The dust disk of HR 4049

Another brick in the wall

¹ Sterrenkundig Instituut “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: dominik@science.uva.nl
² Max Planck Institut für Astrophysik, Karl Schwarzschild Strasse 1, 85748 Garching, Germany e-mail: dullemon@mpa-garching.mpg.de
³ SRON-Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: cami@astro.rug.nl
⁴ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Heverlee, Belgium e-mail: Hans.VanWinckel@ster.kuleuven.ac.be

Corresponding author: C. Dominik, dominik@science.uva.nl

Received: 6 March 2002
Accepted: 8 October 2002

Abstract

We present the Spectral Energy Distribution of HR 4049 based on literature data and new continuum measurements at 850 μm. The SED shows variable absorption in the UV, and a large IR excess, both caused by circumstellar dust. The shape of the IR excess from 1 μmall the way down to 850 μmcan be nearly perfectly fitted with a single blackbody function at  K or alternatively with a sum of blackbodies in a narrow temperature range. The energy emitted in this IR continuum radiation is about one-third of the stellar luminosity. We show that this blackbody radiation must be due to the presence of a circumbinary disk with a large height. This disk must also be gas-rich, in agreement with the observations of molecular bands in the ISO-SWS spectrum. We present two possible scenario's for explaining the shape and the intensity of the IR excess. The first scenario involves large grains ( mm) that each radiate like a blackbody. The second scenario argues that the blackbody radiation is due to a very optically thick circumbinary disk. We investigate if such a disk would indeed produce blackbody radiation by presenting results from radiative transfer calculations. We further quantify the properties of such a disk and its stability in the framework of (hydro)dynamics, grain settling, radiation pressure and grain drift. The virtues and shortcomings of both models for the origin of the IR blackbody are discussed by contrasting them with other observations and assessing them in the framework of (binary) (post-)AGB evolution.