ACCESS I. AN OPTICAL TRANSMISSION SPECTRUM OF GJ 1214b REVEALS A HETEROGENEOUS STELLAR PHOTOSPHERE

Abstract

GJ1214b is the most studied sub-Neptune exoplanet to date. Recent measurements have shown its near-infrared transmission spectrum to be flat, pointing to a high-altitude opacity source in the exoplanetʼs atmosphere, either equilibrium condensate clouds or photochemical hazes. Many photometric observations have been reported in the optical by different groups, though simultaneous measurements spanning the entire optical regime are lacking. We present an optical transmission spectrum (4500-9260Å) of GJ1214b in 14 bins, measured with Magellan/IMACS repeatedly over three transits. We measure a mean planet-to-star radius ratio of ( ) in the spectral bins. The optical transit depths are shallower on average than observed in the near-infrared. We present a model for jointly incorporating the effects of a composite photosphere and atmospheric transmission through the exoplanetʼs limb (the CPAT model), and use it to examine the cases of absorber and temperature heterogeneities in the stellar photosphere. We find the optical and near-infrared measurements are best explained by the combination of (1) photochemical haze in the exoplanetary atmosphere with a mode particle size r=0.1μm and haze-forming efficiency = f 10% haze and (2) faculae in the unocculted stellar disk with a temperature contrast D = -+ T 354 46 46 K, assuming 3.2% surface coverage. The CPAT model can be used to assess potential contributions of heterogeneous stellar photospheres to observations of exoplanet transmission spectra, which will be important for searches for spectral features in the optical.

Evaluating Climate Variability of the Canonical Hot-Jupiters HD 189733b and HD 209458b through Multi-epoch Eclipse Observations

“…references within) whose variability potentially impacts the observed transit depth of the planet (McCullough et al 2014;Knutson et al 2012). Although the amplitudes of these variations are reduced as our Spitzer/IRAC observations are in the infrared (Oshagh et al 2014;Rackham et al 2017;Morris et al 2018) and of eclipses (Zellem et al 2017), we examine the amplitude of these changes in comparison to the scatter and uncertainty in our eclipse depth measurements.…”

“…Stellar variability in the form of spots or plages can affect the measured planetary transit signal, potentially altering one's interpretation of its atmospheric composition, particularly for bright targets with large transit signals (Pont et al 2008;Silva-Valio 2008;Czesla et al 2009;Wolter et al 2009;Agol et al 2010b;Berta et al 2011;Carter et al 2011;Désert et al 2011a;Sing et al 2011;Fraine et al 2014;McCullough et al 2014;Oshagh et al 2014;Damasso et al 2015;Barstow et al 2015;Zellem et al 2015Zellem et al , 2017Cauley et al 2017Cauley et al , 2018Rackham et al 2017Rackham et al , 2018Rackham et al , 2019Morris et al 2018). HD 189733 is an active K0 star which has been observed to vary by as much as ± 1.5% at visible wavelengths with a rotation period of 11.95 days (Knutson et al 2012, and FIG.…”

The Influence of Stellar Contamination on the Interpretation of Near-infrared Transmission Spectra of Sub-Neptune Worlds around M-dwarfs

“…An important consideration in characterizing such planets is the nature of the host star . Transmission spectroscopy of hot-Jupiters to terrestrial worlds alike have demonstrated evidence of stellar activity (Pont et al 2007;Alonso et al 2008;Zhang et al 2018;Rackham et al 2017). M-dwarf hosts in particular, exhibit higher recorded detections of stellar activity in the form of flares (Hawley & Pettersen 1991;Schmidt et al 2014;Hawley et al 2014) and H α chromospheric emissions (Delfosse et al 1998) relative to F, G, K-type host stars, indicative of frequent occurrence of photospheric heterogeneities such as stellar spots and faculae.…”

“…Unocculted stellar spots as well as homogeneously covered regions of smaller spots affect the transit baseline in addition to causing discrepancies in the transit depth measurements depending on the fractional coverage in the area of the spots and their temperature contrast relative to the photosphere Silva-Valio 2008;Czesla et al 2009;Wolter et al 2009;Agol et al 2010;Berta et al 2011;Carter et al 2011;Désert et al 2011;Sing et al 2011;Fraine et al 2014;McCullough et al 2014;Oshagh et al 2014;Barstow et al 2015a;Damasso et al 2015;Zellem et al 2015;Rackham et al 2017). Attempts to understand the nature of unocculted spots have thus far been done with photometric monitoring of the star, i.e., tracking the rotational variability in the modulating brightness on the stellar photosphere translating onto the light curve Berta et al 2011;Désert et al 2011;Sing et al 2011;Knutson et al 2012;Pont et al 2013;Zellem et al 2015).…”

“…Attempts to understand the nature of unocculted spots have thus far been done with photometric monitoring of the star, i.e., tracking the rotational variability in the modulating brightness on the stellar photosphere translating onto the light curve Berta et al 2011;Désert et al 2011;Sing et al 2011;Knutson et al 2012;Pont et al 2013;Zellem et al 2015). Despite reasonable success, efforts with photometric monitoring of unocculted stellar spots have been inadequate in teasing out underlying heterogeneities spread across the entire disk of the star, especially those that are not significant enough to show variability effects on the stellar light curve (Jackson & Jeffries 2012;Rackham et al 2017Rackham et al , 2018. Studies by Chapman 1987 andShapiro et al 2014, have also shown that every differential region of the star's photosphere-whether the transit chord or the entire disk has its own unique spectrum.…”