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Discovery of Extreme Asymmetry in the Debris Disk Surrounding HD 15115

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Discovery of Extreme Asymmetry in the Debris Disk Surrounding HD 15115 Submitted to ApJ Letters December 18, 2006; Accepted April 04, 2007 A Preprint typeset using LTEX style emulateapj v. 08/22/09 DISCOVERY OF EXTREME ASYMMETRY IN THE DEBRIS DISK SURROUNDING HD 15115 Paul Kalas1,2, Michael P. Fitzgerald1,2, James R. Graham1,2 Submitted to ApJ Letters December 18, 2006; Accepted April 04, 2007 ABSTRACT We report the first scattered light detection of a dusty debris disk surrounding the F2V star HD 15115 using the Hubble Space Telescope in the optical, and Keck adaptive optics in the near-infrared. The most remarkable property of the HD 15115 disk relative to other debris disks is its extreme length asymmetry. The east side of the disk is detected to ∼315 AU radius, whereas the west side of the disk has radius >550 AU. We find a blue optical to near-infrared scattered light color relative to the star that indicates grain scattering properties similar to the AU Mic debris disk. The existence of a large debris disk surrounding HD 15115 adds further evidence for membership in the β Pic moving group, which was previously argued based on kinematics alone. Here we hypothesize that the extreme disk asymmetry is due to dynamical perturbations from HIP 12545, an M star 0.5◦ (0.38 pc) east of HD 15115 that shares a common proper motion vector, heliocentric distance, galactic space velocity, and age. Subject headings: stars: individual(HD 15115) - circumstellar matter 1. INTRODUCTION is consistent with a single temperature dust belt at ∼35 Volume-limited, far-infrared surveys of the solar neigh- AU radius with an estimated dust mass of 0.047 M⊕ borhood suggest that ∼15% of main sequence stars have (Zuckerman & Song 2004; Williams & Andrews 2006). excess thermal emission indicative of circumstellar grains Recently, Mo´or et al. (2006) identified HD 15115 as a (Aumann 1985; Backman & Paresce 1993; Meyer et al. candidate for membership in the 12 Myr-old β Pic mov- 2007). Direct imaging of dust scattered light reveals the ing group (BPMG), based on new radial velocity mea- geometry of the grain population relative to the star, surements that resulted in galactic kinematics similar to which further elucidates the origin of dust. In some those of the BPMG. cases, a circumstellar nebulosity may be amorphous with 2. OBSERVATIONS & DATA ANALYSIS asymmetric striated features produced when stellar ra- diation pressure deflects interstellar dust (Kalas et al. We first detected the HD 15115 disk in scattered light 2002). In other cases, such as β Pictoris and Fomal- using the HST ACS High Resolution Camera (HRC) on 2006 July 17. We used the F606W broadband filter and haut, the geometry of dust is consistent with a circum- ′′ stellar disk or belt related to the formation of planetesi- the 1.8 diameter occulting spot to artificially eclipse the mals (Smith & Terrile 1984; Kalas et al. 2005). Though star. Three flatfielded frames of 700 seconds each from standard pipeline processing of the HST data archive larger bodies such as comets and asteroids are not di- rectly observed, they most likely exist as a reservoir for were median combined for cosmic ray rejection. The injecting fresh debris into the system over the lifetime point spread function was then subtracted iteratively us- ing five other stars of similar spectral type obtained from of the star. Furthermore, circumstellar debris disks dis- play significant structure and asymmetry that may be the HST archive. The relative intensity scaling between linked, in principle, to dynamical perturbations from images was iteratively adjusted until the residual image arXiv:0704.0645v1 [astro-ph] 4 Apr 2007 showed a mean radial profile equal to zero intensity per- a planetary system (Roques et al. 1994; Liou & Zook 1999; Moro-Mart´ın& Malhotra 2002). Unfortunately, pendicular to the circumstellar disk. The images were only ∼10% of stars with excess thermal emission have then corrected for geometric distortion, giving a 25 mas pixel−1 scale. detected scattered light disks due to the high contrast between the host star and the low surface brightness neb- The resulting optical images revealed a needle-like fea- ulosity at optical and near-infrared wavelengths. Fortu- ture projecting westward from the star to the edge of the field, but with almost no counterpart to the east. nately, the observational capabilities have improved in recent years due to instrument upgrades on the Hubble Given the high degree of asymmetry that could conceiv- Space Telescope (HST) and the implementation of adap- ably arise from instrumental scattering, we endeavored to confirm the disk using the Keck II telescope with tive optics (AO) on large, ground-based telescopes. AO on 2006 October 07 and 2007 January 26. Utilizing Here we show new scattered light images of a debris ′′ disk surrounding HD 15115, an F2 star at 45 pc (Table the near-infrared camera NIRC2, a 0.4 radius occulting spot and a 10 mas pixel scale, we confirmed the exis- 1), first reported as a source of thermal excess emission µ µ ′ by Silverstone (2000). The spectral energy distribution tence of the disk in J (1.2 m), H (1.6 m), and K (2.2 µm). PSF subtraction is accomplished by allowing the 1 Astronomy Department and Radio Astronomy Laboratory, 601 sky to rotate relative to the detector, thereby separat- Campbell Hall, Berkeley, CA 94720 ing the stellar PSF from the disk. The observing pro- 2 National Science Foundation Center for Adaptive Optics, Uni- cedure and data reduction procedure are fully described versity of California, Santa Cruz, CA 95064 in Fitzgerald et al. (2007). Due to poor observing condi- 2 Kalas, Fitzgerald, Graham. tions in October, including intermittent cirrus clouds, we show evidence for significant flattening inward toward used only the best fraction of data by visually selecting the star (Fig. 3). All four surface brightness profiles frames of relatively constant intensity and PSF sharp- are well-represented by a single power law decrease with ness. The resulting effective integration times are 450 s, radius. If there is an inner dust depletion, then it resides 980 s, and 600 s for J, H, and K′, respectively. Stan- within 40 AU radius. This constraint is consistent with dard star observations were obtained under similar, non- model fits of the spectral energy distribution that place photometric conditions and processed in a similar man- the dominant emitting dust component at ∼35 AU radius ner. In January 2007 we re-observed HD 15115 (1930 (Zuckerman & Song 2004; Williams & Andrews 2006). s cumulative integration time) and two standard stars The color of the disk may be estimated in the 2.0′′ − under photometric conditions from Keck using the same 3.3′′ region where the H-band and V-band data overlap − instrumentation with the H broadband filter. However, (Fig. 3). At face value, ΣV − ΣH ≈−0.6 mag arcsec 2 the observations were made after meridian transit and at 2′′ radius, increasing to −1.9 mag arcsec−2 at 3.3′′ the limited rotation of the sky relative to the instrumen- radius for the West disk extension. The east ansa has tal PSF causes disk emission to be included in the PSF similar blue scattering at 2′′ radius, but the V-band sur- estimate, resulting in disk self-subtraction at small radii. face brightness profile is steeper in the east than in the Our analysis of the 2007 January data therefore yields a west, giving a roughly constant blue color with increasing detection of the west ansa in the region 1.3′′ − 3.3′′ ra- radius in the east. dius. The photometry in this second epoch agrees well In a future paper we will present a detailed model of with that of the first epoch (on average, the 2007 Jan- dust scattering and thermal emission properties, which uary disk photometry is 0.13 mag arcsec−2 fainter than requires a more complicated treatment of the obvious 2006 October), suggesting that our frame selection tech- disk asymmetry. However, for isotropically scattering nique for the first epoch of cloudy conditions effectively grains in an edge-on disk, an analytic approach shows filtered out non-photometric data. that the grain number density distribution as a function 3. of radius within the disk midplane follows a power-law RESULTS with index equivalent to one minus the sky-projected ra- Fig. 1 shows the PSF-subtracted images of HD 15115 dial midplane power-law index. From the Keck data in with HST and with Keck. The west side of the disk in Fig. 3, we estimate that the disk number density dis- the optical HST data has PA = 278◦.5 ± 0.5 and is de- tribution decreases with disk radius as r−3 in the inner tected from the edge of the occulting spot at 1.5′′ (67 region up to ∼3.3′′ radius for both sides of the disk. At AU) radius to the edge of the field at 12.38′′ (554 AU) > 3.3′′ radius, the optical data show that this profile radius. The east midplane is detected as far as ∼7′′ (315 continues for the east extension, but that the disk num- AU) radius. At this radius the east midplane begins to ber density profiles flattens for the west extension, as intersect the outer portion of the coronagraph’s 3.0′′ oc- described in Fig. 3. A precise measurement of the color culting spot. Further east, past the spot and to the edge and polarization of the disk scattered light is necessary of the field, no nebulosity is detected 9.0′′ − 14.9′′ radius. to further constrain the grain size distribution, the corre- The appearance of the disk is more symmetric in the sponding scattering phase function and albedo, and the 2006 October Keck data, which show the disk between effect on the disk number density profile.
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