THE SOLAR NEIGHBORHOOD. XXXVI. THE LONG-TERM PHOTOMETRIC VARIABILITY OF NEARBY RED DWARFS IN THEVRIOPTICAL BANDS

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 Dec. = +30 with V − K = 3.96-9.16 and M V ≈ 10-20, corresponding to spectral types M2V-M8V, most of which are within 25 pc. The stars have been observed in the V RI filters for ∼4-14 years at the CTIO/SMARTS 0.9m 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 V RI bands. Only four stars have been found to vary by more than 50 mmag, including GJ 1207 at 8.6 pc that experienced a single extraordinary flare, and GJ 2006A, TWA 8A, and TWA 8B, 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 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 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. ). These results and ours will be discussed in more detail in §8 to provide a portrait of the dominant stellar component of our Galaxy, the red dwarfs. SampleFor this study, we focus on a sample of 264 red dwarfs south of Dec. = +30 observed at the CTIO/SMARTS 0.9m, some starting as long ago as 1999. The sample is listed in Table 1, with stars falling in the ranges of V − K = 3.96-9.16 and M V ≈ 10-20, corresponding to spectral types M2V-M8V. Known cool subdwarfs have been removed, as they have been previously discussed in detail in Jao et al. (2011). Multiple systems with separations in the range 1-3 ′′ have been omitted because even if two sources can be seen in some images, accurate photometry for each source cannot always be determined, given variable seeing. Known multiples with separations less than 1 ′′ are treated as single sources and noted with component letters in Table 1, e.g., AB, o...

THE SOLAR NEIGHBORHOOD. XXXVIII. RESULTS FROM THE CTIO/SMARTS 0.9 m: TRIGONOMETRIC PARALLAXES FOR 151 NEARBY M DWARF SYSTEMS

“…G 169-29 (1650+2226), Figure 7: As illustrated in the second panel of Figure 7, and as first reported in Hosey et al (2015), this object shows a remarkable increase in brightness of ∼60 mmag (6%) that now stretches to 15 years in R.…”

“…The right panel shows the long-term increasing brightness of G 169-29, originally presented in Hosey et al (2015). Five additional years of data are available for analysis, confirming the brightening trend that now extends to 15 years.…”

“…For these single stars, the absolute mean offset is 21%, slightly higher than the 15% systematic error determined by Henry et al (2004). methodology can be found in Jao et al (2011) andHosey et al (2015).…”

The Solar Neighborhood. XLII. Parallax Results from the CTIOPI 0.9 m Program—Identifying New Nearby Subdwarfs Using Tangential Velocities and Locations on the H–R Diagram

“…Jao et al (2011) first reported the long-term variability of our parallax stars with coverage from 2-10 years and found that the 22 cool subdwarfs investigated at the time were substantially less photometrically variable than the 108 main sequence red dwarf examined. Hosey et al (2015) then expanded the variability study to 264 M dwarfs and found that only 8% of M dwarfs are photometrically variable by at least 20 mmag. Details of the data-reduction processes used to determine variability can be found in those two papers.…”

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

“…However, there are several indications in the literature that this system is old. Hosey et al (2015) showed that it is very quiet with variation amplitudes of only 17.2 mmag in the optical. Its absolute magnitude is about 0.7 mag brighter than the main sequence, which could be an indication of youth, but this can be explained by its high metallicity alone (e.g., Neves et al 2013 measure [Fe/H] ≈ 0.12-40 dex).…”