Obscured clusters^*

III. Follow-up observations of Mercer 23

¹ Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011, USA e-mail: margaret.hanson@uc.edu
² Departamento de Fisíca y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Casilla 5030, Valparaíso, Chile e-mail: [radostin.kurtev; jura.borissova]@uv.cl
³ Instituto de Astronomia, Universidad Nacional Autónoma de México, Apartado Postal 70-254, CD Universitaria, CP 04510 Mexico DF, Mexico e-mail: georgiev@astro.unam.mx
⁴ European Southern Observatory, Ave. Alonso de Cordova 3107, Casilla 19, Santiago 19001, Chile e-mail: vivanov@eso.org
⁵ Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA e-mail: Hillier@pitt.edu
⁶ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile and Vatican Observatory, V00120 Vatican City State, Italy e-mail: dante@astro.puc.cl

Received: 15 August 2009
Accepted: 13 April 2010

Abstract

Context. New infrared surveys have revealed over 1000 new open cluster candidates in the Milky Way, but these candidates need to be confirmed with follow up observations. Of particular interest are young, massive star clusters because they serve as nearby analogues to the distant super star clusters studied as point sources in other galaxies.

Aims. We determine the physical parameters and investigate the high-mass stellar content of the infrared star cluster Mercer 23, situated near the Galactic plane (l = 53772, b = +0164).

Methods. Our analysis is based on new Baade/PANIC JHK_S and ISAAC/VLT imaging of Mercer 23 and ISAAC/VLT moderate resolution (R ≈ 4000) spectroscopy of the brightest cluster members in the H- and K-bands. The cluster age is determined from isochrone main-sequence (MS) and pre-MS fitting. We derive stellar parameters for eight of the stellar members, using a full non-LTE modeling of the obtained spectra.

Results. Mercer 23 is a very young cluster, with age of t = 2–4 Myr. The cluster suffers reddening of E(J–K_S) = 1.35, A_V =7.2 mag. The derived distance is d = 6.5±0.3 kpc. Our spectral modeling allows us to conclude that the three most luminous member are evolved highly massive stars: a WR star, and two mid-O supergiant stars, based on their derived luminosity.

Conclusions. Mercer 23 is not a super-massive cluster such as those recently recognized to exist in the Milky Way. However, its mass estimate of 4–6×10³  and possibly more, puts it in the class of young Galactic clusters hosting WR stars.