WIYN open cluster study

XXVI. Improved kinematic membership and spectroscopy of IC 2391

¹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: imants@pha.jhu.edu
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁴ Laboratoire d'Astrophysique de l'École polytechnique fédérale de Lausanne, 1290 Chavannes-des-Bois, Switzerland
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Vilnius University Observatory, Ciurlionio 29, Vilnius 2009, Lithuania

Received: 2 June 2006
Accepted: 21 August 2006

Abstract

Context.Young open clusters provide important clues to the interface between the main sequence and pre-main-sequence phases of stellar evolution. The young and nearby open cluster IC 2391 is well-suited to studies of these two evolutionary phases.

Aims.We establish a bona fide set of cluster members and then analyze this set in terms of binary frequency, projected rotational velocities, [Fe/H], and lithium abundance. In the wake of the Hipparcos distance controversy for the Pleiades, we compare the main-sequence fitting distance modulus to the Hipparcos mean parallax for IC 2391.

Methods.We have obtained new proper motions for 6991 stars down to –16 over a ~9-deg² area of the sky comprising IC 2391. The precision of proper motions,  mas yr^-1, allowed us to calculate reliable membership probabilities. We also obtained precise radial velocity and  measurements with Coravel and FEROS for 76 probable cluster members. The cluster's mean radial velocity is  km s^-1. The FEROS high-resolution spectra were used to determine both the [Fe/H] abundance in the four main sequence dwarfs of IC 2391 and the Li abundance in 47 stars. In addition, new BV CCD photometry was obtained for the majority of probable cluster members.

Results.The proper-motion survey covers a 6 times larger sky area than the prior targeted searches for cluster members in IC 2391. A total of 66 stars are considered bona fide cluster members down to a mass equivalent to 0.5 . A quarter of them have been newly identified with many in the F2-K5 spectral range, which is crucial for a main-sequence fit. We find a mean [Fe/H] value of , when a solar abundance of (Fe) = 7.45 is adopted. The main sequence fitting yields a distance modulus that is 0.19 mag larger than that derived from Hipparcos parallaxes; thus this offset nearly has the size of a similar offset found for the Pleiades. The Li abundance pattern is similar to the earlier findings and is typical for a 40 Myr old open cluster.

Conclusions.A variety of new data on the probable members of IC 2391 improve essentially all observational parameters of this young open cluster.