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Gas-phase CO depletion and N2H+ abundances in starless cores | | N. Lippok
; R. Launhardt
; D. Semenov
; A. M. Stutz
; Z. Balog
; Th. Henning
; O. Krause
; H. Linz
; M. Nielbock
; Ya. N. Pavlyuchenkov
; M. Schmalzl
; A. Schmiedeke
; H. J. Bieging
; | | Date: |
13 Aug 2013 | | Abstract: | Seven isolated, nearby low-mass starless molecular cloud cores have been
observed as part of the Herschel key program Earliest Phases of Star formation
(EPoS). By applying a ray-tracing technique to the obtained continuum emission
and complementary (sub)mm emission maps, we derive the physical structure
(density, dust temperature) of these cloud cores. We present observations of
the 12CO, 13CO, and C18O (2-1) and N2H+ (1-0) transitions towards the same
cores. Based on the density and temperature profiles, we apply time-dependent
chemical and line-radiative transfer modeling and compare the modeled to the
observed molecular emission profiles. CO is frozen onto the grains in the
center of all cores in our sample. The level of CO depletion increases with
hydrogen density and ranges from 46% up to more than 95% in the core centers in
the core centers in the three cores with the highest hydrogen density. The
average hydrogen density at which 50% of CO is frozen onto the grains is
1.1+-0.4 10^5 cm^-3. At about this density, the cores typically have the
highest relative abundance of N2H+. The cores with higher central densities
show depletion of N2H+ at levels of 13% to 55%. The chemical ages for the
individual species are on average 2+-1 10^5 yr for 13CO, 6+-3 10^4 yr for C18O,
and 9+-2 10^4 yr for N2H+. Chemical modeling suggests that the UV-radiation at
the outer boundaries of the cores is weaker than the nominal value; this could
be caused by an unresolved outer cold envelope of small dust grains or a weaker
than canonically assumed UV-component of the interstellar radiation field
(ISRF) itself. We observationally confirm chemical models of CO-freezeout and
nitrogen chemistry. We find clear correlations between the hydrogen density and
CO depletion and the emergence of N2H+. The chemical ages indicate a core
lifetime of less than 1 Myr. | | Source: | arXiv, 1308.2958 | | Services: | Forum | Review | PDF | Favorites | |
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