Great spectra and great questions. I think that as UVEX becomes popular you will see a lot more spectra like yours.
ALPY has a fixed (short) backfocus from the grism to the camera so there definitely is a limit on how deep in the NIR it can go. I am nowhere near that limit because I have a small (inexpensive?) ASI178MM Camera. Here's Gamma Cas.
For most chemists, I think the composite nature of these spectra is disorienting. On earth, we put a sample in a machine and obtain a spectrum. We put starlight from Beta Mon A in our telescope and we get a spectrum.
On earth, with our sample at equilibrium, it isn't possible to observe both an emission peak and an absorption trough at the same time, from the same transition (at the same wavelength). But look closer at H-Beta in Beta Mon A and we see an emission peak inside of a wider absorption peak. What is going on?
Even the continuum comes from different places. So the far UV continuum is closer to the core and the visible continuum comes from further out, where it's cooler. Above the visible photosphere the hydrogen is even cooler, so many more electrons are in n=2 level (and cause Balmer absorptions) than in the n=3 level (that cause Paschen absorptions). That's why Balmer absorptions are (usually) deeper in these B-type stars. In Be stars there is also hydrogen in the decretion disk. This hydrogen is moving much slower (has a lower velocity dispersion) than in the star, so this absorption peak is narrower. I think in Beta Mon A you can see this composite in some higher Balmer lines with wide absorption at the top and narrower absorption at the bottom.
From the UV photosphere there is a continuum of light including at 102.5 nm (Ly-ß). These photons could cause an electron to go from n=1 to n=3, but in a normal B star, closer to the star, where the H density is high enough for significant absorption, it's too hot and most H atoms are at n=2. Further out there are neutral H atoms, but the density is too low. Net result is that these Ly-ß photons come streaming out of B stars. However, when they hit the denser hydrogen in the cool Be disk, they excite electrons to n=3, which can decay to n=2 and produce H-α emission (by fluorescence). Other wavelengths (and mechanisms) can also cause H-α. Disk density and inclination angle also affect the observed H-α. More energetic photons are needed for the other emission lines in the Balmer series. Less energetic photons are needed for Paschen emission (these will be more easily absorbed by the star and thus cause less emission).
It just happens that oxygen, O I, has a transition at 102.6 nm, so O I in the disk could absorb Ly-ß and re-emit at 844.6 nm (by fluorescence). If a Be star has the disk density and inclination angle favorable for Ly-ß to produce H-α, it should also produce O I at 844.6 nm. Ly-ß could also cause emission at [O I] 630.0 nm and 634.6 nm (by fluorescence), but the density would have to be much lower to avoid collisions so this doesn't happen in Be disks.
At least this is my understanding so far.
Edited by Organic Astrochemist, 14 November 2020 - 05:34 PM.