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 {it Spitzer} archive. We focus on the tw
o brighter {H~{sc i}} lines observed in the {it Spitzer} spectra, the {H~{sc i}}(7-6) at 12.37$mu$m and the {H~{sc i}}(9-7) at 11.32$mu$m. We detect the {H~{sc i}}(7-6) line in 46 objects, and the {H~{sc i}}(9-7) in 11. We compare these lines with the other most common gas line detected in {it Spitzer} spectra, the {[Ne~{sc iii}]} at 12.81$mu$m. We argue that it is unlikely that the {H~{sc i}} emission originates from the photoevaporating upper surface layers of the disk, as has been found for the {[Ne~{sc iii}]} lines toward low-accreting stars. Using the {H~{sc i}}(9-7)/{H~{sc 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~{sc i}} line luminosity. These two results suggest that the observed mid-IR {H~{sc 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~{sc 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~{sc 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$_{odot}$/yr. We discuss some advantages of extending accretion indicators to longer wavelengths.
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 sigma Orionis cluster. The sample includes stars with masses ranging from 0.08 to 0.3 M$_odot$. The aim of this fi
rst paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass, young stars. 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$_{odot}$ yr$^{-1}$ down to 5$times10^{-11}$ M$_{odot}$ 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, obtained simultaneously. Most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We confirm the validity of the correlations between accretion luminosities and line luminosities given in the literature, with the possible exception of Halpha. 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 accretion luminosities, often in better agreement than the uncertainties introduced by the adopted correlations. The average accretion luminosity 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.
