Letter to the Editor

Grain growth in the inner regions of Herbig Ae/Be star disks^*

¹ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
⁴ CEA, DSM, DAPNIA, Service d'Astrophysique, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstrasse 1, Postfach 1317, 85748 Garching bei München, Germany

Corresponding author: R. van Boekel, rvanboek@eso.org

Received: 19 September 2002
Accepted: 2 February 2003

Abstract

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in Herbig Ae/Be stars. Our survey significantly extends the sample that was studied by Bouwman et al. (2001). We find a correlation between the strength of the silicate feature and its shape. We interpret this as evidence for the removal of small (0.1 μm) grains from the disk surface while large (1–2 μm) grains persist. If the evolution of the grain size distribution is dominated by gravitational settling, large grains are expected to disappear first, on a timescale which is much shorter than the typical age of our programme stars. Our observations thus suggest a continuous replenishment of micron sized grains at the disk surface. If the grain replenishment is due to the dredge-up of dust from the disk interior, the mineralogy we observe is representative of the bulk composition of dust in these stars.