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.