Hunting pre-stellar cores with APEX: IRAS16293E (Oph464)

Abstract

Pre-stellar cores are the first steps in the process of star and planet formation. However, the dynamical and chemical evolution of pre-stellar cores is still not well understood. We aim at estimating the central density of the pre-stellar core IRAS16293E and at carrying out an inventory of molecular species towards the density peak of the core. We observed high-$J$ rotational transitions of N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$, and several other molecular lines towards the dust emission peak using the Atacama Pathfinder EXperiment (APEX) telescope, and derived the density and temperature profiles of the core using far-infrared surface brightness maps from $Herschel$. The N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$ lines were analysed by non-LTE radiative transfer modelling. Our best-fit core model consists in a static inner region, embedded in an infalling envelope with an inner radius of approximately 3000 au (21" at 141 pc). The observed high-J lines of N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$ (with critical densities greater than 10${}^6$ cm${}^{-3}$) turn out to be very sensitive to depletion; the present single-dish observations are best explained with no depletion of N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$ in the inner core. The N${}_2$D${}^{+}$/N${}_2$H${}^{+}$ ratio that best reproduces our observations is 0.44, one of the largest observed to date in pre-stellar cores. Additionally, half of the molecules that we observed are deuterated isotopologues, confirming the high-level of deuteration towards this source. Non-LTE radiative transfer modelling of N${}_2$H${}^{+}$ and N${}_2$D${}^{+}$ lines proved to be an excellent diagnostic of the chemical structure and dynamics of a pre-stellar core. Probing the physical conditions immediately before the protostellar collapse is a necessary reference for theoretical studies and simulations with the aim of understanding the earliest stages of star and planet formation and the time scale of this process.

arXiv commentAccepted for publication in A&A

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