Physics And Chemistry In UV Illuminated Regions: The Horsehead Case

Abstract

Molecular lines are used to trace the structure of the interstellar

medium and the physical conditions of the gas in different

environments, from protoplanetary disks to high-z galaxies. To fully

benefit from the diagnostic power of molecular lines, the formation

and destruction paths of the molecules, including the interplay

between gas-phase and grain surface chemistry, must be quantitatively

understood. Well-defined sets of observations of simple template

sources are key to benchmark the theoretical models.

In this context the PDR of the Horsehead mane is a particularly

interesting case because it has a simple geometry (almost 1D, viewed

edge-on) and the density profile across the PDR is well constrained.

In this talk, I will summarize our recent results on the ISM physics

and chemistry in the Horsehead, from a complete and unbiased line

survey at 1, 2 and 3mm performed with the IRAM-30m telescope, where

approximately 30 species (plus their isotopologues) are detected with

up to 7 atoms. I will show the importance of the interplay between the

solid and gas phase chemistry in the formation of (complex) organic

molecules, like H$_2$CO, CH$_3$OH, and CH$_3$CN, which reveal that

photo-desorption of ices is an efficient mechanism to release

molecules into the gas phase. The case of CH$_3$CN is especially

surprising, as it is 40 times more abundant in the warm ($T_\mathrm{kin}\sim60$~K)

UV-illuminated edge of the nebula, than in the shielded and colder

($T_\mathrm{kin}\sim20$~K) inner layers. I will show that complex molecules, such as

HCOOH, CH$_2$CO, CH$_3$CHO, and CH$_3$CCH are easily detected in the

PDR. I will also discuss new diagnostics of the UV-illuminated gas,

like CF$^+$ (for which we recently resolved its hyperfine structure

for the first time), which is observable from the ground, and we

propose it can be used as a proxy of C$^+$. I will finish by reporting

the first detection of a new molecule, recently confirmed to be the

small hydrocarbon C$_3$H$^+$, which shows that photo-erosion of PAHs

is needed to explain the enhanced abundance of other small

hydrocarbons, like C$_3$H and C$_3$H$_2$.