A continuum study of the NGC 2024 molecular ridge

variations?

Figure 1 shows a spectral index map made from the 450 micron and 800 micron maps. It is striking that the spectral index is lower at the positions of the FIR cores compared with the surrounding molecular cloud. This may be explained as follows: if the dust opacity is written in the usual long-wavelength form, k = k(/), and the dust is optically thin, the spectral index , (F ), can be written as = 2 + + , where is a Rayleigh-Jeans correction factor. An increase in thus indicates an increase in or . The typical difference in the spectral index between the ridge and the FIR cores is found to be too great to be caused by only, and we therefore have to conclude that variations in are the most likely cause. In T Tauri disks is observed to be in the range 0 to 1, which is lower than the theoretical limit for small grains ( = 2). This has been interpreted as an evolution of the dust properties, perhaps an increase in grain size, in T Tauri disks compared to the interstellar medium. In the case of NGC 2024 the decreased in the FIR cores might also suggest an evolution of the dust properties. It is tempting to draw the conclusion that the grain particles are larger in the denser regions.

Depletion or hot dust?

In Figure 2 the integrated CS(2-1) emission measured using the Owens Valley millimeter array is displayed in greyscale with 450 micron continuum contours overlayed. There is a good correlation between the 450 micron dust continuum and the CS(2-1) emission along the ridge, except within 7" of FIR5, where there is bright continuum but no CS emission. This discrepancy between dust and molecular line emission has been reported in previous studies, but these new data show the discrepancy to be extremely localised. It has been suggested that the discrepancy may be caused by depletion of the CS molecules onto cold dust grains in the dense cold condensations. An alternative explanation is that the temperature of the gas and dust in and near FIR5 is somewhat higher than in the surrounding ridge. Emission from optically thin dust scales with T(dust), and emission from CS scales with 1/T, if CS is optically thin and thermalized. At the high densities present here, T(dust) and T are expected to be coupled, so a higher temperature would cause an increase in dust emission and a decrease in molecular line emission. High resolution measurements at 50 micron and 100 micron will be essential for determining the spectral energy distributions, and hence the dust temperatures of the individual FIR cores.

Figure 1: Spectral index map from the 450 micron and 800 micron continuum images. Regions were blanked in both images if the intensity was less than twice the mean rms noise in either image. The contours are from the 450 micron continuum map.

With SCUBA it should be possible to observe large regions with a higher sensitivity making it possible to study variations of dust properties on larger scales.

A. E. Visser(1), J. S. Richer(1), C. J. Chandler(1), J. E. Carlstrom(2) & R. Padman(1)

1. MRAO, Cambridge, UK

2. University of Chicago, USA

Figure 2: The grey scale plot shows integrated CS(2-1) emission (Chandler and Carlstrom, 1996, ApJ, 466, 338), the contours are 450 micron continuum emission.