E. Rigliaco

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: 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.

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.

X-shooter spectroscopy of young stellar objects - V. Slow winds in T Tauri stars

Disks around T Tauri stars are known to lose mass, as best shown by the profiles of forbidden emission lines of low ionization species. At least two separate kinematic components have been identified, one characterised by velocity shifts of tens to hundreds km/s (HVC) and one with much lower velocity of few km/s (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low mass young stars in Lupus and Ori observed with the X-Shooter ESO spectrometer. We detect forbidden line emission of Oi, Oii, Sii, Ni, and Nii, and characterize the line profiles as LVC, blue-shifted HVC and red-shifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both Lstar and the accretion luminosity (or the mass-accretion rate) over a large interval of values (Lstar 10 2 1 L ; Lacc 10 5 10 1 L ; ˙ Macc 10 11 10 7 M /yr). The lines show the presence of a slow wind (Vpeak 10 8 cm 3 ), warm (T 5000 10000 K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from 10 12 M and 0:01 AU 3 for Mstar 0:1 M , to 3 10 10 M and 1 AU 3 for Mstar 1 M , respectively. These results provide quite stringent

The ultracool dwarf DENIS-P J104814.7-395606 - Chromospheres and coronae at the low-mass end of the main-sequence

Context. Several diagnostics ranging from the radio to the X-ray band re suitable for investigating the magnetic activity of lat etype stars. Empirical connections between the emission at d ifferent wavelengths place constraints on the nature and ef ficiency of the emission mechanism and the physical conditions in differen t atmospheric layers. The activity of ultracool dwarfs, at t he low-mass end of the main-sequence, is poorly understood. Aims. We have performed a multi-wavelength study of one of the near est M9 dwarfs, DENIS-P J104814.7-395606 ( 4 pc), with the aim to examine its position within the group of magnetically ctive ultracool dwarfs, and, in general, to advance our und erstanding of these objects comparing them to early-M type dwarf stars and the Sun. Methods. We have obtained an XMM-Newton observation of DENIS-P J1048 4.7-395606 and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter. From this d ataset we obtain the X-ray properties, stellar parameters, kinematics and the emission line spectrum tracing chromospheric activity . We integrate these data by a compilation of activity parame ters for ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation has led to the first X-ray det ection of DENIS-P J104814.7-395606 ( logLx = 25.1) as well as the first measurement of its V band brightness ( V = 17.35 mag). Flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreeme nt of DENISP J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker a s judged from their lower fluxes. The observed Balmer decreme nts of DENIS 1048-3956 are compatible with optically thick plasma in LTE at low, nearly photospheric temperature or optically thin LTE plasma at20000K. Describing the decrements with Case B recombination requ i s different emitting regions for H α and the higher Balmer lines. The high observed H α/Hβ flux ratio is also poorly fitted by the optically thin models. W e derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a chara teristic of ultracool dwarfs. We add DENISP J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relatio n between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the prese nce of two types of ultracool dwarfs with distinct radio and X-ray beha vior.We have obtained an XMM-Newton observation and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter for one of the nearest M9 dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of activity parameters for ultracool dwarfs from the literature with the aim to advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Our deep XMM-Newton observation has led to the first X-ray detection of DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V band brightness (V = 17.35mag). Flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low, nearly photospheric temperature or optically thin LTE plasma at 20000K. Describing the decrements with CaseB recombination requires different emitting regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J1048-3956 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behavior.

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.

Probing Stellar Accretion with Mid-infrared Hydrogen Lines

In this paper we investigate the origin of the mid-infrared (IR) hydrogen recombination lines for a sample of 114 disks in different evolutionary stages (full, transitional, and debris disks) collected from the Spitzer archive. We focus on the two brighter H I lines observed in the Spitzer spectra, the H I (7-6) at 12.37 μm and the H I (9-7) at 11.32 μm. We detect the H I (7-6) line in 46 objects, and the H I (9-7) in 11. We compare these lines with the other most common gas line detected in Spitzer spectra, the [Ne II] at 12.81 μm. We argue that it is unlikely that the H I emission originates from the photoevaporating upper surface layers of the disk, as has been found for the [Ne II] lines toward low-accreting stars. Using the H I (9-7)/H I (7-6) line ratios we find these gas lines are likely probing gas with hydrogen column densities of 10^(10)-10^(11) cm^(–3). The subsample of objects surrounded by full and transitional disks show a positive correlation between the accretion luminosity and the H I line luminosity. These two results suggest that the observed mid-IR H I lines trace gas accreting onto the star in the same way as other hydrogen recombination lines at shorter wavelengths. A pure chromospheric origin of these lines can be excluded for the vast majority of full and transitional disks. We report for the first time the detection of the H I (7-6) line in eight young (<20 Myr) debris disks. A pure chromospheric origin cannot be ruled out in these objects. If the H I (7-6) line traces accretion in these older systems, as in the case of full and transitional disks, the strength of the emission implies accretion rates lower than 10^(–10) M_☉ yr^(–1). We discuss some advantages of extending accretion indicators to longer wavelengths, and the next steps required pinning down the origin of mid-IR hydrogen lines.

Disks, accretion and outflows of brown dwarfs

Characterization of the properties of young brown dwarfs are important to constraining the formation of objects at the extreme low-mass end of the initial mass function. While young brown dwarfs share many properties with solar-mass T Tauri stars, differences may be used as tests of how the physics of accretion/outflow and disk chemistry/dissipation depend on the mass of the central object. This article summarizes the presentations and discussions during the splinter session on “Disks, accretion and outflows of brown dwarfs” held at the CoolStars17 conference in Barcelona in June 2012. Recent results in the field of brown dwarf disks and outflows include the determination of brown dwarf disk masses and geometries based on Herschel far-IR photometry (70–160 µm), accretion properties based on X-Shooter spectra, and new outflow detections in the very low-mass regime. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An X-Shooter Analysis of Chromospheric Activity of Class III Low Mass Sources

The knowledge of the photospheric parameters and the level of chromospheric activity in young pre-main sequence stars is one of the main limitations when trying to measure mass accretion rates in Class II YSOs. A detailed characterization of photospheres and chromospheric activities in low-mass, young stars without disks (late K and M type Class III YSOs) is still missing. Using VLT/X-Shooter spectra we have analyzed a sample of Class III in the Spectral Type range between K5 and M9.5. We report on the characterization of the chromospheric emission in the stars in our sample and on the implications that our work has on accurate measurements of mass accretion rates in YSOs.

Chasing disk dispersal indicators: The origin of the [OI] low-velocity components from young stars

Understanding how a disk surrounding a young star evolves and disperses is crucial in order to understand the subsequent planet formation. In this proceeding, we summarize the results reported by Rigliaco et al . (2013) on the origin of the [OI] low-velocity component as a possible disk dispersal indicator.