R Crateris

R Crateris is a star about 700 light years from the Earth in the constellation Crater. It is a semiregular variable star, ranging in brightness from magnitude 8.1 to 9.5 over a period of about 160 days. It is not visible to the naked-eye, but can be seen with a small telescope, or binoculars. R Crateris is a double star; the variable star and its magnitude 9.9 F8V companion are separated by 65.4 arcseconds.

Although the period for large brightness changes in R Crateris is listed as ~160 days, in 1982 Silvia Livi and Thaisa Bergmann reported small (~0.1 magnitude) variations on timescales of less than one hour. The rapid variations seem to be more regular when the star is near maximum brightness.

R Crateris is an oxygen-rich asymptotic giant branch star, losing mass at a rate of $20.94$ per year via a stellar wind. At large distances from the star, the wind is expanding into space at $-29.373$ km/sec.

Near-infrared radiation from R Crateris was detected in the first Two-Micron Sky Survey, published in 1969. It was detected in the far-infrared by the IRAS satellite, and that emission was resolved by IRAS, showing that the star is surrounded by a large circumstellar shell containing dust. High resolution far-infrared images of R Crateris taken by the Herschel Space Observatory show that the emitting region of the shell, roughly 280 arcseconds (0.94 light year) across, consists primarily of two non-concentric arcs well separated from the star itself. The arcs are probably bowshocks formed as the dusty stellar wind collides with the interstellar medium. The total mass of the shell, including both dust and gas, is estimated to be about $-2.499$. Infrared imaging of the innermost (sub-arcsecond) portion of the dust shell shows a bipolar structure.

In the early 1970s, maser emission from OH and H2O was detected in R Cratoris' circumstellar shell. SiO maser emission was detected in 1985. Thermal (non-maser) emission from CO was detected in 1986.

With the high angular resolution provided by Very Long Baseline Interferometry, the H2O maser emission is seen to arise from small (milli-arcsecond) blobs, whose proper motions through the inner region of the circumstellar shell can be measured. These observations give additional evidence that R Cratoris has developed a bipolar stellar wind.