A 3D view of the Taurus star-forming region by Gaia and Herschel

Context. Recently published high-quality OmegaCAM photometry of the 3 × 3 deg around the Orion Nebula Cluster (ONC) in r, and i filters revealed three well-separated pre-main sequences in the color-magnitude diagram (CMD). The objects belonging to the individual sequences are concentrated toward the center of the ONC. The authors concluded that there are two competitive scenarios: a population of unresolved binaries and triples with an exotic mass ratio distribution, or three stellar populations with different ages (≈1 Myr age differences). Aims. We use Gaia DR2 in combination with the photometric OmegaCAM catalog to test and confirm the presence of the putative three stellar populations. We also study multiple stellar systems in the ONC for the first time using Gaia DR2. Methods. We selected ONC members based on parallaxes and proper motions and take advantage from OmegaCAM photometry that performs better than Gaia DR2 photometry in crowded regions. We identify two clearly separated sequences with a third suggested by the data. We used Pisa stellar isochrones to estimate ages of the stellar populations with absolute magnitudes computed using Gaia parallaxes on a star by star basis. Results. (1) We confirm that the second and third sequence members are more centrally concentrated toward the center of the ONC. In addition we find an indication that the parallax and proper motion distributions are different among the members of the stellar sequences. The age difference among stellar populations is estimated to be 1−2 Myr. (2) We use Gaia proper motions and other measures to identify and remove as many unresolved multiple system candidates as possible. Nevertheless we are still able to recover two well-separated sequences with evidence for the third one, supporting the existence of the three stellar populations. (3) Due to having ONC members with negligible fore- or background contamination we were able to identify a substantial number of wide binary objects (separation between 1000 and 3000 au) and with relative proper motions of the binary components consistent with zero. This challenges previously inferred values that suggested no wide binary stars exist in the ONC. Our inferred wide-binary fraction is ≈5%. Conclusions. We confirm the three populations correspond to three separated episodes of star formation. Based on this result, we conclude that star formation is not happening in a single burst in this region. In addition we identify 5% of wide-binary stars in the ONC that were thought not to be present.

Context. The Magellanic Clouds are a nearby pair of interacting dwarf galaxies and satellites of the Milky Way. Studying their kinematic properties is essential to understanding their origin and dynamical evolution. They have prominent tidal features and the kinematics of these features can give hints about the formation of tidal dwarfs, galaxy merging and the stripping of gas. In addition they are an example of dwarf galaxies that are in the process of merging with a massive galaxy. Aims. The goal of this study is to investigate the kinematics of the Magellanic Bridge, a tidal feature connecting the Magellanic Clouds, using stellar proper motions to understand their most recent interaction. Methods. We calculated proper motions based on multi-epoch Ks-band aperture photometry, which were obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA), spanning a time of 1−3 yr, and we compared them with Gaia Data Release 2 (DR2) proper motions. We tested two methods for removing Milky Way foreground stars using Gaia DR2 parallaxes in combination with VISTA photometry or using distances based on Bayesian inference. Results. We obtained proper motions for a total of 576 411 unique sources over an area of 23 deg2 covering the Magellanic Bridge including mainly Milky Way foreground stars, background galaxies, and a small population of possible Magellanic Bridge stars (< 15 000), which mostly consist of giant stars with 11.0 < Ks < 19.5 mag. The first proper motion measurement of the Magellanic Bridge centre is 1.80 ± 0.25 mas yr−1 in right ascension and −0.72 ± 0.13 mas yr−1 in declination. The proper motion measurements of stars along the Magellanic Bridge from the VISTA survey of the Magellanic Cloud system (VMC) and Gaia DR2 data confirm a flow motion from the Small to the Large Magellanic Cloud. This flow can now be measured all across the entire length of the Magellanic Bridge. Conclusions. Our measurements indicate that the Magellanic Bridge is stretching. By converting the proper motions to tangential velocities, we obtain ∼110 km s−1 in the plane of the sky. Therefore it would take a star roughly 177 Myr to cross the Magellanic Bridge.

The first and most important stage in studying open clusters is the detection of reliable members. Since open clusters form and evolve within the inner disk of the galaxy, they are surrounded by numerous field stars, making membership determination challenging. Because cluster members originate from the same molecular clouds, they exhibit similar physical parameters—such as proper motion and parallax—and align along a single main sequence in the color-magnitude diagram. For this reason, machine learning algorithms can identify cluster members as familiar data among field stars. In this work, we used a combination of unsupervised machine learning algorithms—DBSCAN and GMM—based on astrometric parameters, proper motion, parallax, and position from the latest Gaia data release (GDR3). After selecting reliable members within the tidal radius, we applied the Random Forest algorithm to detect members beyond the tidal radius, utilizing proper motion, parallax, G-band magnitude, and BP-RP color index as classification features. By leveraging accurate data and a suitable method capable of handling large datasets, we identified members both inside and beyond the tidal radius of clusters. We observed clusters with a comprehensive field of view and analyzed their morphology. All members outside the tidal radius fall within the range of proper motion, parallax, and the main sequence of members inside the tidal radius.

NGC 6791 is a remarkable stellar system that stands out among Galactic open clusters due to its dual distinction as one of the oldest and the most metal-rich clusters. This cluster is among the most thoroughly investigated open clusters, boasting a mass of approximately 4000 ${\rm M}_{\odot }$, an age of about 8 billion years, and a high metallicity of [Fe/H] $\approx 0.4$, making it the most metal-rich cluster identified in the Milky Way. This paper aims to revisit its kinematic, orbital, photometric properties, and stellar populations by using Gaia Data Release 3 (DR3) and BV photometry. To assess membership, we employed the pyupmaskpython package with the HDBSCAN algorithm. The cluster’s proper motions ($\mu _{\alpha }\cos \delta$, $\mu _{\delta }$) and parallax ($\varpi$) are −0.419 $\pm$ 0.12 mas yr$^{-1}$, −2.284 $\pm$ 0.21 mas yr$^{-1}$, and 0.192 $\pm$ 0.042 mas, respectively. Utilizing the PARSEC stellar isochrone fit, we derived the physical parameters of the cluster. The average cluster age and relaxation time are estimated to be 8.2 $\pm$ 0.5 Gyr and 153.9$\pm$ 12.54 Myr, respectively. For the Gaia data, the colour excess $E(G_{BP}-G_{RP})$ is determined to be 0.22 $\pm$ 0.04 mag, and the distance modulus is 13.1 $\pm$ 0.08 mag, corresponding to a distance of 4170 $\pm$ 262 pc. But for BV data, the distance modulus and the colour excess E(B − V) are 12.98 $\pm$ .08 and 0.17 $\pm$ 0.04 mag. The orbit of the cluster has been established. Finally, the mass function is well matched by the two Gaussian functions, which suggest that there are two stellar mass populations in NGC 6791 cluster.

Context. Type II Cepheids (T2Cs) are radially pulsating variables that trace old stellar populations and provide distance estimates through their period-luminosity (PL) relation. Aims. We trace the structure of old stellar population in the Galactic bulge using new distance estimates and kinematic properties of T2Cs. Methods. We present new near-infrared photometry of T2Cs in the bulge from the VISTA Variables in the Vía Láctea survey (VVV). We provide the largest sample (894 stars) of T2Cs with JHKs observations that have accurate periods from the Optical Gravitational Lensing Experiment (OGLE) catalog. Our analysis makes use of the Ks-band time-series observations to estimate mean magnitudes and individual distances by means of the PL relation. To constrain the kinematic properties of our targets, we complement our analysis with proper motions based on both the VVV and Gaia Data Release 2. Results. We derive an empirical Ks-band PL relation that depends on Galactic longitude and latitude: Ks0 = (10.66 ± 0.02) − (2.21 ± 0.03)·(log P−1.2)−(0.020±0.003)·l+(0.050±0.008)·|b| mag; individual extinction corrections are based on a 3D reddening map. Our targets display a centrally concentrated distribution, with solid evidence of ellipsoidal symmetry – similar to the RR Lyræ ellipsoid – and a few halo outliers up to ≳100 kpc. We obtain a distance from the Galactic center of R0 = 8.46 ± 0.03(stat.) ± 0.11(syst.) kpc. We also find evidence that the bulge T2Cs belong to a kinematically hot population, as the tangential velocity components (συl∗ = 104.2 ± 3.0kms−1 and συb = 96.8 ± 5.5kms−1) agree within 1.2σ. Moreover, the difference between absolute and relative proper motion is in good agreement with the proper motion of Sgr A✻ from VLBA measures. Conclusions. We conclude that bulge T2Cs display an ellipsoidal spatial distribution and have kinematics similar to RR Lyræ stars, which are other tracers of the old, low-mass stellar population. T2Cs also provide an estimate of R0 that agrees excellently well with the literature, taking account of the reddening law.

We present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the Gaia Data Release 2 catalog and from Hubble Space Telescope (HST) Advanced Camera for Surveys data. For the Gaia data, we identify and select two populations of stars in the Bridge region, young main-sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known H i gas structure, finding a correlation between the MS stars and the H i gas. In the HST fields our signal comes mainly from an older MS and turnoff population, and the PM baselines range between ∼4 and 13 yr. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud toward the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision.

We present the three-dimensional kinematics of classical Cepheids (CCs) in the Small Magellanic Cloud (SMC) using Gaia DR3 data. By crossmatching the CCs obtained from the fourth phase of the Optical Gravitational Lensing Experiment with Gaia DR3, we obtain distances and proper motions (PMs) for 4236 CCs. Among them, radial velocities (RVs) are available for 91 stars, measured by Gaia, enabling the construction of an accurate relationship between distance and RV. Furthermore, we calculate the internal PMs of the CCs, providing the internal dynamics of the SMC while removing the distance projection effects. The CCs exhibit a northeast–southwest distance gradient, with internal PMs pointing northeast for nearer stars and southwest for more distant stars, indicating a northeast–southwest elongation. The RVs of the CCs show a northwest–southeast gradient, consistent with RVs of other stellar populations, and suggesting that the SMC’s northwest–southeast elongation results from interactions with the Large Magellanic Cloud. The distances and RVs of the CCs are nearly uncorrelated, implying that the two elongations arise from distinct causes.

Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozens of individual objects, both photometrically and spectroscopically, look for signs of multiple generations of young stars and investigate the role of the massive stars in this process. We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate ages and masses of individual objects. We identify the population of embedded young stellar objects (YSO) by their infrared colors and emission line spectra. RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 +/- 0.6 Myr. Of the stars which could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps, and two exposed jets.

The background of the Gaia Data Release 2 (DR2) is introduced and research progress based on Gaia DR2 is reviewed. Gaia DR2 coordinates and proper motions are transformed to Galactic coordinates to allow for a comparison of the Gaia data to models of the Galaxy. A comparison of the absolute predicted and measured numbers of stars per unit area on the sky as well as the corresponding proper motions in Galactic X, Y, and Z are made. It is found that there is a very big difference between the measurements taken from the Gaia Survey and the predictions made by the Galaxia Model of the Galaxy for the proper motions. A further in depth analysis, taking into account the proper motions as a function of distances, is required to fully understand the discovered discrepancies.

The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure ∼1167 kinematic members of Serpens, few of which have been previously identified, to evaluate the star formation history of the complex. The optical members of Serpens are concentrated in three distinct groups located at 380–480 pc; the densest clusters are still highly obscured by optically thick dust and have few optical members. The total population of young stars and protostars in Serpens is at least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may be much higher. Distances to dark clouds measured from deficits in star counts are consistent with the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila Rift, dark clouds located at 600–700 pc, and behind patchy extinction, here called the Serpens Cirrus, located at ∼250 pc. Based on the lack of a distributed population of older stars, the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of 20 within the past ∼5 Myr. The optically bright stars in Serpens Northeast are visible because their natal molecular cloud has been eroded, not because they were flung outwards from a central factory of star formation. The separation between subclusters of 20–100 pc and the absence of an older population together lead to speculation that an external forcing was needed to trigger the active star formation.

Jets and outflows are an integral part of the star formation process. While there are many detailed studies of molecular outflows towards individual star-forming sites, few studies have surveyed an entire star-forming molecular cloud for this phenomenon. The 100-deg2 Five College Radio Astronomy Observatory CO survey of the Taurus Molecular Cloud provides an excellent opportunity to undertake an unbiased survey of a large, nearby, molecular cloud complex for molecular outflow activity. Our study provides information on the extent, energetics and frequency of outflows in this region, which are then used to assess the impact of outflows on the parent molecular cloud. The search identified 20 outflows in the Taurus region, eight of which were previously unknown. Both 12CO and 13CO data cubes from the Taurus molecular map were used, and dynamical properties of the outflows are derived. Even for previously known outflows, our large-scale maps indicate that many of the outflows are much larger than previously suspected, with eight of the outflows (40 per cent) being more than a parsec long. The mass, momentum and kinetic energy from the 20 outflows are compared to the repository of turbulent energy in Taurus. Comparing the energy deposition rate from outflows to the dissipation rate of turbulence, we conclude that outflows by themselves cannot sustain the observed turbulence seen in the entire cloud. However, when the impact of outflows is studied in selected regions of Taurus, it is seen that locally outflows can provide a significant source of turbulence and feedback. The L1551 dark cloud which is just south of the main Taurus complex was not covered by this survey, but the outflows in L1551 have much higher energies compared to the outflows in the main Taurus cloud. In the L1551 cloud, outflows can not only account for the turbulent energy present, but are probably also disrupting their parent cloud. We conclude that for a molecular cloud like Taurus, an L1551-like episode occurring once every 105 years is sufficient to sustain the turbulence observed. Five of the eight newly discovered outflows have no known associated stellar source, indicating that they may be embedded Class 0 sources. In Taurus, 30 per cent of Class I sources and 12 per cent of flat-spectrum sources from the Spitzer young stellar object (YSO) catalogue have outflows, while 75 per cent of known Class 0 objects have outflows. Overall, the paucity of outflows in Taurus compared to the embedded population of Class I and flat-spectrum YSOs indicates that molecular outflows are a short-lived stage marking the youngest phase of protostellar life. The current generation of outflows in Taurus highlight an ongoing period of active star formation, while a large fraction of YSOs in Taurus have evolved well past the Class I stage.

A new proper motion study for about 1400 stars with R< 18 mag in a one square degree region around the young open cluster IC 348 based on Schmidt plates is presented. With an overall accuracy of about 3 mas/yr (2.5 mas/yr, respectively for all stars with R< 17 mag) we have obtained membership probabilities divid- ing our sample into three groups: foreground stars, clus- ter stars and background stars. This separation is also supported by the proper motion distribution with respect to the direction of the standard antapex and an increase of mean proper motion cluster membership probabilities with smaller cluster radii. 240 stars of our sample have cluster membership probabilities larger than 80%. The re- sults are compared with the highly accurate proper mo- tion study of Fredrick (1956) and with other catalogues (PPM, ACT, Hipparcos), all containing only bright stars. From PPM and ACT proper motions a cluster radius of about 30 arcmin can be assumed. Hipparcos proper mo- tions and parallaxes allow the separation of foreground stars, cluster stars and background stars in a somewhat larger region around IC 348 (with distances from the clus- ter centre of up to 85 arcmin). On the basis of Hipparcos data we calculate a mean distance of 261 +27 23 pc for the cluster stars with common proper motion. This is nearly the same distance as obtained by de Zeeuw et al. (1999) for the Per OB 2 association covering more than 15 10 square degrees. The mean proper motion of the cluster IC 348 obtained in our study is in good agreement with that of the highly probable members of the Per OB 2 asso- ciation according to de Zeeuw et al. (1999). Therefore, we Send oprint requests to: R.-D. Scholz conclude that the cluster IC 348 is embedded in the Per OB 2 association. From our proper motion mem- bership probabilities we found a cluster radius of 10 15 arcmin. There is a concentration of Fredrick's highly probable cluster stars just in between our cluster stars and the distant eld stars proper motion distributions. On the other hand, two groups of stars in the proper motion di- agram of Fredrick (1956) also seem to be indicated if all his possible cluster members are considered. The X-ray sources from Preibisch et al. (1996) iden- tied with optical counterparts from our proper motion sample show a strong concentration in the proper mo- tion diagram. For these objects which are likely T Tauri stars we obtained high membership probabilities so that we conclude that they belong to the cluster IC 348 and to the Per OB 2 association. The proper motion foreground stars and cluster members which are counterparts of near- infrared (NIR) sources (Lada & Lada 1995) are located in dierent parts of the (J H) (H K )t wo colour diagram, respectively.

(abridged) We want to provide further evidence of the origin of the proposed stellar members of Chamaeleon and to identify interlopers from the foreground \epsilon Cha and \eta Cha associations. To this aim, we compile lists of spectroscopically confirmed members of Chamaeleon I and II, \epsilon Cha and \eta Cha, and of background objects in the same line of sight. Using Virtual Observatory tools, we cross-match these lists with the UCAC3 catalogue to get the proper motions of the objects. In the vector point diagram, we identify the different moving groups, and use this information to study the membership of proposed candidate members of the associations from the literature. For those objects with available radial velocities, we compute their Galactic space velocities. We look for correlations between the known properties of the objects and their proper motions. The members of the dark clouds exhibit clearly different proper motions from those of the foreground associations and of the background stars. The data suggest that Chamaeleon II could have different dynamical properties from Chamaeleon I. Although the two foreground clusters \epsilon and \eta Chamaeleontis constitute two different proper motion groups, they have similar spatial motions, which are different from the spatial motion of Chamaeleon I. On the other hand, the space motions of the Chamaeleon II stars look more similar to those of the foreground clusters than to the Chamaeleon I stars, but the numbers are low. Hence, with the available data it is unclear to what extent the stellar populations in both clouds are physically connected to each other. We find no correlations between the proper motions and the properties of the objects in either of the clouds.

Multiple structures within stellar groups are an intriguing subject for theoretical and observational studies of stellar formation. With the accuracy and completeness of data from Gaia Data Release 3, we now have new opportunities to detect reliable members of stellar groups across a larger field of view than in previous studies. In this work, using machine learning methods and high-accuracy data, we investigate the possibility of detecting multiple structures within 500 arcmin of ASCC 32. We first applied DBSCAN to proper motion and parallax, as multiple structures tend to share similar values for these parameters. Next, we applied GMM to position, proper motion, and parallax for the members detected by DBSCAN. This approach allowed us to identify a filamentary structure among the DBSCAN-detected members. This structure contains all stellar groups previously identified in this region. Subsequently, based on the BIC score, we applied GMM to this filamentary structure. Since multiple structures exhibit distinct positional distributions, GMM was able to effectively separate all groups within the filament. Our methods successfully identified ASCC 32, OC 0395, and HSC 1865 within a 500 arcmin radius. Additionally, we found two distinct substructures within ASCC 32. These four groups exhibit a single main-sequence distribution in the CMD, with proper motion values within three times the standard deviation and slightly differing parallax values, despite having distinct spatial structures. Furthermore, these four groups share the same radial velocity distribution. We provide documentation demonstrating the formation of these stellar groups as a multiple structure, with improved membership identification compared to previous studies.

Context. Blue horizontal-branch stars evolve from low-mass stars that have completed their main-sequence lifetimes and undergone a helium flash at the end of their red-giant phase. As such, blue horizontal-branch stars are very old objects that can be used as markers in studies of the Galactic structure and formation history. To create a clean sky catalogue of blue horizontal-branch stars, we cross-matched the Gaia data release 2 (DR2) dataset with existing reference catalogues to define selection criteria based on Gaia DR2 parameters. Following the publication of Gaia early data release 3 (EDR3), these methods were verified and subsequently applied to this latest release. Aims. Previous catalogues of blue horizontal-branch stars were developed using spectral analyses or were restricted to individual globular clusters. The purpose of this catalogue is to identify a set of blue horizontal-branch star candidates that have been selected using photometric and astrometric observations and exhibits a low contamination rate. This has been deemed important as the success of the Gaia mission has changed the way that targets are selected for large-scale spectroscopic surveys, meaning that far fewer spectra will be acquired for blue horizontal-branch stars in the future unless they are specifically targeted. Methods. We cross-matched reference blue horizontal-branch datasets with the Gaia DR2 database and defined two sets of selection criteria. Firstly, in Gaia DR2 – colour and absolute G magnitude space, and secondly, in Gaia DR2 – colour and reduced proper motion space. The main-sequence contamination in both subsets of the catalogue was reduced, at the expense of completeness, by concentrating on the Milky Way’s Galactic halo, where relatively young main-sequence stars were not expected. The entire catalogue is limited to those stars with no apparent neighbours within 5 arcsec. These methods were verified and subsequently applied to the Gaia EDR3. Results. We present a catalogue, based on Gaia EDR3, of 57 377 blue horizontal-branch stars. The Gaia EDR3 parallax was used in selecting 16 794 candidates and the proper motions were used to identify a further 40 583 candidates.