Outflows from young objects observed with the ISO-LWS I. Fine structure lines [O I] 63, [O I]145 and [C II]157 micron

TL;DR

This study analyzes far-infrared fine structure lines from ISO data of young stellar objects and outflows, revealing limitations in their diagnostic utility due to effects like self-absorption and masing.

Contribution

It provides a comprehensive statistical analysis of [O I] and [C II] lines in star-forming regions, highlighting their limited diagnostic power and exploring causes of line ratio anomalies.

Findings

[O I] 145 micron line can mase in interstellar conditions.

Self-absorption by foreground gas explains line ratio anomalies.

[O I] and [C II] lines have limited diagnostic value in star-forming regions.

Abstract

Far infrared fine structure line data from the ISO archive have been extracted for several hundred YSOs and their outflows, including molecular (CO) outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of these lines to astrophysics, their excitation and transfer ought to be investigated in detail and, at this stage, the reliability of the diagnostic power of the fine structure transitions of O I and C II has been examined. Several issues, such as the extremely small intensity ratios of the oxygen 63 micron to 145 micron lines, are still awaiting an explanation. It is demonstrated that, in interstellar cloud conditions, the 145 micron line is prone to masing, but that this effect is likely an insufficient cause of the line ratio anomaly observed from cold dark clouds. Very optically thick emission could in principle also account for this, but would need similar, prohibitively high column densities and must therefore be abondoned as a viable explanation. One is left with [O I] 63 micron self absorption by cold and tenuous foreground gas, as has been advocated for distant luminous sources. Recent observations with the submillimeter observatory Odin support this scenario also in the case of nearby dark molecular clouds. On the basis of this large statistical material we are led to conclude that in star forming regions, the [O I] and [C II] lines generally have only limited diagnostic value.