Formation and Evolution of Planetary Systems FEPS: Cold Outer Disks Associated with Sunlike Stars ht

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Formation and Evolution of Planetary Systems
(FEPS) Cold Outer Disks Associated with
Sun-like Starshttp//feps.as.arizona.edu/
J.Serena Kim (Steward, U. of Arizona), D. C.
Hines (Space Science Institute), D. E. Backman
(NASA Ames/SOFIA), L. A. Hillenbrand (CalTech),
M. R. Meyer (Steward), J. Rodmann (MPIA), A.
Moro-Martin (Princeton), J.M. Carpenter
(CalTech), M.D. Silverstone (Steward), J. Bouwman
(MPIA), E. E. Mamajek (CfA), S. Wolf (MPIA), R.
Malhotra (LPL, U of A), I. Pascucci (Steward), J.
Najita (NOAO), D. L. Padgett (SSC), T. Henning
(MPIA), T. Y. Brooke (CalTech), M. Cohen
(UC-Berkeley), S. E. Strom (NOAO), E. B. Stobie,
C. W. Engelbracht, K. D. Gordon, K. Misselt, J.
E. Morrison, J. Muzerolle, K. Y. L. Su (Steward)
Abstract We present the discovery of Kuiper Belt
(KB)-like debris systems around three Sun-like
stars based on observations with the Spitzer
Space Telescope as part of the legacy science
program, "The Formation and Evolution of
Planetary Systems (FEPS)". Two other debris
disks systems suggested by observations with ISO
and IRAS are also confirmed. All five stars show
infrared emission excess at 70um compared to
expected stellar photospheres while no excess
emission is detected at lt33um. These systems are
relatively old (0.7 - 3 Gyr) among the FEPS
targets. We performed simple blackbody modeling
for the spectral energy distributions of the
mid-infrared excess sources, and interpret these
systems as likely to possess debris belts
generated by planetesimal collisions similar to
those in our Kuiper-Belt. In this study we also
report observations of the relatively nearby
system HD 13974 (at 11 pc), that is consistent
with stellar photosphere model within the
uncertainties at 24um and 70um.
Spitzer Data IRAC (imaging at 3.6 ?m,
4.8 ?m, 8.0 ?m) IRS (spectroscopy at 5
?m - 35 ?m) - both Low and high resolution
MIPS (imaging at 24 ?m, 70 ?m, 160 ?m) MIPS
Data were reduced using MIPS_DAT pipeline (Gordon
et al. 2005), and the photometry were done using
IDP3 (Schneider Stobie 2002).
Sample 70mm excess sources discussed here were
selected from 10 of FEPS data, that were
publicly available since April 2005.
Figure 1.Flux density ratio of 24 ?m/Ks vs. 70
?m/Ks of 37 sources with MIPS 70 ?m detection
(pink filled circles). Small dots are not
detected in 70 ?m. We used 1 sigma upper limits
for the 70 ?m non detection. Note that HD 13974
has 70 ?m flux consistent with photosphere (Kim
et al. 2005).
Figure 3. Example 70 ?m mosaic images
70 ?m
Figure 2. Histogram of (observed - photospheric
70 ?m fluxes) divided by ? (random uncertainty).
Four excess sources (filled histogram) are easily
noticed in this histogram. Note that HD 8907
shows very strong excess flux at 70 ?m, which was
detected by IRAS and ISO. HD 6963, HD 122652, HD
145229, and HD 206374 discovered by FEPS program
are nearer to the rest of the stars without IR
excess (Kim et al. 2005).
5.2
Blackbody Model Blackbody models are based on
color temperatures of excess flux measured in IRS
and MIPS bands fluxes. The relation between
grain temperature, position, and primary stellar
luminosity (Backman Paresce 1993) is for
blackbody grains larger than the longest
wavelength of observation. Grain albedo is
assumed to be zero. Lack of data beyond the peak
of emission prevents useful characterization of
outer boundary (ROUT). Information from
mineralogical features can be used to help
characterizing grain size. The total radiating
masses can be considered lower limits, calculated
for single particle size and fixed grain density
(e.g., 10 um radius and density 2.5 g/cm3).
Here we use 10 ?m grain size, which is the
smallest grain size that emit efficiently at 70
?m.
Table 1. Source properties, blackbody modeling
results, Radiation blowout size (ablowout), and
P-R drag time scale
On-going analysis with more complete data
Summary (Kim et al. 2005)
1. The five excess sources have SEDs that are
consistent with photospheric models out to 33 ?m,
but show clear excesses at 70 ?m, which was the
selection criterion. We find that these stars
are all "old" (three sources are in our 1 - 3
Gyr age bin, while two are in the 0.3 - 1 Gyr
age bin). 2. As seen in Figure 1 2, the
improved sensitivity of Spitzer allows us to
detect debris disk systems that are much fainter
than those detected by IRAS and ISO. The overall
impression is that Kuiper-Belt (KB)-like systems
detectable by Spitzer and considered in this
paper are less massive and more distant than
systems detected with IRAS and ISO. 3. HD
13974 has a MIPS 70 ?m flux consistent with
photospheric emission within 1 ? total
uncertainty (including 20 absolute calibration
uncertainty). The upper limit of log(LIR/L) is lt
-5.2, similar to that of inferred for the solar
systems' KB. 4. Simple blackbody grain modeling
of our 5 excess SEDs yielded log(LIR/L) lt -4.5
-3.5, color temperatures between 55 - 58 K, and
inner radii of outer disks between 18 and 46
AU. 5. A solar system KB evolution model
predicts Spitzer 70 ?m fluxes (see Kim et al.
2005, Backman et al. 2006) from hypothetical
planetesimal assemblages around our target stars
that are within factors of 2 - 3 of the observed
fluxes. We infer that these systems have outer
remnant planetesimal belts that are consistent in
scale and starting masses to our KB. 6. The
absence of a disk around the 1 Gyr old star HD
13974 suggests that either this object does not
contain the parent bodies that produce
infrared-emitting debris, or perhaps the debris
has been cleared out already. 7. We placed upper
limits on warm dust masses interior to RIN for
each of these systems, and showed that the
depletion of the disk lt RIN is significant. We
commented on several possible causes for RIN. We
speculate that the RIN of exo-KBs presented in
this study could be explained by the existence of
one or more Jupiter mass planets at 10 - 20 AU
from each star.
Nexcess/Ntot ()
--- KB-like disk only
Figure 4. Spectral energy distributions (SEDs) of
HD 6963, HD 8907, HD 122652, HD 145229, HD
206374, and HD 13974. Blackbody model SEDs are
over-plotted with red dotted lines. (Kim et al.
2005)
log (age)
Figure 6.(preliminary) Fraction of 70 ?m excess
sources vs. age using nearly complete FEPS data
(320 sources). Note the peak is gt 1 Gyr old
age bin. Black solid line includes all sources
with 70?m excess, while blue line include ONLY
KB-like excess sources. Stay tuned for upcoming
paper!
Debris Disk Model (WH03) for HD 8907
Disk Radius (AU) lt0.1 0.3-1 1-10 30-100
Dusty Disk lifetimeWhile inner disks are
seen in mostly young sources (Silverstone et al.
2005, Bouwman et al. 2006 for hot and warm disks
in the FEPS sample), cold outer disks like our KB
are seen in old (1 - 4 Gyr) sun-like stars.
a 6um - 1mm Astronomical silicate Rin 42.5
AU log (LIR/L) -3.6 Md 0.02 Mearth
3-10 10-30
100-300
1 Gyr-4 Gyr
Age (Myr)
disk
star
References Backman, D. E. Paresce, F. 1993,
Protostars and Planets III. 1253 Backman et al.
2005, in preparation Bouwman et al. 2006, in
preparation Carpenter et al. 2005, ApJ, 129,
1049 Gordon et al. 2005, PASP, in
press Hillenbrand et al. 2005, in preparation Kim
et al. 2005, ApJ, 632, 659 Meyer, M.R. et al.
2004. ApJS, 154, 422 Schneider, G. Stobie, E.
2002, ADASS, 11, 382 (IDP3) Silverstone et al.
2005, ApJ, submitted Wolf, S. Hillenbrand, L.A.
2003, ApJ, 596, 603 (WH03)
Debris Disk Model Wolf Hillenbrand (2003)
Figure 5. Spectral energy distribution (SED) of
HD 8907. The model SED is a fit from detailed
modeling using WH03 models (see section 4.2 Kim
et al. 2005). Blue filled circles are IRAC data,
red filled circles are MIPS data, box points are
ISO fluxes, and filled upside-down-triangles are
3 ? upper limits of 2.9mm and 3.1mm (Carpenter et
al. 2005). The spectrum (green) is IRS spectrum.
10 - 20 AU
20-40 AU