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

the massive deeply embedded protostellar cores and the bright rimmed globules detected in the compressed
surrounding cloud, to the “naked” cores and the protoplanetary disks exposed to the ionizing radiation in
the nebula (Cernicharo et al. 1998; Lefloch & Cernicharo 2000, Lefloch et al. 2001, 2002). Recent Spitzer
observations have unveiled the young stellar population of M20 and the protostellar cluster forming in the
massive cores (see Fig. 3.2; Rho et al. 2005).
ii) In collaboration avec L. Deharveng, A. Zavagno (Marseille), A. Whitworth (Cardiff), we have undertaken
a multi-wavelength survey for evidences of induced star formation around young HII regions, to study the
molecular emission of the parental cloud and compare the properties of the protostellar condensations and the
young stellar clusters with the predictions of the scenarios of triggered star formation (see e.g. Deharveng et al.
2004).
Figure 3.2: Images of the Trifid Nebula in the visible (left panel) and in the Infrared (right panels). The IR
images have been obtained by SPITZER Rho et al. 2005 and show the presence of many young stars invisible
in the optical, because obscured by the thick dusty molecular cloud.
3.2.6 The X-rays from young protostars
• X-ray irradiation of circumstellar disks
X-ray emission is ubiquitous among young stars (see Feigelson & Montmerle 1999, ARAA, 37, 363). In
low-mass stars, the main X-ray emission mechanism is due to solar-type magnetic activity, which manifests
itself mainly in the form of hour-long flares. The average X-ray luminosity (normalized to the stellar bolometric
luminosity) is very high (LX/Lbol ∼ 10 −4 − 10 −3 ), i.e., three to four times higher than for the present-day Sun.
In the case of young stars, the prime target for the X-rays is the circumstellar disk. The main effect is ionization
(induced by the photoelectric effect): although the results depend to some extent on the density-radius relation,
in most cases the disk is ionized throughout its extent, with representative values of the ionization fraction
comparable to the average ISM (xe ≈ 10 −7 ) (Glassgold, Feigelson, & Montmerle 2000, Protostars & Planets IV,
University of Arizona Press, p.429). The fact that the circumstellar disk is ionized (albeit weakly) is important
because it provides a coupling between the disk material and magnetic fields, especially in the inner parts of
the disk, close to the “accretion-ejection” engine. Note however that the densest parts of the disk, along the
equatorial plane, may be too thick for X-rays to penetrate them, resulting in an embedded “dead zone”. It is
possible that such a dead zone, being neutral, be favorable to the formation of planets (see also the discussion
in §3.5.
Another effect is that of X-ray heating. Glassgold et al. (2005) have shown that this heating is efficient in the
outer, tenuous layers of the disk, resulting in an extended (several tens of AU) warm photosphere (T ∼ 3, 000 K),
explaining the observed CO overtone IR lines. As a result, the vertical temperature structure of circumstellar
disks must be “inverted”, i.e., cold along the horizontal plane and warm at high altitudes.
54