The millimeter wavelength emissivity in IC 5146

We have observed a core in the northern streamer of the IC5146 cloud using the MPIfR 19-channel Bolometer system on the IRAM 30m telescope at a wavelength of 1.2 mm. We smoothed our observations to the same angular resolution (30) as a new map of the NIR extinction of this cloud (Lada, Alves, and Lada 1998) and in this way obtained direct measurements of the ratio of millimeter emissivity to dust extinction. Our results are compatible with a ratio of the visual extinction coefficient κ V to the 1.2 mm grain absorption coefficient κ 1.2 of (4 ± 2) × 10 4. This is in good agreement with the theoretical predictions for bare interstellar grains (Draine & Lee 1984; Mathis 1990). Within the errors our measurements are also consistent with recent models of grains with ice mantles whose size distribution has been modified due to coagulation but whose millimeter emissivities are not a strong function of grain size (Ossenkopf & Henning 1994). We find that the ratio of millimeter grain emissivity to visual extinction varies roughly inversely with the visual extinction over our map and interpret this as implying a gradient in dust temperature from roughly 8 K in the core interior (A V > 20 mag) to 20 K in regions of low (5 mag) extinction. One consequence of our result is that the 1.2 mm intensity, by itself, is not a good tracer of mass in this cloud.