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

Since X-rays mainly come from flares, as in the case of the Sun, one must also consider the effect of energetic
particles hitting the disk. In particular, energetic protons and α nuclei generate nuclear spallation reactions on
the gas, the resulting particles being subsequently trapped in macroscopic bodies like meteorites. This “internal
irradation” scenario has successfully explained almost all the “extinct radioactivities” in solar system meteorites
(Gounelle et al. 2001, ApJ, 548, 1051; Montmerle 2002, Feigelson et al. 2005).
Taking a broader perspective, the irradiation phenomena that must have taken place during the very young
stages of circumstellar disks around solar-type stars may set interesting boundary conditions for the origin of
life (Montmerle 2005).
• X-ray irradiation of molecular clouds
After nearly three decades of theoretical work and expectations, diffuse X-ray emission has been discovered
massive star-forming regions (M17 : Townsley et al. 2004). This region is excited by about a dozen of O3
stars (the most massive stars in the Galaxy). The diffuse X-ray emission was predicted as coming from a large
hot bubble (T ∼ 10 7 K) of low-density gas, inflated by the intense and fast stellar winds from these stars.
With stellar mass-loss rates reaching ˙ M ∼ 10 −5 M⊙/yr and velocities vw ∼ 4, 000 km s −1 , bubbles several pc
in size or more were expected. It took the sharp, subarcsec resolution of Chandra to distinguish truly diffuse
X-ray emission from the unresolved X-ray emission of the thousands of low-mass stars present in such massive
star-forming regions (Fig. 3.3).
Figure 3.3: ISO pointings of M17, from the O star cluster into the molecular cloud, superimposed on the
Chandra image. Square: ISOCAM field of view; Sn: SWS pointings; Ln: LWS pointings (dashed ellipse: fields
in the direction of the molecular cloud). Thin, closely spaced isophotes: 330 MHz emission; loose isophotes:
IRAS 100 µm emission.
With an intense diffuse X-ray flux, spread over a large volume, it was important to search for large-scale Xray
irradiation effects on the parent giant molecular cloud of M17. Several possible tracers, observed at different
wavelengths, from the radio cm range to the mid-IR range (ISO spectroscopy), were examined from archival
data (Montmerle & Vuong 2005). The results were however inconclusive (like extended excess C + emission),
because of the presence, simultaneous with X-rays, of UV photons from the massive stars, able to travel to large
distances in the tenuous, external layers of the molecular cloud. New, targeted observations in the mm range,
are planned to probe the dense parts of the molecular cloud, which only X-rays can penetrate.
• X-ray absorption and metallicity of molecular clouds
While X-rays are emitted by the young stars born in a molecular cloud, they are absorbed by the surrounding
material, as explained above. Then one can take advantage of this absorption to map the column density NH,X
towards each star, by fitting the observed X-ray spectrum. X-rays are absorbed by heavy atoms, whatever the
material (gas or grains) in which they are located. 1-10 keV X-rays can penetrate up to the equivalent of several
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