Research Note

The spectral type of CHS 7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
e-mail: vicrodriguez@mpia.de
² Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Instituto Superior de Corellación Geológica, CONICET, Miguel Lillo 205, 4000, San Miguel de Tucumán, Argentina
⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
⁶ Astronomy Department, Wesleyan University, Middletown, CT 06459, USA

Received: 4 June 2012
Accepted: 18 January 2013

Abstract

Aims. We present the spectroscopic characterization of the unusual high-amplitude very low mass pre-main-sequence periodic variable CHS 7797.

Methods. This study is based on optical medium-resolution (R = 2200) spectroscopy in the 6450 − 8600   Å range, carried out with GMOS-GEMINI -S in March 2011. Observations of CHS 7797 have been carried out at two distinct phases of the 17.8 d period, namely at maximum (I ≈ 17.4 mag) and four days before maximum (I ≈ 18.5 mag). Four different spectral indices were used for the spectral classification at these two phases, all of them well-suited for spectral classification of young and obscured late M dwarfs. In addition, the gravity-sensitive Na I (8183/8195 Å) and K I (7665/7699 Å) doublet lines were used to confirm the young age of CHS 7797.

Results. From the spectrum obtained at maximum light we derived a spectral type (SpT) of M 6.05  ±  0.25, while for the spectrum taken four days before maximum the derived SpT is M 5.75  ±  0.25. The derived SpTs confirm that CHS 7797 has a mass in the stellar-substellar boundary mass range. In addition, the small differences in the derived SpTs at the two observed phases may provide indirect hints that CHS 7797 is a binary system of similar mass components surrounded by a tilted circumbinary disk, a system similar to KH 15D.