Natural Law Institute Proceedings Vol. 2

https://scholarship.law.nd.edu/naturallaw_proceedings/1002/thumbnail.jp

Polarization in young open cluster NGC 6823

We present multiwavelength linear polarimetric observations of 104 stars towards the region of young open cluster NGC 6823. The polarization towards NGC 6823 is dominated by foreground dust grains and we found the evidence for the presence of several layers of dust towards the line of sight. The first layer of dust is located approximately within 200 pc towards the cluster, which is much closer to the Sun than the cluster (~ 2.1 kpc). The radial distribution of the position angles for the member stars are found to show a systematic change while the polarization found to reduce towards the outer parts of the cluster and the average position angle of coronal region of the cluster is very close to the inclination of the Galactic parallel (~ 32 degree). The size distribution of the grains within NGC 6823 is similar to those in general interstellar medium. The patchy distribution of foreground dust grains are suggested to be mainly responsible for the both differential reddening and polarization towards NGC 6823. The majority of the observed stars do not show the evidence of intrinsic polarization in their light.Comment: 16 pages, 6 tables, 11 figures, Accepted for publication in MNRA

Comparison of 13CO Line and Far-Infrared Continuum Emission as a Diagnostic of Dust and Molecular Gas Physical Conditions: III. Systematic Effects and Scientific Implications

Far-infrared continuum data from the {\it COBE}/{\it DIRBE} instrument were combined with Nagoya 4-m \cOone spectral line data to infer the multiparsec-scale physical conditions in the Orion$$A and B molecular clouds, using 140\um/240\um dust color temperatures and the 240\um/\cOone intensity ratios. In theory, the ratio of far-IR, submillimeter, or millimeter continuum to that of a \cO (or \Co) rotational line can place reliable upper limits on the temperature of the dust and molecular gas on multi-parsec scales; on such scales, both the line and continuum emission are optically thin, resulting in a continuum-to-line ratio that suffers no loss of temperature sensitivity in the high-temperature limit as occurs for ratios of CO rotational lines or ratios of continuum emission in different wavelength bands. Two-component models fit the Orion data best, where one has a fixed-temperature and the other has a spatially varying temperature. The former represents gas and dust towards the surface of the clouds that are heated primarily by a very large-scale (i.e. $\sim 1$kpc) interstellar radiation field. The latter represents gas and dust at greater depths into the clouds and are shielded from this interstellar radiation field and heated by local stars. The inferred physical conditions are consistent with those determined from previously observed maps of \COone and \Jtwo that cover the entire Orion$$A and B molecular clouds. The models require that the dust-gas temperature difference is 0$\pm 2$K. If this surprising result applies to much of the Galactic ISM, except in unusual regions such as the Galactic Center, then there are a number implications.Comment: The work of Schnee et al. 2006 is relevant here. This is now mentioned in the Discussion and in the Conclusions. In the third version, I've corrected a few typos and slightly changed the emphasis on the cold gas/dust. In the version of March 2007, I've fixed a few typos and updated a few reference