Formation and Evolution of Planetary Systems: Upper Limits the Gas Mass in HD105

Astrophysics, abstract
astro-ph/0506252

From: Uma Gorti [view email]

Date: Fri, 10 Jun 2005 22:13:27 GMT   (77kb)

We report infrared spectroscopic observations of HD 105, a nearby ($\sim 40$
pc) and relatively young ($\sim 30$ Myr) G0 star with excess infrared continuum
emission, which has been modeled as arising from an optically thin
circumstellar dust disk with an inner hole of size $\gtrsim 13$ AU. We have
used the high spectral resolution mode of the Infrared Spectrometer (IRS) on
the Spitzer Space Telescope to search for gas emission lines from the disk. The
observations reported here provide upper limits to the fluxes of H$_2$ S(0)
28$\mu$m, H$_2$ S(1) 17$\mu$m, H$_2$ S(2) 12 $\mu$m, [FeII] 26$\mu$m, [SiII]
35$\mu$m, and [SI] 25$\mu$m infrared emission lines. The H$_2$ line upper
limits directly place constraints on the mass of warm molecular gas in the
disk: $M({\rm H_2})< 4.6$, 3.8$\times 10^{-2}$, and $3.0\times 10^{-3}$ M$_J$
at $T= 50$, 100, and 200 K, respectively. We also compare the line flux upper
limits to predictions from detailed thermal/chemical models of various gas
distributions in the disk. These comparisons indicate that if the gas
distribution has an inner hole with radius $r_{i,gas}$, the surface density at
that inner radius is limited to values ranging from $\lesssim 3$ gm cm$^{-2}$
at $r_{i,gas}=0.5$ AU to 0.1 gm cm$^{-2}$ at $r_{i,gas}= 5-20$ AU. These values
are considerably below the value for a minimum mass solar nebula, and suggest
that less than 1 M$_J$ of gas (at any temperature) exists in the 1-40 AU
planet-forming region. Therefore, it is unlikely that there is sufficient gas
for gas giant planet formation to occur in HD 105 at this time.

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