1302.7257 (Akihiko Fukui et al.)

Optical-to-Near-Infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint for Cloud-free Atmosphere    [PDF]

Akihiko Fukui, Norio Narita, Kenji Kurosaki, Masahiro Ikoma, Kenshi Yanagisawa, Daisuke Kuroda, Yasuhiro Shimizu, Yasuhiro H. Takahashi, Hiroshi Ohnuki, Masahiro Onitsuka, Teruyuki Hirano, Takuya Suenaga, Kiyoe Kawauchi, Shogo Nagayama, Kouji Ohta, Michitoshi Yoshida, Nobuyuki Kawai, Hideyuki Izumiura

We present optical (g', R_c, and I_c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, the second lowest-mass planet among transiting planets around nearby M dwarfs, by using the 50-cm MITSuME telescope and the 188-cm telescope both at Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 \pm 1.3 M_earth, 4.32^{+0.21}_{-0.10} R_earth, and 0.94 \pm 0.12 g cm^{-3}, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5-micron photometry (0.72^{+0.13}_{-0.12} g cm^{-3}). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R_p/R_s) in the J band (0.07577^{+0.00072}_{-0.00075}) is smaller than that in the I_c (0.0802 \pm 0.0013) and 4.5-micron (0.07806^{+0.00052}_{-0.00054}) bands by 5.9 \pm 2.0 % and 3.0 \pm 1.2 %, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorptions and/or scattering by atmospheric molecules. Although the significance of the observed R_p/R_s variations is still low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission spectroscopic observations of this planet to search for certain molecular features without being prevented by clouds.

View original: http://arxiv.org/abs/1302.7257