The FEPS Spitzer Legacy Program Goals Characterize transition from primordial to debris disks

1. J.Serena Kim (Steward Obs.) & FEPS collaboration(M.R.Meyer (PI), D. Backman (NASA-Ames, D.P.I.) , S.V.W. Beckwith (STScI), J. Bouwman (MPIA), J.M. Carpenter (Caltech), M. Cohen (UC-Berkeley), U. Gorti (NASA-Ames), T. Henning (MPIA), L. Hillenbrand (Caltech, D.P.I.), D. C. Hines (Steward), D. Hollenbach (NASA-Ames), J. Lunine (LPL), R. Malhotra (LPL), E. Mamajek (Steward), A. Moro-Martin (Steward), P. Morris (SSC), J. Najita (NOAO), D. Padgett (SSC), I. Pascucci (MPIA), J. Rodmann (MPIA), M.D. Silverstone (Steward), D. Soderblom (STScI), J.R. Stauffer (SSC), E. Stobie (Steward), S. Strom (NOAO), D. Watson (Rochester), S. Weidenschilling (PSI), S. Wolf (MPIA), and E. Young (Steward)) FEPS (The Formation and Evolution of Planetary Systems):The First Results from a Spitzer Legacy Program Abstract We present 3-160um photometry obtained with the IRAC and MIPS instruments, spectro-photometry from 5-35um using IRS low resolution spectrograph, and IRS high resolution spectrum of HD105. Our report includes updates on new detections at 70um and 160um of the candidate debris disk around HD 105 (G0V, ~30 Myr old) as well as a newly discovered debris disk, HD 150706 (G3V, ~1 Gyr old). We also place preliminary upper limits on the remnant molecular gas in the disk surrounding HD 105. 30+- 10 Myr 45 to ??? AU ~1x10-7 Msun • The FEPS Spitzer Legacy Program • Goals • Characterize transition fromprimordial to debrisdisks • Constrain timescale ofgas disk dissipation • Examine thediversity of planetary systems • Try to answer a question: Is our Solar System Unique? • Targets 700+- 300 Myr 20 to <100 AU ~6.9x10-8 Msun Figure 1.MIPS 70 & 160um images of HD 105 and HD 150706. Figure 2.SED for HD 105 and HD 150706. Representative models for the dust debris surrounding HD 105 (RIN/ROUT = 45/300 AU; aMIN/aMAX = 5/100um) and HD 150706 (RIN/ROUT = 20/100 AU; aMIN/aMAX = 1/100um) are also shown. Table 1.FEPS targets No lines detected for HD 105 Mass in H2 < 4 Mjup. <- Figure 3.Mgas upper limits for HD 105 Table 2.Adopted stellar and derived circumstellar properties Figure 4.SED for HD 161897, HD 157664, and HD 47875. Kurucz models are overplotted. • HD 150706 • Grain size: 0.3 or 1 um (smaller than HD 105’s case) • Inner radius ~ 45 or 20 AU with outer radius < 100 AU. • P-R drag time scale for 1 um grains at 20AU < 1 Myr (<< age of • the star ~700Myr ) -> suggest regeneration of small grains via • collision. • HD 150706 is less likely to have gas-rich disk compared to HD 105. • FOR BOTH HD 105 & HD 150706 • Lack of circumstellar materials in the inner disk suggest: • Something is preventing dust at 20-40AU from reasching the sublimation radius in the inner disk • a lack of significant numbers of colliding planetesimals inside of 20-45AU. • These suggest that the inner region is relatively clear of small bodies, consistent with timescale of terrestrial planet formation (e.g., Kenyon & Bromley 2004) • The presence of >= 1 large planet < 20-40AU from a star may explain the iner edge of the outer dust disk (e.g., Moro-Martin & Malhotra 2003) • Models • Toy Model: based on Backman & Paresce (1993) • Assumptions:The IR excess emission is from grains • orbiting, and in thermal equilibrium with radiation from the • centeral stars. • The model RIN and ROUT of HD 105 and HD 150706 containing cold materials are calculated with assumptions regarding grain compositions, size distributions, and spatial distributions. Without mineralogical features in the observed IRS spectra, the solution is not an unique model, but a range of models. • DDS (Debris Disk Simulator): following Wolf & Hillenbrand (2003) • parameters and Assumptions: • - Initial input parameters are based on results from Toy models • - grain composition: “astronomical” silicates + graphite in the ISM • ratio and surface density distribution proportional to r0. • Mdisk was adjusted to match the peak flux in the IR excesses. • grain size distribution n(a) ~ a-p power-law exponent, amin & amax, • and RIN and ROUT were varied to find the range of values. • Models are relatively insensitive to the radial density distribution • exponent. • l from which dust re-emission spectrum begins to deviate from • stellar photosphere significantly was used to find smallest amin • and smallest RIN consistent with the data. HD 105 Range of fitting results (amin & RIN are degenerated) : Grain size (amin) : 0.3, 5, 8 um Inner radius (RIN): 1000, 120, & 42 AU Best fit models: Adopting amin = 5 um, allowing grain size distribution up to 100 or 1000um, RIN range 120 – 45 AU (32 AU for amin = 8um). amax and ROUT are not well constrained. The mass in grains < 1mm for these models: 9x10-8 and 4x10-8 Msun. PR drag time scale (assuming grain density of 2.5 g/cm3): < 15 Myr for 5um grain at 45 AU ( < 30 Myr, the stellar age) -> suggest any such small grains are regenerated, perhaps, via collisions of planetesimals. However given the optical depth of dust (15-300AU2, the radiating cross-sectional area), the time scale for dust to collide is < 1Myr. -> suggest that collisions as well as P-R drag are important in determining the actual size distribution of dust as well as its radial surface density profile. Gas Mass Upper limit for H2 < 4 MJUPITER (Fig. 3) * note: IRS High resolution data of HD 105 are still under analysis… References: Backman, D. E. & Paresce, F. 1993, Protostars and Planets III. 1253 Keynon, S.J. & Bromley, B.C. 2004, ApJ, 602, L133 Meyer, M.R. et al. 2004. ApJS, in press Moro-Martin, A. & Malhotra, R. 2003, AJ, 125, 2255 Wolf, S. & Hillenbrand, L.A. 2003, ApJ, 596, 603