  We present the obliquity of the warm Neptune HD 106315c measured via a series
of spectroscopic transit observations. HD 106315c is a 4.4 REarth warm Neptune
orbiting a moderately rotating late F-star with a period of 21.05 days. HD
106315 also hosts a 2.5 REarth super-Earth on a 9.55 day orbit. Our Doppler
tomographic analyses of four transits observed by the Magellan/MIKE, HARPS, and
TRES facilities find HD 106315c to be in a low stellar obliquity orbit,
consistent with being well aligned with the spin axis of the host star at
lambda = -10 +3.6/-3.8 deg. We suggest, via dynamical N-body simulations, that
the two planets in the system must be co-planar, and thus are both well aligned
with the host star. HD 106315 is only the fourth warm Neptune system with
obliquities measured. All warm Neptune systems have been found in well aligned
geometries, consistent with the interpretation that these systems are formed
in-situ in the inner protoplanetary disk, and also consistent with the majority
of Kepler multi-planet systems that are in low obliquity orbits. With a transit
depth of 1.02 mmag, HD 106315c is among the smallest planets to have been
detected in transit spectroscopy, and we discuss its detection in the context
of TESS and the next generations of spectrographs.
