凌星类热木行星大气的地面观测研究

陈果

(中国科学院紫金山天文台 南京 210008)

博士学位论文摘要选登

凌星类热木行星大气的地面观测研究

陈果†

(中国科学院紫金山天文台 南京 210008)

太阳系外凌星类行星是目前进行系外行星大气研究的最佳目标之一.在特殊的轨道构型条件下才可以进行凌星主食和次食的观测,它们分别对应于行星位于宿主恒星正前方和背后两种情况.相应的观测可以产生大气透射谱和热发射谱,在大气组成和温度结构的观测研究中扮演着重要的角色.基于这两种谱线性质,本文使用马普2.2 m望远镜的GROND(Gamma-Ray Burst Op tical and Near-infrared Detector)设备对3颗热木星的次食进行了测光观测，又使用帕洛玛5.1 m海尔望远镜的DBSP(Double Spectrograph)、TSpec(Trip le Spectrograph)和COSM IC(Carnegie Observatories Spectroscopic M u ltislit and Im aging Cam era)设备对另两颗热木星的主食进行了光谱观测,以求搜寻到任何可能的来自行星大气的信号、并用于了解拥有不同物理性质的热木星的大气多样性.

本文第3章至第5章分别对WASP-5 b、WASP-46 b和WASP-43 b的次食的测光观测进行了详细说明.3颗热木星的次食均在K波段被显著探测到，同时在J和H波段还存在一些可能性较高的非显著探测或者3σ上限.这些近红外次食测量所反映的是来自行星昼半球大气的热发射,由于相应波段的不透明度较低,所探测的大气层也相应较深.这是首次对WASP-5 b和WASP-46 b进行近红外热发射探测,我们利用GROND测得的数据显示WASP-5 b的较深层大气呈大致等温的状态,温度约为2700 K.当与斯皮策空间望远镜获取的中红外数据结合在一起分析时,我们发现在数据可探测压强范围内不存在逆温现象.斯皮策数据反映的是较高层大气,对应的温度约为1900 K.在大气模型分析中,氧丰大气与观测数据并不相符,而碳丰模型虽能很好地进行数据拟合,却不能保证能量平衡.WASP-46 b的观测显示其昼半球大气很可能也处于等温状态,温度大约在2386 K,虽高于全局热再分布的平衡温度,但在误差范围内仍与局部最高平衡温度相符.我们在K波段测得的WASP-43 b的热发射信号确认了此前其他研究组在2.09µm窄带和KS波段获得的测量结果.观测表明,这3颗热木星在昼夜半球间的热再分布效率都非常低.我们还分析了WASP-46 b和WASP-43 b在光学波段的次食测量结果,如果这些信号并非来自系统效应,那么很可能意味着高层大气中存在反射云层.此外,借助于GROND多波段同步观测能力,我们还测得了一条WASP-43 b昼夜交界时大气宽带透射谱.但限于宽带的光谱分辨率和测量精度,目前还难以获得大气组成的有效信息,这显示了相应研究工作使用光谱观测的必要性.

在第6章和第7章中,我们分别总结了针对HAT-P-1 b和HAT-P-32 b的主食进行的光谱观测.恒星星光因穿透行星昼夜交界处的高层大气而携带了相应的化学组成信息.我们通过分析HAT-P-1 b的透射谱,发现了钾共振双线的存在,这是首次在1颗系外行星的光谱中分辨出钾双线特征.所测得的钾双线只存在窄线心轮廓,缺乏理论中预期的压力致宽线翼,暗示可能存在云霾.我们的光学透射谱覆盖了310–520 nm和610–1000 nm,对其的分析显示:HAT-P-1 b高层大气中不存在TiO和VO,也不存在纯瑞丽散射云霾.结合哈勃望远镜W FC3设备在1.1–1.7µm获取的透射谱,我们可以排除其大气中存在光学厚云的可能性.现有的数据表明其高层大气很可能由近太阳丰度组成,并含有Na、K、H2O和CH4等成分.在针对HAT-P-32 b的观测中,我们测得的光学透射谱在一定程度上呈现了TiO/VO的吸收特征,但多云大气的情况并不能被排除.

Transiting exop lanets are currently am ong them ost favorable targets for atm ospheric studies of exop lanets.Such special orbital geom etry enables transits and secondary eclipses to be observable,which refer to the events when p lanetsmove in front of or behind host stars.Corresponding observations would result in transm ission spectroscopy or em ission spectroscopy,which are extrem ely powerful in the investigation of atm ospheric com positions and tem perature structures.Based on these two techniques,this thesispresentsphotometric observations on the secondary eclipses of three hot Jupiters using GROND(Gamma-Ray Burst Opticaland Near-infrared Detector)mounted on the MPG 2.2m telescope,and spectroscopic observations on the transits of another two hot Jupiters using DBSP(Double Spectrograph),TSpec(Triple Spectrograph),and COSM IC(Carnegie Observatories Spectroscopic Multislit and Imaging Camera)mounted on the Palomar 5.1 Hale telescope.The primary goal is to search for any detectable signals of atm ospheric origin,and to study potential atmospheric diversity among hot Jupitersw ith various physical properties.

The photometric observations on the secondary eclipses of WASP-5 b,WASP-46 b, and WASP-43 b are detailed in Chapter 3,4,and 5,respectively.The dips of secondary eclipse have been significantly detected for all three hot Jupiters in the K band,along w ith some possible detection or 3σupper lim it in the J or H band.These near-infrared eclipse detection measures the thermal em ission from the deep dayside atmosphere.It is the first time to detect any therm al em ission in the near infrared for WASP-5 b and WASP-46 b.Our GROND measurements indicate a roughly isothermal temperature profile of around 2700 K in the deep layers of WASP-5 b’s dayside atmosphere.Together w ith Spitzer observations,which probe higher layers w ith a tem perature of around 1900 K,a tem perature inversion is ruled out in the probed pressure range.W hile an oxygen-richm odel is unable to explain all the data,a carbon-rich model provides a reasonable fit but violates the energy balance.The GROND em ission measurements of WASP-46 b also suggest an isothermal tem perature profile for its dayside atm osphere.Our K-band measurement for WASP-43 b confirm s previous detections obtained in the 2.09µm narrow band and KSband.A ll three hot Jupiters seem to have very poor day to nightside heat redistribution. Furtherm ore,we present the analysis of optical eclipse measurements for WASP-46 b and WASP-43 b,which are either false positivesor indication of reflective cloudsat high altitude. Additionally,thanks to the simultaneous multi-band observing capability of GROND,a broad-band transm ission spectrum is derived for WASP-43 b.Lim ited by current spectral resolution and precision,the atm ospheric com positions can not be distinguished,indicating the necessity of the spectroscopic observations.

In Chapter 6 and 7,the spectroscopic observations on the transits of HAT-P-1 b and HAT-P-32 b are summarized,respectively.Stellar light that has been transm itted through

planetary term inator atmosphere carries information of atmospheric com positions.We report the first detection of the K resonance doublet absorption in HAT-P-1 b.This is the first time to resolve the K doublets in any p lanet.The K absorption profile consists of only narrow coreswhile lacks pressure-broadened w ings,indicating clouds or hazes.Our optical transm ission spectrum,covering the wavelength ranges of 310–520 nm and 610–1000 nm, rules out the presence of TiO and VO and the p resence of pure Rayleigh-scattering hazes. Together w ith the 1.1–1.7µm transm ission spectrum derived by HST/W FC3,we also rule out the possibility of optically thick clouds.The current combined dataset iswellexp lained by a nearly solar-abundance atm osphere of Na,K,H2O,and CH4.For HAT-P-32 b,the measured optical transm ission spectrum shows signatures of TiO/VO absorption to some degree,but a cloudy atmosphere is not comp letely ruled out.

G round-based Observational Characterization of Transiting Hot-Jup iter A tm osphere

CHEN Guo

(Pu rp le M oun tain O bserva to ry,Chinese A cadem y o f Sciences,Nan jing 210008)

10.15940/j.cnki.0001-5245.2016.05.011

†2014-07-06获得博士学位,导师:紫金山天文台王红池研究员;guochen@pm o.ac.cn