C. F. Manara

X-shooter spectroscopy of young stellar objects - IV. Accretion in low-mass stars and substellar objects in Lupus

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Macc.

X-shooter spectroscopy of young stellar objects - II. Impact of chromospheric emission on accretion rate estimates

Context. The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) Young Stellar Objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars. Aims. Our goal is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have analyzed a sample of 24 non-accreting young stellar objects of spectral type between K5 and M9.5. We identify the main emission lines normally used as tracers of accretion in Class II objects, and we determine their fluxes in order to estimate the contribution of the chromospheric activity to the line luminosity. Results. We have used the relationships between line luminosity and accretion luminosity derived in the literature for Class II objects to evaluate the impact of chromospheric activity on the accretion rate measurements. We find that the typical chromospheric activity would bias the derived accretion luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of the objects. The noise on Macc depends on stellar mass and age, and the typical values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions. Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution as the line emission may be dominated by the contribution of chromospheric activity.

Gas content of transitional disks: a VLT/X-Shooter study of accretion and winds

Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. n nAims. Using X-Shooter broad band − UV to near-infrared − medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. n nMethods. We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets. We then compared our findings with results for classical T Tauri stars. n nResults. The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. n nConclusions. The sample analyzed here suggests that, at least for some objects, the process responsible of the inner disk clearing allows for a transfer of gas from the outer disk to the inner region. This should proceed at a rate that does not depend on the physical mechanisms that produces the gap seen in the dust emission and results in a gas density in the inner disk similar to that of unperturbed disks around stars of similar mass.

HUBBLE SPACE TELESCOPE MEASURES OF MASS ACCRETION RATES IN THE ORION NEBULA CLUSTER

The present observational understanding of the evolution of the mass accretion rates (Ṁ_acc) in pre-main-sequence stars is limited by the lack of accurate measurements of Ṁ_acc over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of Ṁ_acc for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M_* ~ 2 M_☉. We derive Ṁ_acc from both the U-band excess and the Hα luminosity (L_Hα), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L acc) and L_Hα, which is L_acc/L_☉ = (1.31 ± 0.03) ⋅ L_Hα/L_☉ + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between Ṁ_acc and the parameters of the central star such as mass and age. We clearly find Ṁ_acc to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of Ṁ_acc with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log(Ṁ_acc)/M_☉ yr) = (–5.12 ± 0.86) – (0.46 ± 0.13) ⋅ log(t/yr) – (5.75 ± 1.47) ⋅ log(M */M ☉) + (1.17 ± 0.23) ⋅ log(t/yr) ⋅ log(M_*/M_☉). These results also suggest that the similarity solution model could be revised for sources with M_* ≳ 0.5 M_☉. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources.

Accurate determination of accretion and photospheric parameters in young stellar objects: The case of two candidate old disks in the Orion Nebula Cluster

Context. Current planet formation models are largely based on the observational constraint that protoplanetary disks have a lifetime of ~3 Myr. Recent studies, however, report the existence of pre-main-sequence stars with signatures of accretion (strictly connected with the presence of circumstellar disks) and photometrically determined ages of 30 Myr or more. n nAims. Here, we present a spectroscopic study of two major age outliers in the Orion Nebula Cluster. We use broadband, intermediate resolution VLT/X-shooter spectra combined with an accurate method to determine the stellar parameters and the related age of the targets to confirm their peculiar age estimates and the presence of ongoing accretion. n nMethods. The analysis is based on a multicomponent fitting technique, which derives simultaneously spectral type, extinction, and accretion properties of the objects. With this method, we confirm and quantify the ongoing accretion. From the photospheric parameters of the stars, we derive their position on the H-R diagram and the age given by evolutionary models. With other age indicators like the lithium-equivalent width, we estimate the age of the objects with high accuracy. n nResults. Our study shows that the two objects analyzed are not older than the typical population of the Orion Nebula Cluster. While photometric determination of the photospheric parameters are an accurate method to estimate the parameters of the bulk of young stellar populations, our results show that those of individual objects with high accretion rates and extinction may be affected by large uncertainties. Broadband spectroscopic determinations should thus be used to confirm the nature of individual objects. n nConclusions. The analysis carried out shows that this method allows us to obtain an accurate determination of the photospheric parameters of accreting young stellar objects in any nearby star-forming region. We suggest that our detailed, broadband spectroscopy method should be used to derive accurate properties of candidate old and accreting young stellar objects in star-forming regions. We also discuss how a similarly accurate determination of stellar properties can be obtained through a combination of photometric and spectroscopic data.

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.

The Ṁ–M* relation of pre-main-sequence stars: a consequence of X-ray driven disc evolution

We analyse current measurements of accretion rates onto pre-main sequence stars as a function of stellar mass, and conclude that the steep dependance of accretion rates on stellar mass is real and not driven by selection/detection threshold, as has been previously feared. These conclusions are reached by means of statistical tests including a survival analysis which can account for upper limits. The power-law slope of the Mdot-M* relation is found to be in the range of 1.6-1.9 for young stars with masses lower than 1 Msun. The measured slopes and distributions can be easily reproduced by means of a simple disc model which includes viscous accretion and X-ray photoevaporation. We conclude that the Mdot-M* relation in pre-main sequence stars bears the signature of disc dispersal by X-ray photoevaporation, suggesting that the relation is a straight- forward consequence of disc physics rather than an imprint of initial conditions.

THE HUBBLE SPACE TELESCOPE TREASURY PROGRAM ON THE ORION NEBULA CLUSTER

The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster (ONC) has used 104 orbits of HST time to image the Great Orion Nebula region with the Advanced Camera for Surveys (ACS), the Wide-Field/Planetary Camera 2 (WFPC2), and the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) instrument in 11 filters ranging from the U band to the H band equivalent of HST. The program has been intended to perform the definitive study of the stellar component of the ONC at visible wavelengths, addressing key questions like the cluster initial mass function, age spread, mass accretion, binarity, and cirumstellar disk evolution. The scanning pattern allowed us to cover a contiguous field of approximately 600 arcmin^2 with both ACS and WFPC2, with a typical exposure time of approximately 11 minutes per ACS filter, corresponding to a point source depth AB(F435W) = 25.8 and AB(F775W) = 25.2 with 0.2 mag of photometric error. We describe the observations, data reduction, and data products, including images, source catalogs, and tools for quick look preview. In particular, we provide ACS photometry for 3399 stars, most of them detected at multiple epochs; WFPC2 photometry for 1643 stars, 1021 of them detected in the U band; and NICMOS JH photometry for 2116 stars. We summarize the early science results that have been presented in a number of papers. The final set of images and the photometric catalogs are publicly available through the archive as High Level Science Products at the STScI Multimission Archive hosted by the Space Telescope Science Institute.

Strong biases in estimating the time dependence of mass accretion rates in young stars

The temporal decay of mass accretion in young stars is a fundamental tracer of the early evolution of circumstellar disks. Through population syntheses, we study how correlated uncertainties between the estimated parameters of young stars (luminosity, temperature, mass, age) and mass accretion rates Mdot, as well as observational selection effects, can bias the temporal decay of mass accretion rates (Mdot t^-eta) inferred from a comparison of measured Mdot with isochronal ages in young stellar clusters. We find that the presence of realistic uncertainties reduces the measured value of eta by up to a factor of 3, leading to the inference of shallower decays than the true value. This suggests a much faster temporal decay of Mdot than generally assumed. When considering the minimum uncertainties in ages affecting the Orion Nebula Cluster, the observed value eta~1.4, typical of Galactic star forming regions, can only be reproduced if the real decay exponent is eta>4. This effect becomes more severe if one assumes that observational uncertainties are larger, as required by some fast star formation scenarios. Our analysis shows that while selection effects due to sample incompleteness do bias eta, they can not alter this main result and strengthen it in many cases. A remaining uncertainty in our work is that it applies to the most commonly used and simple relationship between Mdot, the accretion luminosity and the stellar parameters. We briefly explore how a more complex interplay between these quantities might change the results.

Time Monitoring of Radio Jets and Magnetospheres in the Nearby Young Stellar Cluster R Coronae Australis

We report Karl G. Jansky Very Large Array 8-10xa0GHz (λ = 3.0-3.7xa0cm) monitoring observations toward the young stellar object (YSO) cluster R Coronae Australis (Rxa0CrA), taken from 2012 March 15 to 2012 September 12. These observations were planned to measure the radio flux variabilities in timescales from 0.5xa0hr to several days, to tens of days, and up to ~200xa0days. We found that among the YSOs detectable in individual epochs, in general, the most reddened objects in the Spitzer observations show the highest mean 3.5xa0cm Stokes I emission, and the lowest fractional variabilities on <200xa0day timescales. The brightest radio flux emitters in our observations are the two reddest sources IRS7W and IRS7E. In addition, by comparing our observations with observations taken from 1996 to 1998 and 2005, we found that the radio fluxes of these two sources have increased by a factor of ~1.5. The mean 3.5xa0cm fluxes of the three Classxa0I/II sources, IRSI, IRS2, and IRS6, appear to be correlated with their accretion rates derived by a previous near-infrared line survey. The weakly accreting Classxa0I/II YSOs, or those in later evolutionary stages, present radio flux variability on <0.5xa0hr timescales. Some YSOs were detected only during occasional flaring events. The source Rxa0CrA went below our detection limit during a few fading events.