Gas-phase CO depletion and N₂H⁺abundances in starless cores

Abstract

Context. In the dense and cold interiors of starless molecular cloud cores, a number of chemical processes allow for the formation of complex molecules and the deposition of ice layers on dust grains. Dust density and temperature maps of starless cores derived from Herschel continuum observations constrain the physical structure of the cloud cores better than ever before. We use these to model the temporal chemical evolution of starless cores. Aims. We derive molecular abundance profiles for a sample of starless cores. We then analyze these using chemical modeling based on dust temperature and hydrogen density maps derived from Herschel continuum observations. Methods. We observed the 12 CO (2-1), 13 CO (2-1), C 18 O (2-1) and N 2 H + (1-0) transitions towards seven isolated, nearby low-mass starless molecular cloud cores. Using far infrared (FIR) and submillimeter (submm) dust emission maps from the Herschel key program Earliest Phases of Star formation (EPoS) and by applying a ray-tracing technique, we derived the physical structure (density, dust temperature) of these cores. Based on these results we applied time-dependent chemical modeling of the molecular abundances. We modeled the molecular emission profiles with a line-radiative transfer code and compared them to the observed emission profiles. Results. 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 of 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 N 2 H + . The cores with higher central densities show depletion of N 2 H + at levels of 13% to 55%. The chemical ages for the individual species are on average (2 ± 1) × 10 5 yr for 13 CO, (6 ± 3) × 10 4 yr for C 18 O, and (9 ± 2) × 10 4 yr for N 2 H + . Chemical modeling indirectly suggests that the gas and dust temperatures decouple in the envelopes and that the dust grains are not yet significantly coagulated. Conclusions. 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 N 2 H + . The chemical ages indicate a core lifetime of less than 1 Myr.

Gas and dust in the star-forming regionρ Oph A

“…For instance, temperatures based on such observations are around the (for a dense core) surprisingly high value of ∼25 K. At temperatures below this, which are commonly encountered in dense cores, the CO gas will be depleted because of freeze-out of the molecules onto the surfaces of the accompanying grains. In contrast, N 2 H + molecules remain in the gas phase down to much lower temperatures (Lippok et al 2013, and references therein).…”

“…However, at temperatures below about 25 K, the abundance of the dominant gas tracer, CO and its isotopologues, could also become highly diminished by freeze-out (e.g., Lippok et al 2013, and references therein). One might therefore select spectral lines of a molecule that is seen in regions where CO is thought to be depleted.…”

THE ARIZONA RADIO OBSERVATORY CO MAPPING SURVEY OF GALACTIC MOLECULAR CLOUDS. IV. THE NGC 1333 CLOUD IN PERSEUS IN CO J = 2-1 AND 13 CO J = 2-1

“…Previous papers in this series made such data available for the giant molecular clouds near W51 and W3 (Bieging et al 2010;Bieging & Peters 2011); and for the intermediate mass Serpens cloud (Burleigh et al 2013). Observations of Bok globules are shown in Stutz et al (2009) and Lippok et al (2013). We have also given summary results for GMCs associated with the H ii regions Sh 254-258 ) and M17 (Povich et al 2009).…”

THE ARIZONA RADIO OBSERVATORY CO MAPPING SURVEY OF GALACTIC MOLECULAR CLOUDS. V. THE SH2-235 CLOUD IN CO J = 2 − 1, 13CO J = 2 − 1, AND CO J = 3 − 2

“…Previous papers reported on the giant molecular clouds (GMCs) associated with H II regions W51 (Bieging et al 2010) and W3 (Bieging & Peters 2011); and on the lower-mass clouds in Serpens (Burleigh et al 2013) and the NGC1333 region in Perseus (Bieging et al 2014). In addition, summary results have been given for molecular clouds associated with the H II regions Sh2-254-258 ) and M17 (Povich et al 2009), and for a selection of Bok Globules (Stutz et al 2009;Lippok et al 2013). …”