系外行星系统55 Cnc和Tau Boo的Planetpol偏振测量
Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo
摘要：
我们提出了非常敏感的偏振55 CNC和Tau Boo在试图检测部分偏振的反射光从行星绕这两颗恒星。55 Cnc被一颗0.038天文单位的热海王星行星（55 Cnc e）、一颗0.11天文单位的热木星行星（55 Cnc b）和至少3颗更远的行星环绕。这个系统的偏振非常稳定，没有显示出周期性变化的迹象，如果发现一个短周期行星的话。夜间平均Stokes Q/I和U/I参数的实测标准差为2.2 × 10 ^{-6}。利用蒙特卡罗方法对一个简单大气模型进行多次散射模拟，得到了行星的几何半径、A_G和行星半径的上限。我们假设类瑞利散射（由90度处的最大偏振p_m缩放）和压力不敏感消光。在多次散射作用很小的大气中，偏振与A_G几乎呈线性关系。在这种情况下，对于55 Cnc e，4 σ上限为A_G<0.13（R/1.2 R_{Jup}）^{-2}p_m^{-1}。这表明55 Cnc e相对较小，并且不是纯H-He行星。这些数据没有给出55 Cnc b的有用上限。Tau Boo被一颗异常巨大的热木星行星环绕。夜间平均斯托克斯Q/I和U/I偏振参数的标准偏差为5.1 × 10 ^{-6}。对于τ Boo b，4 σ上限为A_G<0.37（R/1.2 R_{Jup}）^{-2}p_m^{-1}。这将之前报道的这颗行星的类似上限扩展到了更长的波长。Tau Boo数据中较大的散射可能是由于MOST卫星通过光度测量检测到的斑点活动。这些结果与最近声称的对来自HD 189733 b的周期性偏振信号的3 sigma检测的结果形成鲜明对比，该信号的振幅P= 2x 10 ^{-4}。
英文摘要：
We present very sensitive polarimetry of 55 Cnc and tau Boo in an attempt to detect the partially polarised reflected light from the planets orbiting these two stars. 55 Cnc is orbited by a hot Neptune planet (55 Cnc e) at 0.038 AU, a hot Jupiter planet (55 Cnc b) at 0.11 AU, and at least 3 more distant planets. The polarisation of this system is very stable, showing no sign of the periodic variations that would be expected if a short period planet were detected. The measured standard deviation of the night averaged Stokes Q/I and U/I parameters is 2.2x10^{-6}. We derive upper limits on the geometric albedo, A_G and planetary radius using Monte Carlo multiple scattering simulations of a simple model atmosphere. We assume Rayleigh-like scattering (scaled by the maximum polarisation, p_m at 90 degrees) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarisation and A_G. In this case, the 4 sigma upper limits is A_G<0.13(R/1.2 R_{Jup})^{-2}p_m^{-1} for 55 Cnc e. This suggests that 55 Cnc e is relatively small and not a pure H-He planet. The data do not give a useful upper limit for 55 Cnc b. tau Boo is orbited by an unusually massive hot Jupiter planet. The standard deviation in the night averaged Stokes Q/I and U/I polarisation parameters is 5.1x10^{-6}. The 4 sigma upper limit is A_G<0.37(R/1.2 R_{Jup})^{-2}p_m^{-1} for tau Boo b. This extends the similar upper limits reported previously for this planet to longer wavelengths. The larger scatter in the tau Boo data may be due to the spot activity detected photometrically by the MOST satellite. These results contrast markedly with the recent claim of a 3 sigma detection of a periodic polarisation signal from HD189733 with amplitude P=2x10^{-4}, attributed to the planet HD189733 b.
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