A high-resolution study of episodic mass loss from the carbon star TT Cygni

Abstract

CO radio line observations with the IRAM Plateau de Bure interferometer show that the carbon star TT Cyg is surrounded by a large (radius ~ 35arcsec or 2.7x10¹⁷ cm), geometrically thin (average width ~ 2farcs 5 or 1.9x10¹⁶ cm) shell of gas, which has a remarkable overall spherical symmetry (e.g., its radius varies by less than +/-3%). It expands with a velocity of ~ 12.6 km s^-1. The emitting gas is very evenly distributed in the shell when averaged over a solid angle of about 0.2 steradians. We estimate a molecular hydrogen density of ~ 250 cm^-3, a gas kinetic temperature of ~ 100 K, and a mass of ~ 0.007 M_sun for the shell if the medium is homogeneous. There is no evidence for matter immediately inside or outside the shell, nor is there any evidence for structure in the radial direction of the shell brightness distribution (it is essentialy perfectly fitted with Gaussians). The shell centre is displaced ~ 1farcs 7 (position angle ~ -20degr ) with respect to the star. We favour an interpretation of this displacement in terms of TT Cyg being a member of a binary system. We put forward several arguments for a shell medium that consists almost entirely of a large number of small (la 1arcsec ) clumps (in which case the density required to fit the observational data is much higher, ~ 10⁴ cm^-3, and the kinetic temperature is considerably lower, la 20 K). TT Cyg is presently losing mass at a modest rate, ~ 3x10^-8 M_sunpyr, and with a low expansion velocity, ~ 3.8 km s^-1. This is inferred from CO line emission from a region centred on the present position of the star. The systemic velocity is estimated, from both the centre and the shell emission, to be -27.3+/-0.1 km s^-1 in the LSR system. All quantitative results are obtained assuming the Hipparcos distance of 510 pc. These data strongly support that TT Cyg has recently ( ~ 7x10³ yr ago) gone through a period of drastically varying mass loss properties. We discuss briefly two scenarios: a short period (a few hundred years) of very intense mass loss (a rate in excess of 10^-5 M_sunpyr), and a related scenario with a more modest mass ejection and where most of the shell gas is swept-up from a previous, slower stellar wind. It is presently not possible to favour any of these two scenarios, but we suggest that in either case it is a coordinated mass ejection that caused the shell formation. The He-shell flash phenomenon in AGB-stars can provide this coordination, and it also fits the time scales involved.