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

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.

The identification of 93 day periodic photometric variability for YSO YLW 16A

“…This implies that a wide companion offers a plausible, if not required, mechanism to explain how a circum-binary disk could be warped with respect to the orbit of a hypothesized inner binary, to produce the observed periodic disk extinction. These two systems join KH-15D in NGC 2264 and CHS 7797 in the Orion star-forming region (Herbst et al 2010;Rodriguez-Ledesma et al 2013.…”

“…A circum-binary disk, tilted with respect to the binary's orbital plane by the gravitational influence of the third companion, eclipses each member of the inner binary in turn. Periodic eclipsing of a binary by a circum-binary disk is also the preferred model of the well-studied system KH-15D (Herbst et al 2010, and references therein), as well as the recently discovered object CHS 7797 in the Orion star-forming region (Rodriguez-Ledesma et al 2013.…”

Near-Infrared Variability in the Orion Nebula Cluster

“…This star's period was accurately derived to be 17.786 days by Rodríguez-Ledesma et al (2012) using six years of I-band monitoring; its variability was also characterized at R, z , [3.6] and [4.5] bands, showing variability amplitudes ∼ 1.7 in R, I and z and decreasing slightly at longer wavelengths. The star's SED indicates that it is a Class I object, and its spectral features show that it is a low-mass object near the stellar/substellar boundary (Rodríguez-Ledesma et al 2013). The star was noted to show colorless variability at wavelengths λ ≤ 2µm, but at λ ≥ 2µm it became redder when fainter; this was interpreted as evidence for dust grains that had grown to 1 − 2 µm.…”

“…The star was noted to show colorless variability at wavelengths λ ≤ 2µm, but at λ ≥ 2µm it became redder when fainter; this was interpreted as evidence for dust grains that had grown to 1 − 2 µm. In Rodríguez- Ledesma et al (2013), the authors identify spectral differences between two light curve phases that suggest the system has a similar geometry to KH 15D.…”

“…This also sets ONCvar 1226 clearly apart from ONCvar 479 (CHS 7797), whose light curve in bands shorter than 2µm, have similar shape and amplitude including R, I and z . Unlike ONCvar 149, which is faint and highly reddened (K ≈ 15, J ≈ 20), making optical followup impossible, ONCvar 1226 is bright and relatively unreddened (K ≈ 11, J ≈ 12), so it would be an excellent target for optical spectroscopic followup, as was done with CHS 7797 in Rodriguez- Ledesma et al (2013).…”