Intrinsic Johnson U-B Colors of B8V-M0V Field Stars

Intrinsic Johnson U-B Colors of B9V-M0V Field Stars

Eric Mamajek (U. Rochester)

Last updated: 13 November 2009

B8V-M8V stars: I cross-referenced the revised Hipparcos catalog (van Leeuwen 2007) with J.C. Mermilliod's Catalogue of Homogeneous Means in the UBV system to derive median U-B colors of unreddened main sequence dwarf stars as a function of spectral type. Many, if not most, of the spectral types in the Hipparcos catalogs are from the first four volumes of the Michigan Spectral Survey (e.g. Houk & Cowley 1975). Only stars with parallaxes greater than 13.33 mas and relative parallax error less than 12.5% were included (d < 75 pc), to select those stars ostensibly within the "Local Bubble" which should negligible reddening. I only list median U-B colors for subtypes with >=3 stars with U-B colors from Mermilliod's catalog.

The median U-B color is listed, along with the standard error of the median (Gott et al. 2001), and an estimate of the standard deviation from the probit method (which is rather insensitive to the inclusion of outlier points: Lutz & Upgren 1980).

O9V-B8V stars: Hot main sequence stars are rare in the solar vicinity, as are unreddened examples. Using published Johnson B-V and U-B for B-type stars (Mermilliod 1991), I defined an intrinsic dwarf color sequence that was consistent with the colors of the few B-type dwarfs within 75 pc, and the "blue envelope" among B-type dwarfs within hundreds of pc (reddened and unreddened; see Figure below). The best fit between -0.28 < (B-V)o < -0.02 is: (U-B)o = 3.977(B-V)o + 0.065. The intrinsic B-V colors of reddened B-type stars is found using the widely used reddening-free index Q = (U-B) - 0.72(B-V). The Q index maps into an intrinsic B-V color is defined as (B-V)o = 0.294 Q - 0.023. Using published U-B and B-V values from Mermilliod (1991), I calculated intrinsic (U-B)o and (B-V)o values using these equations for vetted MK spectral standard stars (i.e. those that appear to have consistent spectral types assigned to them by the majority of classifiers).

SpT	U-B	s.e.m.	st.dev.	N*s
O9V	-1.063	...	...	6 standards
B0V	-1.051	...	...	3 standards
B1V	-0.955	...	...	4 standards
B2V	-0.777	...	...	4 standards
B3V	-0.650	...	...	9 standards
B5V	-0.560	...	...	6 standards
B6V	-0.472	...	...	2 standards
B7V	-0.489	...	...	2 standards
B8V	-0.344	...	...	7 standards
B9V	-0.183	0.067	0.155	8
B9.5V	-0.102	0.012	0.011	4
A0V	-0.005	0.013	0.055	35
A1V	0.030	0.007	0.037	34
A2V	0.048	0.010	0.038	43
A3V	0.084	0.004	0.023	45
A4V	0.091	0.008	0.021	15
A5V	0.093	0.007	0.034	28
A6V	0.100	0.016	0.011	3
A7V	0.090	0.006	0.027	27
A8V	0.077	0.006	0.038	10
A9V	0.069	0.019	0.058	11
F0V	0.046	0.007	0.057	61
F1V	-0.008	0.021	0.041	12
F2V	-0.009	0.006	0.047	44
F3V	-0.033	0.007	0.048	53
F4V	-0.025	0.005	0.055	33
F5V	-0.027	0.005	0.040	117
F6V	-0.019	0.004	0.038	106
F7V	0.000	0.007	0.044	99
F8V	0.010	0.010	0.056	115
F9V	0.037	0.011	0.064	42
G0V	0.057	0.005	0.058	133
G1V	0.082	0.011	0.084	64
G2V	0.100	0.009	0.062	107
G3V	0.114	0.013	0.086	60
G4V	0.200	0.030	0.086	19
G5V	0.188	0.012	0.082	135
G6V	0.180	0.024	0.082	21
G7V	0.224	0.018	0.047	8
G8V	0.270	0.009	0.081	91
G9V	0.379	0.041	0.112	14
K0V	0.450	0.011	0.123	95
K1V	0.497	0.029	0.126	84
K2V	0.645	0.010	0.111	138
K3V	0.790	0.016	0.164	137
K4V	1.013	0.011	0.126	52
K5V	1.060	0.016	0.162	125
K7V	1.188	0.011	0.105	80
M0V	1.197	0.010	0.059	42