A New Nearby Candidate Star Cluster in Ophiuchus atd ≃ 170 pc

The recent discoveries of nearby star clusters and associations within a fewhundred pc of the Sun, as well as the order of magnitude difference in theformation rates of the embedded and open cluster populations, suggests thatadditional poor stellar groups are likely to be found at surprisingly closedistances to the Sun. Here I describe a new nearby stellar aggregate found byvirtue of the parallel proper motions, similar trigonometric parallaxes, andconsistent color-magnitude distribution of its early-type members. The 120Myr-old group lies in Ophiuchus at $d$ $\simeq$ 170 pc, with its most massivemember being the 4th-magnitude post-MS B8II-III star $\mu$ Oph. The group mayhave escaped previous notice due to its non-negligible extinction ($A_V$$\simeq$ 0.9 mag). If the group was born with a normal initial mass function,and the nine B- and A-type systems represent a complete system ofintermediate-mass stars, then the original population was probably of order$\sim$200 systems. The age and space motion of the new cluster are very similarto those of the Pleiades, $\alpha$ Per cluster, and AB Dor Moving Group,suggesting that these aggregates may have formed in the same star-formingcomplex some $\sim10^8$ yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A