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Astron. Astrophys. 352, 645-658 (1999)


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Optical images of MCLD123.5+24.9: a cloud illuminated by the North star?

F. Zagury 1,2, F. Boulanger 2 and V. Banchet 3

1 Department of Astrophysics, Nagoya University, Nagoya, 464-01 Japan (zagury@a.phys.nagoya-u.ac.jp)
2 Institut d'Astrophysique Spatiale, Université Paris Sud, Bat. 121, 91405 Orsay Cedex, France
3 Institut d'Astrophysique de Paris, 65 Bd. Arago, 75005 Paris, France

Received 15 July 1999 / Accepted 22 September 1999

Abstract

We present B R I images of a [FORMULA] field centered on the molecular cloud (MCLD123.5+24.9) located in the large infrared cirrus known as the Polaris Flare. The optical images are compared with IRAS images and an extinction map derived from stellar reddenings. We analyse the possibility for the North star (HD8890), only [FORMULA] North of the field, to be the source of MCLD123.5+24.9 optical emission. For widely accepted values of the dust albedo ([FORMULA]) and the assymetry parameter of the Henyey Greenstein phase function ([FORMULA]), forward scattering of Polaris light explains the cloud optical brightness, colors, and the surface brightness ratios derived from comparison of optical and [FORMULA]m data. The cloud colors are redder than what is expected from diffusion of Polaris light by a low column density medium, but the color difference may be explained by extinction with no need for red luminescence. Within this interpretation, the cloud is located between 105 and 125 pc from the Sun and is 6 to 25 pc in front of Polaris. Over this range of distances, due to forward scattering, Polaris is the main source of the cloud optical emission but a minor source of dust heating relative to the general Galactic interstellar radiation field of the Solar neighborhood (ISRF).

Key words: ISM: clouds – ISM: dust, extinction – ISM: individual objects: MCLD123.5+24.9 – stars: individual: Polaris = HD 8890 – scattering

Send offprint requests to: F. Zagury

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999

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