We have performed a 5-colour surface photometric study of the
high-galactic-latitude area of dark nebula LDN 1642. Scattered light properties
are presented of diffuse, translucent and opaque dust over the range of 3500 --
5500 A. Far infrared absolute photometry at 200 um improves the precision of
and provides a zero point to the extinction. The intensity of the scattered
light depends on dust column density in a characteristic way: for optically
thin dust the intensity first increases linearly, then turns to a saturation
value; at still larger extinctions the intensity turns down to a slow decrease.
The AV value of the saturated intensity maximum shifts in a systematic way,
from AV≈ 1.5 mag at 3500 A, to ∼3 mag at 5500 A. The intensity
curves offer a straight-forward explanation for the behaviour of the
scattered-light colours. At the intensity peak the colour agrees with the
integrated starlight colour, while it is bluer at the low- and redder at the
high-column-density side of the peak, respectively. These colour changes are a
direct consequence of the wavelength dependence of the extinction. We have
compared the colours of the LDN 1642 area with other relevant observational
studies: high-latitude diffuse/translucent clouds, wide-field cirrus dust; and
externally illuminated AGB-star envelopes. For extragalactic
low-surface-brightness sources cirrus is an unwanted foreground contaminant.
Our results for cirrus colours can help to distinguish cases where a diffuse
plume or stream, apparently associated with a galaxy or a group or cluster, is
more likely a local cirrus structure.
Keywords: ISM: dust, extinction -- ISM: clouds, individual LDN 1642 --
Galaxy: solar neighbourhood -- Astronomical instruments, methods and
techniques: methods -- Physical data and processes: scatteringComment: 20 pages, 10 figures, accepted for publication in MNRAS, published as
MNRAS advance article on June 27 202