Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

et al. 2002, 2003). When CO disappears from the gas phase (because condensed onto the grain mantles),
ratio is highly enhanced and can reach the unity (Caselli et al. 2003). Besides, even the
the H2D + /H + 3
multiple deuterated forms of H + 3 , namely HD+ 2 and D+ 3 , can become more abundant than H+ 3 (Roberts,
Herbst & Millar 2003, ApJ 591, L41). Since the deuterated forms of H + 3 react with all molecules in the
gas phase exchanging the D atom, they transmit the deuteration to molecules (e.g. Roberts, Herbst &
Millar 2004, A&A 424, 905).
• Molecules like H2CO, CH3OH and H2S, which are thought to form on the grain surfaces rather than in
the gas phase, have been found to show up the largest multiple deuteration factors (Parise et al. 2002,
2004a; Vastel et al. 2003). Very likely this is because they are formed by hydrogenation of CO (and S)
during the last phases of the pre-collapse, when the CO depletion is larger (this is a definition!).
• Water though does not follow the same route. Indeed, the gaseous HDO/H2O ratio is less than 3% in low
mass protostars (Parise et al. 2005), whereas HDCO/H2CO and CH2DOH/CH3OH ratios are around
30%. Even the solid HDO/H2O ratio is indeed less than 3% (Parise et al. 2004b), which confirms the
lower deuteration of water. One possible interpretation is that water forms on the grain surfaces at an
higher temperature, because the H2O condensation occurs already at about 100 K. This would limit the
H2D + /H + 3 ratio.
• At the center of the Pre-Stellar Cores, all CO is now believed to almost completely freeze-out onto the
grain mantles. Therefore, CO and all the other heavy-bearing molecules disappear from the gas phase,
and the best tool to probe those regions is the H2D + emission (Caselli et al. 2003). The study of the
profile of the H2D + ground state transition provides a valuable tool to probe the velocity field at the
center of the condensation, and, consequently, whether and how the collapse sets in (van der Tak, Caselli
& Ceccarelli 2005).
• Similarly to the centers of the Pre-Stellar Cores, the gas in the midplane of the proto-planetary disks
surrounding solar type protostars can only be probed by the H2D + emission (Ceccarelli et al. 2004,
Ceccarelli & Dominik 2005). This aspect will be discussed in more detail in §3.5.
3.2.3 The hot corinos of solar type protostars
In the first phases of the star birth, the future star is embedded in and totally obscured by the infalling material,
which forms a thick envelope. The density in the envelope ranges between 10 4 to 10 8 cm −3 , and the temperature
ranges between 10 and about 200 K. The exact density and temperature structure depends on the dynamics of
the collapse. It is therefore extreme important to study the density and temperature profiles of these envelopes
and how they evolve with time. Furthermore, giving the involved densities and temperatures, protostellar
envelopes are privileged sites for a rich chemistry to occur. The chemical composition of these envelopes is
not only interesting in itself. The matter in protostellar envelopes forms the proto-planetary disks, which will
eventually form the planets and comets. During the comets bombarding phase, molecules formed during the
protostellar collapse phase may be preserved ( for example frozen-out onto the dust grains) and brought to
the formed planets. It is therefore of paramount importance to know the molecular complexity reached in the
protostellar envelopes. In this context, Astromol members, in collaboration with WAGOS, have contributed to
three major aspects:
• The study of the physical structure. Astromol members published the very first articles reconstructing
the density and temperature profiles of protostellar envelopes ( Ceccarelli et al. 2000 A&A 355, 1129;
2000 A&A 357, L9; 2001 A&A 372, 998; Maret et al. 2002, 2004, 2005). Based on these studies, the
protostellar envelopes have approximatively the structure of free-infalling envelopes, as predicted by the
inside-out theory by Shu and colleagues. In addition, they possess warm and dense regions close to the
central object where the icy grain mantles sublimate, injecting into the gas phase the molecules formed
and/or trapped into the mantles.
• The study of the chemical composition. Chemically, the protostellar envelopes are formed by two components:
i) an outer envelope (defined by where the dust temperature is less than ∼ 100 K), whose chemical
composition is similar to that in molecular clouds; ii) an inner envelope, where the chemistry is dominated
by the material sublimated from the icy grain mantles. In this inner envelopes, large quantities of H2O,
H2CO, CH3OH, SO and SO2 molecules are found (Maret et al. 2002, 2004, 2005; Wakelam et al. 2004a,
2004b).
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