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New, deep HST photometry allowed us to identify and study eight compact and bright (M_V< -5.8) star clusters in the outskirts of the star-forming isolated dwarf galaxy NGC5238 (M_*= 10^8 M_sun). Five of these clusters are new discoveries, and six appear projected onto, and/or aligned with the tidal tails recently discovered around this galaxy. The clusters are partially resolved into stars and their colour magnitude diagrams reveal a well developed red giant branch, implying ages older than 1-2~Gyr. Their integrated luminosity and structural parameters are typical of classical globular clusters and one of them has M_V=-10.56 +/- 0.07, as bright as Omega Cen, the brightest globular cluster of the Milky Way. Since the properties of this cluster are in the range spanned by those of nuclear star clusters we suggest that it may be the nuclear remnant of the disrupted satellite of NGC5238 that produced the observed tidal tails., Comment: Accepted for publication by A&A Letters. Latex, 10 pages, 6 figures, 5 appendices

Pre-main sequence disk accretion is pivotal in determining the final stellar properties and the early conditions for close-in planets. We aim to establish the impact of internal (stellar mass) and external (radiation field) parameters on disk evolution in the Lagoon Nebula massive star-forming region. We employ simultaneous $u,g,r,i,H\alpha$ time series photometry, archival infrared data, and high-precision $K2$ light curves, to derive stellar, disk, and accretion properties for 1012 Lagoon Nebula members. Of all young stars in the Lagoon Nebula, we estimate $34\%-37\%$ have inner disks traceable down to $\sim 12$ $\mu$m, while $38\%-41\%$ are actively accreting. We detect disks $\sim$1.5 times more frequently around G/K/M stars than higher-mass stars, which appear to deplete their inner disks on shorter timescales. We find tentative evidence for faster disk evolution in the central regions of the Lagoon Nebula, where the bulk of the O/B population is located. Conversely, disks appear to last longer at its outskirts, where the measured fraction of disk-bearing stars tends to exceed those of accreting and disk-free stars. The derived mass accretion rates show a non-uniform dependence on stellar mass between $\sim 0.2-5$ $M_\odot$. In addition, the typical accretion rates appear to differ across the Lagoon Nebula extension, with values two times lower in the core region than at its periphery. Finally, we detect tentative density gradients in the accretion shocks, with lags in the appearance of brightness features as a function of wavelength that can amount to $\sim7\%-30\%$ of the rotation period., Comment: 30 pages, 11 figures, two tables; accepted for publication in The Astronomical Journal

In this work we present the highest spatial and spectral resolution integral field observations to date of the bipolar jet from the Orion proplyd 244-440 using MUSE NFM) observations on the VLT. We observed a previously unreported chain of six distinct knots in a roughly S-shaped pattern, and by comparing them with HST images we estimated proper motions in the redshifted knots of 9.5 mas yr$^{-1}$ with an inclination angle of $73^{\circ}$, though these quantities could not be measured for the blueshifted lobe. Analysis of the [FeII] and [NiII] lines suggests jet densities on the order of $\sim 10^5$ cm$^{-3}$. We propose that the observed S-shaped morphology originates from a jet launched by a smaller source with $M_\star < 0.2$ M$_{\odot}$ in orbital motion around a larger companion of $M_\star \simeq 0.5$ M$_{\odot}$ at a separation of 30-40 au. The measured luminosities of the knots using the [OI]$\lambda6300$ and [SII]$\lambda6731$ lines were used to estimate a lower limit to the mass-loss rate in the jet of $1.3 \times 10^{-11}$ M$_{\odot}$ yr$^{-1}$ and an upper limit of $10^{-9}$ M$_{\odot}$ yr$^{-1}$, which is typical for low-mass driving sources. While the brightness asymmetry between the redshifted and blueshifted lobes is consistent with external irradiation, further analysis of the [NiII] and [FeII] lines suggests that photoionization of the jet is not likely to be a dominant factor, and that the emission is dominated by collisional excitation. The dynamical age of the jet compared to the anticipated survival time of the proplyd demonstrates that photoevaporation of the proplyd occurred prior to jet launching, and that this is still an active source. These two points suggest that the envelope of the proplyd may shield the jet from the majority of external radiation, and that photoionization of the proplyd does not appear to impact the ability of a star to launch a jet., Comment: 30 pages, 20 figures

We characterize twelve young stellar objects (YSOs) located in the Lupus I region, spatially overlapping with the Upper Centaurus Lupus (UCL) sub-stellar association. The aim of this study is to understand whether the Lupus I cloud has more members than what has been claimed so far in the literature and gain a deeper insight into the global properties of the region. We selected our targets using Gaia DR2 catalog, based on their consistent kinematic properties with the Lupus I bona fide members. In our sample of twelve YSOs observed by X-Shooter, we identified ten Lupus I members. We could not determine the membership status of two of our targets, namely Gaia DR2 6014269268967059840 and 2MASS J15361110-3444473 due to technical issues. We found out that four of our targets are accretors, among them 2MASS J15551027-3455045, with a mass of ~0.03 M_Sun, is one of the least massive accretors in the Lupus complex to date. Several of our targets (including accretors) are formed in-situ and off-cloud with respect to the main filaments of Lupus I, hence, our study may hint that there are diffused populations of M-dwarfs around Lupus I main filaments. In this context, we would like to emphasize that our kinematic analysis with Gaia catalogs played a key role in identifying the new members of the Lupus I cloud., Comment: 20 pages, 15 Tables, 13 Figures, accepted for publication in A&A

Context: Tidal tails of star clusters are commonly understood to be populated symmetrically. Recently, the analysis of Gaia data revealed large asymmetries between the leading and trailing tidal tail arms of the four open star clusters Hyades, Praesepe, Coma Berenices and NGC 752. Aims: As the evaporation of stars from star clusters into the tidal tails is a stochastic process, the degree of stochastic asymmetry is quantified in this work. Methods: For each star cluster 1000 configurations of test particles are integrated in the combined potential of a Plummer sphere and the Galactic tidal field over the life time of the particular star cluster. For each of the four star clusters the distribution function of the stochastic asymmetry is determined and compared with the observed asymmetry. Results: The probabilities for a stochastic origin of the observed asymmetry of the four star clusters are: Praesepe ~1.7 sigma, Coma Berenices ~2.4 sigma, Hyades ~6.7 sigma, NGC 752 ~1.6 sigma. Conclusions: In the case of Praesepe, Coma Berenices and NGC 752 the observed asymmetry can be interpreted as a stochastic evaporation event. However, for the formation of the asymmetric tidal tails of the Hyades additional dynamical processes beyond a pure statistical evaporation effect are required., Comment: accepted for publication by A&A

We use panoramic optical spectroscopy obtained with MUSE@VLT to investigate the nature of five candidate extremely isolated low-mass star forming regions (Blue Candidates, BCs hereafter) toward the Virgo cluster of galaxies. Four of the five (BC1, BC3, BC4, BC5) are found to host several HII regions and to have radial velocities fully compatible with being part of the Virgo cluster. All the confirmed candidates have mean metallicity significantly in excess of that expected from their stellar mass, indicating that they originated from gas stripped from larger galaxies. In summary, these four candidates share the properties of the prototype system SECCO 1, suggesting the possible emergence of a new class of stellar systems, intimately linked to the complex duty cycle of gas within clusters of galaxies. A thorough discussion on the nature and evolution of these objects is presented in a companion paper, where the results obtained here from MUSE data are complemented with Hubble Space Telescope (optical) and Very Large Array (HI) observations., Comment: accepted for publication by ApJ. Latex. 16 pages, 8 color figures

We study the accretion properties of pre-main sequence low-mass stars in the LH 91 association within the Large Magellanic Clouds. Using optical multiband photometry obtained with the Hubble Space Telescope, we identify 75 candidates showing H$\alpha$ excess emission above the 3$\sigma$ level with equivalent width $EW_{H\alpha}$ $\ge$ 10 \AA. We estimate the physical parameters (effective temperature, luminosity, age, mass, accretion luminosity and mass accretion rate) of the pre-main sequence stellar candidates. The age distribution suggests a star formation ranging from few Myr up to $\sim$ 60 Myr with a gap between $\sim$ 5 Myr and 10 Myr. The masses of the PMS candidates span from 0.2 $M_{\odot}$ for the cooler objects to 1.0 $M_{\odot}$ with a median of $\sim$ 0.80 $M_{\odot}$. The median value of the accretion luminosity of our 75 PMS stars is about 0.12 $L_{\odot}$, the median value of the mass accretion rate is about 4.8 $\times$ $10^{-9}$ $M_{\odot} yr^{-1}$ with higher values for the younger population ($\sim$ 1.2 $\times$ $10^{-8}$ $M_{\odot} yr^{-1}$), and lower values for the older candidates ($\sim$ 4.7 $\times$ $10^{-9}$ $M_{\odot} yr^{-1}$). We compare our results with the findings on LH 95, the closer region to LH 91 for which accretion properties of PMS candidates were derived. An interesting qualitative outcome is that LH 91 seems to be in a more evolved stage. Moreover, we find that the PMS candidates are distributed homogeneously, without any evidence of clumps around more massive stars., Comment: A&A, 2022

Using Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar "bar" coincides with an HI high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km/s, compatible with an inflow/outflow, and derive a steeply-rising rotation curve and high HI surface density at the center, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy center and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended HI disc and the central HI spiral arms. The high central HI density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf., Comment: Accepted for publication in MNRAS. Corrected typo in the list of authors

We present a detailed 3D kinematic analysis of the central regions ($R<30''$) of the low-mass and dynamically evolved galactic globular cluster NGC 6362. The study is based on data obtained with ESO-VLT/MUSE used in combination with the adaptive optics module and providing $\sim3000$ line-of-sight radial velocities, which have been complemented with Hubble Space Telescope proper motions. The quality of the data and the number of available radial velocities allowed us to detect for the first time a significant rotation signal along the line of sight in the cluster core with amplitude of $\sim 1$ km/s and with a peak located at only $\sim20''$ from the cluster center, corresponding to only $\sim10\%$ of the cluster half-light radius. This result is further supported by the detection of a central and significant tangential anisotropy in the cluster innermost regions. This is one of the most central rotation signals ever observed in a globular cluster to date. We also explore the rotational properties of the multiple populations hosted by this cluster and find that Na-rich stars rotate about two times more rapidly than the Na-poor sub-population thus suggesting that the interpretation of the present-day globular cluster properties require a multi-component chemo-dynamical approach. Both the rotation amplitude and peak position would fit qualitatively the theoretical expectations for a system that lost a significant fraction of its original mass because of the long-term dynamical evolution and interaction with the Galaxy. However, to match the observations more quantitatively further theoretical studies to explore the initial dynamical properties of the cluster are needed., Comment: 13 pages, 12 figures, 2 tables; accepted for publication in MNRAS

The evolution of young stars and disks is driven by the interplay of several processes, notably accretion and ejection of material. Critical to correctly describe the conditions of planet formation, these processes are best probed spectroscopically. About five-hundred orbits of the Hubble Space Telescope (HST) are being devoted in 2020-2022 to the ULLYSES public survey of about 70 low-mass (M<2Msun) young (age<10 Myr) stars at UV wavelengths. Here we present the PENELLOPE Large Program that is being carried out at the ESO Very Large Telescope (VLT) to acquire, contemporaneous to HST, optical ESPRESSO/UVES high-resolution spectra to investigate the kinematics of the emitting gas, and UV-to-NIR X-Shooter medium-resolution flux-calibrated spectra to provide the fundamental parameters that HST data alone cannot provide, such as extinction and stellar properties. The data obtained by PENELLOPE have no proprietary time, and the fully reduced spectra are made available to the whole community. Here, we describe the data and the first scientific analysis of the accretion properties for the sample of thirteen targets located in the Orion OB1 association and in the sigma-Orionis cluster, observed in Nov-Dec 2020. We find that the accretion rates are in line with those observed previously in similarly young star-forming regions, with a variability on a timescale of days of <3. The comparison of the fits to the continuum excess emission obtained with a slab model on the X-Shooter spectra and the HST/STIS spectra shows a shortcoming in the X-Shooter estimates of <10%, well within the assumed uncertainty. Its origin can be either a wrong UV extinction curve or due to the simplicity of this modelling, and will be investigated in the course of the PENELLOPE program. The combined ULLYSES and PENELLOPE data will be key for a better understanding of the accretion/ejection mechanisms in young stars., Comment: Accepted for publication on Astronomy & Astrophysics. 15 pages + appendix, language edited version

The formation and evolutionary processes of galaxy bulges are still unclear, and the presence of young stars in the bulge of the Milky Way is largely debated. We recently demonstrated that Terzan 5, in the Galactic bulge, is a complex stellar system hosting stars with very different ages and a striking chemical similarity to the field population. This indicates that its progenitor was likely one of the giant structures that are thought to generate bulges through coalescence. Here we show that another globular cluster-like system in the bulge (Liller 1) hosts two distinct stellar populations with remarkably different ages: only 1-3 Gyr for the youngest, 12 Gyr for the oldest, which is impressively similar to the old component of Terzan 5. This discovery classifies Liller 1 and Terzan 5 as sites of recent star formation in the Galactic bulge and provides clear observational proof that the hierarchical assembly of primordial massive structures contributed to the formation of the Milky Way spheroid., Comment: 29 pages, 10 figures, typo in author affiliation fixed, published in Nature Astronomy

We present the first analysis of the stellar content of the structures and substructures identified in the peculiar star-forming galaxy NGC5474, based on Hubble Space Telescope resolved photometry from the LEGUS survey. NGC5474 is a satellite of the giant spiral M101, and is known to have a prominent bulge that is significantly off-set from the kinematic center of the underlying HI and stellar disc. The youngest stars (age~ 100 Myr) trace a flocculent spiral pattern extending out to ~8 kpc from the center of the galaxy. On the other hand intermediate-age (age > 500 Myr) and old (age > 2 Gyr) stars dominate the off-centred bulge and a large substructure residing in the South Western part of the disc and not correlated with the spiral arms (SW over-density). The old age of the stars in the SW over-density suggests that this may be another signature of the dynamical interaction/s that have shaped this anomalous galaxy. We suggest that a fly by with M101, generally invoked as the origin of the anomalies, may not be sufficient to explain all the observations. A more local and more recent interaction may help to put all the pieces of this galactic puzzle together., Comment: Accepted for publication on Astronomy & Astrophysics. Latex, 10 pages, 12 figures (colors)

Blue Stragglers are stars located in an unexpected region of the color-magnitude diagram of a stellar population, as they appear bluer and more luminous than the stars in the turnoff region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the blue straggler population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with FLAMES@VLT. We also analyze the radial distribution of the blue straggler population to probe the dynamical status of the cluster. Blue straggler candidates were identified first with Gaia DR2, according to their position on the CMD, proper motions, and parallaxes. Their radial distribution was compared with those of the main sequence, red giant, and red clump stars, to evaluate mass segregation. Additionally, their radial velocities (and the associated uncertainties) were compared with the mean radial velocity and the velocity dispersion of the cluster. When possible, close binaries and long-period binaries were also identified, based on the radial velocity variations for the different epochs. We also looked for yellow stragglers, i.e., possible evolved blue stragglers. We found 53 blue stragglers members of Collinder 261, six of them already identified in previous catalogs. Among the blue straggler candidates with radial velocity measurements, we found one long-period binary, five close-binary systems, three non-variable stars; we also identified one yellow straggler., Comment: Accepted for publication in the Astronomical Journal (16 pages, 8 figures)

The Smallest Scale of Hierarchy (SSH) survey is an ongoing strategic large program at the Large Binocular Telescope, aimed at the detection of faint stellar streams and satellites around 45 late-type dwarf galaxies located in the Local Universe within $\simeq$10 Mpc. SSH exploits the wide-field, deep photometry provided by the Large Binocular Cameras in the two wide filters $g$ and $r$. This paper describes the survey, its goals, and the observational and data reduction strategies. We present preliminary scientific results for five representative cases (UGC 12613, NGC 2366, UGC 685, NGC 5477 and UGC 4426) covering the whole distance range spanned by the SSH targets. We reach a surface brightness limit as faint as $\mu(r)\sim$ 31 mag arcsec$^{-2}$ both for targets closer than 4$-$5 Mpc, which are resolved into individual stars, and for more distant targets through the diffuse light. Our analysis reveals the presence of extended low surface brightness stellar envelopes around the dwarfs, reaching farther out than what traced by the integrated light, and as far out as, or even beyond, the observed HI disk. Stellar streams, arcs, and peculiar features are detected in some cases, indicating possible perturbation, accretion, or merging events. We also report on the discovery of an extreme case of Ultra Diffuse Galaxy ($\mu_g(0)=27.9$~mag/arcsec$^2$) in the background of one of our targets, to illustrate the power of the survey in revealing extremely low surface brightness systems., Comment: Accepted for publication in MNRAS

We used resolved star counts from Hubble Space Telescope images to determine the center of gravity and the projected density profiles of 6 old globular clusters in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257 and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more than 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similarly to what observed for Galactic globular clusters, the ratio between the effective and the core radii has been found to anti-correlate with the cluster dynamical age., Comment: 15 pages, 12 figures, in press on the ApJ

The distribution of size as a function of age observed for star clusters in the Large Magellanic Cloud (LMC) is very puzzling: young clusters are all compact, while the oldest systems show both small and large sizes. It is commonly interpreted as due to a population of binary black holes driving a progressive expansion of cluster cores. Here we propose, instead, that it is the natural consequence of the fact that only relatively low-mass clusters have formed in the last ~3 Gyr in the LMC and only the most compact systems survived and are observable. The spread in size displayed by the oldest (and most massive) clusters, instead, can be explained in terms of initial conditions and internal dynamical evolution. To quantitatively explore the role of the latter, we selected a sample of five coeval and old LMC clusters with different sizes, and we estimated their dynamical age from the level of central segregation of blue straggler stars (the so-called dynamical clock). Similarly to what found in the Milky Way, we indeed measure different levels of dynamical evolution among the selected coeval clusters, with large-core systems being dynamically younger than those with small size. This behaviour is fully consistent with what expected from internal dynamical evolution processes over timescales mainly set by the structure of each system at formation., Comment: In press in Nature Astronomy, 9 September 2019 issue. 24 pages, 7 figures, 2 tables

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses being represented by the canonical IMF. As a consequence the galaxy-wide IMF (gwIMF), defined as the sum of the IMFs of all star forming regions, should also be invariant. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation we use the IMF determined in resolved star clusters and apply the IGIMF-theory to calculate a grid of gwIMF models for metallicities, -3<[Fe/H]<1, and galaxy-wide star formation rates, $10^{-5}$ 1\,M_\odot$/yr, which is a common condition in the early Universe, we find that the gwIMF is top-heavy (more massive stars), when compared to the canonical IMF. For a SFR $< 1\,\mathrm{M_{\odot}/yr}$ the gwIMF becomes top-light regardless of the metallicity. For metallicities $\mathrm{[Fe/H]} > 0$ the gwIMF can become bottom-heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top-heavy within the first few hundred Myr of the galaxy's life and that it evolves into a bottom-heavy gwIMF in the metal-enriched galactic center. We study the SFR$-$H$\alpha$ relation, its dependency on metallicity and the SFR, the correction factors to the Kennicutt SFR$_{\rm K}-$H$\alpha$ relation, and provide new fitting functions Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for the gas-consumption time scales and for the main sequence of galaxies. The Leo P and ultra-faint dwarf galaxies are discussed explicitly. [abridged], Comment: Astronomy and Astrophysics (A&A) in press. 15 pages, 8 figures

As part of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters, we present a detailed investigation of the internal kinematics of NGC 5986. The analysis is based on about 300 individual radial velocities of stars located at various distances from the cluster center, up to 300 arcseconds (about 4 half-mass radii). Our analysis reveals the presence of a solid-body rotation extending from the cluster center to the outermost regions probed by the data, and a velocity dispersion profile initially declining with the distance from the cluster's center, but flattening and staying constant at ~5 km/s for distances larger than about one half-mass radius. This is the first globular cluster for which evidence of the joint presence of solid-body rotation and flattening in the outer velocity dispersion profile is found. The combination of these two kinematical features provides a unique opportunity to shed light on fundamental aspects of globular cluster dynamics and probe the extent to which internal relaxation, star escape, angular momentum transport and loss, and the interaction with the Galaxy tidal field can affect a cluster's dynamical evolution and determine its current kinematical properties. We present the results of a series of N-body simulations illustrating the possible dynamical paths leading to kinematic features like those observed in this cluster and the fundamental dynamical processes that underpin them., Comment: 22 pages, 8 figure, ApJ in press

The parameter A+, defined as the area enclosed between the cumulative radial distribution of blue straggler stars (BSSs) and that of a reference population, is a powerful indicator of the level of BSS central segregation. As part of the Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GCs), here we present the BSS population and the determination of A+ in 27 GCs observed out to about one half-mass radius. In combination with 21 additional clusters discussed in a previous paper this provides us with a global sample of 48 systems (corresponding to \sim 32\% of the Milky Way GC population), for which we find a strong correlation between A+ and the ratio of cluster age to the current central relaxation time. Tight relations have been found also with the core radius and the central luminosity density, which are expected to change with the long-term cluster dynamical evolution. An interesting relation is emerging between A+ and the ratio of the BSS velocity dispersion relative to that of main sequence turn-off stars, which measures the degree of energy equipartition experienced by BSSs in the cluster. These results provide further confirmation that BSSs are invaluable probes of GC internal dynamics and A+ is a powerful dynamical clock., Comment: Accepted for publication in the ApJ

In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters, we present the line-of-sight rotation curve and velocity dispersion profile of M5 (NGC 5904), as determined from the radial velocity of more than 800 individual stars observed out to 700" (~ 5 half-mass radii) from the center. We find one of the cleanest and most coherent rotation patterns ever observed for globular clusters, with a very stable rotation axis (having constant position angle of 145^o at all surveyed radii) and a well-defined rotation curve. The density distribution turns out to be flattened in the direction perpendicular to the rotation axis, with a maximum ellipticity of 0.15. The rotation velocity peak (~3 km/s in projection) is observed at ~0.6 half-mass radii, and its ratio with respect to the central velocity dispersion (~0.3-0.4 at 4 projected half-mass radii) indicates that ordered motions play a significant dynamical role. This result strengthens the growing empirical evidence of the kinematic complexity of Galactic globular clusters and motivates the need of fundamental investigations of the role of angular momentum in collisional stellar dynamics., Comment: 27 pages, 11 figures, ApJ submitted. Text overlap with arXiv:1804.08618 because the two papers are part of the MIKiS Survey series

We present the first results of the Multi-Instrument Kinematic Survey of Galactic Globular Clusters, a project aimed at exploring the internal kinematics of a representative sample of Galactic globular clusters from the radial velocity of individual stars, covering the entire radial extension of each system. This is achieved by exploiting the formidable combination of multi-object and integral field unit spectroscopic facilities of the ESO Very Large Telescope. As a first step, here we discuss the results obtained for 11 clusters from high and medium resolution spectra acquired through a combination of FLAMES and KMOS observations. We provide the first kinematical characterization of NGC 1261 and NGC 6496. In all the surveyed systems, the velocity dispersion profile declines at increasing radii, in agreement with the expectation from the King model that best fits the density/luminosity profile. In the majority of the surveyed systems we find evidence of rotation within a few half-mass radii from the center. These results are in general overall agreement with the predictions of recent theoretical studies, suggesting that the detected signals could be the relic of significant internal rotation set at the epoch of the cluster's formation., Comment: 39 pages, 12 figures, ApJ, in press

SECCO1 is an extremely dark, low-mass (M_star=10^5 M_sun), star-forming stellar system lying in the Low Velocity Cloud (LVC) substructure of the Virgo cluster of galaxies, and hosting several HII regions. Here we review our knowledge of this remarkable system, and present the results of (a) additional analysis of our panoramic spectroscopic observations with MUSE, (b) the combined analysis of Hubble Space Telescope and MUSE data, and (c) new narrow-band observations obtained with OSIRIS@GTC to search for additional HII regions in the surroundings of the system. We provide new evidence supporting an age as young as 4 Myr for the stars that are currently ionising the gas in SECCO1. We identify only one new promising candidate HII region possibly associated with SECCO1, thus confirming the extreme isolation of the system. We also identify three additional candidate pressure-supported dark clouds in Virgo among the targets of the SECCO survey. Various possible hypotheses for the nature and origin of SECCO1 are considered and discussed, also with the help of dedicated hydrodynamical simulations showing that a hydrogen cloud with the characteristics of SECCO1 can likely survive for >1 Gyr while traveling within the LVC Intra Cluster Medium., Comment: Accepted for publication by MNRAS. PDFLatex. 21 pages, 16 figures, 4 tables

As part of the Accretion Discs in H$\alpha$ with OmegaCAM (ADHOC) survey, we imaged in r, i and H-alpha a region of 12x8 square degrees around the Orion Nebula Cluster. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster's center. Although several explanations can be invoked to explain these sequences we are left with two competitive, but intriguing, scenarios: a population of unresolved binaries with an exotic mass ratio distribution or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt for a revised look at the formation mode and early evolution of stars in clusters., Comment: A&A in print

We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high velocity clouds. Our analysis shows that the two systems are distant (D>20 Mpc) dwarf irregular galaxies unrelated to the local HI clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified Ultra Diffuse Galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ~2.0 Mpc. Moreover, one of them has a very high gas content compared to galaxies of similar stellar mass, with a HI to stellar mass ratio M_HI/M_* ~90, typical of almost-dark dwarfs. Expanding on this finding, we show that extended dwarf irregulars overlap the distribution of UDGs in the M_V vs. log(r_e) plane and that the sequence including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously populated in this plane., Comment: Accepted for publication by MNRAS. PDFLatex. 8 pages, 4 figures, 2 tables

We present the results of VLT-MUSE integral field spectroscopy of SECCO1, a faint, star-forming stellar system recently discovered as the stellar counterpart of an Ultra Compact High Velocity Cloud (HVC274.68+74.0), very likely residing within a substructure of the Virgo cluster of galaxies. We have obtained the radial velocity of a total of 38 individual compact sources identified as HII regions in the main and secondary body of the system, and derived the metallicity for 18 of them. We provide the first direct demonstration that the two stellar bodies of SECCO1 are physically associated and that their velocities match the HI velocities. The metallicity is quite uniform over the whole system, with a dispersion sigma_12+log(O/H/)=0.08, lower than the uncertainty on individual metallicity estimates. The mean abundance, 12+log(O/H)=8.44, is much higher than the typical values for local dwarf galaxies of similar stellar mass. This strongly suggests that the SECCO~1 stars were born from a pre-enriched gas cloud, possibly stripped from a larger galaxy. Using archival HST images we derive a total stellar mass of ~1.6 X 10^5 M_sun for SECCO1, confirming that it has a very high HI to stellar mass ratio for a dwarf galaxy, M_HI/M_*~ 100. The star formation rate, derived from the H_alpha flux is a factor of more than 10 higher than in typical dwarf galaxies of similar luminosity., Comment: Accepted for publication by MNRAS. pdfLaTeX. 10 pages, 4 figures, 4 tables

We present the results of the analysis of deep photometric data for a sample of three Galactic globular clusters (NGC5466, NGC6218 and NGC6981) with the aim of estimating their degree of mass segregation and testing the predictions of analytic dynamical models. The adopted dataset, composed by both Hubble Space Telescope and ground based data, reaches the low-mass end of the mass functions of these clusters from the center up to their tidal radii allowing to derive their radial distribution of stars with different masses. All the analysed clusters show evidence of mass segregation with the most massive stars more concentrated than low-mass ones. The structures of NGC5466 and NGC6981 are well reproduced by multimass dynamical models adopting a lowered-Maxwellian distribution function and the prescription for mass segregation given by Gunn & Griffin (1979). Instead, NGC6218 appears to be more mass segregated than model predictions. By applying the same technique to mock observations derived from snapshots selected from suitable N-body simulations we show that the deviation from the behaviour predicted by these models depends on the particular stage of dynamical evolution regardless of initial conditions., Comment: 12 pages, 7 figures, accepted for publication by MNRAS

Studies during the last decade have revealed that nearly all Globular Clusters (GCs) host multiple populations (MPs) of stars with a distinctive chemical patterns in light elements. No evidence of such MPs has been found so far in lower-mass ($< \sim 10^4$ M$_{\odot}$) open clusters nor in intermediate age (1-2 Gyr) massive ($> 10^5$ M$_{\odot}$) clusters in the Local Group. Young massive clusters (YMCs) have masses and densities similar to those expected of young GCs in the early universe, and their near-infrared (NIR) spectra are dominated by the light of red super giants (RSGs). The spectra of these stars may be used to determine the cluster's abundances, even though the individual stars cannot be spatially resolved from one another. We carry out a differential analysis between the Al lines of YMC NGC 1705: 1 and field Small Magellanic Cloud RSGs with similar metallicities. We exclude at high confidence extreme [Al/Fe] enhancements similar to those observed in GCs like NGC 2808 or NGC 6752. However, smaller variations cannot be excluded., Comment: 7 pages, 4 figures, 1 table, MNRAS accepted

SECCO is a survey devoted to the search for stellar counterparts within Ultra Compact High Velocity Clouds. In this contribution we present the results of a set of simulations aimed at the quantitative estimate of the sensitivity of the survey as a function of the total luminosity, size and distance of the stellar systems we are looking for. For all our synthetic galaxies we assumed an exponential surface brightness profile and an old and metal-poor population. The synthetic galaxies are simulated both on the images and on the photometric catalogs, taking into account all the observational effects. In the fields where the available observational material is of the top quality we detect synthetic galaxies as >=5 sigma over-densities of resolved stars down to muV,h=30.0 mag/arcsec2, for D<=1.5 Mpc, and down to muV,h~29.5 mag/arcsec2, for D<=2.5 Mpc. In the field with the worst observational material of the whole survey we detect synthetic galaxies with muV,h<=28.8 mag/arcsec2 out to D<=1.0 Mpc, and those with muV,h<=27.5 mag/arcsec2 out to D<=2.5 Mpc. Dwarf galaxies with MV=-10, with sizes in the range spanned by known dwarfs, are detected by visual inspection of the images up to D=5 Mpc independently of the image quality. In the best quality images dwarfs are partially resolved into stars up to D=3.0 Mpc, and completely unresolved at D=5 Mpc. As an independent test of the sensitivity of our images to low surface brightness galaxies we report on the detection of several dwarf spheroidal galaxies probably located in the Virgo cluster with MV<=-8.0 and muV,h<=26.8 mag/arcsec2. The nature of the previously discovered SECCO 1 stellar system, also likely located in the Virgo cluster, is re-discussed in comparison with these dwarfs. While specific for the SECCO survey, our study may also provide general guidelines for detection of faint stellar systems with 8m class telescopes., Comment: accepted for publication on A&A

We used a combination of Hubble Space Telescope and ground based data to probe the dynamical state of the low mass Galactic globular cluster NGC 6101. We have re-derived the structural parameters of the cluster by using star counts and we find that it is about three times more extended than thought before. By using three different indicators, namely the radial distribution of Blue Straggler Stars, that of Main Sequence binaries and the luminosity (mass) function, we demonstrated that NGC 6101 shows no evidence of mass segregation, even in the innermost regions. Indeed, both the BSS and the binary radial distributions fully resemble that of any other cluster population. In addition the slope of the luminosity (mass) functions does not change with the distance, as expected for non relaxed stellar systems. NGC 6101 is one of the few globulars where the absence of mass segregation has been observed so far. This result provides additional support to the use of the "dynamical clock" calibrated on the radial distribution of the Blue Stragglers as a powerful indicator of the cluster dynamical age., Comment: Accepted for publication by ApJ; 33 pages, 13 figures

The presence of multiple populations in globular clusters has been well established thanks to high-resolution spectroscopy. It is widely accepted that distinct populations are a consequence of different stellar generations: intra-cluster pollution episodes are required to produce the peculiar chemistry observed in almost all clusters. Unfortunately, the progenitors responsible have left an ambiguous signature and their nature remains unresolved. To constrain the candidate polluters, we have measured lithium and aluminium abundances in more than 180 giants across three systems: NGC~1904, NGC~2808, and NGC~362. The present investigation along with our previous analysis of M12 and M5 affords us the largest database of simultaneous determinations of Li and Al abundances. Our results indicate that Li production has occurred in each of the three clusters. In NGC~362 we detected an M12-like behaviour, with first and second-generation stars sharing very similar Li abundances favouring a progenitor that is able to produce Li, such as AGB stars. Multiple progenitor types are possible in NGC~1904 and NGC~2808, as they possess both an intermediate population comparable in lithium to the first generation stars and also an extreme population, that is enriched in Al but depleted in Li. A simple dilution model fails in reproducing this complex pattern. Finally, the internal Li variation seems to suggest that the production efficiency of this element is a function of the cluster's mass and metallicity - low-mass or relatively metal-rich clusters are more adept at producing Li., Comment: Accepted for publication in MNRAS. 10 pages, 8 figures

Within the SECCO survey we identified a candidate stellar counterpart to the Ultra Compact High Velocity Cloud (UCHVC) HVC274.68+74.70-123, that was suggested by Adams et al. (2013) as a possible mini-halo within the Local Group of galaxies. The spectroscopic follow-up of the brightest sources within the candidate reveals the presence of two HII regions whose radial velocity is compatible with physical association with the UVHVC. The available data does not allow us to give a definite answer on the nature of the newly identified system. A few alternative hypotheses are discussed. However, the most likely possibility is that we have found a new faint dwarf galaxy residing in the Virgo cluster of galaxies, which we name SECCO-1. Independently of its actual distance, SECCO-1 displays a ratio of neutral hydrogen mass to V luminosity of M_{HI}/L_V>= 20, by far the largest among local dwarfs. Hence, it appears as a nearly star-less galaxy and it may be an example of the missing links between normal dwarfs and the dark mini halos that are predicted to exist in large numbers according to the currently accepted cosmological model., Comment: Accepted for publication in ApJ Letters. Pdflatex, emulateapj.cls. 6 pages, 3 figures, 2 tables

Two separated sequences of blue straggler stars (BSSs) have been revealed by Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way globular cluster M30. Their presence has been suggested to be related to the two BSS formation channels (namely, collisions and mass-transfer in close binaries) operating within the same stellar system. The blue sequence was indeed found to be well reproduced by collisional BSS models. In contrast, no specific models for mass transfer BSSs were available for an old stellar system like M30. Here we present binary evolution models, including case-B mass transfer and binary merging, specifically calculated for this cluster. We discuss in detail the evolutionary track of a $0.9+0.5 M_\odot$ binary, which spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old cluster. We also run Monte-Carlo simulations to study the distribution of mass transfer BSSs in the CMD and to compare it with the observational data. Our results show that: (1) the color and magnitude distribution of synthetic mass transfer BSSs defines a strip in the CMD that nicely matches the observed red BSS sequence, thus providing strong support to the mass transfer origin for these stars; (2) the CMD distribution of synthetic BSSs never attains the observed location of the blue BSS sequence, thus reinforcing the hypothesis that the latter formed through a different channel (likely collisions); (3) most ($\sim 60\%$) of the synthetic BSSs are produced by mass-transfer models, while the remaining $< 40\%$ requires the contribution from merger models., Comment: 8 pages, 5 figures, accepted to ApJ

We present an imaging survey aimed at searching for the stellar counterparts of recently discovered ultra-compact high-velocity HI clouds (UCHVC). Adams et al. (2013) proposed these clouds to be candidate mini-haloes in the Local Group and/or its surroundings, within a distance range of 0.25-2.0 Mpc. Using the Large Binocular Telescope we obtain wide-field (~ 23' X 23') g- and r-band images of the twenty-five most promising and most compact clouds among the fifty-nine identified by Adams et al. Careful visual inspection of all the images does not reveal any stellar counterpart even slightly resembling LeoP, the only local dwarf galaxy that was found as a counterpart to a previously detected high velocity cloud. Only a possible distant (D>3.0 Mpc) counterpart to HVC274.68+74.70-123 has been identified on our images. The point source photometry in the central 17.3' X 7.7' chips reaches r<= 26.5, and is expected to contain most of the stellar counterparts to the UCHVCs. However, no obvious stellar over-density is detected in any of our fields, in marked contrast to our comparison LeoP field in which the dwarf galaxy is detected at a >30 sigma significance level. Only HVC352.45+59.06+263 may be associated with a weak over-density, whose nature cannot be ascertained with our data. Sensitivity tests shows that our survey would have detected any dwarf galaxy dominated by an old stellar population, with an integrated absolute magnitude M_V<= -8.0, a half-light radius r_h<= 300 pc, and lying within 1.5 Mpc from us, thereby confirming that it is unlikely that the observed UCHVCs are associated with stellar counterparts typical of known Local Group dwarf galaxies., Comment: Accepted for publication by A&A. PdfLaTex. 24 pages, 27 figures. Minor changes; A&A language editing

We present a study of the mass accretion rates of pre-Main Sequence (PMS) stars in the cluster Trumpler 14 (Tr14) in the Carina Nebula. Using optical multi-band photometry we were able to identify 356 PMS stars showing H-alpha excess emission with equivalent width EW(H-alpha)>20\AA. We interpret this observational feature as indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate ($\dot{M}_{acc}$) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PSM stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario in which old and evolved debris discs still host a significant amount of gas., Comment: 12 pages, 13 figures, accepted for publication on A&A

We present a detailed study of the stellar and HI structure of the dwarf irregular galaxy Sagittarius. We use new deep and wide field photometry to trace the surface brightness profile of the galaxy out to ~5.0' (corresponding to ~1600 pc) and down to $\mu_V\simeq 30.0$ mag/arcsec$^2$, thus showing that the stellar body of the galaxy is much more extended than previously believed, and it is similarly (or more) extended than the overall HI distribution. The whole major-axis profile is consistent with a pure exponential, with a scale radius of $\simeq 340$ pc. The surface density maps reveal that the distribution of old and intermediate-age stars is smooth and remarkably flattened out to its edges, while the associated HI has a much rounder shape, is off-centred and presents multiple density maxima and a significant hole. No clear sign of systemic rotation is detectable in the complex HI velocity field. No metallicity gradient is detected in the old and intermediate age population of the galaxy, and we confirm that this population has a much more extended distribution than young stars (age$\lt 1$ Gyr)., Comment: 11 pages, 14 figures, accepted for publication on A&A. arXiv admin note: text overlap with arXiv:1404.1697

The existence of blue straggler stars (BSS), which appear younger, hotter, and more massive than their siblings, is at odds with a simple picture of stellar evolution, as such stars should have exhausted their nuclear fuel and evolved long ago to become cooling white dwarfs. As such, BSS could just be some quirks but in fact their understanding requires a deep knowledge of many different areas in astronomy, from stellar evolution through cluster dynamics, from chemical abundances to stellar populations. In November 2012, a workshop on this important topic took place at the ESO Chilean headquarters in Santiago. The many topics covered at this workshop were introduced by very comprehensive invited reviews, providing a unique and insightful view on the field. These reviews have now become chapters of the first ever book on BSS., Comment: Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springer

In this chapter we briefly review the basics of the theory of stellar evolution., Comment: Chapter 1, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springer

We present a detailed study of the stellar and HI structure of the dwarf irregular galaxies SextansA and SextansB, members of the NGC3109 association. We use newly obtained deep (r~26.5) and wide field g,r photometry to extend the Surface Brightness (SB) profiles of the two galaxies down to mu_V~ 31.0 mag/arcsec^2. We find that both galaxies are significantly more extended than what previously traced with surface photometry, out to ~4 kpc from their centers along their major axis. Older stars are found to have more extended distribution with respect to younger populations. We obtain the first estimate of the mean metallicity for the old stars in SexB, from the color distribution of the Red Giant Branch, <[Fe/H]>=-1.6. The SB profiles show significant changes of slope and cannot be fitted with a single Sersic model. Both galaxies have HI discs as massive as their respective stellar components. In both cases the HI discs display solid-body rotation with maximum amplitude of ~50 km/s (albeit with significant uncertainty due to the poorly constrained inclination), implying a dynamical mass ~10^{9}~M_sun, a mass-to-light ratio M/L_V~25 and a dark-to-barionic mass ratio of ~10. The distribution of the stellar components is more extended than the gaseous disc in both galaxies. We find that the main, approximately round-shaped, stellar body of Sex~A is surrounded by an elongated low-SB stellar halo that can be interpreted as a tidal tail, similar to that found in another member of the same association (Antlia). We discuss these, as well as other evidences of tidal disturbance, in the framework of a past passage of the NGC3109 association close to the Milky Way, that has been hypothesized by several authors and is also supported by the recently discovered filamentary configuration of the association itself., Comment: Accepted for publication by A&A. PdfLateX, 16 pages, 11 figures, 2 appendices

Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation. Aims. To investigate the chemical abundance pattern of the old open cluster Trumpler\,5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum. Methods. One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416. Results. Star #3416 is super Li-rich with A(Li)=3.75\,dex. The lack of $^6$Li enrichment ($^6$Li/$^7$Li$<$2%), the low carbon isotopic ratio ($^{12}$C/$^{13}$C=14$\pm$3), and the lack of evidence for radial velocity variation or enhanced rotational velocity ($v\sin i = 2.8\,$\kms) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism. Conclusions. We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB., Comment: 7 pages, 2 figures. Accepted for publication in A&A Letters

In this paper we analyse the evolutionary status of three open clusters: NGC 1817, NGC 2141, and Berkeley 81. They are all of intermediate age, two are located in the Galactic anti-centre direction while the third one is located in the Galactic centre direction. All of them were observed with LBC@LBT using the Bessel B, V, and I filters. The cluster parameters have been obtained using the synthetic colour-magnitude diagram (CMD) method, i.e. the direct comparison of the observational CMDs with a library of synthetic CMDs generated with different evolutionary sets (Padova, FRANEC, and FST). This analysis shows that NGC 1817 has subsolar metallicity, age between 0.8 and 1.2 Gyr, reddening E(B-V) in the range 0.21 and 0.34, and distance modulus (m-M)_0 of about 10.9; NGC 2141 is older, with age in the range 1.25 and 1.9 Gyr, E(B-V) between 0.36 and 0.45, (m-M)_0 between 11.95 and 12.21, and subsolar metallicity; Berkeley~81 has metallicity about solar, with age between 0.75 and 1.0 Gyr, has reddening E(B-V)~0.90 and distance modulus (m-M)_0~12.4. Exploiting the large field of view of the instrument we derive the structure parameters for NGC 2141 and Be 81 by fitting a King profile to the estimated density profile. Combining this information with the synthetic CMD technique we estimate a lower limit for the cluster total mass for these two systems., Comment: 20 pages, 24 figures

We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the H{\alpha} emission properties of main sequence stars in the globular cluster 47 Tucanae. Using a combination of multi-band observations in the F606W, F814W and F658N bands, we search for stars showing H{\alpha} excess emission. An accurate photometric measurement of their H{\alpha} equivalent width allows us to identify objects with large H{\alpha} emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources and their location in the colour-magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs). We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterise candidate CVs., Comment: 13 pages, 7 figure, accepted for publication on MNRAS

We re-consider the association of dwarf galaxies around NGC3109, whose known members were NGC3109, Antlia, Sextans A and Sextans B, based on a new updated list of nearby galaxies and the most recent data. We find that the original members of the NGC3109 association, together with the recently discovered and adjacent dwarf irregular Leo P, form a very tight and elongated configuration in space. All these galaxies lie within ~100 kpc of a line that is ~1070 kpc long, from one extreme (NGC3109) to the other (Leo P), and they show a gradient in the Local Group standard of rest velocity with a total amplitude of 43 km/s Mpc and a r.m.s. scatter of just 16.8 km/s. It is shown that the reported configuration is exceptional given the known dwarf galaxies in the Local Group and its surroundings. We conclude that (a) Leo P is likely an additional member of the NGC3109 association, and (b) the association is highly ordered in space and velocity, and it is very elongated, suggesting that it has been originated by a tidal interaction or it was accreted as a filamentary sub-structure., Comment: Accepted for publication by Astronomy & Atrophysics Letters. 5 pages, 3 figures, pdflatex. Language editor corrections included

We used high-quality images acquired with the WFC3 on board the HST to probe the blue straggler star (BSS) population of the Galactic globular cluster NGC 362. We have found two distinct sequences of BSS: this is the second case, after M 30, where such a feature has been observed. Indeed the BSS location, their extension in magnitude and color and their radial distribution within the cluster nicely resemble those observed in M 30, thus suggesting that the same interpretative scenario can be applied: the red BSS sub-population is generated by mass transfer binaries, the blue one by collisions. The discovery of four new W UMa stars, three of which lying along the red-BSS sequence, further supports this scenario. We also found that the inner portion of the density profile deviates from a King model and is well reproduced by either a mild power-law (\alpha -0.2) or a double King profile. This feature supports the hypothesis that the cluster is currently undergoing the core collapse phase. Moreover, the BSS radial distribution shows a central peak and monotonically decreases outward without any evidence of an external rising branch. This evidence is a further indication of the advanced dynamical age of NGC 362: in fact, together with M 30, NGC 362 belongs to the family of dynamically old clusters (Family III) in the "dynamical clock" classification proposed by Ferraro et al. (2012). The observational evidence presented here strengthens the possible connection between the existence of a double BSS sequence and a quite advanced dynamical status of the parent cluster., Comment: Accepted for publication by ApJ; 39 pages, 16 figures, 1 table

We present a detailed investigation of the radial distribution of blue straggler star and binary populations in the Galactic globular cluster NGC 5466, over the entire extension of the system. We used a combination of data acquired with the ACS on board the Hubble Space Telescope, the LBC-blue mounted on the Large Binocular Telescope, and MEGACAM on the Canadian-France-Hawaii Telescope. Blue straggler stars show a bimodal distribution with a mild central peak and a quite internal minimum. This feature is interpreted in terms of a relatively young dynamical age in the framework of the "dynamical clock" concept proposed by Ferraro et al. (2012). The estimated fraction of binaries is 6-7% in the central region (r<90") and slightly lower (5.5%) in the outskirts, at r>200". Quite interestingly, the comparison with the results of Milone et al. (2012) suggests that also binary systems may display a bimodal radial distribution, with the position of the minimum consistent with that of blue straggler stars. If confirmed, this feature would give additional support to the scenario where the radial distribution of objects more massive than the average cluster stars is primarily shaped by the effect of dynamical friction. Moreover, this would also be consistent with the idea that the unperturbed evolution of primordial binaries could be the dominant BSS formation process in low-density environments., Comment: Accepted for publication on ApJ

Current planet formation models are largely based on the observational constraint that protoplanetary disks have lifetime 3Myr. Recent studies, however, report the existence of PMS stars with signatures of accretion (strictly connected with the presence of circumstellar disks)and photometrically determined ages of 30 Myr, or more. Here we present a spectroscopic study of two major age outliers in the ONC. We use broad band, intermediate resolution VLT/X-Shooter spectra combined with an accurate method to determine the stellar parameters and the related age of the targets to confirm their peculiar age estimates and the presence of ongoing accretion.The analysis is based on a multi-component fitting technique, which derives simultaneously SpT, extinction, and accretion properties of the objects. With this method we confirm and quantify the ongoing accretion. From the photospheric parameters of the stars we derive their position on the HRD, and the age given by evolutionary models. Together with other age indicators like the lithium equivalent width we estimate with high accuracy the age of the objects. Our study shows that the two objects analyzed are not older than the typical population of the ONC. Our results show that, while photometric determination of the photospheric parameters are an accurate method to estimate the parameters of the bulk of young stellar populations, those of individual objects with high accretion rates and extinction may be affected by large uncertainties. Broad band spectroscopic determinations should thus be used to confirm the nature of individual objects. Our analysis shows that this method allows us to obtain an accurate determination of the photospheric parameters of accreting YSOs in any nearby star-forming region. We suggest that our detailed, broad- band spectroscopy method should be used to derive accurate properties of candidate old and accreting YSOs., Comment: Accepted for publication in Astronomy & Astrophysics

We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This is the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that: (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13 and M62 is not confirmed; (2) the majority of clusters in our sample are fitted equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distances; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a secondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates with an empirical dynamical age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters., Comment: Accepted for publication in The Astrophysical Journal; 19 pages (emulateapj style), 15 figures, 2 tables

We report on the identification of the optical counterpart to the recently detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster M28. From the analysis of a multi epoch HST dataset we have identified a strongly variable star positionally coincident with the radio and Chandra X-ray sources associated to the INTEGRAL transient. The star has been detected during both a quiescent and an outburst state. In the former case it appears as a faint, unperturbed main sequence star, while in the latter state it is about two magnitudes brighter and slightly bluer than main sequence stars. We also detected Halpha excess during the outburst state, suggestive of active accretion processes by the neutron star., Comment: Accepted for publication by ApJ; 15 pages, 4 figures, 1 table

We present a combination of high-resolution Hubble Space Telescope and wide-field ground-based and Galaxy Evolution Explorer data of the Galactic Globular Cluster M10 (NGC6254). By using this large data-set we determined the center of gravity of the cluster and we built its density profile from star counts over its entire radial extension. We find that the density profile is well reproduced by a single-mass King model with structural parameters c=1.41 and r_c=41". We also studied the Blue Straggler Star population and its radial distribution. We count a total number of 120 BSS within the tidal radius. Their radial distribution is bimodal: highly peaked in the cluster center, decreasing at intermediate distances and rising again outwards. We discuss these results in the context of the dynamical clock scheme presented by Ferraro et al. (2012) and of recent results about the radial distribution of binary systems in this cluster., Comment: Accepted for publication by ApJ; 26 pages, 11 figures, 1 table

We present accurate wide-field uVI photometry of the remote and massive Galactic globular cluster NGC 2419, aimed at searching for the u-V color spread along the Red Giant Branch (RGB) that is generally interpreted as the photometric signature of the presence of multiple populations in globular clusters. Focusing on the RGB stars in the magnitude range 19.80.0 lie preferentially on the blue side., Comment: 13 pages, 12 figure, accepted for publication on MNRAS