A Search for Pre-Transit Absorption Features in Spectra of Hot Jupiters

Abstract

In the past it was claimed that the transit of the hot Jupiter HD 189733 b is preceded by a bow shock. The authors arrived at this conclusion by the comparison of CaII H&K light curve with the light curves of the Balmer lines. While the former did not show any evidence for pre-transit absorption signals, the latter does show a distinct absorption signal preceding the planetary transit. The authors rule out a stellar origin because of the discrepancy of the CaII and Balmer lines. This work deals with the question if bow shocks are a ubiquitous phenomenon and if they are temporally stable. In the case of HD 189733 b no indications for bow shocks could be found in TIGRE data of the year 2017. Further, it could be shown that the existence of bow shocks cannot be proven in the planetary systems WASP-69, WASP-131, KELT-7 as well as KELT-20. Instead, the observational data allows to derive upper limits for the strengths of the bow shocks. These depend on the telescope and on the brightness of the host star and range between 2 und 6 milliangstroms. This is considerably lower than the value of 13 milliangstroms postulated in the literature for HD 189733 b. For this reason the question is if an alternative explanation exists for the observed phenomena in the HD 189733 system. A possible explanation is stellar activity. An example are prominences of sufficient size and temperature. If they are sufficiently cool and extended they are faint in the CaII K light but bright in the H-alpha light. Similar problems can also occur during transit. An example are different transit observations of KELT-7 b. These show that the transit depths in H-alpha are variable. This variability is caused by active regions on the stellar surface. A fundamental problem for the interpretation of the observations is that the relevant extrasolar planetary systems cannot yet be spatially resolved although this is essential to explore the spatial structure of prominences of other stars. For this reason the astrophysical nature of the observed peculiarities currently remains an open question. The absence of the bow shock allows to impose limits on the strength of the planetary magnetic field. The assumption that the bow shock occured before observations started yields planetary magnetic fields between 155G and more than 18kG. These values are significantly higher than the value of 4G of Jupiter and thus seem to be unrealistic. Alternatively, the passage of bow shock and planet could be temporally so close that the observation data cannot resolve the components. In this case the limits for the planetary magnetic field range between 3,2mG and 0,3G. On the other hand, these values seem to be unreasonable low. The observed spectra not only depend on the physical processes in the planetary system, but are affected by various processes while on their way towards the spectrograph. First, the starlight has to traverse the interplanetary medium. If this possesses its own absorption lines, distorsions are introduced. For nearby stars such distorsions should be negligible. This assumption has been doubted in the literature and therefore it has been validated for a few examples. These show that a correct stellar model is essential to avoid misinterpretations. Another problem is the passage of stellar light through the atmosphere of the Earth. The atmosphere imprints various absorption lines on the stellar spectrum. This is especially problematic in the vicinity of the H-alpha line when stellar and telluric lines blend. These line blends yield an increase in the equivalent width of the H-alpha line. Its strength depends on the local meteorological condition and is therefore temporally variable. All these observations require considerable accuracies of the spectrographs employed. Even small instrumental distorsions can lead to significant distorsions of the observed spectra. Especially spectrographs with a resolution around 20,000 are prone to this. For instance, a decrease in spectral resolution to 15,000 in combination with a radial velocity shift of +900m/s leads to an increase of the equivalent width of the H-alpha line by 5 milliangstroms. Possible reasons for this are an increase in seeing in combination with thermal expansion of spectrograph components. If this happens during pre-transit phase such a signal can be misinterpreted as a planetary absorption feature. At higher spectral resolutions of about 80,000 these nuisances are on the order of 2 milliangstroms and thus smaller.