A Planets Rocky Road to Success:Spitzer Observations of Debris DisksG. H. Rieke, for the MIPS teammajor contributors are Chas Beichman, Geoff Bryden, Nadya Gorlova, Beth Holmes, James Muzerolle, John Stansberry, Karl Stapelfeldt, Kate Su, David Trilling, Erick Young, Jane Morrison, Karl Gordon, and Karl Misselt

Summary: Spitzer provides the means to study the first stages of planet formation in some detail, and to connectthem with theories for the evolution of the early Solar System.A Planets Rocky Road to Success:Spitzer Observations of Debris DisksG. H. Rieke, for the MIPS teammajor contributors are Chas Beichman, Geoff Bryden, Nadya Gorlova, Beth Holmes, James Muzerolle, John Stansberry, Karl Stapelfeldt, Kate Su, David Trilling, Erick Young, Jane Morrison, Karl Gordon, and Karl Misselt

Summary: Spitzer provides the means to study the first stages of planet formation in some detail, and to connectthem with theories for the evolution of the early Solar System.Can all stars (excepting certain binaries) form terrestrial planets?

How are terrestrial planets built around other stars?

Can we use other systems to test theories for the formation ofthe Teerrestrial planets?*A Planets Rocky Road to Success:Spitzer Observations of Debris DisksG. H. Rieke, for the MIPS teammajor contributors are Chas Beichman, Geoff Bryden, Nadya Gorlova, Beth Holmes, James Muzerolle, John Stansberry, Karl Stapelfeldt, Kate Su, David Trilling, Erick Young, Jane Morrison, Karl Gordon, and Karl Misselt* in the solar system

Protoplanetary Disksare the residual interstellar gas and dust left from the formation of a star.

HH 30HST has imaged some famous protoplanetary andtransitional circumstellar disks. Still, our understanding of themis generally limited to a few examples.

HH 30Do all stars form with potentiallyplanet-forming disks?HST has imaged some famous protoplanetary andtransitional circumstellar disks. Still, our understanding of themis generally limited to a few examples.

Low resolutionspectra andeven photometrywith Spitzercan constrainthe structure ofprotoplanetary disks.

From Uchida et al. 2004,ApJS, 154, 439 Spectral Comparisons

TW Hya has amorphoussilicates and a dustclearing very close(roughly < 1 AU) to the star (noteabsence of excessemission at 4 - 7mm)

Hen 3-600 has crystallinesilicates, and morematerial very closeto the star.silicatesphotospherelarge grains

From Watson et al. 2004,ApJS, 154, 391Disk Clearing in Young Stars:Class I T Tau stars (i.e, with largefar infrared excess) showa large variety of clearingof the inner disks, includingCoKu Tau 4, thoroughly cleared to well beyond 1 AU. These stars are very young,probably < 1 Myr.

In some cases, spectrademonstrate very thoroughclearing at < 1 MYr

From Watson et al. 2004,ApJS, 154, 391Disk Clearing in Young Stars:Class I T Tau stars (i.e, with largefar infrared excess) showa large variety of clearingof the inner disks, includingCoKu Tau 4, thoroughly cleared to well beyond 1 AU. These stars are very young,probably < 1 Myr.

In some cases, spectrademonstrate very thoroughclearing at < 1 MYrHow common is this behavior?

NGC 7129:Embeddedcluster atan age of~ 1 Myr anddistance of~ 1 kpc.Spitzer PhotometricProbes of Protoplanetary Disks:From Muzerolle et al.2004, ApJS, 154, 379

Combined IRAC/MIPS survey of NGC 7129 showsa number of sources with small excesses out to8mm, but strong ones at 24mm. It appears thatprotoplanetary disk clearing has already occurred within an AU of these stars. Spitzer PhotometricProbes of Proto-planetary Disks:From Muzerolle et al.2004, ApJS, 154, 379

Disk clearing inNGC 2068/2071: ~ 0.5 MYr

dashed line - average T Taudotted line - photospheredot-dash line - TW HyaMuzerolle, private communication

Other approaches also indicate that someprotoplanetary disks clear very quickly. 16 + 8% of 0.5 Myr-old stars have no 3.6mm excess (Haisch et al. (2001))

In r Oph, we have selected 17 stars (from Bontemps et al. 2001) withluminosity > 1 Lsun and of class II or III. Six have negligibleexcesses between 2 and 14mm. These objects are probably~ 0.5 MYr old.

A CO (J = 2-1) survey of 12 weak-lined T Tau stars yielded, for 11 of them,upper limits of 6 X 10-7 Msun for the mass of gas in their circumstellar disks. A 1.3mm continuum survey set upper limits of 2 X 10-4 Msun to thedust mass in the disks (Duvert et al 2000)

It is estimated that the solar system required a disk mass of about0.01 Msun (Carpenter 2002)We are discovering that many of these systems clear quickly close-in to the star

Other approaches also indicate that someprotoplanetary disks clear very quickly. 16 + 8% of 0.5 Myr-old stars have no 3.6mm excess (Haisch et al. (2001))

In r Oph, we have selected 17 stars (from Bontemps et al. 2001) withluminosity > 1 Lsun and of class II or III. Six have negligibleexcesses between 2 and 14mm. These objects are probably~ 0.5 MYr old.

A CO (J = 2-1) survey of 12 weak-lined T Tau stars yielded, for 11 of them,upper limits of 6 X 10-7 Msun for the mass of gas in their circumstellar disks. A 1.3mm continuum survey set upper limits of 2 X 10-4 Msun to thedust mass in the disks (Duvert et al 2000)

It is estimated that the solar system required a disk mass of about0.01 Msun (Carpenter 2002)We are discovering that many of these systems clear quickly close-in to the starWhat happens in the ones thatretain disks? How do they form planets?

After the gas has cleared from the protoplanetary disk,terrestrial planet building continues through collisions of the planet embryos.Artists concept by Chris Butler

This stage may have eventually led to colossal collisions between large bodies, such as the one responsible for the formation of the moon.From Bill Hartmann, 1 hour after the collision.. four hours later

This accretion end game has only been accessible in computersimulations* - the traces are obviously largely erased in the solarsystem and out of reach around other stars.

Chambers, 2001, Icarus, 152, 205 Kenyon & Bromley 2004, ApJL, 602, L133* and space art

Kenyon & Bromley 2004Detailed numerical simulations suggest that the 20mm flux from asystem in the accretion end game will show spikes as a result of majorcollisions that throw debris into circumstellar space.Q - Q0 is the excess 20mm emission above the stellar photosphere, Q0.

Detailed numerical simulations suggest that the 20mm flux from asystem in the accretion end game will show spikes as a result of majorcollisions that throw debris into circumstellar space.Q - Q0 is the excess 20mm emission above the stellar photosphere, Q0.But is this picture correct? If so, how long does it last and what is the detailed behavior?

Detailed numerical simulations suggest that the 20mm flux from asystem in the accretion end game will show spikes as a result of majorcollisions that throw debris into circumstellar space.Q - Q0 is the excess 20mm emission above the stellar photosphere, Q0.But is this picture correct? If so, how long does it last and what is the detailed behavior?One test is to image nearby systems and look for evidence of large collisions.

Spitzer images of the nearest systems oftenshow a familiar structure.e Eri

850mm to the left (Greaves et al. 1998); 70mm to the right (MIPS)

Zodiacal cloudThe pattern is similar, but much elevated in brightness,to that in the solar system.

Fomalhaut

Vega

1.3-mmdataindicatesimilarstructureto Fomalhaut.

(Wilner et al. 2002,ApJL, 569, L115)high resolutionstarsmoothedsmoothed,star removedmodelcomparison with observation

Predicted Spitzer view of Vega system at 24mm(model from submm data of Wilner et al. 2002)Spitzer images of Vega should be nearly identical to Fomalhaut, but face-on.

ActualSpitzerImage at70mm(Su et al., in prep.)(upper left shows model image to same scale)

AlthoughVega andFomalhautare twinstars*, theirdebris systemslook completelydifferent!* They havesimilar masses, ages, distances,and spectralenergy distributions.

The Vega system is huge!~ 600 AU

For Vega, a constant color temperature with radius from 24 to 70mm indicates we are seeing small grains heated stochastically and being driven out of the system by photon pressure. Photon pressure is dominant for grains < 10mm in sizerun of color temp with radiusgrains in thermal equilibrium

Fits to the radialprofile at the three MIPS bands areconsistent witha radiation-pressure-driven wind, witha central holesimilar in sizeto the hole inthe ring seen inthe mm-wave.24mm

70mmFits to the radialprofile at the three MIPS bands areconsistent witha radiation-pressure-driven wind, witha central holesimilar in sizeto the hole inthe ring seen inthe mm-wave.

160mmFits to the radialprofile at the three MIPS bands areconsistent witha radiation-pressure-driven wind, witha central holesimilar in sizeto the hole inthe ring seen inthe mm-wave.

The infrared radiometric properties of the Vega systemare dominated by small grains (< 10mm) that have ashort lifetime within the system (~ 300 yr) and mayoriginate in the ring of larger grains seen in the mm-wave.

Another eccentric debris system: HD 69830 is a ~ 3 Gyr old K0V star. Its debris system excess is strong at 24mm (unusual for a star of this age) and non-existent at 70mm (even more unusual)

Subtracting the photosphere, it is apparent that the excess ispurely crystalline silicate grains of size 1mm or less (Beichman et al.)

This star poses a similar problem to Vega:such small grains have a short lifetime around the star. Assume that the small grains are maintained by a standard collisionalcascade. Then the grain size distribution is given by

n(a) da = C a-3.5 da (Dohnanyi 1968)

Integrating up to 100km parent bodies, the required mass is ~ 104times the mass actually seen in the radiating grains

The Poynting-Robertson loss time for this system and grain size is~ 105 years

To maintain the system in its current configuration for 3GYr wouldrequire ~ 10Mearth or more (if we assume the activity has tendedto decay over this time)

For Vega, the problem is similar to an order of magnitude worsebecause of the ~ 300 yr grain lifetime against photon pressure

The most likely explanation for both stars is thatthe debris systems have been greatly augmentedby some recent, major event.

The most likely explanation for both stars is thatthe debris systems have been greatly augmentedby some recent, major event.Do we see anything similar in other stars?

The SEDs of debris systems show a huge variety, consistentwith many of them being dominated by a single, recent event.from Rieke et al. 2005youngestintermediateoldest

Enough anecdotal examples! What is the overall pattern?

We now look at debris disk behavior on a statistical basis Use sample of 266 stars within a factor of 1.5 of 2.5 Msun

Take ages from cluster, moving group membership

Supplement with ages from HR diagram

Concentrate on 24mm excesses, since can detect photospheresat high SNR at this wavelength, so debris disk sample is complete.

Probes debris systems from ~ 5 to ~ 50 AU.Enough anecdotal examples! What is the overall pattern?

NGC 2547is a clusterof age ~ 25Myrand at adistance of ~ 450pc.(Young et al.2004, ApJS, 154, 428)

NGC 2547: 25 Myr old clusterand many other ~A stars have modest excess

M47 is a ~ 100 Myr old cluster

P1121has aK - [24] excessof ~ 3.7 magnitudes!It is a late F star.

(Gorlova et al.2004, ApJS, 154,448)

Many ~A starshave modest excesses.

Determining overall behavior requires a larger sample at 24mm:Sample of 266 stars on the HR Diagram (from Rieke et al., ApJ in press).

Ages have large uncertainties, but are adequate to examinedebris system behavior.

24mm excesses decay over ~ 200 Myr - in fact, many stars of all ages have no, or very little excess.

Upper envelope of the excesses goes as 1/t.

Pattern of Excesses with Stellar AgeLarge excesses decay from ~ 25% incidence to very low incidenceSmall excesses grow from ~ 50% incidence to ~ 80% or more

The pattern is consistent with ~ 30% of the starshaving essentially no 24mm excess when they emergefrom the protoplanetary disk state,

with the really major collisions generallyoccurring in the first 100 million years,

.and with moderate collisions persistingfor much, much longer.

This picture agrees very well with recent theoriesand discoveries in the solar system. Detailed numerical simulations of the evolution of the asteroidbelt show that its infrared output should still be dominated about10% of the time by recent collisions that break up largishasteroids and initiate collisional cascades to produce dust

Zodiacal bands discovered by IRAS appear to result from two recent events of this nature. It is hypothesized that there was an era of major collisions for100 - 200 million years (Chambers, 2001; 2003) or ~ 10 millionyears (Kenyon and Bromley 2004)

Asteroid belt evolution, Grogan et al. 1997

Zodiacal bands, Sykes, 1988

The band at 2.1o has beenassociated with the breakupof an asteroid 5.8Myr ago.It accounts for ~ 5% of thedust in the asteroid belt.-- Nesvorn et al. 2002

The band at 9o may be froma breakup ~ 8.3 Myr ago ofthe Veritas family precursor.It accounts for ~ 25% of thedust in the asteroid belt.-- Nesvorn et al. 2003Breakup times fordust band parentbodies can be estimatedby extrapolating the orbitsof fragment backwards.

Breakup times fordust band parentbodies can be estimatedby extrapolating the orbitsof fragment backwards. The band at 2.1o has beenassociated with the breakupof an asteroid 5.8Myr ago.It accounts for ~ 5% of thedust in the asteroid belt.-- Nesvorn et al. 2002

The band at 9o may be froma breakup ~ 8.3 Myr ago ofthe Veritas family precursor.It accounts for ~ 25% of thedust in the asteroid belt.-- Nesvorn et al. 2003From Bill Hartmann

Summary: Spitzer provides the means to study the first stages of planet formation in some detail, and to connectthem with theories for the evolution of the early Solar System.Many stars appear to lose the material in the terrestrial zone of theircircumstellar disks too early to make planets

Terrestrial planets are built around other stars in a series of violent collisions for ~ 200 million years, after which the systems settle down.

We can use other systems to test theories for the formation ofthe Teerrestrial planets.Summary