The “second stellar spectrum:” rotating hot massive star linear spectropolarimetry with the Öhman effect

Abstract

To understand better the polarized radiative transfer near the surface of rotating massive stars that remain nearly spherically symmetric, we use plane-parallel stellar atmosphere models to explore the unique opportunity presented by the Öhman effect. This effect refers to the predicted variation in linear polarization across a rotationally broadened absorption line, due to the interaction of that line with the spatially varying continuum polarization across the face of a strongly scattering photosphere, such as found in hot stars. Even if the rotation is weak enough for the star to remain spherically symmetric, the Öhman effect persists because differential absorption induced by the rotational Doppler shift of the line breaks the symmetry that would otherwise cancel the continuum polarization in the absence of that line. Neglecting rotational distortion effects, the net polarization across the line vanishes, yet resolved line profiles display a telltale triple-peak polarization pattern, with one strong polarization peak at line center and two smaller ones in the line wings at a position angle that is rotated 90 degrees from the line center. The far ultraviolet (FUV) is emphasized because both the polarization amplitude and the specific luminosity are greatest there for photospheres with effective temperatures between about 15,000 and 20,000 K. Additionally, larger polarizations result for lower-gravity atmospheres. There is a high density of spectral lines in the FUV, leading to a rich “second stellar spectrum” in linear polarization (analogous to the “second solar spectrum”) that is made observable with stellar rotation. Some hot stars exhibit extreme rotation, which suppresses the polarimetric amplitude for the forest of weaker FUV lines, but a few strong lines such as the Siiv 140 nm doublet still give observable polarizations at high rotation speeds even before rotational distortion effects of the atmosphere are considered. Thus polarizations at the level of 0.1% to 1% are achievable across individual lines for a wide variety of B-type stars. We highlight the prospects for accessing the unique information encoded in the Öhman effect with future moderate-resolution spaceborne spectropolarimetric missions in the FUV.