Showing papers by "Jay Farihi published in 2023"
TL;DR: In this article , 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.
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○. 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.
TL;DR: In this article , the mass-loss rates and upper limits are tied to accretion in the white dwarfs, based on limiting cases for how the wind is captured, and compared with known cases of wind pollution from close M dwarfs.
Abstract:
White dwarfs stars are known to be polluted by their active planetary systems, but little attention has been paid to the accretion of wind from low-mass companions. The capture of stellar or substellar wind by white dwarfs is one of few methods available to astronomers which can assess mass-loss rates from unevolved stars and brown dwarfs, and the only known method to extract their chemical compositions. In this work, four white dwarfs with closely-orbiting, L-type brown dwarf companions are studied to place limits on the accretion of a substellar wind, with one case of a detection, and at an extremely non-solar abundance mNa/mCa > 900. The mass-loss rates and upper limits are tied to accretion in the white dwarfs, based on limiting cases for how the wind is captured, and compared with known cases of wind pollution from close M dwarf companions, which manifest in solar proportions between all elements detected. For wind captured in a Bondi-Hoyle flow, mass-loss limits $\dot{M}\lesssim 5\times 10^{-17}$ M⊙ yr−1are established for three L dwarfs, while for M dwarfs polluting their hosts, winds in the range 10−13 − 10−16 M⊙ yr−1are found. The latter compares well with the $\dot{M}\sim 10^{-13} - 10^{-15}$ M⊙ yr−1estimates obtained for nearby, isolated M dwarfs using Lyα to probe their astropsheres. These results demonstrate that white dwarfs are highly-sensitive stellar and substellar wind detectors, where further work on the actual captured wind flow is needed.