Far-Infrared Polarization Spectrum of Rho OphA

In this work, we studied the polarization spectrum of a molecular cloud, meaning that we looked for variations of the polarization levels as a function of wavelength. This is an important property that may help to test the most promising grain alignment theory, as explained below. We have observed one of the closest star-forming regions to the Solar System, known as Rho Ophiuchi, located at only about 130 parsecs (424 light-years). In the central parts of the cloud, known as Rho Oph A, several young stars are currently being formed, some of which will probably become stars with planetary systems much like our Sun.

With HAWC+ on SOFIA, we have observed for the first time that systematic variations of the far-infrared polarization spectrum exist within an interstellar cloud. More specifically, the observations show that as we go from the more diffuse to the denser regions of Rho Oph A, the slope of the polarization spectrum smoothly changes from positive to negative. The observed change is consistent with the grain alignment theory known as radiative torques (RATs). According to this theory, dust particles in the interstellar medium need to be exposed to light waves in order to become aligned. The change in the polarization spectrum can be predicted from the RATs theory, and our observations of Rho Oph A match the prediction.