A COLD DETACHED DUST ENVELOPE AROUND AN OXYGEN-RICH MIRA-TYPE AGB STAR R-HYDRAE

The circumstellar structure of R Hydrae has been investigated in the I RAS survey scan data. R Hya is a late M-type Mira variable star whose IRAS LRS spectrum is classified as 15 (no feature). But, it shows a re markable infrared excess in the IRAS photometric data, which cannot be explained by an oxygen-rich circumstellar dust envelope of a mass los ing late-type star without showing a significant 10 mu m silicate feat ure in the LRS spectrum. An extended emission component around R Hya i s clearly found in the 60 and 100 mu m images produced by Pyramid Maxi mum Entropy (PME) image reconstruction techniques. It is also visible in the individual IRAS scan data at 25, 60 and 100 mu m. It firmly ind icates the existence of an extended circumstellar dust envelope around R Hya. The IRAS data further suggest the dust envelope being detached from the central star. The detached circumstellar dust envelope is co nsistent with both the featureless LRS spectrum and the red colors in the IRAS photometric data. R Hya is likely to be experiencing a cessat ion of mass loss which has lasted for about one hundred years, and whi ch has made the inner boundary of the dust envelope about sixty stella r radii, far from the region which is warm enough to produce the 10 mu m feature.