Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so to minimize systematic differences in the results. We study trends of L_X and L_X/L_bol as a function of stellar mass and association age. For low mass stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ~10^7 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&

X-ray observations of IC348 in light of an updated cluster census

IC348 is an excellent laboratory for studies of low-mass star formation being nearby, compact and rich. A Chandra observation was carried out early in the satellite's lifetime. The extensive new data in optical and infrared wavelengths accumulated in subsequent years have changed the cluster census calling for a re-analysis of the X-ray data.Comment: poster paper to appear in Proc. of the 15th Workshop on Cool Stars, Stellar Systems and the Su

The History of the Mysterious Eclipses of KH 15D: Asiago Observatory, 1967-1982

We are gathering archival observations to determine the photometric history of the unique and unexplained eclipses of the pre-main-sequence star KH 15D. Here we present a light curve from 1967-1982, based on photographic plates from Asiago Observatory. During this time, the system alternated periodically between bright and faint states, as observed today. However, the bright state was 0.9 mag brighter than the modern value, and the fractional variation between bright and faint states (Delta I = 0.7 mag) was smaller than observed today (3.5 mag). A possible explanation for these findings is that the system contains a second star that was previously blended with the eclipsing star, but is now completely obscured.Comment: Accepted to AJ. 24 pages, 10 figures, 2 tables. v2: Phase error corrected in figures 8 and 1

Observational clues for a role of circumstellar accretion in PMS X-ray activity

We revisit the published analyses of ROSAT X-ray observations of the star forming regions NGC 2264 and Chamaeleon I (~3 and ~5 Myr old respectively) in the light of newly published optical data. At odds with previous results on Chamaeleon I members, we find that low mass stars in both regions have near-saturated emission levels. Similarly to what previously found in the Orion Nebula Cluster, Weak Line T-Tauri Stars in NGC 2264 and in the Chamaeleon I cloud have higher X-ray activity levels respect to Classical T Tauri Stars, arguing in favor of a role of the disk and/or accretion in determining X-ray emission

Dynamical star-disk interaction in the young stellar system V354 Mon

The main goal of this work is to characterize the mass accretion and ejection processes of the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. In March 2008, photometric and spectroscopic observations of V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm telescope at the Observat\'orio Pico dos Dias (LNA - Brazil) equipped with a CCD camera and Johnson/Cousins BVRI filters, and the SOPHIE \'echelle spectrograph at the Observatoire de Haute-Provence (CNRS - France). The light curve of V354 Mon shows periodical minima (P = 5.26 +/- 0.50 days) that vary in depth and width at each rotational cycle. From the analysis of the photometric and spectroscopic data, it is possible to identify correlations between the emission line variability and the light-curve modulation of the young system, such as the occurrence of pronounced redshifted absorption in the H_alpha line at the epoch of minimum flux. This is evidence that during photometric minima we see the accretion funnel projected onto the stellar photosphere in our line of sight, implying that the hot spot coincides with the light-curve minima. We applied models of cold and hot spots and a model of occultation by circumstellar material to investigate the source of the observed photometric variations. We conclude that nonuniformly distributed material in the inner part of the circumstellar disk is the main cause of the photometric modulation, which does not exclude the presence of hot and cold spots at the stellar surface. It is believed that the distortion in the inner part of the disk is created by the dynamical interaction between the stellar magnetosphere, inclined with respect to the rotation axis, and the circumstellar disk, as also observed in the classical T Tauri star AA Tau and predicted by magnetohydrodynamical numerical simulations.Comment: Accepted by Astronomy and Astrophysic

KH 15D: A Spectroscopic Binary

We present the results of a high-resolution spectroscopic monitoring program of the eclipsing pre-main-sequence star KH 15D that reveal it to be a single-line spectroscopic binary. We find that the best-fit Keplerian model has a period P = 48.38 days, which is nearly identical to the photometric period. Thus, we find the best explanation for the periodic dimming of KH 15D is that the binary motion carries the currently visible star alternately above and below the edge of an obscuring cloud. The data are consistent with the models involving an inclined circumstellar disk, as recently proposed by Winn et al. (2004) and Chiang & Murray-Clay (2004). We show that the mass ratio expected from models of PMS evolution, together with the mass constraints for the visible star, restrict the orbital eccentricity to 0.68 < e < 0.80 and the mass function to 0.125 < Fm < 0.5 Msun.Comment: 8 pages, 4 figures, 3 tables, accepted for publication in September AJ. Discussion of rotational velocity deferred to Hamilton, et al. (2004, in prep). Previously reported vsini value in error; Replaced Table 3 with new Figure 3; Added new Table 2 showing individual radial velocities w.r.t. each reference star; Fixed typo in Figure

Multi-wavelength observing of a forming solar-like star

V2129 Oph is a 1.35 solar mass classical T Tauri star, known to possess a strong and complex magnetic field. By extrapolating from an observationally derived magnetic surface map, obtained through Zeeman-Doppler imaging, models of V2129 Oph's corona have been constructed, and used to make predictions regarding the global X-ray emission measure, the amount of modulation of X-ray emission, and the density of accretion shocks. In late June 2009 we will under take an ambitious multi-wavelength, multi-observing site, and near contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray, spectropolarimetric and photometric monitoring. This will allow the validity of the 3D field topologies derived via field extrapolation to be determined.Comment: 4 pages, proceedings of the 3rd MSSL workshop on High Resolution X-ray Spectroscopy: towards IX

XMM-Newton observations of the sigma Ori cluster. II. Spatial and spectral analysis of the full EPIC field

We present the results of an XMM-Newton observation of the young (~2-4 Myr) cluster around the hot star sigma Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars down to the substellar limit. We detected eleven new possible candidate members from the 2MASS catalogue. We find that late-type stars have a median log L_X/L_bol ~ -3.3, i.e. very close to the saturation limit. We detected significant variability in ~40% of late-type members or candidates, including 10 flaring sources; rotational modulation is detected in one K-type star and possibly in another 3 or 4 stars. Spectral analysis of the brightest sources shows typical quiescent temperatures in the range T_1 ~ 0.3-0.8 keV and T_2 ~ 1-3 keV, with subsolar abundances Z ~ 0.1-0.3 Z_sun, similar to what is found in other star-forming regions and associations. We find no significant difference in the spectral properties of classical and weak-lined T Tauri stars, although classical T Tauri stars tend to be less X-ray luminous than weak-lined T Tauri stars.Comment: 20 pages, 10 figures, 6 tables. Accepted by A&