4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Poster (WITHDRAWN)

Poster session

The JCMT dense gas survey of the Ophiuchus Molecular Cloud: Modelling density using HCO$^+$ $J=4\rightarrow3$



Primary authors



Uncovering the density of molecular clouds is essential for understanding star formation. Past observations of Gould Belt clouds have focused on understanding the cloud column density, supporting a column density threshold for forming protostars. In reality the volume, not column, density should determine whether star formation takes place. To investigate this link between column density, volume density and star formation, we have developed a technique to derive density from molecular line data using radiative transfer codes and assumed density profiles. We apply this technique to HCO$^+$ $J=4\rightarrow3$ data of the Ophiuchus molecular cloud using the James Clerk Maxwell Telescope. After constraining the column density using C$^{18}$O data, the HCO$^+$ emission is modelled using the non-LTE radiative transfer codes RADEX and TORUS under the assumptions of (a) constant densities and (b) variable densities along the line-of-sight. Models with varying density are based on densities that peak at the centre of the cloud and/or are derived from assuming a log-normal density PDF. We find peak densities across the cloud that range from 10$^4$ to 10$^6$ cm$^{−3}$, reaching the critical density of the HCO+ molecule. Though our results are consistent with past column density thresholds for star formation ($>$10$^{22}$ cm$^{−2}$), we find that protostellar and starless/pre-stellar cores do not necessarily form at the highest peak densities in the cloud, but are rather found at densities of order $>$10$^5$ cm$^{−3}$ (using the varying-sigma log-normal density profiles).