Imaging the Water Snowline around Protostars with Water and HCO+ Isotopologues

The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent (≲1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H13CO+. We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular-line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at ∼0.″2−0.″7 (60–210 au) resolution. B1-c provides a textbook example of compact H 2 18 O (31,3−22,0) and HDO (31,2−22,1) emission surrounded by a ring of H13CO+ (J = 2−1) and HC18O+ (J = 3−2). Compact HDO surrounded by H13CO+ is also detected toward B1-bS. The optically thick main isotopologue HCO+ is not suited to trace the snowline, and HC18O+ is a better tracer than H13CO+ due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H13CO+ or HC18O+ emission, their true value as a snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H 2 18 O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency.

Identification and architecture of a putative secretion tube across mycobacterial outer envelope

Tuberculosis-causing mycobacteria have thick cell-wall and capsule layers that are formed from complex structures. Protein secretion across these barriers depends on a specialized protein secretion system, but none has been reported. We show that Mycobacterium tuberculosis Rv3705c and its homologous MSMEG_6251 in Mycobacterium smegmatis are tube-forming proteins in the mycobacterial envelope (TiME). Crystallographic and cryo-EM structures of these two proteins show that both proteins form rotationally symmetric rings. Two layers of TiME rings pack together in a tail-to-tail manner into a ring-shaped complex, which, in turn, stacks together to form tubes. M. smegmatis TiME was detected mainly in the cell wall and capsule. Knocking out the TiME gene markedly decreased the amount of secreted protein in the M. smegmatis culture medium, and expression of this gene in knocked-out strain partially restored the level of secreted protein. Our structure and functional data thus suggest that TiME forms a protein transport tube across the mycobacterial outer envelope.

Influence of air layer on temperature and humidity characteristics of outer envelope structure with internal heat insulation

Objective. The article addresses the process of heat and mass transfer in outer envelope structure with internal heat insulation. To prevent condensate formation, it is proposed to use a ventilated air duct inside the wall structure. Methods. The study was carried out using numerical simulation methods. Air movement in the duct was described by solving the Navier-Stokes equation in k-ε approximation. Condensate formation was analysed via joint resolution of heat conduction and diffusion equations. The problem of heat and mass transfer was addressed for a structure with internal heat insulation, behind the layer of which an air duct was located with air movement occurring due to natural or forced convection. Results. As part of the work, it was shown that the use of an air duct significantly reduces the thermal resistance of the structure, and an increase in the airflow rate leads to a decrease in thermal resistance and the likelihood of condensate formation. A decrease in thermal resistance with an increase in air flow rate into interlayers occurs faster than an increase in the amount of air-entrained moisture. Conclusion. Results of the work have shown that the use of an air duct throughout the entire period of operation of a building is not effective, but it is proposed to use this duct periodically in winter to dry the condensate.

BonA from Acinetobacter baumannii Forms a Divisome-Localized Decamer That Supports Outer Envelope Function

The pathogen Acinetobacter baumannii is considered an urgent threat to human health. A. baumannii is highly resistant to treatment with antibiotics, in part due to its protective cell envelope. This bacterium is only distantly related to other bacterial pathogens, so its cell envelope has distinct properties and contains components distinct from those of other bacteria that support its function.

Insertion of plastidic β-barrel proteins into the outer envelopes of plastids involves an intermembrane space intermediate formed with Toc75-V/OEP80

The insertion of organellar membrane proteins with the correct topology requires the following: First, the proteins must contain topogenic signals for translocation across and insertion into the membrane. Second, proteinaceous complexes in the cytoplasm, membrane, and lumen of organelles are required to drive this process. Many complexes required for the intracellular distribution of membrane proteins have been described, but the signals and components required for the insertion of plastidic β-barrel-type proteins into the outer membrane are largely unknown. The discovery of common principles is difficult, as only a few plastidic β-barrel proteins exist. Here, we provide evidence that the plastidic outer envelope β-barrel proteins OEP21, OEP24, and OEP37 from pea (Pisum sativum) and Arabidopsis thaliana contain information defining the topology of the protein. The information required for translocation of pea proteins across the outer envelope membrane is present within the six N-terminal β-strands. This process requires the action of TOC (translocon of the outer chloroplast membrane). After translocation into the intermembrane space, β-barrel proteins interact with TOC75-V, as exemplified by OEP37 and P39, and are integrated into the membrane. The membrane insertion of plastidic β-barrel proteins is affected by mutation of the last β-strand, suggesting that this strand contributes to the insertion signal. These findings shed light on the elements and complexes involved in plastidic β-barrel protein import.

Stopping Super-Earths from growing into Jupiters

<p>Super-Earths are by far the most dominant type of exoplanet, yet their formation is<br />still not well understood. In particular, planet formation models predict that many<br />of them should have accreted enough gas to become gas giants. Here we examine the<br />role of the protoplanetary disk in the cooling and contraction of the protoplanetary<br />envelope. In particular, we investigate the effects of 1) the thermal state of the disk as<br />set by the relative size of heating by accretion or irradiation, and whether its energy is<br />transported by radiation or convection, and 2) advection of entropy into the outer envelope by disk flows that penetrate the Hill sphere, as found in 3D global simulations.<br />We find that, at 0.1 AU, the envelope quickly becomes fully radiative, nearly isothermal, and thus cannot cool down, stalling gas accretion. This<br />effect is significantly more pronounced in convective disks, leading to envelope mass or-<br />ders of magnitude lower. Entropy advection at 0.1 AU in either radiative or convective<br />disks could therefore explain why super-Earths failed to undergo runaway accretion.</p> <p>Ali-Dib, Cumming, & Lin (MNRAS 2020)</p>

Turbulence in the Sun is suppressed on large scales and confined to equatorial regions

Convection in the Sun’s outer envelope generates turbulence and drives differential rotation, meridional circulation, and the global magnetic cycle. We develop a greater understanding of these processes by contrasting observations with simulations of global convection. These comparisons also enhance our comprehension of the physics of distant Sun-like stars. Here, we infer toroidal flow power as a function of wave number, frequency, and depth in the solar interior through helioseismic analyses of space-based observations. The inferred flows grow with spatial wave number and temporal frequency and are confined to low latitudes, supporting the argument that rotation induces systematic differences between the poles and equator. In contrast, the simulations used here show the opposite trends—power diminishing with increasing wave number and frequency while flow amplitudes become weakest at low latitudes. These differences highlight gaps in our understanding of solar convection and point to challenges ahead.

The expansion of stripped-envelope stars: Consequences for supernovae and gravitational-wave progenitors

Massive binaries that merge as compact objects are the progenitors of gravitational-wave sources. Most of these binaries experience one or more phases of mass transfer, during which one of the stars loses all or part of its outer envelope and becomes a stripped-envelope star. The evolution of the size of these stripped stars is crucial in determining whether they experience further interactions and understanding their ultimate fate. We present new calculations of stripped-envelope stars based on binary evolution models computed with MESA. We use these to investigate their radius evolution as a function of mass and metallicity. We further discuss their pre-supernova observable characteristics and potential consequences of their evolution on the properties of supernovae from stripped stars. At high metallicity, we find that practically all of the hydrogen-rich envelope is removed, which is in agreement with earlier findings. Only progenitors with initial masses below 10 M⊙ expand to large radii (up to 100 R⊙), while more massive progenitors remain compact. At low metallicity, a substantial amount of hydrogen remains and the progenitors can, in principle, expand to giant sizes (> 400 R⊙) for all masses we consider. This implies that they can fill their Roche lobe anew. We show that the prescriptions commonly used in population synthesis models underestimate the stellar radii by up to two orders of magnitude. We expect that this has consequences for the predictions for gravitational-wave sources from double neutron star mergers, particularly with regard to their metallicity dependence.

The imprint of the protoplanetary disc in the accretion of super-Earth envelopes

ABSTRACT Super-Earths are by far the most dominant type of exoplanet, yet their formation is still not well understood. In particular, planet formation models predict that many of them should have accreted enough gas to become gas giants. Here, we examine the role of the protoplanetary disc in the cooling and contraction of the protoplanetary envelope. In particular, we investigate the effects of (1) the thermal state of the disc as set by the relative size of heating by accretion or irradiation, and whether its energy is transported by radiation or convection, and (2) advection of entropy into the outer envelope by disc flows that penetrate the Hill sphere, as found in 3D global simulations. We find that, at 5 and 1 au, this flow at the level reported in the non-isothermal simulations where it penetrates only to ∼0.3 times the Hill radius has little effect on the cooling rate since most of the envelope mass is concentrated close to the core, and far from the flow. On the other hand, at 0.1 au, the envelope quickly becomes fully radiative, nearly isothermal, and thus cannot cool down, stalling gas accretion. This effect is significantly more pronounced in convective discs, leading to envelope mass orders of magnitude lower. Entropy advection at 0.1 au in either radiative or convective discs could therefore explain why super-Earths failed to undergo runaway accretion. These results highlight the importance of the conditions and energy transport in the protoplanetary disc for the accretion of planetary envelopes.