Precise Infrared Radial Velocities From Keck/Nirspec and the Search for Young Planets

Abstract

We present a high-precision infrared radial velocity study of late-type stars using spectra obtained with NIRSPEC at the W. M. Keck Observatory. Radial velocity precisions of 50 m/s are achieved for old field mid-M dwarfs using telluric features for precise wavelength calibration. Using this technique, 20 young stars in the β Pic (age ∼ 12 Myr) and TW Hya (age ∼ 8 Myr) Associations were monitored over several years to search for low mass companions; we also included the chromospherically active field star GJ 873 (EV Lac) in this survey. Based on comparisons with previous optical observations of these young active stars, radial velocity measurements at infrared wavelengths mitigate the radial velocity noise caused by star spots by a factor of ∼ 3. Nevertheless, star spot noise is still the dominant source of measurement error for young stars at 2.3 µm, and limits the precision to ∼ 77 m/s for the slowest rotating stars (vsini < 6 km/s), increasing to ∼ 168 m/s for rapidly rotating stars (vsini > 12 km/s). The observations reveal both GJ 3305 and TWA 23 to be single-lined spectroscopic binaries; in the case of GJ 3305, the motion is likely caused by its 0. ′′ 09 companion, identified after this survey began. The large amplitude, short-timescale variations of TWA 13A are indicative of a hot Jupiter-like companion, but the available data are insufficient to confirm this. We label it as a candidate radial velocity variable. For the remainder of the sample, these observations exclude the presence of any 'hot' (P < 3 days) companions more massive than 8 M Jup , and any 'warm' (P < 30 day) companions more massive than 17 M Jup , on average. Assuming an edge-on orbit for the edge-on disk system AU Mic, these observations exclude the presence of any hot Jupiters more massive than 1.8 M Jup or warm Jupiters more massive than 3.9 M Jup .

A High-Precision Near-Infrared Survey for Radial Velocity Variable Low-Mass Stars Using Cshell and a Methane Gas Cell

“…We achieved short-term RV precisions (within a single night) as low as ∼8-15 m s −1 for higher-S/N observations, which represents a net improvement over previous studies that used similar small facilities (e.g., ≈45-90 m s −1 ; Bailey et al 2012;Davison et al 2015) but that did not benefit from a methane gas cell or our novel RV extraction method. These precisions are almost comparable to what is achieved using 8 m-class telescopes with a spectral grasp ∼10 times larger, although we can only achieve similar S/Ns on brighter targets (e.g., 5-10 m s −1 on targets with…”

“…Such a level of jitter comparable to that of Bailey et al (2012) is not strong enough to reproduce the RV variability of stars with known or candidate companions (GJ876, HD160934AB) or the single stars V577Per, BD+201790, and LHS374. Those three targets display a level of RV variability that is thus unlikely to be explained by the combined loss of information that is due to stellar broadening and stellar jitter, but additional follow-up work will be needed to assess this with certainty.…”

“…This correlation is independent of the survey sample (i.e., independent of age), although younger stars are faster rotators on average. It can be noted that the large RV variability of LOPeg might be explained by RV information loss that is due to the rotational broadening of stellar lines, whereas that of EQPegA would require a significant jitter term at the higher end of what is admitted in the relation proposed by Bailey et al (2012). Further observations will be required to determine whether EQPegA can plausibly host a substellar or planetary companion.…”

“…Six others were already followed either at optical or NIR wavelengths, albeit at a 50 m s −1 precision (AGTri, ATMicA, ATMicB, auMic, BD+012447, V1005Ori; Paulson & Yelda 2006;Bailey et al 2012), and three targets benefited from a precise RV follow-up at optical wavelengths (BD+201790,εEridani, GJ3305AB; e.g., Campbell et al 1988;Figueira et al 2010;Elliott et al 2014;Hernán-Obispo et al 2015).…”

“…As an example, iodine gas cells, which have been used extensively as wavelength calibrators in optical RV surveys, do not offer a sufficient density of absorption lines in the NIR domain. For this reason, most existing NIR RV studies have used telluric lines to achieve wavelength calibration (Blake et al 2010;Crockett et al 2011;Bailey et al 2012;Tanner et al 2012;Davison et al 2015), with the exception of Bean et al (2010), who used an ammonia gas cell with CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) at the VLT to obtain unprecedented long-term precisions of ≈5-10 m s −1 in the NIR on targets with K S -band magnitudes between 4.4 and 8.0.…”