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Context. The observations of the proplyds in the Orion Nebula Cluster, exhibiting clear evidence of ongoing photoevaporation, have provided clear proof of the role of externally induced photoevaporation in the evolution of circumstellar disks. NGC 6611 is an open cluster suitable for study of disk photoevaporation, due to its significant population of massive members and stars with disk. In a previous paper, we obtained evidence of the influence of the strong UV field generated by the massive cluster members on the evolution of disks around low-mass Pre-Main Sequence members. Our study was based on a multiband BVIJHKand X-ray catalog compiled for the purpose of selecting cluster members with and without disk.

Aims. We identify and characterize low-mass stars in the ~3 Myr old Trumpler 16 region by means of a deep ChandraX-ray observation, and study their optical and near-IR properties. We compare the X-ray activity of Trumpler 16 stars with the known characteristics of Orion and Cygnus OB2 stars.

Aims.The aim of this work is to identify the so far unknown low mass stellar population of the ~2 Myr old Cygnus OB2  star forming region, and to investigate the X-ray and near-IR stellar properties of its members.

Context.Mechanisms regulating the evolution of pre-main sequence stars can be understood by studying stellar properties such as rotation, disk accretion, internal mixing and binarity. To investigate such properties, we studied a sample of 332 candidate members of the massive and populous star forming region NGC 6530.

Context.The observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale.

Context.We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium.

We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530, complementing our previous studies of this cluster. We consider different indicators of IR excesses, which can be taken as an indicator of circumstellar disks and therefore of pre-main-sequence status. We used reddening-free indices to ensure that our results are unaffected by highly irregular, differential reddening. More than the study of the $\it JHK$bands alone (33 IR-excess stars found), we found it useful to compare various optical and IR colors, finding overall 333 stars with optical-IR excess. In the field of the previously studied ChandraACIS-I observation, we found 196 stars with optical-IR excess, of which 120 have not been previously found in X-rays. The total number of estimated cluster members thus becomes ≥1100. The estimated disk frequency in the ACIS field is ~20%. By considering different optical-IR excess indices, we find only partially overlapping subsamples, corresponding to different characteristics of their spectral energy distributions (longer- or shorter-wavelength IR excesses, or blue-violet excess). In a region displaced towards northwest with respect to the known cluster center, we find an unexpected concentration of stars with optical-IR excesses, most of which are not detected in X-rays. The spectral energy distribution of some of these objects shows more extreme excesses with respect to most ordinary T Tauri stars, which suggests circumstellar disks with very small inner holes and high accretion rates; some objects are best interpreted as being surrounded by reflection nebulosities, as found in some optically detected Class I objects in nearer star-forming regions. These reflection-nebulae candidates have the lowest X-ray detection rate among all subsamples considered here. Optical-IR excess stars in the north of NGC 6530 are nearly co-spatial with a sub-population of cluster stars older than the central cluster stars. This leads to the conclusion that in these northern regions of NGC 6530, far from massive cluster stars, star formation (and disk evolution) has proceeded rather undisturbed over longer time periods than near the cluster center, where most massive stars are found, and most stars lack substantial disks and strong accretion.

We present a ChandraACIS-I X-ray observation of the region near the Herbig early-Be star MWC 297, where we detect a tight group of point X-ray sources. These are probably physically associated to MWC 297, because of their obvious clustering with respect to the more scattered field-source population. These data are compared to earlier ASCA data with much poorer spatial resolution, from which the detection of strong quiescent and flaring emission from MWC 297 itself was claimed. We argue that this star, contributing only 5% to the total X-ray emission of the group, was probably not the dominant contributor to the observed ASCA emission, while the X-ray brightest star in the group is a much better candidate. This is also supported by the spectral analysis of the Chandradata, with reference to the ASCA spectra. We conclude that none of the X-ray data available for MWC 297 justify the earlier claim of strong magnetic activity in this star. The X-ray emission of MWC 297 during the Chandraobservation is even weaker than that found in other Herbig stars with the same spectral type, even accounting for its large line-of-sight absorption.

We present astrometry and BVIphotometry, down to $V\simeq22$, of the very young open cluster NGC 6530, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the Vvs. $B-V$and the Vvs. $V-I$color-magnitude diagrams (CMD) show that the upper main sequence is dominated by very bright cluster stars, while, because of the high obscuration of the giant molecular cloud surrounding the cluster, the blue envelopes of the diagrams at $V\gtrsim 14$are limited to the main sequence stars at the distance of NGC 6530. This particular structure of the NGC 6530 CMD allows us to conclude that its distance is about $d \simeq 1250$pc, significantly lower than the previous determination of $d=1800$pc. We have positionally matched our optical catalog with the list of X-ray sources found in a Chandra-ACIS observation, finding a total of 828 common stars, 90% of which are pre-main sequence stars in NGC 6530. Using evolutionary tracks of [CITE], mass and age values are inferred for these stars. The median age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0)$\,M_\odot$, the Initial Mass Function (IMF) shows a power law index $x=1.22\pm0.17$, consistent with both the Salpeter index (1.35), and with the index derived for other young clusters; towards smaller masses the IMF shows a peak and then it starts to decrease.

We present an X-ray study of the high metallicity young open cluster Blanco 1 based on XMM-Newtondata. X-ray spectroscopy of cluster members is presented for the first time as well as new X-ray distribution functions of late-type stars. We detected all known dF and dG stars in the EPIC field and 80% and 90% of dK and dM stars, respectively. The X-ray spectral analysis of the X-ray brightest cluster stars and X-ray color analysis of a larger sample show that a model with two temperatures (at about 0.3 and 1 keV) explains the quiescent activity phase spectra.
We discuss also the nature of unidentified X-ray sources in the observed region and their X-ray spectral properties.

We present a study of the X-ray sources present in the nearby L1551 star forming region, based on a deep XMM-Newtonobservation complemented with Chandradata for the brightest sources. Most known pre-main sequence stars in the region have been detected, most of them with sufficient statistics to allow a detailed study of the temporal and spectral characteristics of their X-ray emission. Significant temporal (and spectral) variability on both short and long time scales is visible for most of the stars. In particular XZ Tau shows large-amplitude variations on time scales of several hours with large changes in the intervening absorption, suggestive of the X-ray emission being eclipsed by the accretion stream (and thus of the X-ray emission being partly or totally accretion-induced). The coronal metal abundance of the WTTS sources is clustered around $Z \simeq 0.2$, while the CTTS sources span almost two orders of magnitudes in coronal Z, even though the photospheric abundance of all stars in the L1551 is likely to be very similar. Some individual elements (notably Ne) appear to be systematically enhanced with respect to Fe in the WTTS stars. The significant differences between the spectral and temporal characteristics of the CTTS and WTTS populations suggest that a different emission mechanism is (at least partly) responsible for the X-ray emission of the two types of stars.

We present the results of a ROSAT study of NGC 2422, a southern open cluster at a distance of about 470 pc, with an age close to the Pleiades. Source detection was performed on two observations, a 10-ks PSPC and a 40-ks HRI pointing, with a detection algorithm based on wavelet transforms, particularly suited to detecting faint sources in crowded fields. We have detected 78 sources, 13 of which were detected only with the HRI, and 37 detected only with the PSPC. For each source, we have computed the $0.2{-}2.0$keV X-ray flux. Using optical data from the literature and our own low-dispersion spectroscopic observations, we find candidate optical counterparts for 62 X-ray sources, with more than 80% of these counterparts being late type stars. We have assigned to the optical sources an astrometric and/or a photometric membership flag, depending on which data are available to us. The number of sources (38 of 62) with high membership probability counterparts is consistent with that expected for Galactic plane observations at our sensitivity. We have computed maximum likelihood X-ray luminosity functions (XLF) for F and early-G type stars with high membership probability. Heavy data censoring due to our limited sensitivity permits determination of only the high-luminosity tails of the XLFs; the distributions are indistinguishable from those of the nearly coeval Pleiades cluster.

We present the first results of an XMM-Newton EPICsurvey of NGC 2516, a southern low-metallicity open cluster with an age close to the Pleiades. The attained limiting sensitivity is of ~2.4 10-15erg s-1cm-2in the 0.1-4.0 keV bandpass. This has been achieved by summing the data of the MOS and PN cameras of two distinct observations for a total exposure time of ~33 ks and by analyzing the summed data set with the wavelet detection algorithm developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over 200 X-ray detections. Using data just from a single exposure or from a single camera would have reduced by a factor 2-4 our limiting sensitivity and would have resulted in 25-40% less X-ray detections. To date, 129 detections have as counterparts one or more of the 540 photometrically selected cluster members in the surveyed region, for a total of 147 likely detected members, with unique identification in 112 cases. We derive the X-ray luminosity functions (XLF) of NGC 2516 members of different spectral types and compare them with those of the more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516 photometrically selected dG and dK stars less luminous than the Pleiades. The XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We compare the XMM-Newtonresults with those recently obtained with Chandra.

Aims.We report a deep X-ray survey of the young (~140?Myr), rich open cluster NGC?2516 obtained with the EPIC camera on board the XMM-Newton satellite.Methods.By combining data from six observations, a high sensitivity, greater than a factor of?5 with respect to recent Chandra observations, has been achieved. Kaplan-Meier estimators of the cumulative X-ray luminosity distribution are built, statistically corrected for non members contaminants and compared to those of the nearly coeval Pleiades. The EPIC spectra of the X-ray brightest stars are fitted using optically thin model plasma with one or two thermal components.Results.We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC?2516 Main Sequence. On the basis of X-ray emission and optical photometry, we indicate 20 new candidate members of the cluster; at the same time we find 49 X-ray sources without known optical or infrared counterpart. The X-ray luminosities of cluster stars span the range logLX(erg s-1)?=?28.4?30.8. The representative temperatures span the 0.3?0.6?keV (3.5?8?MK) range for the cool component and 1.0?2.0?keV (12?23?MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC?2391, and Blanco?1. While no significant differences are found in X-ray spectra, NGC?2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey reveals the lack of variability amplitudes larger than a factor of?2 in solar type stars in a ? 11?yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or have a different period and amplitude in young stars.