A 14‐step synthesis of (+)‐cochlearol B is reported. This renoprotective meroterpenoid features a unique core structure containing a densely substituted cyclobutane ring with three stereocenters. Our ...strategy employed an organocatalytic Kabbe condensation in route to the key chromenyl triflate. A subsequent Catellani reaction incorporated the remaining carbon atoms featured in the skeleton of cochlearol B. An ensuing visible‐light‐mediated 2+2 photocycloaddition closed the cyclobutane and formed the central bicyclo3.2.0heptane core. Notably, careful design and tuning of the Catellani and photocycloaddition reactions proved crucial in overcoming undesired reactivity, including cyclopropanation reactions and 4+2 cycloadditions.
A 14‐step approach to (+)‐cochlearol B is reported. The strategy involves an organocatalytic Kabbe condensation, Catellani reaction, and visible‐light‐mediated 2+2 cycloaddition to rapidly access the core of this natural product. Careful design and tuning of the Catellani and photocycloaddition reactions proved crucial in overcoming undesired reactivity, including cyclopropanation reactions, and 4+2 cycloadditions.
Animal studies suggest that obesity-related diets induce structural changes in the hypothalamus, a key brain area involved in energy homeostasis. Whether this translates to humans is however largely ...unknown. Using a novel multimodal approach with manual segmentation, we here show that a higher body mass index (BMI) selectively predicted higher proton diffusivity within the hypothalamus, indicative of compromised microstructure in the underlying tissue, in a well-characterized population-based cohort (n
= 338, 48% females, age 21-78 years, BMI 18-43 kg/m²). Results were independent from confounders and confirmed in another independent sample (n
= 236). In addition, while hypothalamic volume was not associated with obesity, we identified a sexual dimorphism and larger hypothalamic volumes in the left compared to the right hemisphere. Using two large samples of the general population, we showed that a higher BMI specifically relates to altered microstructure in the hypothalamus, independent from confounders such as age, sex and obesity-associated co-morbidities. This points to persisting microstructural changes in a key regulatory area of energy homeostasis occurring with excessive weight. Our findings may help to better understand the pathomechanisms of obesity and other eating-related disorders.
We review observations of extended regions of radio emission in clusters; these include diffuse emission in ‘relics’, and the large central regions commonly referred to as ‘halos’. The spectral ...observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.
Iron(III)-catalyzed carbonyl-olefin ring-closing metathesis represents a new approach toward the assembly of molecules traditionally generated by olefin–olefin metathesis or olefination. Herein, we ...report detailed synthetic, spectroscopic, kinetic, and computational studies to determine the mechanistic features imparted by iron(III), substrate, and temperature to the catalytic cycle. These data are consistent with an iron(III)-mediated asynchronous, concerted 2+2-cycloaddition to form an intermediate oxetane as the turnover-limiting step. Fragmentation of the oxetane via Lewis acid-activation results in the formation of five- and six-membered unsaturated carbocycles.
We study the case of a bright (L > L
) barred spiral galaxy from the rich cluster A3558 in the Shapley supercluster core (z = 0.05) undergoing ram-pressure stripping. Integral-field spectroscopy with ...Wide Field Spectrograph (WiFeS) at the 2.3-m Australian National University telescope, complemented by imaging in ultraviolet (GALEX), B and R European Southern Observatory (ESO) 2.2-m WFI, Hα (Magellan), K (United Kingdom Infrared Telescope), 24 and 70 μm (Spitzer), allows us to reveal the impact of ram pressure on the interstellar medium. With these data we study in detail the kinematics and the physical conditions of the ionized gas and the properties of the stellar populations. We observe one-sided extraplanar ionized gas along the full extent of the galaxy disc, extending ∼13 kpc in projection from it. Narrow-band Hα imaging resolves this outflow into a complex of knots and filaments, similar to those seen in other cluster galaxies undergoing ram-pressure stripping. The gas velocity field is complex with the extraplanar gas showing signature of rotation, while the stellar velocity field is regular and the K-band image shows a symmetric stellar distribution. We use line-ratio diagnostics to ascertain the origin of the observed emission. In all parts of the galaxy, we find a significant contribution from shock excitation, as well as emission powered by star formation. Shock-ionized gas is associated with the turbulent gas outflow and highly attenuated by dust (A
v
= 1.5-2.3 mag). All these findings cover the whole phenomenology of early-stage ram-pressure stripping. Intense, highly obscured star formation is taking place in the nucleus, probably related to the bar, and in a region 12 kpc south-west (SW) from the centre. These two regions account for half of the total star formation in the galaxy, which overall amounts to 7.2 ± 2.2 M yr−1. In the SW region we identify a starburst characterized by a ∼5× increase in the star formation rate over the last ∼100 Myr, possibly related to the compression of the interstellar gas by the ram pressure. The scenario suggested by the observations is supported and refined by ad hoc N-body/hydrodynamical simulations which identify a rather narrow temporal range for the onset of ram-pressure stripping around t ∼ 60 Myr ago, and an angle between the galaxy rotation axis and the intracluster medium wind of ∼45°. The ram pressure is therefore acting at an intermediate angle between face-on and edge-on. Taking into account that the galaxy is found ∼1 Mpc from the cluster centre in a relatively low density region, this study shows that ram-pressure stripping still acts efficiently on massive galaxies well outside the cluster cores, as also recently observed in the Virgo cluster.
Aims. We investigate the influence of ram-pressure stripping on the star formation and the mass distribution in simulated spiral galaxies. Special emphasis is put on the question of where the newly ...formed stars are located. The stripping radius from the simulation is compared to analytical estimates. Methods. Disc galaxies are modelled in combined N-body/hydrodynamic simulations (GADGET-2) with prescriptions for cooling, star formation, stellar feedback, and galactic winds. These model galaxies move through a constant density and temperature gas, which has parameters comparable to the intra-cluster medium (ICM) in the outskirts of a galaxy cluster (T = 3 keV ≈ 3.6 $\times$ 107 K and ρ = 10-28 g/cm3). With this numerical setup we analyse the influence of ram-pressure stripping on the star formation rate of the model galaxy. Results. We find that the star formation rate is significantly enhanced by the ram-pressure effect (up to a factor of 3). Stars form in the compressed central region of the galaxy, as well as in the stripped gas behind the galaxy. Newly formed stars can be found up to hundred kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter and with masses of up to 107 $M_{\odot}$. As they do not possess a dark matter halo due to their formation history, we name them “stripped baryonic dwarf” galaxies. We also find that the analytical estimate for the stripping radius from a Gunn & Gott (1972) criterion agrees well with the numerical value from the simulation. Like in former investigations, edge-on systems lose less gas than face-on systems, and the resulting spatial distribution of the gas and the newly formed stars is different.
To date, exploiting the full bandwidth of state-of-the-art InGaAs photomixers for generation and detection of continuous-wave (CW) THz radiation (typ. ~50 GHz to ~3 THz) required complex and costly ...external-cavity diode lasers with motorized resonator control. Distributed feedback (DFB) lasers, by contrast, are compact and inexpensive, but the tuning range per diode is limited to ~600 GHz at 1.5 μm. In this paper, we show that a combination of three DFB diodes covers the complete frequency range from 0 – 2750 GHz without any gaps. In combination with InGaAs-based photomixers for terahertz generation and detection, the system achieves a dynamic range of > 100 dB at 56 GHz, 64 dB at 1000 GHz, and 26 dB at 2500 GHz. A field-programmable gate array (FPGA)-based lock-in amplifier permits a flexible adjustment of the integration time from 0.5 ms to 600 ms. Employing an optimized “fast scan” mode, a spectrum of ~1200 GHz – the bandwidth of each subset of two lasers – and 40 MHz steps is acquired in less than one minute, still maintaining a reasonable dynamic range. To the best of our knowledge, the bandwidth of 2.75 THz presents a new record for DFB-based CW-terahertz systems.
Aims. We investigate the influence of stellar bulges on the star formation and morphology of disc galaxies that suffer from ram pressure. Several tree-SPH (smoothed particle hydrodynamics) ...simulations have been carried out to study the dependence of the star formation rate on the mass and size of a stellar bulge. In addition, different strengths of ram pressure and different alignments of the disc with respect to the intra-cluster medium (ICM) are applied. Methods. The simulations were carried out with the combined N-body/hydrodynamic code GADGET-2 with radiative cooling and a recipe for star formation. The same galaxy with different bulge sizes was used to accomplish 31 simulations with varying inclination angles and surrounding gas densities of 10-27g cm-3 and 10-28g cm-3. For all the simulations a relative velocity of 1000 km s-1 for the galaxies and an initial gas temperature for the ICM of 107K were applied. Besides galaxies flying edge-on and face-on through the surrounding gas, various disc tilt angles in between were used. To allow a comparison, the galaxies with the different bulges were also evolved in isolation to contrast the star formation rates. Furthermore, the influence of different disc gas mass fractions has been investigated. Results. As claimed in previous works, when ram pressure is acting on a galaxy, the star formation rate (SFR) is enhanced and rises up to four times with increasing ICM density compared to galaxies that evolve in isolation. However, a bulge suppresses the SFR when the same ram pressure is applied. Consequently, fewer new stars are formed because the SFR can be lowered by up to 2M⊙ yr-1. Furthermore, the denser the surrounding gas, the more interstellar medium (ISM) is stripped. While at an ICM density of 10-28g cm-3 about 30% of the ISM is stripped, the galaxy is almost completely (more than 90%) stripped when an ICM density of 10-27g cm-3 is applied. But again, a bulge prevents the stripping of the ISM and reduces the amount being stripped by up to 10%. Thereby, fewer stars are formed in the wake if the galaxy contains a bulge. The dependence of the SFR on the disc tilt angle is not very pronounced. Merely a slight trend of decreasing star formation with increasing inclination angle can be determined. Furthermore, with increasing disc tilt angles, less gas is stripped and therefore fewer stars are formed in the wake. Reducing the disc gas mass fraction results in a lower SFR when the galaxies evolve in vacuum. On the other hand, the enhancement of the SFR in case of acting ram pressure is less pronounced with increasing gas mass fraction. Moreover, the fractional amount of stripped gas does not depend on the gas mass fraction.
Ultrafast pump–probe experiments open the possibility to track fundamental material behaviour, such as changes in electronic configuration, in real time. To date, most of these experiments are ...performed using an electron or a high-energy photon beam that is synchronized to an infrared laser pulse. Entirely new opportunities can be explored if not only a single, but multiple synchronized, ultrashort, high-energy beams are used. However, this requires advanced radiation sources that are capable of producing dual-energy electron beams, for example. Here, we demonstrate simultaneous generation of twin-electron beams from a single compact laser wakefield accelerator. The energy of each beam can be individually adjusted over a wide range and our analysis shows that the bunch lengths and their delay inherently amount to femtoseconds. Our proof-of-concept results demonstrate an elegant way to perform multi-beam experiments in the future on a laboratory scale.A laser-based scheme for the simultaneous generation of two temporally synchronized electron beams with individually adjustable energies offers new opportunities for ultrafast pump–probe experiments.
Plasma wakefield acceleration (PWFA) is a novel acceleration technique with promising prospects for both particle colliders and light sources. However, PWFA research has so far been limited to a few ...large-scale accelerator facilities worldwide. Here, we present first results on plasma wakefield generation using electron beams accelerated with a 100-TW-class Ti:sapphire laser. Because of their ultrashort duration and high charge density, the laser-accelerated electron bunches are suitable to drive plasma waves at electron densities in the order of1019cm−3. We capture the beam-induced plasma dynamics with femtosecond resolution using few-cycle optical probing and, in addition to the plasma wave itself, we observe a distinctive transverse ion motion in its trail. This previously unobserved phenomenon can be explained by the ponderomotive force of the plasma wave acting on the ions, resulting in a modulation of the plasma density over many picoseconds. Because of the scaling laws of plasma wakefield generation, results obtained at high plasma density using high-current laser-accelerated electron beams can be readily scaled to low-density systems. Laser-driven PWFA experiments can thus act as miniature models for their larger, conventional counterparts. Furthermore, our results pave the way towards a novel generation of laser-driven PWFA, which can potentially provide ultralow emittance beams within a compact setup.
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