Determining the magnitudes & spectral types of the components of the binary Mira star X Ophiuchi

2017 July 21

David Boyd

Several values for the magnitudes and spectral types of the components of the binary Mira-type variable star X Ophiuchi have been published in the literature over the last century.  Analysis of new photometry and spectroscopy of the star between 2016 May and 2016 Dec indicates that the V magnitude of the constant star is 9.0 and its spectral type K1III. The spectral type of the Mira changed from M6III at maximum to M7III as it faded and passed through minimum. The Mira’s V magnitude varied between 6.47 at maximum and 9.83 at minimum, a range of 3.36 magnitudes.

What do(n’t) we know about X Oph?
X Ophiuchi was first found to be variable by T.E. Espin observing at Darlington in 1886. In 1900 W.J. Hussey discovered the star to be a visual double using a power of 1000 on the 36-in refractor at Lick Observatory, and measured the separation as 0.22 arcsec. In the Annals of the Harvard College Observatory in 1907 Annie J. Cannon reported its magnitude range as 6.5 to 9.0, the period of its brightness variation as 335 days and its spectral type as Md on the Draper spectral classification, indicating the presence of Hg and Hd emission lines. G. Van Biesbroeck, observing at Yerkes Observatory in 1920-’24, deduced from brightness estimates that the more northerly component was the variable, a conclusion later independently confirmed by C.H. Gingrich at Mt Wilson Observatory. Van Biesbroeck visually estimated the magnitude of the southern constant star as 8.9 and the minimum magnitude reached by the variable component as 9.9, but in any case not fainter than 10.0.

In 1921 P.W. Merrill at Mt Wilson Observatory reported that the Hg and Hd emission lines peaked around maximum light and disappeared as the star faded. Two years later he reported the spectral type as M6e at maximum, becoming K0 at minimum as the constant component dominated. Spectral type M6e is an extension of the Draper classification scheme adopted by the IAU in 1922, with the letter e indicating the presence of emission lines. Merrill gave the visual magnitude of the constant star as 8.9 and the visual range of the variable as 6.8 to 12.  E. Pettit & S.B. Nicholson found a visual magnitude at minimum of 11.5 for the variable based on radiometric measurements, and a magnitude of 8.9 for its constant companion.

In a comprehensive review of current knowledge about X Oph, J.D. Fernie reported an analysis of the relative intensities of spectral lines which gave the spectral type of the constant star as K1III. Based on several assumptions, he obtained a V magnitude for this star of 8.51 and a maximum V magnitude of the variable as 7.12. He also derived the distance to X Oph as 240±35 pc and its colour excess as E(B-V)= 0.15.

Based on the MK (Morgan & Keenan) spectral classification system using relative intensities of specific absorption and emission features, P.C. Keenan reported a range of spectral types for X Oph between M6e and M8e+K. These observations covered the full magnitude range of X Oph and showed the spectral type becoming later as the star faded, with the M-type spectrum almost disappearing relative to the K-type spectrum at minimum light. (continued…)

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