A&A 589, A99 (2016) Astronomy DOI: 10.1051/0004-6361/201525675 & c ESO 2016 Astrophysics A CRIRES-search for H+ emission from the hot Jupiter atmosphere 3 of HD 209458 b? (Research Note) L. F. Lenz1, A. Reiners1, A. Seifahrt2, and H. U. Käufl3 1 Institute for Astrophysics, University of Goettingen, Friedrich-Hund-Platz 1, 37077 Goettingen, Germany e-mail: [email protected] 2 Department of Astronomy and Astrophysics, University of Chicago, IL 60637, USA 3 European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany Received 16 January 2015 / Accepted 11 March 2016 ABSTRACT + Close-in extrasolar giant planets are expected to cool their thermospheres by producing H3 emission in the near-infrared (NIR), but + simulations predict H3 emission intensities that differ in the resulting intensity by several orders of magnitude. We want to test the + observability of H3 emission with CRIRES at the Very Large Telescope (VLT), providing adequate spectral resolution for planetary + 0 atmospheric lines in NIR spectra. We search for signatures of planetary H3 emission in the L band, using spectra of HD 209458 + obtained during and after secondary eclipse of its transiting planet HD 209458 b. We searched for H3 emission signatures in spectra containing the combined light of the star and, possibly, the planet. With the information on the ephemeris of the transiting planet, we derive the radial velocities at the time of observation and search for the emission at the expected line positions. We also apply a cross-correlation test to search for planetary signals and use a shift and add technique combining all observed spectra taken after + secondary eclipse to calculate an upper emission limit. We do not find signatures of atmospheric H3 emission in the spectra containing + the combined light of HD 209458 and its orbiting planet. We calculate the emission limit for the H3 line at 3953.0 nm [Q(1; 0)] to be 8:32 × 1018 W and a limit of 5:34 × 1018 W for the line at 3985.5 nm [Q(3; 0)]. Comparing our emission limits to the theoretical + predictions suggests that we lack 1 to 3 magnitudes of sensitivity to measure H3 emission in our target object. We show that under more favorable weather conditions the data quality can be improved significantly, reaching 5 × 1016 W for star-planet systems that are close to Earth. We estimate that pushing the detection limit down to 1015 W will be possible with ground-based observations with future instrumentation, for example, the European Extremly Large Telescope. Key words. stars: individual: HD 209458 – infrared: stars – planetary systems 1. Introduction contribute to balance the heating due to the host star radiation + (Yelle 2004). H3 emission was detected from the auroral regions Methods of exoplanet detection have become more and more of Jupiter by Drossart et al.(1989). The ion was successfully successful in recent years and various kinds of exoplanets, from used as a tool to measure atmospheric temperatures and ion den- hot Jupiters to earth-sized planets, have recently been detected. sities of Jupiters atmosphere (Yelle 2004). Miller et al.(2000) However, it is still difficult to gain information about the atmo- argue this ion is the main coolant in the ionosphere and ther- spheric composition of these exoplanets. Only a small fraction mosphere of Jupiter in both the auroral and nonauroral regions. of the detected exoplanets can be accessed for investigations of + H3 was also detected in Saturn (Geballe et al. 1993) and Uranus the atmosphere, for example, transits and secondary eclipses. (Trafton et al. 1993). The temperature for the thermosphere of HD 209458 b was the first transiting exoplanet for which an Saturn is measured at around 800 K (Geballe et al. 1993), and atmosphere was detected in a spectrophotometric observation of for Jupiter a temperature range of approximately 700–1000 K Na i (Charbonneau et al. 2002). Most exoplanets with a detec- was measured by Lam et al.(1997). Koskinen et al.(2010) mea- tion of atmospheric molecules are gas giant planets in close-in sure a mean temperature of 8000–11 000 K for the thermosphere orbits. The atmospheres of these extrasolar giant planets (EGP), + of HD 209458b. H3 is an effective coolant even for high temper- such as HD 209458 b, are expected to be severely influenced by atures up to 10 000 K (Neale et al. 1996). the stellar extreme ultraviolet (EUV) radiation that drives a hy- H+ is formed by a reaction of H+, under EUV radiation and drodynamic escape in the upper atmosphere. Different models 3 2 have been derived to estimate the evaporation rate of close-in energetic electron precipitation along magnetic field lines, react- giant planets (e.g., Lammer et al. 2003). ing with neutral H2, i.e., Some of these models propose that infrared (IR) emissions of H+ ions cool the thermospheres of close-in EGPs and, thus, ∗ + 3 H2 + e ! H2 + e + e ? + Based on observations collected at the European Organisa- H2 + hν ! H2 + e tion for Astronomical Research in the Southern Hemisphere, Chile, + + 086.C-0045. H2 + H2 ! H3 + H: Article published by EDP Sciences A99, page 1 of7 A&A 589, A99 (2016) Table 1. Wavelength coverage of the observed spectra for each of the The ion is destroyed by the dissociative recombinations + four CRIRES detectors and the H3 emission line positions and inten- + + sities from the southern aurorae of Jupiter as given in Maillard et al. H3 + e ! H2 + H + e (1990). + H3 + e ! H + H + H: + Detector Wavelength range Line position Intensity So far H3 emission has not been detected in the atmosphere of an h W i EGP on a planet outside of our solar system. A detection would [nm] [nm] cm2 sr help to understand the thermal structure of EGPs. Additionally, the planetary radial velocity could be measured directly: since 1 3947.3–3968.6 3953.0 42.8 + 2 3974.6–3995.1 3985.5 45.2 H3 is not formed in stellar atmospheres, a detection could safely be assigned to the exoplanet of the observed stellar system. 3987.0 23.5 + 3994.6 11.5 The theoretical predictions for H3 emitted from exoplanet atmospheres vary by several magnitudes in the different mod- 3 4000.6–4020.3 4012.0 19.3 els. Miller et al.(2000) estimated an emission limit of ∼1017 W 4013.3 17.3 for a planet similar to τBoo b. Yelle(2004) investigated the case 4 4025.4–4044.3 4043.2 10.2 of HD 209458 b with a one-dimensional model of an EGP at- mosphere. He calculated an emission limit of 1×1016 W emitted Table 2. System parameters (Southworth 2010). from the lower thermosphere. At higher altitudes he derived tem- + peratures of 10 000 to 15 000 K, where H becomes dominant Object Spectral type Distance MStar MP Porb + and suppresses the formation of H3 . [pc] [M ][MJup] [d] The lowest estimations for the H+ emission limits of close-in 3 HD 209458 G0 47.1 1.01 0.685 3.5247 EGPs are derived by Koskinen et al.(2007): With their model of a coupled thermosphere and ionosphere they performed three- dimensional, self-consistent global simulations for different or- Maillard et al.(1990). Three of the emission lines of the atmo- bital distances of EGPs around a sunlike host star. They con- + sphere of Jupiter were measured with an intensity larger than clude, that EGPs are cooled efficiently by H inside 0.2–1 AU − − 3 20 Wcm 2sr 1 and another four emission lines with an intensity orbits and state that thermal dissociation and dissociative pho- above 10 Wcm−2sr−1. toionization of H2 hampers the emission for orbits closer than The line positions are listed in Table1 with the correspond- 0:1 AU. With their simulation they derived a total power output ing intensities from Maillard et al.(1990) and the detector num- ∼ 15 of 10 W for a hot Jupiter planet in a 0:1 AU orbit and calcu- ber and wavelength coverage of the four CRIRES detectors, on ∼ 12 lated a spectral line output of 10 W for the intensity of the which the lines fall in the observed spectra. The H+ emission Q(3; 0)-transition. 3 + line at 3953.0 nm was also used by Shkolnik et al.(2006) in their Brittain & Rettig(2002) reported the detection of H 3 emis- + search for H3 of hot Jupiter atmospheres. sion signals from observations of HD 141569 A. However As a result of unfavorable weather conditions, we were able these observations could not be confirmed by observations to record only 16 spectra of HD 209458. The stellar and plan- made by Goto et al.(2005), who measured upper emission lim- etary parameters of HD 209458 are summarized in Table2. its that were significantly lower than the signals reported by Twelve of the observed spectra were taken during secondary Brittain & Rettig(2002) while achieving comparable data qual- eclipse. Hence, planetary emission might be visible in the four ity. The most extensive observation attempt for H+ emission 3 remaining spectra available for analysis; these four spectra are from hot Jupiter systems was carried out by Shkolnik et al. hereafter called combined light spectra. The mean signal-to- (2006). They observed six close-in EGPs with CSHELL at the noise ratio with exposure times of 150 s is ≈71 for the 12 eclipse Q ; NASA IRTF. They searched for emission from the (1 0)- spectra and for the four combined light spectra. transition at 3953.0 nm and calculated emission limits around The observations for HD 209458 were carried out in an 1×1018 W from their measurements.