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HD 189733 B: Bow Shock Or No Shock?? S A&A 619, A96 (2018) https://doi.org/10.1051/0004-6361/201833567 Astronomy & © ESO 2018 Astrophysics HD 189733 b: bow shock or no shock?? S. Kohl, M. Salz, S. Czesla, and J. H. M. M. Schmitt Hamburger Sternwarte, Universität Hamburg, Gojenbergweg 112, 21029 Hamburg, Germany e-mail: [email protected] Received 4 June 2018 / Accepted 23 August 2018 ABSTRACT Context. Hot Jupiters are surrounded by extended atmospheres of neutral hydrogen. Observations have provided evidence for in-transit hydrogen Hα absorption as well as variable pre-transit absorption signals. These have been interpreted in terms of a bow shock or an accretion stream that transits the host star before the planet. Aims. We test the hypothesis of planetary-related Hα absorption by studying the time variability of the Hα and stellar activity-sensitive calcium lines in high-resolution TIGRE (Telescopio Internacional de Guanajuato Robótico Espectroscópico) spectra of the planet host HD 189733. Methods. In the framework of an observing campaign spanning several months, the host star was observed several times per week randomly sampling the orbital phases of the planet. We determine the equivalent width in the Hα and Ca IRT(calcium infrared triplet) lines, and subtract stellar rotationally induced activity from the Hα time series via its correlation with the IRT evolution. The residuals are explored for significant differences between the pre-, in-, and out-of-transit phases. Results. We find strong stellar rotational variation with a lifetime of about 20–30 days in all activity indicators, but the corrected Hα time series exhibits no significant periodic variation. We exclude the presence of more than 6.2 mÅ pre-transit absorption and 5.6 mÅ in-transit absorption in the corrected Hα data at a 99% confidence level. Conclusions. Previously observed Hα absorption signals exceed our upper limits, but they could be related to excited atmospheric states. The Hα variability in the HD 189733 system is dominated by stellar activity, and observed signals around the planetary transit may well be caused by short-term stellar variability. Key words. planets and satellites: atmospheres – planets and satellites: gaseous planets – planets and satellites: individual: HD 189733 1. Introduction hydrogen into elevated atmospheric layers. For example, inter- actions with the stellar wind could increase the local excitation Today our knowledge about extrasolar planets is not only lim- level, which would lead to enhanced absorption in the Balmer ited to basic parameters like mass and radius, but information lines. In contrast to the Lyα line, the Balmer series can be studied on the chemical composition has also been derived from trans- with ground-based telescopes equipped with medium-resolution mission spectroscopy in multiple cases (e.g., Sing et al. 2016). spectrographs covering the visible wavelength range. Particu- During transit, a planetary atmosphere causes excess absorp- larly, excess hydrogen Hα absorption has been claimed to occur tion in atomic or molecular lines. In the extended atmospheres around the transit of HD 189733 b (Jensen et al. 2012; Cauley of large close-in gas planets, hydrogen remains mostly neu- et al. 2015). tral and in the ground state, despite equilibrium temperatures Here we investigate possible Hα absorption in the of several thousand Kelvin, resulting in strong hydrogen line HD 189733 system with a very general observing approach. In absorption. In fact, observations verified Lyα excess absorption Sect.2, we present the properties of HD 189733 and introduce around the transits of HD 189733 b (Lecavelier Des Etangs et al. the concept of a bow shock. In Sect.3 we describe our dataset 2010; Bourrier et al. 2013), HD 209458 b (Vidal-Madjar et al. and go through the data reduction steps. We present our results 2003; Ehrenreich et al. 2008), and GJ 436 b (Kulow et al. 2014; and conclusions in Sects.4 and5. Ehrenreich et al. 2015). For the aforementioned planets, transit depths of more than 10% were measured in the Lyα line wings, in contrast to broadband optical transit depth measurements of 2. The case of HD 189733 b 0.6% for GJ 436 b and 2.5% for HD 189733 b (Han et al. 2014). Clearly, these planets must host extended envelopes of hydrogen. The K dwarf HD 189733 is orbited by a Jupiter-sized planet in These close-in planets are exposed to extreme high-energy 2.2 days, a so-called hot Jupiter. The host star is considerably irradiation levels, which are thought to drive a significant loss more active than the Sun, although with a presumed age of of atmospheric material through an energy-limited planetary 5.3 ± 3.8 Gyr it is not a young star (Bonfanti et al. 2016); we list wind (Watson et al. 1981; Lammer et al. 2003; Salz et al. the stellar and planetary properties in Table1. The high level of 2016). Such a wind can carry considerable amounts of neutral stellar irradiation in the close planetary orbit leads to a bloated planetary atmosphere, which has been subject to intensive stud- ? Full Table 2 is only available in electronic form at the CDS via ies because of the stellar brightness (mV = 7.65). For example, anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via Boisse et al.(2009) present a spectroscopic observing campaign, http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A96 which was carried out with the high-resolution spectrograph Article published by EDP Sciences A96, page 1 of8 A&A 619, A96 (2018) Table 1. Parameters of HD 189733 and its planet b. that the transit of a bow shock could precede the transit of the planetary disk. The presence of pre-transit absorption seems to be confirmed Parameter Value References by additional data (Cauley et al. 2016), where an excess absorp- HD 189733 tion depth of 7 mÅ over a duration of 2.5 h was detected in mV 7.65 1 the pre-transit phase. Cauley et al. interpret this signal as accre- MV–MB 0.93 1 tion clumps spiraling toward the star, but also point out that this Spectral type K0V + M4V 1 interpretation has serious limitations. Further investigations by a Teff 5052 ± 46 2 Barnes et al.(2016) challenge the results and note that the H α log (g)a 4.49 ± 0.13 2 variations are well correlated with the Ca H&K lines, indicat- (M=H)a 0.02 ± 0.02 2 ing that the Hα time series are dominated by stellar activity. Yet, −1 vrot sin(i)(km s ) 3.5 ± 1.0 3 additional out-of-transit time series taken by Cauley et al.(2017) Rotation period (d) 11.953 ± 0.009 11 show significantly lower Hα residual equivalent width than the −1 previously presented pre-transit signals. Thus, in summary, the vrad(km s ) −2.55 ± 0.21 1 Parallaxes (mas) 50.40 ± 0.22 1 origin of the Hα features has to be treated as an open issue. 0 log(RHK) −4.524· · · −4.458 9 age (Gyr) 5.30 ± 3.80 10 3. Observations and data reduction Mass (M ) 0.846 ± 0.049 6 Radius (R ) 0.805 ± 0.016 6 3.1. Observations HD 189733 b To test the nature of the Balmer line absorption features in Planet mass [MJup] 1.144 ± 0.056 3 HD 189733, we initiated an observing campaign to monitor Semi-major axis (AU) 0.03100 ± 0.00062 3 chromospheric and possibly planet-induced variations. The cam- Transit midpoint (MJD) 53955.0· · · ±8.8 × 10−6 4 paign was performed with our 1.2 m TIGRE (Telescopio Inter- Orbital period (d) 2.2· · · ±7.7 × 10−8 4 nacional de Guanajuato Robótico Espectroscópico) telescope located in the central Mexican highlands. TIGRE is equipped Planetary radius (RJup) 1.138 ± 0.027 3 Duration of transit (h) 1.8036 ± 0.0023 4 with a spectrograph with a nominal resolution of 20 000. For Transit depth (%) 2.413 ± 0.003 8 details about the spectrograph and telescope, we refer to Schmitt Planetary density (g cm−3) 0.963+0:088 3 et al.(2014). The brightness of the host star allows us to reach −0:079 a signal-to-noise ratio (S/N) of about 84 inside the Hα line Observations core with typically 900 s exposures. The actual exposure times Epoch of obs. (yr) 2017.32–2017.88 were varied according to the observing conditions to reach the Exposure time (s) ≈700–2500 required S/N. S/N at Hα ≈85 (line core) We typically obtained several spectra of HD 189733 per No. of spectra 108 week, randomly sampling the stellar rotation phase and the plan- etary orbit. Because the pre- and in-transit phases cover only a Notes. (a)When several values were listed for a stellar parameter, we small fraction of the planetary orbit, we randomly increase the averaged all the given values, however discarding outliers. sampling rate around the transit phase. Therefore, during tran- References. (1) Wenger et al.(2000); (2) Soubiran et al.(2010); sit nights our goal was to take three spectra; if no transit was (3) Han et al.(2014); (4) Baluev et al.(2015); (5) Gray et al. expected, we took a single exposure per night. The target is only (2006); (6) Głe¸bocki & Gnacinski´ (2005); (7) Akeson et al.(2013); observable from spring to fall and the weather is generally more (8) Morello et al.(2014); (9) Pace(2013); (10) Bonfanti et al.(2016); unstable during the summer, causing larger observation gaps (see (11) Henry & Winn(2008). Fig.1). Nevertheless, this is one of the most extensive spectral time series of HD 189733 to our knowledge.
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