Properties of dust and detection of Hα emission in LDN 1780

Abstract

We present ISOPHOT observations between 60 and 200 μm and a near-infrared extinction map of the small intermediate-density cloud Lynds Dark Nebula (LDN) 1780 (Galactic coordinates l = 359° and b = 36°8). For an angular resolution of 4 arcmin, the visual extinction maximum is A V = 4.4 mag. We have used the ISOPHOT data together with the 25-, 60- and 100-μm IRIS maps to disentangle the warm and cold components of large dust grains that are observed in translucent clouds and dense clouds. The warm and cold components in LDN 1780 have different properties (temperature, emissivity) and spatial distributions, with the warm component surrounding the cold component. The warm component is mainly in the illuminated side of the cloud facing the Galactic plane and the Scorpius-Centaurus (Sco-Cen) OB association, as in the case of the H I excess emission. The cold component is associated with the 13 CO(J = 1-0) line integrated (W 13 ), which trace molecular gas at densities of ∼10 3 cm -3 . The warm component has a uniform colour temperature of 25 ± 1 K (assuming β = 2), and the colour temperature of the cold component slightly varies between 15.8 and 17.3 K (β = 2, AT = 0.5 K). The ratio between the emission at 200 μm of the cold component [I c v (200)] and A V is I c v (200)/A V = 12.1 ± 0.7 MJy sr -1 mag -1 and the average ratio τ 200 /A V = (2.0 ± 0.2) x 10 -4 mag -1 . The far-infrared emissivity of the warm component is significantly lower than that of the cold component. The Ha emission [I v (Hα)] and A V correlate very well; a ratio I v (Hα)/A V = 2.2 ± 0.1 Rayleigh mag -1 is observed. This correlation is observed for a relatively large range of column densities and indicates the presence of a source of ionization that can penetrate deep into the cloud (reaching zones with optical extinctions A v of 2 mag). Based on modelling predictions, we reject out a shock front as precursor of the observed Ha surface brightness although that process could be responsible of the formation of LDN 1780. Using the ratio I v (Hα)/A v , we have estimated an ionization rate for LDN 1780 that results to be ∼10 -16 γs -1 . We interpret this relatively high value as due to an enhanced cosmic ray radiation rate of ∼10 times the standard value. This is the first time such an enhancement is observed in a moderately dense molecular cloud. The enhancement in the ionization rate could be explained as the result of a confinement of low-energy (∼100 Mev) cosmic rays by self-generated magnetohydrodynamics waves in agreement with the recent modelling results of Padoan & Scalo. The origin of the cosmic rays could be from supernovae in the Sco-Cen OB association and/or the runaway ζ Ophiuchus. The observed low- 13 CO abundance and relatively high temperatures of the dust in LDN 1780 support the existence of a heating source that can come in through the denser regions of the cloud.