"Comparison between Bessell-Brett (1988, "BB88") V-K colors and Johnson-2MASS V-Ks colors for dwarf stars, or a check on the Carpenter (2001) transformation from 2MASS Ks to BB88 K-band" Eric Mamajek 6/22/2011 [updated 1/24/2017] In this short memo, I basically confirm the transformation between 2MASS Ks and Bessell & Brett K-band by Carpenter (2001) appears to be valid, through comparison of Bessell-Brett (BB88) intrinsic V-K colors for dwarf stars and the author's Johnson-2MASS V-Ks colors for field dwarfs, and that one can simply apply an offset of ~0.04 mag when comparing BB88 intrinsic V-K colors to V-Ks colors measured using 2MASS and any V-band photometry which is claimed to be tied to Johnson's system. ______ Bessell & Brett (1988; PASP 100, 1134; hereafter "BB88") lists a table (Table II) of "Intrinsic Colors for Dwarfs". Given the systematic differences between the numerous near-IR natural JHK photometric systems that appeared during the 1960s-1980s, BB88 generated a homogenized system, "essentially the Johnson-Glass system", and estimated conversions between the other near-IR photometric system and this new homogenized system. BB88 "decided to adopt the system of Glass (with zero-point corrections) as the base system for homogenizing the photometry". Many authors use the BB88 colors as the basis for their modeling of spectral energy distributions and estimates of interstellar reddening and extinction (e.g. perhaps most notably Kenyon & Hartmann 1995 ApJS 101, 117, which itself is heavily cited as a source for intrinsic dwarf stellar colors). Surprisingly little published work has been done on the intrinsic colors of stars on the 2MASS JHKs system, which given its extensive reach (99.998% sky coverage, 0.47 billion point sources; Skrutskie et al 2006, AJ, 131, 1163). The author (EEM) has derived median (V-Ks; i.e. Johnson V minus 2MASS "Ks") colors for normal dwarf stars based on examination of the median colors for nearby, unreddened field dwarfs classified on the MK system, and the colors of well-accepted dwarf MK standard stars (see extensive notes, listed by spectral type, in directory at http://www.pas.rochester.edu/~emamajek/spt/ ). The majority of median V-Ks colors for a given spectral type were in fact estimated by adopting a best Johnson B-V color, and then referring to a dense stellar B-V vs. V-Ks locus of stellar dwarf colors generated by the author. [updated following section 1/24/2017] The inferred V-Ks colors also agree well with straight median colors of samples of nearby dwarfs with MK classifications by expert classifiers Gray, Corbally, Garrison for the BAFGK dwarfs and Henry and Kirkpatrick for the M dwarfs (essentially based on the modern MK system as defined by the standards of Morgan from the 1970s and Keenan & McNeil 1989, and Kirkpatrick et al. 1991 for the M dwarfs). I have decided to not factor in classifications from Houk (Michigan Spectral Survey) as these spectral types are systematically off of the modern grid of Morgan/Gray/Garrison/Keenan/Kirkpatrick/Henry that have been commonly used since around the early 1990s. Pecaut & Mamajek (2016; http://adsabs.harvard.edu/abs/2016MNRAS.461..794P) provide a look up table to convert between Houk/Michigan types and the modern MK grid defined by the Morgan/Gray/Garrison/Keenan/Kirkpatrick/Henry works of the 1970s-1990s. Here is a table of the V-K colors from Bessell & Brett (1988) listed by dwarf (V) spectral type and the median colors estimated by Mamajek [updated 1/24/2017]. #SpT V-K(BB88) V-Ks(EEM) B8 -0.36 -0.254 A0 0.00 0.041 A2 0.14 0.188 A5 0.38 0.403 A7 0.50 0.528 F0 0.70 0.732 F2 0.82 0.925 F5 1.10 1.079 F7 1.32 1.244 G0 1.41 1.440 G2 1.46 1.564 G4 1.53 1.621 G6 1.64 1.691 K0 1.96 1.953 K2 2.22 2.155 K4 2.63 2.733 K5 2.85 2.835 K7 3.16 3.418 M0 3.65 3.790 M1 3.87 4.065 M2 4.11 4.265 M3 4.65 4.600 M4 5.26 5.250 M5 6.12 5.942 M6 7.30 7.300 Let's quantify the differences in the BB88 V-K and Johnson-2MASS V-Ks colors as derived by Mamajek. A least-squares fit to the data suggests [updated 1/24/2017]: (V-Ks) = 0.061 + 0.99236*(V-K_BB88) for -0.36 < (V-K)_BB88 < 7.30 and -0.254 < (V-Ks)_2M < 7.30 There does not seem to be sufficient curvature to the trend to warrant a color term (indeed the reddest sample of M6 dwarfs appears to have identical V-K colors on the BB88 and 2MASS systems!). If we further assume that the V magnitudes are identical (a safe assumption at the <0.01 mag level, as Cousins and Hipparcos V magnitudes are tied to Johnson's system; see documentation on V-band photometry from Hipparcos; ESA 1997), then this suggests: Ks = -0.061 + 0.99236*(K_BB88) Ks ~ -0.061 + K_BB88 Note that if I only fit the B8 through A dwarfs, I get essentially the same result: Ks ~ -0.061 + K_BB88 with rms scatter of only 0.015 mag. Looking at the 25 spectral type bins as an ensemble, and neglecting any color effects (which appear to be negligible given the near unity slope in the previous equation), we find that via median statistics (Gott et al. 2001 ApJ 549, 1): Ks = (-0.032+-0.012) + K_BB88 This is within 1sigma of equation A1 of Carpenter (2001; AJ, 121, 2851): Ks = (-0.044+-0.003) + K_BB88 + (0.000+-0.005)*(J-K)_BB88 BB88 tied their homogenized system to the SAAO photometry from Glass (1974), but since these stars were saturated in 2MASS, Carpenter tied 2MASS to the Glass system via his own transformations and those listed in BB88. Unsurprisingly then, Carpenter (2001) also found a similar transformation (his equation 4) between the SAAO system and 2MASS, but with a slight color term: Ks = (-0.032+-0.012) + K_AAO + (-0.021+0.008)(J-K)_AAO Conclusion: We have shown using an independent method (comparing intrinsic dwarf colors for 25 spectral subclasses) that Carpenter's (2001) conversion between the 2MASS Ks magnitudes and Bessell-Brett (BB88) K magnitudes is validated. This can be helpful in converting between other colors dependent on either the BB88 K band or 2MASS Ks band and other colors published in the pre-2MASS literature. ______