Dusty Circumstellar Disks: From IRAS to Spitzer

• Collaborators:

• Joseph Rhee, Inseok Song (Gemini Observatory),

• Michael McElwain, Eric Becklin (UCLA)

• Alycia Weinberger (Carnegie Institution)

Why should one care about dusty debris disks?

In 1983 when IRAS first discovered dust particles orbiting Vega and many other main sequence stars, it was not clear whether these “Vega-like” stars were signposts for planetary systems or, rather, signified failed planetary systems. Now, it is evident that these dusty disks are associated with planets.

Solar system time scales and ages of young nearby stars

• Formation of Jupiter < 10 Myr• Formation of Earth’s core ~ 30 Myr• Era of heavy bombardment

in inner solar system ~ 600 Myr

Cha cluster 8 Myr• TW Hydrae Assoc. 8 Myr Pictoris moving group 12 Myr• Tucana/Horologium Assoc. 30 Myr• AB Dor moving group 70 Myr

Debris disk discoveries in the far-infrared: IRAS, ISO, Spitzer

• IRAS was an all-sky survey and was first. ISO and Spitzer that followed are pointed telescopes. In addition, it appears that the frequency of disks does not rise rapidly with decreasing dust mass. Thus, not withstanding their superior sensitivity, ISO did not and, so far, Spitzer has not added very many newly detected debris disks to those found by IRAS. New dusty systems found:

• IRAS ~170 ISO 22 Spitzer ~few dozen

Disk Imaging

Thermal emission at submillimeter wavelengths (with SCUBA at JCMT)and at mid-Infrared wavelengths (e.g. with Keck).

Reflected light at visual and near-IR wavelengths with HST (ACS & NICMOS) and with AO on large telescopes (Keck, VLT, Gemini).

HST ACS planet search

Hubble Space Telescope

JCMT SCUBA 450 micron map (Wyatt & Dent 2002)

HST Fomalhaut detection -- consistent with sub-mm maps

HST ACS planet search

Fomalhaut

F814W: 80 min., 17 May, 02 Aug, 27 Oct, 2004F606W: 45 min., 27 Oct. 200425 mas / pix, FWHM = 60 mas = 0.5 AU

Kalas, Graham & Clampin2005, Nature, Vol. 435, pp. 1067

• Semi-major axis: a =140.7± 1.8 AU• Semi-minor axis: b = 57.5 ± 0.7 AU• PA major axis: 156.0˚±0.3˚• Inclination: i = 65.9˚± 0.4˚• Projected Offset: 13.4 ± 1 AU• PA of offset: 156.0˚ ± 0.3˚• Deprojected Offset f = 15.3 AU• Eccentricity: e = f / a = 0.11

orbital period at 140 AU = 1200 yr

No inner clumps

AU Mic

From HST GO/10228; Kalas PI (in prep)

HR 4796A

Schneider et al 1999

18 Micron Image of HR 4796

TW Hya

Weinberger at al 2002

HD 181327

Pic Group Member (Schneider et al 2006, submitted to ApJ)

Finding new dusty systems

• Establishing evolutionary sequences requires large/clean samples of dusty systems of various ages, spectral types, association with binary systems where the secondary might be of stellar or planetary mass or both, etc.

• IRAS surveys for new dusty disks have been plagued by limited search spaces (stellar catalogs), false positives, poor knowledge of stellar ages, etc.

• Over 900 IR excess stars claimed in literature since 1983 (ROE debris disk database).

• > 50% false positives due to mis-identification (galaxy contamination, IS cirrus, etc.)- HD 43954 (M&B 1998)

• Need for a clean list of bona fide IR excess stars

• IRAS being the only IR all sky survey for next 4+ yrs until Astro-F

History/Motivation

HD 43954

Nearbygalaxy

Search Methods MS stars (68054) from Hipparcos Catalog

Hip MS X IRAS(60m detection)

Visual Check using GAIAMis-identification

Contamination (galaxies, ISM cirrus, etc.)

SED CheckBinaryPre-main sequence

Mv > 6.0(B-V) - 2.0Sp type ≥ B6 (B-V > -0.15)Distance ≤ 120 pc

PSC(|b| < 10º): 76, r ≤ 10”

FSC: 481, r ≤ 45” PSC(|b| > 10º): 65, r ≤ 45”

• ~170 IRAS Identified Hipparcos dwarfs

• ~40 new candidates

• Tstar, Tdust, , &

• Age estimate

– Zuckerman & Song (2004)

Bona Fide IR Excess Stars

Solar system time scales and ages of young nearby stars

• Formation of Jupiter < 10 Myr• Formation of Earth’s core ~ 30 Myr• Era of heavy bombardment

in inner solar system ~ 600 Myr

Cha cluster 8 Myr• TW Hydrae Assoc. 8 Myr Pictoris moving group 12 Myr• Tucana/Horologium Assoc. 30 Myr• AB Dor moving group 70 Myr

The age of dusty, nearby, G-type star HD207129?

• HD207129 is a good example of how uncertain stellar age estimates can be. In their ISO study of the evolution of dust abundances around main-sequence stars, Habing’s group estimated that HD207129 is older than the Sun, while Zuckerman & Webb estimated an age of only 40 Myr!

Mv

B-V

Galactic Space Motions

Group Name U V W (km/s)

• TW Hydrae -11 -18 -5

• Tucana/Hor -11 -21 0 Pictoris -11 -16 -9

• AB Doradus -8 -27 -14 Cha -12 -19 -10

Disk Mass and Semi-major axis (as a function of time)

• Probably the most interesting macroscopic properties of the dusty debris disks are their masses (M) and dimensions (semi-major axis = R).

• M = Na3 /3=N a2 / R2 (= LIR/Lbol)

/M= 1/a R2

How good a proxy for disk mass is the more easily measured

quantity “tau”?

• For a variety of reasons, total disk mass is best measured at submillimeter wavelengths. But tau, which is a measure of far-IR excess emission, is much easier to measure and has been determined for an order of magnitude more stars than has dust mass.

Kuiper Belt vs asteroid belt

• The dust at almost all Vega-like stars is sufficiently cold to be orbiting with semi-major axes of 50 AU or more from the central star. Thus, the debris disks are almost always to be considered (young) analogs of the Sun’s Kuiper Belt.

• Until the past year, among the 100+ main sequence stars with far-IR excess, only one example of warm dust signifying a potential asteroid belt analog had been reliably established – at the A-type star zeta Lep, of age a few 100 Myr (Jura & Chen). Tau ~10-4

• Absence of warm dust is true even for stars with ages as young as tens of Myr. Thus, dust in the terrestrial region dissipates very quickly.

In the past year, three more stars with warm dust in the terrestrial

region have been identified

With Spitzer, Beichman et al 2005 found an ~2 Gyr old K-type star (HD 69830) with tau ~10-4 and silicate emission features seen in the wavelength range accessible to IRS. (Note: excess emission at 25 micron was marginally detected by IRAS!)

From old IRAS data, we identified two solar-mass, adolescent stars -- a Pleiad and a field star (age >~few 100 Myr); Follow-up at Keck and at Gemini revealed a huge tau (4%) and evidence for micron-size crystalline and amorphous silicate particles.

Comparison of Tau in Sun’s zodiacal cloud and in analogous regions at 4 stars with IR excess emission first detected by IRAS

• Zodiacal dust: 10-7

• Zeta Lep: 2 x 10-4

• HD 69830: 2 x 10-4

• BD+20 307: 0.04

• Pleiad: 0.03

Zodiacal dust properties

• In our solar system, the typical zodiacal dust particle is 30-100 microns in size.

• In HD 69830 and BD+20 307, the strong silicate emission features indicate the dust particles are of micron size (due to a collisional cascade?).

• As a result, at these stars, PR lifetimes from <~1 AU, are only ~1000 years.

Era of heavy bombardment in early solar system

• Until ~600 Myr following the formation of the Sun, the bombardment rate in the early solar system was sporadically heavier than at present by factors up to 1000.

• At BD+20 307, which is ~1,000,000 times dustier than the present solar system, the current bombardment rate might be incredibly large!

Very recent collision of two planet-mass objects??

• To account for the estimated dust mass at BD+20 307, one must pulverize a 300 km diameter object (e.g., Davida, the 5th largest asteroid) into micron-size particles.

• Perhaps something analogous to the collision postulated to explain Earth’s moon has occurred within the past few 1000 years in a planetary system at BD+20 307.

• BD+20 307 is an excellent target for mid-IR interferometers and, perhaps, for radial-velocity planet searches.

Solar System Asteroids

• Total mass 2 1024 g (0.0003 Mass of the Earth)

• Ceres is largest with half of the total mass

• Other notables include Jura = 42113, three Stooges; Moe = 30439, Larry = 30440, Curly = 30441

• Will survive Sun’s evolution to a white dwarf because > 2 AU from the Sun

Zeta Lep: Another Asteroid Belt?

• A-type main-sequence star, Teff = 8500 K • L* = 14 L(sun)• LIR = 1.7 10-4 L*

• D = 21 pc, M = 2 M(sun)• 12th closest main-sequence A-type star• Upper limit to size of excess emitting region 6 AU• Grain temperature near 200 K

Fluxes from Zeta Lep

Asteroid Belt Around Zeta Lep

• Steady state: Poynting-Roberston drag balanced by dust production

• LIR ~ (dM/dt) c2

• Zeta Lep: dM/dt ~ 1010 g s-1

• Solar System zodiacal light: 3 106 g s-1

• If steady state then mass of asteroids around zeta Lep about 200 times mass of solar system’s asteroids

HST ACS planet search

Fomalhaut's Belt: Significance to Astronomy1. Fomalhaut's belt is the closest that has been resolved in scattered light.

2. Inclination 66˚ means that it can be studied around its entire circumference

3. Belt characteristics that are consistent with planet-mass objects orbiting Fomalhaut:

1) The belt center is offset from the stellar center by 15 AU ± 1 AU, demanding

apsidal alignment by a planet,

2) Disk edges are sharper on the inner boundary compared to the outer boundary

and consistent with our scattered light model that simulates a knife-edge inner

boundary and dynamical models of planet-disk interactions.

1. Age 200-300 Myr, this is one of the oldest debris disk seen in scattered light. It is probably

leaving the clean-up phase and progressing to a configuration similar to that of our solar

system.

2. Replace Beta Pictoris as the debris

disk Rosetta Stone?

• Astrophysical Mirror to our

Kuiper Belt?

Summary Questions:

1. Outer extent of the disk?

2. Color? Main belt vs. inner dust?

3. Width as a function of azimuth?

4. Azimuthal asymmetries?

5. Plausible companion properties?

6. Planet at large radii?

7. Exterior companion?

8. Co-moving blobs?

Contact Info:Kalas (at) astron.berkeley.edu

More information:http://www.disksite.com/

Reference:Kalas et al. 2005, Nature, Vol. 435, pp. 1067