A self-synthesized origin for heavy metals in hot subdwarf stars

Context. A number of He-rich hot subdwarf stars present high abundances for trans-iron elements, such as Sr, Y, Zr, and Pb. Diffusion processes are important in hot subdwarf stars and it is generally believed that the high abundances of heavy elements in these peculiar stars are due to the action of radiative levitation. However, during the formation of He-rich hot subdwarf stars, hydrogen can be ingested into the convective zone driven by the He-core flash. It is known that episodes of protons being ingested into He-burning convective zones can lead to neutron-capture processes and the formation of heavy elements.Aims. In this work, we aim to explore, for the first time, whether neutron-capture processes can occur in late He-core flashes taking place in the cores of the progenitors of He-rich hot subdwarfs. We aim to explore the possibility of a self-synthesized origin for the heavy elements observed in some He-rich hot subdwarf stars.Methods. We computed a detailed evolutionary model for a stripped red-giant star using a stellar evolution code with a nuclear network comprising 32 isotopes. Then we post-processed the stellar models in the phase of helium and hydrogen burning using a post-processing nucleosynthesis code with a nuclear network of 1190 species, which allowed us to follow the neutron-capture processes in detail.Results. We find the occurrence of neutron-capture processes in our model, with neutron densities reaching a value of ∼5 × 1012 cm−3. We determined that the trans-iron elements are enhanced in the surface by 1 to 2 dex, as compared to initial compositions. Moreover, the relative abundance pattern [Xi/Fe] produced by neutron-capture processes closely resembles those observed in some He-rich hot subdwarf stars, hinting at a possible self-synthesized origin for the heavy elements in these stars.Conclusions. We conclude that intermediate neutron-capture processes can occur during a proton ingestion event in the He-core flash of stripped red-giant stars. This mechanism offers a natural channel for the production of the heavy elements observed in certain He-rich hot subdwarf stars

Symmetrical electroadhesives independent of different interfacial surface conditions

Current electroadhesive actuators cannot produce stable electroadhesive forces on the same substrate with different interfacial surface interactions. It is, therefore, desirable to develop electroadhesive actuators that can generate stable adhesive forces on different surface conditions. A symmetrical electroadhesive pad that is independent of different interfacial scratch directions is developed and presented. A relative difference of only 6.4% in the normal force direction was observed when the electroadhesive was facing an aluminium plate with surface scratch directions of 0°, 45°, 90°, and 135°. This step-change improvement may significantly promote the application of electroadhesion technology. In addition, this manifests that significant performance improvements could be achieved via further investigations into electroadhesive designs

Low coherence interferometric fibre based in-situ measurements of surface roughness in air and in water

In many high-value manufacturing applications there is increased demand for component surfaces with enhanced geometric complexity. In order to ensure accurate adherence to design specifications and to compensate for any deviations, an in-situ measurement system capable of providing feedback on surface geometry within reasonable range is an important tool for manufacturing processes. While there have been many sensing techniques used for monitoring the manufacturing process, many of these are limited in terms of their ability to provide accurate reliable real-time measurements, are off-line systems, or are inappropriate for the hostile machining environment. In this work, a fibre based common path low-coherence interferometric technique has been successfully developed to measure surface step heights and surface roughness, in air, but more importantly in water. Results are produced from slip gauge sets and reference roughness comparator standards, demonstrating the ability to achieve close to nominal value agreement in air, but water based step height and surface roughness Ra values depart further from nominal as a function of media refractive index and dispersion characteristics

Geometric optimisation of electroadhesive actuators based on 3D electrostatic simulation and its experimental verification

A systematic research methodology for the performance evaluation of different electroadhesive pad geometries is demonstrated in this paper. The proposed research method for the investigation was based on a 3D electrostatic simulation using COMSOL Multiphysics, a cost-effective electroadhesive pad design and manufacturing process based on solid-ink printing, chemical etching, conformal coating, and an advanced and mechatronic electroadhesive force testing platform and procedure. The method has been validated using 2 novel pad designs, approximate 21 cm x 19 cm, compared with the normal comb design, on the glass and aluminium plate. The experimental results showed that: 1) on the glass substrate, a relative increase of 1% and 28% in the electroadhesive forces obtainable can be seen in the curve-comb pad and the worm-comb pad respectively; and 2) on the Al substrate, a relative increase of 5% and 12% can be seen. This manifests that the two new pad designs, especially the worm-comb shape design, are better at generating larger electroadhesive forces. The comparison between the simulation results and experimental results proved that proposed method is promising for evaluating the pad design before spending time and money on pad manufacture and testing

In-situ fiber-based surface profile measurement system using low coherence interferometer

For many high-value manufacturing applications, advanced control systems are required to ensure product quality is maintained; this requires accurate data to be collected from in-situ sensors. Making accurate in-situ measurements is challenging due to the aggressive environments found within manufacturing machines and processes. This paper investigates a method to obtain surface profile measurements in a spectral-domain, common-path, low-coherence system. A fibre based Low Coherence Interferometer was built and was used to experimentally measure surface profiles. The fringes obtained from interferograms were transformed into the Fourier domain to obtain a trackable peak relating to the surface depth. This has been illustrated with ideal step height measurements and referenced specimens as well as more challenging surface roughness measurements, which have produced complex signal processing issues. This work opens up avenues for a metrology based system where both machining and measurement system can coexist on the same plane, in aggressive environments

Visualization methods for understanding the dynamic electroadhesion phenomenon

Experimental investigation into the surface potential and electric field visualization of an electroadhesion system is presented for understanding the dynamic electroadhesion phenomenon. The indirect experimental approach has been based on measuring surface potentials on the surface of an electroadhesive pad by an electrostatic voltmeter. The direct approach has been based on charging and discharging the electroadhesive pad in a viscous oil mixed with lightweight particles. The visualization of the dynamic field distribution of electroadhesive pads can be a useful method to understand the dynamic electroadhesion phenomenon. In addition, indication of different field distributions of different pad geometries can be obtained through the method demonstrated here. Furthermore, the method is useful for instructors or lecturers to showcase or teach the dynamic electroadhesion phenomenon

Shell helium-burning hot subdwarf B stars as candidates for blue large-amplitude pulsators

Blue large-amplitude pulsators (BLAPs) are a newly discovered type of variable star. Their typical pulsation periods are on the order of a few tens of minutes, with relatively large amplitudes of 0.2-0.4 mag in optical bands, and their rates of period changes are on the order of $10^{-7} yr^{-1}$ (both positive and negative). They are extremely rare objects and attempts to explain their origins and internal structures have attracted a great deal of attention. Previous studies have proposed that BLAPs may be pre-white dwarfs, with masses around $0.3M_\odot$, or core-helium-burning stars in the range of $\sim 0.7-1.1M_\odot$. In this work, we use a number of MESA models to compute and explore whether BLAPs could be explained as shell helium-burning subdwarfs type B (SHeB sdBs). The models that best match existing observational constraints have helium core masses in the range of $\sim 0.45-0.5M_\odot$. Our model predicts that the positive rate of period change may evolve to negative. The formation channels for SHeB sdBs involve binary evolution and although the vast majority of BLAPs do not appear to be binaries (with the exception of HD 133729), the observational constraints are still very poor. Motivated by these findings, we explored the Roche lobe overflow channel. Of the 304 binary evolution models we computed, about half of them are able to produce SHeB sdBs in long-period binaries that evade detection from the limited observations that are currently available.Comment: A&A accepte

No peaks without valleys: The stable mass transfer channel for gravitational-wave sources in light of the neutron star-black hole mass gap

Gravitational-wave (GW) detections are starting to reveal features in the mass distribution of double compact objects. The lower end of the black hole (BH) mass distribution is especially interesting as few formation channels contribute here and because it is more robust against variations in the cosmic star formation than the high mass end. In this work we explore the stable mass transfer channel for the formation of GW sources with a focus on the low-mass end of the mass distribution. We conduct an extensive exploration of the uncertain physical processes that impact this channel. We note that, for fiducial assumptions, this channel reproduces the peak at $\sim9 \mathrm{M_{\odot}}$ in the GW-observed binary BH mass distribution remarkably well, and predicts a cutoff mass that coincides with the upper edge of the purported neutron star BH mass gap. The peak and cutoff mass are a consequence of unique properties of this channel, namely (1) the requirement of stability during the mass transfer phases, and (2) the complex way in which the final compact object masses scale with the initial mass. We provide an analytical expression for the cutoff in the primary component mass and show that this adequately matches our numerical results. Our results imply that selection effects resulting from the formation channel alone can provide an explanation for the purported neutron star--BH mass gap in GW detections. This provides an alternative to the commonly adopted view that the gap emerges during BH formation.Comment: Accepted for publication in ApJ associated code is available at https://github.com/LiekeVanSon/LowMBH_and_StableChanne

Supernova Shock Breakout from a Red Supergiant

Massive stars undergo a violent death when the supply of nuclear fuel in their cores is exhausted, resulting in a catastrophic "core-collapse" supernova. Such events are usually only detected at least a few days after the star has exploded. Observations of the supernova SNLS-04D2dc with the Galaxy Evolution Explorer space telescope reveal a radiative precursor from the supernova shock before the shock reached the surface of the star and show the initial expansion of the star at the beginning of the explosion. Theoretical models of the ultraviolet light curve confirm that the progenitor was a red supergiant, as expected for this type of supernova. These observations provide a way to probe the physics of core-collapse supernovae and the internal structures of their progenitor starsComment: Science, in press. 32 pages, 7 figure