A SPITZER TRANSMISSION SPECTRUM FOR THE EXOPLANET GJ 436b, EVIDENCE FOR STELLAR VARIABILITY, AND CONSTRAINTS ON DAYSIDE FLUX VARIATIONS
A SPITZER TRANSMISSION SPECTRUM FOR THE EXOPLANET GJ 436b, EVIDENCE FOR STELLAR VARIABILITY, AND CONSTRAINTS ON DAYSIDE FLUX VARIATIONS
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系外行星 GJ 436b 的斯皮策透射光谱、恒星变率的证据以及对日侧通量变化的限制
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
S. Seager
中科院分区：
文献类型：
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作者：
H. Knutson;N. Madhusudhan;N. Cowan;J. Christiansen;E. Agol;D. Deming;J. Désert;D. Charbonneau;G. Henry;D. Homeier;J. Langton;G. Laughlin;S. Seager
本文描述了在UT 2007年6月29日至UT 2009年2月4日期间，利用斯皮策太空望远镜上的IRAC仪器在3.6、4.5和8.0 μm波段获得的太阳系外行星GJ 436b的8次凌日和11次次日食的统一分析。我们发现，在同一带通中，访问的最佳拟合过境深度从一个时代到下一个时代可以变化高达总深度的8% （4.7σ显著性）。虽然我们不能完全排除残余探测器效应或行星大气中随时间变化的高空云层是这些变化的原因，但我们认为，在凌日观测的一个子集中，恒星上活跃区域的掩星是最有可能的解释。我们发现，对于最深的3.6 μm凌日，凌日数据比凌日外数据具有更高的标准偏差，如果行星遮蔽了恒星黑子，这是可以预期的。我们还比较了该天体所有已发表的凌日观测结果，发现在红外波段观测到的凌日通常比在可见光波段观测到的凌日有更小的时间偏移。在这种情况下，三个最深的斯皮策凌日都是在五天内测量的，与恒星活动增加的单一时期一致。我们通过提出恒星的自转轴相对于我们的视线是倾斜的，因此行星的轨道很可能是错位的，来调和磁活动区域的存在与恒星缺乏明显的可见光或红外通量变化。与Beaulieu等人报告的结果相反，我们没有发现行星透射光谱中甲烷吸收的令人信服的证据。如果我们排除我们认为受恒星活动影响最大的凌日现象，我们发现我们更喜欢CO增加和甲烷减少的模型，这与史蒂文森等人的GJ 436b的日面成分一致。也有可能所有的凌日都受到这种活动的显著影响，在这种情况下，使用多个时期获得的宽带测光技术来表征行星的透射光谱可能是不可行的。这些观测结果有助于说明围绕晚型恒星运行的行星的透射光谱所面临的挑战；我们预计其他星系，如gj1214，可能会显示出相对可变的凌日深度。我们比较了PHOENIX和ATLAS恒星大气模型预测的翼缘暗化系数，并讨论了这些系数对GJ 436b近掠凌日几何形状下测量的行星-恒星半径比的影响。我们测量到的8 μm次日食深度与一个恒定值一致，并且我们将该波段的行星日面通量变化的1σ上限定为17%。这些结果与该行星的一般环流模型的预测一致，该模型发现该行星的日面通量在8 μm波段内变化了几个百分点或更少。对11次观测的平均值使我们对二次月食深度的估计提高了0.0452%±0.0027%；我们还检查了日食入口和出口的残余，并对GJ 436b表面亮度不均匀造成的偏差设置了上限。我们将观测所得的时间信息与先前公布的数据结合起来，生成了一份精细的轨道星历表，并确定最合适的凌日和日食时间与恒定的轨道周期一致。我们发现，次日食发生在0.58672±0.00017的相位，对应于ecos (ω) = 0.13754±0.00027，其中e为行星的轨道偏心率，ω为周心经度。我们还提出了对其他系统参数的改进估计，包括轨道倾角，a/R -和行星-恒星半径比。
In this paper, we describe a uniform analysis of eight transits and eleven secondary eclipses of the extrasolar planet GJ 436b obtained in the 3.6, 4.5, and 8.0 μm bands using the IRAC instrument on the Spitzer Space Telescope between UT 2007 June 29 and UT 2009 February 4. We find that the best-fit transit depths for visits in the same bandpass can vary by as much as 8% of the total (4.7σ significance) from one epoch to the next. Although we cannot entirely rule out residual detector effects or a time-varying, high-altitude cloud layer in the planet's atmosphere as the cause of these variations, we consider the occultation of active regions on the star in a subset of the transit observations to be the most likely explanation. We find that for the deepest 3.6 μm transit the in-transit data have a higher standard deviation than the out-of-transit data, as would be expected if the planet occulted a star spot. We also compare all published transit observations for this object and find that transits observed in the infrared typically have smaller timing offsets than those observed in visible light. In this case, the three deepest Spitzer transits are all measured within a period of five days, consistent with a single epoch of increased stellar activity. We reconcile the presence of magnetically active regions with the lack of significant visible or infrared flux variations from the star by proposing that the star's spin axis is tilted with respect to our line of sight and that the planet's orbit is therefore likely to be misaligned. In contrast to the results reported by Beaulieu et al., we find no convincing evidence for methane absorption in the planet's transmission spectrum. If we exclude the transits that we believe to be most affected by stellar activity, we find that we prefer models with enhanced CO and reduced methane, consistent with GJ 436b's dayside composition from Stevenson et al. It is also possible that all transits are significantly affected by this activity, in which case it may not be feasible to characterize the planet's transmission spectrum using broadband photometry obtained over multiple epochs. These observations serve to illustrate the challenges associated with transmission spectroscopy of planets orbiting late-type stars; we expect that other systems, such as GJ 1214, may display comparably variable transit depths. We compare the limb-darkening coefficients predicted by PHOENIX and ATLAS stellar atmosphere models and discuss the effect that these coefficients have on the measured planet–star radius ratios given GJ 436b's near-grazing transit geometry. Our measured 8 μm secondary eclipse depths are consistent with a constant value, and we place a 1σ upper limit of 17% on changes in the planet's dayside flux in this band. These results are consistent with predictions from general circulation models for this planet, which find that the planet's dayside flux varies by a few percent or less in the 8 μm band. Averaging over the eleven visits gives us an improved estimate of 0.0452% ± 0.0027% for the secondary eclipse depth; we also examine residuals from the eclipse ingress and egress and place an upper limit on deviations caused by a non-uniform surface brightness for GJ 436b. We combine timing information from our observations with previously published data to produce a refined orbital ephemeris and determine that the best-fit transit and eclipse times are consistent with a constant orbital period. We find that the secondary eclipse occurs at a phase of 0.58672 ± 0.00017, corresponding to ecos (ω) = 0.13754 ± 0.00027, where e is the planet's orbital eccentricity and ω is the longitude of pericenter. We also present improved estimates for other system parameters, including the orbital inclination, a/R⋆, and the planet–star radius ratio.
最近提出的超级地球轨道 GJ 436 的观测结果
影响因子：
6.5
作者：
Seifahrt
通讯作者：
Seifahrt