Precision Radial Velocity Observations Research Articles

Characterization of K2-167 b and CALM, a new stellar activity mitigation method

ABSTRACT We report precise radial velocity (RV) observations of HD 212657 (= K2-167), a star shown by K2 to host a transiting sub-Neptune-sized planet in a 10 d orbit. Using Transiting Exoplanet Survey Satellite (TESS) photometry, we refined the planet parameters, especially the orbital period. We collected 74 precise RVs with the HARPS-N spectrograph between August 2015 and October 2016. Although this planet was first found to transit in 2015 and validated in 2018, excess RV scatter originally limited mass measurements. Here, we measure a mass by taking advantage of reductions in scatter from updates to the HARPS-N Data Reduction System (2.3.5) and our new activity mitigation method called CCF Activity Linear Model (CALM), which uses activity-induced line shape changes in the spectra without requiring timing information. Using the CALM framework, we performed a joint fit with RVs and transits using exofastv2 and find Mp = $6.3_{-1.4}^{+1.4}$ $\, M_{\hbox{$\oplus $}}$ and Rp = $2.33^{+0.17}_{-0.15}$ $\, R_{\hbox{$\oplus $}}$, which places K2-167 b at the upper edge of the radius valley. We also find hints of a secondary companion at a ∼22 d period, but confirmation requires additional RVs. Although characterizing lower mass planets like K2-167 b is often impeded by stellar variability, these systems especially help probe the formation physics (i.e. photoevaporation, core-powered mass-loss) of the radius valley. In the future, CALM or similar techniques could be widely applied to FGK-type stars, help characterize a population of exoplanets surrounding the radius valley, and further our understanding of their formation.

Discovery and characterization of two Neptune-mass planets orbiting HD 212729 with TESS

ABSTRACT We report the discovery of two exoplanets orbiting around HD 212729 (TOI 1052, TIC 317060587), a Teff = 6146 K star with V = 9.51 observed by TESS in Sectors 1 and 13. One exoplanet, TOI-1052b, is Neptune-mass and transits the star, and an additional planet TOI-1052c is observed in radial velocities but not seen to transit. We confirm the planetary nature of TOI-1052b using precise radial velocity observations from HARPS and determined its parameters in a joint RV and photometry analysis. TOI-1052b has a radius of $2.87^{+0.29}_{-0.24}$ R⊕, a mass of 16.9 ± 1.7 M⊕, and an orbital period of 9.14 d. TOI-1052c does not show any transits in the TESS data, and has a minimum mass of $34.3^{+4.1}_{-3.7}$ M⊕ and an orbital period of 35.8 d, placing it just interior to the 4:1 mean-motion resonance. Both planets are best fit by relatively high but only marginally significant eccentricities of $0.18^{+0.09}_{-0.07}$ for planet b and $0.24^{+0.09}_{-0.08}$ for planet c. We perform a dynamical analysis and internal structure model of the planets as well as deriving stellar parameters and chemical abundances. The mean density of TOI-1052b is $3.9^{+1.7}_{-1.3}$ g cm−3 consistent with an internal structure similar to Neptune. A nearby star is observed in Gaia DR3 with the same distance and proper motion as TOI-1052, at a sky projected separation of $\scriptstyle \sim$1500 au, making this a potential wide binary star system.

TOI-969: a late-K dwarf with a hot mini-Neptune in the desert and an eccentric cold Jupiter

Context. The current architecture of a given multi-planetary system is a key fingerprint of its past formation and dynamical evolution history. Long-term follow-up observations are key to complete their picture. Aims. In this paper, we focus on the confirmation and characterization of the components of the TOI-969 planetary system, where TESS detected a Neptune-size planet candidate in a very close-in orbit around a late K-dwarf star. Methods. We use a set of precise radial velocity observations from HARPS, PFS, and CORALIE instruments covering more than two years in combination with the TESS photometric light curve and other ground-based follow-up observations to confirm and characterize the components of this planetary system. Results. We find that TOI-969 b is a transiting close-in (Pb ~ 1.82 days) mini-Neptune planet (mb = 9.1−1.0+1.1 M⊕, Rb = 2.765−0.097+0.088 R⊕), placing it on the lower boundary of the hot-Neptune desert (Teq,b = 941 ± 31 K). The analysis of its internal structure shows that TOI-969 b is a volatile-rich planet, suggesting it underwent an inward migration. The radial velocity model also favors the presence of a second massive body in the system, TOI-969 c, with a long period of Pc = 1700−280+290 days, a minimum mass of mc sin ic = 11.3−0.9+1.1 MJup, and a highly eccentric orbit of ec = 0.628−0.036+0.043. Conclusions. The TOI-969 planetary system is one of the few around K-dwarfs known to have this extended configuration going from a very close-in planet to a wide-separation gaseous giant. TOI-969 b has a transmission spectroscopy metric of 93 and orbits a moderately bright (G = 11.3 mag) star, making it an excellent target for atmospheric studies. The architecture of this planetary system can also provide valuable information about migration and formation of planetary systems.

Identifying Exoplanets with Deep Learning. IV. Removing Stellar Activity Signals from Radial Velocity Measurements Using Neural Networks

Exoplanet detection with precise radial velocity (RV) observations is currently limited by spurious RV signals introduced by stellar activity. We show that machine-learning techniques such as linear regression and neural networks can effectively remove the activity signals (due to starspots/faculae) from RV observations. Previous efforts focused on carefully filtering out activity signals in time using modeling techniques like Gaussian process regression. Instead, we systematically remove activity signals using only changes to the average shape of spectral lines, and use no timing information. We trained our machine-learning models on both simulated data (generated with the SOAP 2.0 software) and observations of the Sun from the HARPS-N Solar Telescope. We find that these techniques can predict and remove stellar activity both from simulated data (improving RV scatter from 82 to 3 cm s−1) and from more than 600 real observations taken nearly daily over 3 yr with the HARPS-N Solar Telescope (improving the RV scatter from 1.753 to 1.039 m s−1, a factor of ∼1.7 improvement). In the future, these or similar techniques could remove activity signals from observations of stars outside our solar system and eventually help detect habitable-zone Earth-mass exoplanets around Sun-like stars.

TESS and HARPS reveal two sub-Neptunes around TOI 1062

The Transiting Exoplanet Survey Satellite (TESS) mission was designed to perform an all-sky search of planets around bright and nearby stars. Here we report the discovery of two sub-Neptunes orbiting around TOI 1062 (TIC 299799658), a V = 10.25 G9V star observed in the TESS Sectors 1, 13, 27, and 28. We use precise radial velocity observations from HARPS to confirm and characterize these two planets. TOI 1062b has a radius of 2.265−0.091+0.096 R⊕, a mass of 10.15 ± 0.8 M⊕, and an orbital period of 4.1130 ± 0.0015 days. The second planet is not transiting, has a minimum mass of 9.78−1.18+1.26 M⊕ and is near the 2:1 mean motion resonance with the innermost planet with an orbital period of 7.972−0.024+0.018 days. We performed a dynamical analysis to explore the proximity of the system to this resonance, and to attempt further constraining the orbital parameters. The transiting planet has a mean density of 4.85−0.74+0.84 g cm−3 and an analysis of its internal structure reveals that it is expected to have a small volatile envelope accounting for 0.35% of the mass at most. The star’s brightness and the proximity of the inner planet to what is know as the radius gap make it an interesting candidate for transmission spectroscopy, which could further constrain the composition and internal structure of TOI 1062b.

A hot terrestrial planet orbiting the bright M dwarf L 168-9 unveiled by TESS

We report the detection of a transiting super-Earth-sized planet (R= 1.39 ± 0.09R⊕) in a 1.4-day orbit around L 168-9 (TOI-134), a bright M1V dwarf (V= 11,K= 7.1) located at 25.15 ± 0.02 pc. The host star was observed in the first sector of the Transiting Exoplanet Survey Satellite (TESS) mission. For confirmation and planet mass measurement purposes, this was followed up with ground-based photometry, seeing-limited and high-resolution imaging, and precise radial velocity (PRV) observations using the HARPS andMagellan/PFS spectrographs. By combining the TESS data and PRV observations, we find the mass of L 168-9 b to be 4.60 ± 0.56M⊕and thus the bulk density to be 1.74−0.33+0.44times higher than that of the Earth. The orbital eccentricity is smaller than 0.21 (95% confidence). This planet is a level one candidate for the TESS mission’s scientific objective of measuring the masses of 50 small planets, and it is one of the most observationally accessible terrestrial planets for future atmospheric characterization.

TESS Spots a Hot Jupiter with an Inner Transiting Neptune

Abstract Hot Jupiters are rarely accompanied by other planets within a factor of a few in orbital distance. Previously, only two such systems have been found. Here, we report the discovery of a third system using data from the Transiting Exoplanet Survey Satellite (TESS). The host star, TOI-1130, is an eleventh magnitude K-dwarf in Gaia G-band. It has two transiting planets: a Neptune-sized planet (3.65 ± 0.10 ) with a 4.1 days period, and a hot Jupiter ( ) with an 8.4 days period. Precise radial-velocity observations show that the mass of the hot Jupiter is . For the inner Neptune, the data provide only an upper limit on the mass of 0.17 (3σ). Nevertheless, we are confident that the inner planet is real, based on follow-up ground-based photometry and adaptive-optics imaging that rule out other plausible sources of the TESS transit signal. The unusual planetary architecture of and the brightness of the host star make TOI-1130 a good test case for planet formation theories, and an attractive target for future spectroscopic observations.

Searching for Planets Orbiting α Cen A with the James Webb Space Telescope

α Centauri A is the closest solar-type star to the Sun and offers an excellent opportunity to detect the thermal emission of a mature planet heated by its host star. The MIRI coronagraph on the James Webb Space Telescope can search the 1–3 au (1″–2″) region around α Cen A which is predicted to be stable within the α Cen AB system. We demonstrate that with reasonable performance of the telescope and instrument, a 20 hr program combining on-target and reference star observations at 15.5 μm could detect thermal emission from planets as small as ∼5 R⊕. Multiple visits every 3–6 months would increase the geometrical completeness, provide astrometric confirmation of detected sources, and push the radius limit down to ∼3 R⊕. An exozodiacal cloud only a few times brighter than our own should also be detectable, although a sufficiently bright cloud might obscure any planet present in the system. While current precision radial velocity (PRV) observations set a limit of 50–100 M⊕ at 1–3 au for planets orbiting α Cen A, there is a broad range of exoplanet radii up to 10 R⊕ consistent with these mass limits. A carefully planned observing sequence along with state-of-the-art post-processing analysis could reject the light from α Cen A at the level of ∼10−5 at 1″–2″ and minimize the influence of α Cen B located 7″–8″ away in the 2022–2023 timeframe. These space-based observations would complement on-going imaging experiments at shorter wavelengths as well as PRV and astrometric experiments to detect planets dynamically. Planetary demographics suggest that the likelihood of directly imaging a planet whose mass and orbit are consistent with present PRV limits is small, ∼5%, and possibly lower if the presence of a binary companion further reduces occurrence rates. However, at a distance of just 1.34 pc, α Cen A is our closest sibling star and certainly merits close scrutiny.

TESS Spots a Compact System of Super-Earths around the Naked-eye Star HR 858

Abstract Transiting Exoplanet Survey Satellite (TESS) observations have revealed a compact multiplanet system around the sixth-magnitude star HR 858 (TIC 178155732, TOI 396), located 32 pc away. Three planets, each about twice the size of Earth, transit this slightly evolved, late F-type star, which is also a member of a visual binary. Two of the planets may be in mean motion resonance. We analyze the TESS observations, using novel methods to model and remove instrumental systematic errors, and combine these data with follow-up observations taken from a suite of ground-based telescopes to characterize the planetary system. The HR 858 planets are enticing targets for precise radial velocity observations, secondary eclipse spectroscopy, and measurements of the Rossiter–McLaughlin effect.

Toward precision radial velocity measurements using Echelle spectrograph at Vainu Bappu Telescope (Erratum)

The Echelle spectrograph operating at Vainu Bappu Telescope is a general purpose instrument designed for high-resolution spectroscopy. It is being considered for precision Doppler measurements without altering the existing design and basic usage. However, the design level limitations and environmental perturbations are a major source of instability and systematic errors. As a result, a small Doppler signal in the stellar spectra is completely swamped by the large and uncontrolled instrumental drift. We discuss some of the remedial measures we took to improve the radial velocity performance of the spectrograph. We show that an autoguider assembly has greatly reduced the mechanical jitter of the star image at the fiber input, making the illumination of the spectrograph slit at the other end stable. We have also installed an iodine absorption cell to track and eliminate the instrumental drifts to facilitate precision radial velocity observations. Furthermore, we have developed a generic algorithm that uses iodine exposures to extract the stellar radial velocities without the need for the complex forward modeling. Our algorithm is not accurate to the level of traditional iodine technique. However, it is convenient to use on a low-cost general-purpose spectrograph targeting a moderate radial velocity (RV) precision at a few 10 to 100 ms − 1 level. Finally, we have demonstrated the usefulness of our approach by measuring the RV signal of a well-known short-period, planet-hosting star.

Toward precision radial velocity measurements using Echelle spectrograph at Vainu Bappu Telescope

The Echelle spectrograph operating at Vainu Bappu Telescope (VBT) is a general purpose instrument designed for high resolution spectroscopy. It is being considered for precision Doppler measurements without altering the existing design and basic usage. However, the design level limitations and environmental perturbations are a major source of instability and systematic errors. As a result, a small Doppler signal in the stellar spectra is completely swamped by the large and uncontrolled instrumental drift. In this paper, we discuss some of the remedial measures we took to improve the radial velocity performance of the spectrograph. We show that a new auto-guider assembly has greatly reduced the mechanical jitter of the star image at the fibre input, making the illumination of the spectrograph slit at the other end stable. We have also installed an iodine absorption cell to track and eliminate the instrumental drifts to facilitate precision radial velocity observations. Furthermore, we have developed a generic algorithm that uses iodine exposures to extract the stellar radial velocities without the need for the complex forward modeling. Our algorithm is not accurate to the level of traditional iodine technique. However it is convenient to use on a low-cost general-purpose spectrograph targeting a moderate Radial Velocity (RV) precision at a few 10-100~$\textrm{ms}^{-1}$ level. Finally, we have demonstrated the usefulness of our approach by measuring the RV signal of a well known short-period, planet-hosting star.

Diffuser-assisted Photometric Follow-up Observations of the Neptune-sized Planets K2-28b and K2-100b

Abstract We present precision transit observations of the Neptune-sized planets K2-28b and K2-100b using the Engineered Diffuser on the Astrophysical Research Council Telescope Imaging Camera (ARCTIC) imager on the ARC 3.5 m Telescope at Apache Point Observatory. K2-28b is a R p = 2.56 R ⊕ mini-Neptune transiting a bright (J = 11.7) metal-rich M4 dwarf offering compelling prospects for future atmospheric characterization. K2-100b is an R p = 3.45 R ⊕ Neptune in the Praesepe Cluster and is one of few planets known in a cluster transiting a host star bright enough (V = 10.5) for precision radial velocity observations. Using the precision photometric capabilities of the diffuser/ARCTIC system allows us to achieve a precision of 365 ppm and 70 ppm in 30-minute bins for K2-28b and K2-100b, respectively. Our joint-fits to the K2 and ground-based light curves give an order-of-magnitude improvement in the orbital ephemeris for both planets, yielding a timing precision of 2 minutes in the James Webb Space Telescope (JWST) era. Although we show that the currently available broad-band measurements of K2-28b’s radius are currently too imprecise to place useful constraints on K2-28b’s atmosphere, we demonstrate that JWST/NIRISS will be able to discern between a cloudy/clear atmosphere in a modest number of transit observations. Our light curve of K2-100b marks the first transit follow-up observation of this challenging-to-observe transit, where we obtain a transit depth of 822 ± 50 ppm in the SDSS i′ band. We conclude that diffuser-assisted photometry can play an important role in the Transiting Exoplanet Survey Satellite (TESS) era to perform timely and precise follow-up of the expected bounty of TESS planet candidates.

Precise Masses in the WASP-47 System

Abstract We present precise radial velocity observations of WASP-47, a star known to host a hot Jupiter, a distant Jovian companion, and, uniquely, two additional transiting planets in short-period orbits: a super-Earth in a ≈19 hr orbit, and a Neptune in a ≈9 day orbit. We analyze our observations from the HARPS-N spectrograph along with previously published data to measure the most precise planet masses yet for this system. When combined with new stellar parameters and reanalyzed transit photometry, our mass measurements place strong constraints on the compositions of the two small planets. We find that, unlike most other ultra-short-period planets, the inner planet, WASP-47 e, has a mass (6.83 ± 0.66 ) and a radius (1.810 ± 0.027 ) that are inconsistent with an Earth-like composition. Instead, WASP-47 e likely has a volatile-rich envelope surrounding an Earth-like core and mantle. We also perform a dynamical analysis to constrain the orbital inclination of WASP-47 c, the outer Jovian planet. This planet likely orbits close to the plane of the inner three planets, suggesting a quiet dynamical history for the system. Our dynamical constraints also imply that WASP-47 c is much more likely to transit than a geometric calculation would suggest. We calculate a transit probability for WASP-47 c of about 10%, more than an order of magnitude larger than the geometric transit probability of 0.6%.

A MISALIGNED PROGRADE ORBIT FOR KEPLER-13 Ab VIA DOPPLER TOMOGRAPHY

Transiting planets around rapidly rotating stars are not amenable to precise radial velocity observations, such as are used for planet candidate validation, as they have wide, rotationally broadened stellar lines. Such planets can, however, be observed using Doppler tomography, wherein the stellar absorption line profile distortions during transit are spectroscopically resolved. This allows the validation of transiting planet candidates and the measurement of the stellar spin-planetary orbit (mis)alignment, an important statistical probe of planetary migration processes. We present Doppler tomographic observations which provide a direct confirmation of the hot Jupiter Kepler-13 Ab, and also show that the planet has a prograde, misaligned orbit, with lambda = 58.6 +/- 2.0 degrees. Our measured value of the spin-orbit misalignment is in significant disagreement with the value of lambda = 23 +/- 4 degrees previously measured by Barnes et al. (2011) from the gravity-darkened Kepler lightcurve. We also place an upper limit of 0.75 solar masses (95% confidence) on the mass of Kepler-13 C, the spectroscopic companion to Kepler-13 B, the proper motion companion of the planet host star Kepler-13 A.

Radii of LMC and SMC Cepheids

AbstractWe collected long sets of precise radial velocity observations of radial velocities for 8 SMC and 14 LMC Cepheids. Based on these series of radial velocity measurements and on multicolor photoelectric observations, we calculated radii of the program extragalactic Cepheids using a modification of Balona's method. Our main goal was to determine the long‐period branch of the period‐radius diagram and reveal a dependence on metallicity. We derived the following period‐radius relation: log R = 1.06(±0.03)+0.74(±0.02) log P. We did not find any significant difference between the period‐radius relation of extragalactic Cepheids and that of fundamental‐mode galactic Cepheids. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterizing Long‐Period Transiting Planets Observed byKepler

Kepler will monitor a sufficient number of stars that it is likely to detect single transits of planets with periods longer than the mission lifetime. We show that by combining the exquisite Kepler photometry of such transits with precise radial velocity observations taken over a reasonable timescale (~ 6 months) after the transits, and assuming circular orbits, it is possible to estimate the periods of these transiting planets to better than 20%, for planets with radii greater than that of Neptune, and the masses to within a factor of 2, for planets with masses larger than or about equal to the mass of Jupiter. Using a Fisher matrix analysis, we derive analytic estimates for the uncertainties in the velocity of the planet and the acceleration of the star at the time of transit, which we then use to derive the uncertainties for the planet mass, radius, period, semimajor axis, and orbital inclination. Finally, we explore the impact of orbital eccentricity on the estimates of these quantities.

Getting More For Your Money: Identifying and Confirming Long-Period Planets with Kepler

AbstractKepler will monitor enough stars that it is likely to detect single transits of planets with periods longer than the mission lifetime. We show that by combining the Kepler photometry of such transits with precise radial velocity (RV) observations taken over ~3 months, and assuming circular orbits, it is possible to estimate the periods of these transiting planets to better than 20% (for planets with radii greater than that of Neptune) and the masses to within a factor of 2 (for planet masses mp ≥ MJup). We also explore the effects of eccentricity on these quantities.

Stability constraints in modeling of multi-planet extrasolar systems

AbstractWe present an analysis of high precision radial velocity (RV) observations of stars hosting multi-planet systems with Jovian companions. We use dynamical stability constraints and quasi-global methods of optimization. As an illustration, we present new results derived for the RV data of the Sun-like dwarfs HD 155358 and τ1 Gruis.

On the Extrasolar Multiplanet System around HD 160691

We reanalyze the precision radial velocity (RV) observations of HD 160691 (μ Ara) by the Anglo-Australian Planet Search Team. The star is supposed to host two Jovian companions (HD 160691b, HD 160691c) in long-period orbits (~630 days and ~2500 days, respectively) and a hot Neptune (HD 160691d) in ~9 day orbit. We perform a global search for the best fits in the orbital parameter space with a hybrid code employing the genetic algorithm and simplex method. The stability of Keplerian fits is verified with the N-body model of the RV signal that takes into account the dynamical constraints (so called GAMP method). Our analysis reveals a signature of the fourth, as yet unconfirmed, Jupiter-like planet HD 160691e in ~307 day orbit. Overall, the global architecture of four-planet configuration recalls the solar system. All companions of μ Ara move in quasi-circular orbits. The orbits of two inner Jovian planets are close to the 2 : 1 mean motion resonance. The alternative three-planet system involves two Jovian planets in eccentric orbits (e ~ 0.3), close to the 4 : 1 MMR, but it yields a significantly worse fit to the data. We also verify a hypothesis of the 1 : 1 MMR in the subsystem of two inner Jovian planets in the four-planet model.

Long‐Term Spectroscopic and Precise Radial Velocity Monitoring of Arcturus

This dissertation presents an analysis of data obtained from long-term spectroscopic monitoring of the bright, nearby K giant star Arcturus (a Bootis, K1.5 III, HR 5340, HD 124897), conducted using the University of Western Ontario’s Elginfield Observatory. Over 1300 high-resolution spectra ( ) 5 l/Dl ≈ 10 were recorded in the 6250 A spectral region during the 1984– 2006 observing seasons, with 311 simultaneous, precise radial velocity measurements (j ∼ 25 m s ) obtained during the 2002–2006 observing seasons. Temperature variations were measured using the well-established line-depth ratio V i l6251.83 to Fe i l6252.56. Velocity spans were obtained from the line bisector of the Fe i l6252.56 line. Precise radial velocities were measured using the telluric absorption technique outlined by Gray & Brown (2006, PASP, 118, 399). Spectroscopic observations and precise radial velocity measurements reveal variability in Arcturus from hours to years. From velocity span observations, three distinct periods of variability are observed: (1) a semiregular 5 1 yr period interpreted as evidence for a long-term stellar magnetic cycle, (2) a 2.0 0.2 yr modulation identified as the rotation period (see also Gray & Brown 2006, PASP, 118, 1112), and (3) a semicoherent 200 5 day period interpreted as one-fourth of the rotation period, indicating the presence of four magnetically active longitudes on the surface of Arcturus. One of these four active longitudes is postulated to dominate the other three, inducing the observed 2 yr rotational modulation in the velocity span measurements. Line-depth ratio measurements indicate no evidence for variations in temperature in excess of 10 K over the observed magnetic cycle, rotation period, and short-term variability of Arcturus. Precise radial velocity measurements exhibit a long-term period of 222 3 days, with an amplitude of 150 20 m s . This long-term period and the corresponding amplitude are consistent with the radial velocity observations of Hatzes & Cochrane (1993, ApJ, 413, 339). The 222 day period is coherent over the 14 yr time span between the Hatzes & Cochrane (1993, ApJ, 413, 339) data and the observations of this work, and does not correlate with observed line bisector or line-depth ratio variations, suggesting that this observed period is not associated with stellar variability. Hatzes & Cochrane (1993, ApJ, 413, 339) suggested that the observed long-term radial velocity period was due either to rotational modulation or a low-mass companion, but were unable to differentiate between the two explanations, due to a lack of ancillary spectroscopic data. A detection of a low-mass companion is postulated by this work. Assuming a mass of 1 M, for Arcturus, a best-fit Keplerian orbit to the 222 day period is found to have approximately zero eccentricity, a projected semimajor axis ( ) of 0.72 AU, and a companion a sin i mass ( ) of ≥4.4 MJ. M sin i 2 Precise radial velocity observations also demonstrate a scatter (j) of ∼145 m s 1 superimposed on the observed 222 day period, resulting from short-term variability on the timescale of individual nights to days. This short-term variability has been interpreted as being the result of solar-type p-mode oscillations by previous studies, such as Hatzes & Cochrane (1994, ApJ, 422, 366), Merline (1995, Ph.D. thesis, Univ. Arizona), and Retter et al. (2003, ApJ, 591, L151). Observed scatter during individual nights ranges from ∼50 to 180 m s , while the scatter over consecutive nights ranges from ∼70 to 160 m s . These observations are consistent with previous studies of the short-term radial velocity variability of Arcturus. Ultimately, this scatter of ∼145 m s 1 limits the accuracy of studies of the long-term radial velocity variability of Arcturus. Evidence for eruptive events is also suggested from the observed radial velocities.