Far-infrared CO and H₂O emission in intermediate-mass protostars

Abstract

Context. Intermediate-mass young stellar objects (YSOs) provide a link to understanding how feedback from shocks and UV radiation scales from low- to high-mass star forming regions.
Aims: Our aim is to analyze excitation of CO and H₂O in deeply embedded intermediate-mass YSOs and compare it with similar studies on low-mass and high-mass YSOs.
Methods: Herschel/PACS spectral maps are analyzed for six YSOs with bolometric luminosities of L_bol ~ 10²-10³L_⊙. The maps cover spatial scales of ~10⁴ AU in several CO and H₂O lines located in the ~55-210 μm range.
Results: Rotational diagrams of CO show two temperature components at T_rot ~ 320 K and T_rot ~ 700-800 K, comparable to low- and high-mass protostars probed at similar spatial scales. The diagrams for H₂O show a single component at T_rot ~ 130 K, as seen in low-mass protostars, and about 100 K lower than in high-mass protostars. Since the uncertainties in T_rot are on the same order as the difference between the intermediate and high-mass protostars, we cannot conclude whether the change in rotational temperature occurs at a specific luminosity or whether the change is more gradual from low- to high-mass YSOs.
Conclusions: Molecular excitation in intermediate-mass protostars is comparable to the central 10³ AU of low-mass protostars and consistent within the uncertainties with the high-mass protostars probed at 3 × 10³ AU scales, suggesting similar shock conditions in all those sources.

Appendix A is available in electronic form at http://www.aanda.org