《自然》(20230323出版)一周論文導讀

科學網 發佈 2023-03-27T07:09:47.777469+00:00

編譯 | 未玖Nature, 23 March 2023, VOL 615, ISSUE 7953《自然》2023年3月23日,第615卷,7953期天文學AstronomyA helium-burning white dwarf binary as a supersoft X-

編譯 | 未玖


Nature, 23 March 2023, VOL 615, ISSUE 7953

《自然》2023年3月23日,第615卷,7953期




天文學Astronomy








A helium-burning white dwarf binary as a supersoft X-ray source

氦燃燒白矮星雙星可作為超軟X射線源

▲ 作者:J. Greiner, C. Maitra, F. Haberl, R. Willer, J. M. Burgess, N. Langer, et al.

▲ 連結:

https://www.nature.com/articles/s41586-023-05714-4

▲ 摘要:

Ia型超新星是宇宙距離的指示器,也是宇宙中鐵的主要來源,但它們的形成路徑仍有爭議。幾十個超軟X射線源(其中一顆白矮星從一顆非簡併供體恆星中吸積富氫物質)已被觀測到,並被認為是Ia型超新星的前體。


然而人們預期的氫在超新星爆炸期間從供體恆星上剝離的觀測證據仍然缺乏。30多年前就有人預測,吸積氦的白矮星可避開這個問題,包括它們作為超軟X射線源的出現,但迄今為止尚未探測到。


研究組報導了一個吸積盤光譜完全由氦主導的超軟X射線源,這表明供體恆星不含氫。他們認為明亮的超軟X射線因吸積白矮星表面附近氦燃燒產生。


該系統的性質為基於氦吸積的錢德拉塞卡質量爆炸擴展路徑提供了證據,尤其是在白矮星中以低於目前預期的吸積率穩定燃燒。在這種情況下,人們有望恢復亞高能被稱為Iax型超新星的數量,高達所有Ia型超新星總數的30%。

▲ Abstract:

Type Ia supernovae are cosmic distance indicators, and the main source of iron in the Universe, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed and suggested as Type Ia supernovae progenitors. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years, including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae, within this scenario.








物理學Physics








Diode effect in Josephson junctions with a single magnetic atom

單個磁原子約瑟夫森結中的二極體效應

▲ 作者:Martina Trahms, Larissa Melischek, Jacob F. Steiner, Bharti Mahendru, Idan Tamir, Nils Bogdanoff, et al.

▲ 連結:

https://www.nature.com/articles/s41586-023-05743-z

▲ 摘要:

電子器件中的電流流動可能與偏置方向不對稱,這是二極體效用的一種潛在現象,被稱為非互易電荷傳輸。最近,無耗散電子學的前景刺激了人們對超導二極體的追求,非互易超導器件已在各種非中心對稱系統中實現。


研究組通過在掃描隧道顯微鏡中創建原子尺度的Pb-Pb約瑟夫森結來研究微型化的最終極限。由單個Pb原子穩定的原始結表現出遲滯行為,證實了結的高質量,但偏置方向之間沒有不對稱性。當將單個磁原子嵌入結中時,會出現非互易超電流,其首選方向取決於原子種類。


藉助理論建模,研究組追蹤了超導能隙內電子空穴不對稱Yu-Shiba-Rusinov態流動的准粒子電流的非互易性,並確定了約瑟夫森結中二極體行為的新機制。該研究結果為創造原子級約瑟夫森二極體和通過單原子操作調整其特性開闢了新途徑。

▲ Abstract:

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Here we investigate the ultimate limits of miniaturization by creating atomic-scale Pb–Pb Josephson junctions in a scanning tunnelling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behaviour, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing by means of electron–hole asymmetric Yu–Shiba–Rusinov states inside the superconducting energy gap and identify a new mechanism for diode behaviour in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation.








信息科學Information Science








Dense reinforcement learning for safety validation of autonomous vehicles

密集強化學習助力自動駕駛汽車安全性驗證

▲ 作者:Shuo Feng, Haowei Sun, Xintao Yan, Haojie Zhu, Zhengxia Zou, Shengyin Shen, et al.

▲ 連結:

https://www.nature.com/articles/s41586-023-05732-2

▲ 摘要:

阻礙自動駕駛汽車開發和部署的一個關鍵瓶頸是,由於安全關鍵事件很罕見,在自然駕駛環境中驗證其安全性需要高昂的經濟和時間成本。


研究組開發了一個智能測試環境,其中基於人工智慧的後台代理經過訓練,以驗證自動駕駛汽車在加速模式下的安全性能,而不損失無偏性。從自然駕駛數據中,後台代理通過密集深度強化學習(D2RL)方法學習要執行何種對抗性操作,通過刪除非安全關鍵狀態並重新連接關鍵狀態來編輯馬爾可夫決策過程,從而使訓練數據中的信息密集化。


D2RL使神經網絡能夠從具有安全關鍵事件的密集信息中學習,並完成傳統深度強化學習方法難以解決的任務。研究組通過在增強現實環境下在高速公路和城市測試軌道上測試高度自動化的車輛,並將模擬背景車輛與物理道路基礎設施和真實自動駕駛測試車輛相結合,證明了該方法的有效性。


研究結果表明,經過D2RL訓練的代理可將評估過程加快多個數量級(快103到105倍)。此外,D2RL將實現與其他安全關鍵自主系統一起加速測試和培訓。


▲ Abstract:

One critical bottleneck that impedes the development and deployment of autonomous vehicles is the prohibitively high economic and time costs required to validate their safety in a naturalistic driving environment, owing to the rarity of safety-critical events. Here we report the development of an intelligent testing environment, where artificial-intelligence-based background agents are trained to validate the safety performances of autonomous vehicles in an accelerated mode, without loss of unbiasedness. From naturalistic driving data, the background agents learn what adversarial manoeuvre to execute through a dense deep-reinforcement-learning (D2RL) approach, in which Markov decision processes are edited by removing non-safety-critical states and reconnecting critical ones so that the information in the training data is densified. D2RL enables neural networks to learn from densified information with safety-critical events and achieves tasks that are intractable for traditional deep-reinforcement-learning approaches. We demonstrate the effectiveness of our approach by testing a highly automated vehicle in both highway and urban test tracks with an augmented-reality environment, combining simulated background vehicles with physical road infrastructure and a real autonomous test vehicle. Our results show that the D2RL-trained agents can accelerate the evaluation process by multiple orders of magnitude (103 to 105 times faster). In addition, D2RL will enable accelerated testing and training with other safety-critical autonomous systems.








化學Chemistry








Catalytic asymmetric synthesis of cannabinoids and menthol from neral

從檸檬醛催化不對稱合成大麻素和薄荷醇

▲ 作者:Joyce A. A. Grimm, Hui Zhou, Roberta Properzi, Markus Leutzsch, Giovanni Bistoni, Johanna Nienhaus, et al.

▲ 連結:

https://www.nature.com/articles/s41586-023-05747-9

▲ 摘要:

將天然或合成的檸檬醛選擇性轉化為(1R,6S)-反式異戊醇將使薄荷醇和大麻素的可持續途徑得以實現和加速。然而,人們一直認為該反應無法實現,因為其產物對所需酸性催化劑的反應性比原料更強,導致一些副產物生成。


研究組表明,一種不對稱、強而空間受限的手性酸(即高度氟化的亞胺-亞胺二磷酸)能以優異的效率和選擇性催化這一過程。將該策略擴展到其他α、β-不飽和醛,可獲得此前不易獲得的新型大麻素和薄荷醇衍生物。


機理研究表明,空間受限催化劑通過將產物以非活性構象結合,阻止其分解,從而完成該反應。研究組還展示了(1R,6S)-反式異戊醇如何高效地轉化為藥用大麻素和薄荷醇,每種策略都是迄今為止最短和最原子經濟性的路線。


▲ Abstract:

The selective conversion of natural or synthetic neral to (1R,6S)-trans-isopiperitenol would enable and expedite sustainable routes to menthol and cannabinoids. However, this reaction has been considered impossible because its product is more reactive to the required acid catalysts than its starting material, resulting in several side products. We now show that an unsymmetric, strong and confined chiral acid, a highly fluorinated imino-imidodiphosphate, catalyses this process with excellent efficiency and selectivity. Expanding the method to other α,β-unsaturated aldehydes could enable access to new cannabinoids and menthol derivatives not readily accessible previously. Mechanistic studies suggest that the confined catalyst accomplishes this reaction by binding the product in an unreactive conformation, thereby preventing its decomposition. We also show how (1R,6S)-trans-isopiperitenol can be readily converted to pharmaceutically useful cannabinoids and menthol, each in the shortest and most atom-economic routes so far.








地球科學Earth Science








Basin-scale reconstruction of euxinia and Late Devonian mass extinctions

缺氧硫化與晚泥盆紀大滅絕的盆地尺度重建

▲ 作者:Swapan K. Sahoo, Geoffrey J. Gilleaudeau, Kathleen Wilson, Bruce Hart, Ben D. Barnes, Tytrice Faison, et al.

▲ 連結:

https://www.nature.com/articles/s41586-023-05716-2

▲ 摘要:

泥盆紀-石炭紀的過渡標誌著地表環境的根本轉變,主要與海洋-大氣氧化態的變化有關,這是由於維管陸生植物的持續增殖刺激了水文循環和大陸風化、冰川海面升降、陸表海富營養化和缺氧擴張,以及大規模滅絕事件。


研究組展示了整個巴肯頁岩(北美威利斯頓盆地)90個岩芯地球化學數據的綜合時空彙編。該數據集詳細記錄了有毒的缺氧硫化水逐步侵入淺海,導致了一系列晚泥盆紀滅絕事件。


顯生宙的其他滅絕也與淺水缺氧化擴張有關,這表明硫化氫毒性是顯生宙生物多樣性的關鍵驅動因素。

▲ Abstract:

The Devonian–Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean–atmosphere oxidation states, resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering, glacioeustasy, eutrophication and anoxic expansion in epicontinental seas, and mass extinction events. Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biodiversity.












Core origin of seismic velocity anomalies at Earth’s core–mantle boundary

地核-地幔邊界地震速度異常的地核起源

▲ 作者:Suyu Fu, Stella Chariton, Vitali B. Prakapenka & Sang-Heon Shim

▲ 連結:

https://www.nature.com/articles/s41586-023-05713-5

▲ 摘要:

地震研究在地核-地幔邊界(CMB)發現了精細尺度異常,如超低速帶(ULVZs)和地核剛性帶。ULVZs被歸因於地幔相關過程,但對可能的地核起源知之甚少。人們提議用外核輕元素沉澱來解釋核心剛性區,但仍不清楚是什麼過程導致了這種沉澱。儘管Fe-Si-H對外核很重要,但其在相關壓力-溫度條件下的熔化行為尚未明確。


研究組報導了在125 GPa和3700 K的氫氣存在下,通過雷射加熱的金剛石壓砧從Fe-9wt %Si熔體中觀察到B2 FeSi結晶。氫極大地增加了B2晶體中Si的濃度,使Si:Fe摩爾比達到≈1,而Si大部分保留在共存的Fe液中。


B2相中的高Si含量使其在最外層地核溫度下以固體形式穩定存在,且密度小於周圍的液體。因此,富矽晶體可在CMB界面下方形成、漂浮和沉積,這很好地解釋了地核側剛性異常。如果少量FeSi晶體能被地幔吸收,它們將在CMB上方形成緻密的低速結構,這可能是一些ULVZs形成的原因。


最外層地核H促進的B2 FeSi沉澱為CMB的兩類異常提供了單一地核驅動來源。這種情況也可用於解釋,與沉澱物平衡的物質通過與ULVZ相連的地幔羽流被夾帶到最上面地幔後,海洋島嶼玄武岩中的類核鎢同位素特徵。

▲ Abstract:

Seismic studies have found fine-scale anomalies at the core–mantle boundary (CMB), such as ultralow velocity zones (ULVZs) and the core rigidity zone. ULVZs have been attributed to mantle-related processes, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core, the melting behaviour of Fe–Si–H at relevant pressure–temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe–9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.

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