The Corona of Hd 189733 and Its X-Ray Activity

Abstract

Testing whether close-in massive exoplanets (hot Jupiters) can enhance the stellar activity in their host primary is crucial for the models of stellar and planetary evolution. Among systems with hot Jupiters, HD 189733 is one of the best studied because of its proximity, strong activity and the presence of a transiting planet, that allows transmission spectroscopy, a measure of the planetary radius and its density. Here we report on the X-ray activity of the primary star, HD 189733 A, using a new XMM-Newton observation and a comparison with the previous X-ray observations. The spectrum in the quiescent intervals is described by two temperatures at 0.2 keV and 0.7 keV, while during the flares a third component at 0.9 keV is detected. With the analysis of the summed RGS spectra, we obtain estimates of the electron density in the range n e = 1.6 − 13 × 10 10 cm −3 and thus the corona of HD 189733 A appears denser than the solar one. For the third time, we observe a large flare that occurred just after the eclipse of -2the planet. Together with the flares observed in 2009 and 2011, the events are restricted to a small planetary phase range of φ = 0.55 − 0.65. Although we do not find conclusive evidence of a significant excess of flares after the secondary transits, we suggest that the planet might trigger such flares when it passes close to locally high magnetic field of the underlying star at particular combinations of stellar rotational phases and orbital planetary phases. For the most recent flares, a wavelet analysis of the light curve suggests a loop of length of four stellar radii at the location of the bright flare, and a local magnetic field of order of 40-100 G, in agreement with the global field measured in other studies. The loop size suggests an interaction of magnetic nature between planet and star, separated by only ∼ 8R * . The X-ray variability of HD 189733 A is larger than the variability of field stars and young Pleiades of similar spectral type and X-ray luminosity. We also detect the stellar companion (HD 189733 B, ∼ 12 from the primary star) in this XMM-Newton observation. Its very low X-ray luminosity (L X = 3.4 × 10 26 erg s −1 ) confirms the old age of this star and of the binary system. The high activity of the primary star is best explained by a transfer of angular momentum from the planet to the star.

No X-rays from WASP-18

“…We have observed this system three times with XMM-Newton and found strong evidence of SPI at work in X-rays (Pillitteri et al 2010(Pillitteri et al , 2011(Pillitteri et al , 2014. The first evidence comes from the overall activity of the host star.…”

“…However, in the cases of stars with hot Jupiters, the rotation and the activity tracers can be affected by the interaction with the planet, thus biasing the estimate of the age of the host star. Pillitteri et al (2010Pillitteri et al ( , 2011Pillitteri et al ( , 2014; Poppenhaeger et al (2013);Poppenhaeger & Wolk (2014) and Schröter et al (2011) have found that the hot Jupiter hosting stars HD 189733 and CoRoT-2A have most likely been spun up by their close in planets, and thus their activity and rotation have been enhanced, thereby mimicking the behavior of younger stars. In HD 189733, activity tracers like X-ray luminosity would assign an age in the range 0.6-1.1 Gyr (Melo et al 2006;Sanz-Forcada et al 2011), while the stellar companion is much older.…”

Time-resolved UVES observations of a stellar flare on the planet host HD 189733 during primary transit

“…For example, Pillitteri et al (2010) present a study of an X-ray flare event observed with XMM-Newton, occurring 3 ks after the end of the planetary secondary transit (at orbital phase ∼ 0.54); their analysis suggests flare temperatures of ∼ 10 MK, similar to temperatures of solar flares, yet the size of the flaring loop is estimated to be on the order of the stellar radius. Using again XMM-Newton, the same group of authors report a further flare observed near secondary transit (at phase 0.52; Pillitteri et al (2011)) and Pillitteri et al (2014) report the occurrence of a third flare at phase 0.65, with similar characteristics as the previous two flares, with a loop of length of four stellar radii at the location of the flare.…”

“…As described in Sect. 1, Pillitteri et al (2010Pillitteri et al ( , 2011Pillitteri et al ( , 2014 caused by the orbiting exoplanet. Their findings are supported by the models of Lanza (2012), who predict a modulation of coronal flaring activity with the orbital phase of the planet.…”

Analysis of combined radial velocities and activity of BD+20 1790: evidence supporting the existence of a planetary companion

“…This suggests that close-in massive planets may affect the level of chromospheric activity of its host star. Some authors suggested some excess of flaring activity in phase with the planet rotation (Shkolnik et al 2008;Pillitteri et al 2011Pillitteri et al , 2014. A recent review by Lanza (2014) recaps the open questions in this field and the theoretical models made to explain the different observational results obtained.…”