Title: Molecular Hydrogen Emission from Protoplanetary Disks

Authors: H. Nomura^1, Y. Takagi^2, Y. Itoh^2, K. Sugitani^3,
M. Watanabe^4, J.S. Bary^5, D.A. Weintraub^6, Y. Aikawa^2,
M. Tsujimoto^7, Y. Nakagawa^2, T.J. Millar^8

1. Kyoto Univ., 2. Kobe Univ., 3. Nagoya City Univ.,
4. Subaru Telescope, NAOJ, 5. Colgate Univ., 6. Vanderbilt Univ.,
7. JAXA/ISAS, 8. Queen's Univ. Belfast


Abstract:
It is believed that planets are being formed from dust and gas
in protoplanetary disks. Recent high spectral resolution and high
sensitivity observations have made it possible to detect the 2.12
micron v=1-0 S(1) ro-vibrational transition line of molecular
hydrogen, which is the major component in the disks, toward
T Tauri and Herbig Ae/Be stars.

Historically, near-infrared H2 line ratios have been used as a tool
to derive the physical properties of various astronomical objects.
In this work we have measured the v=2-1 S(1)/v=1-0 S(1) H2 line
ratios toward T Tauri stars to diagnose the H2 excitation
mechanisms and the evolutionary status of dust grains in
protoplanetary disks.

Thanks to K-band echelle spectroscopy with wide spectral
coverage of Subaru/IRCS together with the new adaptive optics
system, AO188, we were able to observe the 2.25 um 2-1 S(1),
the 2.22 um 1-0 S(0), and the 2.12 um 1-0 S(1) lines
simultaneously with sufficiently high sensitivity and high
spectral resolution.

As a result, we have succeeded in constraining the
2-1 S(1)/1-0 S(1) H2 line ratio to less than 0.1 toward a
T Tauri star, UY Aur. Our result suggests that the molecular
hydrogen is excited by thermal collisions, that is, the gas
temperature is sufficiently high in the disk surface. The high
gas temperature will mean that there are enough amount of small
dust grains which heat the gas via photoelectric effect
induced by FUV photons. Considering that the dust coagulation
and settling toward the midplane is efficient in the disk,
our result suggests continuous supply of small dust grains
by some mechanisms. More detailed analysis of the observed
data together with model calculations will be shown in the
poster.