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

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.

THE BERLIN EXOPLANET SEARCH TELESCOPE II CATALOG OF VARIABLE STARS. II. CHARACTERIZATION OF THE CoRoT SRc02 FIELD

“…This star has a similar light curve to the new class of variable young stellar objects found by Rodríguez-Ledesma et al (2012) in the Orion Nebula Cluster, Klagyivik et al (2013) in the young open cluster NGC 2264, and Pawlak et al (2013) in the Small Magellanic Cloud. This kind of light curve shape can be reproduced with a hot spot either on the star or on an accretion disk around it.…”

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

“…This is also the case for CHS 7797, an unusual high-amplitude 17.8 d periodic variable in the ONC (Rodríguez-Ledesma et al 2012). As described in detail by Rodríguez-Ledesma et al (2012), CHS 7797 has a variation amplitude of ≈1.7 mag in the R, I, and z bands, which decreases only slightly at longer wavelengths (data available up to 4.5 μm). The measured period over the six years (2004 to 2010) of monitoring in the I band is 17.786 ± 0.03 d with a very low false-alarm probability (FAP = 1 × 10 −15 %).…”

“…The best-fitted model predicts an inner disc edge that is located farther from the star than the radius corresponding to a period of 17.8 d, and therefore no dust structure at this radius can account for the observed obscuration. We consider that the second scenario presented by Rodríguez-Ledesma et al (2012) might be the most likely for CHS 7797, namely that CHS 7797 is a binary system with an orbital period of 17.8 d and a circumbinary disc that is slightly inclined with respect to the orbital plane of the binary. In the binary scenario the system is always occulted by the disc but the amount of extinction varies depending on the position of the two stars with respect to the more transparent outer region of the disc.…”

“…However, the object becomes redder when fainter at λ > ∼ 2 μm, which indicates that CHS 7797 is occulted by circumstellar matter in which grains have grown from typical 0.1 μm to ≈1−2 μm sizes. Rodríguez-Ledesma et al (2012) proposed two scenarios to explain the periodic variability observed over many years. In both scenarios the internal clock for the variability is orbital motion in combination with extinction by a surrounding disc.…”

Indian Astronomical Observatory, Leh-Hanle

“…The prime motivations of this project are (a) to explore various manifestations of stellar magnetic activity in very young, low-mass stars, (b) to search for new pre-main sequence eclipsing binaries, and (c) to look for EX Or and FU Or like transient activities associated with young stellar objects (YSOs). Several new variables have been detected in the region, and this work clearly demonstrates the need for a systematic, long term monitoring to detect variablility in YSOs (Parihar et al, 2009;Rodríguez-Ledesma, 2012). YSOs have been identified and characterized in several other starforming regions as well (e.g.…”