A spatial study of the mid-IR emission features in four Herbig Ae/Be stars

¹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV, Groningen, The Netherlands e-mail: boersma@astro.rug.nl
² Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
³ SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA
⁴ NASA-Ames Research Center, Mail Stop 245-6, Moffet Field, CA 94035, USA
⁵ Instituto de Astrofísica de Canarias, vía Láctea s/n, E38205 La Laguna, Spain
⁶ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁷ NASA-Ames Research Center, Mail Stop 245-3, Moffet Field, CA 94035, USA

Received: 10 February 2009
Accepted: 9 May 2009

Abstract

Context. Infrared (IR) spectroscopy and imaging provide a prime tool to study the characteristics of polycyclic aromatic hydrocarbon (PAH) molecules and the mineralogy in regions of star formation. Herbig Ae/Be stars are known to have varying amounts of natal cloud material present in their vicinity.

Aims. Our aim is to study the characteristics of the mid-IR emission originating in Herbig Ae/Be stars, especially the extent of the emission and how this relates to the (proto-)stellar characteristics.

Methods. Today's powerful ground- and space-based telescopes provide images and spectra at unprecedented spectral and spatial resolution. We analyse the images and spectra from four Herbig Ae/Be stars (IRAS 06084-0611, CD-42 11721, TY CrA, and HD 176386), as obtained with TIMMI2 on the ESO 3.6 m telescope and VISIR on the VLT. These observations are supplemented with data from ISO-SWS and Spitzer-IRAC.

Results. We find evidence for large-scale structure and extended emission in all four sources, except for HD 176386, which only shows silicate emission, all sources show PAH emission in their spectra. In addition, a spatially resolved silicate and PAH spectrum could be extracted for TY CrA.

Conclusions. The variety in emission scales distinguishes two classes. In the first, the morphology and spectral characteristics resemble those of reflection nebulae. In the second, the characteristics are in-line with Herbig A stars. This separation simply reflects a difference in stellar characteristics (e.g. luminosity). In Herbig B stars, dust emission from the surroundings dominates, where for Herbig A stars, the disk dominates the emission. In this scheme, IRAS 06084-0611 and CD-42 11721 resemble reflection nebulae and HD 176386 a more typical Herbig Ae/Be star. TY CrA shows characteristics common to both genuine reflection nebulae and Herbig B stars. We propose a geometry for TY CrA, with most notably, a ~70 AU inner gap in the 340 AU circumtertiary disk cleared by a fourth stellar companion.