We present ISOPHOT observations between 60 and 200 \mu m and a near-infrared extinction map of the small intermediate-density cloud LDN 1780 ( Galactic coordinates l=359 ^ { o } and b = 36.8 ^ { o } ) .
For an angular resolution of 4 ^ { \prime } , the visual extinction maximum is A _ { V } = 4.4 mag .
We have used the ISOPHOT data together with the 25 , 60 and 100 \mu m IRIS maps ( Miville-Deschênes & Lagache 2005 ) to disentangle the warm and cold components of large dust grains that are observed in translucent clouds ( Cambrésy et al .
2001 , del Burgo et al .
2003 ) and dense clouds ( del Burgo & Laureijs 2005 ) .
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 OB association , as in the case of the HI excess emission ( Mattila & Sandell 1979 ) .
The cold component is associated with the ^ { 13 } CO ( J=1-0 ) line integrated ( W _ { 13 } ) , which trace molecular gas at densities of \sim 10 ^ { 3 } cm ^ { -3 } .
The warm component has a uniform colour temperature of 25 \pm 1 K ( assuming \beta = 2 ) , 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 emission at 200 \mu m of the cold component ( I ^ { c } _ { \nu } ( 200 ) ) and A _ { V } is I ^ { c } _ { \nu } ( 200 ) / A _ { V } =12.1 \pm 0.7 MJy sr ^ { -1 } mag ^ { -1 } and the average ratio \tau _ { 200 } / A _ { V } = ( 2.0 \pm 0.2 ) \times 10 ^ { -4 } mag ^ { -1 } .
The far infrared emissivity of the warm component is significantly lower than that of the cold component .
The H \alpha emission ( I _ { \nu } ( { \mathrm { H } } \alpha ) ) and A _ { V } correlate very well ; a ratio I _ { \nu } ( { \mathrm { H } } \alpha ) / A _ { V } = 2.2 \pm 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 ionisation 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 H \alpha surface brightness although that process could be responsible of the formation of LDN 1780 .
Using the ratio I _ { \nu } ( { \mathrm { H } } \alpha ) / A _ { V } we have estimated a ionisation rate for LDN 1780 that results to be \sim 10 ^ { -16 } \gamma s ^ { -1 } .
We interpret this relatively high value as due to an enhanced cosmic ray flux of \sim 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 ionisation rate could be explained as result of a confinement of low energy ( \sim 100 MeV ) cosmic rays by self generated MHD waves ( Padoan & Scalo 2005 ) .
The origin of the cosmic rays could be from supernovae in the Scorpio-Centaurus OB association and/or the runaway \zeta 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 .