2. EP Aqr
EP Aqr is an M8 star classified as an SRb variable with a period of 55 days (although this period is uncertain), at a distance of 135 parsec Knapp et al. 1998, Perryman et al. 1997. The light curve shows a variable amplitude.
Knapp et al. (1998) obtained high velocity resolution profiles of the CO(2-1) and CO(3-2) lines in this object, clearly revealing the presence of two components in the outflow, both centered at a radial velocity of -33 km/s. The first (broad) component has an outflow velocity of 10.8 1.0 km/s while the second (narrow) component has a velocity width of only 1.5 0.3 km/s. The mass loss rates derived from these observations are 2.3 10 and 1.7 10 respectively. The authors suggest that the narrow component may be due to the resumption of mass loss after it has been stopped by some change in the stellar properties. However, as the line profile of the narrow component has a gaussian rather than a parabolic profile, it is also possible that this star has a wide circumstellar disk; similar observations are analyzed by Jura & Kahane (1999).
Both the IRAS LRS and ISO/SWS spectra of EP Aqr clearly show silicate dust emission at 10 and 18 µm and a dust feature at 13 µm. Justtanont et al. (1998) showed that there is a correlation between the strength of the 13 µm dust feature in the spectra of O-rich AGB stars and the strength of the CO2 bands. The equivalent width of the 13 µm feature is the largest in EP Aqr. Many possible carriers for this feature have been proposed, but MgAl2O4 (spinel) seems to be the most convincing candidate Posch et al. 1999. The correlation of the strength of the 13 µm feature with the presence of CO2 suggests that the formation of both species is favoured by the same physical conditions.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000