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Table 7

Synthetic table giving the characteristic times of destruction and formation processes of the aliphatic hydrogenated carbon component in the different phases of the interstellar medium.

Interstellar phase Diffuse Interface a Dense

Visual extinction AV(mag)  <10-4 10-4 − 1  >1
Density n (cm-3) 1−100 102−104 104−108
Energy deposited UV  ≫  CR UVCRinduced  ~ 10  ×  CRb
Atomic or molecular hydrogen H  ≫  H2 H  < H2 H  ≪  H2

Dynamical time tdyn (years) 108c  ≲107d  ~few 107e
Destruction time by CR td,CR (years) 108 108 108
Destruction time by UV photons td,UV (years) 4 × 103f  ≳4.103exp(AV)  ≳107g
Formation time by atomic H tf,H (years) 2 × 103h less efficient than in diffuse ISM inefficient?i

Destruction/Formation Efficient formation Efficient destruction? Slow destruction

Notes. The destruction and formation characteristic times can be compared to the dynamical time, i.e., the approximate time spent in each of these phases.


The figure is adapted from Le Petit et al. (2006). The blue zone emphasises the transition zone between atomic and molecular hydrogen.


The local UV field induced by cosmic rays deposits more energy than cosmic rays themselves (Shen et al. 2004). However, because the penetration depth of photons is smaller, the ice mantles are more UV processed, whereas the internal part of grains is dominated by cosmic ray energy deposition.


Mennella et al. (2001) (σd,UV = 1.0 × 10-19 cm2/photon), Mathis et al. (1983) (interstellar radiation flux of about 8  ×  107 photons cm-2 s-1).


Mennella et al. (2001) (σd,UV = 1.0 × 10-19 cm2/photon), Prasad & Tarafdar (1983) (flux of internal UV field due to CR of 103−104 photons cm-2 s-1). This results in a characteristic destruction time by an internal UV field of 107−108 yr. Once the ice mantles appear on the dust grains (AV ≳ 3, (Whittet et al. 1988; Smith et al. 1993; Murakawa et al. 2000)), the refractory material is partly protected from the UV via the photochemical interaction in these mantles.


Mennella (2006) (σf,H = 1.7 × 10-18 cm2/H atom), Sorrell (1990) (flux of 8  ×  106 H atoms cm-2 s-1 in diffuse clouds).


See the first two paragraphs of Sect. 4.6.2 for details.

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