Extinction versus Attenuation¶
All extinction curves are attenuation curves, but not all attenuation curves are extinction curves.
Interstellar dust extinction is the result of photons being absorbed or scattered out of the line-of-sight by dust grains.
Extinction can be directly measured by observing a star with dust along the line-of-sight. Knowledge of the intrinsic spectrum of the star from either an observation of a similar star without foreground dust along the line-of-sight or a stellar atmosphere model is used.
Extinction can be directly measured towards individual stars. The specific geometry is a star with a column of dust between the observer and the star. The two effects present are dust grains absorbing photons or scattering photons out of the line-of-sight. Since the dust grains are not near the star, scattering of photons from the star into the line-of-sight is very small and can be safely ignored.
Both dust absorption and scattering out of the line-of-sight are processes that are directly proportional to the amount of dust along the line-of-sight. As a result, the ratio of dust extinctions at two different wavelengths does not vary with different amounts of otherwise identical dust. In other words, extinction curves normalized by dust column measured towards different different stars with different amounts of identical dust grains are equivalent. This is illustrated below where the left plot shows the total extinction as a function of wavelength and the right plot shows the same curves normalized by A(V).
Note that there is dust scattered light throughout a galaxy due to the all the stars and dust in that galaxy. For our Galaxy, this is called the Diffuse Galactic Light (DGL). This overall scattered light is removed from observations of a single star by the standard practice of subtracting a local background. The overall scattered light is smooth on the usual spatial scales involved in measuring a single star.
Dust attenuation refers to the general impact on the spectrum of an object due to the presence of dust. In general, attenuation is used to indicate that the geometry of the sources and dust in a system is more complex than a single star with a foreground screen of dust. Examples of such systems include dusty galaxies (composed of many stars) and stars with circumstellar dust.
Attenuation includes two additional effects not included in extinction. These are scattering of photons into the observation beam and sources extinguished by different columns of dust. These two additional sources results in the ratio of dust attenuations at two different wavelengths varying with different amounts total system dust. These two additional effects mean that the measurement and/or theoretical calculation of attenuation is significantly more complex than for extinction.
The separate package dust_attenuation package exists to provide attenuation models.