Astronomy and Astrophysics – Astrophysics – Earth and Planetary Astrophysics
Scientific paper
2009-02-09
Astrophys.J.696:241-253,2009
Astronomy and Astrophysics
Astrophysics
Earth and Planetary Astrophysics
18 pages, 13 figures, accepted for publication in ApJ
Scientific paper
10.1088/0004-637X/696/1/241
The transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure, because the planet's relatively small size has been interpreted as evidence for a highly metal-enriched composition. We present observations of 4 transits with the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, within a wavelength range of 1.1--2.0 $\mu$m. Analysis of the light curve gives the most precise estimate yet of the stellar mean density, $\rho_\star = 0.497^{+0.042}_{-0.057}$ g cm$^{-3}$. By requiring agreement between the observed stellar properties (including $\rho_\star$) and stellar evolutionary models, we refine the estimate of the stellar radius: $R_\star = 1.541^{+0.046}_{-0.042}$ $R_\sun$. We also find a deeper transit than has been measured at optical and mid-infrared wavelengths. Taken together, these findings imply a planetary radius of $R_p = 0.813^{+0.027}_{-0.025}$ $R_{\rm Jup}$, which is larger than earlier estimates. Models of the planetary interior still require a metal-enriched composition, although the required degree of metal enrichment is reduced. It is also possible that the deeper NICMOS transit is caused by wavelength-dependent absorption by constituents in the planet's atmosphere, although simple model atmospheres do not predict this effect to be strong enough to account for the discrepancy. We use the 4 newly-measured transit times to compute a refined transit ephemeris.
Carter Joshua A.
Gilliland Ronald
Holman Matthew J.
Winn Joshua N.
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