Download 1 - University of Arizona

Spitzer Cycle-2 GO Proposal
1 Science Justification
OVERVIEW. We request 1.1 hours of Spitzer Director’s Discretionary (DD) time to
obtain IRAC and MIPS imaging photometry and IRS low resolution spectroscopy of a
newly discovered, nearly edge-on, debris disk around HD 32297. The disk, discovered in
our currently executing HST/NICMOS GO 10177 coronagraphic survey program,
extends at least 3.3" (400 AU) from the star along its major axis and has a 1.1m flux
density of F(1.1m) = 4.81 ± 0.57 mJy beyond 0.3". While providing crucial
information on the disk morphology, and limits on the scattering grain sizes, our scattered
light images alone cannot unambiguously inform on the properties of the dust in this
potentially planet-forming environment. Thermal emission from the disk was detected at
25 & 60 m with IRAS, but these measures alone permit only a very rudimentary thermal
model of the system, leaving many degeneracies, allowing a large range of particle sizes
and disk masses. No other IR photometry is available, and indeed this object has been
largely ignored in the past at all wavelengths. To further elucidate the physical
characteristics of the disk and its constituent grains, we propose to obtain photometry and
spectra from 3.6 – 160 m with Spitzer. This will allow us to develop a well-sampled
spectral energy distribution (SED) for both the star and the disk emission, including midIR spectral coverage sufficient to characterize mineralogical features that will place
strong constraints on the dust constituents. In addition, the large angular extent of the disk
makes it one of the few systems so far known that is large enough to spatially resolve in
thermal emission with Spitzer (at s < 24 m). Given the 1.1 m scattering fraction of
fdisk/fstar = 0.33% (from our NICMOS observations), an IRAS F(60 m) = 1.12 Jy and
25+60 m fractional dust emission excess luminosity of Lir/L* ~ 0.27%, this object is
easy to observe with Spitzer and will consume minimal resources. As one of only a small
handful of disks resolved in both thermal and scattered light, these observations will
provide a crucial laboratory for understanding disks and planet formation.
INTRODUCTION. After decades of concerted effort applied to understanding the
formation processes that gave birth to our solar system, only recently has it become
possible to discern the detailed morphology of circumstellar material that must eventually
form planets. Circumstellar debris disks are identified by the presence of (mid-to-far) IR
emission in excess of their stellar photospheres. This emission arises from ~0.5-100m
dust grains that are heated by the star. For A-K stars older than a few million years, this
dust must come primarily from collisional erosion of planetesimals (as opposed to being
primordial grains left over from stellar formation), since the timescale for grains to be
removed from the system by radiation pressure and Poynting-Robertson drag is very
short compared with the star’s age. These “debris disks” mark an important phase in
planet formation, and provide insight into the properties and evolution of similar
structures that still exist in our solar system: the asteroid and Kuiper Belts.
The spatial distribution of the dust in debris disks has come mostly from optical and nearIR images of light scattered by dust grains in the disks. The advent of high contrast
coronagraphic imaging, as implemented with the instruments aboard HST, has
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Spitzer Cycle-2 GO Proposal
dramatically enhanced our ability to image these disks. Even so, only a handful of
evolved dusty disks have thus far been imaged and spatially resolved in light scattered
from their constituent grains (e.g., Schneider et al. 1999; Weinberger et al. 1999; Ardila
et al. 2004; Krist et al. 2005; Kalas et al. 2005).
While debris disks have been readily identified in the IR by IRAS & ISO, the sensitivity
and angular resolution of these previous observatories limited our ability to characterize
the thermal emission in detail, or to resolve the thermally emitting dust. The
extraordinary sensitivity and improved spatial resolution of Spitzer over IRAS and ISO
now permits us to examine the morphology of even low emissivity disks in detail. In
addition, Spitzer spectrophotometry over a wide wavelength range provides spectral
information with sufficient resolution investigate the mineralogy of the dust grains.
Combining spatially resolved thermal and scattered light observations provides direct
measurement of the dust properties including the albedo and particle size. To date,
however, this capability has been applied only to four systems with disks previously seen
by dust-scattered light. Our discovery of the large nearly edge-on disk associated with
HD 32297 provides the fifth object that can be investigated with this technique.
The diameter of the HD 32997 disk is least 6.6", making it one of the few disks that can
be resolved by Spitzer. The images will allow us to directly compare the morphology of
the scattered light with the thermally emissive dust, while the spectra will assist us in
determining the grain composition. The overall SED from 3.6-160 m will further
constrain the disk morphology, especially the presence an inner hole and its inner radial
distance from the star. The IRAC images may reveal the disk in scattered light (especially
at 3.6m), and will also enable us to look for nearby low-mass companions (planets) that
may influence the structure of the disk. Below we briefly review the current paradigm in
debris disk investigations and how our proposed program will advance the field.
SPECTRAL ENERGY DISTRIBUTIONS (SED)s & IMAGING. Comprehending
disk structures and compositions, and how they interact with any planets forming within
or encircling them, is essential to determining planet formation processes. Mid- to far-IR
SEDs such as those observed by Spitzer, greatly inform on the possible nature of the
constituent grains in disk systems. However, grain properties and systemic geometries
cannot be independently determined from SED fitting alone, and thus the likely
evolutionary paths taken by such systems are not well determined. With spatially
resolved scattered light imagery and SEDs combined, these degeneracies can be broken
using simultaneous radiative transfer and light scattering models (e.g., Schneider et al
2003). The high spatial resolution of Spitzer (compared with previous IR space
telescopes) affords the added benefit of resolving the thermal emission as well. Indeed,
this capability has shown that objects having similar IR SEDs can have very different
morphologies (Fomalhaut: Stapelfeldt et al. 2004; Vega: Su et al. 2005). Importantly, at
least in the case of Fomalhaut, the scattered light and (cool) thermally emitting dust have
very similar morphology, thus allowing a good estimate of the grain albedos and sizes.
Together, the geometrical and physical properties of the disks (e.g., total masses, sizes,
morphologies, vertical height distributions) and of their constituent grains (e.g., the
particle size and density distributions, grain albedos) can be ascertained. With the disk
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Spitzer Cycle-2 GO Proposal
grain properties in individual systems elucidated, current theories of disk evolution and
planet formation scenarios can be tested directly. These results can also be used to infer
the structure of disks that are too small to be resolved by imaging at any wavelength.
HST/NICMOS IMAGING SURVEY. With HST/NICMOS we are currently conducting
a highly sensitive circumstellar disk imaging survey of a well-defined and carefully
selected sample of twenty-six main sequence main sequence stars, with large IR excesses
(found by IRAS & ISO), to probe the posited epochs of planetary system evolution, and to
provide this critically needed imagery (HST/GO 10177; Schneider, PI). Our resolved
images probe the circumstellar environments as close as 0.3" from the target stars and are
shedding light on the spatial distribution of the dust in these systems. In most cases, the
IR excess was determined by detection at only one wavelength band (typically 60m),
and therefore constructing anything more than a very crude SED is impossible. This in
turn severely limits our ability to develop anything but the simplest models of the disks,
which this survey is discovering. The superior sensitivity of Spitzer thus provides a
critical tool for measuring the SEDs over a wide wavelength range, which then provides
much tighter constraints on the disk structure.
HD 32297 — GEOMETRIC/PHOTOMETRIC PROPERITES. We have very
recently discovered a nearly edge-on, high surface brightness (SB), debris disk about the
A0V star HD 32297 (d = 112 ± 12 pc, Jmag = 7.69) seen in light scattered by the disk
grains (see Figure 1: Schneider, Silverstone & Hines 2005). The disk has a 1.1 m flux
density of 4.81 ± 0.57 mJy beyond a radius of 0.3" from the coronagraphically occulted
star, and extends to a distance of at least 3.3" (400 AU) along its major axis. The fraction
of 1.1 m starlight scattered by the disk, 0.0033 ± 0.0004, is comparable to its fractional
excess emission at 25 + 60 m of ~ 0.0027 as measured by IRAS. The disk appears to be
inclined by 10.5° ± 2.5° from an edge-on viewing geometry, with its major axis oriented
56.5° ± 1° eastward of north. Radial SB profiles along the disk major axis (Figure 2)
suggests azimuthal asymmetries in the dust distribution which might implicate the
existence of one or more (unseen) planetary mass companions.
EVIDENCE FOR PLANET(S)? The break in the HD 32297 radial SB profile in Figure
2 could arise from a change in the surface density of scattering grains, or differentiation
in their properties, with distance from the star. The latter cannot explain the NE/SW SB
profile asymmetries. In the cases of HR 4796A and HD 141569A the presence of their
M-star companions may explain the outer "truncation'' of their disks (e.g., see Clampin et
al. 2003). The explanations for more complex asymmetries may rest in disk/planet
dynamics. Evidence for planets in the previously imaged debris systems, and possibly
also in HD 141569A, has been offered given the various forms of anisotropies in their
disks. Azimuthal asymmetries could be explained by the presence of undetected planets
altering an otherwise azimuthally isotropic dust density distribution by gravitational
perturbations (e.g., Ozernoy et al. 2000). Such a mechanism might also be responsible for
the SB asymmetries in HD 32297's disk. While still represented by a very small sample,
the occurrence (and diversity) of azimuthal asymmetries in the circumstellar disks of Astar seems to be the rule rather than the exception. With IRAC imagery we will probe for
sub-stellar companions undetected by NICMOS.
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Spitzer Cycle-2 GO Proposal
2 Technical Plan
SPITZER OBSERVATIONS. We propose to obtain IRAC and MIPS photometry, and
low resolution IRAS spectroscopy, of HD 33297 and its debris disk. The IRAC and MIPS
photometry will (in conjunction with our HST and other measures) define the SED of the
disk above the stellar photosphere from the near to far IR. We note that given the spatial
extent of the disk of ~ 7", it would be resolvable in IRAC, though both the 1.1 m
scattering fraction (0.33% from NICMOS) and the IRAS 25 + 60 m fractional excess
emission (0.27%) suggest the contrast challenge might be problematic. If HD 33297 is
sufficiently young, however, IRAC imagery, might detect (or set an age-dependent mass
limit on) a thermally emissive giant planet companion (undetectable at 1.1 m with
NICMOS) if one exists. The NICMOS imaging data also offer no compositional
information on the HD 32297 disk grains. While defining the SED at mid spectral range,
IRS spectroscopy will also provide key compositional and mineralogical information on
the disk grains (e.g., PAHs, crystalline silicates, and other complex species one may
posit) and may also better constrain its age by the presence or absence of gas. The MIPS
images will provide the long wavelength SED and will resolve the disk at 24m and
possibly also at 70m if cold material extends beyond the scattered light disk.
The observations are detailed in the AORs provided with this DD request.
TIME ESTIMATION. We have used the SENS-PET and SPEC-PET software tools to
estimate integration times. For IRAC, we use high dynamic range mode with a 5 position
random dither pattern to mitigate against cosmic rays and image artifacts. With 12 sec
frame time, we achieve a 10s point-source sensitivity of 16 Jy, which will allow us to
detect a close companion as cool as ~ 800 K, ~ 1-10MJ if the star’s age is ~ 10 - 100
Myrs (see Schneider, Silverstone & Hines 2005). If detected, IRAC colors in conjunction
with a well-determined 1.1 m NIMOS flux density limit, will inform on the nature of a
putative companion. For MIPS, we use 3 sec DCEs for 24 and 70 m to avoid possible
saturation, but add enough cycles so that we can detect extended emission. Here we have
assumed that the IRAS-observed flux densities [F(25m) ~ 0.2 Jy, F(60m) ≈ 1.1 Jy] is
distributed evenly over 4 square arcsecond surface area of our scattered light disk image.
For the IRS, we have set the integration times in order to achieve at least a SNR on the
photosphere.
DDT REQUEST. We are requesting 1.1 hours of Spitzer time to observe HD 32297. As
all other currently known debris disks resolved in scattered light imagery are targets of
intense scrutiny by many Spitzer programs, HD 32297 is unique in that it has not yet been
observed by Spitzer. Furthermore, despite its substantial 60m emission (1.12 Jy), its
SED is very poorly defined, with a marginal detection at 25m, and no other IR
photometric data.
We are asking for time allocated from the DD pool, rather than waiting for the next GO
CP for the following reasons. As noted above, this object is rare in its large angular size
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Spitzer Cycle-2 GO Proposal
and is thus an important object for studying the spatial relationship between the scattered
and thermal emission. We further note, from SPOT, that the first opportunity to observe
HD 32297 following the next Spitzer GO call would come in September, 2006, (i.e., a
year and a quarter from now). Given the uncertainties in predicting the availability of
technical capabilities for any space missions (despite our great faith in Spitzer) we would
find it untenable to risk the loss of an opportunity to secure the observations we propose
given the imaging data now in hand and the modest amount of Spitzer time which would
be required. Moreover, we wish to obtain our proposed Spitzer observations sufficiently
ahead of the next HST submission deadline to enable us to prepare an informed
optical/near-IR follow-up program of imaging observations (possibly with coronagraphic
polarization observations as well as optical/near-IR multi-color imaging with ACS and
NICMOS) predicated on our Spitzer results. In the "normal" course of HST and Spitzer
GO cycles, the first opportunity for such HST follow-up to Spitzer observations would be
in July 2007 - June 2008. Given the longevity uncertainties for HST we feel obligated to
expedite the Spitzer/HST follow-up opportunities.
DATA ANALYSIS (Post-Pipeline). Co-I Hines will assume responsibility for the postBCD calibration and (re)-processing of the MIPS and IRS data. The raw DCEs will be
processed with the Data Analysis Tool (DAT) developed by the MIPS instrument team
(Gordon et al 2005) to obtain the maximum possible sensitivity. Co-I Silverstone will be
responsible for the photometric calibration of the IRAC disk and companion imaging
photometry, and with PI Schneider will assess instrumental systematics which might
impeed detectability at short wavelength. Photometry will be performed using the IDL p
IDP3 software (Schneider & Stobie 2002), developed by the NICMOS instrument science
team and optimized for Spitzer imaging data by the FEPS team.
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Spitzer Cycle-2 GO Proposal
3 Figures, Tables & References
Figure 1. The HD 32297 debris disk revealed with NICMOS PSF-subtracted coronagraphy. Linear (left)
and log(right) display stretches (peak = 11 mJy/sq.arcsec).
Figure 2. Radial surface brightness profiles along NE (black) and SW (red) "halves'' of the HD 32297 disk
major axis (gray region obscured by the NICMOS coronagraph). The readily noted asymmetry may be due
to the presence of a planetary perturber modifying the azimuthal distribution of circumstellar scattering
grains.
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Spitzer Cycle-2 GO Proposal
REFERENCES:
Ardila, D. R., et al., 2004, ApJ, 617, L147
Clampin, M., et al., 2003, AJ, 126, 385.
Gordon, K., et al, 2005, PASA, in press.
Kalas, P., & Jewitt D., 1995, AJ, 110, 794.
Krist, J., et al., 2005, AJ, 129, 1008.
Schneider, G., et al., 1999, ApJ, 513,L127
Schneider, G., et al, 2003, AJ, 125, 1467.
Schneider, G., & Stobie, E. , 2002, ASP Conf. Ser., 281, 382.
Schneider, Silverstone & Hines 2005, ApJ (Letters), in press1.
Weinberger, A. J., et al., 1999, ApJ, 525, L53
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Preprint available upon request: email to [email protected]
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Spitzer Cycle-2 GO Proposal
4 Brief Resume/Bibliography
Glenn Schneider (Ph.D. 1985, Univeristy of Florida, Gainesville); Associate
Astronomer and NICMOS Project Instrument Scientist at Steward Observatory,
University of Arizona; HST/NICMOS IDT and GTO team - Environments of Nearby
Stars Team Lead; HST Program PI: Solar Systems in Formation (GO 10177). 73 refereed
and contributed publications (http://nicmosis.as.arizona.edu:8000/Publications.html)
Dean C. Hines (Ph.D. 1994, University of Texas at Austin); Research Scientist at the
Space Science Institute; NICMOS/HST Instrument & Science Team; MIPS/Spitzer
Instrument and Science Team; FEPS Legacy Program Data Lead; 67 refereed
publications on quasars, polarimetry, infrared astronomy, and debris disks.
Murray D. Silverstone (Ph.D. 2000, UCLA); Research Associate, Univ. of Arizona;
FEPS IRAC Data Lead; Co-I on HST/NICMOS Coronagraphic Imaging Survey for
Debris Disks; former member of NICMOS Science Team; 10 publications on debris disks
and T-Tauri-transition disks.
SELECTED BIBLIOGRAPHY:
Schneider, G., et al., 1999, "NICMOS Imaging of the HR 4796A Circumstellar Disk",
ApJ, 513, L127
Schneider, G., et al., 2003, "NICMOS Coronagraphic Observations of the GM Aurigae
Circumstellar Disk", AJ, 125, 1467.
Schneider, G., et al., 2001, "NICMOS Coronagraphic Observations of 55 Cancri", AJ,
121, 525
Weinberger, A. J., Becklin, E. E., Schneider, G., Smith, B. A., Lowrance, P. J.,
Silverstone, M. D., and Zuckerman, B., 1999, "The Circumstellar Disk of HD 141569
Imaged with NICMOS", ApJ, 522, L53.
Weinberger, A. J., Becklin, E. E., Schneider, G., Chaing, E. I., Lowrance, P. J.,
Silverstone, M. D., Zuckerman, B., Hines, D. C., and Smith, B. A., 2002, "Infrared
Views of the TW Hya Disk", ApJ,566, 409.
Cotera, A. S., Whitney, B. A., Young, E., Wolff, M. J., Wood, K., Povich, M.,
Schneider, G., Rieke, M., Thompson, R., 2001, "High-resolution Near-Infrared Images
and Models of the Circumstellar Disk in HH 30", ApJ, 556, 958.
Weinberger, A. J., Becklin, E. E., Schneider, G., Smith, B. A., Lowrance, P. J.,
Silverstone, M. D., and Zuckerman, B., 1999, "The Circumstellar Disk of HD 141569
Imaged with NICMOS", ApJ, 522, L53.
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Spitzer Cycle-2 GO Proposal
5 Observation Summary Table
This is an example for the extragalactic First Look Survey Field. It should be tailored to
your proposed observations.
Target/
Field
Position
(J2000)
Flux Density/Surface
Brightness
AOT/
Bands
Integration
Time/Pixel
FLSIRACMain
17:18:00+59:30:00
(4 square degrees)
5uJy-5mJy
IRAC
-all
60 sec
FLSMIPSMain
17:18:00+59:30:00
(4 square degrees)
24um: 0.3-100mJy
70um: .03-1 Jy
160um: 0.1-1 Jy
MIPS
Scan
-all
80 sec
(medium
rate)
Estimated
AOR
Duration
5000
# of
AORs
8000
8
9
6 Status of Existing Observatory Programs
DDT Proposal PI Schneider is not currently PI of Technical Contact for any Spitzer
proposals.
7 Proprietary Period Modification
None.
8 Justification of Duplicate Observations
None.
9 Justification of Scheduling Constraints
None.
10 Data Analysis Funding Distribution
Uof A: PI Schneider & Co-I Silverstone (50%)
SSI: co-I Hines (50%)
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Spitzer Cycle-2 GO Proposal
11 Financial Contact Information
For UofA - Schneider and Silverstone:
Leslie P. Tolbert
Vice President for Research
University of Arizona
P.O. Box 3308
Tucson, AZ 85722-3308
(520) 626-6000
Fax: (520) 626-4137
email: sponsor @email.arizona.edu
For SSI - Hines:
Carl Wuth, Business Manager
720 974-5885
[email protected]
Space Science Institute
4750 Walnut Street, Suite 205
Boulder, CO 80301
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