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Finch C.T.,U.S. Naval Observatory | Zacharias N.,U.S. Naval Observatory | Subasavage J.P.,U.S. Naval Observatory | Henry T.J.,RECONS Institute | And 2 more authors.
Astronomical Journal

We use data from the U.S. Naval Observatory fourth CCD Astrograph Catalog (UCAC4) in combination with photometry from the AAVSO Photometric All-Sky Survey and Two Micron All-Sky Survey to identify stars within 25 pc of the Sun. A sample of nearby stars with accurate trigonometric parallaxes from the Research Consortium On Nearby Stars is used to generate a set of 16 new photometric color-MKs relations that provide distance estimates with uncertainties of 15%. This work expands the available suites of well-calibrated photometric distance relations that can be used to identify nearby stellar systems. The distance relations are used with quality cuts to extract an initial sample of stars from the UCAC4 estimated to be within 25 pc. Color, proper motion, and existing literature sources are then used to obtain a clean sample of red dwarfs, while limiting the amount of contamination from background giants, resulting in a sample of 1761 candidate nearby stars within 25 pc. Of these, 339 are new discoveries with no previously known published parallax or distance estimate, primarily with proper motions less than 0.2 arcsec yr-1. Five stars are estimated to be within 10 pc, with the nearest, TYC 3980 1081 1, with V = 10.50 estimated to be at 5.9 pc. That several hundred new stars have been revealed so close to the Sun illustrates once again that there is considerable work yet to be done to map the solar neighborhood and that additional nearby stars are likely still to be discovered. © 2014. The American Astronomical Society. All rights reserved. Source

Chatelain J.P.,Georgia State University | Chatelain J.P.,Northern Arizona University | Henry T.J.,RECONS Institute | French L.M.,Illinois Wesleyan University | And 3 more authors.

The L5 Jupiter Trojan asteroids are minor bodies that orbit 60 degrees behind Jupiter. Because these orbits are stable over the lifetime of the Solar System, the properties of these objects may inform us about the conditions under which the Solar System formed. We present BVRKCIKC photometry for the 42 intrinsically brightest and presumably largest members of the L5 Jupiter Trojans. We define a new principal color component aT* that is indicative of taxonomic types relevant to the Jupiter Trojan asteroids. We find that 76% of the largest L5 Jupiter Trojans are consistent with a D-type classification, while 24% show shallower slopes more consistent with X-type and C-Type classifications. Such a breakdown is consistent with other surveys and will help to place the Trojans in the larger context of the Solar System. © 2016 Elsevier Inc. Source

Davison C.L.,Georgia State University | White R.J.,Georgia State University | Henry T.J.,RECONS Institute | Riedel A.R.,York College | And 7 more authors.
Astronomical Journal

We present a carefully vetted equatorial (±30° decl.) sample of all known single (within 4′) mid M dwarfs (M2.5 V-M8.0 V) extending out to 10 pc; their proximity and low masses make them ideal targets for planet searches. For this sample of 58 stars, we provide VJ, RKC, and IKC photometry, new low-dispersion optical (6000-9000 Å) spectra from which uniform spectral types are determined, multi-epoch Hα equivalent widths, and gravity-sensitive Na I indices. For 12 of these 58 stars, strict limits are placed on the presence of stellar and substellar companions based on a pioneering program described here that utilizes precise infrared radial velocities (RVs) and optical astrometric measurements in an effort to search for Jupiter-mass, brown dwarf, and stellar-mass companions. Our infrared RV precision using CSHELL at NASA's Infrared Telescope Facility is ∼90 m s-1 over timescales from 13 days to 5 yr. With our spectroscopic results the mean companion masses that we rule out of existence are 1.5MJUP or greater in 10 day orbital periods and 7MJUP or greater in 100 day orbital periods. We use these spectra to determine rotational velocities and absolute RVs of these 12 stars. Our mean astrometric precision using Research Consortium on Nearby Stars (RECONS; www.recons.org) data from the 0.9 m telescope at Cerro Tololo Inter-American Observatory is ∼3 mas over baselines ranging from 9 to 13 yr. With our astrometric results the mean companion masses that we rule out of existence are greater than 11.5MJUP with an orbital period of 4 yr and greater than 7.5MJUP with an orbital period of 8 yr. Although we do not detect companions around our subsample of 12 stars, we demonstrate that our two techniques probe a regime that is commonly missed in other companion searches of late-type stars. © 2015. The American Astronomical Society. All rights reserved. Source

Lurie J.C.,University of Washington | Henry T.J.,RECONS Institute | Jao W.-C.,Georgia State University | Quinn S.N.,Georgia State University | And 5 more authors.
Astronomical Journal

Astrometric measurements are presented for seven nearby stars with previously detected planets: six M dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ 1214) and one K dwarf (BD-10 -3166). Measurements are also presented for six additional nearby M dwarfs without known planets, but which are more favorable to astrometric detections of low mass companions, as well as three binary systems for which we provide astrometric orbit solutions. Observations have baselines of 3 to 13 years, and were made as part of the RECONS long-term astrometry and photometry program at the CTIO/SMARTS 0.9 m telescope. We provide trigonometric parallaxes and proper motions for all 16 systems, and perform an extensive analysis of the astrometric residuals to determine the minimum detectable companion mass for the 12 M dwarfs not having close stellar secondaries. For the six M dwarfs with known planets, we are not sensitive to planets, but can rule out the presence of all but the least massive brown dwarfs at periods of 2-12 years. For the six more astrometrically favorable M dwarfs, we conclude that none have brown dwarf companions, and are sensitive to companions with masses as low as 1 MJup for periods longer than two years. In particular, we conclude that Proxima Centauri has no Jovian companions at orbital periods of 2-12 years. These results complement previously published M dwarf planet occurrence rates by providing astrometrically determined upper mass limits on potential super-Jupiter companions at orbits of two years and longer. As part of a continuing survey, these results are consistent with the paucity of super-Jupiter and brown dwarf companions we find among the over 250 red dwarfs within 25 pc observed longer than five years in our astrometric program. © 2014. The American Astronomical Society. All rights reserved. Source

Hosey A.D.,RECONS Institute | Henry T.J.,RECONS Institute | Jao W.-C.,Georgia State University | Dieterich S.B.,Carnegie Institution for Science | And 4 more authors.
Astronomical Journal

We present an analysis of long-term photometric variability for nearby red dwarf stars at optical wavelengths. The sample consists of 264M dwarfs south of decl. = +30 with V - K = 3.969.16 and MV ≈ 1020, corresponding to spectral types M2VM8V, most of which are within 25 pc. The stars have been observed in the VRI filters for ∼ 414 yr at the CTIO/SMARTS 0.9 m telescope. Of the 238 red dwarfs within 25 pc, we find that only ∼ 8% are photometrically variable by at least 20 mmag (∼ 2%) in the VRI bands. Only four stars have been found to vary by more than 50 mmag, including GJ 1207 at 8.6 pc, which experienced a single extraordinary flare, and GJ 2006 A, TWA 8 A, and TWA 8 B, which are all young stars beyond 25 pc linked to moving groups. We find that high variability at optical wavelengths over the long term can in fact be used to identify young stars. Overall, however, the fluxes of most red dwarfs at optical wavelengths are steady to a few percent over the long term. The low overall rate of photometric variability for red dwarfs is consistent with results found in previous work on similar stars on shorter timescales, with the body of work indicating that most red dwarfs are only mildly variable. As expected, we find that the degree of photometric variability is greater in the V band than in the R or I bands, but we do not find any obvious trends in variability over the long term with red dwarf luminosity or temperature. We highlight 17 stars that show long-term changes in brightness, sometimes because of flaring activity or spots, and sometimes because of stellar cycles similar to our Suns solar cycle. Remarkably, two targets show brightnesses that monotonically increase (G 169-029) or decrease (WT 460AB) by several percent over a decade. We also provide long-term variability measurements for seven M dwarfs within 25 pc that host exoplanets, none of which vary by more than 20 mmag. Both as a population, and for the specific red dwarfs with exoplanets observed here, photometric variability is therefore often not a concern for planetary environments, at least at the optical wavelengths where they emit much of their light. © 2015. The American Astronomical Society. All rights reserved. Source

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