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We outline the impact of a small two-band UV-photometry satellite mission on the field of stellar physics, magnetospheres of stars, binaries, stellar clusters, interstellar matter, and exoplanets. On specific examples of different types of stars and stellar systems, we discuss particular requirements for such a satellite mission in terms of specific mission parameters such as bandpass, precision, cadence, and mission duration. We show that such a mission may provide crucial data not only for hot stars that emit most of their light in UV, but also for cool stars, where UV traces their activity. This is important, for instance, for exoplanetary studies, because the level of stellar activity influences habitability. While the main asset of the two-band UV mission rests in time-domain astronomy, an example of open clusters proves that such a mission would be important also for the study of stellar populations. Properties of the interstellar dust are best explored when combining optical and IR information with observations in UV. It is well known that dust absorbs UV radiation efficiently. Consequently, we outline how such a UV mission can be used to detect eclipses of sufficiently hot stars by various dusty objects and study disks, rings, clouds, disintegrating exoplanets or exoasteroids. Furthermore, UV radiation can be used to study the cooling of neutron stars providing information about the extreme states of matter in the interiors of neutron stars and used for mapping heated spots on their surfaces.

Membership determination of open clusters in high-noise environments is still an open question. This paper aims to evaluate the effectiveness of the Gaussian mixture model (GMM) in segregating likely cluster members of open clusters in high-noise environments. We use the GMM method to segregate likely cluster members of four low Galactic latitude open clusters: NGC 2112, NGC 2477, NGC 7789, and Collinder 261, based on the high-precision astrometric data of the Gaia Data Release 2 (Gaia-DR2). The GMM method is used to calculate the membership probabilities of the stars in the field of each cluster; five astrometric parameters (positions, parallaxes, and proper motions) are taken into account. We quantitatively evaluate the goodness of the cluster-field segregation for each cluster, and find that the GMM method is effective for segregating likely cluster members of these clusters, even if these clusters suffer from heavy field star contamination. We estimate the distances and absolute proper motions of these clusters using reliable cluster members; our results suggest the existence of a significant zero-point offset in Gaia-DR2 parallaxes. NGC 2112, NGC 2477, NGC 7789, and Collinder 261 are found to have a mean distance of 〈 D 〉 = 1104 4 , 1437 2 , 2067 4 and 2802 21 pc, respectively. Mean proper motions of ( 〈 cos δ 〉 , 〈 δ 〉 ) = ( − 2.714 0.012 , 4.272 0.012 ) , (−2.449 0.006,0.876 0.006), (−0.919 0.004,−1.938 0.004), and (−6.348 0.006,−2.714 0.006) mas/yr are determined for NGC 2112, NGC 2477, NGC 7789, and Collinder 261, respectively.

The 'white dwarf' clock reset White dwarfs are the most common endpoint of stellar evolution, so they convey important information about the structure and evolution of a galaxy — its age for instance. NGC 6791 is a metal-rich open cluster that is so close to us that it can be imaged down to the very faint luminosities of white dwarfs. Its age determined from turn-off ages of its main sequence stars is estimated at ∼8 billion years, but its 'white dwarf luminosity' age, reflecting termination of the white dwarf cooling sequence, is ∼6 billion years. This apparent inconsistency casts doubts on the reliability of white dwarfs as galactic chronometers. One possible explanation is that as white dwarfs cool, 22 Ne produced as helium burns sinks into the star's interior, causing crystallization and phase separation of 12 C and 16 O, which delays cooling. García-Berro et al . use numerical modelling of the entire white dwarf evolution process to show that physical separation does occur in the core of NGC 6791. This confirms 8 billion years as the cluster's age, and restores the reputation of white dwarfs as reliable chronometers. NGC 6791 is a well studied open cluster that is so close to us that it can be imaged down to very faint luminosities. Two different ages have been proposed for this cluster, one based on the white dwarf luminosity function and one derived from its main-sequence stars. The discrepancy in age is now resolved by the finding that, as theoretically anticipated, physical separation processes occur in the core of white dwarfs. NGC 6791 is a well studied open cluster 1 that it is so close to us that can be imaged down to very faint luminosities 2 . The main-sequence turn-off age (∼8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (∼6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22 Ne, sinks in the deep interior of these stars 3 , 4 , 5 . At lower temperatures, white dwarfs are expected to crystallize and phase separation of the main constituents of the core of a typical white dwarf ( 12 C and 16 O) is expected to occur 6 , 7 . This sequence of events is expected to introduce long delays in the cooling times 8 , 9 , but has not hitherto been proven. Here we report that, as theoretically anticipated 5 , 6 , physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.

This paper presents an investigation on fundamental astrophysical properties of the Pleiades cluster (M 45) using high-precision astrometric and photometric data from the Gaia Data Release 2 (Gaia-DR2). To obtain reliable cluster members, a machine-learning (ML) method is used to compute membership probabilities for 31462 sample stars within a radius of 6.5° from the cluster center, both the astrometric and photometric data are taken into account. We obtain a total number of 1454 likely cluster members with membership probabilities larger than 0.6, including a well-known white dwarf (LB 1497) with a high membership probability of ∼0.96. We find a well-defined relationship between the parallaxes and proper motions of the cluster members, the most likely explanation for the relationship is that the depth effect of the cluster along the line of sight must be taken into consideration. Using Monte Carlo simulations, the most likely distance, proper motion, and radial velocity of the cluster are determined to be D = 136.0 0.1 pc, ( 〈 cos δ 〉 , 〈 δ 〉 ) = (+20.141 0.093, −45.536 0.081) mas yr−1, and 〈 V r 〉 = + 5.8 0.1 km s − 1 , respectively. It is found that the likely cluster members extend outward to a limiting radius of Rlim = 310′ 12′ (12.3 0.5 pc) from the cluster center, and the total mass of the cluster within this radius is Mtot = 721 93 M . We find clear evidence for the presence of spatial mass segregation in this young cluster by analyzing the photometry and spatial positions of the likely cluster members. Interestingly, we also find that four high-mass cluster members with high membership probabilities (>0.99) are being ejected from the inner region of the cluster, they may have formed via close encounters between single and binary stars.

Companions sustained blue stragglers Blue straggler stars challenge the standard theory of stellar evolution, as these main sequence stars are brighter and bluer than others in a cluster thought to have formed at about the same time. In theory, they should have already evolved into giants and stellar remnants. Explanations offered to account for these stragglers include stellar collisions, mass transfer from a companion star or mergers in binaries. Aaron Geller and Robert Mathieu have combined precise observations spanning more than a decade with numerical simulations to show that the binary companions of the majority of the blue stragglers in the old open cluster NGC 188 are consistent with a mass transfer origin, and inconsistent with predictions of the other suggested formation channels. In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary star fraction 1 , core mass 2 and radial position 3 suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models 4 , 5 , detailed observations 6 and sophisticated N -body simulations 7 , however, argue in favour of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old 8 (7 × 10 9 -year) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion mass distribution with significantly higher masses. Mergers in hierarchical triple stars 9 are marginally permitted by the data, but the observations do not favour this hypothesis. The data are highly consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass.

A review of the properties of the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud is presented, primarily from the perspective of its massive star content. The proximity of the Tarantula and its accessibility to X-ray through radio observations permit it to serve as a Rosetta Stone amongst extragalactic supergiant HII regions since one can consider both its integrated characteristics and the individual properties of individual massive stars. Recent surveys of its high mass stellar content, notably the VLT FLAMES Tarantula Survey (VFTS), are reviewed, together with VLT/MUSE observations of the central ionizing region NGC 2070 and HST/STIS spectroscopy of the young dense cluster R136, provide a near complete Hertzsprung-Russell diagram of the region, and cumulative ionizing output. Several high mass binaries are highlighted, some of which have been identified from a recent X-ray survey. Brief comparisons with the stellar content of giant HII regions in the Milky Way (NGC 3372) and Small Magellanic Cloud (NGC 346) are also made, together with Green Pea galaxies and star forming knots in high-z galaxies. Finally, the prospect of studying massive stars in metal poor galaxies is evaluated.

There is a growing interest in the automated characterization of open clusters using data from the Gaia mission. This work evidences the importance of choosing an appropriate sampling radius (the radius of the circular region around the cluster used to extract the data) and the usefulness of additional multiband photometry in order to achieve accurate results. We address this issue using as a case study the cluster Alessi-Teutsch 9. The optimal sampling is determined by counting the number of assigned members at different sampling radii. By using this strategy with data from Gaia EDR3 and with observed photometry in 12 bands spanning the optical range from 3000 to 10000 Å, approximately, we are able to obtain reliable members and to determine the properties of the cluster. The spatial distribution of stars show a two-component structure with a central core of radius ∼12−13 arcmin and an outer halo extending out to 35 arcmin. With the derived cluster distance (654 pc) we obtain that the number density of stars is ≃0.06 star/pc3, making Alessi-Teutsch 9 one of the less dense known open clusters. The short relaxation time reveals that it is a dynamically relaxed and gravitationally bound system.

We analysed the open clusters Czernik 2 and NGC 7654 using CCD UBV photometric and Gaia Early Data Release 3 (EDR3) photometric and astrometric data. Structural parameters of the two clusters were derived, including the physical sizes of Czernik 2 being r=5′ and NGC 7654 as 8′. We calculated membership probabilities of stars based on their proper motion components as released in the Gaia EDR3. To identify member stars of the clusters, we used these membership probabilities taking into account location and the impact of binarity on main-sequence stars. We used membership probabilities higher than P=0.5 to identify 28 member stars for Czernik 2 and 369 for NGC 7654. The mean proper motion components (μαcosδ, μδ) of Czernik 2 were derived as (−4.03±0.04, −0.99±0.05) mas yr−1 and for NGC 7654 as (−1.89±0.03, −1.20±0.03) mas yr−1. We estimated colour-excesses and metallicities separately using (U−B)×(B−V) two-colour diagrams to derive homogeneously determined parameters. The derived E(B−V) colour excess is 0.46±0.02 mag for Czernik 2 and 0.57±0.04 mag for NGC 7654. [Fe/H] metallicities were obtained for the first time for both clusters, −0.08±0.02 dex for Czernik 2 and −0.05±0.01 dex for NGC 7654. Keeping the reddening and metallicity as constant quantities, we fitted PARSEC models using V×(B−V) and V×(U−B) colour-magnitude diagrams, resulting in estimated distance moduli and ages of the two clusters. We obtained the distance modulus for Czernik 2 as 12.80±0.07 mag and for NGC 7654 as 13.20±0.16 mag, which coincide with ages of 1.2±0.2 Gyr and 120±20 Myr, respectively. The distances to the clusters were calculated using the Gaia EDR3 trigonometric parallaxes and compared with the literature. We found good agreement between the distances obtained in this study and the literature. Present day mass function slopes for both clusters are comparable with the value of Salpeter (1955), being X=−1.37±0.24 for Czernik 2 and X=−1.39±0.19 for NGC 7654.

We observed the open clusters M35 and NGC 2158 during the initial K2 campaign (C0). Reducing these data to high-precision photometric timeseries is challenging due to the wide point-spread function (PSF) and the blending of stellar light in such dense regions. We developed an image-subtraction-based K2 reduction pipeline that is applicable to both crowded and sparse stellar fields. We applied our pipeline to the data-rich C0 K2 super stamp, containing the two open clusters, as well as to the neighboring postage stamps. In this paper, we present our image subtraction reduction pipeline and demonstrate that this technique achieves ultra-high photometric precision for sources in the C0 super stamp. We extract the raw light curves of 3960 stars taken from the UCAC4 and EPIC catalogs and de-trend them for systematic effects. We compare our photometric results with the prior reductions published in the literature. For de-trended TFA-corrected sources in the 12-12.25 K p magnitude range, we achieve a best 6.5-hour window running rms of 35 ppm, falling to 100 ppm for fainter stars in the 14-14.25 K p magnitude range. For stars with K p > 14 , our de-trended and 6.5-hour binned light curves achieve the highest photometric precision. Moreover, all our TFA-corrected sources have higher precision on all timescales investigated. This work represents the first published image subtraction analysis of a K2 super stamp. This method will be particularly useful for analyzing the Galactic bulge observations carried out during K2 campaign 9. The raw light curves and the final results of our de-trending processes are publicly available at http://k2.hatsurveys.org/archive/.

Since recent years, mass segregation driven by two-body relaxation in star clusters has been proposed to be measured by the so-called dynamical clock, A + , a measure of the area enclosed between the cumulative radial distribution of blue straggler stars and that of a reference population. Since star clusters spend their lifetime immersed in the gravitational potential of their host galaxy, they are also subject to the effects of galactic tides. In this work, I show that the A + index of a star cluster depends on both its internal dynamics in isolation and the effects of galactic tides. Mainly, I focused on the largest sample of open clusters harboring blue straggler stars with robust cluster membership. I found that these open clusters exhibit an overall dispersion of the A + index in diagnostic diagrams, whereas Milky Way globular clusters show a clear linear trend. However, as also experienced by globular clusters, A + values of open clusters show some dependence on their galactocentric distances, in the sense that clusters located closer or farther than ∼ 11 kpc from the Galactic center have larger and smaller A + values, respectively. This different response to two-body relaxation and galactic tides in globular and open clusters, which happen concurrently, can be due to their different masses. More massive clusters can protect their innermost regions from galactic tides more effectively.