Studies of interstellar reddening

Abstract

The wavelength dependence of interstellar obscuration, in magnitudes against reciprocal wavelength, is known to be well represented, over the visible part of the spectrum, by two straight lines intersecting at or near the broad interstellar absorption band at 4430A. The existing measures, obtained principally from low dispersion objective prism spectra, have a bandwidth of about 50A so that the question arises as to whether the observed "corner" is simply the unresolved band or is a separate feature of the extinction law. If the latter, it is important to know how sharp the corner is and to see if the linearity of the two parts of the curve holds good at higher resolution. The extinction law has therefore been observed with a continuous distribution of measured points at a resolution of 6A which is sufficient to give a number of points within the interstellar band.

The data were obtained from photographic slit spectra of reddened and comparison stars in Perseus, taken at a dispersion of 120A/mm.

It appears that the corner of the extinction law and the 4430A interstellar band are separate features, provided that the band is completely contained within 50A. Between 2.0 and 2.7 I at a resolution of 6A, the intersecting straight lines representation is still valid, while the intersection point is defined to 17A. At the level of the observational errors in extinction, the change of slope is shown to occur within the resolution bandwidth.

The sharpness of the corner and the straightness of each section of the curve, as found here, confirms the suggestion that existing theories of extinction, based on classical scattering by interstellar grains, are not entirely adequate.

A summary of the results of the investigation recorded here has been communicated to "Nature" and is now in press.