Observations of 6 - 200 {\mu}m emission of the Ophiuchus cloud LDN 1688
  We examine two positions, ON1 and ON2, within the Ophiuchus cloud LDN 1688
using observations made with the ISOPHOT instrument aboard the ISO satellite.
The data include mid-IR spectra (~6-12{\mu}m) and several photometric bands up
to 200{\mu}m. The data probe the emission from molecular PAH-type species,
transiently-heated Very Small Grains (VSGs), and large classical dust grains.
We compare the observations to earlier studies, especially those carried out
towards an isolated translucent cloud in Chamaeleon (Paper I). The spectra
towards the two LDN 1688 positions are very similar to each other, in spite of
position ON1 having a larger column density and probably being subjected to a
stronger radiation field. The ratios of the mid-IR features are similar to
those found in other diffuse and translucent clouds. Compared to paper I, the
7.7/11.3{\mu}m band ratios are lower, ~2.0, at both LDN 1688 positions. A
continuum is detected in the ~10{\mu}m region. This is stronger towards the
position ON1 but still lower than on any of the sightlines in Paper I. The
far-infrared opacities are higher than for diffuse medium. The value of the
position ON2, {\tau}200/N(H) = 3.9 x 10^{-25} cm^2/H, is twice the value found
for ON1. The radiation field of LDN 1688 is dominated by the two embedded B
type double stars, {\rho} Oph AB and HD 147889, with an additional contribution
from the Upper Sco OB association. The strong heating is reflected in the high
colour temperature, ~24 K, of the large grain emission. Radiative transfer
modelling confirms a high level of the radiation field and points to an
increased abundance of PAH grains. However, when the hardening of the radiation
field caused by the local B-stars is taken into account, the observations can
be fitted with almost no change to the standard dust models. However, all the
examined models underestimate the level of the mid-IR continuum.
