Speakers
Description
About 20% of the B-type stars rotates very rapidly. Among these are the classical Be stars, which eject material and build up gaseous circumstellar ‘decretion’ disks. Rotation plays a significant role in the structure and evolution of massive stars, yet in many aspects is poorly understood. One viable evolutionary pathway to acquire near-critical rotation is through binary interaction - namely mass and angular momentum transfer, during which the remaining core of the mass donor becomes a hot stripped star and the mass gainer spins up to become the Be star. Some models suggest the binary channel is dominant in creating rapid rotators, yet only about 15 rapidly rotating Be + stripped star binaries are known.
HD 92406 is the first late-type Be + stripped star binary, occupying a unique niche in the rapidly rotating binary population. HD 92406 is exceptionally valuable as there are two stellar eclipses and two disk occulations per binary orbit - this is the only known eclipsing binary that hosts a Be star. As flux from the stripped star passes through the disk, the local disk properties are encoded in the observed spectrum. This offers the unique possibility of probing the disk conditions with exquisite details.
Ongoing analysis of this system allowed us to determine some of its parameters with great precision. HD 92406 is composed of a 4.8 $\rm \pm \ 0.3 \ M_{\odot}$ primary Be star with temperature ranging from 12000 K (equator) to 18000 K (pole) and polar radius of $\rm 3.5 \ R_{\odot}$, confirming it as a late type Be star. The parameters of the secondary remains to be better constrained, but it is confirmed as a 0.7 $\rm \pm \ 0.2 \ M_{\odot}$ hot subdwarf with temperatures around 20000 -- 30000 K and radius $\rm R < 0.7 \ R_{\odot}$. From TESS photometry and NRES spectra, we were able to determine the orbital parameters as $\rm P = 32.1854 \ d$, $\rm i = 88 \pm 1 ^{\circ}$, $\rm K_1 = 15 \pm 3 \ km/s$ and $\rm K_2 = 100 \pm 5 \ km/s$.
For this contribution, we present the results of a imaging polarimetric campaign for HD 92406. The rich field of this star allowed us to determine the interstellar polarization as 0.71 $\pm$ 0.02 % @ 128.2 degrees. The intrinsic polarization is very low (average polarization of $\approx$ 0.085 %), consistent we the fact that late type Be stars possess tenuous disks. Despite these low levels, we were able to detect polarimetric modulations as a function of orbital phase. A weak modulation was observed during the disks occulations and a strong, fast variation was observed during the primary stellar eclipse, consistent with the expectation that, during the eclipse, the unpolarized flux is reduced, thus boosting the polarization level. The diagnostic potential of this data to constrain the disk parameters cannot be underestimated. Disk models calculated with the HDUST code will be presented.