The peculiar nebula Simeis 57

I. Ionized gas and dust extinction

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
² Dominion Radio Astrophysical Observatory, Box 248, Penticton, B.C., V2A 6K3, Canada
³ Department of Astrophysical and Planetary Sciences and Center for Astrophysics and Space Astronomy, University of Colorado, Campus Box 389, Boulder, CO 80309-0389, USA

Corresponding author: F. P. Israel, israel@strw.leidenuniv.nl

Received: 11 October 2002
Accepted: 14 November 2002

Abstract

We present high resolution radio continuum maps of the Galactic nebula Simeis 57 (= HS 191 = G 80.3+4.7) made at the Westerbork Synthesis Radio Telescope and the Dominion Radio Astrophysical Observatory at frequencies of 609, 1412 and 1420 MHz. At optical and at radio wavelengths, the nebula has a peculiar “S” shape, crossed by long, thin and straight filaments. The radio maps, combined with other maps from existing databases, show essentially all radio emission from the peculiar and complex nebula to be thermal and optically thin. Although neither the distance nor the source of excitation of Simeis 57 are known, the nebula can only have a moderate electron density of typically . Its mass is also low, not exceeding some tens of solar masses. Peak emission measures are 5000 pc cm^-6. Obscuring dust is closely associated with the nebula, but seems to occur mostly in front of it. Extinctions vary from A_V = 1.0 mag to A_V = 2.8 mag with a mean of about 2 mag. The extinction and the far-infrared emission at m are well-correlated.