EU Andromedae

Lore & Background

EU Andromedae was reported to be a carbon star by Oliver J. Lee et al. in 1947, based on objective prism observations from a Dearborn Observatory study of faint red stars. Years later, its variability was discovered by French amateur astronomer Roger Weber, who examined photographic plates taken with Giuliano Romano from May 1959 through October 1961. Weber announced the discovery in 1962, noting it was probably a long period variable but could not determine if it was semi-regular or Mira. It is number 149 in his catalog. There is disagreement in the literature about its variable star class, with some listing it as a slow irregular variable and others as a semi-regular variable.

Infrared observations show the presence of silicate grains, indicating an oxygen-rich circumstellar shell around the star—a combination known as a silicate star. Subsequently, a water maser was detected around this star, the first time around a carbon star, confirming the shell. The most recent observations suggest the maser originated in a circumstellar disc, seen nearly edge-on, around an unseen companion with a minimum mass of 0.5 M☉. Carbon dioxide has been detected for the first time in a silicate carbon star around EU Andromedae.

EU Andromedae is given as the standard star for the C-J5− spectral class. C-J spectral types are assigned to stars with strong isotopic bands of carbon molecules, defined as the ratio of 12C to 13C being less than four. A more complete spectral type includes the abundance indices C25 j3.5, which indicate the Swan band strength and the isotopic band ratio.

Reader's Guide

EU Andromedae holds significance as a rare silicate carbon star, combining carbon-rich stellar chemistry with an oxygen-rich circumstellar shell. Its detection of a water maser—the first around a carbon star—confirmed the presence of this shell and opened a new avenue for studying such hybrid objects. The maser's likely origin in a circumstellar disc around an unseen companion of at least 0.5 solar masses suggests a binary system, adding to its astrophysical interest. The discovery of carbon dioxide in its environment, also a first for a silicate carbon star, further underscores its unique chemistry. The star's disputed classification—whether it is a slow irregular or semi-regular variable—reflects ongoing uncertainty in the literature, and its role as the standard star for the C-J5− spectral class makes it a reference point for studies of carbon stars with low 12C/13C ratios. Its variability was identified by amateur astronomer Roger Weber from photographic plates, highlighting the contributions of non-professional observers to stellar astronomy.

Did You Know?

• EU Andromedae was the first carbon star around which a water maser was detected.
• The star is the standard for the C-J5− spectral class, defined by a 12C/13C ratio less than four.