Spitzer Space Telescope - General Observer Proposal #3401


Study of an ISO-based sample of Vega stars:  towards a better
understanding of the evolutionary sequence

Principal Investigator: Peter Abraham
           Institution: Konkoly Observatory of the Hungarian Academy of Sc

Co-Investigators: 
Attila Moor, Konkoly Observatory                                      
Carol Grady, Eureka Scientific, NASA GSFC                             
Csaba Kiss, Max-Planck-Institut fuer Astronomie                       
Thomas Henning, Max-Planck-Institut fuer Astronomie                   
Guy Stringfellow, Univ. Colorado, Boulder                             

Science Category: circumstellar/debris disks 
 Observing Modes: IrsStare MipsPhot MipsSed 
  Hours Approved: 12.2 

Abstract:
One of the main discoveries of the IRAS mission was that main-sequence
stars can be accompanied by circumstellar dust disks (Vega
phenomenon).  In this proposal we investigate a sample of 16 Vega
candidate stars, whose far-infrared excess above the photosphere -
indication for the presence of a disk - was already well measured
(5-10  sigma) at 60  and 90  micrometer by ISO.  All stars are in the
spectral range of F0-F9, therefore, due to the narrow mass range, the
stellar luminosities do not change much within the sample.  The
temporal evolution of the density and temperature structure of the
disks, as derived from the proposed observations, can be compared with
models which predict that 1) the debris dust production site
propagates outwards with time;  2) the relative contributions from
small and large grains (which can be deduced from the shape of the
disks spectrum) is also evolving with time.  There are a number of GTO
programmes plus a Legacy programme which are devoted to the Vega
phenomenon.  However, these proposals focus either on intermediate
mass A-type or late F-, G-, K- type stars.  Our proposed sample of 16
F stars would fill the gap between these two major categories.  Debris
disks with fractional luminosity higher than $10^{-3}$ are in a very
special and short state of disk evolution.  The sample contains five
stars belonging to this interesting category of stars with debris
disks, providing an excellent opportunity to study in details these
very important cases.  We propose to observe 16 F-type main-sequence
stars with Spitzer.  We obtain MIPS photometry and IRS low resolution
spectra for each target in order to measure Vega-type excess at MIPS
wavelengths, and to measure the short wavelength cut-off of the excess
as well as search for solid-state features at the mid-infrared
wavelengths.  MIPS SED is requested for five objects to determine
detailed SEDs in the far-infrared for the most spectacular (high
fractional luminosity) targets.  The total programme takes 12.2 hours.