Kate Su, George Rieke, Karl Misselt, John Stansberry, Amaya Moro-Martin, David Trilling… etc. (U. of A) Karl Stapelfeldt, Michael Werner (NASA JPL) Mark Wyatt, Wayne Holland (UK Royal Observatory) People from Spitzer Science Center, and CfA..
Why study Debris Disks? Debris disks are the most visible signposts of other planetary systems, representing indirect evidence of planetary system formation. We can learn about the diversity of planetary systems from the study of debris disks, leading to a better understanding of the formation of our Solar system. Vega is one of the closest and brightest debris disks, which provides us an opportunity to study it in details spatially, as a foundation for understanding other unresolved debris disks.
Lyrae = HD , A0V, d= 7.76 pc infrared excess discovered by IRAS (Aumann et al. 1984) a pole-on star (Gulliver, Hill & Adelman 1994) age of ~350 Myr (Barrado y Navascues 1998, Song et al. 2001) disk size from IRAS 60 m is 18” 3” in radius (van der Bliek et al. 1994) ISO observations show a smooth, resolved, face-on disk with a radius of 22” 2” at 60 m, and 36” 3” at 90 m (Heinrichsen et al. 1998) Facts about Vega and Its Debris Disk SCUBA 850 m map 24”x21” (Holland et al. 1998)
Predicted Spitzer view of Vega system at 24 m (model from 1.3 mm data of Wilner et al. 2002) courtesy of G. Rieke 2002, ApJL, 569, L115 3 M J at 40 AU with e= mm data of Vega disk
MIPS 24 m image of Vega disk expected image
MIPS 70 m image of Vega disk
MIPS 160 m image of Vega disk contamination by filter spectral leak
physical size of the disk: radius of 43” (330 AU), 70” (543 AU) and 105” (815 AU) at 24, 70 and 160 m. disk appears very smooth, no clumpiness. disk flux: F 24 = 1.5 Jy ( 10%), F 70 = 7 Jy ( 20%), F 160 = 4 Jy ( 20%) presence of material warm enough to be detected at a large distance.
24 um Log r from starr in logS Radial Profile Analysis
r 0 = 11” 2” (86 16 AU) 70 m 160 m modelsA24A70A160 hole – r –4 5.5 ± 7%11.5 ± 5%15 ±30% hole – r –3 2.9 ± 17%9.4 ± 7%14 ±30% flat – r –4 8.0 ± 5%18 ± 6%23 ±30% flat – r –3 4.4 ± 14%16 ± 6%21 ±30%
Simple Disk Model axially symmetric, geometrically thin, face-on disk surface number density : (r) = 0 (r/r 0 ) -p, r 0 = 86 AU emission from grains at a given radius r,wavelength is: dF = dn(r) Q(a, ) B (T r ) Optical constants for astronomical silicates (Laor & Draine 1993), and amorphous carbon grains (Zubko et al. 1996) each grain size has its own T r (a) computed by energy balancing convolved with with smoothed STinyTim PSFs to match the observed resolution 6”, 18” and 40” for 24, 70 and 160 m. a2a2 d2d2
Size Distribution Grain Model – silicates Disk is composed of grains with size distribution: n(a) a q with size cut-offs at a min and a max p = 1.0 0.2 q = -3.0 0.5 a min = m a max = 46 11 m M d =2.8 1.1 M Su et al. 2005, ApJ, 628, 487
How sensitive the model fits to grain composition? Silicates vs. amorphous carbon grains Su et al. 2005, ApJ, 628, 487
Size Distribution Grain Model – 70% silicates + 30 % amorphous carbon p = 1.0 (fixed) q = -3.0 0.6 a min = 3.2 0.8 m a max = 29 14 m M d =2.6 1.5 M Su et al. 2005, ApJ, 628, 487
A Complete Model – disk + ring Our main Goal is not to model the ring’s exact physical structure; instead we only try to find a structure where its resultant emission profile is consistent with the 850 m data and does not violate the existing 24, 70 and 160 m profiles. Disk – composed of grains with small grains : (with radii between ~1 to ~50 um) (r) = 0 (r/r 0 ) -1 from 86 to 1000 AU Ring – containing very large grains (r) = ’ 0 86 AU < r <100 AU ’ 0 (r/ 100AU ) AU < r < 200 AU small grains in a large extended disk large grains confined in a ring
A Complete Model – disk + ring Su et al. 2005, ApJ, 628, 487
Summary – Modeling Results (r) = 0 (r/r 0 ) -1, p =1 explains the data best: Expected from a disk contains grains driven outward by radiation pressure A range of models using amorphous silicate and/or carbon grains of sizes from ~1 to ~50 m can fit the infrared radiometric behavior of the disk out to ~800 AU. All these models require 3.0 1.5 M in grains Models that also fit the 850 m data require an inner ring (near 100 AU) with larger grains (>180 m) and a total mass M MIPS is seeing small grains blown out by radiation pressure while SCUBA is seeing larger grains that are gravitationally bound to the star. They are distinctly different grain populations.
Implications of the modeling 3L * /16 GM * c a > 0.5 for particles in escaping orbits fluffy grains with porosity > 0.5 highly non-spherical grains terminal velocity, v r sqrt(2GM * [ -0.5]/r i ) if r i ~80 AU and ~1 v r > 5 km/s t residence R disk /v r < 1000 years if R disk ~ 1000 AU dust production rate ~ 6 g/s Comet Hale-Bopp dust production rate ~ 2 10 8 g/s ~ 3 million Hale-Bopp-like comets to account for the dust production rate in Vega! (not likely)
T. Pyle (SSC/Caltech) disk we imaged is short-lived, and is the aftermath of a large and relatively recent collision. Subsequent collisional cascades occur in the source region (“ring”) generating small debris that is blown outward by radiation pressure. steady state collision grinding throughout Vega’s life (~350 Myr)? Unlikely, because it would require >300 M J mass of dust in the asteroidal reservoir. Origin of the debris in Vega
Spitzer detects a very extended, transient disk in Vega, caused by the aftermath of a large and relatively recent collision. More evidence that the evolution of planetary systems is a pretty chaotic process. Resolving the disk provides critical information about its structure and composition, and hence its origin. Rieke et al Conclusion Vega and other Spitzer observations (eg., Rieke et al. 2005) suggest that often single collision(s) can dominate the radiometric properties of the disk.
[24] [70] radial color distribution
Two-Component Grain Model – silicates Disk is composed of grains with two different sizes n(a) = f 1 n(a 1 ) + f 2 n(a 2 ) p = 1.0 0.2 f 1 = –3.2 % a 1 = 2.0 0.7 m a 2 = –6 m M d =2.9 M Su et al. 2005, ApJ, 628, 487
Vega Disk Spectral Energy Distribution