Multi-Color Transit Photometry of GJ 1214b through BJHKs-Bands and a Long-Term Monitoring of the Stellar Variability of GJ 1214

TL;DR

This study presents new multi-band transit observations of GJ 1214b and long-term stellar variability monitoring, suggesting a high molecular weight atmosphere and revealing stellar brightness variations with a 44-day period.

Contribution

It provides new transit data in BJHKs-bands and long-term stellar brightness monitoring, improving understanding of GJ 1214b's atmosphere and stellar activity.

Findings

Transit depths favor high molecular weight atmosphere models.

Detected a 44-day stellar brightness variation period.

Observed possible spot-crossing feature in Subaru data.

Abstract

We present 5 new transit light curves of GJ 1214b taken in BJHKs-bands. Two transits were observed in B-band using the Suprime-Cam and the FOCAS instruments onboard the Subaru 8.2m telescope, and one transit was done in JHKs-bands simultaneously with the SIRIUS camera on the IRSF 1.4m telescope. MCMC analyses show that the planet-to-star radius ratios are, Rp/Rs = 0.11651 \pm 0.00065 (B-band, Subaru/Suprime-Cam), Rp/Rs = 0.11601 \pm 0.00117 (B-band, Subaru/FOCAS), Rp/Rs = 0.11654 \pm 0.00080 (J-band, IRSF/SIRIUS), Rp/Rs = 0.11550 ^{+0.00142}_{-0.00153} (H-band, IRSF/SIRIUS), and Rp/Rs = 0.11547 \pm 0.00127 (Ks-band, IRSF/SIRIUS). The Subaru Suprime-Cam transit photometry shows a possible spot-crossing feature. Comparisons of the new transit depths and those from previous studies with the theoretical models by Howe & Burrows (2012) suggest that the high molecular weight atmosphere (e.g., 1% H$_2$O + 99% N$_2$) models are most likely, however, the low molecular weight (hydrogen dominated) atmospheres with extensive clouds are still not excluded. We also report a long-term monitoring of the stellar brightness variability of GJ 1214 observed with the MITSuME 50cm telescope in g'-, Rc-, and Ic-bands simultaneously. The monitoring was conducted for 32 nights spanning 78 nights in 2012, and we find a periodic brightness variation with a period of Ps = 44.3 \pm 1.2 days and semi-amplitudes of 2.1% \pm 0.4% in g'-band, 0.56% \pm 0.08% in Rc-band, and 0.32% \pm 0.04% in Ic-band.