A connection between the morphology of the X-ray emission and rotation for solar-type stars in open clusters

It is suggested that the three-segmented morphology of the soft X-ray emission from cluster and field stars may be understood in terms of the recent classification of rotating stars into three kinds: those lying on the convective sequence, on the interface sequence, or in the gap between them.Comment: 7 pages, 1 (color) figure, accepted by ApJ Letter

On the dispersion in lithium and potassium among late-type stars in young clusters: IC 2602

We have measured the equivalent width (EW) of the K I 7699 A line in a sample of G and K-type members of the ~35 Myr old cluster IC 2602 for which a dispersion in Li EWs had been reported by previous studies. Active cluster stars with 0.75 < (B-V)o < 1 are characterized by a dispersion in the EW of the K I 7699 A, while earlier and later-type stars do not show any significant scatter. Cluster stars at all colors show potassium EW excesses with respect to field inactive stars; furthermore, a statistically significant relationship is found between differential potassium EWs and log Lx/Lbol ratios, indicating that the EWs of the potassium feature are altered by activity. Our results suggest that the dispersion in Li EWs observed among cluster stars later than (B-V)o ~ 1 cannot be fully explained by the effects of activity. No final conclusion can instead be drawn for earlier-type stars.Comment: accepted by A&

Lithium evolution in intermediate age and old open clusters: NGC 752 revisited

We present new high resolution spectroscopic observations of the intermediate age (~2 Gyr) open cluster NGC 752. We investigate the Li vs. Teff distribution and we obtain a new accurate determination of the cluster metallicity. We compare the results for NGC 752 with other intermediate age and old clusters spanning the age range from the Hyades (~0.6 Gyr) to NGC 188 (~6-8 Gyr). We find that NGC 752 has a solar iron content ([Fe/H]=+0.01+/-0.04), at variance with early reports of sub-solar metallicity. We find that NGC 752 is only slightly more Li depleted than the younger Hyades and has a Li pattern almost identical to that observed in the ~2 Gyr old IC 4651 and NGC 3680. As for the latter clusters, we find that NGC 752 is characterized by a tight Li vs. Teff distribution for solar-type stars, with no evidence for a Li spread as large as the one observed in the solar age solar metallicity M 67. We discuss these results in the framework of mixing mechanisms and Li depletion on the main sequence (MS). We conclude that the development of a large scatter in Li abundances in old open clusters might be an exception rather than the rule (additional observations of old clusters are required), and that metallicity variations of the order of ~0.2 dex do not affect Li depletion after the age of the Hyades.Comment: A&A accepted, 10 pages, 5 ps figure

Rotation periods of late-type stars in the young open cluster IC 2602

We present the results of a monitoring campaign aimed at deriving rotation periods for a representative sample of stars in the young (30 Myr) open cluster IC 2602. Rotation periods were derived for 29 of 33 stars monitored. The periods derived range from 0.2d (one of the shortest known rotation periods of any single open cluster star) to about 10d (which is almost twice as long as the longest period previously known for a cluster of this age). We are able to confirm 8 previously known periods and derive 21 new ones, delineating the long period end of the distribution. Despite our sensitivity to longer periods, we do not detect any variables with periods longer than about 10d. The combination of these data with those for IC 2391, an almost identical cluster, leads to the following conclusions: 1) The fast rotators in a 30 Myr cluster are distributed across the entire 0.5 < B-V < 1.6 color range. 2) 6 stars in our sample are slow rotators, with periods longer than 6d. 3) The amplitude of variability depends on both the color and the period. The dependence on the latter might be important in understanding the selection effects in the currently available rotation period database and in planning future observations. 4) The interpretation of these data in terms of theoretical models of rotating stars suggests both that disk-interaction is the norm rather than the exception in young stars and that disk-locking times range from zero to a few Myr.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical Journa

Classical T Tauri-like Outflow Activity in the Brown Dwarf Mass Regime

Over the last number of years spectroscopic studies have strongly supported the assertion that protostellar accretion and outflow activity persists to the lowest masses. In this paper we present the results of our latest investigation of brown dwarf (BD) outflow activity and report on the discovery of two new outflows. Here ISO-Oph 32 is shown to drive a blue-shifted outflow with a radial velocity of 10-20 km/s and spectro-astrometric analysis constrains the position angle of this outflow to 240 +/- 7 degrees. The BD candidate ISO-Cha1 217 is found to have a bipolar outflow bright in several key forbidden lines (radial velocity = -20 km/s, +40 km/s) and with a PA of 190-210 degrees. A striking feature of the ISO-Cha1 217 outflow is the strong asymmetry between the red and blue-shifted lobes. This asymmetry is revealed in the relative brightness of the two lobes (red-shifted lobe is brighter), the factor of two difference in radial velocity (the red-shifted lobe is faster) and the difference in the electron density (again higher in the red lobe). Such asymmetries are common in jets from low mass protostars and the observation of a marked asymmetry at such a low mass supports the idea that BD outflow activity is scaled down from low mass protostellar activity. In addition to presenting these new results, a comprehensive comparison is made between BD outflow activity and jets launched by CTTSs. In particular, the application of current methods for investigating the excitation conditions and mass loss rates in CTT jets to BD spectra is explored.Comment: Accepted by Astrophysical Journa

The double population of Chamaeleon I detected by Gaia DR2

Context. Chamaeleon I represents an ideal laboratory to study the cluster formation in a low-mass environment. Recently, two sub clusters spatially located in the northern and southern parts of Chamaeleon I were found with different ages and radial velocities. Aims. In this letter we report new insights into the structural properties, age, and distance of Chamaeleon I based on the astrometric parameters from Gaia data-release 2 (DR2). Methods. We identified 140 sources with a reliable counterpart in the Gaia DR2 archive. We determined the median distance of the cluster using Gaia parallaxes and fitted the distribution of parallaxes and proper motions assuming the presence of two clusters. We derived the probability of each single source of belonging to the northern or southern sub-clusters, and compared the HR diagram of the most probable members to pre-main sequences isochrones. Results. The median distance of Chamaeleon I is ~190 pc. This is about 20 pc larger than the value commonly adopted in the literature. From a Kolmogorov-Smirnov test of the parallaxes and proper-motion distributions we conclude that the northern and southern clusters do not belong to the same parent population. The northern population has a distance dN = 192.7+/-0.4 pc, while the southern one dS = 186.5+/-0.7 pc. The two sub-clusters appear coeval, at variance with literature results, and most of the sources are younger than 3 Myr. The northern cluster is more elongated and extends towards the southern direction partially overlapping with the more compact cluster located in the south. A hint of a relative rotation between the two sub-clusters is also found.Comment: Letter accepted by A&