Exploring the Potential of Integral Field Spectroscopy for Observing Extrasolar Planet Transits: Ground‐based Observations of the Atmospheric Na in HD 209458b

We explore the use of Integral Field Spectroscopy (IFS) to observe extrasolarplanet transits. Although this technique should find its full potential inspace observations (e.g. JWST, TPF), we have tested its basics with groundbased time series observations of HD209458b obtained with WHT+INTEGRAL during atransit. For this analysis we used 5550 spectra, obtained in 150 exposuresduring a period of >7 hours. We found that IFS offers 3 fundamental advantageswith respect to previously used methods. (i) It improves the effective S/N inphoton limited observations by distributing the light coming from the star intothe 2 dimensions of the detector. (ii) This type of IFS data allows to'auto-calibrate' instrumental and background effects. (iii) Since the starimage characteristics as well as its photometric properties are extracted fromthe same data-cube, it is possible to decorrelate photometric instabilitiesinduced by PSF variations. These data have also allowed us to explore theaccuracy limits of ground based 'relative' spectrophotometry. This was doneusing a photometric index that probes the NaD lines, for which we obtained anominal 1-sigma error of ~1.0x10^-4. This result, based on observations of only1 transit, indicates that this type of ground observation can constrain thecharacterization of the transmission spectrum of extrasolar planets. Thepresent observations are compatible with no extra NaD depression during thetransit. Though this result seems to be inconsistent with the recently reportedHST-STIS findings we point out its limited statistical meaning: the resultsdisagree within 1-sigma, but agree within 2-sigma. We also give somerecommenda-tions to instrument developers in order to enhance the efficiency ofthe method.Comment: 15 pages, 2 tables, 10 figures. To appear in PASP (January 2006