A bright-rimmed cloud sculpted by the H II region Sh2-48

Abstract

Aims. We characterize a bright-rimmed cloud embedded in the H II region Sh2-48 while searching for evidence of triggered star formation.

Methods. We carried out observations towards a region of 2' x 2' centered at RA = 18(h)22(m)11.39(s), Dec = -14 degrees 35'24.81 '' (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE; Chile) in the (CO)-C-12 J = 3-2, (CO)-C-13 J = 3-2, HCO+ J = 4-3, and CS J = 7-6 lines with an angular resolution of about 22 ''. 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 7 '' x 5 ''. The molecular transitions were used to study the distribution and kinematics of the molecular gas of the bright-rimmed cloud. The radio continuum data was used to characterize the ionized gas located on the illuminated border of this molecular condensation. Combining these observations with infrared public data allowed 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 a relatively dense clump with n(H-2) similar to 3 x 10(3) cm(-3), located in projection onto the interior of the H II region Sh2-48. The emission distribution of the four observed molecular transitions has, at V-LSR similar to 38 km s(-1), morphological anticorrelation 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 on the border of the clump that is facing 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 (n(c) similar to 25 cm(-3)), above which an ionized boundary layer can be formed and maintained. This 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 prepressure 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 by the radiation-driven implosion process.