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Most Distant Symbiotic Star Pair Discovered

December 3, 2008

Figure 1. The observed spectrum of IC10 StSy-1 as a composition of the blue and red spectra obtained with GMOS-North. The spectra of the galactic symbiotic star Mira Hen 2-147 (from Munari and Zwitter 2002) is plotted for comparison. The spectrum of IC10 StSy-1 has been Gaussian filtered to reduce the noise and match in resolution that of Hen 2-147. The observations were made on October 11, 14 and 18, 2007 as part of the queue mode on GMOS-North using two different gratings: three exposures of 1,700 seconds with R400+G5305 (‘red’) grating and four exposures of 1,700 seconds with the B600+G5303 grating. Slit widths were 1 arcsecond. See high-resolution version of this figure here.

An international team led by Brazilian astronomer Denise Gonçalves used the Gemini Multi-Object Spectrograph (GMOS) on Gemini North to discover the most distant known symbiotic star. The star is part of a dwarf starburst galaxy in the Local Group called IC10 and is located about 750 kiloparsecs (about 2.5 million light-years) away.

A symbiotic star is a variable binary star in which one star has expanded its outer envelope, in the late stages of its evolution, and is shedding mass quickly on another hot star (often a white dwarf). Some time in the future, the pair will end catastrophically with the white dwarf detonating as a Type Ia supernova as more material is dumped on it by the red giant.

Symbiotic stars are critical in understanding Type Ia supernovae and the evolution of binary star systems. However, relatively few systems of this sort are known so finding them outside of our galaxy is an important step in developing a cohesive theory of how symbiotic stars interact over their lifetimes in different environments.

This particular star pair (IC10 StSy-1) shines at apparent magnitude V=24.62+/-0.02 and the cooler component matches that of M8III giants in our solar neighborhood. Its observed emission lines belong to the Balmer series, [SII], [NII] and [OIII] which suggests a low electronic density, negligible optical depth effects and a temperature (Teff) for the ionizing source of between 35,000 and 90,000 K. The spectrum of the new symbiotic star in the pair is an almost perfect copy of the well-known nearby symbiotic star Hen 2-147 and is also similar to the famous naked-eye star Mira in the constellation Cetus.

Unlike many other symbiotic stars, the nebular lines of IC10 StSy-1 show no evidence of high density ionized gas, but could originate in an extended, low density nebula. This would be similar to what is seen around many symbiotic Mira-type stars in our galaxy.

The paper is currently in-press for publication in the Monthly Notices of the Royal Astronomical Society. See astro-ph pre-print here.