An unusual very low-mass high-amplitude pre-main sequence periodic variable

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 Ulugh Bek Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan
⁴ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ Indian Institute of Astrophysics, 560034 Bangalore, India
⁶ Herschel Science Centre, European Space Astronomy Centre (ESA), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ Astronomy Department, Wesleyan University, Middletown, CT 06459, USA

Received: 15 December 2011
Accepted: 26 June 2012

Abstract

Aims. We have investigated the nature of the variability of CHS 7797, an unusual periodic variable in the Orion Nebula Cluster.

Methods. An extensive I-band photometric data set of CHS 7797 was compiled between 2004−2010 using various telescopes. Further optical data have been collected in R and z′ bands. In addition, simultaneous observations of the ONC region including CHS 7797 were performed in the I,J,K_s & IRAC 3.6 and 4.5   μm bands over a time interval of  ≈40 d.

Results. CHS 7797 shows an unusual large-amplitude variation of  ≈1.7 mag in the R,I, and z′ bands with a period 17.786  ±  0.03 d (FAP = 1  ×  10^-15%). The amplitude of the brightness modulation decreases only slightly at longer wavelengths. The star is faint during  ≈2/3 of the period and the shape of the phased light-curves for the seven different observing seasons shows minor changes and small-amplitude variations. Interestingly, there are no significant colour-flux correlations for λ ≲ 2   μm, while the object becomes redder when fainter at longer wavelengths. CHS 7797 has a spectral type of M 6 and an estimated mass between 0.04−0.1 M_⊙.

Conclusions. The analysis of the data suggests that the periodic variability of CHS 7797 is most probably caused by an orbital motion. Variability as a result of rotational brightness modulation by a hot spot is excluded by the lack of any colour-brightness correlation in the optical. The latter indicates that CHS 7797 is most probably occulted by circumstellar matter in which grains have grown from typical 0.1 μm to  ≈1−2 μm sizes. We discuss two possible scenarios in which CHS 7797 is periodically eclipsed by structures in a disc, namely that CHS 7797 is a single object with a circumstellar disc, or that CHS 7797 is a binary system, similar to KH 15D, in which an inclined circumbinary disc is responsible of the variability. Possible reasons for the typical 0.3 mag variations in I-band at a given phase are discussed.