Retrieval of Precise Radial Velocities from Near-infrared High-resolution Spectra of Low-mass Stars

Abstract

Given that low-mass stars have intrinsically low luminosities at optical wavelengths and a propensity for stellar activity, it is advantageous for radial velocity (RV) surveys of these objects to use near-infrared (NIR) wavelengths. In this work we describe and test a novel RV extraction pipeline dedicated to retrieving RVs from low mass stars using NIR spectra taken by the CSHELL spectrograph at the NASA Infrared Telescope Facility, where a methane isotopologue gas cell is used for wavelength calibration. The -2pipeline minimizes the residuals between the observations and a spectral model composed of templates for the target star, the gas cell, and atmospheric telluric absorption; models of the line spread function, continuum curvature, and sinusoidal fringing; and a parameterization of the wavelength solution. The stellar template is derived iteratively from the science observations themselves without a need for separate observations dedicated to retrieving it. Despite limitations from CSHELL's narrow wavelength range and instrumental systematics, we are able to (1) obtain an RV precision of 35 m s −1 for the RV standard star GJ 15 A over a time baseline of 817 days, reaching the photon noise limit for our attained SNR, (2) achieve ∼ 3 m s −1 RV precision for the M giant SV Peg over a baseline of several days and confirm its long-term RV trend due to stellar pulsations, as well as obtain nightly noise floors of ∼2-6 m s −1 , and (3) show that our data are consistent with the known masses, periods, and orbital eccentricities of the two most massive planets orbiting GJ 876. Future applications of our pipeline to RV surveys using the next generation of NIR spectrographs, such as iSHELL, will enable the potential detection of Super-Earths and Mini-Neptunes in the habitable zones of M dwarfs.Subject headings: techniques: radial velocities, planets and satellites: detection temperature contrast between starspots and the rest of the chromosphere is also reduced, decreasing the periodic modulation of chromospheric activity on the Doppler measurements (Martín et al. 2006;Reiners et al. 2010;Mahmud et al. 2011;Crockett et al. 2012;Anglada-Escudé et al. 2013). This effect can help lower the rate of false positive optical planet detections by discriminating actual planets from the effects of spots (Huélamo al. 2008;Prato et al. 2008). This also allows for NIR RV surveys to focus on younger stars, which tend to be more active. Several NIR RV surveys of M dwarfs have been conducted recently. Rodler et al. (2012) used the NIRSPEC spectrograph at the Keck II Telescope to observe eight late-M dwarfs in the J band (∼ 1.1-1.4 µm) and obtained an RV precision of 180-300 m s −1 . Meanwhile, Bailey et al. (2012) and Blake et al. (2010) used NIRSPEC in the K band (∼ 2.0-2.4 µm) to obtain an RV precision of 50 m s −1 for mid-and late-M dwarfs. Tanner et al. (2012) also used NIRSPEC in the K band to observe a sample of late-M dwarfs, with a resulting RV precision of 45 m s −1 . By comparison, simulations have shown that an RV precision...

Precise Radial Velocities of Cool Low-mass Stars with iSHELL

“…T g is our gas cell spectrum, obtained with a Fourier Transform Spectrometer (FTS) at the NASA Jet Propulsion Laboratory (JPL) at R~500,000 (Anglada-Escudé et al 2012;Plavchan et al 2013). Like Gao et al (2016), we find the gas cell optical depth τ g = 0.97 (vs. unity) because of the off-axis angle the gas cell was placed in the FTS, as opposed to CSHELL & iSHELL where the path length is minimized. T t (λ t ) corresponds to the Doppler shifted telluric absorption spectrum with optical depth τ t .…”

“…Known sources of external error arise from the fact that iSHELL is mounted at Cassegrain focus, and thus mechanically flexes as the telescope moves. Finally, iSHELL has both standard and non-standard fringing sources that will induce errors of > 10−20 ms −1 if not modeled sufficiently, and >50 ms −1 if not modeled at all (Gao et al 2016). Determining telluric induced error on RVs is the subject of a future investigation, but regions of large residuals are not found to be correlated with regions of high telluric absorption.…”

Influence of thermal annealing on the microstructure, hardness and corrosion behavior of TiAlSiCuN nanocomposite films

“…c and 2d showed the similar peaks at 74.4 eV and 74.2 eV, respectively, which could be attributed to Al 2 O 3. It suggested that the film annealed at 800 °C had been fully oxidized and almost no nitride phases existed. Analysis of the binding energy from Si 2p peak, as shown in Figs.…”

“…Si doped TiN or TiAlN coatings could provide higher hardness, more excellent high oxidation resistance and better thermal stability owing to their special nc‐TiAlN/a‐Si 3 N 4 structure, which have attracted a broad attention . It is reported that both the dense amorphous Si 3 N 4 boundary phase and Al 2 O 3 top layer formed during high temperature annealing can act as barriers against oxygen diffusion along the grain boundaries and internally . TiAlSiN coating also exhibits better corrosion resistance than TiAlN coating because of the reduced crystal size and strain force caused by Si incorporation .…”

A robust, template-free approach to precise radial velocity extraction

“…The work of Allende Prieto (2007) similarly foreshadowed aspects of our technique in terms of performing multiple pairwise comparisons, but their technique seeks only to improve RVs that have already been measured by some other technique. Our technique is also hardly the first to deliberately avoid using a synthetic template or template from a different star, and to favour instead using only the science observations themselves to construct a template; this approach has been taken elsewhere for both absorption cell spectrographs (Sato et al 2002;Johnson et al 2006;Gao et al 2016) and stabilized spectrographs (Anglada-Escudé & Butler 2012;Astudillo-Defru et al 2015;Zechmeister et al 2018), and has been demonstrated using both CCFs and ML (or least squares) approaches. HARPS-TERRA and SERVAL, which we used for RV extraction in Section 6.3, both adopt such a least-squares template matching approach.…”