We have detected excess absorption in the emission cores of Ca ii H & K during transits of HD 189733b for the first time. Using observations of three transits we investigate the origin of the absorption, which is also seen in Hα and the Na i D lines. Applying differential spectrophotometry methods to the Ca ii H and Ca ii K lines combined, using respective passband widths of ∆λ = 0.4 & 0.6Å yields excess absorption of t d = 0.0074 ± 0.0044 (1.7σ; Transit 1) and 0.0214 ± 0.0022 (9.8σ; Transit 2). Similarly, we detect excess Hα absorption in a passband of width ∆λ = 0.7Å, with t d = 0.0084 ± 0.0016 (5.2σ) and 0.0121 ± 0.0012 (9.9σ). For both lines, Transit 2 is thus significantly deeper. Combining all three transits for the Na i D lines yields excess absorption of t d = 0.0041 ± 0.0006 (6.5σ). By considering the time series observations of each line, we find that the excess apparent absorption is best recovered in the stellar reference frame. These findings lead us to postulate that the main contribution to the excess transit absorption in the differential light curves arises because the normalising continuum bands form in the photosphere, whereas the line cores contain a chromospheric component. We can not rule out that part of the excess absorption signature arises from the planetary atmosphere, but we present evidence which casts doubt on recent claims to have detected wind motions in the planet's atmosphere in these data.
At a distance of 1.8 parsecs 1 , Barnard's star (Gl 699) is a red dwarf with the largest apparent motion of any known stellar object. It is the closest single star to the Sun, second only to the a Centauri triple stellar system. Barnard's star is also among the least magnetically active red dwarfs known 2,3 and has an estimated age older than our Solar System. Its properties have made it a prime target for planet searches employing techniques such as radial velocity 4,5,6 , astrometry 7,8 , and direct imaging 9 , all with different sensitivity limits but ultimately leading to disproved or null results. Here we report that the combination of numerous measurements from high-precision radial velocity instruments reveals the presence of a low-amplitude but significant periodic signal at 233 days. Independent photometric and spectroscopic monitoring, as well as the analysis of instrumental systematic effects, show that this signal is best explained as arising from a planetary companion. The candidate planet around Barnard's star is a cold super-Earth with a minimum mass of 3.2 Earth masses orbiting near its snow-line. The combination of all radial velocity datasets spanning 20 years additionally reveals a long-term modulation that could arise from a magnetic activity cycle or from a more distant planetary object. Because of its proximity to the Sun, the proposed planet has a maximum angular separation of 220 milliarcseconds from Barnard's star, making it an excellent target for complementary direct imaging and astrometric observations.Barnard's star is the second closest red dwarf to the Solar System, after Proxima Centauri, and thus an ideal target to search for exoplanets with potential for further characterisation 10 . Its very low X-ray flux, lack of Ha emission, low chromospheric emission indices, slow rotation rate, slightly sub-solar metallicity, and membership of the thick disc kinematic population indicate an extremely low magnetic activity level and suggest an age older than the Sun. Because of its apparent brightness and very low variability, Barnard's star is often regarded as a benchmark for intermediate M-type dwarfs. Its basic properties are summarized in Table 1.An early analysis of archival radial velocity datasets of Barnard's star up to 2015 indicated the presence of at least one significant signal with a period of ~230 days but with rather poor sampling. To elucidate its presence and nature we undertook an intensive monitoring campaign with the CARMENES spectrometer 11 , collecting precise radial velocity measurements on every possible night during 2016-2017, and we obtained overlapping observations with the ESO/HARPS and HARPS-N instruments. The combined Doppler monitoring effort of Barnard's star, including archival and newly acquired observations, resulted in 771 radial velocity epochs (nightly averages) with typical individual precisions of 0.9 to 1.8 m s -1 , obtained over a timespan exceeding 20 years from seven different facilities and yielding eight independent datasets (ED Table 1).While e...
Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R ′ HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca ii lines, broadening b-parameter, and Ca ii column density). We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km s −1 and for ISM Ca ii column densities log N CaII 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca ii column density of log N CaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R ′ HK value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars.
We measured the chromospheric activity of the four hot Jupiter hosts WASP-43, WASP-51/HAT-P-30, WASP-72 & WASP-103 to search for anomalous values caused by the close-in companions. The Mount Wilson Ca ii H & K S -index was calculated for each star using observations taken with the Robert Stobie Spectrograph at the Southern African Large Telescope. The activity level of WASP-43 is anomalously high relative to its age and falls among the highest values of all known main sequence stars. We found marginal evidence that the activity of WASP-103 is also higher than expected from the system age. We suggest that for WASP-43 and WASP-103 star-planet interactions (SPI) may enhance the Ca ii H & K core emission. The activity levels of WASP-51/HAT-P-30 and WASP-72 are anomalously low, with the latter falling below the basal envelope for both main sequence and evolved stars. This can be attributed to circumstellar absorption due to planetary mass loss, though absorption in the ISM may contribute. A quarter of known short period planet hosts exhibit anomalously low activity levels, including systems with hot Jupiters and low mass companions. Since SPI can elevate and absorption can suppress the observed chromospheric activity of stars with close-in planets, their Ca ii H & K activity levels are an unreliable age indicator. Systems where the activity is depressed by absorption from planetary mass loss are key targets for examining planet compositions through transmission spectroscopy.
Earth mass exoplanets are difficult to detect. The Dispersed Matter Planet Project (DMPP) identifies stars which are likely to host the most detectable low mass exoplanets. DMPP-3 (HD 42936) shows signs of circumstellar absorption, indicative of mass loss from ablating planets. Here we report the radial velocity (RV) discovery of a highly eccentric 507 d binary companion and a hot super-Earth planet in a 6.67 d orbit around the primary star. DMPP-3A is a solar type star while DMPP-3B is just massive enough to fuse hydrogen. The binary, with semi-major axis 1.22 ± 0.02 AU, is significantly tighter than others known to host planets orbiting only one of the component stars. The configuration of the DMPP-3 planetary system is rare and indicates dynamical interactions, though the evolutionary history is not entirely clear. DMPP-3Ab is possibly the residual core of a giant planet precursor, consistent with the inferred circumstellar gas shroud.
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