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Spatially Resolved MidIR Observations of the Disk Surrounding Par 21 Mario van den Ancker
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European Southern Observatory, KarlSchwarzschildStrasse 2, D85748 Garching bei MЭnchen, Germany

1. Introduction The small group of YSOs known as FU Orionis stars provide striking evidence for the importance and irregularity of disk accretion during early stellar evolution. FU Orionis star were originally identified as a class of young stars with large (> 4 magnitudes) outbursts in optical light. All are surrounded by reflection nebulae. More recently it has been realized that the physical reason for such an FU Orionis outburst is that the accretion rate onto the central star changes, within a period of less than a month, from those commonly found around T Tauri stars into values of 103104 M yr1. Intriguingly, meteoritic evidence suggests that chondritic material has formed when our own proto solar nebula went through an episode of enhanced temperatures. The study of FU Orionis objects may therefore not only constitute a crucial phase in the evolution of protoplanetary disks, but may also be directly relevant for the condensation of the protoplanetary disk into solids. At a distance of ~ 400 pc, the object Parsamian 21 (HBC 687) is perhaps most wellknown for its prominent cometary reflection nebula. On the basis of polarimetric maps Draper et al. (1985, MNRAS 212, 1P) suggested the presence of a circumstellar disk. Although no outburst has been recorded for this object, Par 21 was identified as a FU Orionis source on the basis of P Cygni line profiles indicating mass loss and double absorption lines indicating disk rotation (Staude & Neckel 1992, ApJ 400, 556).
Fig. 3. VLT/NACO Hband (1.6 m) image of the immediate vicinity of Par 21 extracted from the ESO Science Archive. The cometary nebula of which the innermost core is depicted here extends over several arcminutes on the sky. Note the ``finger'' of faint emission extending to the SouthEast.

2. Observations
Diffractionlimited images in several filters between 9.0 and 18.7 m of the FU Orionis type object Parsamian 21 were obtained with the newly commissioned midinfrared imager/spectrometer VISIR on the VLT in July 2005. Par 21 is clearly resolved into an elongated structure with a size of a few hundred AU (Figs. 12). This morphology of the region in the midinfrared is quite distinct from that see in archive nearinfrared (1.62.2 m) observations, which are dominated by scattered light in the dusty envelope (Fig. 3).

3. Discussion and Conclusions
An elongated structure is clearly present in our new midinfrared images of Par 21. This structure appears much larger in the midinfrared (radius ~ 300 AU) than what may be expected for simple models for active accretion disks, which predict that the majority of the luminosity of a FU Orionis system is generated within the innermost few AU. The measured falloff of intensity with radius from the center (Fig. 2) can be described by a broken powerlaw, with I ~ R0.8 below 100 AU and I ~ R0.5 between 100 and 300 AU. The observed structure appears to have a sharp edge, with essentially no flux coming from the area beyond 300 AU. We conclude that most likely we are seeing evidence for the presence of a dusty torus surrounding the accretion disk of Par 21. If this structure is circularly symmetric, the axis ratio of the detected structure gives a disk inclination of 19° to the lineofsight and a position angle east of north of 42 degrees. Surprisingly, the observed structure is not oriented perpendicular to the polar axis of the large cometary nebula, but appears to have a relative position angle of ~40 degrees compared to this structure. We note however, that apart from the base of the cometary nebula, the archive nearIR data also show a small (~ 1") linear structure extending to the SouthEast (Fig. 3). This structure is oriented perpendicular to the dusty torus seen in our midinfrared images. We hypothesize that precesion may be responsible for this difference in system orientation on small and large spatial scales.

Fig. 1. VLT/VISIR image of the immediate vicinity of Par 21 in the PAH2 (11.3 m) band. An extended structure with a size of a few hundred Au is visible along the NESW axis.

Fig. 2. Falloff of the radially averaged intensity profile around Par 21 (yellow) with that of a PSF reference star (magenta curve). The ``bump'' that can be seen in the profile of the PSF between 0.2 and 0.5" is the first Airy ring of the diffraction pattern of the VLT.

Background Image: DSS image of the region surrounding Par21