Spectroscopic Variations of the Be-shell Star EW Lac in the V/R Variation Periods

TL;DR

This study investigates the spectroscopic variations of the Be-shell star EW Lac during its V/R variation periods, revealing complex behaviors, wave phenomena, and disk structures through analysis of emission and absorption lines over multiple decades.

Contribution

It provides the first detailed spectroscopic analysis of EW Lac's V/R variations, identifying wave propagation, disk structure, and long-term behavior changes.

Findings

V/R variations differ among Balmer lines in timing and duration.

The 1976-1986 event showed retrograde wave and spiral disk structure.

Recent 2007 variation indicates a less developed disk with minimal lag.

Abstract

EW Lac has shown remarkable V/R variations in 1976 - 1986, and a similar V/R variation has started again in around 2007, after some quasi-periodic V/R variation. For the first V/R variation event, we analyze the spectroscopic behaviors of emission lines and shell absorption lines of the Balmer series. The V/R variations of the H_alpha through H_delta lines are characterized by the different manner of variations of the individual emission lines in time lag and in duration of the V/R phases. Weak correlation is also notable between the V/R variations and other variations of the line-profile parameter such as the peak velocities, emission line intensities and peak separations. We analyze shell absorption lines for higher members of the Balmer series for their central depths and radial velocities. The optical depth of the H_alpha line is in a range of 2000 to 6000, and its long-term variation discloses different behaviors as compared to the V/R variations. Combining the analyses of emission and shell absorption lines, and regarding the V/R variation as wave propagation phenomena, we find for the 1976 - 1986 event that the V/R variation is of retrograde structure and a spiral structure is likely formed inside the disk in the latter half of this event. The weak correlations among physical parameters are suggestive of the disk truncated at some radius. It is noticed that remarkable stellar brightening has occurred in the latter half of the event, accompanying marked decrease of emission line intensities. As for the V/R variation appeared in around 2007, which looks like a recurrence of the previous event, we found less developed state of the disk without appreciable time lag.