The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC 6656 (M22), with observations spanning a range of twenty-two years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are P RR0 =0.66±0.02 d and P RR1 =0.33±0.01 d, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1-to all RR-type variables is N 1 /N RR =0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E(B-V )=0.36±0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V +R)=+0.97±0.1 and at least one "gap" located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.

We report initial results of a large radial velocity survey of K dwarfs up to a distance of 50 pc from the solar system, to look for stellar, brown dwarf, and Jovian planets using radial velocities from the CHIRON spectrograph on the CTIO/SMARTS 1.5 m telescope. We identify three new exoplanet candidates orbiting host stars in the K dwarf survey and confirm a hot Jupiter from TESS orbiting TOI 129. Our techniques are confirmed via five additional known exoplanet orbiting K dwarfs, bringing the number of orbital solutions presented here to 9, each hosting an exoplanet candidate with a minimum mass of 0.5–3.0 M Jup. In addition, we provide a list of 186 nearby K dwarfs with no detected close companions that are ideal for more sensitive searches for lower-mass planets. This set of stars is used to determine CHIRON’s efficiency, stability, and performance for radial velocity work. For K dwarfs with V = 7–12, we reach radial velocity precisions of 5–20 ms−1 under a wide range of observing conditions. We demonstrate the stability of CHIRON over hours, weeks, and years using radial velocity standards, and describe instrumental capabilities and operation modes available for potential users.

We present follow-up optical imaging and spectroscopy of one of the light echoes of η Carinae's 19th-century Great Eruption discovered by Rest et al. (2012a). By obtaining images and spectra at the same light echo position between 2011 and 2014, we follow the evolution of the Great Eruption on a three-year 1 This paper includes data gathered with the 6.5 meter Magellan telescope at Las Campanas Observatory, Chile.

IC 2391 and IC 2602 are important benchmarks for testing early star and planet evolution theories, both structural and dynamical, because they are the nearest open clusters with ages of ∼50 Myr. We refine membership lists for these clusters by identifying candidate members using Gaia DR2 kinematic and distance information. We identify 451 candidate members of IC 2602 and 350 candidate members of IC 2391. If confirmed, this would increase the known populations of these clusters by 275% and 130%, respectively. We use CHIRON on the CTIO/SMARTS 1.5 m telescope via fiber mode which yields a resolution of 27,400 to acquire high-resolution spectra of 26 new candidate cluster members brighter than G = 13 magnitude, as well as an additional 12 previously known members. Measures of lithium, Hα, stellar properties (T eff, log(g), [Fe/H]), radial velocities, and v sin i values from these spectra are used to confirm cluster membership. We find that 37 of 38 stars we observe are bona fide cluster members, of which four are new candidate photometric binaries and 10 are new candidate spectroscopic binaries.

We present light curves and flares from a 7 day, multiwavelength observational campaign of AU Mic, a young and active dM1e star with exoplanets and a debris disk. We report on 73 unique flares between the X-ray to optical data. We use high-time-resolution near-UV (NUV) photometry and soft X-ray (SXR) data from the X-ray Multi-Mirror Mission to study the empirical Neupert effect, which correlates the gradual and impulsive phase flaring emissions. We find that 65% (30 of 46) flares do not follow the Neupert effect, which is 3 times more excursions than seen in solar flares, and propose a four-part Neupert effect classification (Neupert, quasi-Neupert, non-Neupert types I and II) to explain the multiwavelength responses. While the SXR emission generally lags behind the NUV as expected from the chromospheric evaporation flare models, the Neupert effect is more prevalent in larger, more impulsive flares. Preliminary flaring rate analysis with X-ray and U-band data suggests that previously estimated energy ratios hold for a collection of flares observed over the same time period, but not necessarily for an individual, multiwavelength flare. These results imply that one model cannot explain all stellar flares and care should be taken when extrapolating between wavelength regimes. Future work will expand wavelength coverage using radio data to constrain the nonthermal empirical and theoretical Neupert effects to better refine models and bridge the gap between stellar and solar flare physics.