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.^[3] It is not visible to the naked-eye, but can be seen with a small telescope, or binoculars.^[11] R Crateris is a double star; the variable star and its magnitude 9.9 F8V companion are separated by 65.4 arcseconds.^[12]

R Crateris

A visual band light curve for R Crateris, plotted from ASAS data.[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Crater
Right ascension	11h 00m 33.85257s[2]
Declination	−18° 19′ 29.5827″[2]
Apparent magnitude (V)	8.1 - 9.5[3]
Characteristics
Evolutionary stage	AGB[4]
Spectral type	M7/8III[5]
Variable type	SRb[6]
Astrometry
Radial velocity (Rv)	20.94±1.50[2] km/s
Proper motion (μ)	RA: −29.373±0.180[2] mas/yr Dec.: −2.499±0.172[2] mas/yr
Parallax (π)	4.7027±0.1528 mas[2]
Distance	690 ± 20 ly (213 ± 7 pc)
Details
Mass	1.91[7] M☉
Radius	633[8] R☉
Luminosity	8,151[8] L☉
Surface gravity (log g)	−0.86[8] cgs
Temperature	3,295[9] K
Other designations
HD 95384, HIP 53809, SAO 156389, IRC −20222, RAFGL 1450[10]
Database references
SIMBAD	data

Friedrich August Theodor Winnecke discovered that the star is variable in 1861. In 1907 it appeared with its variable star designation, R Crateris, in Annie Jump Cannon's Second Catalog of Variable Stars.^[13] 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.^[14]

R Crateris is an oxygen-rich asymptotic giant branch star, losing mass at a rate of 8×10⁻⁷ solar masses per year via a stellar wind.^[4] At large distances from the star, the wind is expanding into space at 11.7±0.3 km/s.^[15]

Near-infrared radiation from R Crateris was detected in the first Two-Micron Sky Survey, published in 1969.^[16] 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.^[17] 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.^[18] The total mass of the shell, including both dust and gas, is estimated to be about (6.4±2)×10⁻² solar masses.^[19] Infrared imaging of the innermost (sub-arcsecond) portion of the dust shell shows a bipolar structure.^[4][20]

In the early 1970s, maser emission from OH and H₂O was detected in R Crateris' circumstellar shell.^[21] SiO maser emission was detected in 1985.^[22] Thermal (non-maser) emission from CO was detected in 1986.^[23]

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 Crateris has developed a bipolar stellar wind.^[24]