Photometric properties of low-mass stars and brown dwarfs

The colour-magnitude relations plotted here have been constructed by combining data from Mike Bessell's catalogue of BVRI photometry of stars from the CNS1 and CNS2 catalogues (we refer to these as the Gliese stars), together with data for stars within 8 parsecs of the Sun and observations of a small number of ultracool dwarfs. Photometry for the 8-parsec stars is taken mainly from Leggett (2), while the data on ultracool dwarfs are from Dahn et al (3). Both the Gliese stars and the 8-parsec sample have been cross-referenced against the Hipparcos catalogue, and against the USNO parallax program. The CMDs plotted below include only stars with Hipparcos parallaxes accurate to (sigma_pi/pi) < 15%: the USNO measurements, primarily of late-type dwarfs in the 8-parsec and ultracool dwarf samples, are of much higher precision.
Cross-referencing Bessell's catalogue against the Hipparcos catalogue gives 831 matches. All stars known to have suspect photometry (i.e. binaries with joint colours, or primaries with companions of similar luminosities (delta V < 3 mag) at small separations) have been excluded from the datasets - but it is clear that some still remain. That leaves 739 stars in the Gliese dataset, and 120 (out of 150) in 8-parsec sample.
Dave Monet has cross-referenced the Gliese catalogue against the 2MASS database (Oct 22, 1999): 224 stars correlated, and ~150 have JHKS data with acceptable accuracy (most of the remainder are too bright at near-infrared wavelengths for 2MASS!). The sample includes a higher proportion than usual of southern stars (from Bessell and Cousins) and a fair number of bright stars - G dwarfs within 25 parsecs are kind of bright. This sample will be extended in the near future. There are about 2000 stars in the CNS3 that have Hipparcos parallaxes measured to a precision of better than 20%, including faint companions whose distances can be inferred from the measurement of the primary. A large number of those stars have at least BV photometry, so it will be possible to obtain (V-J), (V-K) etc colours once those 2MASS data for those sources.
(1) Bessell, M.S., 1991, AAS 83, 357
(2) Leggett, S.K., 1992, ApJS 82, 351
(3) Dahn, C.C. et al, 2001, in prep.

Color-magnitude diagrams

1. Nearby stars - disk dwarfs

In the following figures, CNS3 stars are plotted as green crosses; stars from the 8-parsec sample as blue crosses; M dwarfs from the USNO ultracool sample (Dahn et al, 2002) as red crosses; L dwarfs (from Dahn et al) as solid magenta points; and T dwarfs as 5-point stars.

The (MV, (V-I)) relation can be represented as
MV = 3.98 + 1.437(V-I) + 1.073 (V-I)2 - 0.192 (V-I)3, 0.85<(V-I)<2.85
MV = 3.66 + 4.66(V-I) - 0.517(V-I)2 + 0.0448 (V-I)3, 2.96<(V-I)<3.45
The discontinuity at (V-I)~2.9 is real, and is evident in several other colour-magnitude diagrams, notably (MJ, (I-J)). There is no theoretical explanation at present, but a change in luminosity at a fixed colour (temperature) suggests a change in radius, and the break happens to match the mass where M dwarfs are predicted to become fully convective.

Colour-colour plots

A couple of the above diagrams appeared in Sky & Telescope's August (September?) article on L & T dwarfs. Slightly modified versions of those diagrams are given below:

The correlations between broadband colours and spectral types are illustrated on this page.

2. Halo subdwarfs

Metal-poor halo main-sequence stars appear to lie below the main-sequent outlined by nearby (near-solar abundance) disk dwarfs. In fact, reduced metal-line blanketting moves the stars to higher luminosities and higher temperatures (bluer colours), so a metal-poor star of a given mass actually lies up and to the left of its disk counterpart in the HR diagram. Multicolour broadband photometry (at least, reliable photometry) exists for relatively few FGK dwarfs; most of the available data are included in the datasets listed by Reid et al (2001). Later-type M subdwarfs are classified under two heading by Gizis (1997): sdM, intermediate abundance subdwarfs, probably with -0.5 > [m/H] > -1.5; and esdM, extreme subdwarfs, [m/H] < -1.5. These stars have substantially bluer colours than their main-sequence counterpart - for example, the hydrogen burning limit for esdM subdwarfs lies at MV~15, (V-I)~3, rather than MV~20, (V-I)~6. The following colour-magnitude diagrams illustrate the general behaviour:

Bolometric corrections

Bolometric corrections are vital in comparing observations of low-mass stars and brown dwarfs to theoretical predictions of fundamental properties.

Photometric data for low-mass stars and brown dwarfs

The above plots are based on the following datafiles. In each case the format is

N, MU, MB, MV, MR, MI, MJ, MH, MK, distance, uncertainty in distance, +dm, -dm (the corresponding uncertainty in absolute magnitude), name
As noted above, all distances are based on trig parallax measurements, primarily Hipparcos or USNO; RI magnitudes are on Cousins system; JHK on the CIT or 2MASS system. Negative numbers mean (obviously) no measurements available.

Metal-poor subdwarfs: UBV photometry is available for a large sample of metal-poor stars, notably from the surveys of Giclas proper motion stars by Sandage & Fouts, 1986, AJ 91, 1189 and by Carney et al, 1994, AJ 107, 2240. RI data are much less readily available. The following files include most of the reliable photometry currently available for metal-poor stars with reasonably accurate trigonometric parallaxes. All of the data are on the Johnson/Cousins system:

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page by Neill Reid, last updated 15/07/02