A Search for Sodium Absorption from Comets Around HD209458

We monitored the planet-bearing solar-type star HD209458 for sodium absorption in the region of the stellar NaI D1 line that would be indicative of cometary activity in the system. We observed the star using the HET HRS with high S/N and high spectral resolution for 6 nights over the course of two years, from July 2001 to July 2003. From modelling we determine a 20% likelihood of a detection, based on a predicted number of comets similar to that of the solar system. We find that our analytical method is able to recover a signal and our S/N is sufficient to detect this feature in the spectral regions on either side of the core of the D1 line, where it is most likely to appear. No significant absorption was detected for any of the nights based on a 3 sigma detection limit. We derive upper limits on the column density of sodium of < ~6 x 10^{9} cm^{-2} for a signal in the region around the line core and < \~2 x 10^{10} cm^{-2} for a signal in the core of the photospheric D1 line. These numbers are consistent with the sodium released in a single periodic comet in our own system, though higher S/N may be necessary to uncover a signal in the core of the D1 line. Implications for cometary activity in the HD209458 system are discussed.