Transmission Spectrum Of HD 189733b Research Articles

CaRM: Exploring the chromatic Rossiter-McLaughlin effect

Aims. In this paper we introduce CaRM, a semi-automatic code for the retrieval of broadband transmission spectra of transiting planets through the chromatic Rossiter-McLaughlin method. We applied it to HARPS and ESPRESSO observations of two exoplanets to retrieve the transmission spectrum and we analyze its fitting transmission models. Methods. We used the strong radius dependence of the Rossiter-McLaughlin (RM) effect amplitude, caused by planetary companions, to measure the apparent radius change caused by the exoplanet atmosphere. In order to retrieve the transmission spectrum, the radial velocities, which were computed over wavelength bins that encompass several spectral orders, were used to simultaneously fit the Keplerian motion and the RM effect. From this, the radius ratio was computed as a function of the wavelength, which allows one to retrieve the low-resolution broadband transmission spectrum of a given exoplanet. CaRM offers the possibility to use two Rossiter-McLaughlin models taken from ARoME and PyAstronomy, associated with a Keplerian function to fit radial velocities during transit observations automatically. Furthermore it offers the possibility to use some methods that could, in theory, mitigate the effect of perturbation in the radial velocities during transits. Results. We applied CaRM to recover the transmission spectrum of HD 189733b and WASP-127b, with HARPS and ESPRESSO data, respectively. Our results for HD 189733b suggest that the blue part of the spectrum is dominated by Rayleigh scattering, which is compatible with former studies. The analysis of WASP-127b shows a flat transmission spectrum. Conclusions. The CaRM code allows one to retrieve the transmission spectrum of a given exoplanet using minimal user interaction. We demonstrate that it allows one to compute the low-resolution broadband transmission spectra of exoplanets observed using high-resolution spectrographs such as HARPS and ESPRESSO.

Broadband transmission spectroscopy of HD 209458b with ESPRESSO: evidence for Na, TiO, or both

Context. The detection and characterization of exoplanet atmospheres is currently one of the main drivers pushing the development of new observing facilities. In this context, high-resolution spectrographs are proving their potential and showing that high-resolution spectroscopy will be paramount in this field. Aims. We aim to make use of ESPRESSO high-resolution spectra, which cover two transits of HD 209458b, to probe the broadband transmission optical spectrum of the planet. Methods. We applied the chromatic Rossiter–McLaughin method to derive the transmission spectrum of HD 209458b. We compared the results with previous HST observations and with synthetic spectra. Results. We recover a transmission spectrum of HD 209458b similar to the one obtained with HST data. The models suggest that the observed signal can be explained by only Na, only TiO, or both Na and TiO, even though none is fully capable of explaining our observed transmission spectrum. Extra absorbers may be needed to explain the full dataset, though modeling approximations and observational errors can also be responsible for the observed mismatch. Conclusions. Using the chromatic Rossiter–McLaughlin technique, ESPRESSO is able to provide broadband transmission spectra of exoplanets from the ground, in conjunction with space-based facilities, opening good perspectives for similar studies of other planets.

The widest broadband transmission spectrum (0.38–1.71μm) of HD 189733b from ground-based chromatic Rossiter–McLaughlin observations

Multiband photometric transit observations (spectro-photometric) have been used mostly so far to retrieve broadband transmission spectra of transiting exoplanets in order to study their atmospheres. An alternative method was proposed, and has only been used once, to recover broadband transmission spectra using chromatic Rossiter–McLaughlin observations. We use the chromatic Rossiter–McLaughlin technique on archival and new observational data obtained with the HARPS and CARMENES instruments to retrieve transmission spectra of HD 189733b. The combined results cover the widest retrieved broadband transmission spectrum of an exoplanet obtained from ground-based observation. Our retrieved spectrum in the visible wavelength range shows the signature of a hazy atmosphere, and also includes an indication for the presence of sodium and potassium. These findings all agree with previous studies. The combined visible and near-infrared transmission spectrum exhibits a strong steep slope that may have several origins, such as a super-Rayleigh slope in the atmosphere of HD 189733b, an unknown systematic instrumental offset between the visible and near-infrared, or a strong stellar activity contamination. The host star is indeed known to be very active and might easily generate spurious features in the retrieved transmission spectra. Using our CARMENES observations, we assessed this scenario and place an informative constraint on some properties of the active regions of HD 189733. We demonstrate that the presence of starspots on HD 189733 can easily explain our observed strong slope in the broadband transmission spectrum.

The High-resolution Transmission Spectrum of HD 189733b Interpreted with Atmospheric Doppler Shifts from Three-dimensional General Circulation Models

Abstract The signature of wind patterns caused by the interplay of rotation and energy redistribution in hot Jupiters is detectable at high spectral resolution, yet no direct comparison has been attempted between predictions from general circulation models (GCMs) and observed high-resolution spectra. We present the first such comparison on near-infrared transmission spectra of the hot Jupiter HD 189733b. Exploring 12 rotation rates and two chemical regimes, we have created model spectra from 3D GCMs and cross-correlated them with the observed spectra. Comparing our models against those of HD 189733b, we obtain three key results: (1) we confirm CO and H2O in the planet’s atmosphere at a detection significance of 8.2σ; (2) we recover the signature of day-to-night winds with speeds of several km s−1 at pressures of several millibars; and (3) we constrain the rotation period of the planet to between 1.2 and 4.69 days (synchronous rotation (2.2 days) remains consistent with existing observations). Our results do not suffer from the shortcomings of 1D models as cross-correlation templates—these models mainly tend to overconstrain the slower rotation rates and show evidence for anomalous blueshifts. Our 3D models instead match the observed line-of-sight velocity of this planet by self-consistently including the effects of high-altitude day-to-night winds. Overall, we find a high degree of consistency between observations of HD 189733b and our GCM-based spectra, implying that the physics and chemistry are adequately described in current 3D forward models for the purpose of interpreting observations at high spectral resolution.

A transmission spectrum of HD 189733b from multiple broad-band filter observations

We present new multi-broadband transit photometry of HD 189733b obtained at the Wyoming Infrared Observatory. Using an ensemble of five-band Sloan filter observations across multiple transits we have created an "ultra-low" resolution transmission spectrum to discern the nature of the exoplanet atmosphere. The observations were taken over three transit events and total 108 u', 120 g', 120 r', 110 i', and 116 z' images with an average exposure cadence of seven minutes for an entire series. The analysis was performed with a Markov-Chain Monte-Carlo method assisted by a Gaussian processes regression model. We find the apparent planet radius to increase from 0.154 +0.000920-0.00096 R* at z'-band to 0.157 +0.00074-0.00078 R* at u'-band. Whether this apparent radius implies an enhanced Rayleigh scattering or clear or grey planet atmosphere is highly dependent on stellar spot modeling assumptions, but our results are consistent with the literature for HD 189733b. This set of observations demonstrates the ability of our 2.3-m ground-based observatory to measure atmospheres of large exoplanets.

Stellar contributions to the line profiles of high-resolution transmission spectra of exoplanets

Context. In-depth studies of exoplanetary atmospheres are starting to become reality. In order to unveil their properties in detail, we need spectra with a higher signal-to-noise ratio (S/N) and also more sophisticated analysis methods. Aims. With high-resolution spectrographs, we can not only detect the sodium feature in the atmosphere of exoplanets, but also characterize it by studying its line profile. After finding a clearly w-shaped sodium line profile in the transmission spectrum of HD 189733b, we investigated the possible sources of contamination given by the star and tried to correct for these spurious deformations. Methods. By analyzing the single transmission spectra of HD 189733b in the wavelength space, we show that the main sodium signal that causes the absorption in the transmission spectrum is centered on the stellar rest frame. We concentrate on two main stellar effects that contaminate the exoplanetary transmission spectrum: center-to-limb variations (CLVs) and stellar rotation. We show the effects on the line profile: while we correct for the CLV using simulated theoretical stellar spectra, we provide a new method, based directly on observational data, to correct for the Rossiter–McLaughlin contribution to the line profile of the retrieved transmission spectrum. Results. We apply the corrections to the spectra of HD 189733b. Our analysis shows line profiles of the Na D lines in the transmission spectrum that are narrower than reported previously. The correction of the sodium D2 line, which is deeper than the D1 line, is probably still incomplete since the planetary radius is larger at this wavelength. A careful detrending from spurious stellar effects followed by an inspection in the velocity space is mandatory when studying the line profile of atmospheric features in the high-resolution transmission spectrum of exoplanets. Since the line profile is used to retrieve atmospheric properties, the resulting atmospheric parameters could be incorrectly estimated when the stellar contamination is not corrected for. Data with higher S/N coupled with improved atmospheric models will allow us to adapt the magnitude of the corrections of stellar effects in an iterative way.

A Model of the Hα and Na Transmission Spectrum of HD 189733b

Abstract This paper presents a detailed hydrostatic model of the upper atmosphere of HD 189733b, with the goal of constraining its temperature, particle densities, and radiation field over the pressure range 10–4 to 10 μbar, where the observed Hα transmission spectrum is produced. The atomic hydrogen level population is computed including both collisional and radiative transition rates. The Lyα resonant scattering is computed using a Monte Carlo simulation. The model transmission spectra are in broad agreement with the data. Excitation of the H(2ℓ) population is mainly by Lyα radiative excitation due to the large Lyα intensity. The density of H(2ℓ) is nearly flat over two decades in pressure and is optically thick to Hα. Additional models computed for a range of the stellar Lyman continuum (LyC) flux suggest that the variability in Hα transit depth may be due to the variability in the stellar LyC. Since metal lines provide the dominant cooling of this part of the atmosphere, the atmosphere structure is sensitive to the density of species such as Mg and Na, which may themselves be constrained by observations. Since the Hα and Na D lines have comparable absorption depths, we argue that the centers of the Na D lines are also formed in the atomic layer where the Hα line is formed.

Search for water vapor in the high-resolution transmission spectrum of HD 189733b in the visible

Ground-based telescopes equipped with state-of-the-art spectrographs are able to obtain high-resolution transmission and emission spectra of exoplanets that probe the structure and composition of their atmospheres. Various atomic and molecular species, such as Na, CO, H2O have been already detected. Molecular species have been observed only in the near-infrared while atomic species have been observed in the visible. In particular, the detection and abundance determination of water vapor bring important constraints to the planet formation process. We search for water vapor in the atmosphere of the exoplanet HD189733b using a high-resolution transmission spectrum in the visible obtained with HARPS. We use Molecfit to correct for telluric absorption features. Then we compute the high-resolution transmission spectrum of the planet using 3 transit datasets. We finally search for water vapor absorption using a cross-correlation technique that combines the signal of 800 individual lines. Telluric features are corrected to the noise level. We place a 5-sigma upper limit of 100 ppm on the strength of the 6500 A water vapor band. The 1-sigma precision of 20 ppm on the transmission spectrum demonstrates that space-like sensitivity can be achieved from the ground. This approach opens new perspectives to detect various atomic and molecular species with future instruments such as ESPRESSO at the VLT. Extrapolating from our results, we show that only 1 transit with ESPRESSO would be sufficient to detect water vapor on HD189733b-like hot Jupiter with a cloud-free atmosphere. Upcoming near-IR spectrographs will be even more efficient and sensitive to a wider range of molecular species. Moreover, the detection of the same molecular species in different bands (e.g. visible and IR) is key to constrain the structure and composition of the atmosphere, such as the presence of Rayleigh scattering or aerosols.

Chromatic line-profile tomography to reveal exoplanetary atmospheres: application to HD 189733b

Transmission spectroscopy can be used to constrain the properties of exoplanetary atmospheres. During a transit, the light blocked from the atmosphere of the planet leaves an imprint in the light coming from the star. This has been shown for many exoplanets with different techniques, with both photometry and spectroscopy. We aim at testing chromatic line-profile tomography as a new tool to investigate exoplanetary atmospheres. The signal imprinted on the cross-correlation function (CCF) by a planet transiting its star is dependent on the planet-to-star radius ratio. We want to verify if the precision reachable on the CCF obtained from a subset of the spectral orders of the HARPS spectrograph is enough to discriminate the radius of a planet at different wavelengths. Methods. We analyze HARPS archival data of three transits of HD 189733b. We divide the HARPS spectral range in 7 broadbands, calculating for each band the ratio between the area of the out-of-transit CCF and the area of the signal imprinted by the planet on it during the full part of the transit. We take into account the effect of the limb darkening using the theoretical coefficients of a linear law. Averaging the results of three different transits allows us to obtain a good quality broadband transmission spectrum of HD 189733b, with a precision greater than that of the chromatic RM effect. Results. We proved chromatic line-profile tomography to be an interesting way to reveal broadband transmission spectra of exoplanets: our analysis of the atmosphere of HD 189733b is in agreement with other ground- and space-based observations. The independent analysis of different transits puts in evidence the probability that stellar activity plays a role in the extracted transmission spectrum. Care has thus to be taken when claiming for Rayleigh scattering in the atmosphere of exoplanets orbiting active stars using only one transit.

Using the chromatic Rossiter-McLaughlin effect to probe the broadband signature in the optical transmission spectrum of HD 189733b

Transmission spectroscopy is a powerful technique for probing exoplanetary atmospheres. A successful ground-based observational method uses a differential technique based on high-dispersion spectroscopy, but that only preserves narrow features in transmission spectra. Here we use the chromatic Rossiter-McLaughlin (RM) effect to measure the Rayleigh-scattering slope in the transmission spectrum of HD 189733b with the aim to show that it can be effectively used to measure broadband transmission features. The amplitude of the RM effects depends on the effective size of the planet, and in the case of an atmospheric contribution therefore depends on the observed wavelength. We analysed archival HARPS data of three transits of HD 189733b, covering a wavelength range of 400 to 700 nm. We measured the slope in the transmission spectrum of HD 189733b at a $2.5\sigma$ significance. Assuming it is due to Rayleigh scattering and not caused by stellar activity, it would correspond to an atmospheric temperature, as set by the scale height, of $T = 2300 \pm 900 \mathrm{K}$, well in line with previously obtained results. This shows that ground-based high-dispersion spectral observations can be used to probe broad-band features in the transmission spectra of extrasolar planets, by using the chromatic RM effect. This method will be particularly interesting in conjunction with the new echelle spectrograph ESPRESSO, which currently is under construction for ESOs Very Large Telescope and will provide a gain in signal-to-noise ratio of about a factor 4 compared to HARPS. This will be of great value because of the limited and uncertain future of the Hubble Space Telescope and because the future James Webb Space Telescope will not cover this wavelength regime.

A search for TiO in the optical high-resolution transmission spectrum of HD 209458b: Hindrance due to inaccuracies in the line database

The spectral signature of an exoplanet can be separated from the spectrum of its host star using high-resolution spectroscopy. During such observations, the radial component of the planet's orbital velocity changes, resulting in a significant Doppler shift which allows its spectral features to be extracted. Aims: In this work, we aim to detect TiO in the optical transmission spectrum of HD 209458b. Gaseous TiO has been suggested as the cause of the thermal inversion layer invoked to explain the dayside spectrum of this planet. Method: We used archival data from the 8.2m Subaru Telescope taken with the High Dispersion Spectrograph of a transit of HD209458b in 2002. We created model transmission spectra which include absorption by TiO, and cross-correlated them with the residual spectral data after removal of the dominating stellar absorption features. We subsequently co-added the correlation signal in time, taking into account the change in Doppler shift due to the orbit of the planet. Results: We detect no significant cross-correlation signal due to TiO, though artificial injection of our template spectra into the data indicates a sensitivity down to a volume mixing ratio of ~10E-10. However, cross-correlating the template spectra with a HARPS spectrum of Barnard's star yields only a weak wavelength-dependent correlation, even though Barnard's star is an M4V dwarf which exhibits clear TiO absorption. We infer that the TiO line list poorly match the real positions of TiO lines at spectral resolutions of ~100,000. Similar line lists are also used in the PHOENIX and Kurucz stellar atmosphere suites and we show that their synthetic M-dwarf spectra also correlate poorly with the HARPS spectra of Barnard's star and five other M-dwarfs. We conclude that the lack of an accurate TiO line list is currently critically hampering this high-resolution retrieval technique.

CONSTRAINING THE ATMOSPHERIC COMPOSITION OF THE DAY-NIGHT TERMINATORS OF HD 189733b: ATMOSPHERIC RETRIEVAL WITH AEROSOLS

A number of observations have shown that Rayleigh scattering by aerosols dominates the transmission spectrum of HD 189733b at wavelengths shortward of 1 $\mu$m. In this study, we retrieve a range of aerosol distributions consistent with transmission spectroscopy between 0.3-24 $\mu$m that were recently re-analyzed by Pont et al. (2013). To constrain the particle size and the optical depth of the aerosol layer, we investigate the degeneracies between aerosol composition, temperature, planetary radius, and molecular abundances that prevent unique solutions for transit spectroscopy. Assuming that the aerosol is composed of MgSiO$_3$, we suggest that a vertically uniform aerosol layer over all pressures with a monodisperse particle size smaller than about 0.1 $\mu$m and an optical depth in the range 0.002-0.02 at 1 $\mu$m provides statistically meaningful solutions for the day/night terminator regions of HD 189733b. Generally, we find that a uniform aerosol layer provide adequate fits to the data if the optical depth is less than 0.1 and the particle size is smaller than 0.1 $\mu$m, irrespective of the atmospheric temperature, planetary radius, aerosol composition, and gaseous molecules. Strong constraints on the aerosol properties are provided by spectra at wavelengths shortward of 1 $\mu$m as well as longward of 8 $\mu$m, if the aerosol material has absorption features in this region. We show that these are the optimal wavelengths for quantifying the effects of aerosols, which may guide the design of future space observations. The present investigation indicates that the current data offer sufficient information to constrain some of the aerosol properties of HD189733b, but the chemistry in the terminator regions remains uncertain.

Probing the haze in the atmosphere of HD 189733b with Hubble Space Telescope/WFC3 transmission spectroscopy

We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting exoplanet HD 189733b, using Wide Field Camera 3. This consists of time-series spectra of two transits, used to measure the wavelength dependence of the planetary radius. These observations aim to test whether the Rayleigh scattering haze detected at optical wavelengths extends into the near-infrared, or if it becomes transparent leaving molecular features to dominate the transmission spectrum. Due to saturation and non-linearity affecting the brightest (central) pixels of the spectrum, light curves were extracted from the blue and red ends of the spectra only, corresponding to wavelength ranges of 1.099-1.168 um and 1.521-1.693 um, respectively, for the first visit, and 1.082-1.128 um and 1.514-1.671 um for the second. The light curves were fitted using a Gaussian process model to account for instrumental systematics whilst simultaneously fitting for the transit parameters. This gives values of the planet-to-star radius ratio for the blue and red light curves of 0.15650\pm0.00048 and 0.15634\pm0.00032, respectively, for visit one and 0.15716\pm0.00078 and 0.15630\pm0.00037 for visit 2 (using a quadratic limb darkening law). The planet-to-star radius ratios measured in both visits are consistent, and we see no evidence for the drop in absorption expected if the haze that is observed in the optical becomes transparent in the infrared. This tentatively suggests that the haze dominates the transmission spectrum of HD 189733b into near-infrared wavelengths, although more robust observations are required to provide conclusive evidence.

Determining Atmospheric Conditions at the Terminator of the Hot Jupiter HD 209458b

We present a theoretical model fit to the HST STIS optical transit transmission spectrum of HD 209458b. In our fit, we use the sodium absorption line profile along with the Rayleigh scattering by H2 to help determine the average temperature-pressure profile at the planetary terminator and infer the abundances of atomic and molecular species. The observed sodium line profile spans an altitude range of ~3500 km, corresponding to pressures between ~0.001 and 50 mbar in our atmospheric models. We find that the sodium line profile requires either condensation into sodium sulfide or ionization, necessary to deplete atomic sodium only at high altitudes below pressures of ~3 mbar. The depletion of sodium is supported by an observed sudden abundance change, from 2 times solar abundance in the lower atmosphere to 0.2 solar or lower in the upper atmosphere. Our findings also indicate the presence of a hot atmosphere near stratospheric altitudes corresponding to pressures of 33 mbar, consistent with that of the observed dayside temperature inversion. In addition, we find that a separate higher altitude temperature rise is necessary within both the condensation and ionization models, corresponding to pressures below ~0.01 mbar. This hot higher altitude temperature indicates that absorption by atomic sodium can potentially probe the bottom of the thermosphere and is possibly sensitive to the temperature rise linked to atmospheric escape.

Ground-based detections of sodium in HD 209458b's atmosphere in two data sets

AbstractWe present two separate ground-based detections of sodium in the transmission spectrum of HD 209458b. First we reanalyzed an archival data set from the HDS spectrograph on Subaru, which shows sodium at a >5σ level. Secondly, our preliminary results of a UVES/VLT data set indicate sodium absorption at a similar level, although the data cover the eclipse only partially. Both results are fully consistent with the HST results of Charbonneau et al. (2002). The Na D absorption feature seems to be resolved in the narrowest passband.

A Search for CO Absorption in the Transmission Spectrum of HD 209458b

ABSTRACTWe observed one transit of the extrasolar planet HD 209458b with the NIRSPEC spectrograph on the Keck II telescope. Using time series of low‐noise observations in the wavelength range 2.0–2.5 μm, we searched for extra absorption from the first‐overtone rotation‐vibration band of CO near 2.3 μm. This was not detected with a detection limit that fails to test simple models of the planetary atmosphere by a factor of about 3. Great improvements in the detectability of the CO spectrum features could be realized by observing a transit that is centered near stellar meridian passage, and in better weather. Since it appears that similar observations taken under better circumstances might succeed, we describe our analysis procedures in detail.