Debris Disks: A Brief Observational History Thomas Oberst April 19, 2006 A671

Transcription

1 Debris Disks: A Brief Observational History Thomas Oberst A671 Debris Disk; Artist s rendition (T. Pyle (SSC), JPL-Caltech, & NASA er.caltech.edu/m edia/happenings / /)

2 Debris Disks (also: Dust Disks, Planetary Disks, Stellar Disks, Vega Phenomenon ) Debris Disk (DD) = solid particles surrounding a star after gas has been absorbed by giant planets or expelled by radiation pressure or Poynting-Robertson drag = Solar System-like or Kuiper Belt-like like system Sizes up to several 100 AU Temperatures ~ K Age is typically > few 100 Myr,, but can also refer to protoplanetary disk (~10 Myr ~100 Myr) Product of collisional grinding of planetesimals Likely episodic in nature Tracer of orbital dynamics (analogous to Saturn s rings) 2

3 Debris Disk Observations Usually detected as a MIR-FIR excess in a stellar spectral energy distribution (SED) First detection: 25, 60 & 100um IRAS excesses found in Vega (Aumann( ApJ 84) Hundreds of follow-up detections (or possible detections) with IRAS & ISO (examples: Plets & Vynckier AA 99, Spangler ApJ 01, Habing AA 01) DDs of several nearby stars have been spatially resolved with ground based & HST optical observations 901 candidate DDs as of Aug 03, prior to Spitzer launch (ROE database: < entification.html>) Aumann ApJ 84 Fig 1 β Pictoris DD optical image (Smith & Terrile 84) 3

4 1 um IR Excess 1000 mjy Artist s Rendition (T. Pyle (SSC), NASA, & JPL-Caltech 4

5 Quick summary of IRAS DDs (1) Plets & Vynckier AA 99, Figure 4: Cross-sections of a a 3D plot of 12, 25 & 60um excess magnitudes for 634 objects from Bright Star Catalogue (Hoffleit and Warren 1991) with reliable IRAS Faint Source Catalog association Ellipsoid represents the 99% quantile 99% chance that star inside ellipsoid does not have IR excess (based on given errors in observations and photosphere models) 5

6 Quick summary of IRAS DDs (2) Plets & Vynckier AA 99, Figure 6: Estimated cumulative distribution function for a subsample of 69 stars of types A-K & luminosity classes IV-V. fraction of main-sequence stars displaying IR excess = 13 +/- 10% (i.e. 95% chance that incidence is between 3 and 23%) Represents best statistical analysis of DDs possible with IRAS results 6

7 Quick Summary of ISO DDs (1) Habing AA 01 Figure 3: Histogram of 60um excess for 84 nearby A-K stars. Top: distribution of ISO flux densities; 3 stars have an excess higher than 500 mjy; the drawn curve is a Gaussian with average = 4 mjy and dispersion = 21 mjy. Bottom: the same for stars where only IRAS data are available; two stars have an excess higher than 500 mjy 7

8 Quick Summary of ISO DDs (2) Habing AA 01 Figure 7: Cumulative distribution of excess stars as a function of the index after sorting by age. The two linear segments are predicted by assuming that the rate of DDs is much higher in the first 400 Myr than afterwards 8

9 Quick Summary of ISO DDs (3) Habing et al A&A 01 Conclusions: Overall DD incidence is 17% Stars < 400 Myr ~1/2 have DD Stars > 400 Myr ~1/10 have DD Most stars arrive on the main sequence surrounded by a DD; DD then decays in about 400 Myr. Because dust is removed by radiation pressure and Poynting-Robertson drag on timescales shorter than stellar ages, dust in DDs must be recently produced The collision of planetesimals is a good source of new dust rapid decay of the disks is caused by the destruction and escape of planetesimals. 9

10 Quick Summary of Spatially Resolved DDs (1) AU Microscopii: M0, d=33 Ly, Age ~ 12 Myr. slight warping & variations in dust density tugging from unseen companion, possibly large planet blue color smaller dust size ( HD : G2V, d=88 Ly, Age ~ Myr. Red color slightly larger dust sizes Simulations using ring diameter & dust quantity suggest unlikely to evolve similar to solar system similar stars may have very different DDs 10

11 Quick Summary of Spatially Resolved DDs (2) High-Res Keck II IR image of AU Microscopii, 100 AU on a side, central black mask = 30 AU diam. ( ers/mliu/research/) Sharp change in structure at 35 AU & spatially localized enhancements and deficits at 25 to 40 AU separations influence of unseen larger bodies and structures expected from recent planet formation. 11

12 Quick Summary of Spatially Resolved DDs (3) ( 12

13 Spitzer FGK Survey (1) GTO program to search for IR excess around well- defined sample of 150 F5-K5 main-sequence field stars Goals: 1. Investigate distribution of IR excess around unbiased sample of solar-type stars (no selection bias for metallicity,, age, or previous IR excess detection) 2. Relate observations of DDs to the presence of planets in the same system Sensitivity: L dust /L ~ 10-5 Solar System s Kuiper Belt: L dust /L ~ (Stern AA 96) Solar System s s Asteroid Belt: L dust /L ~ (Dermott 02) Wavelengths: MIPS: 24 & 70um IRS: 8-40um8 13

14 Spitzer FGK Survey (2) Completed Components: 1. MIPS 24 & 70um observations of 26 (of the 150?) stars known to have one or more planets from radial velocity studies (Beichman et al ApJ 05) 2. Preliminary MIPS 24 & 70um results for 69/150 sources (Bryden( et al ApJ 1/10/06) 3. Preliminary 8-40um 8 IRS results for 41/150 sources (Beichman( et al ApJ 3/10/06) 14

15 MIPS Results (1) Bryden ApJ 06 Fig 1: Distribution of stellar distances. Stars found to have 70um excess are flagged as arrows at the top of the plot. The length of the arrow is an indicator of the strength of 70um excess. Bryden ApJ 06 Fig 5: Distribution of 70um fluxes relative to the expected photospheric values. While most stars cluster around unity, where their flux is photospheric, several stars show a high degree of excess emission 15

16 MIPS Results (2) Bryden ApJ 06 Fig 2: Distribution of stellar ages. Stars found to have 70um excess are flagged as arrows at the top of the plot. The length of the arrow is an indicator of the strength of 70um excess. Weak correlation showing greater DDs in younger stars Bryden ApJ 06 Fig 3: Distribution of stellar metallicities. Stars found to have 70um excess are flagged as arrows at the top of the plot. The length of the arrow is an indicator of the strength of 70um excess. No obvious correlation. 16

17 MIPS Results (3) Bryden ApJ 06 Fig 9: Constraints on T and L of the dust around six stars with 70um excess. Grey region = 3σ limit; Black = 1σ limit. 17

18 MIPS Results (4) Bryden ApJ 06 Fig 11: DD detection frequency compared with theoretical DD distributions. Grey area = 1σ limit. Three possibilities are considered: (1) all stars have DDs with the solar system's average emission, (dotted); (2) all stars have DDs with average 10 times solar (dashed); (3) all stars have DDs with average 10 times < solar (dot-dashed). Models assume Gaussian dist. with 12% frequency of DDs with L dust /L > 10-5 = 12%. Of the three curves, the distribution with solar as average (dotted line) is the best fit to the data. 18

19 Preliminary MIPS Conclusions Have detected 70um excess to 3σ3 confidence level in 7/69 main sequence field stars. Excess emission is produced by cool (<100K) material located beyond 10AU --- consistent with Kuiper Belt analog with 100x more emitting surface than Solar System s K-belt. K Disk frequency: 2% +/- 2% for L dust /L > % +/- 5% for L dust /L > 10-5 Will % increase further with more sensitive surveys? Models suggest average L dust /L ~ (= solar system average) Weak correlation between stellar age and IR excess with stars younger than 1Gyr more likely to have excess emission No correlation between metallicity and IR excess No correlation between spectral type and IR excess 19

20 References Beichman et. al., 2006, ApJ 639, 1166 Beichman et. al., 2005, ApJ 622, 1160 Bryden et. al., 2006, ApJ 636, 1098 Habing et. al., 2001, A&A, 365, 545 Plets & Vynckier,, 1999, A&A, 343, 496 CCAT slides from Terry Herter All other references can be found within the main references listed on this page. 20

21 Extra Slides 21

22 MIPS planet-bearing systems (1) Beichman ApJ 05 Figs 1 & 2: Distribution of 24um (left) and 70um (right) fluxes relative to the expected photospheric values for a sample of 84 stars. While most stars cluster around 1, several show a high degree of excess emission at 70um. Although planet-bearing stars make up less than a third of the sample, four of the five stars with the highest factor of excess 70 um emission are known to have planets. 22

23 MIPS planet-bearing systems (2) Beichman ApJ 05 Fig 3: SEDs for HD and HD In addition to 24 and 70um Spitzer data (dark circles), optical measurements and IRAS fluxes at 12 and 25um are shown. For HD , a submm (850um) constraint is also available (Greaves 04). The emission from dust at a given temperature (dashed lines) is added to the stellar Kurucz model (dotted line) in order to fit the observed excess emission at 70um. In each plot, two separate fits (two different dust temperatures) are considered. In the case of HD hot dust (150 K) is ruled out by the 24um observations, while for HD cold dust (20 K) is excluded by the submm upper limit 23

24 MIPS planet-bearing systems (3) Beichman ApJ 05 Figs 7 & 8: Distribution of stellar ages (left) and metallicities (right) for sample of 84 stars. Stars with 70um excess are flagged as arrows at the top of the plot. For planet-bearing stars the arrows are filled; for stars without known planets they are open. The length of the arrow is an indicator of the strength of 70um excess. No obvious age difference between the overall planet-bearing and non planetbearing samples Distribution of metallicities for planet-bearing stars is clearly higher than solar: [Fe/H] = /

25 MIPS planet-bearing systems (4) Beichman ApJ 05 Fig 10: Constraints on T and L of the dust around HD , as provided by 24 and 70 m MIPS + submm data. Grey region = 3σ limit; Black = 1σ limit. Note that although the formal 3σ error limits extend as low as the Kuiper Belt's luminosity, the Kuiper Belt itself would be too faint to detect. 25