  Photoevaporation driven by the central star is expected to be a ubiquitous
and important mechanism to disperse 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 {\mu}m line confirms that this gas diagnostic
traces the unbound wind component within 10AU from 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 4AU 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
OI, SII, and MgI are centered at the stellar velocity and have symmetric
profiles. The only way these lines could 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 suggest 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 versus a
photoevaporation-induced gap.
