Elvira Covino

X-shooter spectroscopy of young stellar objects: I - Mass accretion rates of low-mass T Tauri stars in \sigma Orionis

We present high-quality, medium resolution X-shooter/VLT spectra in the range 300-2500 nm for a sample of 12 very low-mass stars in the Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without, with masses ranging from 0.08 to 0.3 M . The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass, young stars and the accuracy of the correlations between these secondary tracers (mainly accretion line luminosities) found in the literature. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10 9 M yr 1 down to 5 10 11 M yr 1 , allowing us to investigate the behavior of the accretion-driven emission lines in very-low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines, from the UV to the near-IR, all obtained simultaneously. In general, most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We recompute the relationships between the accretion luminosities and the line luminosities, we confirm the validity of the correlations given in the literature, with the possible exception of H . Metallic lines, such as the CaII IR triplet or the Na I line at 589.3 nm, show a larger dispersion. When looking at individual objects, we find that the Hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of Lacc often in better agreement than the uncertainties introduced by the adopted correlations. The average Lacc derived from several Hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of non-simultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud ⋆

We present the ROSAT All-Sky Survey data in a 126 deg 2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentied ROSAT sources we could nd 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be 10 Myrs old, by comparing their location in the H R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new o-cloud weak-line T Tauri stars may have formed in similar cloudlets, which have dispersed re- cently. High-resolution spectra of our new T Tauri stars show that they have signicantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion { south relative to the galatic plane { of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered Send oprint requests to :R. Neuh¨ ? Partly based on observations collected at the 1.52 m and 3.5 m telescopes of the European Southern Observatory, Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and on ob- servations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope. the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.

The star formation in the L1615/L1616 Cometary Cloud

The present work aims at performing a comprehensive census and characterization of the pre-main-sequence (PMS) population in the cometary cloud L1615/L1616, in order to assess the significance of the triggered star formation scenario and investigate the impact of massive stars on its star formation history and mass spectrum. Our study is based on UBVRCIC and JHKs photometry, as well as optical multiobject spectroscopy. We performed a physical parameterization of the young stellar population in L1615/L1616. We identified 25 new T Tauri stars mainly projected on the dense head of the cometary cloud, almost doubling the current number of known members. We studied the spatial distribution of the cloud members as a function of the age and H? emission. The star formation efficiency (SFE) in the cloud is ~7%-8%, as expected for molecular clouds in the vicinity of OB associations. The slope of the initial mass function (IMF), in the mass range -->0.1 M? ? M ? 5.5 M?, is consistent with that of other T and OB associations, providing further support of a universal IMF down to the hydrogen-burning limit, regardless of environmental conditions. The cometary appearance, as well as the high SFE, can be explained in terms of triggered star formation induced by the strong UV radiation from OB stars or supernova shock waves. The age spread and both the spatial and age distribution of the PMS objects provide strong evidence of sequential, multiple events and possibly still ongoing star formation activity in the cloud.

Timescales of Disk Evolution and Planet Formation: HST, Adaptive Optics, and ISO Observations of Weak-Line and Post-T Tauri Stars*

We present high spatial resolution HST and ground-based adaptive optics observations and high-sensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post? (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ?30 AU from the primary stars. No such objects were found within 300 AU of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTSs and PTTSs in ScoCen reveal infrared excesses which are clearly above photospheric levels and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTSs in ScoCen compared with the younger classical and weak-line TTSs in Cha can be attributed to a deficiency of smaller size (0.1 to 1 ?m) dust grains relative to larger size (?5 ?m) dust grains in the disks of the PTTSs. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to become dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.

The debris disk host star HD 61005: a member of the Argus association?

Context. HD 61005 is a nearby young solar type star that shows a large infrared excess due to a debris disk. The disk has been recently imaged from ground and space, with indications of several components. Some characteristics of the disk suggest the presence of planetary companions around the star, that remain undetected in deep adaptive optics imaging. Aims. For a better understanding of the system we aim to refine the determination of the stellar parameters, with emphasis on the stellar age and system orientation. Methods. We used ASAS and Hipparcos photometry and FEROS spectra to determine the rotation period, radial and rotational velocity, chromospheric emission, effective temperature, and chemical composition. Results. We find no indication of any misalignment between the star rotation axis and the disk. The standard age calibrations applied to several indicators yield an age close to that of the Pleiades (120 Myr); however the kinematic properties strongly support its membership in the younger (40 Myr) Argus association, which also includes the IC 2391 open cluster. Detailed comparison of the properties of HD 61005 and IC 2391 members shows that the characteristics of HD 61005 are compatible with membership to the Argus association, once its rather slow rotation is taken into account, because lithium and other age indicators are somewhat correlated with stellar rotation at a fixed age. We also identify systematic differences between the field and cluster population of the Argus association, which are probably selection effects, so we suggest that additional members with slower rotation and lower activity level are waiting to be identified.

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. I. The Ground-based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS 2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superposed on longer timescale variations. Rapid variations of ~0.01 mag minute-1, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wave bands (UBVRI) track each other well over the period of observation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular, B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence (polarization increases toward the blue, PU/PI = 1.31) and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the UV flux. For the fastest variability timescale observed, we estimate a central black hole mass of 1.5 × 109(δ/10) M☉, consistent with UV and X-ray constraints and smaller than previously calculated for this object.

X-shooter spectroscopy of young stellar objects - III. Photospheric and chromospheric properties of Class III objects

Context. Traditionally, the chromospheres of late-type stars are studied through their strongest emission lines, Hα and Ca ii HK emission. Our knowledge on the whole emission line spectrum is more elusive as a result of the limited spectral range and sensitivity of most available spectrographs. Aims. We intend to reduce this gap with a comprehensive spectroscopic study of the chromospheric emission line spectrum of a sample of non-accreting pre-main sequence stars (Class III sources). Methods. We analyzed X-shooter/VLT spectra of 24 Class III sources from three nearby star-forming regions (σ Orionis, Lupus III, and TW Hya). We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Results. For some objects in the sample the atmospheric and kinematic parameters are presented here for the first time. The effective temperatures are consistent with those derived for the same stars from an empirical calibration with spectral types. Small differences in the surface gravity found between the stars can be attributed to differences in the average age of the three star-forming regions. The strength of lithium absorption and radial velocities confirm the young age of all but one object in the sample (Sz 94). Both X-ray and Hα luminosity as measured in terms of the bolometric luminosity are independent of the effective temperature for early-M stars but decline toward the end of the spectral M sequence. For the saturated early-M stars the average emission level is almost one dex higher for X-rays than for Hα :l og (Lx/Lbol) = −2.85 ± 0.36 vs. log (LHα/Lbol) = −3.72 ± 0.21. When all chromospheric emission lines (including the Balmer series up to H11, Ca ii HK, the Ca ii infrared triplet, and several He i lines) are summed up the coronal flux still dominates that of the chromosphere, typically by a factor 2−5. Flux-flux relations between activity diagnostics that probe different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequence stars from the bulk of field M dwarfs studied in the literature. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. The Balmer decrements can roughly be reproduced by an NLTE radiative transfer model devised for another young star of similar age. Future, more complete chromospheric model grids can be tested against this data set.Context. Traditionally, the chromospheres of late-type stars are st udied through their strongest emission lines, H α and CaII HK emission. Our knowledge on the whole emission line spectrum is ore elusive as a result of the limited spectral range and s e sitivity of most available spectrographs. Aims. We intend to reduce this gap with a comprehensive spectrosco pic study of the chromospheric emission line spectrum of a sample of non-accreting pre-main sequence stars (Class III sources). Methods. We analyzed X-Shooter/VLT spectra of 24 Class III sources from three nearby star-forming regions ( σ Orionis, Lupus III, and TW Hya). We determined the effective temperature, surfa ce gravity, rotational velocity, and radial velocity by com paring the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data t o llow for a comparison between chromospheric and coronal e missions. Results. For some objects in the sample the atmospheric and kinematic parameters are presented here for the first time. The effecti ve temperatures are consistent with those derived for the same tars from an empirical calibration with spectral types. Sm all differences in the surface gravity found between the stars can be attribu ted to differences in the average age of the three star-formi ng regions. The strength of lithium absorption and radial velocities confir m the young age of all but one object in the sample (Sz 94). Both X-ray and Hα luminosity as measured in terms of the bolometric luminosit y are independent of the effective temperature for early-M s tars but decline toward the end of the spectral M sequence. For the sat urated early-M stars the average emission level is almost on e dex higher for X-rays than for Hα: log (Lx/Lbol) = −2.85 ± 0.36 vs. log (LHα/Lbol) = −3.72 ± 0.21. When all chromospheric emission lines (including the Balmer series up to H11, Ca II HK, the Ca I I infrared triplet, and several He I lines) are summed up the c oronal flux still dominates that of the chromosphere, typically by a factor2− 5. Flux-flux relations between activity diagnostics that pro be different atmospheric layers (from the lower chromosphere to the corona) separate our sample of active pre-main sequen ce stars from the bulk of field M dwarfs studied in the literature. Flux rati os between individual optical emission lines show a smooth d ependence on the effective temperature. The Balmer decrements can rou ghly be reproduced by an NLTE radiative transfer model devis ed for another young star of similar age. Future, more complete chr omospheric model grids can be tested against this data set.

Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component

We present results of an investigation of the X-ray properties, age distribution, and kinematical characteristics of a high-galactic latitude sample of late-type field stars selected from the ROSAT All-Sky Survey (RASS). The sample com- prises 254 RASS sources with optical counterparts of spectral types F to M distributed over six study areas located at |b| > 20 ◦ , and Dec ≥− 9 ◦ . A detailed study was carried out for the subsample of ∼200 G, K, and M stars. Lithium abundances were de- termined for 179 G-M stars. Radial velocities were measured for most of the 141 G and K type stars of the sample. Combined with proper motions these data were used to study the age distribution and the kinematical properties of the sample. Based on the lithium abundances half of the G-K stars were found to be younger than the Hyades (660 Myr). About 25% are comparable in age to the Pleiades (100 Myr). A small subsample of 10 stars is younger than the Pleiades. They are therefore most likely pre-main sequence stars. Kinematically the PMS and Pleiades-type stars appear to form a group with space velocities close to the Castor moving group but clearly distinct from the Local Association.

The young active star SAO 51891 (V383 Lacertae)

Aims. The aim of this work is to investigate the surface inhomogeneities of a young, late-type star, SAO 51891, at different atmospheric levels, from the photosphere to the upper chromosphere, analyzing contemporaneous optical high-resolution spectra and broad-band photometry. Methods. The full spectral range of FOCES@CAHA (R � 40 000) is used to perform the spectral classification and to determine the rotational and radial velocities. The lithium abundance is measured to obtain an age estimate. The BVRIJHKs photometric bands are used to construct the spectral energy distribution (SED). The variations in the observed BV fluxes and effective temperature are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Hα ,C aii H& K, H� ,a nd Caii IRT lines. Results. We find SAO 51891 to be a young K0-1V star with a lithium abundance close to the Pleiades upper envelope, confirming its youth (∼100 Myr), which is also inferred from its kinematical membership of the Local Association. No infrared excess is detected from analysis of its SED, limiting the amount of remaining circumstellar dust. We detect a rotational modulation of the luminosity, effective temperature, Ca ii H& K, H� ,a nd Caii IRT total fluxes. A simple spot model with two main active regions, about 240 K cooler than the surrounding photosphere, fits the observed light and temperature curves very well. The small-amplitude radial velocity variations are also well reproduced by our spot model. The anti-correlation of light curves and chromospheric diagnostics indicates

Multi-wavelength observations of the star forming region in L1616

We present the results of a multi-wavelength study of the star forming region in L1616. Our observations include ROSAT All-Sky Survey (RASS) and High Resolution Imager (HRI) X-ray observations, optical wide-field imaging and near-IR imaging data and optical long-slit and multi-object spectroscopic follow-up. 22 new low-mass pre-main sequence (PMS) stars are found to be distributed mainly to the East of the L1616 cometary cloud, in about a one-square-degree field. We find that the class-III infrared sources outnumber the class-II infrared sources by a factor of about three. The X-ray properties of the PMS stars in L1616 are quite similar to those of PMS stars detected in the Orion Nebula Cluster. The comparison of the position of the L1616 PMS stars in the HR diagram with theoretical PMS evolutionary tracks yields an average age of 1–2 Myr, with a very small age spread of about 1 Myr. Unlike the fossil star forming regions in Orion, L1616 appears to be a region of on-going star formation relatively far from the Orion A and B clouds. Given the small age spread, the spatial distribution of the PMS stars relative to the head of the cloud, as well as its cometary shape and high star formation efficiency, we conclude that the star formation in L1616 was most likely induced by a single event, the impact of the winds of the massive stars of the Orion OB association or a supernova explosion being the possible triggers. The Initial Mass Function (IMF) in L1616 is roughly consistent with that of the field in the mass range 0.3 < M/M_⊙ < 2.5. Several faint objects, detected in our optical images, are good candidates for young Brown Dwarfs (BDs). We might expect the number of BDs in L1616 to be intermediate between Taurus and the Trapezium.