Title:

Deuterated molecules: a new tool to probe the structure and  
evolution of proto-planetary disks


Cecilia Ceccarelli
Laboratoire d'Astrophysique, Observatoire de Grenoble



Abstract:

Proto-Planetary disks are the sites of planet formation. Their
physical, dynamical and chemical structure and evolution determine if,
when, how, where and what planets form. Particularly important are the
disks surrounding solar type protostars (T Tau stars), for they likely
represent the progenitor of the Solar System. Their study can,
therefore, shed light on the origin of our own system. To constrain
theories, key parameters to measure are:

i) the gas mass of the disk, which, when compared to the dust mass,
measures the gas dispersal and/or dust coagulation - two key processes
for the formation of rocky and gaseous planets;

ii) the ionization degree across the disk, which determines the
regions of magneto-rotational instability - thus, whether viscous
accretion across the disk can occur and where;

iii) the water content across the disk, and its degree of
fractionation, which are critical to understand the origin of the
oceans on the Earth and comets;

iv) in addition, being water the major constituent of the grain
mantles its gas phase abundance is an important diagnostics of the
processes occurring in the disk.

In this talk, I will present observations and models of deuterated
molecules which help to address the above issues. Specifically, I will
discuss the cases of the deuterated forms of H3+ (H2D+) and H2O (HDO).

H2D+: This molecule has been recently detected for the first time in a
proto-planetary disk (Ceccarelli et al. 2004). This is, so far, the
only detected molecule able to probe the gas in the outer (>20 AU)
disk mid-plane, where the bulk of the disk mass resides. In fact, the
outer midplane is so cold (<20 K) and dense (> 10^7 cm^{-3}) that all
heavy-element bearing molecules (including CO) freeze out onto the
grain mantles and disappear from the gas phase. Observing the disk
midplane has proved, therefore, to be a formidable challenge. I will
discuss the diagnostic value of the H2D+ detection (Ceccarelli &
Dominik 2005) and the potentiality of future H2D+ observations .

HDO: The recent detection of HDO in a proto-planetary disk shows that
the HDO/H2O ratio (~1%) in the outer disk is ten times larger than in
the oceans and comets, and rather similar to what measured in low mass
protostars (Ceccarelli et al. 2005). However, the most important
aspect of this detection is that, being the disks surrounding solar
type protostars so cold and dense, water vapor should be present only
in the inner (<20 AU) disk. The detection of HDO at a distance up to
700 AU challenged our understanding of the basic processes occurring
in disks.  Indeed, a new model, triggered by this detection, predicts
that the FUV photons of the Interstellar Field can cause
photo-desorption of the icy mantles, injecting water vapor in the gas
phase in the observed amount (Dominik et al. 2005). This is the first
time that recycling of icy mantles in the proto-planetary disks is
proved. I will discuss the implications of these new findings.