The wavelength dependence of interstellar polarization in the Local Hot
  Bubble
  The properties of dust in the interstellar medium (ISM) nearest the Sun are
poorly understood because the low column densities of dust toward nearby stars
induce little photometric reddening, rendering the grains largely undetectable.
Stellar polarimetry offers one pathway to deducing the properties of this
diffuse material. Here we present multi-wavelength aperture polarimetry
measurements of seven bright stars chosen to probe interstellar polarization
near the edge of the Local Hot Bubble (LHB) - an amorphous region of relatively
low density interstellar gas and dust extending ~70-150 pc from the Sun. The
measurements were taken using the HIgh Precision Polarimetric Instrument
(HIPPI) on the 3.9-m Anglo-Australian Telescope. HIPPI is an aperture stellar
polarimeter with a demonstrated sensitivity of 4.3 parts-per-million (ppm). Of
the stars observed two are polarized to a much greater degree than the others;
they have a wavelength of maximum polarization ($\lambda_{max}$) of ~550 $\pm$
20 nm - similar to that of stars beyond the LHB - and we conclude that they are
in the wall of the LHB. The remaining five stars have polarizations of ~70 to
160 ppm, of these four have a much bluer $\lambda_{max}$, ~350 $\pm$ 50 nm.
Bluer values of $\lambda_{max}$ may indicate grains shocked during the
evolution of the Loop I Superbubble. The remaining star, HD 4150 is not well
fit by a Serkowski curve, and may be intrinsically polarized.
