6. Detection of H ₂ emission from the GG Tau circumbinary disk

Thi, van Dishoeck, van Zadelhoff, Blake (Caltech) and Hogerheijde (UC Berkeley) obtained the first detection of the low-lying pure rotational emission lines of H₂ from circumstellar disks around T Tauri stars, using the ISO-SWS, see Fig. 2.9. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J=2$\to$0 S(0) line at 28.218 $\mu$m and the J=$3\to$1 S(1) line at 17.035 $\mu$m have been observed toward the double binary system GG Tau. Together with limits on the J=$5\to$3 S(3) and J=$7\to$5 S(5) lines, the data suggest the presence of gas at $T_{\rm kin}\approx 110\pm 10$ K with a mass of $(3.6\pm 2.0)\times 10^{-3}$ M$_{\odot}$. This amounts to $\sim 3$ % of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but is larger than the estimated mass of the circumstellar disk(s). The origin of this warm gas is still unclear; possibilities include photon and wind-shock heating mechanisms of the circumbinary material, but existing model calculations fall short by a factor of a few. The first results for GG Tau were published in ApJ Letters; the analysis of a larger sample of objects in underway.

  

Figure 2.9: Detection of H₂ J=$2\to$0 S(0) and S(1) J=$3\to$1 emission from the circumbinary disk around GG Tau obtained with the ISO-SWS. The solid lines indicate Gaussian fits to the data with a width fixed at the instrumental resolution (from: Thi et al.).