Download 1 - ESO

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
• Formation of Earth’s core
• Era of heavy bombardment
in inner solar system
•
•
•
•
•
 Cha cluster
TW Hydrae Assoc.
 Pictoris moving group
Tucana/Horologium Assoc.
AB Dor moving group
< 10 Myr
~ 30 Myr
~ 600 Myr
8 Myr
8 Myr
12 Myr
30 Myr
70 Myr
Debris disk discoveries in the farinfrared: 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
HST Fomalhaut detection -- consistent with sub-mm maps
Hubble Space Telescope
JCMT SCUBA 450 micron map (Wyatt & Dent 2002)
HST ACS planet search
Fomalhaut
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
• Semi-major axis:
• Semi-minor axis:
• PA major axis:
• Inclination:
• Projected Offset:
• PA of offset:
• Deprojected Offset
• Eccentricity:
a =140.7± 1.8 AU
b = 57.5 ± 0.7 AU
156.0˚±0.3˚
i = 65.9˚± 0.4˚
13.4 ± 1 AU
156.0˚ ± 0.3˚
f = 15.3 AU
e = f / a = 0.11
No inner clumps
orbital period at 140 AU = 1200 yr
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.
History/Motivation
• 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
HD 43954
Nearby
galaxy
Search Methods
MS stars (68054) from
Hipparcos Catalog
Mv > 6.0(B-V) - 2.0
Sp type ≥ B6 (B-V > -0.15)

Distance
Hip
≤ 120 pc
MS X IRAS(60m detection)
FSC: 481, r ≤ 45”
PSC(|b| > 10º): 65,
r ≤ 45”
PSC(|b| < 10º): 76, r ≤ 10”
Visual Check using GAIA
Mis-identification
Contamination (galaxies,
ISM cirrus, etc.)
SED Check
Binary
Pre-main sequence
Bona Fide IR Excess
Stars
• ~170 IRAS Identified
Hipparcos dwarfs
• ~40 new candidates
• Tstar, Tdust, , & 
• Age estimate
– Zuckerman & Song
Solar system time scales and
ages of young nearby stars
• Formation of Jupiter
• Formation of Earth’s core
• Era of heavy bombardment
in inner solar system
•
•
•
•
•
 Cha cluster
TW Hydrae Assoc.
 Pictoris moving group
Tucana/Horologium Assoc.
AB Dor moving group
< 10 Myr
~ 30 Myr
~ 600 Myr
8 Myr
8 Myr
12 Myr
30 Myr
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 mainsequence 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
•
•
•
•
•
TW Hydrae
Tucana/Hor
 Pictoris
AB Doradus
 Cha
U
V
W
(km/s)
-11 -18 -5
-11 -21 0
-11 -16 -9
-8 -27 -14
-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 = r N 4p a3 /3
  = N p a2 / 4p R2
(= LIR/Lbol)
  / M = 1/ r 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 micronsize 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:
Zeta Lep:
HD 69830:
BD+20 307:
Pleiad:
10-7
2 x 10-4
2 x 10-4
0.04
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 radialvelocity 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?
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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 Astronomy
1.
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.
4.
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.
5.
Replace Beta Pictoris as the debris
disk Rosetta Stone?
6.
Astrophysical Mirror to our
Kuiper Belt?
Summary
Questions:
1.
2.
3.
4.
5.
6.
7.
8.
Outer extent of the disk?
Color? Main belt vs. inner dust?
Width as a function of azimuth?
Azimuthal asymmetries?
Plausible companion properties?
Planet at large radii?
Exterior companion?
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