Estimating distance, pressure, and dust opacity using submillimeter observations of self-gravitating filaments

Abstract

We present a detailed study of the surface brightness profiles of dense filaments in IC 5146 using recent Herschel observations done with SPIRE. We describe the profile through an equilibrium solution of a self-gravitating isothermal cylinder pressure confined by its surrounding medium. In this first analysis we applied a simple modified black body function for the emissivity, neglecting any radiative transfer effects. Overall we found a good agreement of the observed surface brightness profiles with the model. The filaments indicate strong self-gravity with mass line densities M/l\gtrsim 0.5 (M/l)_max where (M/l)_max is the maximum possible mass line density. In accordance with the model expectations we found a systematic decrease of the FWHM, a steepening of the density profile, and for filaments heated by the interstellar radiation field a decrease of the luminosity to mass ratio for higher central column density and mass line density. We illustrate and discuss the possibility of estimating the distance, external pressure, and dust opacity. For a cloud distance D\sim500 pc and a gas temperature of T_cyl=10 K the model implies an external pressure p_ext/k\sim 2x10^4 K cm^-3 and an effective dust emission coefficient at 250 microns given by delta x kappa_0^em \sim 0.0588 cm^2 g^-1 where delta is the dust-to-gas ratio. Given the largest estimate of the distance to the cloud complex, 1 kpc, the model yields an upper limit delta x kappa_0^em \sim 0.12 cm^2 g^-1.