A 75% Occurrence Rate of Debris Discs around F stars in the β Pic Moving Group

Only 20% of old field stars have detectable debris discs, leaving open the question of what disc, if any, is present around the remaining 80%. Young moving groups allow to probe this population, since discs are expected to have been brighter early on. This paper considers the population of F stars in the 23 Myr-old BPMG where we find that 9/12 targets possess discs. We also analyse archival ALMA data to derive radii for 4 of the discs, presenting the first image of the 63au radius disc of HD 164249. Comparing the BPMG results to disc samples from ∼45 Myr and ∼150 Myr-old moving groups, and to discs found around field stars, we find the disc incidence rate in young moving groups is comparable to that of the BPMG and significantly higher than that of field stars. The BPMG discs tend to be smaller than those around field stars. However, this difference is not statistically significant due to the small number of targets. Yet, by analysing the fractional luminosity vs disc radius parameter space we find that the fractional luminosities in the populations considered drop by two orders of magnitude within the first 100 Myr. This is much faster than expected by collisional evolution, implying a decay equivalent to 1/age2. We attribute this depletion to embedded planets which would be around 170 Mearth to cause a depletion on the appropriate timescale. However, we cannot rule out that different birth environments of nearby young clusters result in brighter debris discs than the progenitors of field stars which likely formed in a more dense environment.

Small-scale star formation as revealed by VVVX galactic cluster candidates

ABSTRACT We report a search and analysis of obscured cluster candidates in the ‘VISTA Variables in the Via Lactea eXtended (VVVX)’ ESO Public Survey area encompassing the region between 229${_{.}^{\circ}}$4 < l < 295${_{.}^{\circ}}$2 and −4${_{.}^{\circ}}$3 < b < 4${_{.}^{\circ}}$4 of the southern Galactic disc. We discover and propose 88 new clusters. We improve the completeness of the embedded cluster population in this region, adding small size (linear diameters of 0.2–1.4 pc) and relatively far objects (heliocentric distance between 2 and 4 kpc) to existing catalogues. Nine candidates are proposed to be older open cluster candidates. Three of them (VVVX CL 204, CL 207, CL 208) have sufficient numbers of well-resolved stellar members to allow us to determine some basic cluster parameters. We confirm their nature as older, low-mass open clusters. Photometric analysis of 15 known clusters shows that they have ages above 20 Myr, and masses below 2000 M⊙: in general, their proper motions follow the motion of the disc. We outline some groups of clusters, most probably formed within the same dust complex. Broadly, our candidates follow the network of filamentary structure in the remaining dust. Thus, in this part of the southern disc of the Galaxy, we have found recent star formation, producing small size and young clusters, in addition to the well-known, massive young clusters, including NGC 3603, Westerlund 2, and the Carina Nebula Complex.

The future of IMF studies with the ELT and MICADO

Context. Young stellar cluster cores in the local Universe provide the most pristine information available on the stellar initial mass function (IMF), but their stellar densities are too high to be resolved by present-day instrumentation. With a resolving power 100 times better than the Hubble Space Telescope, the Multi-Adaptive Optics Imaging CameraA for Deep Observations (MICADO), which is the near-infrared camera on the Extremely Large Telescope (ELT), will for the first time provide access to a significant number of dense young stellar clusters that are critical for direct studies on the universality and shape of the IMF. Aims. In this work we aim to estimate the lowest stellar mass that MICADO will be able to reliably detect given a stellar density and distance. We also show that instrumental effects that will play a critical role, and report the number of young clusters that will be accessible for IMF studies in the local Universe with the ELT. Methods. We used SimCADO*, the instrument simulator package for the MICADO camera, to generate observations of 56 dense stellar regions with densities similar to the cores of young stellar clusters. We placed the cluster fields at distances between 8 kpc and 5 Mpc from the Earth, implying core densities from 102 to 105 stars arcsec−2, and determined the lowest reliably observable mass for each stellar field through point-spread function fitting photometry. Results. Our results show that stellar densities of <103 stars arcsec−2 will be easily resolvable by MICADO. The lowest reliably observable mass in the Large Magellanic Cloud will be around 0.1 M⊙ for clusters with densities <103 stars arcsec−2. MICADO will be able to access the stellar content of the cores of all dense young stellar clusters in the Magellanic Clouds, allowing the peak and shape of the IMF to be studied in great detail outside the Milky Way. At a distance of 2 Mpc, all stars with M > 2 M⊙ will be resolved in fields of <104 stars arcsec−2, allowing the high-mass end of the IMF to be studied in all galaxies out to and including NGC 300. Conclusions. We show that MICADO on the ELT will be able to probe the IMF of star clusters that are ten times denser than what the James Webb Space Telescope will be able to access, and over one hundred times denser than the clusters that the Hubble Space Telescope can successfully resolve. While the sensitivity of MICADO will not allow us to study the brown dwarf regime outside the Milky Way, it will enable access to all stellar members of over 1000 young clusters in the Milky Way and the Magellanic Clouds. Furthermore, direct measurements of the Salpeter slope of the IMF will be possible in over 1500 young clusters out to a distance of 5 Mpc. MICADO on the ELT will be able to measure resolved IMFs for a large ensemble of young clusters under starkly different environments and test the universality of the IMF in the local Universe.

The Gaia-ESO Survey: a new approach to chemically characterising young open clusters

Context. Open clusters are recognised as excellent tracers of Galactic thin-disc properties. At variance with intermediate-age and old open clusters, for which a significant number of studies is now available, clusters younger than ≲150 Myr have been mostly overlooked in terms of their chemical composition until recently (with few exceptions). On the other hand, previous investigations seem to indicate an anomalous behaviour of young clusters, which includes (but is not limited to) slightly sub-solar iron (Fe) abundances and extreme, unexpectedly high barium (Ba) enhancements. Aims. In a series of papers, we plan to expand our understanding of this topic and investigate whether these chemical peculiarities are instead related to abundance analysis techniques. Methods. We present a new determination of the atmospheric parameters for 23 dwarf stars observed by the Gaia-ESO survey in five young open clusters (τ < 150 Myr) and one star-forming region (NGC 2264). We exploit a new method based on titanium (Ti) lines to derive the spectroscopic surface gravity, and most importantly, the microturbulence parameter. A combination of Ti and Fe lines is used to obtain effective temperatures. We also infer the abundances of Fe I, Fe II, Ti I, Ti II, Na I, Mg I, Al I, Si I, Ca I, Cr I, and Ni I. Results. Our findings are in fair agreement with Gaia-ESO iDR5 results for effective temperatures and surface gravities, but suggest that for very young stars, the microturbulence parameter is over-estimated when Fe lines are employed. This affects the derived chemical composition and causes the metal content of very young clusters to be under-estimated. Conclusions. Our clusters display a metallicity [Fe/H] between +0.04 ± 0.01 and +0.12 ± 0.02; they are not more metal poor than the Sun. Although based on a relatively small sample size, our explorative study suggests that we may not need to call for ad hoc explanations to reconcile the chemical composition of young open clusters with Galactic chemical evolution models.

Isochrone-cloud fitting of the extended main-sequence turn-off of young clusters

Context. Extended main-sequence turn-offs (eMSTOs) are a commonly observed property of young clusters. A global theoretical interpretation for eMSTOs is still lacking, but stellar rotation is considered a necessary ingredient to explain eMSTOs. Aims. We aim to assess the importance of core-boundary and envelope mixing in stellar interiors for the interpretation of eMSTOs in terms of one coeval population. Methods. We constructed isochrone-clouds based on interior mixing profiles of stars with a convective core calibrated from asteroseismology of isolated galactic field stars. We fitted these isochrone-clouds to the measured eMSTO to estimate the age and core mass of the stars in the two young clusters NGC 1850 and NGC 884, assuming one coeval population and by fixing the metallicity to the one measured from spectroscopy. We assessed the correlations between the interior mixing properties of the cluster members and their rotational and pulsational properties. Results. We find that stellar models based on asteroseismically-calibrated interior mixing profiles lead to enhanced core masses of eMSTO stars. Additionally, these models can explain a significant fraction of the observed eMSTOs of the two considered clusters in terms of one coeval population of stars, which have similar ages to those in the literature, given the large uncertainties. The rotational and pulsational properties of the stars in NGC 884 are not sufficiently well known to perform asteroseismic modelling as it is achieved for field stars from space photometry. The stars in NGC 884 for which we have v sin i and a few pulsation frequencies show no correlation between these properties and the core masses of the stars that set the cluster age. Conclusions. Future cluster space asteroseismology may allow for the interpretation of the core masses in terms of the physical processes that cause them, based on the modelling of the interior mixing profiles for the individual member stars with suitable identified modes.

Flipping-up the field: gravitational feedback as a mechanism for young clusters dispersal

Recent analyses of Gaia data have provided direct evidence that most young stellar clusters are in a state of expansion, with velocities of the order of ∼0.5 km s−1. Traditionally, expanding young clusters have been pictured as entities that became unbound due to the lack of gravitational binding once the gas from the parental cloud that formed the cluster has been expelled by the stellar radiation of the massive stars in the cluster. In the present contribution, we used radiation-magnetohydrodynamic numerical simulations of molecular cloud formation and evolution to understand how stellar clusters form and disperse. We found that the ionizing feedback from the newborn massive stars expels the gas from the collapse centre, flipping-up the gravitational potential as a consequence of the mass removal from the inside–out. Since neither the parental clouds, nor the formed shells are distributed symmetrically around the H ii region, net forces pulling out the stars are present, accelerating them towards the edges of the cavity. We call this mechanism ‘gravitational feedback’, in which the gravity from the expelled gas appears to be the crucial mechanism producing unbound clusters that expand away from their formation centre in an accelerated way in young stellar clusters. This mechanism naturally explains the ‘Hubble flow-like’ expansion observed in several young clusters.

Stellar models and isochrones from low-mass to massive stars including pre-main sequence phase with accretion

Grids of stellar models are useful tools to derive the properties of stellar clusters, in particular young clusters hosting massive stars, and to provide information on the star formation process in various mass ranges. Because of their short evolutionary timescale, massive stars end their life while their low-mass siblings are still on the pre-main sequence (pre-MS) phase. Thus the study of young clusters requires consistent consideration of all the phases of stellar evolution. But despite the large number of grids that are available in the literature, a grid accounting for the evolution from the pre-MS accretion phase to the post-MS phase in the whole stellar mass range is still lacking. We build a grid of stellar models at solar metallicity with masses from 0.8 M⊙ to 120 M⊙, including pre-MS phase with accretion. We use the GENEC code to run stellar models on this mass range. The accretion law is chosen to match the observations of pre-MS objects on the Hertzsprung-Russell diagram. We describe the evolutionary tracks and isochrones of our models. The grid is connected to previous MS and post-MS grids computed with the same numerical method and physical assumptions, which provides the widest grid in mass and age to date.