First detection of CF^+ in the Large Magellanic Cloud
arxiv(2024)
摘要
CF^+ has been established as a valuable diagnostic tool for investigating
photo-dissociation regions (PDRs) and fluorine abundances in the Milky Way.
However, its role in extragalactic environments remains largely uncharted. Our
objective is to explore the significance of CF^+ in the Large Magellanic
Cloud (LMC) and assess its utility as a valuable probe for examining C^+
and fluorine abundances in external galaxies. We performed pointed CF^+
observations toward an active star-forming region, N113 in the LMC, using the
Atacama Pathfinder EXperiment 12 m sub-millimeter telescope. We report the
first discovery of CF^+ in the LMC through the successful detection of the
CF^+ (2→1) and (3→2) lines. The excitation models indicate that
CF^+ emission originates from dense PDRs characterized by an H_2 number
density of (0.5-7.9)× 10^4 cm^-3 in N113. Our observations provide
the first constraint on the fluorine abundance in molecular clouds in the LMC,
disclosing a value of ≲ 1.7× 10^-9. This value is about an
order of magnitude lower than those previously measured toward red giants in
the LMC, indicative of fluorine deficiency in the molecular gas. The estimated
column density ratio between C^+ and CF^+ appears to be lower than the
anticipated equilibrium ratio derived from the fluorine abundance in red
giants. Both phenomena can be explained by the deficiency of CF^+ caused by
the freeze-out of its primary chemical precursor, HF, onto dust grains. The
deficiency of CF^+ within molecular clouds suggests that the measurements
presented in this work serve exclusively as conservative estimates,
establishing lower bounds for both the fluorine abundance and C^+ column
densities in external galaxies.
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