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Chromospheric activity induced by short-period planets : a search for modulation of Ca II H & K emission Shkolnik, Evgenya

Abstract

I have detected the first evidence of magnetic interaction between an extra-solar planet and its parent star. Of the > 100 extrasolar planets discovered to date, approximated 20% of them are '51 Peg'-type with a Jupiter-mass planet orbiting within 0.1 Astronomical Units. The systems with the tightest orbits (P[sub orb] < 5 days) offer the best opportunity to observe a tidal or magnetic interaction between the planet and its parent star. Stellar chromospheric activity could be modulated in two ways. For magnetic interaction, the modulation is predicted to be at the orbital period with enhancement near the sub-planetary point (ø = 0). Tidal interaction would stimulate activity with a period of P[sub orb] /2 with enhancements near both ø = 0 and 0.5. The Ca II H & K line reversals at 3968 and 3933 Å are the best chromospheric activity indicators visible from the ground. I observed the H & K emission cores of five sun-like stars with short period planets: τ Boo, HD179949, HD 209458, 51 Peg and υ And. I acquired 10 nights of high resolution (≈ 110,000), high S/N (~ 500) data at the Canada-France-Hawaii Telescope over three semesters: 2001 August, 2002 July and 2002 August. The superb quality of the data yielded differential radial velocities to better than 20 m s⁻¹. Fitting known orbital parameters such as period and velocity amplitude to the radial velocities, I determined updated ephemerides and accurate orbital phases. Night-to-night modulation of the H & K emission was observed in 4 of the 5 stars. Our two standards, τ Ceti and the sun, showed no such variability. Three of the 4 'active' stars did not appear to show a correlation between activity and orbital phase. However, HD 179949, the star with the tightest planetary orbit (P[sub orb] = 3.093 days), repeatedly showed a 2.5% enhancement in the Ca II K emission leading the sub-planetary point by 0.17 in phase. A decrease was observed when the planet was behind the star. The activity persisted for 108 orbits (or 37 stellar rotations). This is the first detection of magnetic interaction between a star and its giant planet, as well as a first glimpse of an extrasolar planetary magnetosphere. As an exaggerated example of enhanced chromospheric activity induced by a companion, we observed ER Vul, an RS CVn binary system with P[sub orb], = 17 hours. Using the same setup at the CFHT, we obtained Ca II H &; K spectra with nearly complete phase coverage. This system shows increased activity near the sub-binary longitudes of both components. There is also evidence of Ca II emission from between the two stars. Observations of κ¹ Ceti, an active single dwarf star, show periodic H & K activity modulated by the stellar rotation (P[sub rot] = 9.4 d.) with an enhancement level of ≈ 7%. The stimulating mechanism for its activity is unknown and may be evidence of a yet unseen, nearby companion.

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