* Astronomy

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HD32297

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RE: HD 32297

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Title: Keck/NIRC2 Imaging of the Warped, Asymmetric Debris Disk around HD 32297
Authors: Thayne Currie, Timothy J. Rodigas, John Debes, Peter Plavchan, Marc Kuchner, Hannah Jang-Condell, David Wilner, Sean Andrews, Adam Kraus, Scott Dahm, Thomas Robitaille

We present Keck/NIRC2 K_{s} band high-contrast coronagraphic imaging of the luminous debris disk around the nearby, young A star HD 32297 resolved at a projected separation of r = 0.3-2.5" (~ 35-280 AU). The disk is highly warped to the north and exhibits a complex, "wavy" surface brightness profile interior to r ~ 110 AU, where the peaks/plateaus in the profiles are shifted between the NE and SW disk lobes. The SW side of the disk is 50--100% brighter at r = 35-80 AU, and the location of its peak brightness roughly coincides with the disk's mm emission peak. Spectral energy distribution modelling suggests that HD 32297 has at least two dust populations that may originate from two separate belts likely at different locations, possibly at distances coinciding with the surface brightness peaks. A disk model for a single dust belt including a phase function with two components and a 5-10 AU pericenter offset explains the disk's warped structure and reproduces some of the surface brightness profile's shape (e.g. the overall "wavy" profile, the SB peak/plateau shifts) but more poorly reproduces the disk's brightness asymmetry. Although there may be alternate explanations, agreement between the SW disk brightness peak and disk's peak mm emission is consistent with an overdensity of very small, sub-blowout-sized dust and large, 0.1-1 mm-sized grains at ~45 AU tracing the same parent population of planetesimals. New near-IR and submm observations may be able to clarify whether even more complex grain scattering properties or dynamical sculpting by an unseen planet are required to explain HD 32297's disk structure.

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Warped Debris Disks Around Stars Are Blowin' in the Wind
The dust-filled disks where new planets may be forming around other stars occasionally take on some difficult-to-understand shapes. Now, a team led by John Debes at NASA's Goddard Space Flight Centre in Greenbelt, Md., finds that a star's motion through interstellar gas can account for many of them.

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Title: CARMA Millimetre-Wave Aperture Synthesis Imaging of the HD 32297 Debris Disk
Authors: H. L. Maness, M. P. Fitzgerald, R. Paladini, P. Kalas, G. Duchene, James R. Graham
(Version v2)

We present the first detection and mapping of the HD 32297 debris disk at 1.3 mm with the Combined Array for Research in Millimetre-wave Astronomy (CARMA). With a sub-arcsecond beam, this detection represents the highest angular resolution (sub)mm debris disk observation made to date. Our model fits to the spectral energy distribution from the CARMA flux and new Spitzer MIPS photometry support the earlier suggestion that at least two, possibly three, distinct grain populations are traced by the current data. The observed millimetre map shows an asymmetry between the northeast and southwest disk lobes, suggesting large grains may be trapped in resonance with an unseen exoplanet. Alternatively, the observed morphology could result from the recent breakup of a massive planetesimal. A similar-scale asymmetry is also observed in scattered light but not in the mid-infrared. This contrast between asymmetry at short and long wavelengths and symmetry at intermediate wavelengths is in qualitative agreement with predictions of resonant debris disk models. With resolved observations in several bands spanning over three decades in wavelength, HD 32297 provides a unique testbed for theories of grain and planetary dynamics, and could potentially provide strong multi-wavelength evidence for an exoplanetary system.

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Title: A Ring of Warm Dust in the HD 32297 Debris Disk
Authors: Michael P. Fitzgerald, Paul G. Kalas, James R. Graham

We report the detection of a ring of warm dust in the edge-on disk surrounding HD 32297 with the Gemini-N/MICHELLE mid-infrared imager. Our N'-band image shows elongated structure consistent with the orientation of the scattered-light disk. The Fnu(11.2 um) = 49.9+/-2.1 mJy flux is significantly above the 28.2+/-0.6 mJy photosphere. Subtraction of the stellar point spread function reveals a bilobed structure with peaks 0.5"-0.6" from the star. An analysis of the stellar component of the SED suggests a spectral type later than A0, in contrast to commonly cited literature values. We fit three-dimensional, single-size grain models of an optically thin dust ring to our image and the SED using a Markov chain Monte Carlo algorithm in a Bayesian framework. The best-fit effective grain sizes are submicron, suggesting the same dust population is responsible for the bulk of the scattered light. The inner boundary of the warm dust is located 0.5"-0.7" (~65 AU) from the star, which is approximately cospatial with the outer boundary of the scattered-light asymmetry inward of 0.5". The addition of a separate component of larger, cooler grains that provide a portion of the 60 um flux improves both the fidelity of the model fit and consistency with the slopes of the scattered-light brightness profiles. Previous indirect estimates of the stellar age (~30 Myr) indicate the dust is composed of debris. The peak vertical optical depths in our models (~0.3-1 x 1e-2) imply that grain-grain collisions likely play a significant role in dust dynamics and evolution. Submicron grains can survive radiation pressure blow-out if they are icy and porous. Similarly, the inferred warm temperatures (130-200 K) suggest that ice sublimation may play a role in truncating the inner disk.

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