Probing Stellar Accretion with Mid-infrared Hydrogen Lines

March 2015 • 2015ApJ...801...31R

Authors • Rigliaco, Elisabetta • Pascucci, I. • Duchene, G. • Edwards, S. • Ardila, D. R. • Grady, C. • Mendigutía, I. • Montesinos, B. • Mulders, G. D. • Najita, J. R. • Carpenter, J. • Furlan, E. • Gorti, U. • Meijerink, R. • Meyer, M. R.

Abstract • 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 10¹⁰-10¹¹ 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.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2015ApJ...801...31R
• PDF https://iopscience.iop.org/article/10.1088/0004-637X/801/1/31/pdf
• PREPRINT http://arxiv.org/abs/1501.06210
• DATA https://irsa.ipac.caltech.edu/bibdata/2015/R/2015ApJ...801...31R.html
• ELECTR https://doi.org/10.1088/0004-637X/801/1/31