The little-studied cluster Berkeley 90

I. LS III +46 11: a very massive O3.5 If* + O3.5 If* binary

¹ Centro de Astrobiología, CSIC-INTA, campus ESAC, apartado postal 78, 28 691 Villanueva de la Cañada, Madrid, Spain
e-mail: jmaiz@cab.inta-csic.es
² Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica Superior, Universidad de Alicante, PO Box 99, 03 080 Alicante, Spain
³ Departamento de Física y Astronomía, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21 218, USA
⁵ Department of Physics & Astronomy, 1 College Circle, SUNY Geneseo, Geneseo, NY 14 454, USA
⁶ Instituto de Astrofísica de Canarias, 38 200 La Laguna, Tenerife, Spain
⁷ Departamento de Astrofísica, Universidad de La Laguna, 38 205 La Laguna, Tenerife, Spain
⁸ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18 008 Granada, Spain
⁹ Instituto de Astrofísica de La Plata (CCT La Plata-CONICET, Universidad Nacional de La Plata), Paseo del Bosque s/n, 1900 La Plata, Argentina

Received: 18 March 2015
Accepted: 25 April 2015

Abstract

Context. It appears that most (if not all) massive stars are born in multiple systems. At the same time, the most massive binaries are hard to find owing to their low numbers throughout the Galaxy and the implied large distances and extinctions.

Aims. We want to study LS III +46 11, identified in this paper as a very massive binary; another nearby massive system, LS III +46 12; and the surrounding stellar cluster, Berkeley 90.

Methods. Most of the data used in this paper are multi-epoch high S/N optical spectra, although we also use Lucky Imaging and archival photometry. The spectra are reduced with dedicated pipelines and processed with our own software, such as a spectroscopic-orbit code, CHORIZOS, and MGB.

Results. LS III +46 11 is identified as a new very early O-type spectroscopic binary [O3.5 If* + O3.5 If*] and LS III +46 12 as another early O-type system [O4.5 V((f))]. We measure a 97.2-day period for LS III +46 11 and derive minimum masses of 38.80 ± 0.83 M_⊙ and 35.60 ± 0.77 M_⊙ for its two stars. We measure the extinction to both stars, estimate the distance, search for optical companions, and study the surrounding cluster. In doing so, a variable extinction is found as well as discrepant results for the distance. We discuss possible explanations and suggest that LS III +46 12 may be a hidden binary system where the companion is currently undetected.