Cover picture: Two images of the GG Tau circumbinary ring

 

Figure:
Two images of the GG Tau circumbinary ring: 5 years of improvement on the IRAM interferometer.
The blue, white and red contours show the iso-intensity levels observed at $\simeq$ 5.5, 6.5 (systemic velocity) and 7.5 km.s^-1 in the ¹³CO J=1-0 (left) and J=2-1 (right) lines. The background is a false colour image of the $\lambda$ 2.7-mm (left) and 1.3-mm (right) thermal dust emission. The synthetized beam is shown in the bottom left corner. The stars give the position of the binary. Located in a hole within the Taurus cloud complex, GG Tau is a TTauri binary of separation 0.26''. Interferometric observations of the ¹³CO and dust $\lambda$ 2.7-mm emissions ( left picture) revealed five years ago a large ( $R_{out}\sim$800 AU), massive ( $0.15 \mbox{M$_\odot$ }$) circumbinary disk of gas and dust, whose central R= 180 AU have been cleared up by tidal effects (Dutrey et al. 1994). The disk was shown to be in Keplerian rotation around a binary star of mass $\simeq 1.3 \mbox{M$_\odot$ }$. Subsequent near-IR observations, made at the CFHT with adaptive optics, confirmed the existence of the central hole and the value of inclination angle determined at mm wavelengths (Roddier et al. 1996). In the last 5 years, the sensitivity of the IRAM interferometer has increased by a factor of 3 at $\lambda$ 3-mm in 5 years, while access to the $\lambda$ 1-mm band and baseline extensions have increased the angular resolution by a factor of 3. The new IRAM observations ( right picture and Sec. 5) have a resolution of $\simeq 0.6''$. They further resolve the circumbinary disk in the ¹³CO(2-1) line and in the $\lambda$ 1.3-mm continuum and show that 80 % of the disk mass is concentrated in a narrow ring of width 80 AU. An extrapolation of this example to the gains in sensitivity and resolution that ALMA will provide shows it will be possible to study protoplanetray disks at the scale of planetary formation.