Rotation in NGC 2264: a study based on CoRoT photometric observations

Rotation is one of the key stellar parameters which undergo substantial evolution during the stellar lifetime, in particular during the early stages. Stellar rotational periods can be determined on the basis of the periodic modulation of starlight produced by non-uniformities on the surface of the stars, due to manifestation of stellar activity. We present the results of an extensive search for rotational periods among NGC 2264 cluster members, based on photometric monitoring using the CoRoT satellite, with a particular attention to the distribution of classical and weak-line T-Tauri stars. NGC 2264 is one of the nearest and best studied star forming region in the solar neighbourhood, with an estimated age of 3 Myr, and is the object of a recent simultaneous multiband campaign including a new CoRoT observation with the aim to assess the physical origin of the observed variability. We find that the rotational distributions of classical and weak-line T-Tauri star are different, suggesting a difference in the rotational properties of accreting and non-accreting stars.Comment: 15 pages, 10 figure

X-ray Photoevaporation-starved T Tauri Accretion

X-ray luminosities of accreting T Tauri stars are observed to be systematically lower than those of non-accretors. There is as yet no widely accepted physical explanation for this effect, though it has been suggested that accretion somehow suppresses, disrupts or obscures coronal X-ray activity. Here, we suggest that the opposite might be the case: coronal X-rays modulate the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in the Orion Nebula Cluster and find that not only are accreting stars systematically fainter, but that there is a correlation between mass accretion rate and stellar X-ray luminosity. We use the X-ray heated accretion disk models of Ercolano et al. to show that protoplanetary disk photoevaporative mass loss rates are strongly dependent on stellar X-ray luminosity and sufficiently high to be competitive with accretion rates. X-ray disk heating appears to offer a viable mechanism for modulating the gas accretion flow and could be at least partially responsible for the observed correlation between accretion rates and X-ray luminosities of T Tauri stars.Comment: 4 pages 3 figures, ApJ Letters, in pres

Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster*

We measured lithium in a sample of low-mass stars (~0.1-0.3 M☉) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster

A Deep Chandra X-Ray Observation of the Rich Young Cluster NGC 6530. I. The X-Ray Source Catalog and the Cluster Population

In ad eep 60 ksChandra ACIS X-ray observation of the very young cluster NGC 6530, we detect 884 X-ray point sources and argue that a very large fraction of them (90%-95%) must be pre-main-sequence (PMS) cluster members, mostly of low masses. This is a significant enlargement of the known NGC 6530 stellar population with respect to previous optical studies, including Hsurveys. We identify 220 X-ray sources with cataloged stars down to V ¼ 17, while most unidentified sources have fainter counterparts. Moreover, we find an infrared counterpart in the 2MASS catalog for 731 X-ray sources. The optically identified cluster X-ray sources are found in a band in the H-R diagram above the main sequence, in the locus of 0.5-1.5 Myr PMS stars, with masses down to 0.5-1.5 M� . We find evidence of an age gradient across the field from northwest to south, suggesting a sequence of star formation events qualitatively similar to that found in earlier studies of the same region, but differing in the details. A group of X-ray sources showing frequent flares may be associated with the youngest stars in the cluster, suggesting that X-ray flaring activity is especially intense in the youngest PMS phases of low- mass stars. Subject headings: open clusters and associations: individual (NGC 6530) — stars: coronae — stars: pre-main-sequence — X-rays: star

Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry

We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in order to directly probe the accretion process from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40% are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha, UV and IR excesses). The remaining non-accreting members define the (photospheric+chromospheric) reference UV emission level over which flux excess is detected and measured. We revise the membership status of cluster members based on UV accretion signatures and report a new population of 50 CTTS candidates. A large range of UV excess is measured for the CTTS population, varying from a few 0.1 to 3 mag. We convert these values to accretion luminosities and obtain mass accretion rates ranging from 1e-10 to 1e-7 Mo/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6sigma correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for upper limits, yields M_acc $\propto$ M^{1.4+/-0.3}. At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, much smaller than the observed spread. We suggest that a non-negligible age spread across the cluster may effectively contribute to the observed spread in accretion rates at a given mass. In addition, different accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-flow accretion) may be associated to different M_acc regimes.Comment: 24 pages, 21 figures, accepted for publication in Astronomy & Astrophysic

Young stars in the time domain

Variability is a defining characteristic of young stellar systems, and optical variability has been heavily studied to select and characterize the photospheric properties of young stars. In recent years, multi-epoch observations sampling a wider range of wavelengths and time-scales have revealed a wealth of time-variable phenomena at work during the star formation process. This splinter session was convened to summarize recent progress in providing improved coverage and understanding of time-variable processes in young stars and circumstellar disks. We begin by summarizing results from several multi-epoch Spitzer campaigns, which have demonstrated that many young stellar objects evidence significant mid-IR variability. While some of these variations can be attributed to processes in the stellar photosphere, others appear to trace short time-scale changes in the circumstellar disk which can be successfully modeled with axisymmetric or non-axisymmetric structures. We also review recent studies probing variability at shorter wavelengths that provide evidence for high frequency pulsations associated with accretion outbursts, correlated optical/X-ray variability in Classical T Tauri stars, and magnetic reversals in young solar analogs