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Recent sub-millimeter dust thermal emission observations have unveiled a significant number of inter-arm massive molecular clouds in M31.However,the effectiveness of this technique is limited to its sensitivity,making it challenging to study more distant galaxies.This study introduces an alternative approach,utilizing optical extinctions derived from space-based telescopes,with a focus on the forthcoming China Space Station Telescope(CSST).We first demonstrate the capability of this method by constructing dust extinction maps for 17 inter-arm massive molecular clouds in M31 using the Panchromatic Hubble Andromeda Treasury(PHAT) data.Our analysis reveals that inter-arm massive molecular clouds with an optical extinction(AV) greater than 1.6 mag exhibit a notable AV excess,facilitating their identification.The majority of these inter-arm massive molecular clouds show an AV around 1 mag,aligning with measurements from our JCMT data.Further validation using a mock CSST RGB star catalog confirms the method's effectiveness.We show that the derived AV values using CSST z and y photometries align more closely with the input values.Molecular clouds with AV>1.6 mag can also be identified using the CSST mock data.We thus claim that future CSST observation could provide an effective way for the detection of inter-arm massive molecular clouds with significant optical extinction in nearby galaxies., Comment: 15 pages,9 figures

Thanks to the precise astrometric measurements of proper motions by the Gaia mission, a new tidal stellar stream has been discovered in the northern hemisphere. The distribution of star count shows that the stream is approximately $80$ degrees long and $1.70$ degrees wide. Observations of $21$ member stars, including 14 RR Lyrae stars, indicate that the stream has an eccentric and retrograde orbit with $e=0.58$. The low metallicity, high total energy, and large angular momentum suggest that it is associated with the merging event Sequoia. This discovery suggests the possibility of finding more substructures with high eccentricity orbits, even in the inner halo., Comment: 17 pages, 8 figures, published by ApJ. This the version before proof

Helium variations are common features of globular clusters (GCs) with multiple stellar populations. All the formation scenarios predict that secondary population stars are enhanced in helium but the exact helium content depends on the polluters. Therefore, searching for helium variations in a star cluster is a straightforward method to understand if it hosts multiple populations or not, and constrain the formation scenario. Although this topic has been well explored for Galactic GCs, GCs beyond the Milky Way are challenging to study because of their large distances. This work studies the helium distribution of GK-type main sequence dwarfs in an old ($\sim$12.5 Gyr) GC in the Large Magellanic Cloud, NGC 2210, using the deep photometry observed by the {\sl Hubble Space Telescope}. We compare the observed morphology of the MS with that of synthetic populations with different helium distributions. We confirm that NGC 2210 dwarfs have a helium spread, with an internal dispersion of $\delta{Y}\sim$0.06--0.07. The fraction of helium enriched stars depends on the $\delta{Y}$ distribution. A continuous $\delta{Y}$ distribution would indicate that more than half of MS stars are helium enriched ($\sim$55\%). If the $\delta{Y}$ distribution is discrete (bimodal), a fraction of $\sim$30\% enriched stars is able to explain the observed morphology of the MS. We also find that the He-enriched population stars are more centrally concentrated than He-normal stars., Comment: 15 pages, 13 figures, ApJ accepted

The color-magnitude diagrams (CMDs) of young star clusters show that, particularly at ultraviolet wavelengths, their upper main sequences (MSs) bifurcate into a sequence comprising the bulk population and a blue periphery. The spatial distribution of stars is crucial to understand the reasons for these distinct stellar populations. This study uses high-resolution photometric data obtained with the Hubble Space Telescope to study the spatial distributions of the stellar populations in seven Magellanic Cloud star clusters. The cumulative radial number fractions of blue stars within four clusters are strongly anti-correlated with those of the high-mass-ratio binaries in the bifurcated region, with negative Pearson coefficients < -0.7. Those clusters generally are young or in an early dynamical evolutionary stage. In addition, a supporting N-body simulation suggests the increasing percentage of blue-MS stars from the cluster centers to their outskirts may be associated with the dissolution of soft binaries. This study provides a different perspective to explore the MS bimodalities in young clusters and adds extra puzzles. A more comprehensive study combined with detailed simulations is needed in the future., Comment: 19 pages, 9 figures, 3 tables. Accepted for publication in ApJ

Multiple stellar populations (MPs) with different chemical compositions are not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies reveal that younger clusters ($\sim$2--6 Gyr-old) in the Magellanic Clouds also exhibit star-to-star chemical variations among evolved stars. However, whether MPs are present among less evolved dwarfs of these intermediate-age clusters is still unclear. In this work, we search for chemical variations among GK-type dwarfs in the $\sim$2 Gyr-old cluster NGC 1978, which is the youngest cluster with MPs. We exploit deep ultraviolet and visual observations from the Hubble Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among MS stars. To do this, we compare appropriate photometric diagrams that are sensitive to N and O with synthetic diagrams of simple stellar populations and MPs. We conclude that the G- and K-type MS stars in NGC\,1978 host MPs. Our statistical analysis shows that the fraction of N-rich stars ranges from $\sim$40\% to $\sim$80\%, depending on the detailed distributions of nitrogen and oxygen., Comment: 16 pages, 10 figures, ApJ accepted

Recent studies have revealed that the onset age for the presence of multiple stellar populations (MPs) in star clusters seems to correspond to the disappearance of the extended main-sequence turnoff (eMSTO) in young clusters, a pattern associated with stellar rotations. A speculative suggestion is that MPs might be caused by the magnetic brake, a stellar evolutionary effect linked to the rotation. In this work, we use the young massive cluster NGC 419 as a testbed. We examined if its magnetically baked MS stars would exhibit MPs. Using the deep ultraviolet and visible images observed through the Hubble Space Telescope, combined with a specific color index that is sensitive to the nitrogen (N) abundance, we examined if its late G- and K-type MS stars are affected by N variation. Our analysis reports that the morphology of its GK-type MS is most likely an SSP, and only a negligible probability that indicates a N variation up to 0.4 dex is present. We conclude that there is no significant N variation among its GK-type MS stars. The absence of a significant chemical variation among the late-type MS stars indicates that MPs might not be a specific pattern of magnetically braked stars., Comment: 11 pages, 5 figure, ApJ accepted

Recent discoveries of bimodal main sequences (MSs) associated with young clusters (with ages $\lesssim 1$ Gyr) in the Magellanic Clouds have drawn a lot of attention. One of the prevailing formation scenarios attributes these split MSs to a bimodal distribution in stellar rotation rates, with most stars belonging to a rapidly rotating population. In this scenario, only a small fraction of stars populating a secondary blue sequence are slowly or non-rotating stars. Here, we focus on the blue MS in the young cluster NGC 1850. We compare the cumulative number fraction of the observed blue-MS stars to that of the high-mass-ratio binary systems at different radii. The cumulative distributions of both populations exhibit a clear anti-correlation, characterized by a highly significant Pearson coefficient of $-0.97$. Our observations are consistent with the possibility that blue-MS stars are low-mass-ratio binaries, and therefore their dynamical disruption is still ongoing. High-mass-ratio binaries, on the other hand, are more centrally concentrated., Comment: 8 pages, 6 figures, accepted to ApJ