J/AJ/150/6 Solar neighborhood. XXXVI. VRI variability of M dwarfs (Hosey+, 2015) ================================================================================ The solar neighborhood. XXXVI. The long-term photometric variability of nearby red dwarfs in the VRI optical bands. Hosey A.D., Henry T.J., Jao W.-C., Dieterich S.B., Winters J.G., Lurie J.C., Riedel A.R., Subasavage J.P. =2015AJ....150....6H ================================================================================ ADC_Keywords: Stars, dwarfs ; Stars, nearby ; Stars, M-type ; Photometry, VRI ; Parallaxes, trigonometric Keywords: stars: low-mass - stars: statistics - stars: variables: general - planetary systems - techniques: photometric Abstract: We present an analysis of long-term photometric variability for nearby red dwarf stars at optical wavelengths. The sample consists of 264 M dwarfs south of decl.=+30 with V-K=3.96-9.16 and M_V_~~10-20, corresponding to spectral types M2V-M8V, most of which are within 25pc. The stars have been observed in the VRI filters for ~4-14yr at the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25pc, we find that only ~8% are photometrically variable by at least 20mmag (~2%) in the VRI bands. Only four stars have been found to vary by more than 50mmag, including GJ 1207 at 8.6pc, which experienced a single extraordinary flare, and GJ 2006 A, TWA 8 A, and TWA 8 B, which are all young stars beyond 25pc 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 Sun's 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 25pc that host exoplanets, none of which vary by more than 20mmag. 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. Description: We present an analysis of long-term photometric variability for nearby red dwarf stars at optical wavelengths (Table1). The sample consists of 264 M dwarfs south of decl.=+30 with V-K=3.96-9.16 and M_V_~~10-20, corresponding to spectral types M2V-M8V, most of which are within 25pc. Our 264 dwarf stars have been observed in the VRI filters over the past 14yr (with a median duration in the coverage of 7.9yr). The REsearch Consortium On Nearby Stars (RECONS; www.recons.org) has been using the Cerro Tololo Inter-American Observatory/Small & Moderate Aperture Research Telescope System (CTIO/SMARTS) 0.9m telescope for astrometric and photometric observations since 1999, first as an National Optical Astronomy Observatory (NOAO) Surveys Program, and since 2003 under the auspices of the SMARTS Consortium. The telescope is equipped with a 2048*2048 Tektronix CCD camera. Images taken during the program are used here to investigate the photometric variability of the nearby M dwarfs that have been targeted for parallax and proper motion measurements. Observations are made using the central quarter of the chip, which provides a 6.8' square field of view and pixels 401mas in size. Parallax frames are taken in the V_J_, R_KC_, and I_KC_ filters with magnitudes ranging from 9 to 20. The central wavelengths for the V_J_, R_KC_, and I_KC_ filters used in this study are 5438/5475, 6425, and 8075{AA}, respectively. The subscript "J" indicates Johnson, "KC" indicates Kron-Cousins (usually known as Cousins). VRI photometry from our program is given for the sample stars in Table1. Details of the photometry observations and reductions can be found in Jao et al. (2005AJ....129.1954J) and Winters et al. 2011 (cat. J/AJ/141/21). For astrometry, five images of each star are typically taken per night, usually within 30 minutes of transit. The target star is positioned in the field so that 5-10 reference stars, normally fainter by 1-4mag, surround the target. These stars constitute a reference grid for the astrometric reductions, and are also used for the photometric variability study described here. Additional details of the observations can be found in Jao et al. (2005AJ....129.1954J). File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 127 264 Red dwarfs studied for long-term variability refs.dat 77 16 References -------------------------------------------------------------------------------- See also: J/AJ/149/5 : Solar neighborhood. XXXV. M dwarfs distances (Winters+, 2015) J/AJ/147/94 : Solar neighborhood. XXXII. L and M dwarfs (Dieterich+, 2014) J/AJ/147/85 : Solar neighborhood. XXXIII. 45 M dwarfs (Riedel+, 2014) J/AJ/141/21 : CCD distance estimates of SCR targets (Winters+, 2011) J/AJ/130/1658 : New high proper motion stars (-47