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Spitzer Observations

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The Spitzer Space Telescope: New Views of the CosmosASP Conference Series, Vol. XXX, 2005XXXSpitzer Observations of the Debris Disk Star ² EridaniM. Marengo1,T. Megeath1, G. Fazio1,D. Backman2, K. Stapelfeld3, M.Werner3, C. Beichman3, K. Su4, J. Stansberry4, G. Rieke4,Abstract. ² Eridani is one of the four debris disk stars discovered by IRAS(together with Vega, Fomalhaut and β Pictoris). The disk has been first resolvedin the sub-mm, and shows evidences for clumpy structures possibly associatedto low mass companions. Radial velocity data also suggest the presence of aplanetary companion orbiting at 3.4 AU. With an age of 730 Myr ² Eridanirepresents a particularly interesting object to study the evolution of debris disksand of the associated exoplanetary companions. As part of the Spitzer “FabulousFour” program, we have observed ² Eridani with all Spitzer instrument. Inparticular, the Multiband Imaging Photometer for Spitzer (MIPS) has beenused to image the disk at 24 and 70 µm, in order to study its infrared thermalemission. The InfraRed Array Camera (IRAC) has instead been used to imagethe star at 3.6, 4.5, 5.8 and 8.0 µm in search for substellar companions. Wepresent here the preliminary results of this analysis.1. MIPS observations of the ² Eridani debris diskThe ² Eridani SED is characterized by a strong infrared excess generated by itsdebris disk, which we have measured with MIPS. At 24 µm the disk is unresolved,but we detected a 12% excess emission above the expected photospheric flux,estimated from a Kurucz model of a main sequence star having the ² Eridanispectral type. The disk is resolved at 70 µm, and has a size R ∼ 34” (3σ detectionlevel), elongated in the N-S direction. The MIPS 70 µm flux is ∼ 1.5 Jy, wellabove the expected photospheric flux of ∼ 0.2 Jy. The S/N of the current MIPSimages does not allow the detection of the clumped structures imaged in thesub-mm by SCUBA, which suggest that the disk may have an asymmetric ringstructure with a central cavity of ∼ 65 AU in size. Dynamic models of thesystem suggest that this cavity and asymmetry may be the consequence of anunseen planetary companion orbiting the star at the distance of ∼ 40 AU.1Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge MA 021382NASA Ames Research Center, Moffett Field, CA 940353JPL, 4800 Oak Grove Dr., Pasadena CA 9111094University of Arizona, Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 8572112 Marengo et al.Figure 1. Radial profiles of the IRAC PSF at 3.6 (solid line), 4.5 (dashedline), 5.8 (dot-dashed line) and 8.0 µm (dotted line), scaled to match the² Eridani brightness. The solid horizontal lines indicate model magnitudes ofa substellar companion of 1, 5, 10 and 40 MJat 4.5 µm.2. IRAC search for substellar companionsAttempts to directly detect substellar companions of ² Eridani have so far pro-duced negative results. The main difficulty faced by these searches is the inabilityto suppress the light coming from the central star, many orders of magnitudestronger than the luminosity of the companions. Mid-IR wavelengths, where theemission from substellar companions peaks, offer a better chance for detection.The IRAC camera is particularly suited for this task thanks to its optical stabil-ity, high dynamic range and the choice of its photometric system. Its bandpassesat 3.6, 5.8 and 8.0 µm are located in spectral regions affected by heavy methane,ammonia and CO absorption bands, while its 4.5 µm band rests in a region rel-atively free of molecular absorption. Colors based on the IRAC photometricsystem are thus extremely effective in discriminating between substellar objectsand other sources. Even at IRAC wavelengths, however, the brightness of ² Eri-dani is formidable, as shown in Figure 1, which compares the radial profiles ofthe IRAC PSF rescaled to the ² Eridani flux with the predicted 4.5 µm fluxesof substellar companions of different masses. To deal with this complication, wehave developed special PSF subtraction techniques allowing to remove most ofthe light from the central star. By observing the star in multiple epochs and byconstructing a model PSF from the observation of several stars with differentbrightness we are capable to detect sources as faint as 14th magnitudes at 3.6and 4.5 µm outside a radius of ∼ 35” from the star, equivalent to 5 MJplanetsorbiting at 110 AU from ² Eridani. The analysis of the data in search of actualcompanion candidates is still in progress.Acknowledgments. This work is based on observations made with theSpitzer Space Telescope, operated by JPL/Caltech under NASA contract 1407.Support for this work was provided by NASA through Contract #1256790 issuedby

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