Dust emission, extinction, and scattering in LDN 1642^★

¹ Department of Physics, PO Box 64, 00014, University of Helsinki, Finland
e-mail: mika.juvela@helsinki.fi
² Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Finland
³ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405, Orsay, France
⁴ Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès 1, 08028 Barcelona, Spain

Received: 8 June 2020
Accepted: 9 September 2020

Abstract

Context. LDN 1642 is a rare example of a star-forming, high-latitude molecular cloud. The dust emission of LDN 1642 has already been studied extensively in the past, but its location also makes it a good target for studies of light scattering.

Aims. We wish to study the near-infrared (NIR) light scattering in LDN 1642, its correlation with the cloud structure, and the ability of dust models to simultaneously explain observations of sub-millimetre dust emission, NIR extinction, and NIR scattering.

Methods. We used observations made with the HAWK-I instrument to measure the NIR surface brightness and extinction in LDN 1642. These data were compared with Herschel observations of dust emission and, with the help of radiative transfer modelling, with the predictions calculated for different dust models.

Results. We find, for LDN 1642, an optical depth ratio τ(250 μm)∕τ(J) ≈ 10⁻³, confirming earlier findings of enhanced sub-millimetre emissivity. The relationships between the column density derived from dust emission and the NIR colour excesses are linear and consistent with the shape of the standard NIR extinction curve. The extinction peaks at A_J = 2.6 mag, and the NIR surface brightness remains correlated with N(H₂) without saturation. Radiative transfer models are able to fit the sub-millimetre data with any of the tested dust models. However, these predict an NIR extinction that is higher and an NIR surface brightness that is lower than based on NIR observations. If the dust sub-millimetre emissivity is rescaled to the observed value of τ(250 μm)∕τ(J), dust models with high NIR albedo can reach the observed level of NIR surface brightness. The NIR extinction of the models tends to be higher than in the direct extinction measurements, which is also reflected in the shape of the NIR surface brightness spectra.

Conclusions. The combination of emission, extinction, and scattering measurements provides strong constraints on dust models. The observations of LDN 1642 indicate clear dust evolution, including a strong increase in the sub-millimetre emissivity, which has not been fully explained by the current dust models yet.