Sunday, March 22, 2015

Accreting Material from a Hot Jupiter

HD 189733 b is a hot Jupiter circling a K-type star that is somewhat less massive and less luminous than the Sun. Like Jupiter, HD 189733 b is a gas giant planet comprised mostly of hydrogen and helium. However, HD 189733 b orbits very close to its host star, about 30 times closer than Earth is from the Sun. The planet has an orbital period of only 2.22 days. During each orbit, HD 189733 b also transits across the face of its host star. HD 189733 b is slightly more massive and slightly larger in size than Jupiter. Because it is so close to its host star, HD 189733 b is subjected to very intense irradiation, about 275 times more insolation than Earth gets from the Sun. Due to the intense heating, the outer atmosphere of HD 189733 b is known to be evaporating into space.

Figure 1: Artist’s impression of HD 189733 b. Credit: NASA’s Goddard Space Flight Center.

The host star of HD 189733 b was observed with the Cosmic Origin Spectrograph (COS) on the Hubble Space Telescope (HST). The observations revealed the presence of an active spot on the surface of the star. An indication that HD 189733 b is responsible for this feature on its host star is that the active spot comes in and out of view in phase with the orbital motion of HD 189733 b. The active spot is situated 70 to 90 degrees ahead of the sub-planetary point. Essentially, the sub-planetary point is the point on the star’s surface where HD 189733 b appears directly “overhead”. Magneto-hydrodynamic (MHD) simulations point out that the active spot is consistant with material evaporating from HD 189733 b that is steadily accreting onto the surface of the star. The rate at which material is accreted from HD 189733 b is estimated to be ~200 million kg/s.

Figure 2: MHD simulations showing the interactions between HD 189733 b and its host star. The star rotates counter-clockwise and the planet orbits the star along the same direction. The two “+” symbols shown on the left panel indicate the location of the star (red disk) and the planet (green disk). A close up of the impact region is depicted in the right panel, where the motion of the accreting plasma is marked with arrows. The plasma is funnelled by the magnetized stellar wind in an almost radial trajectory close to the star (A), it forms a “knee” structure that consists of hot and dense plasma (B), and then accretes in a spot ahead of the orbital phase (C). The knee (B) of the stream and the active spot (C) are the main sites emitting the observed enhanced flux of ultraviolet and X-rays. Pillitteri et al. (2015).

Pillitteri et al. (2015), “FUV variability of HD 189733. Is the star accreting material from its hot Jupiter”, arXiv:1503.05590 [astro-ph.SR]