Two Be stars in the Small Magellanic Cloud with B[e]-like properties

TL;DR

This study analyzes two variable blue stars in the Small Magellanic Cloud with B[e]-like features, revealing their circumstellar disks and variability, and introduces a new class called 'bgBe's' with potential links to sgB[e] stars.

Contribution

It presents detailed spectral and photometric analysis of two novel stars, proposing they are prototypes of a new star class, expanding understanding of circumstellar environments in such objects.

Findings

Both stars have similar circumstellar disks with optically thick, stratified structures.

One star shows blue absorption components indicating shock interactions.

The stars are not in close binaries, based on radial velocity stability.

Abstract

We present an analysis of UVES and MIKE (ground based) high resolution and Far Ultraviolet Spectroscopic Explorer spectra of two novel and photometrically variable bright blue stars in the SMC; OGLE004336.91-732637.7 and the periodically occulted star OGLE004633.76-731204.3. The light curves of these stars exhibit multiple frequencies, and their spectra are similar. The latter are dominated by absorption/emission features produced in a circumstellar (CS) envelope whereas photospheric features are barely visible and forbidden emission lines are not visible. Modeling of spectral features indicates similar physical conditions for the CS envelope in both stars. An optically thick, slowly and thermally stratified disk. OGLE004633.76-731204.3 is noteworthy in showing blue discrete absorption components ("BACs") in their spectra possibly indicating the shock interaction between high velocity and low velocity material. Optical spectra from two spectra separated by 5 years show little change in the radial velocity over this period. However because these observations happen to be made in roughly similar phases over their long period, these suggest only that the stars are not in close binaries. We interpret the occultations and additional photometric nearly periodic variability in OGLE004633.76-731204.3 as due to covering of the star by a density modulation in a sector of a quasi-Keplerian circumstellar disk. Altogether, we suggest that these stars are prototypes of a larger group of stars, which we dub the "bgBe's" and discuss their similarities and differences with respect to the well known sgB[e] variables. Although OGLE004336.91-732637.7 appears to be a member the open cluster NGC 242, the evolutionary context of these stars is unclear.