We present measurements of circular polarization from rotational spectral lines of molecular species in Orion KL, most notably 12 CO (J = 2 → 1), obtained at the Caltech Submillimeter Observatory with the Four-Stokes-Parameter Spectra Line Polarimeter. We find levels of polarization of up to 1 to 2% in general, for 12 CO (J = 2 → 1) this level is comparable to that of linear polarization also measured for that line. We present a physical model based on resonant scattering in an attempt to explain our observations. We discuss how slight differences in scattering amplitudes for radiation polarized parallel and perpendicular to the ambient magnetic field, responsible for the alignment of the scattering molecules, can lead to the observed circular polarization. We also show that the effect is proportional to the square of the magnitude of the plane of the sky component of the magnetic field, and therefore opens up the possibility of measuring this parameter from circular polarization measurements of Zeeman insensitive molecules.

We study the correlation of the velocity dispersion of the coexisting molecules H 13 CN and H 13 CO + and the turbulent energy dissipation scale in the DR21(OH) star-forming region. The down-shift of the H 13 CO + spectrum relative to H 13 CN is consistent with the presence of ambipolar diffusion at dissipation length scales that helps the process of turbulent energy dissipation, but at a different cut-off for ions compared to the neutrals. We use our observational data to calculate a turbulent ambipolar diffusion length scale L ′ ≃ 17 mpc and a strength of B pos ≃ 1.7 mG for the plane of the sky component of the magnetic field in DR21(OH).

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, ζ H 2 , and the ionization fraction, χ e , in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH + and HCO + is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H 3 + abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H 3 + . We apply this technique to the case of the star-forming region DR21(OH), where we successfully detected the (J = 3 → 2) and (J = 4 → 3) rotational transitions of HCNH + . We also determine the C and O isotopic ratios in this source to be 12 C/ 13 C = 63 ± 4 and 16 O/ 18 O = 318 ± 64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H + 3 ) and χ e directly from observations, and estimate ζ H 2 accordingly. Our results, ζ H 2 = 3.1 × 10 −18 s −1 and χ e = 3.2 × 10 −8 , are consistent with recent determinations in other objects.

Context. We study interstellar magnetic fields by measuring the polarization in molecular spectral lines and thermal emission of dust. Aims. We report detection of non-Zeeman circular polarization and linear polarization levels of up to 1% in the 12 CO spectral line emission in a shocked molecular clump around the supernova remnant (SNR) IC 443. We examine our polarization results to confirm that the circular polarization signal in CO lines is caused by a conversion of linear to circular polarization, consistent with anisotropic resonant scattering. In this process background, linearly polarized CO emission interacts with similar foreground molecules aligned with the ambient magnetic field and scatters at a transition frequency. The difference in phase shift between the orthogonally polarized components of this scattered emission can cause a transformation of linear to circular polarization. Methods. We compared linear polarization maps from the dust continuum, which were obtained with PolKa at APEX, and 12 CO (J = 2 → 1) and (J = 1 → 0) from the IRAM 30-m telescope. We found no consistency between the two sets of polarization maps. We then reinserted the measured circular polarization signal in the CO lines across the source to the corresponding linear polarization signal to test whether the linear polarization vectors of the CO maps were aligned with those of the dust before this linear to circular polarization conversion. Results. After the flux correction for the two transitions of the CO spectral lines, the new polarization vectors for both CO transitions aligned with the dust polarization vectors, establishing that the non-Zeeman CO circular polarization is due to a linear to circular polarization conversion.

Imaging polarimetry is an important tool for the study of cosmic magnetic fields. In our Galaxy, polarization levels of a few up to ∼10% are measured in the submillimeter dust emission from molecular clouds and in the synchrotron emission from supernova remnants. Only few techniques exist to image the distribution of polarization angles as a means of tracing the plane-of-sky projection of the magnetic field orientation. At submillimeter wavelengths, polarization is either measured as the differential total power of polarization-sensitive bolometer elements, or by modulating the polarization of the signal. Bolometer arrays such as LABOCA at the APEX telescope are used to observe the continuum emission from fields as large as ∼0:°2 in diameter. Here we present PolKa, a polarimeter for LABOCA with a reflection-type waveplate of at least 90% efficiency. The modulation efficiency depends mainly on the sampling and on the angular velocity of the waveplate. For the data analysis, the concept of generalized synchronous demodulation is introduced. The instrumental polarization toward a point source is at the level of ∼0:1%, increasing to a few percent at the À10 db contour of the main beam. A method to correct for its effect in observations of extended sources is presented. Our map of the polarized synchrotron emission from the Crab nebula is in agreement with structures observed at radio and optical wavelengths. The linear polarization measured in OMC1 agrees with results from previous studies, while the high sensitivity of LABOCA enables us to also map the polarized emission of the Orion Bar, a prototypical photon-dominated region.