Dusty%20Circumstellar%20Disks:%20From%20IRAS%20to%20Spitzer

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Dusty Circumstellar Disks From IRAS to Spitzer

• Collaborators
• Joseph Rhee, Inseok Song (Gemini Observatory),
• Michael McElwain, Eric Becklin (UCLA)
• Alycia Weinberger (Carnegie Institution)

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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.

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Solar system time scales and ages of young nearby
stars

• Formation of Jupiter lt 10 Myr
• Formation of Earths 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

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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

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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).

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HST ACS planet search
HST Fomalhaut detection -- consistent with sub-mm
maps
Hubble Space Telescope
JCMT SCUBA 450 micron map (Wyatt Dent 2002)
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HST ACS planet search
Fomalhaut

• Semi-major axis a 140.7 1.8 AU
• Semi-minor axis b 57.5 0.7 AU
• PA major axis 156.00.3
• 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

Kalas, Graham Clampin 2005, Nature, Vol. 435,
pp. 1067
F814W 80 min., 17 May, 02 Aug, 27 Oct,
2004 F606W 45 min., 27 Oct. 2004 25 mas / pix,
FWHM 60 mas 0.5 AU
No inner clumps
orbital period at 140 AU 1200 yr
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AU Mic
From HST GO/10228 Kalas PI (in prep)
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HR 4796A
Schneider et al 1999
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18 Micron Image of HR 4796
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TW Hya
Weinberger at al 2002
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HD 181327
b Pic Group Member (Schneider et al 2006,
submitted to ApJ)
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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.

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History/Motivation

• Over 900 IR excess stars claimed in literature
  since 1983 (ROE debris disk database).
• gt 50 false positives due to mis-identification
  (galaxy contamination, IS cirrus, etc.)
• - HD 43954 (MB 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

HD 43954
Nearby galaxy
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Search Methods

• MS stars (68054) from Hipparcos Catalog

• Mv gt 6.0(B-V) - 2.0

• Sp type B6 (B-V gt -0.15)
• Distance 120 pc

• Hip MS X IRAS(60?m detection)

• FSC 481, r 45
• PSC(b gt 10º) 65, r 45

• PSC(b lt 10º) 76, r 10

• Visual Check using GAIA
• Mis-identification
• Contamination (galaxies, ISM cirrus, etc.)

• SED Check
• Binary
• Pre-main sequence

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Bona Fide IR Excess Stars

• 170 IRAS Identified Hipparcos dwarfs
• 40 new candidates
• Tstar, Tdust, ?, ?
• Age estimate
• Zuckerman Song (2004)

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Solar system time scales and ages of young nearby
stars

• Formation of Jupiter lt 10 Myr
• Formation of Earths 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

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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, Habings group estimated
  that HD207129 is older than the Sun, while
  Zuckerman Webb estimated an age of only 40 Myr!

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Mv
B-V
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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

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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 r N 4p a3 /3
• t N p a2 / 4p R2 ( LIR/Lbol)
• t / M 1/ r a R2

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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.

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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 Suns 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.

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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 gtfew 100 Myr) Follow-up at Keck and at
  Gemini revealed a huge tau (4) and evidence for
  micron-size crystalline and amorphous silicate
  particles.

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Comparison of Tau in Suns 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
• BD20 307 0.04
• Pleiad 0.03

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Zodiacal dust properties

• In our solar system, the typical zodiacal dust
  particle is 30-100 microns in size.
• In HD 69830 and BD20 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
  lt1 AU, are only 1000 years.

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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 BD20 307, which is 1,000,000 times dustier
  than the present solar system, the current
  bombardment rate might be incredibly large!

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Very recent collision of two planet-mass objects??

• To account for the estimated dust mass at BD20
  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 Earths moon has occurred
  within the past few 1000 years in a planetary
  system at BD20 307.
• BD20 307 is an excellent target for mid-IR
  interferometers and, perhaps, for radial-velocity
  planet searches.

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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 Suns evolution to a white dwarf
  because gt 2 AU from the Sun

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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

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Fluxes from Zeta Lep
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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 systems
  asteroids

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HST ACS planet search
Fomalhaut's Belt Significance to Astronomy

• Fomalhaut's belt is the closest that has been
  resolved in scattered light.
• Inclination 66 means that it can be studied
  around its entire circumference
• 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.
• 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.
• Replace Beta Pictoris as the debris
• disk Rosetta Stone?
• Astrophysical Mirror to our
• Kuiper Belt?

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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 Mor
e information http//www.disksite.com/ Reference

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