Discovery of Dipolar Chromospheres in Two White Dwarfs
arXiv:2307.02543 · doi:10.1093/mnras/stad2184
Abstract
This paper reports the ULTRACAM discovery of dipolar surface spots in two cool magnetic white dwarfs with Balmer emission lines, while a third system exhibits a single spot, similar to the prototype GD 356. The light curves are modeled with simple, circular, isothermal dark spots, yielding relatively large regions with minimum angular radii of 20 deg. For those stars with two light curve minima, the dual spots are likely observed at high inclination (or colatitude), however, identical and antipodal spots cannot simultaneously reproduce both the distinct minima depths and the phases of the light curve maxima. The amplitudes of the multi-band photometric variability reported here are all several times larger than that observed in the prototype GD 356; nevertheless, all DAHe stars with available data appear to have light curve amplitudes that increase toward the blue in correlated ratios. This behavior is consistent with cool spots that produce higher contrasts at shorter wavelengths, with remarkably similar spectral properties given the diversity of magnetic field strengths and rotation rates. These findings support the interpretation that some magnetic white dwarfs generate intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion. Spectroscopic time-series data for DAHe stars is paramount for further characterization, where it is important to obtain well-sampled data, and consider wavelength shifts, equivalent widths, and spectropolarimetry.
arXiv comment9 pages, 5 figures, 3 tables, accepted to MNRAS
References in corpus (41)
- The Transiting Exoplanet Survey Satellite
- The 2.5 m Telescope of the Sloan Digital Sky Survey
- Understanding the Lomb-Scargle Periodogram
- Achieving better than 1 minute accuracy in the Heliocentric and Barycentric Julian Dates
- Kepler Presearch Data Conditioning I - Architecture and Algorithms for Error Correction in Kepler Light Curves
- Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements
- Magnetic White Dwarfs
- ULTRACAM: an ultra-fast, triple-beam CCD camera for high-speed astrophysics
- Binary Star Origin of High Field Magnetic White Dwarfs
- White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
- The True Incidence of Magnetism among Field White Dwarfs
- Double degenerate mergers as progenitors of high-field magnetic white dwarfs
- A 12 minute Orbital Period Detached White Dwarf Eclipsing Binary
- General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system
- I Spy Transits and Pulsations: Empirical Variability in White Dwarfs Using Gaia and the Zwicky Transient Facility
- A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
- The formation of high-field magnetic white dwarfs from common envelopes
- A common origin of magnetism from planets to white dwarfs
- New insight into the magnetism of degenerate stars from the analysis of a volume limited sample of white dwarfs
- white dwarfs within 40 pc I: spectroscopic observations of new candidates
- TESS-Gaia Light Curve: a PSF-based TESS FFI light curve product
- The Magnetic and Metallic Degenerate G77-50
- The incidence of magnetic fields in cool DZ white dwarfs
- An Isolated White Dwarf with a 70 second Spin Period
- The polluted atmospheres of cool white dwarfs and the magnetic field connection
- Discovery of 74 new bright ZZ Ceti stars in the first three years of TESS
- Photometric variability of the unique magnetic white dwarf GD356
- An Isolated White Dwarf with 317-Second Rotation and Magnetic Emission
- Discovery of a hot ultramassive rapidly rotating DBA White Dwarf
- Single magnetic white dwarfs with Balmer emission lines: A small class with consistent physical characteristics as possible signposts for close-in planetary companions
- The Shortest Period Detached Binary White Dwarf System
- Are white dwarf magnetic fields in close binaries generated during common-envelope evolution?
- Discovery of weak magnetic fields in four DZ white dwarfs in the local 20pc volume. Implications for the frequency of magnetic fields with cooling age
- DAHe white dwarfs from the DESI survey
- Spin and Magnetism of White Dwarfs
- A test of the planet-star unipolar inductor for magnetic white dwarfs
- The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156
- Hydrodynamic Simulations of Disrupted Planetary Accretion Discs Inside the Core of an AGB Star
- Does GD 356 have a Terrestrial Planetary Companion?
- A Magnetic White Dwarf with Five H Components
- Two New White Dwarfs With Variable Magnetic Balmer Emission Lines