Dust ring around lambda Orionis.

Sky maps of a 12 deg x 12 deg region obtained by IRAS reveal a dust ring which has a diameter of about 9 deg and is centered on the H II region S 264 and its exciting star, Lambda Orionis. The 60/100 micron color temperature in the ring with an assumed dust emissivity law nu exp 1.5 is on average 25 K. The luminosity in the IRAS wavelength range and the total far-infrared luminosity for the ring are 38,000 and 63,000 solar luminosities, respectively. The radiation field responsible for the heating of dust in the ring contains an enhanced UV component with an energy density 2 to 3 times as large as that of the general interstellar radiation field. This enhancement of the UV component is ascribed to the O star Lambda Ori and several other early B stars. A careful analysis of the energetics of the ring shows that its total infrared luminosity can be explained by the energy supply from the Lambda Ori OB association, HD 34989 and the interstellar radiation field.