Precise Radial Velocities of Cool Low-mass Stars with iSHELL

Abstract

The coolest dwarf stars are intrinsically faint at visible wavelengths and exhibit rotationally modulated stellar activity from spots and plages. It is advantageous to observe these stars at near infrared (NIR) wavelengths (1-2.5 µm) where they emit the bulk of their bolometric luminosity and are most quiescent. In this work we describe our methodology and results in obtaining precise radial velocity (RV) measurements of low mass stars using K-band spectra taken with the R~80,000 iSHELL spectrograph and the NASA Infrared Telescope Facility (IRTF) using a methane isotopologue gas cell in the calibration unit. Our novel analysis pipeline extracts RVs by minimizing the RMS of the residuals between the observed spectrum and a forward model. The model accounts for the gas cell, tellurics, blaze function, multiple sources of quasi-sinusoidal fringing, and line spread function of the spectrograph (LSF). The stellar template is derived iteratively using the target observations themselves through averaging barycenter-shifted residuals. We have demonstrated 5 ms −1 precision over oneyear timescales for the M4 dwarf Barnard's Star and K dwarf 61 Cygni A, and 3 ms −1 over a month for the M2 dwarf GJ 15 A. This work demonstrates the potential for iSHELL to determine dynamical masses for candidate exoplanets discovered with the NASA TESS mission, and to search for exoplanets orbiting moderately active and/or young K & M dwarfs.

A planet within the debris disk around the pre-main-sequence star AU Microscopii

“…In this section, we present the utilization of the higher precision radial velocities from iSHELL, HARPS and HIRES to rule out higher mass companions, correlations with stellar activity, and confirm the planetary nature of AU Mic b by placing an upper limit on its mass. iSHELL 30 is a near-infrared echelle spectrometer with resolution of R=70,000 and simultaneous grasp of 300 nm at the 3.0-meter NASA Infrared Telescope Facility (IRTF), and equipped with our custom-built methane isotopologue absorption gas cell for wavelength calibration and instrument characterization 31 . The iSHELL data reduction and RV extraction follows the prescription in Ref 31 .…”

“…iSHELL 30 is a near-infrared echelle spectrometer with resolution of R=70,000 and simultaneous grasp of 300 nm at the 3.0-meter NASA Infrared Telescope Facility (IRTF), and equipped with our custom-built methane isotopologue absorption gas cell for wavelength calibration and instrument characterization 31 . The iSHELL data reduction and RV extraction follows the prescription in Ref 31 . We combine our data with archival observations from the visible wavelength HARPS at the ESO La Silla 3.6-meter telescope 32 , and the visible wavelength HIRES on the 10-meter Keck telescope 33 obtained for the California Planet Survey.…”

Spin-orbit alignment and magnetic activity in the young planetary system AU Mic

“…Their two-minute exposure time results in a photon S/N of ∼60−70 per spectral pixel at 2.4 µm (the approximate peak of the blaze function for the center order), and in turn in a RV precision of 15−27 m s −1 (median 21 m s −1 ) per measurement. These spectra were reduced and their RVs extracted using the methods outlined in Cale et al (2019). The RV data measured by iSHELL are presented in Appendix A. that systematic errors affecting the polarimetric analysis are minimized (we compute the Stokes parameter using the "ratio" method Donati et al 1997;Bagnulo et al 2009).…”

Precise mass and radius of a transiting super-Earth planet orbiting the M dwarf TOI-1235: a planet in the radius gap?

“…The exposure times were 300 s, repeated 9-11 times within a night to reach a cumulative photon S/N per spectral pixel at about 2.4 µm (at the approximate center of the blaze for the middle order) varying from 77 to 98 to achieve a per-night precision of 4-11 m s −1 . Spectra were reduced and RVs extracted using the methods outlined by Cale et al (2019). The resulting wrms andσ of the iSHELL data were 7.2 and 6.1 m s −1 , slightly better than the CARMENES NIR data, but still twice higher than the expected planet semiamplitude.…”