HST hot-Jupiter transmission spectral survey: haze in the atmosphere of WASP-6b
Summary (5 min read)
1.1 The WASP-6b system
- An analysis with new radial velocity data from Husnoo et al. (2012) brought evidence for non-significant eccentricity.
- Typical STIS G430L and G750L spectra (blue and red continuous lines, respectively).
- The atmosphere of WASP-6b has recently been probed in the optical regime from the ground by Jordán et al. (2013) .
- The authors found evidence for an atmospheric haze characterized by a decreasing apparent planetary size with wavelength and no evidence for the pressure-broadened alkali features.
2.1 HST STIS spectroscopy
- The three HST visits were scheduled such that the second and third spacecraft orbits occurred between the second and third contacts of the planetary transit in order to provide good sampling of the planetary radius while three orbits secured the stellar flux level before and after the transit.
- The raw STIS data was reduced (bias-, dark-and flat-corrected) using the latest version of the CALSTIS 1 pipeline and the relevant up-to-date calibration frames.
- In addition, the authors also corrected all pixels identified by CALSTIS as 'bad' with the same procedure, which together with the cosmic ray identified pixels resulted in a total of ∼11 per cent interpolated pixels.
- The STIS spectra were then used to extract both white-light spectrophotometric time series and custom wavelength bands after integrating the appropriate flux from each bandpass.
2.3 Stellar variability monitoring
- Stellar activity can complicate the interpretation of exoplanet transmission spectra, especially when multi-instrument multi-epoch data sets are combined (Pont et al. 2013) .
- Each nightly observation of WASP-6 consists of 4-10 consecutive exposures that include several comparison stars in the same field of view.
- The standard deviations of groups 2, 4 and 5 significantly exceed the measurement precision and thus indicate low-level activity due to star spots.
- The authors take this period to be their best determination of the star's rotation period.
3 ANALYSIS
- The authors adopt similar analysis methods for the whole HST survey that are similar to the approach detailed in Sing et al.
- The authors fit each HST and Spitzer transit light curve in flux employing a two-component model that consists of a transit model multiplied by a systematics model.
- The systematics models are different for the HST and Spitzer data and are detailed below.
- The authors initially fixed the orbital period of the planet to its literature value (Gillon et al. 2009 ) before being updated to the final value reported in Section 3.2.
3.0.1 STIS white light-curve modelling
- The three STIS white-light curves were fitted simultaneously with common inclination, semimajor axis and a planet-to-star radius contrast.
- The final systematics model for the red data include a fourth-order polynomial of HST orbital phase, a linear time term, and ω with BIC of 77.
- Finally, the authors find that the fitted transit parameters are very well constrained (including the transit depth) and are consistent regardless of the exact systematics model (see Fig. 5 ).
- The final results for the uncertainties of the fitted parameters were taken from MPFIT after the authors rescaled the errors per data point based on the standard deviation of the residuals.
- In these fits, the authors report the fitted parameters with uncertainties assuming pure photon noise and no rescaling.
3.1 System parameters and transit ephemeris
- The authors combined the central transit times from their HST data set with the transit times reported by Dragomir et al. (2011) and Jordán et al. (2013) to derive an updated transit ephemeris of WASP-6 b.
- In addition, the authors obtained the archival light curve with full transit coverage reported in Gillon et al. (2009) 5 and performed a fit to determine its central time.
- The authors excluded the two Spitzer IRAC light curves as these originate from incomplete transits and are not useful for determination of accurate transit ephemeris.
- The three measured transit radii in a bin between 5250-5400 Å taken between the two STIS G430L visits and the STIS G750L visit.
- In addition, the authors refined the system parameters including the orbital inclination, i and normalized semimajor axis, (a/R * ).
3.2 An evaluation of the stellar activity from the HST data
- The overlapping region between the two STIS gratings offers the opportunity to measure the photometric variability of the host star over the time interval of 40 d covered by the three HST visits, as significant stellar activity from stellar spots would change the measured transit radii.
- 1 from Nikolov et al. 2014 ) and measured the quantity R p /R * from a fit, keeping the system parameters, ephemeris and limb darkening to their best-fitting white light values and theoretical coefficients, respectively.
- The authors result suggests that the three HST visits occurred at similar stellar flux levels implying no significant offset between the blue and red STIS sections of the transmission spectrum.
- Finally, the very similar radius values for the two blue visits obtained six days apart imply the lack of variability related with the planetary atmosphere itself.
3.3 Fits to transmission spectra
- Light curves from various spectral bins were extracted from the STIS/G430L and G750L data to construct a broad-band transmission spectrum and to search for expected strong absorbers (see Figs 8 and 9 ).
- The activity of the parent star was taken into account by inflating the error bars of the measured R p /R * of each spectral bin assuming stellar variability of 0.3 per cent.
- This number also takes into account the contribution of unocculted spots.
- In the second approach, the authors removed the common-mode systematics from each spectral bin before fitting for residual trends with a parametrized model but with fewer parameters.
- It was found that this method reduces the amplitude of the breathing systematics.
4 D I S C U S S I O N 4.1 A search for narrow-band spectral signatures
- The authors performed a thorough search for narrow-band absorption features including the expected signatures of Na, K, Hα and Hβ.
- The authors find only tentative evidence for sodium and potassium line cores with the largest significance levels of 1.2σ and 2.7σ in 50 and 170 Å bins, respectively.
- The authors also searched but did not find evidence for Na and K pressure-broadened line wings.
4.2 Comparison to existing cloud-free theoretical models
- The authors averaged the model within the transmission spectrum wavelength bins and fitted these theoretical values to the data with a single free parameter that controls their vertical position.
- The models from Burrows et al. (2010) and Hothe authors & Burrows (2012) assume 1D dayside temperature-pressure (T-P) profile with stellar irradiation, in radiative, chemical, and hydrostatic equilibrium.
- Chemical mixing ratios and corresponding opacities assume solar metallicity and local thermodynamical chemical equilibrium accounting for condensation with no ionization, using the opacity data base from Sharp & Burrows (2007) and the equilibrium chemical abundances from Burrows & Sharp (1999) and Burrows et al. (2001) .
- Models were also generated both with and without the inclusion of TiO and VO opacities.
- Because each of the aforementioned models started at wavelength λ = 3500 Å (due to a restriction of the employed opacity data base), the authors extrapolated the models in the range 2700-3500 Å to enable self-content comparisons to the bluest STIS measurements along with all models.
4.3 Interpreting the transmission spectrum with aerosols
- The authors followed Lecavelier Des Etangs et al. (2008a) and performed a fit with a pure scattering model to the transmission spectrum with excluded sodium and potassium measurements as they can bias the result towards smaller slopes.
- These measurements are in modest disagreement (at the 2.2σ confidence level) with the prediction of Rayleigh scattering from the ground-based result of Jordán et al. (2013) who found αT = −10670 ± 3015 K.
- Note that their result significantly improves the error bar of the product αT due to the much broader wavelength coverage.
4.3.1 Rayleigh scattering
- No secondary eclipse observations have been reported in the literature, which prevent an empiric comparison to the planet dayside brightness temperature.
- This value is consistent with their results at the ∼1σ confidence level.
4.3.2 Mie scattering dust
- Good fits to the data are possible assuming a significant opacity from aerosols, i.e. colloids of fine solid particles or liquid droplets in a gas.
- Compared to the enstatite and forsterite, the alkali sulphides and chlorides are expected to be considerably less abundant in solarcomposition atmospheres.
- A variety of different atmospheric models were tested assuming aerosols are the main spectral feature throughout the transmission spectrum.
- In all fits, the authors excluded the radius measurements corresponding to the sodium and potassium spectral bins, as these are not predicted by the Mie scattering theoretical models.
- The authors also estimated condensate grain sizes assuming a planetary temperature of 1145 K and fitting for a and the baseline radius.
4.3.3 High-altitude haze and lower altitude clouds
- The authors also compared the transmission spectrum of WASP-6b with a suite of Fortney et al. (2010) atmospheric models with either an artificially added Rayleigh scattering component simulating a scattering haze, or an added 'cloud deck' produced by a grey flat line at specific altitude.
- The authors performed comparison to these models as they can help better understand their observational results, as clouds and hazes can mask or completely block the expected atomic and molecular features in a transmission spectrum depending on the altitude distribution and particle size.
- In the Solar system planets, the scaleheight of haze is often smaller than the scaleheight of the atmosphere itself, because of sedimentation and the structure of the vertical mixing.
- That would result in a smaller Rayleigh slope for the haze signature in the transmission spectrum.
- It would also add a gas-to-grain scaleheight ratio factor in equation 1 of Lecavelier Des Etangs et al. (2008a) , thus severing the relation between the measured slope and the temperature.
4.4 The atmosphere of WASP-6b
- All of the cloud-free models have been found to produce unsatisfactory fits indicating low level of molecular absorption in their data.
- Rayleigh scattering can potentially be produced by molecular hydrogen, a potential scenario explored for HD 209458b (Lecavelier Des Etangs et al. 2008b ).
- Molecular hydrogen is disfavoured for WASP-6b by the lack of pressure-broadened line wings in its transmission spectrum and the low altitudes of any molecular features as apparent in Fig. 11 .
- The first five best-fitting Mie scattering aerosol models are practically indistinguishable in quality between each other providing good representation of the observed transmission spectrum (Table 5 ).
- The authors find that the particle sizes of all models are sub-micron, regardless the temperature.
4.4.1 Comparison to a ground-based result
- Recently, Jordán et al. (2013) reported WASP-6b transmission spectrum with the Magellan IMACS instrument.
- When fitting the light curves, the authors fixed the system parameters to their best-fitting values.
- The authors find no evidence for the observed discrepancy to be due to the different system parameters (i.e. a/R * and i) nor the limbdarkening laws adopted.
- 6 in Jordán et al. (2013) where white light curves are compared after subtracting the best-fitting systematics models, it can be seen that the final Magellan transit depth varies greatly between the different systematics models used.
- Fig. 13 shows that the three STIS transit light curves obtained at three individual epochs are all in excellent agreement with each other, illustrating the measured HST transit depth is highly repeatable.
4.4.2 Comparison to HD 189733b
- Complemented by the transmission spectra of HD 209458b and HD 189733b, the first results from the large HST programme cou- pled by Spitzer IRAC transit radii start building up an observational data base of hot-Jupiter atmospheres.
- The similarities and differences in the spectra and system parameters of the studied objects could potentially indicate distinct categories of gas giant atmospheres.
- Instead, absorption from the narrow cores of the Na I and K I doublets are evidenced for HD 189733b in the medium resolution STIS data and the revised ACS data from Huitson et al. (2012) and Pont et al. (2013) , respectively.
- In addition, HD 189733b's transmission spectrum exhibits a steeper scattering slope and relatively higher altitude for the hazes on WASP-6b (Fig. 15 ).
- A higher surface gravity means that a given pressure corresponds to less mass.
5 C O N C L U S I O N
- With a broad-coverage optical transmission spectrum measured from HST and Spitzer broad-band transit spectrophotometry, WASP-6b joins the small but highly valuable family of hot-Jupiter exoplanets with atmospheric constraints.
- The data point on the potassium spectral line is ∼2.7σ higher than the spectrum at that wavelength region.
- The authors find no evidence of pressure-broadened wings which is in agreement with the haze model as clouds and hazes can reside at high altitudes which can significantly mask or even completely block the signatures of collisionally broadened wings.
- Given the rather low temperature of WASP-6b, aerosol species are expected to originate from high-temperature condensates similar to these in the prototype HD 189733b.
- Given their condensation temperatures and the results from their analysis, MgSiO 4 , KCl, MgSiO 3 and Na 2 S are candidate condensates.
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Cites background from "HST hot-Jupiter transmission spectr..."
...…details of observations, see Pont et al. 2013; Line et al. 2013a; Huitson et al. 2013; Mandell et al. 2013; Sing et al. 2013, 2015; Wakeford et al. 2013; Nikolov et al. 2014, 2015; McCullough et al. 2014) represent a variety of atmospheres, interpreted as a continuum of clear to cloudy conditions....
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Cites background from "HST hot-Jupiter transmission spectr..."
...Transmission spectra at optical wavelengths show evidence for Rayleigh scattering by molecular hydrogen and high-altitude aerosols, and absorption by alkali metals (e.g. Charbonneau et al. 2002; Pont et al. 2008; Sing et al. 2013, 2015; Nikolov et al. 2014, 2015)....
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Cites result from "HST hot-Jupiter transmission spectr..."
...Similar inferences have been made for several other hot Jupiters in recent years (Sing et al. 2013, 2015; Nikolov et al. 2015)....
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"HST hot-Jupiter transmission spectr..." refers methods in this paper
...Such models were found statistically unjustified based on the Bayesian Information Criterion (BIC; Schwarz 1978)....
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4,305 citations
"HST hot-Jupiter transmission spectr..." refers background in this paper
...…adopted here due to the lack of observations on the planet dayside spectrum. c© 2013 RAS, MNRAS 000, 1–?? of WASP-6b the prime condensate candidate materials include MgSiO3 (enstatite), Mg2SiO4 (forsterite), sulphides and chlorides of the alkali metals Na and K (Lodders 2003; Morley et al. 2012)....
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3,696 citations
"HST hot-Jupiter transmission spectr..." refers methods in this paper
...…data was collected for WASP-6 during two transits of its planet on UT 2013 January 14 and 21 with the Spitzer space telescope (Werner et al. 2004) employing respectively the 4.5 and 3.6µm channels of the Infrared Array Camera (IRAC, Fazio et al. 2004) as part of program 90092 (P.I. J.-M. Desert)....
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