Disk Loss and Disk Renewal Phases in Classical Be Stars I: Analysis of Long-Term Spectropolarimetric Data

TL;DR

This study provides a detailed spectropolarimetric analysis of disk loss and renewal in two classical Be stars, revealing inside-out clearing, episodic outbursts, and deviations from axisymmetry, advancing understanding of disk dynamics.

Contribution

It offers one of the most comprehensive spectropolarimetric observations of Be star disk transitions, highlighting the inside-out clearing process and episodic outbursts with implications for disk injection mechanisms.

Findings

Disk loss in 60 Cyg occurred over 1000 days, consistent with viscous disk models.

pi Aqr's disk loss was episodic, taking about 2440 days.

Minor outbursts suggest injection and dissipation of material blobs in the inner disk.

Abstract

(Abridged) Classical Be stars occasionally transition from having a gaseous circumstellar disk (''Be phase'') to a state in which all observational evidence for the presence of these disks disappears (''normal B-star phase''). We present one of the most comprehensive spectropolarimetric views to date of such a transition for two Be stars, pi Aquarii and 60 Cygni. 60 Cyg's disk loss episode was characterized by a monotonic decrease in emission strength over a time-scale of 1000 days, consistent with the viscous time-scale of the disk, assuming alpha is 0.14. pi Aqr's disk loss was episodic in nature and occurred over a time-scale of 2440 days. An observed time lag between the behavior of the polarization and H-alpha in both stars indicates the disk clearing proceeded in an ''inside-out'' manner. We determine the position angle of the intrinsic polarization to be 166.7 +/- 0.1 degrees for pi Aqr and 107.7 +/- 0.4 degrees for 60 Cyg, and model the observed polarization during the quiescent diskless phase of each star to determine the interstellar polarization along the line of sight. Minor outbursts observed during the quiescent phase of each star shared similar lifetimes as those previously reported for mu Cen, suggesting that the outbursts represent the injection and subsequent viscous dissipation of individual blobs of material into the inner circumstellar environments of these stars. We also observe deviations from the mean intrinsic polarization position angle during polarization outbursts in each star, indicating deviations from axisymmetry. We propose that these deviations might be indicative of the injection (and subsequent circularization) of new blobs into the inner disk, either in the plane of the bulk of the disk material or in a slightly inclined (non-coplanar) orbit.