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1. Numerical Simulations of Tidal Deformation and Resulting Light Curves of Small Bodies: Material Constraints of 99942 Apophis and 1I/‘Oumuamua

   https://doi.org/10.3847/PSJ/acccef  

   Taylor, Aster G.; Seligman, Darryl Z.; MacAyeal, Douglas R.; Hainaut, Olivier R.; Meech, Karen J. (May 2023, The Planetary Science Journal)

   Abstract In this paper, we present an open-source software (Simulator of Asteroid Malformation Under Stress,SAMUS) that simulates constant-density, constant-viscosity liquid bodies subject to tidal forces for a range of assumed viscosities and sizes. This software solves the Navier–Stokes equations on a finite-element mesh, incorporating the centrifugal, Coriolis, self-gravitational, and tidal forces. The primary functionality is to simulate the deformation of minor bodies under the influence of tidal forces. It may therefore be used to constrain the composition and physical structure of bodies experiencing significant tidal forces, such as 99942 Apophis and 1I/‘Oumuamua. We demonstrate thatSAMUSwill be useful to constrain the material properties of Apophis during its near-Earth flyby in 2029. Depending on the material properties, Apophis may experience an area change of up to 0.5%, with similar effects on the photometric brightness. We also applySAMUSto constrain the material dynamic viscosity of 1I/‘Oumuamua, the first interstellar object discovered traversing the inner solar system. ‘Oumuamua experienced a close approach to the Sun at perihelion (q≃ 0.25 au) during which there were significant tidal forces that may have caused deformation of the body. This deformation could have lead to observable changes in the photometric light curve based on the material properties. The application ofSAMUSto produce synthetic observations which incorporate tidal deformation effects demonstrates that no deformation—an infinite dynamic viscosity—best reproduces the photometric data. While these results indicate that ‘Oumuamua did not experience significant tidal deformation, a sophisticated model incorporating nonprincipal axis rotation is necessary to conclusively analyze both ‘Oumuamua and Apophis. 

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2. The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156

   https://doi.org/10.3847/1538-3881/ac8543  

   Williams, Kurtis A.; Hermes, J. J.; Vanderbosch, Zachary P. (September 2022, The Astronomical Journal)

   Abstract The distribution of white dwarf rotation periods provides a means for constraining angular momentum evolution during the late stages of stellar evolution, as well as insight into the physics and remnants of double degenerate mergers. Although the rotational distribution of low-mass white dwarfs is relatively well constrained via asteroseismology, that of high-mass white dwarfs, which can arise from either intermediate-mass stellar evolution or white dwarf mergers, is not. Photometric variability in white dwarfs due to rotation of a spotted star is rapidly increasing the sample size of high-mass white dwarfs with measured rotation periods. We present the discovery of 22.4 minute photometric variability in the light curve of EGGR 156, a strongly magnetic, ultramassive white dwarf. We interpret this variability as rapid rotation, and our data suggest that EGGR 156 is the remnant of a double degenerate merger. Finally, we calculate the rate of period change in rapidly-rotating, massive, magnetic WDs due to magnetic dipole radiation. In many cases, including EGGR 156, the period change is not currently detectable over reasonable timescales, indicating that these WDs could be very precise clocks. For the most highly-magnetic, rapidly-rotating massive WDs, such as ZTF J1901+1450 and RE J0317−853, the period change should be detectable and may help constrain the structure and evolution of these exotic white dwarfs. 

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3. Unstable accretion in TW Hya: 3D simulations and comparisons with observations

   https://doi.org/10.1093/mnras/staf148  

   Romanova, M_M; Espaillat, C_C; Wendeborn, J.; Donati, J_-F; Petrov, P_P; Lovelace, R_V_E (January 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the origin of photometric variability in the classical T Tauri star TW Hya by comparing light curves obtained by Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes with light curves created using three-dimensional (3D) magnetohydrodynamic (MHD) simulations. TW Hya is modelled as a rotating star with a dipole magnetic moment, which is slightly tilted about the rotational axis. We observed that for various model parameters, matter accretes in the unstable regime and produces multiple hotspots on the star’s surface, which leads to stochastic-looking light curves similar to the observed ones. Wavelet and Fourier spectra of observed and modelled light curves show multiple quasi-periodic oscillations (QPOs) with quasi-periods from less than 0.1 to 9 d. Models show that variation in the strength and tilt of the dipole magnetosphere leads to different periodograms, where the period of the star may dominate or be hidden. The amplitude of QPOs associated with the stellar period can be smaller than that of other QPOs if the tilt of the dipole magnetosphere is small and when the unstable regime is stronger. In models with small magnetospheres, the short-period QPOs associated with rotation of the inner disc dominate and can be mistaken for a stellar period. We show that longer period (5–9 d) QPOs can be caused by waves forming beyond the corotation radius. 

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4. The Plutino Population: An Abundance of Contact Binaries

   https://doi.org/https://doi.org/10.3847/1538-3881/aac0ff  

   Thirouin, A; Sheppard, S.S. (May 2018, The Astronomical journal)

   We observed 12 Plutinos over two separated years with the 4.3 m Lowell’s Discovery Channel Telescope. Here, we present the first light-curve data for those objects. Three of them (2014 JL80, 2014 JO80, and 2014 JQ80) display a large light-curve amplitude explainable by a single elongated object, but they are most likely caused by a contact binary system due to their light-curve morphology. These potential contact binaries have rotational periods from 6.3 to 34.9 hr and peak-to-peak light-curve variability between 0.6 and 0.8mag. We present partial light curves, allowing us to constrain the light-curve amplitude and the rotational period of another nine Plutinos. By merging our data with the literature, we estimate that up to ∼40% of the Plutinos could be contact binaries. Interestingly, we found that all of the suspected contact binaries in the 3:2 resonance are small with absolute magnitude H>6mag. Based on our sample and the literature, up to ∼50% of the small Plutinos are potential contact binaries. 

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5. Kepler and TESS Observations of PG 1159-035

   https://doi.org/10.3847/1538-4357/ac8871  

   Oliveira da Rosa, Gabriela; Kepler, S. O.; Córsico, Alejandro H.; Costa, J. E. S.; Hermes, J. J.; Kawaler, S. D.; Bell, Keaton J.; Montgomery, M. H.; Provencal, J. L.; Winget, D. E.; et al (September 2022, The Astrophysical Journal)

   Abstract PG 1159-035 is the prototype of the PG 1159 hot (pre-)white dwarf pulsators. This important object was observed during the Kepler satellite K2 mission for 69 days in 59 s cadence mode and by the TESS satellite for 25 days in 20 s cadence mode. We present a detailed asteroseismic analysis of those data. We identify a total of 107 frequencies representing 32ℓ= 1 modes, 27 frequencies representing 12ℓ= 2 modes, and eight combination frequencies. The combination frequencies and the modes with very highkvalues represent new detections. The multiplet structure reveals an average splitting of 4.0 ± 0.4μHz forℓ= 1 and 6.8 ± 0.2μHz forℓ= 2, indicating a rotation period of 1.4 ± 0.1 days in the region of period formation. In the Fourier transform of the light curve, we find a significant peak at 8.904 ± 0.003μHz suggesting a surface rotation period of 1.299 ± 0.002 days. We also present evidence that the observed periods change on timescales shorter than those predicted by current evolutionary models. Our asteroseismic analysis finds an average period spacing forℓ= 1 of 21.28 ± 0.02 s. Theℓ= 2 modes have a mean spacing of 12.97 ± 0.4 s. We performed a detailed asteroseismic fit by comparing the observed periods with those of evolutionary models. The best-fit model hasT_eff= 129, 600 ± 11 100 K,M_*= 0.565 ± 0.024M_⊙, and $\log g={7.41}_{-0.54}^{+0.38}$, within the uncertainties of the spectroscopic determinations. We argue for future improvements in the current models, e.g., on the overshooting in the He-burning stage, as the best-fit model does not predict excitation for all of the pulsations detected in PG 1159-035. 

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