Tidally excited oscillations in hot white dwarfs

We study the flux variation in helium white dwarfs (WDs) induced by dynamical tides for a variety of WD models with effective temperatures ranging from $T$=10 kK to $T$=26 kK. At linear order, we find the dynamical tide can significantly perturb the observed flux in hot WDs. If the temperature $T\gtrsim14$ kK, then the dynamical tide may induce a fractional change in the flux by >1% when the orbital period is $P_{\rm orb}\simeq 20-60\,{\rm min}$. The ratio between the flux modulation due to the dynamical tide and that due to the equilibrium tide (i.e., ellipsoidal variability) increases as the WD's radius decreases, and it could exceed O(10) if the WD has a radius $R\lesssim0.03 R_\odot$. Unlike the ellipsoidal variability which is in phase with the orbital motion, the pulsation caused by the dynamical tide may have a substantial phase shift. A cold WD with $T\lesssim 10$ kK, on the other hand, is unlikely to show observable pulsations due to the dynamical tide. At shorter orbital periods, the dynamical tide may become highly nonlinear. We approximate this regime by treating the waves as one-way traveling waves and find the flux variation is typically reduced to 0.1%-1% and the excess phase is likely to be 90 degrees (though with large uncertainty). Even in the traveling-wave limit, the flux perturbation due to dynamical tide could still exceed the ellipsoidal variability for compact WDs with $R\lesssim0.02 R_\odot$. We further estimate the nonlinear flux perturbations oscillating at four times the orbital frequency dominated by a self-coupled parent g-mode driving low-order daughter p-modes. The nonlinear flux variation could be nearly 50% of the linear variation for very hot WD models with $T\gtrsim26$ kK and 1% linear flux variation. We thus predict both the linear and nonlinear flux variations due to dynamical tides are likely to have significant observational signatures.Comment: 17 pages, 14 figures, submitted to MNRA

Tidally excited oscillations in hot white dwarfs

We study the flux variation in helium white dwarfs (WDs) induced by dynamical tides for a variety of WD models with effective temperatures ranging from T=10kK to T=26kK⁠. At linear order, we find the dynamical tide can significantly perturb the observed flux in hot WDs. If the temperature T≳14kK⁠, then the dynamical tide may induce a fractional change in the flux by >1 per cent when the orbital period is P_(orb) ≃ 20−60min⁠. The ratio between the flux modulation due to the dynamical tide and that due to the equilibrium tide (i.e. ellipsoidal variability) increases as the WD’s radius decreases, and it could exceed O(10) if the WD has a radius R ≲ 0.03 R_⊙. Unlike the ellipsoidal variability which is in phase with the orbital motion, the pulsation caused by the dynamical tide may have a substantial phase shift. A cold WD with T≃10kK⁠, on the other hand, is unlikely to show observable pulsations due to the dynamical tide. At shorter orbital periods, the dynamical tide may break and become highly non-linear. We approximate this regime by treating the waves as one-way travelling waves and find the flux variation is typically reduced to 0.1–1 per cent and the excess phase is ∼90° (though with large uncertainty). Even in the travelling-wave limit, the flux perturbation due to dynamical tide could still exceed the ellipsoidal variability for compact WDs with R ≲ 0.02 R_⊙. We further estimate the non-linear flux perturbations oscillating at four times the orbital frequency dominated by a self-coupled parent g-mode driving low-order daughter p modes. The non-linear flux variation could be nearly 50 per cent of the linear variation for very hot WD models with T≳26kK and 1 per cent linear flux variation. We thus predict that both the linear and non-linear flux variations due to dynamical tides are likely to have significant observational signatures

Twinkle, Twinkle, Little Stars: Shedding Light on the Population of Galactic Gravitational Wave Sources

Time domain surveys are revolutionizing our understanding of compact binary systems containing a white dwarf and another compact object at short orbital periods. These extreme binaries are astrophysical laboratories which can probe compact object physics, the nature of Type Ia supernova progenitors, accretion physics, tidal physics, the process of binary evolution, and they will dominate the population of objects the Laser Interferometer Space Antenna (LISA) will detect. In this thesis, I present substantial advances in the discovery and characterization of compact binaries using the Zwicky Transient Facility (ZTF). This work has resulted in a ten-fold increase in the discovery rate of such binaries compared to previous work in the field, and has helped lay the groundwork for discovering and characterizing these sources using other facilities, such as the the Transiting Exoplanet Survey Satellite (TESS), the upcoming Vera Rubin Observatory (VRO) and eventually LISA itself.</p

Bunny Bot V2.0

Robotics is an area of engineering that is driven by extensive research and development. Robots are very effective at accomplishing specific tasks they are programmed to execute. Model 3 Attached to the Turtlebot A Turtlebot platform is being used for a robotics and computer vision research project at Boise State University. Turtlebot is an open-source robotics platform controlled by the Robotic Operating System framework. Bunny Bot V2.0 is a community outreach event that will participate in the annual Boise State Easter Egg Hunt

A design method for prismatic prestressed continuous box girder bridges

Prestressed concrete is one of the most difficult building materials to design due to many reasons such as number of different solutions available for the same problem and the time dependent changes that take place. When used for continuous box girder bridges, there are added problems associated with secondary moments. In this paper, a straight forward design method has been presented that localises the iterations involved in the design process as much as possible so that the calculations involved can be minimised. The guidelines to determine the cross sectional dimensions has also been discussed. A complete design example has been presented for a three span continuous bridge. A simple method for modelling a box girder as a grillage of beams also has been presented

A compact X-ray emitting binary in likely association with 4FGL J0935.3+0901

4FGL J0935.3+0901 is a γ-ray source detected by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. We have conducted detailed analysis of the LAT data for this source and multiwavelength studies of the source field. Its γ-ray emission can be described with a power law (Γ = 2.0 ± 0.2) with an exponential cut-off (E_c = 2.9 ± 1.6 GeV), while the flux shows significant long-term variations. From analysis of archival Neil Gehrels Swift Observatory X-Ray Telescope data, we find only one X-ray source in the LAT’s 2σ error region. Within a 3.7arcsec radius error circle of the X-ray source, there is only one optical object down to r′ ∼ 23 mag. Time-resolved photometry of the optical object indicates a likely 2.5 h periodic modulation, while its spectrum shows double-peaked hydrogen and helium emission lines (similar to those seen in accretion discs in low-mass X-ray binaries). Combining these results, we conclude that we have discovered a compact X-ray emitting binary in likely association with 4FGL J0935.3+0901, i.e. a millisecond pulsar (MSP) binary. We discuss the implication of the optical spectral features: this binary could be a transitional MSP system at a subluminous disc state, although the other possibility, the binary in a rotation-powered state showing the optical emission lines due to intrabinary interaction processes, cannot be excluded. Further observational studies will help to determine detailed properties of this candidate MSP binary and thus clarify its current state