Spectroscopy of the open cluster Blanco 1^*

II. Hα emission as an indicator of relative age

¹ Joint Center for Mesoscale Meteorology, Universty of Reading, Reading, UK
² Visiting Fellow, Astronomy Centre, Univ. Sussex, Falmer, Brighton, BN1 9QH, UK
³ Dept. Physics & Astronomy, Univ. St. Andrews, North Haugh, Fife, KY16 9SS, Scotland

Send offprint request to: P.M. Panagi

Received: 13 September 1995
Accepted: 22 May 1996

Abstract

We present the results of Hα(6562Å) and Li  i(6708Å) observations of 114 low-mass stars of the young open cluster Blanco 1. We also present observations of 30 stars in Ca ii(K). This work extends the first Blanco 1 spectroscopic study of Panagi et al. (1994). 
From a sample of four well-studied clusters, including Blanco 1, we find that the fraction of Hα emission-line stars amongst K dwarfs is a good indicator of relative age, with a smaller fraction indicative of older age. Blanco 1 shows a relatively small fraction of emitters, inconsistent with previous age estimates for the cluster. We estimate the cluster age to to be 90 ± 25 Myr, slightly older than the Pleiades. The method is shown to be more sensitive to age than lithium and a useful alternative to other age measurement techniques. 
The variation of Hα with () is similar to that observed in the older solar neighbourhood dwarfs, suggesting that, at least for the absorption-line stars, the contribution of stellar rotation to the equivalent width is unclear. 
We combine both spectroscopy and photometry to revise cluster membership and give accurate positions for all these stars.