We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects. The remaining 26 objects have trigonometric parallaxes from the literature. We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel SED fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. Our uncertainties range from ∼20K to ∼150K in temperature, ∼0.01 to ∼0.06 in log(L/L ⊙ ) and ∼3% to ∼10% in radius. We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry. We find evidence for the local minimum in the radius−temperature and radius−luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T ef f ∼ 2075K, log(L/L ⊙ ) ∼ −3.9, and (R/R ⊙ ) ∼ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ∼400K cooler. The minimum radius happens near the locus of 2MASS J0523-1403, an L2.5 dwarf with V −K = 9.42. We make qualitative arguments as to why the effects of the recent revision in solar abundances accounts for the discrepancy between our findings and the evolutionary models. We also report new color-absolute magnitude relations for optical and -2infrared colors useful for estimating photometric distances. We study the optical variability of all 63 targets and find an overall variability fraction of 36 +9 −7 % at a threshold of 15 milli-magnitudes in the I band, in agreement with previous studies.

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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...

We report findings from a long-term photometric variability study of M dwarfs carried out at the SMARTS 0.9 m telescope at the Cerro Tololo Inter-American Observatory. As part of a multi-faceted effort to investigate the range of luminosities of M dwarfs of a given color on the Hertzsprung-Russell Diagram, 76 M dwarfs have been observed for 3-17 years in the Johnson-Kron-Cousins V band. We find that stars elevated above the center of the main sequence distribution tend to have higher levels of variability, likely caused by magnetic activity, than their fainter counterparts below the center. This study provides insight into how the long-term magnetic activity of these stars may be affecting their sizes, luminosities, and thus positions on the H-R Diagram.