Debris Disks: A Brief Observational Historyastrosun2.astro.cornell.edu/academics/courses/a671... · 2006-04-24 · April 19, 2006 T. Oberst: Spitzer Debris Disks 3 Debris Disk Observations

Debris Disks: A Brief Debris Disks: A Brief Observational HistoryObservational History

Thomas OberstThomas OberstApril 19, 2006April 19, 2006

A671A671

Debris Disk; Artist’s rendition (T. Pyle (SSC), JPL-Caltech, & NASA http://www.spitzer.caltech.edu/Media/happenings/20051214/)

22April 19, 2006April 19, 2006 T. Oberst: Spitzer Debris DisksT. Oberst: Spitzer Debris Disks

Debris DisksDebris Disks (also: Dust Disks, Planetary (also: Dust Disks, Planetary Disks, Stellar Disks, “Vega Phenomenon”)Disks, Stellar Disks, “Vega Phenomenon”)

Debris Disk (DD) = solid particles surrounding a star Debris Disk (DD) = solid particles surrounding a star after gas has been absorbed by giant planets or expelled after gas has been absorbed by giant planets or expelled by radiation pressure or by radiation pressure or PoyntingPoynting--Robertson drag = Robertson drag = Solar SystemSolar System--like or like or KuiperKuiper BeltBelt--like systemlike systemSizes up to several 100 AUSizes up to several 100 AUTemperatures ~ 50Temperatures ~ 50--150K150KAge is typically > few 100 Age is typically > few 100 MyrMyr, but can also refer to , but can also refer to protoplanetaryprotoplanetary disk (~10 disk (~10 MyrMyr –– ~100 ~100 MyrMyr))Product of Product of collisionalcollisional grinding of grinding of planetesimalsplanetesimalsLikely episodic in natureLikely episodic in natureTracer of orbital dynamics (analogous to Saturn’s rings)Tracer of orbital dynamics (analogous to Saturn’s rings)


Debris Disk ObservationsDebris Disk ObservationsUsually detected as a MIRUsually detected as a MIR--FIR “excess” FIR “excess” in a stellar spectral energy distribution in a stellar spectral energy distribution (SED)(SED)

First detection: 25, 60 & 100um IRAS First detection: 25, 60 & 100um IRAS excesses found in Vega (excesses found in Vega (AumannAumann ApJApJ’84)’84)Hundreds of followHundreds of follow--up detections (or up detections (or “possible” detections) with IRAS & ISO “possible” detections) with IRAS & ISO (examples: (examples: PletsPlets & & VynckierVynckier AA ’99, AA ’99, Spangler Spangler ApJApJ ‘01, ‘01, HabingHabing AA ’01)AA ’01)

DDsDDs of several nearby stars have been of several nearby stars have been spatially resolved with ground based & spatially resolved with ground based & HST optical observationsHST optical observations901 901 candidatecandidate DDsDDs as of Aug ’03, prior to as of Aug ’03, prior to Spitzer launch (ROE database: Spitzer launch (ROE database: <<www.roe.ac.uk/ukatc/research/topics/dust/idwww.roe.ac.uk/ukatc/research/topics/dust/identification.htmlentification.html>>))

Aumann ApJ ’84 Fig 1

β Pictoris DD optical image (Smith & Terrile ’84)


IR “Excess”IR “Excess”

Artist’s Rendition (T. Pyle (SSC), NASA, & JPL-Caltech http://www.spitzer.caltech.edu/Media/happenings/20051214/)

1 um 10 100 1000

100

1000 mJy

1 um 10 100 1000

100

1000 mJy

10


Quick summary of Quick summary of IRAS IRAS DDsDDs (1)(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)


Quick summary of IRAS Quick summary of IRAS DDsDDs (2)(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


Quick Summary of ISO Quick Summary of ISO DDsDDs (1)(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



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



Habing et al A&A ’01 Conclusions:Overall DD incidence is 17% Stars < 400 Myr ⇒ ~1/2 have DDStars > 400 Myr ⇒ ~1/10 have DDMost 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 producedThe collision of planetesimals is a good source of new dust ⇒ rapid decay of the disks is caused by the destruction and escape of planetesimals.


Quick Summary of Spatially Quick Summary of Spatially Resolved Resolved DDsDDs (1)(1)

(http://origins.jpl.nasa.gov/news/2004/120904-a.html)

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 107146: G2V, d=88 Ly, Age ~ 30-250 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



High-Res Keck II IR image of AU Microscopii, 100 AU on a side, central black mask = 30 AU diam. (http://www.ifa.hawaii.edu/users/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.



(http://astronomy.swin.edu.au/~smaddiso/research/images/debris/)


Spitzer FGK Survey (1)Spitzer FGK Survey (1)GTO program to search for IR excess around wellGTO program to search for IR excess around well--defined sample of 150 F5defined sample of 150 F5--K5 mainK5 main--sequence field starssequence field starsGoals:Goals:1.1. Investigate distribution of IR excess around unbiased sample of Investigate distribution of IR excess around unbiased sample of

solarsolar--type stars (no selection bias for type stars (no selection bias for metallicitymetallicity, age, or , age, or previous IR excess detection)previous IR excess detection)

2.2. Relate observations of Relate observations of DDsDDs to the presence of planets in the to the presence of planets in the same systemsame system

Sensitivity: Sensitivity: LLdustdust/L/L ~ 10~ 10--55

Solar SystemSolar System’’s s KuiperKuiper Belt: Belt: LLdustdust/L/L ~10~10--77--1010--66 (Stern AA (Stern AA ’’96)96)Solar SystemSolar System’’s Asteroid Belt: s Asteroid Belt: LLdustdust/L/L ~10~10--88--1010--77 (Dermott (Dermott ’’02)02)

Wavelengths:Wavelengths:MIPS: 24 & 70umMIPS: 24 & 70umIRS: 8IRS: 8--40um40um


Spitzer FGK Survey (2)Spitzer FGK Survey (2)Completed Components:Completed Components:1.1. MIPS 24 & 70um observations of 26 (of the 150?) stars known MIPS 24 & 70um observations of 26 (of the 150?) stars known

to have one or more planets from radial velocity studies to have one or more planets from radial velocity studies ((BeichmanBeichman et al et al ApJApJ ’05)’05)

2.2. Preliminary MIPS 24 & 70um results for 69/150 sources (Preliminary MIPS 24 & 70um results for 69/150 sources (BrydenBrydenet al et al ApJApJ 1/10/06)1/10/06)

3.3. Preliminary 8Preliminary 8--40um IRS results for 41/150 sources (40um IRS results for 41/150 sources (BeichmanBeichmanet al et al ApJApJ 3/10/06)3/10/06)


MIPS Results (1)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



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.



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.



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, 10-6.5

(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 Ldust/L > 10-5

= 12%.

Of the three curves, the distribution with solar as average (dotted line) is the best fit to the data.


Preliminary MIPS ConclusionsPreliminary MIPS ConclusionsHave detected 70um excess to 3Have detected 70um excess to 3σσ confidence level in 7/69 main confidence level in 7/69 main sequence field stars.sequence field stars.Excess emission is produced by cool (<100K) material located Excess emission is produced by cool (<100K) material located beyond 10AU beyond 10AU ------ consistent with consistent with KuiperKuiper Belt analog with 100x more Belt analog with 100x more emitting surface than Solar System’s Kemitting surface than Solar System’s K--belt.belt.Disk frequency:Disk frequency:

2% +/2% +/-- 2% for 2% for LLdustdust/L/L > 10> 10--44

12% +/12% +/-- 5% for 5% for LLdustdust/L/L > 10> 10--55

Will % increase further with more sensitive surveys?Will % increase further with more sensitive surveys?Models suggest average Models suggest average LLdustdust/L/L ~ 10~ 10--6.56.5 (= solar system average)(= solar system average)Weak correlation between stellar age and IR excess with stars Weak correlation between stellar age and IR excess with stars younger than 1Gyr more likely to have excess emissionyounger than 1Gyr more likely to have excess emissionNo correlation between No correlation between metallicitymetallicity and IR excessand IR excessNo correlation between spectral type and IR excessNo correlation between spectral type and IR excess


ReferencesReferencesBeichmanBeichman et. al., 2006, et. al., 2006, ApJApJ 639, 1166639, 1166BeichmanBeichman et. al., 2005, et. al., 2005, ApJApJ 622, 1160622, 1160BrydenBryden et. al., 2006, et. al., 2006, ApJApJ 636, 1098636, 1098HabingHabing et. al., 2001, A&A, 365, 545et. al., 2001, A&A, 365, 545PletsPlets & & VynckierVynckier, 1999, A&A, 343, 496, 1999, A&A, 343, 496CCAT slides from Terry CCAT slides from Terry HerterHerterAll other references can be found within the main All other references can be found within the main references listed on this page.references listed on this page.


Extra SlidesExtra Slides


MIPS planetMIPS planet--bearing systems (1)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.



Beichman ApJ ’05 Fig 3: SEDs for HD 82943 and HD 117176. In addition to 24 and 70um Spitzer data (dark circles), optical measurements and IRAS fluxes at 12 and 25um are shown. For HD 117176, 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 82943 hot dust (150 K) is ruled out by the 24um observations, while for HD 117176 cold dust (20 K) is excluded by the submm upper limit



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 planet-bearing samples

• Distribution of metallicities for planet-bearing stars is clearly higher than solar: [Fe/H] = 0.13 +/- 0.05



Beichman ApJ ’05 Fig 10: Constraints on T and L of the dust around HD 117176, 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.

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Debris Disks: A Brief Observational Historyastrosun2.astro.cornell.edu/academics/courses/a671... · 2006-04-24 · April 19, 2006 T. Oberst: Spitzer Debris Disks 3 Debris Disk Observations