The extinction curve in the visible and the value of R_V: Addendum to AN 333, 160

Abstract

This paper corrects and completes a previous study of the shape of the extinction curve in the visible and the value of R V . A continuous visible/infrared extinction law proportional to 1/λ p with p close to 1 (±0.4) is indistinguishable from a perfectly linear law (p = 1) in the visible within observational precision, but the shape of the curve in the infrared can be substantially modified. Values of p slightly larger than 1 would account for the increase of extinction (compared to the p = 1 law) reported for λ > 1µm and deeply affect the value of R V . In the absence of gray extinction R V must be 4.04 if p = 1. It becomes 3.14 for p = 1.25, 3.00 for p = 1.30, and 2.76 for p = 1.40. Values of p near 1.3 are also attributed to extinction by atmospheric aerosols, which indicates that both phenomena may be governed by similar particle size distributions. A power extinction law may harmonize visible and infrared data into a single, continuous, and universal, interstellar extinction law.

Testing new ideas regarding the nature of interstellar extinction

“…A value of p = 1, the simplest situation, would imply an invariant value of R V = 4 throughout the Galaxy [16,17], whereas observed values of R V ≃ 3 are typical of most Galactic fields [8]. Associated values of p for interstellar extinction must therefore lie in the range 1.3−1.4 [17], which is still similar to what is observed for atmospheric aerosols. By implication the size distributions of atmospheric and interstellar dust particles must be governed by similar physical mechanisms.…”

“…In such a situation true interstellar extinction becomes a simple case of Mie scattering over the entire infrared to far ultraviolet wavelength region, with an extinction law described by A(λ) ∝ λ −p , as specified by the size distribution of the dust grains responsible. As an aside, atmospheric aerosols produce an identical dependence for their extinction [16,17]. A value of p = 1, the simplest situation, would imply an invariant value of R V = 4 throughout the Galaxy [16,17], whereas observed values of R V ≃ 3 are typical of most Galactic fields [8].…”

“…As an aside, atmospheric aerosols produce an identical dependence for their extinction [16,17]. A value of p = 1, the simplest situation, would imply an invariant value of R V = 4 throughout the Galaxy [16,17], whereas observed values of R V ≃ 3 are typical of most Galactic fields [8]. Associated values of p for interstellar extinction must therefore lie in the range 1.3−1.4 [17], which is still similar to what is observed for atmospheric aerosols.…”

The 2200 Å bump and the interstellar extinction curve

“…The parameter α p depends on p (p ∼ 1) only and will be deduced from Eq. (11) in Zagury & Turner (2012).…”

“…In the latter alternative the direct light from the star should follow the visible extinction law over the whole spectrum, and therefore scale as e −αpE(B−V )/λ p (λ in μm) (Zagury & Turner 2012). The parameter α p depends on p (p ∼ 1) only and will be deduced from Eq.…”