The Warm Neptunes around HD 106315 Have Low Stellar Obliquities

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 R Earth warm Neptune orbiting a moderately rotating late F-star with a period of 21.05 days. HD 106315 also hosts a 2.5 R Earth 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 λ = −10.9 +3.6 −3.8• . 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. Subject headings: planets and satellites: individual (HD106315c), planets and satellites: dynamical evolution and stability

Physical Parameters of the Multiplanet Systems HD 106315 and GJ 9827* †

“…Similar to GJ 9827, we aim to understand the stellar activity component of the radial velocity data through investigating the possible relationships between the K2 light curve, the Calcium II H&K and H-alpha stellar lines, and our radial velocity data. The projected rotational velocity measurement (v sin i = 13.2 ± 1 km s −1 ) combined with the obliquity measurement (λ = −10.9 ± 3.7, Zhou et al 2018) suggests a stellar rotation period of 4.78±0.15 days. HD 106315 was observed in K2 Campaign 10; this campaign had a 14 day data gap resulting in 49 days of contiguous data.…”

“…Later Zhou et al (2018) investigated the system architecture through measuring the obliquity of HD 106315 c using Doppler tomography and constraining the mutual inclination of HD 106315 b through dynamical arguments. They found that these two planets both have low obliquities, consistent with the few other warm Neptunes with measured obliquities (eg.…”

Two Young Planetary Systems around Field Stars with Ages between 20 and 320 Myr from TESS

“…We also made use of the CHIRON observations to measure the projected rotational velocity v sin I of the host stars. Following Zhou et al (2018), we model the line profiles derived from the CHIRON spectra via a convolution of kernels representing the rotation, radialtangential macroturbulence, and instrument broadening terms. The rotational and radial-tangential macroturbulence kernels follow the prescription in Gray (2005), while the instrument broadening kernel is represented by a Gaussian function of width that of the instrument resolution.…”

EPIC 246851721 b: A Tropical Jupiter Transiting a Rapidly Rotating Star in a Well-aligned Orbit

“…s 1 to account for the instrument broadening. The disk integration process is computationally intensive, as such we only compute a grid of broadening kernels spanning  grid during our model fit(Zhou et al 2018). By modeling the line profiles, we derive a projected stellar rotation rate of …”