G. Novak

Polarized far-infrared emission from the core and envelope of the sagittarius b2 molecular cloud

We have detected linear polarization in the 115 μm continuum radiation from the giant molecular cloud Sagittarius B2. We found polarization at nine positions in the dense cloud core and at 15 positions in the less-dense envelope. The polarization in the core is due to absorption by magnetically aligned grains and that in the envelope is due to emission from magnetically aligned grains. The inferred magnetic field direction is roughly north-south everywhere, but with spatially smooth variations of up to 30°. By considering our data together with Zeeman splitting observations we are able to set a conservative lower limit of 150 μG on the strength of the large-scale field in the envelope. If large-scale fields this strong are common in Galactic center clouds, they could be detectable via large-beam Zeeman measurements. For positions in the envelope that are furthest from the core, the field is nearly parallel to the plane of the Galaxy. This is consistent with the idea of a globally azimuthal magnetic field in the Galactic center neutral gas layer, which is expected if gravitational forces dominate magnetic forces.

POLARIMETRY OF DG TAU AT 350 μm

We present the first 350 μm polarization measurement for the disk of the T Tauri star (TTS) DG Tau. The data were obtained using the SHARP polarimeter at the Caltech Submillimeter Observatory. We measured normalized Stokes parameters q= –0.0086 ± 0.0060 and u = –0.0012 ± 0.0061, which gives a 2σ upper limit for the percent polarization of 1.7%. We obtain information about the polarization spectrum by comparing our 350 μm measurement with an 850 μm polarization detection previously published for this source. Comparing the two measurements in Stokes space (not in percent polarization) shows that the two data points are not consistent, i.e., either the degree of polarization or the angle of polarization (or both) must change significantly as one moves from 850 μm to 350 μm. This conclusion concerning the polarization spectrum disagrees with the predictions of a recent model for TTS disk polarization. We show that this discrepancy can be explained by optical depth effects. Specifically, we demonstrate that if one were to add more mass to the model disk, one would expect to obtain a model polarization spectrum in which the polarization degree falls sharply with increasing frequency, consistent with the observations at the two wavelengths. We suggest that multiwavelength polarimetry of TTS disk emission may provide a promising method for probing the opacity of TTS disks.

TRACING H2 COLUMN DENSITY WITH ATOMIC CARBON (C I) AND CO ISOTOPOLOGS

We present first results of neutral carbon ([CI], 3P1 - 3P0 at 492 GHz) and carbon monoxide (13CO, J = 1 - 0) mapping in the Vela Molecular Ridge cloud C (VMR-C) and G333 giant molecular cloud complexes with the NANTEN2 and Mopra telescopes. For the four regions mapped in this work, we find that [CI] has very similar spectral emission profiles to 13CO, with comparable line widths. We find that [CI] has opacity of 0.1 - 1.3 across the mapped region while the [CI]/13CO peak brightness temperature ratio is between 0.2 to 0.8. The [CI] column density is an order of magnitude lower than that of 13CO. The H2 column density derived from [CI] is comparable to values obtained from 12CO. Our maps show CI is preferentially detected in gas with low temperatures (below 20 K), which possibly explains the comparable H2 column density calculated from both tracers (both CI and 12CO underestimate column density), as a significant amount of the CI in the warmer gas is likely in the higher energy state transition ([CI], 3P2 - 3P1 at 810 GHz), and thus it is likely that observations of both the above [CI] transitions are needed in order to recover the total H2 column density.