Spiral Structure in the Gas Disk of TW Hya

We report the detection of spiral substructure in both the gas velocity and temperature structure of the disk around TW Hya, suggestive of planet-disk interactions with an unseen planet. Perturbations from Keplerian rotation tracing out a spiral pattern are observed in the SE of the disk, while significant azimuthal perturbations in the gas temperature are seen in the outer disk, outside 90 au, extending the full azimuth of the disk. The deviation in velocity is either Delta v(phi)/v(kep) similar to 0.1 or Delta v(z)/v(kep) similar to 0.01 depending on whether the perturbation is in the rotational or vertical direction, while radial perturbations can be ruled out. Deviations in the gas temperature are +/- 4 K about the azimuthally averaged profile, equivalent to deviations of Delta T-gas/T-gas similar to 0.05. Assuming all three structures can be described by an Archimedean spiral, measurements of the pitch angles of both velocity and temperature spirals show a radially decreasing trend for all three, ranging from 9 degrees at 70 au, dropping to 3 degrees at 200 au. Such low pitch-angled spirals are not readily explained through the wake of an embedded planet in the location of previously reported at 94 au, but rather require a launching mechanism that results in much more tightly wound spirals. Molecular emission tracing distinct heights in the disk is required to accurately distinguish between spiral launching mechanisms.