I have been reading some articles on Be stars and I found out an excellent review by Thomas Rivinius , Alex C. Carciofi & Christophe Martayan: Classical Be Stars, Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks. It was published in 2013.
I also liked the more recent article (2017) by Baader et al: Short-term variability and mass loss in Be stars - III - BRITE and SMEI satellite photometry of 28 Cygni - a very nice review of the non radial pulsations seen in this Be star.
Here are some of my notes taken from the article read - of course all mistakes would be mine! But this explains why I have such interest in Be stars: they are bright but still very mysterious, even if some veils were lifted in the past decade... :-)
Ten years after the excellent review by Porter & Rivinius published in 2003, the authors first clarifies the definition of Be stars: a non-supergiant B star whose spectrum has, or had at some time, one or
more Balmer lines in emission.
This is actually the first time that this definition is clarified as they list stars that are NOT Be stars while they could look like from the definition above:
-Herbig Ae/Be: young object with acretion disk, while classical Be stars have actually decretion disks;-mass transfering binaries: typical Algol or W UMa binaries sometimes show emission lines but this comes from the mass transfered from one star to the other one, different mechanism than classical Be stars;
-B[e] stars also show forbidden lines. There are different types of B[e] stars, some could say that thre is almost one category for each B[e] star! But their mechanism is also different than classical Be stars even if they could show a disk shape circum stellar material;
-magnetic B stars can sometimes show emission line. Circum stellar environment shows variability with exactly the same period as the photosphere, thus making them different to classical Be stars;
-Oe stars can sometimes be considered as the blue extension of Be stars but some spectropolarimetry observation tend to show otherwise;
-A & F shell stars could also be considered as the red extension of Be stars group within the HR diagram but some study show that they could be closer to Herbig Ae/Be stars thus they should not included in the classical Be stars group.
Of course, supergiant B stars such as P Cygni while considered originally as part of the same group were quickly look as different mechanism to eject mass (winds typically), are clearly not Be stars as the definition states.
Variability on all time scale is a clear characteristic of classical Be stars. One can see variations over several decades, with for exemple period of quiescence and no emission at all is mixed with period with visible emission line. But variation of several days, hours or even minutes can also be seen, leading to a wide variety of astrohysical phenomena!
Be star spectra (typically Halpha) can lead to different measurements:
-EW (Equivalent Width) is a measurement of the line strength. Some times, emission can lead to tens of Angstroems of EW.
-V/R is the ratio, when the emission line is double peak, of the Violet peak intensity by the Red peak intensity. Sometimes the continuum is substracted, sometimes even the photospheric absorption profile. So if you do similar measurements, make sure to always detail the definition of your V & R measurements.
Usully variable within weeks to decade, short time variation are usually binarity driven and longuer ones linked to the disk itself.
Variations within few days are closer to the photosphere. Transportation of material from the star to the disk usually takes days or weeks at most. Stellar rotation and typical pulsations period fall into that range too.
Variation of less than half a day are often beta Cep pulsations modes, specially in early type Be stars.
Cyclic asymetry of the emission lines can also be linked to global waves over the disk, very similar to acretion disk phenomenon seen in young objects. But remember that for Be stars, disk are decretion ones.
Of course, Be stars are observed with other technics than spectroscopy, such as:
-(spectro)Polarimetry: light emitted from the star isn't polarized but when the light is scattered by the free electrons in the ionized disk, it becomes polarized. Spectropolarimetry is also a powerful tool to measure magnetifc field, even if those are very weak in massive stars such as Be stars.
-Interferometry had been developped in recent years and both stars and disk have been observed with this technics, only available for large and/or close stars.
-Spectroastrometry
-Photometry, specially space based high frequency precise photometry (BRITE, SMEI, CoRoT, Kelper...), leading to very fine study of the pulsations modes of the Be stars. Ground based photometry are also important as geometry changes in how we see the Be star system (in case of binarity) lead to variation of the absolute flux received to us.
Anyway, check out those two articles, they are very interesting to read:
https://arxiv.org/abs/1310.3962
