Hydrogen Recombination Research Articles

Synergistic reductive catalytic effects of an organic and inorganic hybrid covalent organic framework for hydrogen fuel production.

Electrocatalytic hydrogen generation in alkaline medium has become widely used in a variety of sectors. However, the possibility for additional performance improvement is hampered by slow kinetics. Because of this restriction, careful control over processes such as water dissociation, hydroxyl desorption and hydrogen recombination is required. Covalent organic frameworks (COFs) based on porphyrin and polyoxometalates (POMs) show encouraging electrocatalytic performance, offering a viable route for effective and sustainable hydrogen generation. Their specific architectures lead to increased electrocatalytic activity, which makes them excellent choices for developing water electrolysis as a clean energy conversion method in the alkaline medium. In this regard, TTris@ZnPor and Lindqvist POM were coordinated to create a new eco-friendly and highly active covalent organic framework (TP@VL-COF). In order to describe TP@VL-COF, extensive structural and morphological investigations were carried out through FTIR, 1H NMR, elemental analysis, SEM, fluorescence, UV-visible, PXRD, CV, N2-adsorption isotherm, TGA and DSC analyses. In an alkaline medium, the electrocatalytic capability of 20%C/Pt, TTris@ZnPor, Lindqvist POM and TP@VL-COF was explored and compared for the hydrogen evolution reaction (HER). The TP@VL-COF showed the best catalytic efficiency for HER in an alkaline electrolyte, requiring just a 75 mV overpotential to drive 10 mA cm-2 and outperforming 20%C/Pt, TTris@ZnPor, Lindqvist POM and other reported catalysts. The Tafel slope value also indicates faster kinetics for TP@VL-COF (114 mV dec-1) than for 20%C/Pt (182 mV dec-1) TTris@ZnPor (116 mV dec-1) and Lindqvist POM (125 mV dec-1).

Dual Pt atoms stabilized by an optimized coordination environment for propane dehydrogenation

Tailoring atomically dispersed catalysts by various methods has been the subject of intense current research in heterogeneous catalysis. In this work, the structural stability and propane dehydrogenation performance of Pt-TiO2 catalysts with single, dual, and triple Pt atoms doped on the reduced TiO2 surface have been examined. Experimental results indicate the atomically dispersed Pt is thermally very stable and shows exceptionally high activity (TOFpropane = 4.93 s−1) and selectivity (∼98.5 %) for the dehydrogenation reaction. The recently developed machine-learning-based SSW-NN method is then employed to explore the global potential energy surface of the atomically dispersed Pt atom-doped TiO2, which suggests that the single and dual Pt atoms can be tightly held by the vacancies created on the reduced transition-metal oxide, as confirmed by DFT + U calculated energetics. After that, advanced characterization techniques are used in conjunction with microkinetic analysis to establish the quantitative structure–activity relationships. It is found that the dual-Pt-atom structure with two Pt atoms embedded in adjacent Ti and O vacancies is the only active ensemble that dramatically promotes the CH bond activation and hydrogen recombination, as compared to single Pt atoms accommodated by either Ti or O vacancies. These results provide new and direct evidence in support of the idea that specific function can be imparted to specific active sites by tuning their coordination environments, which makes it possible to tailor the catalytic properties of metal oxides to a particular reaction such as propane dehydrogenation.

Performance parameters of infra-red and visible-active MXene photocatalysts for water splitting.

Water splitting reactions through photocatalysis are an efficient and sustainable technique for the generation of green energy. Ability to simultaneously generate hydrogen and oxygen, along with efficiency of utilizing charge carriers, conversion of solar energy to hydrogen, fast migration, and low recombination rates of carriers are the parameters that decide a photocatalyst's suitability in water splitting. In the literature, comprehensive calculation and analysis of all these performance parameters for a potential photocatalyst are rare. In this work, we have performed first-principles-based computations to find new efficient photocatalysts from the family of Janus MXenes and assessed their performance parameters using strain engineering. Out of 14 studied materials, we find 5 materials: Sc2COS, Zr2COS, Hf2COS, and ZrHfCO2 under zero or finite tensile strain and Hf2COSe under 6% tensile strain meeting the requirements of simultaneous reactions to split water. The computations of various efficiency-related parameters demonstrate that Zr2COS, Hf2COS, and Hf2COSe have excellent efficiencies, significantly better than those of the well-known photocatalysts. The origin of such performances lies in their electronic and optical properties, which are analysed systematically.

Initial Results of our Spectro-photometric Monitoring of XZ Tau

We present here initial results of our spectro-photometric monitoring of XZ Tau. During our monitoring period, XZ Tau exhibited several episodes of brightness variations in timescales of months at optical wavelengths in contrast to the mid-infrared wavelengths. The color evolution of XZ Tau during this period suggests that the brightness variations are driven by changes in accretion from the disc. The mid-infrared light curve shows an overall decline in brightness by ∼ 0.5 and 0.7 magnitude respectively in WISE W1 (3.4 μm) and W2 (4.6 μm) bands. The emission profile of the hydrogen recombination lines along with that of Ca II IRT lines points towards magnetospheric accretion of XZ Tau. We have detected P Cygni profile in Hβ indicating of out-flowing winds from regions close to accretion. Forbidden transitions of oxygen are also detected, likely indicating jets originating around the central pre-main sequence star.

Innovative Materials and Techniques for Enhancing Hydrogen Storage: A Comprehensive Review of Damage Detection and Preventive Strategies

Abstract Hydrogen is a promising alternative energy resource, but an improvement of secure and efficient storage solutions must be developed for its increased use. This review will investigate efforts to improve the storage of hydrogen using Solid-State methods such as Activated Carbon, Carbon Nanotubes, Metal-Organic Framework, and Metal Hydrides in comparison with traditional liquid and gaseous storage methods. Solid-state methods rely on the temporary trapping or chemical bonding of the hydrogen atoms and molecules to reduce the reactivity and explosivity of the hydrogen and improve safety and equipment sustainability. To support the research into storage methods and improve the industrial infostructure for an increase in hydrogen use, several methods for detecting hydrogen are explored, including Acoustic Emissions Testing, Scanning Kelvin Probe Testing, and Digital Image Correlation. Lastly, various preventative measures used to improve the performance of material used in Hydrogen environments are researched, including Laser Shock Peening, Hydrogen Recombination Coatings, Hydrogen Diffusion Barriers, Hydrogen Getter Coating, and Microstructure Testing.

Impact of Position and Platinum Loading of Recombination Interlayers in Proton Exchange Membranes on the Anodic Hydrogen Content in Water Electrolysis

Hydrogen crossover through the membrane is a significant safety concern in proton exchange membrane water electrolyzers (PEMWEs). This issue is predominant at low loads, which contradicts the application of PEMWEs in conjunction with fluctuating renewable energy sources that require the electrolyzer to switch between low- and high-load operation. The risk of flammable or explosive oxygen-hydrogen mixtures on the anode side of the electrolyzer prevents the application of membranes as thin as in fuel cells, which results in relatively large Ohmic losses. A means to mitigate the accumulation of hydrogen in the oxygen compartment is the integration of platinum nanoparticles as a recombination catalyst interlayer within the membrane that catalyzes the recombination of hydrogen and oxygen to water. It was shown that such an interlayer effectively minimizes the hydrogen concentration on the anode side without reducing the cell performance.1 Thus, this technology can pave the way toward thinner membranes and more efficient electrolyzers.However, the interlayer should not increase the costs of the membrane electrode assembly by excessive Pt loading. Hence, we investigated the ideal position of this recombination interlayer within the membrane,2 and we performed a parameter study on the minimum required catalyst loading for an effective hydrogen-in-oxygen reduction.3 We find that the best position for the interlayer is close to the anode due to the lower oxygen permeability of the membrane. Also, a loading of 10 µg per cm² of platinum nanoparticles is sufficient to reduce the anodic hydrogen content from more than 2.5 vol.% (without interlayer) to approx. 0.5 vol.% when operating a PEMWE cell with a 110 µm thick membrane at 0.5 A/cm² and a differential pressure of 10 barc/1 bara. In summary, we are able to show that low platinum loadings within the membrane are sufficient to reduce the anodic hydrogen content in PEMWEs to safe values without impairing the electrochemical performance of the cell.References C. Klose, P. Trinke, T. Böhm, B. Bensmann, S. Vierrath, R. Hanke-Rauschenbach and S. Thiele, J. Electrochem. Soc., 165(16), F1271-F1277 (2018).A. Martin, D. Abbas, P. Trinke, T. Böhm, M. Bierling, B. Bensmann, S. Thiele and R. Hanke-Rauschenbach, J. Electrochem. Soc., 168(9), 94509 (2021).D. Abbas, A. Martin, P. Trinke, M. Bierling, B. Bensmann, S. Thiele, R. Hanke-Rauschenbach and T. Böhm, J. Electrochem. Soc., 169(12), 124514 (2022).

In Situ Measurement of Hydrogen Absorption into Copper-Coated Titanium Using Neutron Reflectometry and Electrochemical Impedance Spectroscopy

We set out to investigate the possibility that a small amount of hydrogen, generated by sulfide-driven corrosion or radiolytic processes, might be absorbed into the copper coating proposed for use as corrosion protection for the used nuclear fuel containers (UFCs) to be stored in a deep geological repository (DGR) in Canada. Hydrogen absorption may have the potential to alter the mechanical properties of the copper coating. This contribution discusses our initial study, using in situ neutron reflectometry (NR) and electrochemical impedance spectroscopy (EIS), of hydrogen absorption into a 50-nm layer of copper coated onto 4 nm of Ti on a Si single-crystal substrate. In situ NR-EIS experiments detected no increase in the hydrogen content within the copper layer of the copper-titanium thin film following 55 h of polarization in deoxygenated 0.1 M NaCl solution at pH 9, at potentials ranging from −444 mV/SCE to −1300 mV/SCE (detection limit ~2 at.%). However, these NR results showed that hydrogen can be absorbed into copper when the latter is cathodically polarized below about ─900 mV/SCE, which is the experimentally measured potential threshold for the hydrogen evolution reaction (HER) under these conditions. Although hydrogen did not accumulate in the outer Cu layer, our experiments revealed that the absorbed hydrogen quickly traversed the Cu layer to concentrate in the underlying titanium layer, which favours absorbed hydrogen. This observation is consistent with hydrogen uptake measured using inert gas fusion analysis on oxygen-free, phosphorus-doped (OFP) copper, which showed that in the absence of hydrogen recombination inhibitors (e.g., As(III)), only a small increase of 22 wt. ppm hydrogen was absorbed into the copper after 24 h at E = −1300 mV/SCE, and no increase in absorbed hydrogen occurred at potentials of −1100 mV/SCE or more positive. The initial absorption efficiency for electrolytically produced hydrogen atoms at the copper surface was determined to be 3.2%, suggesting that there may be an upper limit for hydrogen uptake by the copper coating of the UFC, although studies on 3 mm copper-coated steel samples representative of UFC materials will be required to confirm that. This result supports the prediction that hydrogen absorption is unlikely to lead to any failure of the copper barrier within a proposed DGR, in which it will not be possible to achieve such negative electrochemical potentials. Intended future research using in situ NR-EIS to study hydrogen absorption during the corrosion of copper thin films by HS⁻ ions and further studies on hydrogen absorption into 3-mm-thick copper-coated steel samples will help to validate these conclusions.

Enhanced Synergistic Effect by Facile Tuning of Nickel Doping on Cobalt Phosphide as Electrochemical Catalysts for Hydrogen Evolution Reaction in Alkaline Water Electrolysis

The development of efficient and cost-effective electrocatalyst for the hydrogen evolution reaction (HER) is essential for alkaline water electrolysis. Cobalt phosphide (Co2P) is a promising non-noble electrocatalyst material for HER due to its low cost and high stability in alkaline solutions. However, improving the HER performance of Co2P is still challenging due to its catalytic mechanism in which Volmer reaction is the dominated rate-determining step. In this study, we introduced nickel to cobalt phosphide in order to improve its performance as an electrocatalyst because nickel is known to be beneficial for water dissociation (Volmer reaction), while cobalt is beneficial for producing gaseous hydrogen (Heyrovsky or Tafel reaction). This synergistic effect between nickel and cobalt phosphide contributes to enhancing HER performance. Also, it is crucial to find the optimal ratio of nickel and cobalt for maximizing synergistic effect. To facilitate the optimization process, a co-sputtering system was used to fabricate the electrodes. The resulting Ni-doped Co2P showed significantly enhanced HER performance with low overpotential of ~91 mV at 10 mA/cm2 and Tafel slope of ~47 mV/dec compared to pristine Co2P in electrochemical analysis. This enhancement is attributed from the optimized ratio between the sites where water dissociation and adsorbed hydrogen recombination reactions occur. The findings of this study demonstrate that the nickel cobalt phosphide electrode is a promising electrocatalyst for HER in alkaline water electrolysis.

Dust- and inclination-corrected star formation and interstellar medium scaling relations in nearby galaxies

ABSTRACT Following from our recent work, we present a detailed analysis of star formation and interstellar medium (ISM) scaling relations, done on a representative sample of nearby galaxies. H α images are analysed in order to derive the integrated galaxy luminosity, known as a more instantaneous and accurate star formation rate (SFR) tracer, and the required photometric and structural parameters. Dust- and inclination-corrected H α luminosities, SFRs, and related quantities are determined using a self-consistent method based on previous work prescriptions, which does not require the assumption of a dust attenuation curve and use of Balmer decrements (or other hydrogen recombination lines) to estimate the dust attenuation, with the advantage of determining dust opacities and dust masses along the way. We investigate the extent to which the dust and inclination effects bias the specific parameters of these relations, the scatter and degree of correlation, and which relations are fundamental or are just a consequence of others. Most of our results are consistent with errors with other similar studies, while others come in opposition or are inconclusive. By comparing the B-band optical and H α (star-forming) discs scale lengths, we found on average the star formation distribution to be more extended than the stellar continuum emission one (the ratio being 1.10), this difference increasing with stellar mass. Similarly, more massive galaxies have a more compact stellar emission surface density than the star formation one (average ratio of 0.77). The method proposed can be applied in larger scale studies of star formation and ISM evolution, for normal low- to intermediate-redshift galaxies.

Bulk Nanostructured Silicide Thermoelectric Materials by Reversible Hydrogen Absorption-Desorption.

The production of bulk nanostructured silicide thermoelectric materials by a reversible hydrogen absorption-desorption process is demonstrated. Here, high-pressure reactive milling under 100bar hydrogen is used to decompose the Ca2Si phase into CaH2 and Si. Subsequent vacuum heat treatment results in hydrogen desorption and recombination of the constituents into the original phase. By changing the heat treatment temperature, recombination into Ca2Si or Ca5Si3 can be achieved. Most importantly, the advanced synthesis process enables drastic and simple microstructure refinement by more than two orders of magnitude, from a grain size of around 50µm in the initial ingot to 100-200nm after the hydrogen absorption-desorption process. Fine precipitates with sizes ranging from 10-50nm are forming coherently inside the grains. Thus, the route is promising and can be used for reducing thermal conductivity due to phonon scattering from grain boundaries as well as through nanostructuring with second-phase precipitates. Moreover, the process is environmentally friendly since hydrogen is reversibly absorbed, desorbed, and can be fully recovered.

The Detection of Higher-order Millimeter Hydrogen Recombination Lines in the Large Magellanic Cloud

We report the first extragalactic detection of the higher-order millimeter hydrogen recombination lines (Δn > 2). The γ-, ϵ-, and η-transitions have been detected toward the millimeter continuum source N 105–1 A in the star-forming region N 105 in the Large Magellanic Cloud with the Atacama Large Millimeter/submillimeter Array. We use the H40α line, the brightest of the detected recombination lines (H40α, H36β, H50β, H41γ, H57γ, H49ϵ, H53η, and H54η), to determine the electron temperature and study ionized gas kinematics in the region, and the 3 mm free–free continuum emission to determine the physical parameters: the size, emission measure, and electron density. We compare the physical properties of N 105–1 A to a large sample of Galactic compact and ultracompact (UC) H ii regions and conclude that N 105–1 A is similar to the most luminous (L > 105 L ⊙) UC H ii regions in the Galaxy. N 105–1 A is ionized by an O5.5 V star; it is deeply embedded in its natal molecular clump, and likely associated with a (proto)cluster. We incorporate high-resolution molecular line data including CS, SO, SO2, and CH3OH (∼0.12 pc), and HCO+ and CO (∼0.087 pc) to explore the molecular environment of N 105–1 A. Based on the CO data, we find evidence for a cloud–cloud collision that likely triggered star formation in the region. We find no clear outflow signatures, but the presence of filaments and streamers indicates ongoing accretion onto the clump hosting the UC H ii region. Sulfur chemistry in N 105–1 A is consistent with the accretion shock model predictions.

Acceleration of Pd-V intermetallic diffusion by hydrogen

Vanadium-based membranes have great potential for hydrogen purification due to their perfect selectivity, high permeability, and relatively low cost. With appropriate surface cleaning, V efficiently permeates hydrogen at elevated temperature, but performance declines due to its affinity to absorb impurities. The application of palladium thin films maintains a clean surface that catalyzes hydrogen dissociation and recombination. Hydrogen permeation in Pd-V-Pd membranes initially reach theoretical permeability, but declines due to Pd-V interdiffusion. The objective of this work was to quantify the intermetallic diffusion process as a function of temperature and ambient. Pd-V composites were subjected to various annealing treatments and characterized using Auger electron spectroscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy, as well as correlated to measurements of membrane permeability. In an inert environment Pd-V interdiffusion was observable as low as 300 °C, and the diffusion coefficient had an activation energy of 44 kJ/mol. Furthermore, the presence of hydrogen at partial pressures > 10 kPa accelerated interdiffusion six-fold at T = 400 °C. Membrane performance degraded with an activation energy 75 kJ/mol, suggesting that intermetallic diffusion leads to both a loss of catalytic activity and as well as degradation of bulk permeability. These findings provide a baseline for evaluating hydrogen permeable interdiffusion barriers to overcome these challenges.

Thermo-optic response and mechanical properties of hybrid polyurethane elastomers via silicon group-induced microphase separation evolution

In this study, silicon hybrid polyurethane elastomers (SPUs) with flower-like microstructure were constructed by a two-step method according to incorporating diphenylsilanediol (DPSD) into hard segments. The reinforced microphase separation of SPUs was estimated by combining Fourier transform infrared spectrometer (FT-IR) characterization with differential scanning calorimeter (DSC) and dynamic mechanical analyzer (DMA). The silicon heteroatom-inducing flower-withering like morphology for SPUs was found with increasing silicon groups, which revealed an evolution from flowers-blossoming to broken petals like microdomain due to the ordered aggregation of hard segments. In addition, the reversible thermo-optic response of SPUs, a sharp transition from light opaque at room temperature to more than 55% transmission at 160 °C, was observed, attributing to “sea-island” or bi-continuous structural evolution owing to hydrogen bond dissociation or recombination with changing the temperature. Comparing with pure PU, the enhanced mechanical properties of SPUs were achieved, more than 20 times tensile strength (14.75 ± 2.52 MPa) and more than 3 times elongation at break (293.31 ± 2.87%). In sum, the study was of great significance for designing novel functional segmented PU materials.

Investigating electrochemical charging conditions equivalent to hydrogen gas exposure of X65 pipeline steel

AbstractIn this study, we systematically investigate electrochemical hydrogen charging conditions equivalent to hydrogen gas pressures relevant for hydrogen transportation in X65 pipeline steel. By performing hydrogen gas permeation, a relationship for Sieverts' law was established, which was used in combination with electrochemical hydrogen permeation to determine the equivalent hydrogen pressure. The results revealed that cathodic protection simulated condition at –1050 mVAg/AgCl was equivalent to a hydrogen pressure of 12.3 bar. The addition of thiourea, a hydrogen recombination poison, and changing the applied potential in the cathodic direction increased the equivalent hydrogen pressure. In this way, an electrochemical charging condition equivalent to a potential hydrogen gas pressure for hydrogen transportation (200 bar) was determined.

Investigating the effect of spray system on the containment condition of VVER1000/V446 NPP during LBLOCA and TLOFW accidents

The containment system serves as the last barrier against the release of fission products. However, over pressurization or hydrogen detonation can compromise its effectiveness, as evidenced by the Fukushima nuclear power plant (NPP) disaster. This study describes the efficacy of the spray system in reducingcontainment pressure andtemperature, as well as,itsimpacton hydrogen mitigation during large break loss of coolant accident (LBLOCA) and total loss of feed water (TLOFW) accident in the VVER-1000/V446NPP. The evaluation is conductedusing theMELCOR 1.8.6code.The results of the LBLOCA analysis demonstrate that the spray system significantly reduces the pressure and temperature of the containment. It also enhances hydrogen mitigation by increasing the amount of recombined hydrogen by around 4% and reducing the onset time of hydrogen recombination by 90,000 s. In the TLOFW accident scenario, the spray system is effective in reducing the containment pressure from 0.41 MPa to 0.27 MPa. The investigation of hydrogen behavior in the reactor containment atmosphere reveals that the simultaneous operation of the spray and the hydrogen recombiner systems is the most effective strategy for hydrogen mitigation. However, activating the spray system without the passive autocatalytic recombiners (PARs) function can lead to an increase in the hydrogen mole fraction in all containment compartments, thereby increasing the likelihood of hydrogen combustion.

Infrared Spectroscopy of Be Stars: Influence of the Envelope Parameters on Brackett-Series Behaviour

The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there is a lack of synthetic spectra with which to compare observations. Therefore, we computed synthetic spectra with the HDUST code for different disc parameters. Here, we present our results on the spectral region that includes lines of the Brackett series. We discuss the dependence of the line series strengths on several parameters that describe the structure of the disc. We also compared model line profiles, fluxes, and EWs with observational data for two Be stars (MX Pup and π Aqr). Even though the synthetic spectra adequately fit our observations of both stars and allow us to constrain the parameters of the disc, there is a discrepancy with the observed data in the EW and flux measurements, especially in the case of MX Pup. It is possible that by including Brackett lines of higher terms or adding the analysis of other series, we may be able to better constrain the parameters of the observed disc.

Evaluation of hydrogen recombination characteristics of a PAR using SPARC PAR experimental results

Passive auto-catalytic recombiners (PARs) are widely used to mitigate a hydrogen hazard. The first step to evaluate the hydrogen safety by PARs is to obtain qualified test data of the PARs for validation of their analytical model. SPARC PAR tests SP8 and SP9 were conducted to evaluate the hydrogen recombination characteristics of a honeycomb-shaped catalyst PAR. To obtain the hydrogen recombination rate from the PAR test data, two methods, Method-1 and Method-2, introduced by the THAI project, were applied. Since a large gradient of hydrogen concentration developed during hydrogen injection can cause a large error in the hydrogen mass obtained by integrating the measured hydrogen concentrations, a gate was installed at the PAR inlet to homogenize hydrogen in the test vessel before the PAR operation in the tests. A computational fluid dynamics (CFD) code with a PAR model was also applied to evaluate the characteristics of the PAR recombination according to the PAR inlet conditions, and the results were compared with those from Method-1 and Method-2. It was confirmed that the recombination rates from Method-1 require a correction factor to be compatible with results from Method-2 and the CFD simulation in the case of the SPARC-PAR tests.

Hydrogen egress from palladium surfaces: A microelectrode-based investigation

Atomic hydrogen could participate in multiple electrochemical reactions during egress from Palladium (Pd) into aqueous environments. Possible reactions include hydrogen recombination to form H2, the oxygen reduction reaction (ORR) and “hydrogen ionisation” to form H+. Here, bulk measurements and microelectrode-based methods were used to mechanistically investigate such reactions occurring during hydrogen egress from Pd. Significant ORR was detected on hydrogen-charged Pd (Pd-H) surfaces, when using a Platinum (Pt) microelectrode (ME) in the redox competition mode. However, minor hydrogen recombination was also detected while using the Pt ME in the sample generation-tip collection (SG-TC) mode. Hydrogen ionisation is observed to occur when Pd (H) is anodically polarised. These reactions can all be linked to the highly negative equilibrium potential of the atomic H/H+ redox couple. Atomic hydrogen being a strong reducing agent thus tends to efficiently reduce various oxidants in solution resulting in different products, under varying conditions.

Numerical simulations of the random angular momentum in convection – II. Delayed explosions of red supergiants following ‘failed’ supernovae

ABSTRACT When collapse of the iron core in a massive red or yellow supergiant does not lead to an energetic supernova, a significant fraction of the convective hydrogen envelope will fall in towards the black hole formed from the collapsing core. The random velocity field in the convective envelope results in finite specific angular momentum in each infalling shell. Using 3D hydrodynamical simulations, we follow the infall of this material to small radii, resolving the circularization radii of the flow. We show that infall of the convective envelope leads to nearly complete envelope ejection in a ≳1048 erg explosion with outflow speeds of ≳200 km s−1. The light curve of such an explosion would show a characteristic, red plateau as the ejecta cools and a hydrogen recombination front recedes through the expanding ejecta. Adopting supernova IIp scalings, the event would have a plateau luminosity of ≳1040 erg s−1 and a duration of several hundreds of days. These events would appear quite similar to luminous red novae with red or yellow supergiant progenitors; some luminous red novae may, in fact, be signposts of black hole formation. The mechanism studied here produces more energetic explosions than the weak shock generated from radiation of neutrino energy during the protoneutron star phase. Because we cannot simulate all the way to the horizon, our results are likely lower limits on the energy and luminosity of transients produced during the collapse of a red or yellow supergiant to form a black hole.

Detection of a High-velocity Jet from MWC 349A Traced by Hydrogen Recombination Line Maser Emission

MWC 349A is one of the rare stars known to have hydrogen radio recombination line (RRL) masers. The bright maser emission makes it possible to study the dynamics of the system at milliarcsecond (mas) precision. We present Atacama Large Millimeter/submillimeter Array observations of the 1.4 and 0.8 mm continuum emission of MWC 349A, as well as the H30α and H26α RRLs. Using the most extended array configuration of C43-10 with a maximum baseline of 16.2 km, we spatially resolved the H30α line and 1.4 mm continuum emission for the first time. In addition to the known H30α and H26α maser emission from a Keplerian disk at LSR velocities from −12 to 28 km s−1 and from an ionized wind for velocities between −12 to −40 km s−1 and 28 to 60 km s−1, we found evidence of a jet along the polar axis at V LSR from −85 to −40 km s−1 and +60 to +100 km s−1. These masers are found in a linear structure nearly aligned with the polar axis of the disk. If these masers lie close to the polar axis, their velocities could be as high as 575 km s−1, which cannot be explained solely by a single expanding wind as proposed in Báez-Rubio et al. We suggest that they originate from a high-velocity jet, likely launched by a magnetohydrodynamic wind. The jet appears to rotate in the same direction as the rotation of the disk. A detailed radiative transfer modeling of these emissions will further elucidate the origin of these masers in the wind.