1 Introduction

Since its discovery by IRAS (e.g., Aumann 1984), the $\beta ~ {\rm Pic}$toris system has presented the prime example of a dusty disk around a main sequence star, partly because of its high degree of "dustiness'' ( $L_{\rm IR}/L_{\star} = 2.5\times 10^{-3}$, e.g. Lagrange et al. 2000) and partly because of its relatively close distance to the Earth (19.3 pc, Crifo et al. 1997), which makes it possible to obtain high quality data over the entire spectral range. Recent papers reviewing the physics of the disk around $\beta ~ {\rm Pic}$ include those of Artymowicz (2000), Lagrange et al. (2000) and Zuckerman (2001). Considerable uncertainty existed regarding the age of the system, but most recent estimates place the stellar age close to only ten million years ( 12⁺⁸_-4 Myr, Zuckerman et al. 2001). This could open up the possibility that planet formation (nearing its final phases?) might actually become observable.

Since these reviews were written, new relevant information has been added to our knowledge of the $\beta ~ {\rm Pic}$ system: Olofsson et al. (2001) reported the discovery of widespread atomic gas in the disk, a result recently confirmed and extended by Brandeker et al. (2002). These observations revealed the sense of disk rotation and that the northeast (NE) part of the gaseous disk is extending to the limit of the observations by Brandeker et al., viz. to at least 17 $^{\prime \prime }$ (330 AU) from the star. Several difficulties were encountered with these discoveries, such as the observed fact that the gas stays on (quasi-)Keplerian orbits, although radiation pressure forces in the resonance lines should accelerate the gas to high velocities and remove it on time scales comparable to the orbital period.

This needs to be addressed in the context of the origin and evolution of the gas and dust, i.e. whether one or both components are presently produced in situ in the disk or whether they (at least to some degree) constitute "left-overs'' from the star formation process, being of primordial origin. Takeuchi & Artymowicz (2001) have considered the interaction of gas and dust in a circumstellar disk, the dynamical evolution of which is critically dependent on the relative abundance of these species (see also Lecavelier des Etangs et al. 1998). Possible observational consequences, even relatively far from the central star, may become assessible with modern mm/submm cameras. With the aim to compare and to extend the results obtained at 850 $\mu$m with SCUBA by Holland et al. (1998), we performed imaging observations at longer wavelengths and, in this paper, we present the image of the $\beta ~ {\rm Pic}$ disk at 1200 $\mu$m (1.2 mm).

The SIMBA observations and the reduction of the data are presented in Sect. 2. Our basic result, i.e. the 1.2 mm image of $\beta ~ {\rm Pic}$ and its circumstellar disk, is found in Sect. 3, and in Sect. 4 the possible implications of these observations are discussed, where also other data are consulted. Finally, in Sect. 5, we briefly summarise our main conclusions from this work.

Figure 1: The normalised SIMBA 1.2 mm image with 8 $^{\prime \prime }$ pixels of the flux calibrator Uranus, the size of which was $3\hbox{$.\!\!^{\prime\prime}$ }5$ in diameter at the time of our observations and, hence, appeared point-like to SIMBA. Offsets in Right Ascension and Declination are in arcsec and the derived circular Gaussian beam of 25 $^{\prime \prime }$ FWHM is shown in the lower left corner. During the scanning alt-az observations, the image rotates which would smear out any low-level features. Contour levels as in Fig. 2.

Figure 2: The $\beta ~ {\rm Pic}$ disk imaged at 1.2 mm with a pixel sampling of 8 $^{\prime \prime }$. The lowest (dashed) contour corresponds to $2\sigma$ and increments are by $1 \sigma =3$ mJy/beam. The center coordinates (0, 0) refer to the stellar position and offsets are in arcsec. Features discussed in the text are marked and the optical disk midplane is shown by the straight line at position angle 31$.^$5. The circular Gaussian beam of 25 $^{\prime \prime }$ FWHM ($\sim$500 AU) is shown to scale in the lower left corner.