Formation and Evolution of Planetary Systems Meyer, Hillenbrand et al., Formation and Evolution of Planetary Systems (FEPS): First Results from a Spitzer Legacy Science Program ApJ S 154: 422 427 (2004). Presented by James Ledoux on the Ides of March 2006 (Astro 671) Evolution of Planetary Systems http://www.space.com/scienceastronomy/planet_formation_041209.html Slide 1
Intro about Formation of Planetary Systems At formation, most stars are enveloped by circumstellar accretion disks, some of which are known to form planets orbiting nearby stars (quoted 2000 Marcy et al. paper finding M sin i =.2 15 M Jupiter though more recently found planets ~5 M Earth ~.01 M jupiter ). IRAS (1983) first detected excess IR emission from Vega, and IRAS and ISO detected dozens of debris disks comprised of micron sized grains around luminous main sequence stars though neither had the sensitivity to detect disks around solar type stars at reasonable distances. Evolution of debris disks similar to our solar system hard to infer without a large base of known disks with various ages. Slide 2
FEPS Legacy Science Program The FEPS Legacy Science Program will use Spitzer search for debris disks around 330 stars with solar like spectral types (F8 V K3 V) with (solar) ages from 3 Myr to 3 Gyr to help unravel the evolution of these debris disks. Meyer et al., (FEPS: First Results, 2004) looks at five systems detected in Dec. 2003 validation campaign taken to be representative of the entire age range. HD 105, HD 47875, HD 150706, HD 157664, HD 161897 Through photometry of all five systems, it was found two stars had observed IR excess (HD 105 and HD 150706) and low res spectra was taken for these systems. Slide 3
Determination of Stellar Ages Stellar ages determined by Li I 6707 width, position in HR diagram, X ray emission, and CaII HK emission and associations with kinematic groups. HD 157664 and HD 161897 assigned preliminary age of 1 3 Gyr based on no indications of youth and likely age of sun like stars in volume limited samples. Slide 4
Photometric Data Spitzer's Infrared Array Camera (IRAC) used for photometry at 3.6, 4.5, 5.8, 8.0 µm at all five sources (integration time of 2.56 s per channel). Spitzer's Multiband Imaging Photometer (MIPS) was used in the 24, 70, 160 um bands at all five sources (2 cycles totaling 6 20s in each band) Slide 5
Infrared Excess Detections Two of the five systems had infrared excess detections. HD 105 had excesses at 70µm and 160µm HD 150706 had excess at 70µm, but no detected excess at 160µm. Right: MIPS images from which photometry was derived, images were four times oversampled. Slide 6
Spitzer's Infrared Spectrograph (IRS) obtained spectra for systems with the two systems with detected IR excess at 70 µm. Low resolution (R ~ 70 120) spectra scanned the entire IRS range (5.2 38 µm) with integration times for 5.2 14.5 µm of 6 s (both), and for 14 38 µm of 6s (HD 150706) and 14s (HD 105). Spectroscopic Data Neither IRS spectra had distinct mineralogical features nor showed obvious excess at < 35 µm Slide 7
Spectral Energy Distribution Models Modeled stellar photospheric emission using Kurucz atmospheres to available BV Johnson, vby Stromgren, B T V T Tycho, H P Hipparcos, RI Cousins, and J, H, K s 2MASS photometric data. Effective temperature and stellar radius (normalization constant) were free parameters in the model. Metallicity was fixed at solar, and surface gravity was set at value for adopted stellar age and mass. Visual extinction (A V ) was set to zero for d<40pc (dust free Local Bubble), otherwise left as free parameter. Slide 8
Infrared Excess HD 105 and HD 150706 had flux excesses of ~1x10 14 W m 2 and ~3x10 15 W m 2 corresponding to L IR /L star of ~3.9x10 4 and ~5.4x10 5 respectively. Upper limits placed on infrared excesses other three targets. For HD 105, assuming a dust temperature of ~40K, this corresponds to total effective particle cross section of 13 AU 2 with the fraction of radiating area from grains at ~100K less than 3x10 3 and less than 4x10 5 at 300K. For HD 150706, assuming dust temperature of < 84K, the total effective particle cross section is more than.09 AU 2. Slide 9
Simple Blackbody Debris Disk Model Assuming that the IR excess emission is from orbiting dust grains in thermal equilibrium with the stellar radiation, models can be used to determine inner and outer radius of debris disk and dust grain size. As the IRS spectra do not show distinct mineralogical signatures, multiple models can be used to describe the data. A simple model just assuming blackbody dust grains (size larger than longest wavelength of significant emission) gives the limit for HD 150706 R inner ~ 11AU (and a greater distance if the grains are smaller). Slide 10
Blackbody Debris Disk Models (cont'd) For HD105 the Blackbody data is consistent with a narrow ring with R inner of 42 +/ 6 AU and R outer R inner < 4 AU. If the surface density in the inner hole (r < R inner ) is assumed constant, it must be less than 3% the surface density of the model ring density. Greybody models based on intermediate sized grains can be made with emissivity falling as 1/ for > 40 m. These models give for (HD 105) have R inner from 50 to 70 AU, and R outer from 250 to 1500 AU (dependent on assumed power law exponent). Slide 11
Detailed Debris Dust Models Following Wolf & Hillenbrand (2003), they assumed: grain compositions had astronomical silicate and graphite in the ISM ratio surface density distribution (r) r 0 (However, models largely insensitive to changes in radial dependence of distribution) mass of disk adjusted to match peak flux in IR excess and they varied the power law exponent (p) of the grain sized distribution n(a) ~ a p minimum (a min ) and maximum (a max ) grain size the radius inner (R inner ) and outer edge (R outer ) of the disk Slide 12
W&H Model Results Both R inner and a min depend on the wavelength where IR excess becomes significant, so there is a degeneracy in values. Single grain sizes of 0.3, 5, and 8 m require R inner to be 1000, 120, and 42 AU respectively (HD 105). Using a minimum grain size of 5 m and allowing a grain size distribution of up to 100 or 1000 m improved 2 fits and decreased the inner radius from 120 AU to 45 AU (similarly a min ~ 8 m or larger reduced R inner to 32 AU) (HD 105). For HD 150706, we have smaller minimum grain sizes of 0.3 or 1 m with smaller R inner ~45 or 20AU, respectively. Upper grain size and outer radius not well constrained as there are no submillimeter measurements to give upper limit on IR excess. The mass in submillimeter grains in the above models is 9 x 10 8 to 4 x 10 7 M solar. Slide 13
P R drag Time Scale The Poynting Roberson (P R) effect is caused when dust particles absorb the starlight and reradiate it to remain in thermal equilibrium. They lose momentum upon reradiation causing it to leave its orbit and fall inward, so grains of a certain size are depleted on the order of P R time scale. Assuming a grain density of 2.5 g cm 3, the PR drag removal of 5 m occurs < 15 Myr at a radius of 45 AU (HD 105). Compared with its stellar age of 30 Myr, this suggests that the small grains are regenerated through some other process (perhaps collisions of planetesimals). From the radiation cross sectional area (15 300 AU 2 ) the time scale for dust grains to collide is less than 10 6 yr, so collisions are also important in determining the size distribution. For HD 150706, the time scale for removal of 1 m grains at 20 AU is less than 1 Myr compared to an age of 700+/ 300 Myr Slide 14
Summary & Sources As first results of Spitzer Formation and Evolution of Planetary Systems Legacy Science Program, five sun like stars spanning the 30 3000 Myr age range were studied. Two of these had significant IR dust excess HD 105 (G0 V, 30 +/ 10 Myr), and HD 150706 (G3 V, ~700 +/ 300 Myr) from dust debris disks. Both of these disks exhibit evidence of a large inner hole in the dust distribution. Slide 15