All of the scientific results that arise from HUT observations rely to a large extent on the calibration of the instrument (H01a), which means the ability to take the observed counts from the detector as a function of wavelength and convert them into physical units of measurement, such as the flux of energy received from the source per unit wavelength and per unit of time. The various components of HUT are carefully measured in the laboratory prior to launch to verify the expected performance on orbit. However, the integration of the payload and preparations for launch take considerable time and the sensitivity of various components is expected to change with time, and possibly with conditions on orbit. Hence, a method of calibration during the mission is required. Laboratory measurements again after the flight are then used to confirm the on-orbit measurements.
Hot white dwarf stars have long been used for this purpose for various UV instruments like Hubble and the IUE satellite. White dwarfs have relatively simple spectra, and theoretical calculations of their spectral shapes are thought to be the most reliable stellar models available. Hence, by observing certain white dwarfs that have the best existing information about their intrinsic fluxes, scientists can derive a conversion between the instruments performance on orbit and the "real world." This conversion is then used to "calibrate" observations of other objects. (Note: there are many other steps or aspects of calibration that are not discussed here.)
Of course, as the endpoint of stellar evolution for stars of a few solar masses or less, white dwarf stars are of interest scientifically in their own right. These unusual stars contain anywhere from about 0.5 to 1.2 solar masses (a few are known to be more massive, but 1.4 solar masses is the theoretical upper limit for a white dwarf), all packed in an object smaller than the size of the Earth.
White dwarf stars are thought to represent just the cores of their original stars. When red giant stars lose their outer layers and expose their cores, we see an expanding nebula (called a planetary nebula) with a blue star at the center. The nebula expands and disperses in a matter of thousands of years, leaving behind what is essentially a fledgling white dwarf. These young white dwarfs are the hottest known stars in the universe, with temperatures in excess of 150,000 K in some cases. These stars cool down quickly at first (by astronomical standards), so quickly that very few of them have been identified for study. Of the stars that are known, most show small optical brightness variations on regular time periods of 5 - 15 minutes or so that are indicative of pulsations; these are called PG1159 stars, after PG1159-053, the prototype of the class (program H01b). A few stars apparently do not seem to vary, meaning (presumably) that they do not pulsate. HUT will use its "photon counting" detector to make sensitive searches for pulsations in the far UV (where these stars are brightest) for comparison with optical data. HUT scientists will try and understand why some stars of this category pulsate and others do not, as well as comparing these stars with the older, cooler white dwarfs discussed above.
Program G12 will use high quality HUT observations of the hydrogen
Lyman absorption lines in white dwarf stars, which are all located
between 912 and 1216 angstroms in the HUT prime wavelength range,
to derive very accurate measurements of the temperatures and
surface gravities of the stars. HUT observations are planned for
white dwarfs over a large range of intrinsic temperatures and
surface gravities. Interestingly, this will not only improve our
knowledge of the white dwarf star properties, but will ultimately
lead to an improved calibration by providing better inputs to the
theoretical models of the stars used for this purpose.
