The young open cluster Stock 16: an example of star formation in an elephant trunk?

Abstract

UBV photometry is presented for 33 stars in the young open cluster Stock 16 lying in the H II region RCW 75, and these observations are used to obtain new values for the reddening (E[subscript B-V] = 0.49 [plus or minus] 0.01), distance (1.90 [plus or minus] 0.08 kpc), and age (3-5 x 10[superscript 6] yr) of the cluster. The nearby Cepheid V378 Cen and the W-R stars MR 44 and MR 45 are confirmed as nonmembers of Stock 16, although the latter may be members of an anonymous background association which lies at a distance of 3.6 kpc. The relationship of Stock 16 to nearby members of Cen OB1 and Cen R1 is discussed, and arguments are presented for the origin of this sparsely populated cluster from a now-truncated elephant-trunk structure visible in RCW 75. Numbers of faint stars found just within the eastern boundaries of RCW 75 are suggested to be low-mass pre-main-sequence stars formed from the interaction of the ionization/shock front from RCW 75 with the adjacent gas and dust complex. Stock 16 and RCW 75 otherwise constitute what appears to be a rather passive star-formation complex.