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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 1.1m flux density of F(1.1m) = 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 ~0.5-100m 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 1 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 3.6m), 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 2 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 60m), 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. 3 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 24m and possibly also at 70m 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(25m) ~ 0.2 Jy, F(60m) ≈ 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 60m emission (1.12 Jy), its SED is very poorly defined, with a marginal detection at 25m, 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 4 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. 5 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. 6 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 1 Preprint available upon request: email to [email protected] 7 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. 8 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%) 9 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 10