The spectrum of HD46223

TL;DR

This paper analyzes the optical to far UV spectrum of HD46223, revealing the roles of direct starlight and scattered light, and inferring properties of interstellar dust grains from the wavelength dependence of scattering.

Contribution

It presents a detailed spectral decomposition of HD46223, highlighting the significance of scattered light and small grains in the UV extinction curve, with a novel interpretation of the 2200 Å bump.

Findings

Scattered light dominates the far-UV extinction curve.

Wavelength dependence of scattered light follows a 1/λ^4 law.

The 2200 Å bump may be caused by scattered light extinction.

Abstract

The spectrum of HD46223 was established from the optical to the far UV and normalized by the spectrum of a non reddened star of same spectral type. The resulting spectrum is separated into two components. One is the direct starlight. The second is an additional component of light scattered at small angles. In the optical the spectrum is dominated by direct starlight which decreases exponentially due to the linear extinction. Scattered light begins to be noticeable in the near-UV. The near-UV rise of the scattered light is interrupted in the bump region. The wavelength dependence of the scattered light is established in the far-UV, where scattered light dominates the extinction curve. A 1/lambda^4 dependence is found, proving the presence of grains small compared to UV wavelengths. The mathematic expressions of the different components mentionned above give a good fit to the extinction curve in the direction of the star. On a mathematic standpoint the fit can be completed by a Lorentzien for the 2200Ang. bump region. The physical interpretation of the bump may be more difficult to achieve since the paper shows the possibility that only scattered light is extinguished in the bump region. Consequences for the grain size distribution which is necessary to explain the different aspects of scattering in interstellar clouds, for the value of Rv, and problems raised by this interpretation of the spectrum of HD46223, are considered at the end of this paper.