Spitzer Space Telescope - General Observer Proposal #20308


Probing the disk mineralogy and geometry of Herbig Ae/Be stars

Principal Investigator: Bram Acke
           Institution: Katholieke Universiteit Leuven

Co-Investigators:
Mario van den Ancker, ESO
Roy van Boeke, MPIA
Jeroen Bouwman, MPIA
Hans Van Wincke, KU Leuven
Rens Waters, Anton Pannekoek, Amsterdam

Science Category: circumstellar/debris disks 
 Observing Modes: IrsStare MipsSed 
  Hours Approved: 13.0 

Abstract:
We propose to obtain IRS spectra (SH and LH mode), and MIPS SED
observations of a sample of carefully selected Herbig Ae/Be stars.
The observed spectral energy distribution of Herbig stars can be split
up in roughly two groups:  group I contains the strong mid-IR (20-100
micron) emitters, while group II sources have much more moderate
excesses at these wavelengths.  State-of-the-art models of
protoplanetary disks indicate that the observational difference
between the groups reflects the disk's geometry:  group I sources have
flared and group II self-shadowed disks.  Moreover, we have found
evidence that there is a evolutionary connection between both groups
suggesting that flared disks evolve into self-shadowed disks as grain
growth makes the grains settle to the disk's midplane.  Unfortunately,
the observational record till now is strongly biased towards the, by
definition brighter, group I sources.  This hampers our understanding
of the evolution of protoplanetary disks around these stars.  We
therefore propose both IRS and MIPS spectroscopic observations which
cover the vital wavelength region which defines the group I/II
membership.  The MIPS and IRS spectra will provide a powerful tool,
not only to study in detail the connection between disk geometry and
mineralogy of individual sources, but also to study the disk evolution
by confronting our findings with the evolutionary timescales of the
central objects.  Moreover, disks surrounding binary post-AGB stars
show remarkable similarities with those around group II Herbig stars.
This resemblance sets stringent restrictions on the chemical and
geometrical timescales involved in disk evolution.  The unbiased
sample of Herbig Ae/Be group I and II sources and the comparison with
post-AGB binary disks will provide an excellent laboratory to
investigate the phenomena which govern the physical processes in
circumstellar disks.  The ultimate goal of this proposal is to gain
insight in the evolution of the protoplanetarey disks surrounding
Herbig Ae/Be stars.