A bright-rimmed cloud sculpted by the Hii region Sh2-48

Abstract

Aims. To characterize a bright-rimmed cloud embedded in the Hii region Sh2-48 searching for evidence of triggered star formation. Methods. We carried out observations towards a region of 20 × 20 centered at RA=18h 22m 11.39s , dec.=-14 350 24.8100(J2000) using the Atacama Submillimeter Telescope Experiment (ASTE; Chile) in the 12CO J=3?2, 13CO J=3?2, HCO+ J=4?3, and CS J=7?6 lines with an angular resolution of about 2200 . We also present radio continuum observations at 5 GHz carried out with the Jansky Very Large Array (JVLA; EEUU) interferometer with a synthetized beam of 700 × 500 . The molecular transitions are used to study the distribution and kinematics of the molecular gas of the bright-rimmed cloud. The radio continuum data is used to characterize the ionized gas located at the illuminated border of this molecular condensation. Combining these observations with infrared public data allows us to build up a comprehensive picture of the current state of star formation within this cloud. Results. The analysis of our molecular observations reveals the presence of a relatively dense clump with n(H2) ~ 3 × 103cm−3, located in projection onto the interior of the Hii region Sh2-48. The emission distribution of the four observed molecular transitions has, at VLSR ~ 38 kms−1, morphological anti-correlation with the bright-rimmed cloud as seen in the optical emission. From the new radio continuum observations we identify a thin layer of ionized gas located at the border of the clump which is facing to the ionizing star. The ionized gas has an electron density of about 73 cm−3 which is a factor three higher than the typical critical density (nc ~ 25 cm−3) above which an ionized boundary layer can be formed and be maintained. This fact supports the hypothesis that the clump is being photoionized by the nearby O9.5V star, BD-14 5014. From the evaluation of the pressure balance between the ionized and molecular gas, we conclude that the clump would be in a pre-pressure balance state with the shocks being driven into the surface layer. Among the five YSO candidates found in the region, two of them (class I), are placed slightly beyond the bright rim suggesting that their formation could have been triggered via the radiation-driven implosion process.