Near-infrared spectroscopy in NGC 7538^*

E. Puga¹, A. Marín-Franch², F. Najarro¹, A. Lenorzer², A. Herrero², J. A. Acosta Pulido², L. A. Chavarría³, A. Bik⁴, D. Figer⁵ and S. Ramírez Alegría²

¹ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850, Torrejón de Ardoz, Madrid, Spain e-mail: elena@damir.iem.csic.es
² Instituto de Astrofísica de Canarias, C/ vía Láctea s/n, 38200, La Laguna, Spain
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany
⁵ Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY 14623, USA

Received: 14 September 2009
Accepted: 18 February 2010

Abstract

Aims. The characterisation of the stellar population in young high-mass star-forming regions allows fundamental cluster properties like distance and age to be constrained. These are essential when using high-mass clusters as probes for conducting Galactic studies.

Methods. NGC 7538 is a star-forming region with an embedded stellar population unearthed only in the near-infrared (NIR). We present the first near-infrared spectro-photometric study of the candidate high-mass stellar content in NGC 7538. We obtained H and K spectra of 21 sources with both the multi-object and long-slit modes of LIRIS at the WHT, and complement these data with subarcsecond JHK_s photometry of the region using the imaging mode of the same instrument.

Results. We find a wide variety of objects within the studied stellar population of NGC 7538. Our results discriminate between a stellar population associated to the H ii region, but not contained within its extent, and several pockets of more recent star formation. We report the detection of CO bandhead emission toward several sources, as well as other features indicative of a young stellar nature. We infer a spectro-photometric distance of 2.7 ± 0.5 kpc, an age spread in the range 0.5-2.2 Myr and a total mass ~1.7 × 10³  for the older population.