[论文解读] Transit spectrophotometry of the exoplanet HD189733b. II. New Spitzer observations at 3.6 microns

Jean-Michel Désert, David K. Sing+7 more|arXiv (Cornell University)|Aug 14, 2010

Stellar, planetary, and galactic studies被引用 49

一句话总结

本研究利用斯皮itzer/IRAC在3.6 μm波段对系外行星HD 189733b进行了高精度凌日光变曲线观测，精度相比以往测量提升了3倍。通过地面V波段光变曲线校正恒星活动效应，解决了行星半径比的不一致问题，表明3.6 μm半径异常更可能由霾层引起的瑞利散射所致，而非水汽吸收。

ABSTRACT

We present a new primary transit observation of the hot-jupiter HD189733b, obtained at 3.6 microns with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. Previous measurements at 3.6 microns suffered from strong systematics and conclusions could hardly be obtained with confidence on the water detection by comparison of the 3.6 and 5.8 microns observations. We use a high S/N Spitzer photometric transit light curve to improve the precision of the near infrared radius of the planet at 3.6 microns. The observation has been performed using high-cadence time series integrated in the subarray mode. We are able to derive accurate system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from the fits of the transit light curve. We compare the results with transmission spectroscopic models and with results from previous observations at the same wavelength. We obtained the following system parameters: R_p/R_\star=0.15566+0.00011-0.00024, b=0.661+0.0053-0.0050, and a/R_\star=8.925+0.0490-0.0523 at 3.6 microns. These measurements are three times more accurate than previous studies at this wavelength because they benefit from greater observational efficiency and less statistic and systematic errors. Nonetheless, we find that the radius ratio has to be corrected for stellar activity and present a method to do so using ground-based long-duration photometric follow-up in the V-band. The resulting planet-to-star radius ratio corrected for the stellar variability is in agreement with the previous measurement obtained in the same bandpass (Desert et al. 2009). We also discuss that water vapour could not be evidenced by comparison of the planetary radius measured at 3.6 and 5.8 microns, because the radius measured at 3.6 microns is affected by absorption by other species, possibly Rayleigh scattering by haze.

研究动机与目标

• 利用高时间分辨率的斯皮itzer/IRAC观测，提高3.6 μm波段行星与恒星半径比的测量精度。
• 解决以往阻碍水汽可靠探测的系统误差和恒星活动效应。
• 调和先前斯皮itzer在3.6 μm与5.8 μm波段观测结果在水汽存在性上的矛盾。
• 开发并应用基于地面V波段光变跟踪的恒星变异性校正方法。
• 优化系统参数（倾角参数b、轨道标度a/R⋆），以更精确约束大气结构与成分。

提出的方法

• 利用斯皮itzer空间望远镜的IRAC仪器，在3.6 μm波段获取高时间分辨率、子阵列模式的凌日光变曲线。
• 采用边缘暗化凌日模型拟合光变曲线，推导系统参数，包括Rp/R⋆、倾角参数b和a/R⋆。
• 应用念珠法（Prayer Bead method）探索联合参数空间中参数的相关性与不确定性。
• 利用地面V波段光变数据监测恒星变异性，特别追踪由黑子引起的自转调制效应。
• 通过建模凌日期间黑子引起的相对通量下降，对3.6 μm波段的行星半径比进行恒星活动校正。
• 将校正后的Rp/R⋆与先前测量结果及瑞利散射和水汽吸收的理论模型进行比较。

实验结果

研究问题

• RQ1 高精度斯皮itzer观测在3.6 μm波段能否解决由恒星活动引起的行星与恒星半径比不一致问题？
• RQ2 黑子引起的通量变化在凌日光变曲线中在多大程度上会偏差3.6 μm波段的Rp/R⋆测量值？
• RQ3 当与5.8 μm波段比较时，3.6 μm波段观测到的半径比是否支持水汽的存在？还是主要受其他大气效应主导？
• RQ4 本研究新获得的系统参数（Rp/R⋆、b、a/R⋆）与以往斯皮itzer观测及其他研究结果相比如何？
• RQ5 基于地面V波段光变跟踪的校正方法能否可靠地消除高精度系外行星凌日测量中的恒星活动效应？

主要发现

• 3.6 μm波段的行星与恒星半径比测量值为Rp/R⋆ = 0.15566⁺⁰.⁰⁰⁰¹¹₋₀.⁰⁰⁰²⁴，相比以往研究精度提升了3倍。
• 倾角参数b = 0.661⁺⁰.⁰⁰⁵³₋₀.⁰⁰⁵⁰，系统标度a/R⋆ = 8.925⁺⁰.⁰⁴⁹₀₋₀.⁰⁵²³。
• 本研究（第3次观测）的3.6 μm半径比比第1次观测高4σ，这是由于未校正恒星活动所致；经V波段校正后，两者达成一致。
• 在对黑子引起的通量变化（fV-band ≈ -2%）进行校正后，3.6 μm波段的校正后Rp/R⋆与第1次观测及以往测量结果一致。
• 本研究结论认为，仅通过比较3.6 μm与5.8 μm波段的半径比，无法可信地探测水汽，因为3.6 μm信号更可能由霾层引起的瑞利散射主导。
• 基于地面V波段后续观测的校正方法成功分离并消除了恒星活动效应，使大气解释更加可靠。