Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS

Abstract

We present Hubble Space Telescope (HST) optical and near‐ultraviolet transmission spectra of the transiting hot Jupiter HD 189733b, taken with the repaired Space Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover the range 2900–5700 Å and reach per exposure signal‐to‐noise ratio levels greater than 11 000 within a 500‐Å bandwidth. We used time series spectra obtained during two transit events to determine the wavelength dependence of the planetary radius and measure the exoplanet’s atmospheric transmission spectrum for the first time over this wavelength range. Our measurements, in conjunction with existing HST spectra, now provide a broad‐band transmission spectrum covering the full optical regime. The STIS data also show unambiguous evidence of a large occulted stellar spot during one of our transit events, which we use to place constraints on the characteristics of the K dwarf’s stellar spots, estimating spot temperatures around Teff∼ 4250 K. With contemporaneous ground‐based photometric monitoring of the stellar variability, we also measure the correlation between the stellar activity level and transit‐measured planet‐to‐star radius contrast, which is in good agreement with predictions. We find a planetary transmission spectrum in good agreement with that of Rayleigh scattering from a high‐altitude atmospheric haze as previously found from HST Advanced Camera for Surveys. The high‐altitude haze is now found to cover the entire optical regime and is well characterized by Rayleigh scattering. These findings suggest that haze may be a globally dominant atmospheric feature of the planet which would result in a high optical albedo at shorter optical wavelengths.

K s-band secondary eclipses of WASP-19b and WASP-43b with the Anglo-Australian Telescope★

“…Rayleigh scattering due to the presence of hazes and clouds in the upper atmosphere can also lead to enhanced flux ratio at the optical to NIR range of wavelengths, that seems to explain well the nearly-featureless transmission spectrum in this region for HD189733b (Sing et al 2011;Pont et al 2008). A recent measurement of albedo in the secondary eclipse spectrum of HD189733b (Evans et al 2013) with the STIS on the Hubble Telescope seems to support the haze hypothesis.…”

Erratum: Physical properties, star-spot activity, orbital obliquity and transmission spectrum of the Qatar-2 planetary system from multicolour photometry

“…This is easily seen in Fig. 8 in which, using equations (4) and (5) from Sing et al (2011), we plot for the case of Qatar-2, the correction for unocculted spots for a total dimming of 1 per cent at a reference wavelength of 600 nm for different star-spot temperatures. Starspots are modelled with ATLAS9 stellar atmospheric models (Kurucz 1979) of different temperatures ranging from 4450 to 3700 K in 250 K intervals, and T eff = 4645 K for the stellar temperature.…”

“…The effect can be big enough to measure using medium-size telescopes with multiband imagers, assuming they have a good spectral resolution. It is important to obtain the observations at multiple wavelengths simultaneously, to avoid variations in transit depth due to unseen star-spots rather than planetary radius variations, even if one should take into account that unocculted starspots may still cause wavelength dependence of the transit depth (Sing et al 2011). In order to investigate this effect, one should monitor the variability of the parent star for many years 2 or, assuming that stellar activity does not change suddenly, repeatedly measure the transit depth by observing several planetary-transit events a few days away from each other.…”

Variability in the super-Earth 55 Cnc e

“…Second, clouds are less likely to form in the atmospheres of extremely hot exoplanets compared to cooler objects (Madhusudhan & Redfield 2015), which is particularly relevant in the light of the numerous non-detections of spectral features in exoplanets (e.g. Pont et al 2008;Sing et al 2011; Knutson et al 2014; Kreidberg et al 2014). The combination of the high photometric precision possible for transit/occultation obser-vations and the high planetary temperature means that 55 Cnc e currently represents one of the most promising super-Earth for infrared spectroscopic observations of its atmosphere.…”