Light and colour of cirrus, translucent and opaque dust in the high-latitude area of LDN 1642

TL;DR

This study provides a detailed multi-wavelength analysis of dust scattering and colour variations in the high-latitude dark nebula LDN 1642, revealing how dust optical properties influence observed light and colours.

Contribution

It offers new insights into the wavelength-dependent scattering and colour behaviour of dust in high-latitude clouds, with improved infrared calibration and comparison to other dust environments.

Findings

Scattered light intensity varies with dust column density, showing linear increase, saturation, then slow decrease.

Colour of scattered light peaks at the intensity maximum, matching integrated starlight, and shifts with extinction.

Dust colour properties can help distinguish local cirrus from extragalactic low-surface-brightness sources.

Abstract

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 $A_V$ value of the saturated intensity maximum shifts in a systematic way, from $A_V\approx$ 1.5 mag at 3500 A, to $\sim 3$ 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: scattering