A Planet’s Rocky Road to Success: Spitzer Observations of Debris Disks G. H. Rieke, for the MIPS team major 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 connect them with theories for the evolution of the early Solar System. A Planet’s Rocky Road to Success: Spitzer Observations of Debris Disks G. H. Rieke, for the MIPS team major 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 connect them 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 of the Tee rrestrial planets?* A Planet’s Rocky Road to Success: Spitzer Observations of Debris Disks G. H. Rieke, for the MIPS team major 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 Disks are the residual interstellar gas and dust left from the formation of a star.

HH 30 HST has imaged some famous protoplanetary and transitional circumstellar disks. Still, our understanding of them is generally limited to a few examples.

HH 30 Do all stars form with potentially planet-forming disks? HST has imaged some famous protoplanetary and transitional circumstellar disks. Still, our understanding of them is generally limited to a few examples.

Low resolution spectra and even photometry with Spitzer can constrain the structure of protoplanetary disks.

From Uchida et al. 2004, ApJS, 154, 439 Spectral Comparisons TW Hya has amorphous silicates and a dust clearing very close (roughly < 1 AU) to the star (note absence of excess emission at  m) Hen has crystalline silicates, and more material very close to the star. silicates photosphere large grains

From Watson et al. 2004, ApJS, 154, 391 Disk Clearing in Young Stars: Class I T Tau stars (i.e, with large far infrared excess) show a large variety of clearing of the inner disks, including CoKu Tau 4, thoroughly cleared to well beyond 1 AU. These stars are very young, probably < 1 Myr. In some cases, spectra demonstrate very thorough clearing at < 1 MYr

From Watson et al. 2004, ApJS, 154, 391 Disk Clearing in Young Stars: Class I T Tau stars (i.e, with large far infrared excess) show a large variety of clearing of the inner disks, including CoKu Tau 4, thoroughly cleared to well beyond 1 AU. These stars are very young, probably < 1 Myr. In some cases, spectra demonstrate very thorough clearing at < 1 MYr How common is this behavior?

NGC 7129: Embedded cluster at an age of ~ 1 Myr and distance of ~ 1 kpc. Spitzer Photometric Probes of Protoplanetary Disks: From Muzerolle et al. 2004, ApJS, 154, 379

Combined IRAC/MIPS survey of NGC 7129 shows a number of sources with small excesses out to 8  m, but strong ones at 24  m. It appears that protoplanetary disk clearing has already occurred within an AU of these stars. Spitzer Photometric Probes of Proto- planetary Disks: From Muzerolle et al. 2004, ApJS, 154, 379

Disk clearing in NGC 2068/2071: ~ 0.5 MYr dashed line - average T Tau dotted line - photosphere dot-dash line - TW Hya Muzerolle, private communication

Other approaches also indicate that some protoplanetary disks clear very quickly % of 0.5 Myr-old stars have no 3.6  m excess (Haisch et al. (2001)) In  Oph, we have selected 17 stars (from Bontemps et al. 2001) with luminosity > 1 L sun and of class II or III. Six have negligible excesses between 2 and 14  m. 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 M sun for the mass of gas in their circumstellar disks. A 1.3mm continuum survey set upper limits of 2 X M sun to the dust mass in the disks (Duvert et al 2000) It is estimated that the solar system required a disk mass of about 0.01 M sun (Carpenter 2002) We are discovering that many of these systems clear quickly close- in to the star

Other approaches also indicate that some protoplanetary disks clear very quickly % of 0.5 Myr-old stars have no 3.6  m excess (Haisch et al. (2001)) In  Oph, we have selected 17 stars (from Bontemps et al. 2001) with luminosity > 1 L sun and of class II or III. Six have negligible excesses between 2 and 14  m. 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 M sun for the mass of gas in their circumstellar disks. A 1.3mm continuum survey set upper limits of 2 X M sun to the dust mass in the disks (Duvert et al 2000) It is estimated that the solar system required a disk mass of about 0.01 M sun (Carpenter 2002) We are discovering that many of these systems clear quickly close- in to the star What happens in the ones that retain 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. Artist’s 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 computer simulations* - the traces are obviously largely erased in the solar system and out of reach around other stars. Chambers, 2001, Icarus, 152, 205 Kenyon & Bromley 2004, ApJL, 602, L133 * and space art

Kenyon & Bromley 2004 Log time(yr) Detailed numerical simulations suggest that the 20  m flux from a system in the accretion end game will show spikes as a result of major collisions that throw debris into circumstellar space. Q - Q 0 is the excess 20  m emission above the stellar photosphere, Q 0.

Log time(yr) Detailed numerical simulations suggest that the 20  m flux from a system in the accretion end game will show spikes as a result of major collisions that throw debris into circumstellar space. Q - Q 0 is the excess 20  m emission above the stellar photosphere, Q 0. But is this picture correct? If so, how long does it last and what is the detailed behavior?

Log time(yr) Detailed numerical simulations suggest that the 20  m flux from a system in the accretion end game will show spikes as a result of major collisions that throw debris into circumstellar space. Q - Q 0 is the excess 20  m emission above the stellar photosphere, Q 0. 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 often show a familiar structure.  Eri 850  m to the left (Greaves et al. 1998); 70  m to the right (MIPS)

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

Disk becomes more asymmetric from submm to infrared, then fills in at 24  m. The asymmetry might be a resonance maintained by a massive planet. The filling-in is due to PR drag or to other processes that create particles < 100 AU from the star - comets, asteroid collisions Stapelfeldt et al. (2004, ApJS, 154, 458) Fomalhaut

Vega 1.3-mm data indicate similar structure to Fomalhaut. (Wilner et al. 2002, ApJL, 569, L115) high resolution star smoothed smoothed, star removed model comparison with observation

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

Actual Spitzer Image at 70  m (Su et al., in prep.) (upper left shows model image to same scale)

Although Vega and Fomalhaut are “twin” stars*, their debris systems look completely different! * They have similar masses, ages, distances, and spectral energy distributions.

The Vega system is huge! ~ 600 AU

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

Fits to the radial profile at the three MIPS bands are consistent with a radiation- pressure- driven wind, with a central hole similar in size to the hole in the ring seen in the mm-wave. 24  m

70  m Fits to the radial profile at the three MIPS bands are consistent with a radiation- pressure- driven wind, with a central hole similar in size to the hole in the ring seen in the mm-wave.

160  m Fits to the radial profile at the three MIPS bands are consistent with a radiation- pressure- driven wind, with a central hole similar in size to the hole in the ring seen in the mm-wave.

The infrared radiometric properties of the Vega system are dominated by small grains (< 10  m) that have a short lifetime within the system (~ 300 yr) and may originate in the ring of larger grains seen in the mm-wave.

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

Subtracting the photosphere, it is apparent that the excess is purely crystalline silicate grains of size 1  m 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 collisional cascade. 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 ~ 10 4 times the mass actually seen in the radiating grains The Poynting-Robertson loss time for this system and grain size is ~ 10 5 years To maintain the system in its current configuration for 3GYr would require ~ 10M earth or more (if we assume the activity has tended to decay over this time) For Vega, the problem is similar to an order of magnitude worse because of the ~ 300 yr grain lifetime against photon pressure

The most likely explanation for both stars is that the debris systems have been greatly augmented by some recent, major event.

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

The SEDs of debris systems show a huge variety, consistent with many of them being dominated by a single, recent event. from Rieke et al youngest intermediate oldest

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 M sun Take ages from cluster, moving group membership Supplement with ages from HR diagram Concentrate on 24  m excesses, since can detect photospheres at 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 2547 is a cluster of age ~ 25Myr and at a distance of ~ 450pc. (Young et al. 2004, ApJS, 154, 428)

Main sequence Decreasing mass one relatively low mass cluster member has a very large 24  m excess. NGC 2547: 25 Myr old cluster and many other ~A stars have modest excess

M47 is a ~ 100 Myr old cluster

P1121has a K - [24] excess of ~ 3.7 magnitudes! It is a late F star. (Gorlova et al. 2004, ApJS, 154, 448) Many ~A stars have modest excesses.

4 M sun 1.5 M sun 400 MYr 200 MYr 800 MYr age from HRD age from cluster, moving groups Determining overall behavior requires a larger sample at 24  m: Sample of 266 stars on the HR Diagram (from Rieke et al., ApJ in press).

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

24  m 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 Age Large excesses decay from ~ 25% incidence to very low incidence Small excesses grow from ~ 50% incidence to ~ 80% or more

The pattern is consistent with ~ 30% of the stars having essentially no 24  m excess when they emerge from the protoplanetary disk state,

…with the really major collisions generally occurring in the first 100 million years,

….and with “moderate” collisions persisting for much, much longer.

This picture agrees very well with recent theories and discoveries in the solar system. Detailed numerical simulations of the evolution of the asteroid belt show that its infrared output should still be dominated about 10% of the time by recent collisions that break up largish asteroids 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 for million years (Chambers, 2001; 2003) or ~ 10 million years (Kenyon and Bromley 2004) Log time(yr)

Asteroid belt evolution, Grogan et al. 1997

Zodiacal bands, Sykes, 1988

The band at 2.1 o has been associated with the breakup of an asteroid 5.8Myr ago. It accounts for ~ 5% of the dust in the asteroid belt. -- Nesvorný et al The band at 9 o may be from a breakup ~ 8.3 Myr ago of the Veritas family precursor. It accounts for ~ 25% of the dust in the asteroid belt. -- Nesvorný et al Breakup times for dust band parent bodies can be estimated by extrapolating the orbits of fragment backwards.

Breakup times for dust band parent bodies can be estimated by extrapolating the orbits of fragment backwards. The band at 2.1 o has been associated with the breakup of an asteroid 5.8Myr ago. It accounts for ~ 5% of the dust in the asteroid belt. -- Nesvorný et al The band at 9 o may be from a breakup ~ 8.3 Myr ago of the Veritas family precursor. It accounts for ~ 25% of the dust in the asteroid belt. -- Nesvorný et al From Bill Hartmann

Summary: Spitzer provides the means to study the first stages of planet formation in some detail, and to connect them with theories for the evolution of the early Solar System. Many stars appear to lose the material in the terrestrial zone of their circumstellar 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 of the Tee rrestrial planets. Summary