The initial conditions of isolated star formation -- IX. Akari mapping
  of an externally heated pre-stellar core
  We present observations of L1155 and L1148 in the Cepheus molecular cloud,
taken using the FIS instrument on the Akari satellite. We compare these data to
submillimetre data taken using the SCUBA camera on the JCMT, and far-infrared
data taken with the ISOPHOT camera on board the ISO satellite. All of the data
show a relation between the position of the peak of emission and the wavelength
for the core of L1155. We interpret this as a temperature gradient. We fit
modified blackbody curves to the spectral energy distributions at two positions
in the core and see that the central core in L1155 (L1155C) is approximately 2
degrees warmer at one edge than it is in the centre. We consider a number of
possible heating sources and conclude that the A6V star BD+67 1263 is the most
likely candidate. This star is at a distance of 0.7 pc from the front of L1155C
in the plane of the sky. We carry out radiative transfer modelling of the
L1155C core including the effects from the nearby star. We find that we can
generate a good fit to the observed data at all wavelengths, and demonstrate
that the different morphologies of the core at different wavelengths can be
explained by the observed 2 degree temperature gradient. The L1148 core
exhibits a similar morphology to that of L1155C, and the data are also
consistent with a temperature gradient across the core. In this case, the most
likely heating source is the star BD197053. Our findings illustrate very
clearly that the apparent observed morphology of a pre-stellar core can be
highly dependent on the wavelength of the observation, and that temperature
gradients must be taken into account before converting images into column
density distributions. This is important to note when interpreting Akari and
Spitzer data and will also be significant for Herschel data.
