Doucet, C. , Habart, E. , Lagage, P. O. , Pantin, E. , Pinte, C. , Duchêne, G. , and Ménard F.
HIGH RESOLUTION IMAGING OF DUST DISKS STRUCTURES AROUND HERBIG AE STARS WITH VISIR

HIGH RESOLUTION IMAGING OF DUST DISKS STRUCTURES AROUND HERBIG AE STARS WITH VISIR

Doucet, C. (1), Habart, E. (2), Lagage, P. O. (1), Pantin, E. (1), Pinte, C. (3), Duchêne, G. (3), and Ménard F. (3)
(1)CEA/Saclay DSM/DAPNIA/SAp, F-91191 Gif sur Yvette, France
(2)Institut d'Astrophysique Spatial (IAS), 91405 Orsay cedex, France
(3)Laboratoire d'Astrophysique - Observatoire de Grenoble BP53, F38041 Grenoble cedex 9, France

Circumstellar (CS) disks are ubiquitous around stars with intermediate ages around a few million years. They are a natural outcome of the star formation process, because of the need of angular momentum conservation during the collapse of the initial molecular core (Shu, F. H., & Adams, F. C., 1987, IAUS, 115, 417.). We consider normally that disks contain gas and dust. As the star evolved, the disk changes: the gas is dissipated and a variety of processes (collisions...) leads to the growth of dust grains and the formation of planets. In order to characterise planets'formation, it is interesting to understand the physics of the medium where they were born. Herbig Ae (HAe) stars represent a particularly interesting laboratory for studying disks evolution and planet formation since the planets are still in the building process or eventually just formed. HAe stars are believed to be the more massive analogues of T Tauri stars ($\sim$ 2-10 M$_{\odot}$). Although great progress had been made in modelling the disk struture with radiative transfer models able to reproduce the SEDs (Chiang, E. I., & Goldreich, P. 1997, ApJ, 490, 368; Natta, A., Prusti, T., Neri, R., et al. 2001, A&A, 371, 186; Dullemond, C.P., & Dominik, C., & Natta, A. 2001, ApJ, 560, 957), the structure of the disks is not uniquely constrained. Spatially resolved observations are also needed to constrain more the geometry of the disks.

The observations were performed using the ESO mid-infrared instrument VISIR installed on the Very Large Telescope (Paranal, Chili), a mid-IR (around 10 and 20 $\mu$m) camera and spectrometer. With a 8 m telescope, the Full Width Half Maximum due to diffraction gives a spatial resolution of 0.3" at 10 $\mu$m. As a result, we are able to resolve disks with a typical size of 100 AU around Herbig stars at a distance of 100 pc. Around 10 $\mu$m, the emission may be produced by non-equilibrium reprocessing of ultraviolet radiation by very large organic molecules (like PAH: polycyclic aromatic hydrocarbons particle). Such particles undergo transient heating : they do not reach thermal equilibrium with the radiation field, but absorb individual photons, experiencing a rapid increase in temperature, and slowly cool, re-radiating the absorbed energy at longer wavelengths. This radiation allows us to see much further since this grains could reach high temperature far away from the star.

Under the assumption of good seeing (0.5 arc second in the visible), the diffraction is the limitation at 10 $\mu$m on a 8 m telescope. Unfortunatly, during our exposures, we observe a seeing of 0.8 arc second wich restricts our resolution. Indeed, the source is moving on the detector of about 5 pixels (pfov = 0.075"/pixel). In order to get the best spatial resolution, we experiment a new imaging mode on the bright objects, the 'BURST' mode. The principe is to take rapid images (exposure time = 50 ms) shifted and added to correct from seeing effect and 'possible' bad positions of the chopper in order to reach the best spatial resolution.

So, we present high angular imaging and spectroscopic observations of a sample of Herbig stars, extracted from the lists of Thé et al.(1994, A&AS, 104, 315) and Malfait et al.(1998, A&AL, 332, 25). We have spatially resolved most of them and an example, HD97048, is shown in Fig.1. With both imagery and spectroscopy, the object is spatially resolved and is quite extended (around 1.5'') at 11.3 $\mu$m (feature of PAH). We can also resolve an asymmetry in the direction east/west due probably to a geometric effect of the flared disk. We will discuss the comparison of the observed spatial distribution of the 11.3 $\mu$m feature and the adjacent continuum emission with the predictions of a disk model that includes transiently heated small grains PAHs in addition to large grains in thermal equilibrium (Habart, E., Natta, A., Krugel, E., 2004, A&A, 427, 179).