X-shooter spectroscopy of young stellar objects

Abstract

We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ∼0.03 to ∼1.2 M , but mostly with 0.1 M < M < 0.5 M . Our aim is twofold: firstly, to 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 (L acc ), and in turn the accretion rate (Ṁ acc ), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L line ) of many emission lines of H , He , and Ca ii, observed simultaneously in the range from ∼330 nm to 2500 nm. The luminosity of all the lines is well correlated with L acc . We provide empirical relationships between L acc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to L acc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring L acc andṀ acc yield significantly different results: Hα line profile modelling may underestimateṀ acc by 0.6 to 0.8 dex with respect toṀ acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships betweenṀ acc and other YSOs properties reported in the literature. We derivedṀ acc in the range 2 × 10 −12 -4 × 10 −8 M yr −1 and conclude thatṀ acc ∝ M 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 inṀ acc , confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.

A spectroscopic survey of Herbig Ae/Be stars with X-Shooter – II. Accretion diagnostic lines

“…Each panel shows only the HAeBes where both a clear UV-excess and clear emission line detection are made, shown as grey squares;a best-fit to them is shown as a solid-black line (the equation for the line is given below the line descriptor in the top-left of each panel, and is also given in 2). Where applicable, the data on CTTs from Alcalá et al (2014) is plotted for comparison, as red circles, along with their best-fit as a dashed-red line. Figure A1.…”

“…The data on CTTs from Alcalá et al (2014) for the lines in common with this study are also included for comparison, along with their best fit in Figure A1. Overall, the plots appear to show a qualitative agreement in trends between the HAeBes and the CTTs, where Lacc is increasing proportionally to L line , generally a 1:1 correlation.…”

X-Shooter study of accretion inρ-Ophiucus: very low-mass stars and brown dwarfs

“…Given that the spectra have very low or zero S/N in the UVB arm, the analysis cannot be performed using the method described in Manara et al (2013b), which simultaneously determines SpT, A V , and accretion luminosity (L acc ), fitting the UV and visible part of the spectra with a grid of models that includes the contribution of photospheric and accretion-induced emission, and reddening. Similarly, the analysis methods used by Alcalá et al (2014) or Rigliaco et al (2012) cannot be adopted here, as all these need to perform a fit of the excess in the UV part of the spectrum to derive L acc , and are well suited for the SpT determination only for objects with low extinction. The procedure we use here is the following: we collect a grid of photospheric templates, which cover the SpT from K5 to L0, with a typical step of 0.5 spectral subclasses throughout the entire M subclass.…”

“…Data reduction has been carried out using the X-Shooter pipeline (Modigliani et al 2010) version 1.3.7 and the same procedure as in Alcalá et al (2014). For the targets observed on 2010-06-01, 2010-08-20, and 2010-08-28, the photometric standard observed during the night of observation is not supported anymore in the pipeline.…”

“…In this object, the quality of the spectrum in the last ∼100 nm of the VIS arm is not good, and this results in a bad matching of the VIS and NIR arms. Finally, telluric line removal has been performed using the IRAF 2 task telluric following the procedure discussed in Alcalá et al (2014) and using standard telluric spectra obtained close in time and air mass to the targets.…”