Deriving the physical properties of red supergiants ( RSGs ) depends upon accurate corrections for reddening by dust .
We use our recent modeling of the optical spectra of RSGs to address this topic .
First , we find that previous broad-band studies have underestimated the correction for extinction in the visible , and hence the luminosities ( if derived from V ) ; the shift in the effective wavelengths of the standard B and V bandpasses necessitates using an effective value of the ratio R ^ { \prime } _ { V } = 4.2 to correct broad-band photometry of RSGs if R _ { V } = 3.1 for early-type stars viewed through the same dust , where we have assumed the standard reddening law of Cardelli , Clayton , & Mathis ( 1989 ) .
Use of the Fitzpatrick ( 1999 ) reddening law would lead to R ^ { \prime } _ { V } = 3.8 , as well as slightly lower values of extinction derived from spectrophotometry , but results in slightly poorer fits .
Second , we find that a significant fraction of RSGs in Galactic OB associations and clusters show up to several magnitudes of excess visual extinction compared to OB stars in the same regions ; we argue that this is likely due to circumstellar dust around the RSGs .
We also show that the RSG dust production rate ( as indicated by the 12- \mu m excess ) is well-correlated with bolometric luminosity , contrary to what has been found by earlier studies .
The stars with the highest amount of extra visual extinction also show significant near-UV ( NUV ) excesses compared to the stellar models reddened by the standard reddening law .
This NUV excess is likely due to scattering of the star ’ s light by the dust and/or a larger average grain size than that typical of grains found in the diffuse interstellar medium .
Similar excesses have been attributed to circumstellar dust around R Coronae Borealis stars .
Finally , we estimate that the RSGs contribute dust grains at the rate of 3 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } kpc ^ { -2 } in the solar neighborhood , comparable to what we estimate for late-type WCs , 1 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } kpc ^ { -2 } .
In the solar neighborhood this represents only a few percent of the dust production ( which is dominated by low-mass AGBs ) , but we note that in low-metallicity starbursts , dust production by RSGs would likely dominate over other sources .