau and Planck2019-Tau maps have a weak dependence on the dust temperature, they both strongly depend on the dust spectral index. Finally, we present corrections of the SFD and Planck extinction maps within the LAMOST footprint, along with empirical relations for corrections outside the LAMOST footprint. Our results provide important clues for the further improvement of the Galactic all-sky extinction maps and lay an significant foundation for the accurate extinction correction in the era of precision astronomy.
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