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

3.2.4 The outflows of protostars
Astromol members have studied for several years the physics and chemistry of protostellar outflows, which are
one of the most spectacular mass-loss phenomenon that takes place all along the protostellar phase. The current
picture is that these outflows results from the acceleration and the sweeping up of ambient material by a faster,
strongly collimated jet from the protostar. The main issues addressed by Astromol are the nature of the shock
acceleration mechanism of the molecular gas, in particular the type of shock, either purely hydrodynamical (J-)
or magnetohydrodynamical (C-), the relation the flow holds to the high-velocity atomic/ionized (Herbig-Haro)
jet, and the impact of outflows on the parental cloud, both dynamically and chemically.
Until now, most of the information on outflows has been derived from observations probing the cold (20-
50 K), post-shocked molecular material. However, the bulk of mass of the shocked molecular gas lies at much
higher temperatures, in the range 100-2000 K, which can be probed by high-excitation molecular lines in the
mm/submm domain. The data analysis relies strongly on comparison with shock diagnostics, for which we use
state of the art models developed by Pineau des Forets and S. Cabrit in LERMA (Paris).
The confirmation that (MHD) C-shocks play a major role in the dynamics of outflows has been provided
by ISO. The observation of pure H2 rotational lines with ISOCAM/CVF in the archetypal HH 1/2 system in
Orion (Lefloch et al. 2003) has revealed large column densities of warm gas with excitation temperature Tex=
500-1300 K, substantially cooler than the previously known component probed by the near-IR ro-vibrational
H2 lines. Comparison with state of the art grid models (Cabrit et al. 2004) shows that the warm H2 column
density greatly exceeds that predicted for purely hydrodynamical (J-type) shocks, and allows to constrain the
magnetic field. In the leading edge of the jet, where the geometry of the emission allows a simple modeling,
the emission could be satisfyingly reproduced by MHD shock models with neutral-ion decoupling. This result
appears quite general and is one of the most convincing evidences for C-shocks in outflows.
The determination of the physical conditions in the pre-shock and the shocked gas (density, ortho-para ratio)
requires the observations of additional molecular tracers. The molecules formed from the material released from
dust grain mantles in the gas phase through shattering or sputtering in shocks are privileged tools for such study.
SiO has long been recognized as a school case (Schilke et al. 1997 A&A 321, 293; Lefloch et al. 1998 ApJ 504,
L109). The S-bearing species are also of particular interest as their chemistry is relatively fast (τ ∼ 10 4 yr). We
have carried out several observational studies (Wakelam et al. 2004, 2005), which have required the development
of a time-dependent chemical model with up-to-date reaction rate coefficients. We could show that S-bearing
molecular ratios cannot be easily used as chemical clocks, contrary to a suggestion made a few years ago by
various authors, but it allows to constrain the depleted form of sulphur onto grains (Wakelam et al. 2005).
It has long been proposed that the X-ray/UV radiation produced in strong protostellar shocks could affect
the chemical composition of gas and dust in the quiescent surroundings, and the region downstream of HH 2 (see
above) was considered as an illustrative case of such processes. A detailed study based on ISOCAM observations
and the millimeter SO line emission showed however that the gas downstream of HH 2 was not as quiescent
as claimed so far, and that the “anomalous” chemical gas composition was probably the result of previous
protostellar ejections (Lefloch et al. 2005).
3.2.5 The intermediate/high mass protostars
A large number of observations indicate that HII regions play an important role in spreading star formation
throughout the Galaxy. Most of the embedded young stellar clusters in the solar neighborhood are adjacent
to HII regions excited by more evolved stars, like Orion or Perseus OB2, to name but a few. It is estimated
that 10%-30% of stars in mass in the Galaxy are formed under the influence of adjacent HII regions. Various
scenarios of triggered star formation in the molecular layer surrounding the HII regions have been proposed
(Elmegreen & Lada 1977 ApJ 214, 725; Whitworth et al. 1994 MNRAS 268, 291; Fukuda & Hanawa 2000 ApJ
533, 911) but detailed comparison with the observations, especially at early ages, is still missing. Two lines of
investigation have been followed.
i) In collaboration with J. Cernicharo (DAMIR, Madrid), we have started a systematic multi-wavelength
study of the Trifid nebula (M20), from centimeter to near-IR wavelengths, which benefited the advent of ISO,
allowing to study the properties of the Photon-Dominated Region at the interface between the nebula and
the molecular cloud. A comprehensive picture of this young HII region (∼ 0.3 Myr) could be obtained, from
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