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 new class of large-amplitude radial-mode hot subdwarf pulsators

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T eff, and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T eff and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M ⊙ helium-core pre-WDs.Published versio

An 8.8 minute orbital period eclipsing detached double white dwarf binary

We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just $8.8$ minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its $6.91\, \rm min$ counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of $d=2120^{+131}_{-115}\,\rm pc$, LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of $87\pm5$ after four years. We find component masses of $M_A= 0.349^{+0.093}_{-0.074}\,M_\odot$ and $M_B=0.384^{+0.114}_{-0.074}\,M_\odot$, radii of $R_A=0.0308^{+0.0026}_{-0.0025}\,R_\odot$ and $R_B = 0.0291^{+0.0032}_{-0.0024}\,R_\odot$, and effective temperatures of $T_A=22200^{+1800}_{-1600}\,\rm K$ and $T_B=16200^{+1200}_{-1000}\,\rm K$. We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint ($r>21 \, m_{\rm AB}$) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.Comment: 12 pages, 7 figures, 3 tables, submitte

Measurements of Aperture Averaging on Bit-Error-Rate

We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 in. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments

Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen-poor Low-mass White Dwarf

We report the discovery of a detached double white dwarf binary with an orbital period of ≈20.6 minutes, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a ≈0.17 M⊙ mass white dwarf with a helium-dominated atmosphere containing traces of hydrogen. This object exhibits ellipsoidal variations due to tidal deformation, and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of K_B = 618.7 ± 6.9 km s⁻¹. We have detected significant orbital decay due to the emission of gravitational radiation, and we expect that the Laser Interferometer Space Antenna (LISA) will detect this system with a signal to noise of 8.4^(+4.2)_(-3.0) after four years of operation. Because this system already has a well-determined orbital period, radial velocity semi-amplitude, temperature, atmospheric composition, surface gravity, and orbital decay rate, a LISA signal will help fully constrain the properties of this system by providing a direct measurement of its inclination. Thus, this binary demonstrates the synergy between electromagnetic and gravitational radiation for constraining the physical properties of an astrophysical object

ZTF 18aaqeasu (SN 2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar Mass White Dwarf

The detonation of a helium shell on a white dwarf has been proposed as a possible explosion triggering mechanism for Type Ia supernovae. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of $\approx 18$ days from explosion, the transient reached a peak absolute magnitude of $M_R \approx -18.2$ mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like Type Ia supernovae, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color ($g-r \approx 2$ mag) arising from nearly complete line blanketing of flux blue-wards of 5000 \AA. This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after $\approx 30$ days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive ($\approx 0.15$ M$_\odot$) helium shell on a sub-Chandrasekhar mass ($\approx 0.75$ M$_\odot$) white dwarf after including mixing of $\approx 0.2$ M$_\odot$ of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells.Comment: 10 pages, 6 figures. Submitted to ApJ

A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T_(eff), and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T_(eff) and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M_⊙ helium-core pre-WDs

First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary

We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of T_(eff) = 42,400 ± 300 K and a surface gravity of log(g) = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M_(sdOB) = 0.337 ± 0.015 M⊙ and a white dwarf accretor with a mass M_(WD) = 0.545 ± 0.020 M⊙. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 M⊙ star lost its envelope when crossing the Hertzsprung gap. To match its current P_(orb), T_(eff), log(g), and masses, we estimate a post–common envelope period of P_(orb) ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 M⊙, merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star