The Science
Ionization
The DEUCE science centers on ionization and hot stars. Hot stars likely played a major role in reionization, and continue to provide ionizing photons enabling ionization upkeep in the modern Universe.
A summary of the data gap and model-data discrepancy for Epsilon CMa, a hot B star.
Lack of Observations
However, the ionization output of hot stars is not totally clear. No ionizing hot star has ever been reliably observed in the 700-900A bandpass that produces the ionizing photons most readily absorbed by neutral hydrogen, and thus most effective at ionization.
Neutral hydrogen is incredibly good at absorbing these wavelength photons, meaning that even a small amount of neutral hydrogen present between Earth and any nearby hot star would completely absorb any ionizing flux, making direct observation often impossible. All nearby O stars and almost all other stars have sightlines orders of magnitude too dense to allow for their direct observation from 700-900A.
Where we do have observations, we can compare theoretical predictions to the existing data, only to find that stellar models have not always done a great job at predicting the output of these hot stars at ionizing wavelengths. (See left)
The discrepancy between observations and theory fueling the "Photon Underproduction Crisis". See Kollmeier 2014, etc.
Missing Photons
And this is important, because as a whole we currently don't understand the situation concerning ionization in the modern Universe. Our current best models predict significantly fewer ionizing photons in the local Universe than we actually observe with instruments like the Hubble Space Telescope (e.g. Kollmeier 2014). This mismatch is termed the "Photon Underproduction Crisis", and it means we are missing photons-- some astronomical sources out there have to be producing more ionizing photons than we currently predict they do, explaining the discrepancy we see between reality and our best approximations of it.
The DEUCE payload begins to enter the picture with a proposition: maybe hot stars are supplying some of these photons, and maybe we don't understand their ionizing output as well as we think we do. O stars and B stars produce large amounts of ionizing photons, but have proven extremely difficult to observe at the wavelengths central to ionization upkeep.
It would be fantastic to have direct observations of the flux of hot stars at these highly-ionizing wavelengths. Such spectra would allow us to calibrate current stellar models and reduce the uncertainty in hot star ionizing outputs, allowing us to better comment on how well these hot stars influence the environment around them and enable modern ionization upkeep.
Epsilon and Beta CMa
The DEUCE payload centers around the fact that direct observation of ionizing flux from 700-900A is not impossible for ALL stars.
Epsilon and Beta CMa are two nearby, hot B stars that have exceptionally low neutral hydrogen column densities. The sightlines to Epsilon and Beta are orders of magnitude less dense than the average sightline in the Galaxy, allowing for direct observation of their ionizing fluxes.
The main DEUCE science goal is to take the first ever flux-calibrated observations of Epsilon and Beta CMa from 700-900A, helping bridge the data gap for these hot stars and allowing us to better understand the contribution of B stars to ionization in the modern Universe.
Epsilon and Beta are both B stars, which undoubtedly produce significantly less ionizing flux than hotter O stars. Thus, understanding their flux output is likely only contributing to a partial understanding of how hot stars influence ionization upkeep. However, B stars could potentially play a more significant role, considering that ionization is based on not just brightness but escape fraction as well.
Imagine a new generation of hot stars forming inside a cloud of neutral hydrogen. The bigger O stars burn brighter, producing significantly more ionizing photons than their neighboring B stars. However, the new cloud is filled with neutral hydrogen, preventing these ionizing photons from escaping their local environment into the Galaxy. The O stars eventually die off in massive supernova, clearing away much of this neutral hydrogen and blowing 'holes' or 'chimneys' into the interstellar medium. The path is clear-- but the original generation of O stars are dead and gone. Instead, the longer lived B stars now have a clear pathway with very high escape fraction, and will live for a much longer time, potentially allowing them to contribute in a significant way to the ionization of the space around them.
Now, that might be a fanciful scenario, or not how it actually happens, but regardless, it's a good illustration of why we should care about the fluxes of these hot stars, even if they are not as bright as the O stars around them. There's a lot we don't know, and pinning down these B star fluxes is the first step to even understanding what scenarios are reasonable or possible.
At the end of the day, there's an even more significant reason to take a look at Epsilon and Beta CMa. Even if their contribution is small in comparison to O stars, due to their sightline opacity Epsilon and Beta represent our only opportunity to better understand hot star ionizing flux outputs in the near future. If we want to improve our grasp of hot star physics, helping us understand how these hot stars are influencing the environment around them, this is a measurement that we want to make.