X-shooter spectroscopy of young stellar objects

Abstract

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 σ Orionis cluster. The sample includes eight stars with evidence of disks from Spitzer and four without disks, 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, which are 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, and we confirm the validity of the correlations given in the literature, with the possible exception of Hα. 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 L acc that often better agree than the uncertainties introduced by the adopted correlations. The average L acc 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 nonsimultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually.

Doppler probe of accretion onto a T Tauri star

“…The calculated accretion rate is consistent with that derived from the empirical relationship between the accretion rate and the H and He emission line luminosities (e.g., Fang et al 2009;Rigliaco et al 2012). The Hα line luminosity yields an accretion rate of (0.3-0.9) × 10 −7 M /yr, while the He 1 λ5875 line luminosity yields (1-2) × 10 −7 M /yr.…”

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.…”

“…These represent the only suitable way to determine L acc from our spectra, as the UV-excess is not detected in the spectra because of the high extinction of the region. This is an indirect method that is less accurate than direct fitting of the UV-excess, but it has been shown that L acc determined in this way are consistent with direct measurements of L acc when multiple lines are used (e.g., Rigliaco et al 2012;Alcalá et al 2014). For this analysis, we then select 11 emission lines that are usually bright and whose L line has a good correlation with L acc , namely CaK, Hδ, Hγ, Hβ, He λ587nm , Hα, He λ667nm , Paδ, Paγ, Paβ, Brγ.…”

“…The accretion process has been studied for objects located in various nearby star-forming regions (σ-Ori, Rigliaco et al 2012;Lupus, Alcalá et al 2014; Chamaeleon I, Manara et al, in prep. ) and at various phases of disk evolution (e.g., transitional disks; Manara et al 2014).…”

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

“…Correlations for other lines are available in the on-line version of this paper. The line luminosity-accretion luminosity relationship can be approximated by a power law, and has been in numerous works on CTTs and HAeBes (Muzerolle, Hartmann & Calvet 1998;Calvet et al 2004;Dahm 2008;Herczeg & Hillenbrand 2008;Mendigutía et al 2011;Rigliaco et al 2012), where :…”