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Debris Disks around Young Stars

Many extra-solar planets are known to date. It is thought that planets form from the accretion of dust and gas in dusty disks around young stars. However, the details of how these disks produce planets remain unknown.

The discovery of the Vega-phenomenon is considered one of the most important achievements of the IRAS mission. It was discovered that several main sequence (MS) stars, including the A0V star Vega, exhibited large mid- and far-infrared excesses that could not be ascribed to pure photospheric emission (Aumann et al., 1984). Such excesses were attributed to a disc or ring of solid particles surrounding the stars, later termed `debris-disks'. Great interest has been taken in these objects because of their potential relevance to the formation of planetary systems.

Figure 1. Snapshots of the process leading to planet formation. These images are taken from computational modelling of the evolution of a planet forming disk (Wadsley et al., 2003).

Much work has been carried out in identifying main sequence debris disks (e.g Walker & Wolstencroft 1988; Mannings & Barlow 1998) and at UCL we have been active in measuring and modelling their spectral energy distributions from optical to mm wavelengths (Sylvester et al. 1996, MNRAS, 279, 915; 1997, MNRAS, 289, 831; Sylvester & Mannings 2000, MNRAS, 313, 73). However, when only spectrophotometric data are available, not all parameters can be uniquely determined, e.g. the grain-size distribution and the dust density profile are usually not separable.

Direct imaging of protoplanetary disks is crucial for our understanding of the processes leading to planet formation. Despite the efforts of many groups, instrumental capabilities to date have kept the number of directly imaged planet-forming disks to a very small numbers. Only a handful of systems have as yet had their disks resolved by direct imaging at optical, infrared or sub-mm wavelengths (Zuckerman 2001 for a review); some examples are shown in Figure 2.

Figure 2. Imaging of the dust disks for the systems whose SEDs are shown in Figure 3 (from left to right): Beta Pictoris, HR 4796, Epsilon Eridani and HD141569 (Kalas & Jewitt 2000; Schneider et al. 1999; Greaves et al. 1998, Mouillet at al. 2001).

Imaging Polarimetry provides a powerful technique for detecting dust-disks around bright pre-MS and MS stars. Since only the light from the disks is expected to be polarized, the bright central stars are automatically suppressed in polarized light images.

Near-Infrared Imaging Polarimetry

We have recently used imaging polarimetry with IRPOL/UIST on UKIRT (Hales et al., 2005, MNRAS, 365, 1348) to survey the circumstellar (CS) environments of ten late pre-MS and early-MS stars selected from the surveys of Sylvester et al. (1996, MNRAS, 279, 915) and Mannings & Barlow (1998, ApJ, 497, 330). Modelling of their spectral energy distributions (SEDs) had predicted angular sizes that could be resolvable at near-IR wavelengths with sub-arcsecond resolution (Sylvester & Skinner, 1996, MNRAS, 283, 457; Sylvester et al., 1997, MNRAS, 289, 831). For four of the ten targets, the spatial resolution that we achieved with the UKIRT observations (0.9 arcseconds) allowed us to partially resolve the scattered light from the disk away from the stellar Point Spread Function (PSF).

We successfuly detected the disk around the K7Ve star TW Hya. The 'butterfly' pattern seen in both the Q- and U- images (below) indicates the presence of an extended dust disk. This sinusoidal modulation with angle around the star (right-hand graph below) is detected from 0.4" out to 1.7" from the star, as previously reported by Apai et al. (2004) with the VLT.

Figure 3. Q- and U- polarization images of TW Hya at 1.25 microns (left). The right-hand panel shows the best sinusoidal fit to the Q-image.

Our data show that the JHK polarized intensity radial distributions follow a similar behaviour between 0.5-1.3" radius (below, left-hand panel), with the disk being significantly brighter at H- than it is at J- and K-. Radial dependences of the degree of polarization (P) obtained combining HST J- and H- coronographic imaging with our UKIRT imaging polarimetry have been derived and are shown in the right-hand panel. P(J) and P(K) are roughly constant between 0.8" and 1.2" radius, but P(J) falls-off steeply beyond 1.3".

Using the Monte Carlo scattering and polarization code of Whitney & Hartman (1992) we have produced a model fit (Fig. 4) that can successfully match our J-band data (crosses).

Figure 4. Monte-Carlo modelling of TW Hya's disk.

Our preliminary results confirm that the disk around TW Hya is quite massive (0.6 Msun), is seen face-on and extends up to 140 AU in radius. A flared disk-geometry was required in order to fit the slope seen in polarised intensity at J (above, right-hand panel, dots and solid line). For more information, see Hales et al., 2005.

Thanks to Antonio Hales for this article.

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