2011ApJ...736...13P

Photoevaporation driven by the central star is expected to be a ubiquitous and important mechanism for dispersing the circumstellar dust and gas from which planets form. Here, we present a detailed study of the circumstellar disk surrounding the nearby star TW Hya and provide observational constraints to its photoevaporative wind. Our new high-resolution (R ∼ 30,000) mid-infrared spectroscopy in the [Ne II] 12.81 µm line confirms that this gas diagnostic traces the unbound wind component within 10 AU of the star. From the blueshift and asymmetry in the line profile, we estimate that most (>80%) of the [Ne II] emission arises from disk radii where the midplane is optically thick to the redshifted outflowing gas, meaning beyond the 1 or 4 AU dust rim inferred from other observations. We re-analyze high-resolution (R ∼ 48,000) archival optical spectra searching for additional transitions that may trace the photoevaporative flow. Unlike the [Ne II] line, optical forbidden lines from O I, S II, and Mg I are centered at stellar velocity and have symmetric profiles. The only way these lines can trace the photoevaporative flow is if they arise from a disk region physically distinct from that traced by the [Ne II] line, specifically from within the optically thin dust gap. However, the small (∼10 km/s) FWHM of these lines suggests that most of the emitting gas traced at optical wavelengths is bound to the system rather than unbound. We discuss the implications of our results for a planet-induced gap versus a photoevaporation-induced gap.