Calculating R from a reddening ratio requires an assumption about spectral type and therefore intrinsic colors. Spectral type is not a linear function of intrinsic color. We investigate the range in R caused by uncertainty in intrinsic colors due to uncertainty in spectral type.
We start by assigning observer-calculated values of R = 1.6 and foreground reddening, EH–K = 0.1, 0.3, 0.5 mag. By choosing R and EH-K, EJ-H is implied. We then assume that these values are incorrect, assign a second set of real intrinsic colors that are unknown to the observer, and recalculate R across near-infrared intrinsic colors for main-sequence stars.
The real R values are found as a function of assumed spectral type or bin. The range in R values is effectively a measure of uncertainty in R originating from spectral type error.
We initially use classic main-sequence intrinsic colors in the J, H, and K bands of the Johnson photometric system, classified by Morgan-Keenan spectral type. We then recalculate using more recent intrinsic colors in the J, H, and Ks bands from Davenport et al. (2014). These colors are binned by the assumed intrinsic colors (g-i), bypassing spectral type in favor of bins that are chosen so there is an approximately equal number of stars in each bin.
Reddening comparison: We compare the range of R as a function of assumed spectral type at an uncertainty of 10 subclasses for classic main-sequence colors at high, modest, and low reddening.