Constraining RV Variation Using Highly Reddened Type Ia Supernovae from the Pantheon+ Sample

Type Ia supernovae (SNe Ia) are powerful tools for measuring the expansion history of the universe, but the impact of dust around SNe Ia remains unknown and is a critical systematic uncertainty. One way to better understand the impact of dust is to analyze highly reddened (E(B-V)>0.4, roughly equivalent to the fitted SALT2 light-curve parameter c>0.3) SNe Ia that are traditionally discarded from cosmological analyses due to low statistics and assumptions about inadequate model training. With the recently released Pantheon+ sample, there are 71 SNe Ia with c>0.3 that would otherwise be included for cosmological constraints, and these can provide enormous leverage on understanding R_V of the SNe Ia. In this work, we analyze the full set of SNe Ia, with colors up to c=1.61. Studies have found that highly-reddened nearby SNe Ia have very low R_V values (R_V<2). It has been claimed that R_V may decrease with color, and therefore these SNe Ia are not informative about the larger population. To test this claim, we fit two separate color-luminosity relationships, one for the main cosmological sample (c<0.3) and one for the highly reddened (c>0.3) SNe Ia. We find the change in the color-luminosity relationship to be 0.15 ± 0.11 (<1.5σ). Additionally, we also compare the data to forward modeled simulations. From this, it is evident that these highly reddened objects exhibit large selection effects. These results strongly support that R_V varies across the SNe Ia population, and must be accounted for in cosmological analyses. With the incorporation of these highly reddened SNe Ia, there are 15 new possible Hubble constant calibrators.