The warm Neptunes around HD 106315 have low stellar obliquities

TL;DR

This study measures the low stellar obliquity of the warm Neptune HD 106315c, supporting the idea that such planets form in aligned, in-situ disks, and demonstrates the feasibility of detecting small planets via transit spectroscopy.

Contribution

First obliquity measurement of a warm Neptune around HD 106315, confirming low obliquity in such systems and supporting in-situ formation theories.

Findings

HD 106315c has a low stellar obliquity of approximately -10 degrees.

Warm Neptune systems generally exhibit well-aligned, low obliquity orbits.

HD 106315c is among the smallest planets detected in transit spectroscopy.

Abstract

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.