Ring-like features around young B stars

Abstract

Aims. We investigate the nature of two young intermediate mass stars namely IRAS20293+3952 (I20293) and IRAS05358+3843 (I05358), which display clearly defined H 2 emission rings. Methods. High resolution deconvolution of finely sampled 3.6-8.0 μm Spitzer-IRAC images are carried out. Continuum-subtracted 2.122 μm H 2 1-0 S(1) narrow-band images of both targets and HK band spectrum of two main stars in I20293 are presented. The spectral energy distibutions (SED) of the stars enclosed in the H 2 rings are constructed using photometry from the literature and combining with newly obtained Spitzer-MIPS 70 μm photometry. The SEDs are modelled using a grid of radiative transfer models to obtain estimates of the star, disk and envelope physical parameters. Results. The images reveal ring-like structures surrounding isolated young B-type stars. The modelling attributes a mass and age of ∼7 M and ∼0.1 Myr, respectively, to both young stars, in agreement with other observational indicators. The HK band spectra of I20293 display the Brackett series of HI recombination lines, which appears to correspond exclusively to the young star inside this IRAS source. The ring around I20293 appears to be a single well-defined ellipse with inner parts shining brightly in the Spitzer broad band images and outer parts displaying strong H 2 emission. In the source I05358, at least two ring's are found, one circumscribing the other. The emission in the Spitzer bands and the H 2 emission are mostly coincident for the inner ring. The outer ring is visible partly in the Spitzer bands and partly in H 2 emission. The rings in both the sources, I20293 and I05358, have diameters of ∼25 000-30 000 AU. The envelope sizes estimated by the SED modelling are consistent with the size of the rings estimated using the imaging data. The spectrum of the H 2 ring in I20293 is suggestive of fluorescent excitation. In the source I05358, the inner ring displays a combination of PAH and H 2 line emission with excess PAH emission closer to the star.