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The kinematics of the ϵ Cha young stellar association close to the Sun has been studied based on a list of candidate stars from the Dickson-Vandervelde et al. work. The working sample consists of 26 stars with parallaxes, proper motions from the Gaia DR3 catalog and radial velocities taken from literary sources. The orbits of the stars back to the past were constructed, and the moment when the association had a minimum spatial size was determined, as well as an analysis of the dependencies of the velocities on the coordinates was carried out. It is shown that the initial sample is divided into two parts with different kinematic properties. The first sample included 9 stars. Based on the construction of the orbits of these 9 stars, an age estimate of million years was obtained. An expansion coefficient in the plane with the value km/s/kpc was also found for them, on the basis of which another age estimate Ma was obtained. The second sample included 17 stars. The construction of their orbits gave an estimate of age Ma, and based on the gradient 248 km/s/kpc, a second estimate of their age Ma was obtained. This suggests that the Cha association either consists of two groupings of different ages, or a younger one arose as a result of a recent outbreak of star formation within a common star system. The question of the gravitational connection of the groupings has not been considered in the framework of this work. [ABSTRACT FROM AUTHOR]

We present the results from our deep optical photometric observations of Bochum 2 (Boc2) star cluster obtained using the 1.3-m Devasthal Fast Optical Telescope along with archival photometric data from Pan-STARRS2/2MASS/UKIDSS surveys. We also used high-quality parallax and proper motion data from the Gaia Data Release 3. We found that the Boc2 cluster has a small size (∼ 1.1 pc) and circular morphology. Using Gaia parallax of member stars and isochrone fitting method, the distance of this cluster is estimated as 3.8 ± 0.4 kpc. We have found that this cluster holds young ( ∼ 5 Myr) and massive (O7–O9) stars as well as an older population of low mass stars. We found that the massive stars were formed in the inner region of the Boc2 cluster in a recent epoch of star formation. We have derived mass function slope (Γ ) in the cluster region as - 2.42 ± 0.13 in the mass range of ∼ 0.72 < M / M ⊙ < 2.8 . The tidal radius of the Boc2 cluster (∼ 7–9) is much more than its observed radius ( ∼ 1.1 pc). This suggests that most of the low-mass stars in this cluster are the remains of an older population of stars formed via an earlier epoch of star formation. [ABSTRACT FROM AUTHOR]

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens. [ABSTRACT FROM AUTHOR]

Knowledge of the three-dimensional structure of Galactic molecular clouds is important for understanding how clouds are affected by processes such as turbulence and magnetic fields and how this structure affects star formation within them. Great progress has been made in this field with the arrival of the Gaia mission, which provides accurate distances to ∼109 stars. Combining these distances with extinctions inferred from optical–infrared, we recover the three-dimensional structure of 16 Galactic molecular cloud complexes at ∼1 pc resolution using our novel three-dimensional dust mapping algorithm dustribution. Using astrodendro , we derive a catalogue of physical parameters for each complex. We recover structures with aspect ratios between 1 and 11, i.e. everything from near-spherical to very elongated shapes. We find a large variation in cloud environments that is not apparent when studying them in two dimensions. For example, the nearby California and Orion A clouds look similar on sky, but we find California to be more sheet-like, and massive, which could explain their different star formation rates. By calculating the total mass of these individual clouds, we demonstrate that it is necessary to define cloud boundaries in three dimensions in order to obtain an accurate mass; simply integrating the extinction overestimates masses. We find that Larson's relationship on mass versus radius holds true whether you assume a spherical shape for the cloud or take their true extents. [ABSTRACT FROM AUTHOR]

Westerlund 1 (Wd 1) is one of the most relevant star clusters in the Milky Way to study massive star formation, although it is still poorly known. Here, we used photometric and spectroscopic data to model the eclipsing binary W36, showing that its spectral type is O6.5 III  +  O9.5 IV, hotter and more luminous than thought before. Its distance d W36 = 4.03 ± 0.25 kpc agrees, within the errors, with three recent Gaia-EDR3-based distances reported in Paper I, Beasor & Davies, and by Negueruela's group. However, they follow different approaches to fix the zero-points for red sources such as those in Wd 1, and to select the best approach, we used an accurate modelling of W36. The weighted mean distance of our parallax (Paper I) and binary distances results in d wd1 = 4.05 ± 0.20 kpc, with an unprecedented accuracy of 5 per cent. We adopted isochrones based on the Geneva code with supersolar abundances to infer the age of W36B as 6.4 ± 0.7 Myr. This object seems to be part of the prolific star formation burst represented by OB giants and supergiants that occurred at 7.1 ± 0.5 Myr ago, which coincides with the recently published PMS isochrone with age 7.2 Myr. Other BA-type luminous evolved stars and yellow hypergiants spread in the age range of 8–11 Myr. The four red supergiants discussed in paper I represent the oldest population of the cluster with an age of 10.7 ± 1 Myr. The multiple episodes of star formation in Wd 1 are reminiscent of that reported for the R136/30 Dor LMC cluster. [ABSTRACT FROM AUTHOR]

The Orion Nebula Cluster (ONC) is the most massive region of active star formation within a kpc of the Sun. Using Gaia DR3 parallaxes and proper motions, we examine the bulk motions of stars radially and tangentially relative to the cluster centre. We find an age gradient with distance to the stars in the ONC, from 385 pc for the oldest stars, to 395 pc for the younger stars, indicating that the star-forming front is propagating into the cloud. We find an organized signature of rotation of the central cluster, but it is present only in stars younger than 2 Myr. We also observe a net infall of young stars into the centre of the ONC's deep gravitational potential well. The infalling sources lie preferentially along the filament; on the other hand, outflowing sources are distributed spherically around the cluster, and they have larger velocity dispersion. We further propose a solution to a long-standing question of why the ONC shows a weak signature of expansion even though the cluster is likely bound: much of this expansion may be driven by unstable N -body interactions among stars, resulting in low-velocity ejections. Though analysing signatures imprinted on stellar dynamics across different spatial scales, these observation shed new light on the signatures of formation and evolution of young clusters. [ABSTRACT FROM AUTHOR]

BP Psc is an active late-type (sp:G9) star with unclear evolutionary status lying at high-galactic latitude b  = −57°. It is also the source of the well collimated bipolar jet. We present results of the proper motion and radial velocity study of BP Psc outflow based on the archival H α imaging with the GMOS camera at 8.1-m Gemini-North telescope as well as recent imaging and long-slit spectroscopy with the SCORPIO multimode focal reducer at 6-m BTA telescope of SAO RAS. The 3D kinematics of the jet revealed the full spatial velocity up to ∼140 km s−1 and allows us to estimate the distance to BP Psc system as D  = 135 ± 40 pc. This distance leads to an estimation of the central source luminosity L * ≈ 1.2  L ⊙, indicating that it is the ≈1.3M⊙ T Tauri star with an age t ≲ 7 Myrs. We measured the electron density of order Ne ∼ 102 cm−3 and mean ionization fraction f ≈ 0.04 within the jet knots and estimated upper limit of the mass-loss rate in NE lobe as |$\dot{M}_{\rm out}\approx 1.2\cdot 10^{-8}{\rm M}_{\odot }\,{\rm yr}^{-1}$|⁠. The physical characteristics of the outflow are typical for the low-excitation YSO jets and consistent with the magnetocentrifugal mechanism of its launching and collimation. Prominent wiggling pattern revealed in H α images allowed us to suppose the existence of a secondary substellar companion in a non-coplanar orbit and estimate its most plausible mass as Mp ≈ 30 M Jup. We conclude that BP Psc is one of the closest to the Sun young jet-driving systems and its origin is possibly related to the episode of star formation triggered by expanding supershells in second Galactic quadrant. [ABSTRACT FROM AUTHOR]

Open clusters are key coeval structures that help us understand star formation, stellar evolution and trace the physical properties of our Galaxy. In the past years, the isolation of open clusters from the field has been heavily alleviated by the access to accurate large-scale stellar parallaxes and proper motions along a determined line of sight. Still, there are limitations regarding their completeness since large-scale studies rely on optical wavelengths. Here, we extend the open clusters sequences towards fainter magnitudes complementing the Gaia photometric and astrometric information with near-infrared data from the VVV survey. We performed a homogeneous analysis on 37 open clusters implementing two coarse-to-fine characterization methods: extreme deconvolution Gaussian mixture models coupled with an unsupervised machine learning method on eight-dimensional parameter space. The process allowed us to separate the clusters from the field at near-infrared wavelengths. We report an increase of ∼47 per cent new member candidates on average in our sample (considering only sources with high membership probability p  ≧ 0.9). This study is the second in a series intended to reveal open cluster near-infrared sequences homogeneously. [ABSTRACT FROM AUTHOR]

We present a new census of candidate pre-main-sequence stars in the Cepheus flare star-forming region, based on Gaia  EDR3 parallaxes, proper motions, and colour–magnitude diagrams. We identified new candidate members of the previously known young stellar groups associated with NGC 7023, L1177, L1217/L1219, L1228, L1235, and L1251. We studied the 3D structure of the star-forming complex and the distribution of tangential velocities of the young stars. The young stellar groups are located between 330 and 368 pc from the Sun, divided into three kinematic subgroups, and have ages between 1 and 5 million yr. The results confirm the scenario of propagating star formation, suggested by previous studies. In addition to the bulk pre-main-sequence star population between 330 and 370 pc, there is a scattered and more evolved pre-main-sequence population around 150–180 pc. We found new candidate members of the nearby Cepheus Association, and identified a new moving group of 46, 15–20 million yr old pre-main-sequence stars located at a distance of 178 pc, around the A0-type star HD 190833. A few pre-main-sequence stars are located at 800–900 pc, indicative of star-forming regions associated with the Galactic local arm above the Galactic latitude of +10°. [ABSTRACT FROM AUTHOR]

Young stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars and use it to identify ∼3 × 104 candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis of all groups with substructure, using age estimates to reveal each region's star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ∼4 km s−1 (∼4 pc Myr−1). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales. [ABSTRACT FROM AUTHOR]

We present the results of our BVR -band photometric and R -band polarimetric observations of ∼40 stars in the periphery of the dark cloud CB54. From different photometric data, we estimate E (B − V) and E (J − H). After involving data from other sources, we discuss the extinction variations towards CB54. We reveal two main dust layers: a foreground, E (B − V) ≈ 0.1 mag, at ∼200 pc and an extended layer, |$E(B-V) \gtrsim 0.3$| mag, at ∼1.5 kpc. CB54 belongs to the latter. Based on these results, we consider the reason for the random polarization map that we have observed for CB54. We find that the foreground is characterized by low polarization (⁠|$P \lesssim 0.5$|  per cent) and a magnetic field parallel to the Galactic plane. The extended layer shows high polarization (P up to 5–7 per cent). We suggest that the field in this layer is nearly perpendicular to the Galactic plane and both layers are essentially inhomogeneous. This allows us to explain the randomness of polarization vectors around CB54 generally. The data – primarily observed by us in this work for CB54, by A. K. Sen and colleagues in previous works for three dark clouds CB3, CB25 and CB39, and by other authors for a region including the B1 cloud – are analysed to explore any correlation between polarization, the near-infrared, E (J − H), and optical, E (B − V), excesses, and the distance to the background stars. If polarization and extinction are caused by the same set of dust particles, we should expect good correlations. However, we find that, for all the clouds, the correlations are not strong. [ABSTRACT FROM AUTHOR]

Stars born at the same time in the same place should have formed from gas of the same element composition. But most stars subsequently disperse from their birth siblings, in orbit and orbital phase, becoming 'field stars'. Here, we explore and provide direct observational evidence for this process in the Milky Way disc, by quantifying the probability that orbit-similarity among stars implies indistinguishable metallicity. We define the orbit similarity among stars through their distance in action-angle space, Δ(J , θ), and their abundance similarity simply by Δ[Fe/H]. Analysing a sample of main-sequence stars from Gaia DR2 and LAMOST, we find an excess of pairs with the same metallicity (Δ[Fe/H] < 0.1) that extends to remarkably large separations in Δ(J , θ) that correspond to nearly 1 kpc distances. We assess the significance of this effect through a mock sample, drawn from a smooth and phase-mixed orbit distribution. Through grouping such star pairs into associations with a friend-of-friends algorithm linked by Δ(J ,θ), we find 100s of mono-abundance groups with ≥3 (to ≳20) members; these groups – some clusters, some spread across the sky – are over an order-of-magnitude more abundant than expected for a smooth phase-space distribution, suggesting that we are witnessing the 'dissolution' of stellar birth associations into the field. [ABSTRACT FROM AUTHOR]

The region surrounding the well-known reflection nebula, NGC 7023, illuminated by a Herbig Be star, HD 200775, located in the dark cloud L1174 is studied in this work. Based on the distances and proper motion values from Gaia DR2 of 20 previously known young stellar object (YSO) candidates, we obtained a distance of 335 ± 11 pc to the cloud complex L1172/1174. Using polarization measurements of the stars projected on the cloud complex, we show additional evidence for the cloud to be at ∼335 pc distance. Using this distance and proper motion values of the YSO candidates, we searched for additional comoving sources in the vicinity of HD 200775 and found 20 new sources, which show low infrared excess emission and are of age ∼1 Myr. Among these, 10 YSO candidates and 4 newly identified comoving sources are found to show X-ray emission. Three of the four new sources for which we have obtained optical spectra show H α in emission. About 80 per cent of the total sources are found within ∼1 pc distance from HD 200775. Spatial correlation of some of the YSO candidates with the Herschel dust column density peaks suggests that star formation is still active in the region and may have been triggered by HD 200775. [ABSTRACT FROM AUTHOR]

We conducted a high-precision elemental abundance analysis of the twin-star comoving pair HIP 34407/HIP 34426. With mean error of 0.013 dex in the differential abundances (Δ[X/H]), a significant difference was found: HIP 34407 is more metal rich than HIP 34426. The elemental abundance differences correlate strongly with condensation temperature, with the lowest for the volatile elements like carbon around 0.05 ± 0.02 dex, and the highest up to about 0.22 ± 0.01 dex for the most refractory elements like aluminium. Dissimilar chemical composition for stars in twin-star comoving pairs is not uncommon, thus we compile previously published results like ours and look for correlations between abundance differences and stellar parameters, finding no significant trends with average effective temperature, surface gravity, iron abundance, or their differences. Instead, we found a weak correlation between the absolute value of abundance difference and the projected distance between the stars in each pair that appears to be more important for elements that have a low absolute abundance. If confirmed, this correlation could be an important observational constraint for binary star system formation scenarios. [ABSTRACT FROM AUTHOR]

OB associations are the prime star-forming sites in galaxies. However, the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help in tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5–8 parsecs in diameter and display small dispersions in proper motion, while the haloes spread over tens of parsecs with two to three times larger dispersions in proper motion. This is reminiscent of the so-called line width–size relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters. [ABSTRACT FROM AUTHOR]

We present optical photometric, spectroscopic data for the stars in the high Galactic latitude molecular cloud MBM 110. For the complete membership selection of MBM 110, we also analyse Wide-field Infrared Survey Explorer (WISE) mid-infrared data and Gaia astrometric data. Membership of individual stars is critically evaluated using the data mentioned above. The Gaia parallax of stars in MBM 110 is 2.667 ± 0.095 mas (⁠|$d = 375 \pm 13\, \mathrm{pc}$|⁠), which confirms that MBM 110 is a small star-forming region in the Orion–Eridanus superbubble. The age of MBM 110 is between 1.9 and 3.1 Myr depending on the adopted pre-main-sequence evolution model. The total stellar mass of MBM 110 is between 16 M⊙ (members only) and 23 M⊙ (including probable members). The star formation efficiency is estimated to be about 1.4 per cent. We discuss the importance of such small star formation regions in the context of the global star formation rate and suggest that a galaxy's star formation rate calculated from the Hα luminosity may underestimate the actual star formation rate. We also confirm a young brown dwarf member based on photometry, spectroscopy, and astrometry. [ABSTRACT FROM AUTHOR]

We explore the origin of the observed decline in [O/Fe] (and [Mg/Fe]) with Galactocentric distance for high-metallicity stars ([Fe/H] > −1.1), based on a sample of halo stars selected within the Apache Point Observatory Galactic Evolution Experiment (APOGEE) fourteenth data release (DR14). We also analyse the characteristics of the [α/Fe] distributions in the inner-halo regions inferred from two zoom-in Milky Way mass-sized galaxies that are taken as case studies. One of them qualitatively reproduces the observed trend to have higher fraction of α-rich star for decreasing galactocentric distance; the other exhibits the opposite trend. We find that stars with [Fe/H] > −1.1 located in the range [15–30] kpc are consistent with formation in two starbursts, with maxima separated by about ∼1 Gyr. We explore the contributions of stellar populations with different origin to the [α/Fe] gradients detected in stars with [Fe/H] > −1.1. Our analysis reveals that the simulated halo that best matches the observed chemical trends is characterized by an accretion history involving low- to intermediate-mass satellite galaxies with a short and intense burst of star formation, and contributions from a more massive satellite with dynamical masses about ∼1010 M |$\odot$| , distributing low [α/Fe] stars at intermediate radius. [ABSTRACT FROM AUTHOR]

We use Gaia data release 2 (DR2) magnitudes, colours, and parallaxes for stars with G <  12 to explore a parameter space with 15 dimensions that simultaneously includes the initial mass function (IMF) and a non-parametric star formation history (SFH) for the Galactic disc. This inference is performed by combining the Besançon Galaxy Model fast approximate simulations (BGM FASt) and an approximate Bayesian computation algorithm. We find in Gaia DR2 data an imprint of a star formation burst 2–3 Gyr ago in the Galactic thin disc domain, and a present star formation rate (SFR) of ≈1 M⊙/yr. Our results show a decreasing trend of the SFR from 9–10 Gyr to 6–7 Gyr ago. This is consistent with the cosmological star formation quenching observed at redshifts z <  1.8. This decreasing trend is followed by a SFR enhancement starting at ∼5 Gyr ago and continuing until ∼1 Gyr ago which is detected with high statistical significance by discarding the null hypothesis of an exponential SFH with a p-value = 0.002. We estimate, from our best fit model, that about 50% of the mass used to generate stars, along the thin disc life, was expended in the period from 5 to 1 Gyr ago. The timescale and the amount of stellar mass generated during the SFR enhancement event lead us to hypothesise that its origin, currently under investigation, is not intrinsic to the disc. Thus, an external perturbation is needed for its explanation. Additionally, for the thin disc we find a slope of the IMF of α3 ≈ 2 for masses M >  1.53 M⊙ and α2 ≈ 1.3 for the mass range between 0.5 and 1.53 M⊙. This is the first time that we consider a non-parametric SFH for the thin disc in the Besançon Galaxy Model. This new step, together with the capabilities of the Gaia DR2 parallaxes to break degeneracies between different stellar populations, allow us to better constrain the SFH and the IMF. [ABSTRACT FROM AUTHOR]

Precise stellar masses and radii can be determined using asteroseismology, but their accuracy must be tested against independent estimates. Using radii derived from Gaia DR2 parallaxes, we test the accuracy of asteroseismic radii for a sample of 93 dwarfs based on both individual frequency fitting and the seismic scaling relations. Radii from frequency fitting are about 1 per cent smaller than Gaia radii on average; however, this difference may be explained by a negative bias of |$30\, \mu\mathrm{as}$| in the Gaia parallaxes. This indicates that the radii derived from frequency fitting are accurate to within 1 per cent. The scaling relations are found to overestimate radii by more than 5 per cent, compared to the Gaia radii, at the highest temperatures. We demonstrate that this offset is reduced to 3 per cent after applying corrections based on model frequencies to the scaling relation for Δν, but only when the model frequencies are corrected for the surface effect. With corrections to Δν, the scaling relation gives radii accurate to about 2–3 per cent for dwarfs in the temperature range 5400–6700 K. The remaining offset at the highest temperatures may indicate the need for a correction to the scaling relation for νmax. [ABSTRACT FROM AUTHOR]

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the Accretion Disc with OmegaCAM survey, and detailed radial velocity measurements from ESO- Gaia, we analyse in detail a 10 × 5 deg region around the Wolf–Rayet Star γ2 Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions, and parallax, we discover six clusters or associations – four of which appear new. Analysis of the colour–magnitude diagram for these clusters shows that four of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters. [ABSTRACT FROM AUTHOR]

Aims. The formation of high-mass stars is still debated. For this reason, several projects such as Herschel-HOBYS are focussed on the study of the earliest phases of massive star formation. As a result, massive star-forming complexes such as NGC 6334 and NGC 6357 have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to better characterise the environments of these cores we need to understand the previous massive star formation history. To better characterise the environment of these massive dense cores we study the previous high-mass star formation and how these stars act on their environments. Methods. This study is based on the spectral classification of the OB stars identified towards NGC 6334 and NGC 6357 with spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of 109 stars across these regions we were able to evaluate the following: distance, age, mass, global star-forming efficiency (SFE), and star formation rate (SFR) of the regions. The physical conditions of the ionised gas for both complexes was also derived. Results.We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1 kpc to 2.2 kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ~10 Myr are broadly spread across these complexes, while younger stars are mainly located in the H II regions and stellar clusters. Our data also suggests that some of the young stars can be considered runaway stars. We evaluate a SFE of 0.019+0:008 -0:007 and 0.021+0:004 -0:003 and a SFR of 1:1 X 103 ± 300 M☉Myr-1 and 1:7 X 103 ± 400 M☉ Myr-1 for NGC 6334 and NGC 6357, respectively. We note that 29 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 are more impacted by X-ray flux and stellar winds than in NGC 6334. Finally, from the analysis of nebular lines (H , [NII], and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated. [ABSTRACT FROM AUTHOR]

From measuring the annual parallax of water masers over 1.5 yr with VLBI Exploration of Radio Astrometry, we present the trigonometric parallax and corresponding distance of another newly identified water maser source in the region of IRAS 20231+3440 as π = 0.611 ± 0.022 mas and D=1.64±0.06 kpc, respectively. We measured the absolute proper motions of all the newly detected maser spots (30 spots) and presented two pictures describing the possible spatial distribution of the water maser as the morphology marks out an arc of masers whose average proper motion velocity in the jet direction was 14.26 km s-1. As revealed by the ALLWISE composite image and by applying the colour-colour method of young stellar objects (YSO) identification and classification on photometric archived data, we identified the driving source of the north maser group to be a class I, young stellar object. To further probe the nature of the progenitor, we used the momentum rate maximum value (1.2×10-4 M⊙ yr-1 km s-1) of the outflow to satisfy that the progenitor under investigation is a low-mass young stellar object concurrently forming alongside an intermediate-mass YSO ~60 000 au (~37 arcsec) away from it. [ABSTRACT FROM AUTHOR]

We exploit cosmological model-independent measurements of the expansion history of the Universe to provide a cosmic distance ladder. These are supernovae Type Ia used as standard candles (at redshift between 0.01 and 1.3) and baryon acoustic oscillations (at redshifts between 0.1 and 0.8) as standard rulers. We calibrate (anchor) the ladder in two ways: first using the local H0 value as an anchor at z = 0 (effectively calibrating the standard candles) and secondly using the cosmic microwave background-inferred sound-horizon scale as an anchor (giving the standard ruler length) as an inverse distance ladder. Both methods are consistent, but the uncertainty in the expansion history H(z) is smaller if the sound-horizon scale is used. We present inferred values for the sound horizon at radiation drag rd which do not rely on assumptions about the early expansion history nor on cosmic microwave background measurements but on the cosmic distance ladder and baryon acoustic oscillations measurements. We also present derived values of H0 from the inverse distance ladder and we show that they are in very good agreement with the extrapolated value in a Λ cold dark matter model from Planck cosmic microwave background data. [ABSTRACT FROM AUTHOR]

We report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way as part of the BeSSeL Survey with the VLBA. Combining these results with eight others from the literature, we investigated the structure and kinematics of the arm between Galactocentric azimuths β ≈ −2° and 65°. We found that the spiral pitch angle is 7°.3 ± 1°.5; the arm’s half-width, defined as the rms deviation from the fitted spiral, is ≈0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 ± 0.2 kpc from the Sun. Unlike for adjacent spiral arms, we found no evidence for significant peculiar motions of sources in the Sagittarius arm opposite to Galactic rotation. [ABSTRACT FROM AUTHOR]

We report trigonometric parallax measurements of masers in the massive star-forming complex W43 from Very Long Baseline Array observations as part of the Bar and Spiral Structure Legacy Survey. Based on measurements of three 12 GHz methanol maser sources (G029.86–00.04, G029.95–00.01, and G031.28+00.06) and one 22 GHz water maser source (G031.58+00.07) toward W43, we derived a distance of kpc to W43. By associating the masers with CO molecular clouds, and associating the clouds kinematically with CO longitude-velocity spiral features, we assign W43 to the Scutum spiral arm, close to the near end of the Galactic bar. The peculiar motion of W43 is about 20 km s–1 toward the Galactic Center and is very likely induced by the gravitational attraction of the bar. [ABSTRACT FROM AUTHOR]

Context. Accurate parallax measurements allow us to determine physical properties of brown dwarfs and help us constrain evolutionary and atmospheric models, break age-mass degeneracy, and reveal unresolved binaries. Aims. We measured absolute trigonometric parallaxes and proper motions of six cool brown dwarfs using background galaxies to establish an absolute reference frame. We derive the absolute J-band magnitude. The six T brown dwarfs in our sample have spectral types between T2.5 and T8 and magnitudes between 13.9 and 18.0 in the Two Micron All Sky Survey (2MASS) with photometric distances below 25 pc. Methods. The observations were taken in the J-band with the Omega-2000 camera on the 3.5 m telescope at Calar Alto during a time period of 27 months between March 2011 and June 2013. The number of epochs varied between 11 and 12 depending on the object. The reduction of the astrometric measurements was carried out with respect to the field stars. The relative parallax and proper motions were transformed into absolute measurements using the background galaxies in our fields. Results. We obtained absolute parallaxes for our six brown dwarfs with a precision between 3 and 6 mas. We compared our results in a color-magnitude diagram with other brown dwarfs with determined parallax and with the BT-Settl 2012 atmospheric models. For four of the six targets, we found a good agreement in luminosity with objects of similar spectral types. We obtained an improved accuracy in the parallaxes and proper motions in comparison to previous works. The object 2MASS J11061197+2754225 is more than 1 mag overluminous in all bands, which point to binarity or high order multiplicity. [ABSTRACT FROM AUTHOR]

Aims. As a continuation of a previous work, in which we found strong evidence of massive molecular outflows toward a massive star-forming site, we present a new study of this region based on very high angular resolution observations with the aim of discovering the outflow-driven mechanism. Methods. Using near-IR data acquired with Gemini-NIRI at the broad H- and Ks-bands, we studied a region of 22" × 22" around the UCHII region G045.47+0.05, a massive-star forming site at a distance of about 8 kpc. To image the source with the highest spatial resolution possible we employed the adaptative optics system ALTAIR, achieving an angular resolution of about 0".15. Results. We discovered a cone shaped nebula that has an opening angle of about 90° and extends eastward of the IR source 2MASS J19142564+1109283, which is very likely a massive young stellar object (MYSO). This morphology suggests a cavity that was cleared in the circumstellar material, and its emission may arise from scattered continuum light, warm dust, and probably also from emission lines from shock-excited gas. The nebula, which presents arc-like features, is connected with the IR source through a jet-like structure, which is aligned with the blueshifted CO outflow found in a previous study. The near-IR structure lies ~3" north of the radio continuum emission, revealing that it is not spatially coincident with the UCHii region. The observed morphology and structure of the near-IR nebula strongly suggest the presence of a precessing jet. We resolved the circumstellar environment (in scale of a thousand AU) of a distant MYSO, indeed one of the farthest currently known. [ABSTRACT FROM AUTHOR]

Aims. The main theoretical problem for the formation of a Keplerian disk around Be stars is how angular momentum is supplied from the star to the disk, even more so since Be stars probably rotate somewhat subcritically. For instance, nonradial pulsation may transport angular momentum to the stellar surface until (part of) this excess supports the disk-formation/replenishment. The nearby Be star Achernar is presently building a new disk and offers an excellent opportunity to observe this process from relatively close-up. Methods. Spectra from various sources and epochs are scrutinized to identify the salient stellar parameters characterizing the disk life cycle as defined by Hα emission. The variable strength of the non-radial pulsation is confirmed, but does not affect the other results. Results. For the first time it is demonstrated that the photospheric line width does vary in a Be star, by as much as ▵v sin i ≲ 35 km s-1. However, unlike assumptions in which a photospheric spin-up accumulates during the diskless phase and then is released into the disk as it is fed, the apparent photospheric spin-up is positively correlated with the appearance of Hα line emission. The photospheric line widths and circumstellar emission increase together, and the apparent stellar rotation declines to the value at quiescence after the Hα line emission becomes undetectable. [ABSTRACT FROM AUTHOR]

Highlights: [•] We explore D–H exchange among major gas species in an evolving protoplanetary disk. [•] The evolving disk accounts for disk formation and angular momentum transport. [•] The D–H exchange accounts for kinetics among major gas species. [•] We find a non-monotonic distribution D/H ratio in water with distance from the star. [Copyright &y& Elsevier]

Context. Whether the Cygnus X complex consists of one physically connected region of star formation or of multiple independent regions projected close together on the sky has been debated for decades. The main reason for this puzzling scenario is the lack of trustworthy distance measurements. Aims. We aim to understand the structure and dynamics of the star-forming regions toward Cygnus X by accurate distance and proper motion measurements. Methods. To measure trigonometric parallaxes, we observed 6.7GHz methanol and 22 GHz water masers with the European VLBI Network and the Very Long Baseline Array. Results. We measured the trigonometric parallaxes and proper motions of five massive star-forming regions toward the Cygnus X complex and report the following distances within a 10% accuracy: 1.30+0.07 -0.07 kpc for W75N, 1.46+0.09 -0.08 kpc for DR20, 1.50+0.08 -0.07 kpc for DR 21, 1.36+0.12 -0.11 kpc for IRAS 20290+4052, and 3.33+0.11 -0.11 kpc for AFGL 2591. While the distances ofW75N, DR20, DR21, and IRAS 20290+4052 are consistent with a single distance of 1.40 ± 0.08 kpc for the Cygnus X complex, AFGL 2591 is located at a much greater distance than previously assumed. The space velocities of the four star-forming regions in the Cygnus X complex do not suggest an expanding Strömgren sphere. [ABSTRACT FROM AUTHOR]

In this paper, using the Two Micron All Sky Survey (2MASS) photometry, we study the mass functions of a sample of nine clusters of ages varying from 4 Myr to 1.2 Gyr and Galactocentric distances from 6 to 12 kpc. We look for evidence of mass segregation in these clusters by tracing the variation in the value of in different regions of the cluster as a function of the parameter (where is the age of the cluster and is the relaxation time of the cluster), Galactocentric distance, age and size of the cluster. The value of value increases with age and and fits straight lines with slopes and -intercepts given by , and , , respectively, and is a clear indicator of the dynamical processes involved. The confidence level of the Pearson's product-moment correlation of with age is 0.76 with = 0.002 and with is 0.71 with = 0.007. The value of also increases with Galactocentric distance, indicating the presence of a larger relative number of low-mass stars in clusters at larger Galactocentric distances. We find two clusters, namely IC 1805 and NGC 1893, with evidence of primordial or early dynamical mass segregation. Implications of primordial mass segregation on the formation of massive stars and recent results supporting early dynamical mass segregation are discussed. [ABSTRACT FROM AUTHOR]

The first CCD photometric UBVRI observations of the open clusters NGC 1857 (Czernik 20, OC1 427) and Czernik 25 (OC1 493) are presented. The observations were obtained on 28 November 2003 with the 2m Himalayan Chandra Telescope at Hanle using a LN2 cooled 2k x 2k CCD. The colour excess E(B-V) is found to be non-uniform in the line of sight of NGC 1857 ranging between 0.38 and 0.60 mag, while it is 0.69 mag for Cz 25. Further, the clusters NGC 1857 and Cz 25 are estimated to be at a distance of 5.75±0.8 kpc and 4.79±0.5 kpc respectively. As the log (age) of NGC 1857 is estimated to be in the range of 8.00 to 8.25 and that of Cz 25 is 8.45, they may be considered to be of intermediate age. The luminosity function (LF) and the mass function (MF) for both the clusters have been derived. The MF slopes for the clusters NGC 1857 and Cz 25 are -2.39±0.26 and -2.70±0.25 respectively. These values are in fair agreement with that given by Salpeter. Mass segregation is observed in both the clusters and it could be due to the dynamical evolution of the clusters or the imprint of star formation process or both. [ABSTRACT FROM AUTHOR]

ABSTRACT The mean absolute magnitude of the local red clump (RC), M[sup RC][sub λ] , is a very well-determined quantity owing to the availability of accurate Hipparcos parallaxes for several hundred RC stars, potentially allowing it to be used as an accurate extragalactic distance indicator. Theoretical models predict that the RC mean magnitude is dependent on both age and metallicity and, furthermore, that these dependences are non-linear. This suggests that a population correction, ΔM[sup RC][sub λ] , based on the star formation rate (SFR) and age–metallicity relation (AMR) of the system in question, should be applied to the local RC magnitude before it can be compared to the RC in any other system in order to make a meaningful distance determination. Using a sample of eight Galactic open clusters and the Galactic globular cluster 47 Tuc, we determine the cluster distances, and hence the RC absolute magnitude in V, I and K, by applying our empirical main-sequence (MS) fitting method, which utilizes a large sample of local field dwarfs with accurate Hipparcos parallaxes. The nine clusters have metallicities in the range -0.7 ≤[Fe/H]≤+0.02 and ages from 1 to 11 Gyr, enabling us to make a quantitative assessment of the age and metallicity dependences of ΔM[sup RC][sub λ] predicted by the recent theoretical models of Girardi & Salaris and Salaris & Girardi. We find excellent agreement between the empirical data and the models in all three passbands, with no statistically significant trends or offsets, thus fully confirming the applicability of the models to single-age, single-metallicity stellar populations. Since, from the models, ΔM[sup RC][sub λ] is a complicated function of both metallicity and age, if this method is used to derive distances to composite populations, it is essential to have an accurate assessment of the SFR and AMR of the system in question, if errors of several tenths of a magnitude are to be avoided. Using recent determinations of the SFR and AMR for four systems – the Large and Small Magellanic Clouds, Carina and the solar neighbourhood – we examine the quantity I[sup RC][sub obs]-K[sup RC][sub obs] , which is the difference between the mean magnitude of the RC in the I band and the K band. Comparing the theoretical predictions with the most recent observational data, we find complete agreement between the observations and the models, thus confirming even further the applicability of the population corrections predicted from theory. [ABSTRACT FROM AUTHOR]

We present multiband photometry covering ∼5° × 5° across ω Cen collected with the Dark Energy Camera on the 4 m Blanco telescope, combined with Hubble Space Telescope and Wide Field Imager data for the central regions. The unprecedented photometric accuracy and field coverage allows us to confirm the different spatial distribution of blue and red main-sequence stars, and of red giant branch (RGB) stars with different metallicities. The ratio of the number of blue to red main-sequence stars shows that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one, with the frequency of blue main-sequence stars increasing at a distance of ∼20′ from ω Cen’s center. Similarly, the more metal-rich RGB stars show a more extended spatial distribution compared to the more metal-poor ones in the outskirts of the cluster. Moreover, the centers of the distributions of metal-rich and metal-poor RGB stars are shifted in different directions with respect to the geometrical center of ω Cen. We constructed stellar density profiles for the blue and red main-sequence stars; they show that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one in the outskirts of ω Cen, confirming the results based on the number ratio. We also computed the ellipticity profile of ω Cen, which has a maximum value of 0.16 at a distance of ∼8′ from the center, and a minimum of 0.05 at ∼30′; the average ellipticity is ∼0.10. The circumstantial evidence presented in this work suggests a merging scenario for the formation of the peculiar stellar system ω Cen. [ABSTRACT FROM AUTHOR]

We present the results of the Gould’s Belt Distances Survey of young star-forming regions toward the Orion Molecular Cloud Complex. We detected 36 young stellar objects (YSOs) with the Very Large Baseline Array, 27 of which have been observed in at least three epochs over the course of two years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions toward these stars to study the structure and kinematics of the Complex. We measured a distance of 388 ± 5 pc toward the Orion Nebula Cluster, 428 ± 10 pc toward the southern portion L1641, 388 ± 10 pc toward NGC 2068, and roughly ∼420 pc toward NGC 2024. Finally, we observed a strong degree of plasma radio scattering toward λ Ori. [ABSTRACT FROM AUTHOR]

The TW Hya Association (TWA) is a nearby stellar association with an age of ∼5–10 Myr. This is an important age for studying the late stages of star and planet formation. We measure the parallaxes of 14 candidate members of TWA. That brings to 38 the total number of individual stars with fully measured kinematics, i.e., proper motion, radial velocity, and parallax, to describe their motions through the Galaxy. We analyze these kinematics to search for convergence to a smaller volume in the past, but we find that the association is never much more compact than it is at present. We show that it is difficult to measure traceback ages for associations such as TWA that have expected velocity dispersions of 1–2 km s−1 with typical measurement uncertainties. We also use our stellar distances and pre-main-sequence evolutionary tracks to find the average age of the association of 7.9 ± 1.0 Myr. Additionally, our parallax measurement of TWA 32 indicates that it should be considered a bona fide member of TWA. Two new candidate members have high membership probabilities, and we assign them TWA numbers: TWA 45 for 2MASS J11592786–4510192 and TWA 46 for 2MASS J12354615–4115531. [ABSTRACT FROM AUTHOR]