Molecular clumps photoevaporation in ionized regions

Abstract

We study the photoevaporation of molecular clumps exposed to a UV radiation field including hydrogen-ionizing photons (hν > 13.6 eV) produced by massive stars or quasars. We follow the propagation and collision of shock waves inside clumps and take into account self-shielding effects, determining the evolution of clump size and density with time. The structure of the ionization-photodissociation region (iPDR) is obtained for different initial clump masses (M = 0.01 − 10 4 M ) and impinging fluxes (G 0 = 10 2 − 10 5 in units of the Habing flux). The cases of molecular clumps engulfed in the HII region of an OB star and clumps carried within quasar outflows are treated separately. We find that the clump undergoes in both cases an initial shockcontraction phase and a following expansion phase, which lets the radiation penetrate in until the clump is completely evaporated. Typical evaporation time-scales are 0.01 Myr in the stellar case and 0.1 Myr in the quasar case, where the clump mass is 0.1 M and 10 3 M respectively. We find that clump lifetimes in quasar outflows are compatible with their observed extension, suggesting that photoevaporation is the main mechanism regulating the size of molecular outflows.

Photoevaporation of Jeans-unstable molecular clumps

“…R-type DF for Z = 0.5Z (Z = 0.2Z ) when G0 > 3 × 10 4 (G0 > 3 × 10 3 ). This finding agrees with results from Vallini et al (2017) and Nakatani & Yoshida (2018), both pointing out that photoevaporation is more rapid in metalpoor clouds, i.e. the ones expect in high redshift galaxies (e.g.…”

“…The dynamics of photoevaporating clumps can also have important consequences for their Far-Infrared (FIR) line emission (Vallini et al 2017), [C II] in particular. In fact, a strong G0 increases the maximum [C II] luminosity, as FUV radiation ionizes carbon in PDRs, albeit shortlived clumps may contribute less significantly to the parent GMC emission.…”

“…In a previous work (Decataldo et al 2017, hereafter D17), we have constructed a 1D numerical procedure to study the evolution of a molecular clump, under the effects of both FUV and EUV radiation. We have followed the time evolution of the structure of the iPDR (ionizationphotodissociation region) and we have computed the photoevaporation time for a range of initial clump masses and intensity of impinging fluxes.…”

ALMA Reveals Metals yet No Dust within Multiple Components in CR7

“…However, while [C II] is insensitive to dust attenuation, unlike (for example) UV emission, its luminosity is sensitive to metallicity (e.g., Vallini et al 2013;Hemmati et al 2017;Olsen et al 2017). The [C II] luminosity is also dependent on the ionization state of the gas and saturates at high temperatures (Kaufman et al 1999;Decataldo et al 2017;Vallini et al 2017).…”

“…However, [C II] luminosity may be reduced due to photoevaporation by far-UV (6-13.6 eV) and ionizing ( 13.6 > eV) photons emitted from young stars in the vicinity, which typically happens a few tens of megayears after the burst of star formation (e.g., Decataldo et al 2017;Vallini et al 2017), which can affect different clumps differently. The metallicities that we would infer from [C II] ( 0.1 0.2 Z »  -) are inconsistent with the metallicity inferred by Bowler et al (2017), which is 0.005 Z »  .…”