Characterization of Unforeseen Chromospheres in Isolated White Dwarfs

0 Assigned ID: 2212 ----- Attempting Submission 1 (Fri Apr 09 20:30:41 GMT 2021) ----- LONG STIS MAMA SU LIKELY TO INTERSECT THE SAA LONG STIS MAMA SU LIKELY TO INTERSECT THE SAA This version contains an additional visit to compensate for failed Visit 01 (HOPR 92447) 1441198094 ----- Attempting Submission 1 (Mon Jul 19 15:38:38 GMT 2021) ----- ----- Attempting Submission 2 (Thu Feb 23 18:01:21 GMT 2023) ----- It could be possible to schedule the visits to avoid SAA passages, but it needs to be checked. Characterization of Unforeseen Chromospheres in Isolated White Dwarfs In a newly identified class of single, magnetic white dwarfs, an unknown energy transport mechanism powers active chromospheres, transforming their Balmer lines from absorption to emission. The temperature inversion mechanism defies conventional explanation, with the least-unlikely model invoking current loops through conducting exoplanets. However, these emission-line white dwarfs are closely clustered on the HR diagram, thus indicating an intrinsic process must be responsible, and that it occurs at a particular evolutionary stage. We propose spin-resolved ultraviolet spectroscopy of the class prototype to constrain the properties of its chromospherically active region. This is only possible with Hubble, where sensitive constraints on the temperature and emission profile of the emitting region require ultraviolet data. Optical data provide a precise rotation period and phase, with larger amplitudes toward shorter wavelengths, and the ultraviolet will further characterize the continuum, and line emission variability. Ultraviolet observations will distinguish between variability driven by the temperature or magnetic opacity of the emission region. Importantly, anti-phase behaviour is expected between the ultraviolet and optical in the case where spot temperature is the main driver of variability. These observations are crucial to our understanding of this evolutionary-activated chromospheric phenomenon, with implications for not only this whole class of stellar remnants, but for magnetic stars in general. The observational goals are two-fold. First, we will obtain a phase-resolved, low-resolution, FUV and NUV spectra covering two rotational cycles. This will be necessary to observe continuum variation with the rotational phase and obtain temperature information of the heated region. This will be done by probing the spectral slope of the star + spot system, and characterizing the excess due to the heated region. Second, we will re-bin the spectra to obtain a photometric proxy in these regimes. The TIME-TAG mode will be used to produce high-cadence light curves covering two rotational cycles. The amplitude of variations and relative phase information of these light curves would then be compared to the optical results, thus further constraining the nature of the emitting spot. Hence, the use of STIS MAMA detectors in the TIME-TAG mode would provide both photometry and spectroscopy simultaneously allowing us to characterise GD 356 in these regimes. Instrument Choice: We select STIS to obtain a low-resolution, FUV spectrum of GD 356. We must select G140L with TIME-TAG mode to help identify airglow. Using the ETC based on the GALEX NUV flux for the target (16.9 AB mag) and an 8000 K blackbody, it is predicted that S/N = 3.0 can be achieved in 1000 s per resolution element. Considering the resulting spectrum would be of R=1000 (and approximately 1000 elements) then re-binning the whole spectrum would conclude in S/N = 95 after 1000 s or S/N = 47 after 250 s with S/N > 1 in the majority of individual resolution elements. This is superior to imaging which we estimate to achieve optimum S/N = 38 every 250 s. These estimates are in agreement with IUE spectra analysis that estimated GALEX NUV=17.1 AB mag, and thus the same exposure times are sufficient to achieve our goals. We also select STIS to obtain NUV spectra of the science target. STIS is superior to COS for efficient, NUV wavelength coverage. We will use the G230L grating to acquire multiple spectra covering roughly 1600-3100 A, although we expect S/N > 20 below 2200 A, and S/N > 5 below 1800 A, based on the conservative case of photospheric emission alone with the same exposure times as the FUV. Thus, we expect somewhat better S/N, depending on the contribution of the heated region to the UV continuum. Visit Structure: In this proposal we are interested in the phase-resolved variability of GD 356 (P = 116 min), and thus require a complete coverage of a rotational cycle per UV regime. It would take HST 5 consecutive orbits to obtain a fully complete coverage. However, only one more orbit would be required to fully cover two rotational cycles. Therefore, to optimally utilise HST's time we ask for two rotational cycles to be covered per UV band. Thus, a total of 12 orbits would provide a full phase coverage two times in FUV and NUV. The team includes all the necessary expertise to analyze and publish these data. This includes but is not limited to ultraviolet observations and multi-wavelength data, time-series photometry and spectroscopy, white dwarf structure, evolution, magnetism, variability, etc. The team is responsible for the discovery of the intrinsic nature of these emission-line stars using the prototype. Stellar Physics and Stellar Types 6 false false true false false false false SUPPORTED STAR DA This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 Galex NUV = 16.9 AB mag STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 1677173766717 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. 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