Deep imaging observations of the Lupus 3 cloud: Dark cloud revealed as infrared reflection nebula

We carried out deep imaging observations of the Lupus 3 dark cloud in near-infrared J, H, and K-s bands. An area of similar to8' x 8' was observed, which corresponds to a projected area of similar to0.4 x 0.4 pc at the distance of the cloud, similar to150 pc. Lupus 3 showed itself as a near-infrared nebula that has a surface brightness higher than the adjacent sky at all the three wavelengths. In a JHK(s) color composite image ( blue, green, and red are assigned to J, H, and K-s, respectively), three dark red cores are surrounded by a blue halo. The surface brightness was measured with 5 sigma limiting magnitudes of J = 21.6, H = 21.3, and K-s = 20.6 mag arcsec(-2). The appearance of the nebula depends on the wavelength. In the J band, dark cores are surrounded by a brighter halo, while in the K-s band, the dark cores of the J band are bright except for the central part of two of the cores. The appearance in the H band is intermediate between those of the J and K-s bands, having dark cores surrounded by local maxima of the surface brightness and decreased surface brightness farther out. The surface brightness is J = 20.6, H = 19.8, and K-s = 19.4 mag arcsec(-2) at the maximum in each band. Photometry of the point sources was done with 10 sigma limiting magnitudes of J = 20.1, H = 18.8, and K-s = 17.7. We constructed an extinction map of the background stars, using the H-K color of 1974 sources and the standard reddening law of Rieke & Lebofsky. The maximum value for the extinction is A(V) = 47 mag. There are three local maxima of the extinction with A(V) greater than or similar to 30 mag, which we consider to be dense cores. Their positions agree with the cores identified with the surface brightness appearance. The surface brightness and its relationship with the extinction are understood in terms of scattering of starlight by dust. The values of the maximum surface brightness can be explained by scattering of starlight by dust in the cloud if we adopt a model of grain size distribution by Weingartner Draine.