Interstellar matter and star formation in W5-E

Abstract

W5-E has been observed with the Herschel-PACS and -SPIRE photometers, at 100, 160, 250, 350, and 500 microns. The dust temperature map shows a rather uniform temperature, in the range 17.5-20 K in the dense condensations or filaments, 21-22 K in the photodissociation regions, and 24-31 K in the direction of the ionized regions. The column densities are rather low, everywhere lower than 10^23 cm-2, and of the order of a few 10^21 cm-2 in the PDRs. About 8000 solar masses of neutral material surrounds the ionized region, which is low with respect to the volume of this HII region; we suggest that the exciting stars of the W5-E, W5-W, Sh~201, A and B HII regions formed along a dense filament or sheet rather than inside a more spherical cloud. Fifty point sources have been detected at 100 microns. Most of them are Class 0/I YSOs. The SEDs of their envelopes have been fitted using a modified blackbody model. These envelopes are cold, with a mean temperature of 15.7+-1.8K. Their masses are in the range 1.3-47 solar masses. Eleven of these point sources are candidate Class 0 YSOs. Twelve of these point sources are possibly at the origin of bipolar outflows detected in this region. None of the YSOs contain a massive central object, but a few may form a massive star as they have both a massive envelope and also a high envelope accretion rate. Most of the Class 0/I YSOs are observed in the direction of high column density material, for example in the direction of the massive condensations present at the waist of the bipolar Sh 201 HII region or enclosed by the bright-rimmed cloud BRC14. The overdensity of Class 0/I YSOs on the borders of the HII regions strongly suggests that triggered star formation is at work in this region but, due to insufficient resolution, the exact processes at the origin of the triggering are difficult to determine.