Spitzer Space Telescope - General Observer Proposal #40104


Timescale for Gas-giant Planet Formation in A- and B-stars

Principal Investigator: John Carpenter
           Institution: California Institute of Technology

     Technical Contact: John Carpenter, California Institute of Technology

Co-Investigators: 
David Hollenbach, NASA-Ames                                           
Uma Gorti, NASA-Ames                                                  
Scott Dahm, California Institute of Technology                        
Jenny Patience, University of Exeter                                  

Science Category: circumstellar/debris disks 
 Observing Modes: IrsStare 
  Hours Approved: 27.5 

Abstract:
Most 1 Myr old stars are surrounded by circumstellar accretion disks that
undoubtedly represent the formation sites of planetary systems. Based on Spitzer
and ground-based surveys from near-infrared to submillimeter wavelengths, it is
becoming increasingly clear that by an age of 10 Myr, the reservoir of
primordial dust grains in disks has been drastically depleted over all orbital
radii for more than 90% of solar type stars. However, these observations do not
constrain the lifetime of the gas, whose evolution may be decoupled from the
dust. The lifetime of gas in primordial circumstellar disks has fundamental
consequences for the formation of Jovian planets, dynamical evolution of
terrestrial planets, and migration of planetesimals and dust grains. In a recent
Spitzer photometric survey of the 5 Myr Upper Sco OB association, we identified
a population of 10 A- and B- type stars surrounded by circumstellar disks with
large (> 10 AU) inner holes inferred from the dust component. These disks are at
an advanced evolutionary stage relative to the optically-thick, gas-rich
primordial disks found around Herbig Ae/Be stars, and may represent the latter
stages in the dissipation of primordial disks or the formative stages of debris
systems. Given the relative youth of Upper Sco, this sample represents an
important population to establish the gas dissipation time scales around A- and
B- stars. We propose to obtain IRS high resolution spectra of these 10 stars to
search for gas emission lines from Ne II, Ne III, S, Fe II, and molecular
hydrogen. By combining these observations with our complementary ground-based
survey, we can search for gas over the entire disk and determine if these disks
retain sufficient mass of gas to form Jovian planets, or whether we are
observing an end-stage in primordial disk evolution that establishes an upper
limit on the timescale to form gas-giant planets in A- and B-type stars.