Properties of dust and detection of Halpha emission in LDN 1780

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Abstract
We present ISOPHOT observations between 60 and 200 microns and a near-infrared extinction map of LDN 1780. We have used the ISOPHOT data together with the 25, 60 and 100 microns IRIS maps to disentangle the warm and cold components of large dust grains that are observed in translucent and dense clouds. The warm and cold components in LDN 1780 have different properties and spatial distributions, with the warm component surrounding the cold component. The cold component is associated with molecular gas at densities of ~1000 cm^-3. The warm component has a uniform colour temperature of 25 K, and the colour temperature of the cold component slightly varies between 15.8 and 17.3 K (beta=2, Delta(T)=0.5 K). The ratio between the 200 microns emission of the cold component and Av is Ic(200)/Av=12.1 MJy/sr/mag and the average ratio tau(200)/Av=2.0x10^-4 mag^-1. The far-infrared emissivity of the warm component is significantly lower than that of the cold component. The Halpha emission and Av correlate very well; a ratio I(Halpha)/Av=2.2 Rayleigh/mag is observed. This correlation indicates the presence of a source of ionisation that can penetrate deeply into the cloud. Using the ratio I(Halpha)/Av we have estimated a ionisation rate for LDN 1780 that results to be ~10^-16 photons/s. We interpret this relatively high value as due to an enhanced cosmic ray flux of ~10 times the standard value. The origin of the cosmic rays could be from supernovae in the Scorpio-Centaurus OB association and/or the runaway zeta Ophiuchus.

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