Rapid Periodic Spectroscopic Variation of the Hα Line of the Be Shell Star ο Andromedae

Volume 52 number 2 (2024)

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Harry R. Diz

Professor Emeritus, Department of Environmental Science and Engineering, Gannon University, Erie, PA; hrickdiz@gmail.com

Abstract

The variable star omicron Andromedae (ο And) is a Be shell star that has been reported to have rapid variations in its spectrum, including the Hα line at λ6562.85Å. This study provides quantitative documentation of that variation, and identifies periodic behavior over short time frames. The 332 spectra used in this study, unless otherwise attributed, were obtained by the author, an amateur astronomer using a 203-mm SCT telescope equipped with a 3D-printed spectroscope during the period from August 2023 through January 2024. Analysis of a light curve for ο And derived from TESS data yielded a period of 1.56 days, in close agreement with that reported in the literature, as well as a previously unreported period of 0.695 day that fits the data well. Additionally, spectra obtained from the BeSS database established that o And has a recent history of forming circumstellar disks that last about seven years before they dissipate. The current disk began to form about one year prior to the beginning of this study. The spectra of o And revealed continuous rapid changes in the Hα line during the course of individual nights as well as over a longer time frame. Being a shell star, the Hα emission line is complex, consisting of a double peak separated by a central depression. The relationship of the two peaks (wings) is expressed by the V/R ratio. Analysis using CLEANest revealed multiple periods for the variation of the features of the Hα line. The following rapid periods (less than a day) were found: for the central depression a period of 0.683 day, for the V wing a period of 0.49 day, for the R wing a period of 0.66 day, and for the V/R ratio a period of 0.92 day. On a slightly longer timescale, the period for the V wing was found to be 11.92 day and for the V/R ratio it was 12.04 day. The R wing occasionally exhibited a 1.95-day period but often deviated from this. A discussion of possible explanations for the observed variability and periodicity is presented.