  Magnetic (or tidal) interactions between "hot Jupiters" and their host stars
can potentially enhance chromospheric and coronal activity. An ideal testbed
for investigating this effect is provided by the extreme WASP-18 system, which
features a massive (~10 times Jupiter) close-in (~1 day period) transiting
planet orbiting a young F6 star. Optical and X-ray observations of WASP-18 were
conducted in November 2011. The high-resolution echelle spectrograph MIKE was
used on the 6.5m Magellan Clay telescope to obtain 13 spectra spanning
planetary orbital phases of 0.7-1.4, while the X-ray Telescope on Swift
provided contemporaneous monitoring with a stacked exposure of ~50 ks. The
cores of the Ca II H and K lines do not show significant variability over
multiple orbits spanning ~8 d, in contrast to the expectation of
phase-dependent chromospheric activity enhancements for efficient star-planet
interaction. The star is also X-ray faint, with log Lx < 27.6 erg/s (0.3-2
keV), indicating that coronal activity is likewise low. The lack of detectable
star-planet interaction in this extreme system requires that any such effect
must here be transient, if indeed present. We demonstrate that searches for Ca
II H and K variability can potentially mistake a stellar hotspot, if observed
over a short segment of the rotation period, for planet-induced activity. Taken
together, these results suggest that the utility of star-planet interaction as
a robust method of estimating exoplanet magnetic field strengths may be
limited.
