GU Monocerotis: A high-mass eclipsing overcontact binary in the young open cluster Dolidze 25^⋆

¹ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica SuperiorUniversidad de Alicante, Carretera de San Vicente del Raspeig s/n, 03690 San Vicente del Raspeig, Alicante, Spain
e-mail: fjle3@alu.ua.es
² Institut d’Estudis Espacials de Catalunya, Edifici Nexus, c/ Capitá, 2−4, desp. 201, 08034 Barcelona, Spain
³ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, Facultad de Física y Matemáticas, Avda. Astrofísico Francisco Sánchez s/n, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Lenguajes y Sistemas Informáticos, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
⁶ Departamento de Ciencias, IES Arroyo Hondo, c/ Maestro Manuel Casal 2, 11520 Rota, Cádiz, Spain

Received: 30 January 2016
Accepted: 3 March 2016

Abstract

Context. The eclipsing binary GU Mon is located in the star-forming cluster Dolidze 25, which has the lowest metallicity measured in a Milky Way young cluster.

Aims. GU Mon has been identified as a short-period eclipsing binary with two early B-type components. We set out to derive its orbital and stellar parameters.

Methods. We present a comprehensive analysis, including B and V light curves and 11 high-resolution spectra, to verify the orbital period and determine parameters. We used the stellar atmosphere code FASTWIND to obtain stellar parameters and create templates for cross-correlation. We obtained a model to fit the light and radial-velocity curves using the Wilson-Devinney code iteratively and simultaneously.

Results. The two components of GU Mon are identical stars of spectral type B1 V with the same mass and temperature. The light curves are typical of an EW-type binary. The spectroscopic and photometric analyses agree on a period of 0.896640 ± 0.000007 d. We determine a mass of 9.0 ± 0.6 M_⊙ for each component and for temperatures of 28 000 ± 2000 K. Both values are consistent with the spectral type. The two stars are overfilling their respective Roche lobes, sharing a common envelope and, therefore the orbit is synchronised and circularised.

Conclusions. The GU Mon system has a fill-out factor above 0.8, containing two dwarf B-type stars on the main sequence. The two stars are in a very advanced stage of interaction, with their extreme physical similarity likely due to the common envelope. The expected evolution of such a system very probably leads to a merger while still on the main sequence.