Ionizing feedback from an O star formed in a filament

Abstract

ABSTRACT We explore a simple semi-analytic model for what happens when an O star (or cluster of O stars) forms in an isolated filamentary cloud. The model is characterized by three configuration parameters: the radius of the filament, $R_{_{\rm FIL}}$, the mean density of H2 in the filament, $n_{_{\rm FIL}}$, and the rate at which the O star emits ionizing photons, $\dot{\cal N}_{_{\rm LyC}}$. We show that for a wide range of these configuration parameters, ionizing radiation from the O star rapidly erodes the filament, and the ionized gas from the filament disperses into the surroundings. Under these circumstances the distance, L, from the O star to the ionization front (IF) is given approximately by $L(t) \sim 5.2 {\rm pc} [R_{_{\rm FIL}}/0.2 {\rm pc}]^{-1/6} [n_{_{\rm FIL}}/10^4 {\rm cm^{-3}}]^{-1/3} [\dot{\cal N}_{_{\rm LyC}}/10^{49} {\rm s}^{-1}]^{1/6} [t/{\rm Myr}]^{2/3}$, and we derive similar simple power-law expressions for other quantities, for example the rate at which ionized gas boils off the filament, $\dot{M}_{_{\rm IF}}(t)$, and the mass, $M_{_{\rm SCL}}(t)$, of the shock-compressed layer that is swept up behind the IF. We show that a very small fraction of the ionizing radiation is expended locally, and a rather small amount of molecular gas is ionized and dispersed. We discuss some features of more realistic models, and the extent to which they might modify or invalidate the predictions of this idealized model. In particular we show that, for very large $R_{_{\rm FIL}}$ and/or large $n_{_{\rm FIL}}$ and/or low $\dot{\cal N}_{_{\rm LyC}}$, continuing accretion on to the filament might trap the ionizing radiation from the O star, slowing erosion of the filament even further.