Star formation in the Cepheus Flare region: implications from morphology and infrared properties of optically selected clouds

Abstract

Aims. A complex study of visual and infrared properties of ISM and an examination of cloud morphology was carried out using USNO, 2MASS, DIRBE, IRAS, and ISO data of an extended region in Cepheus to explore the conditions of cloud and star formation. Methods. We mapped the distribution of visual extinction in a 256 square degree area of Cepheus. We identified clouds and described their morphology quantitatively to find features such as globular and head-tail shape. We also characterized the region using infrared data and combined the results with those obtained from visual data. Results. Eight cloud complexes and four voids were identified and 208 dark clouds were localised, 86 of which had not been previously catalogued. The observed distribution of cloud axis ratios corresponds to near-prolate ellipsoidal clouds with random alignment. The cloud mass spectrum derived is dN cloud /dM cloud = 6.1 x 10 2 (M cloud /M o ) -1.70 M -1 o . We pointed out a linear relationship between FIR colour temperature and galactic latitude at intermediate galactic latitudes (3° < b < 18°) and a similar relationship for minimum cloud temperatures. The observed FIR colours of dark clouds were found to agree well with model calculations. We showed the signs of connection between the far-IR loop GIRL G109+11 and the active triggered star formation of adjacent cloud complexes. Relationships between the star-forming efficiency, peak extinction, and cloud mass were recognized and described with empirical formulae. YSOs were found in 7 of the 11 clouds with extinction above 4 mag. These 11 clouds represent 21% of the total cloud mass and contain 52% of all the YSO candidates. High star-forming efficiencies, 0.52% and 0.56%, were found in globular and head-tail clouds, respectively, relative to the mean efficiency for the clouds in the region (0.16%). Conclusions. Cloud mass and peak extinction were found to be the most important factors of star-forming efficiency besides cloud morphology. The estimated intrinsic axis-ratio distribution is consistent with cloud formation from large-scale external forcing.