Nature,5 September 2019,Volume 573 Issue 7772
《自然》 2019年9月5日 第573卷7772期
Fast inflows as the adjacent fuel of supermassive black hole accretion disks in quasars
類星體核區為超大質量黑洞吸積盤提供吸積燃料
▲ 連結：
https://www.nature.com/articles/s41586-019-1510-y
Quasars, which are exceptionally bright objects at the centres (or nuclei) of galaxies, are thought to be produced through the accretion of gas into disks surrounding supermassive black holes. There is observational evidence at galactic and circumnuclear scales that gas flows inwards towards accretion disks around black holes, and such an inflow has been measured at the scale of the dusty torus that surrounds the central accretion disk. At even smaller scales, inflows close to an accretion disk have been suggested to explain the results of recent modelling of the response of gaseous broad emission lines to continuum variations. However, unambiguous observations of inflows that actually reach accretion disks have been elusive. Here we report the detection of redshifted broad absorption lines of hydrogen and helium atoms in a sample of quasars. The lines show broad ranges of Doppler velocities that extend continuously from zero to redshifts as high as about 5,000 kilometres per second. We interpret this as the inward motion of gases at velocities comparable to freefall speeds close to the black hole, constraining the fastest infalling gas to within 10,000 gravitational radii of the black hole (the gravitational radius being the gravitational constant multiplied by the object mass, divided by the speed of light squared). Extensive photoionization modelling yields a characteristic radial distance of the inflow of approximately 1,000 gravitational radii, possibly overlapping with the outer accretion disk.
Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b
系外行星LHS 3844b上沒有厚厚的大氣層
▲ AbstractMost known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point (latitude and longitude of 0 degrees)—possibly indicative of atmospheric circulation. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS 3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS 3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040 ± 40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.
Magnitude of urban heat islands largely explained by climate and population
氣候和人口影響城市熱島的大小
Changing climate both increases and decreases European river floods
氣候變化影響歐洲洪水
在這裡，研究人員展示了過去50年歐洲河流洪水量增加和減少的明顯區域模式，這是氣候變化的表現。
來自迄今為止最完整的歐洲洪水資料庫結果表明，秋冬季降雨量的增加導致了歐洲西北部洪水的增加，降水減少和蒸發增加導致南歐大中型流域洪水減少，氣溫升高、積雪和融雪量減少，導致東歐洪水減少。
歐洲區域洪水流量趨勢從每十年增加約11%到減少23%不等。
儘管觀測記錄在空間和時間上存在異質性，研究人員表示，這裡確定的洪水變化與下個世紀的氣候模型預測基本一致，表明氣候驅動的變化已經發生，並支持在洪水風險管理中考慮氣候變化問題。
▲ AbstractClimate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe, because of the limited spatial coverage and number of hydrometric stations.Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results—arising from the most complete database of European flooding so far—suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management.
Fossil insect eyes shed light on trilobite optics and the arthropod pigment screen
昆蟲眼化石揭秘節肢動物視覺能力
Fossilized eyes permit inferences of the visual capacity of extinct arthropods. However, structural and/or chemical modifications as a result of taphonomic and diagenetic processes can alter the original features, thereby necessitating comparisons with modern species. Here we report the detailed molecular composition and microanatomy of the eyes of 54-million-year-old crane-flies, which together provide a proxy for the interpretation of optical systems in some other ancient arthropods. These well-preserved visual organs comprise calcified corneal lenses that are separated by intervening spaces containing eumelanin pigment. We also show that eumelanin is present in the facet walls of living crane-flies, in which it forms the outermost ommatidial pigment shield in compound eyes incorporating a chitinous cornea. To our knowledge, this is the first record of melanic screening pigments in arthropods, and reveals a fossilization mode in insect eyes that involves a decay-resistant biochrome coupled with early diagenetic mineralization of the ommatidial lenses. The demonstrable secondary calcification of lens cuticle that was initially chitinous has implications for the proposed calcitic corneas of trilobites, which we posit are artefacts of preservation rather than a product of in vivo biomineralization. Although trilobite eyes might have been partly mineralized for mechanical strength, a (more likely) organic composition would have enhanced function via gradient-index optics and increased control of lens shape.
Natural selection on the Arabidopsis thaliana genome in present and future climates
擬南芥基因組在當前和未來氣候中的自然選擇
在這種氣候驅動變異的自然選擇中，很大一部分可以從局部適應的特徵中預測出來。
此外，跨物種範圍的實地驗證預測表明，地中海和西伯利亞西部的擬南芥種群目前經歷了最強烈的氣候驅動選擇。
隨著歐洲更頻繁的乾旱和氣溫的上升，我們預測，從歐洲南端向北移動的定向自然選擇將會增加，這將使許多歐洲本土的擬南芥種群面臨進化風險。▲ AbstractThrough the lens of evolution, climate change is an agent of natural selection that forces populations to change and adapt, or face extinction. However, current assessments of the risk of biodiversity associated with climate change do not typically take into account how natural selection influences populations differently depending on their genetic makeup. Here we make use of the extensive genome information that is available for Arabidopsis thaliana and measure how manipulation of the amount of rainfall affected the fitness of 517 natural Arabidopsis lines that were grown in Spain and Germany. This allowed us to directly infer selection along the genome. Natural selection was particularly strong in the hot-dry location in Spain, where 63% of lines were killed and where natural selection substantially changed the frequency of approximately 5% of all genome-wide variants. A significant portion of this climate-driven natural selection of variants was predictable from signatures of local adaptation (R2= 29–52%), as genetic variants that were found in geographical areas with climates more similar to the experimental sites were positively selected. Field-validated predictions across the species range indicated that Mediterranean and western Siberian populations—at the edges of the environmental limits of this species—currently experience the strongest climate-driven selection. With more frequent droughts and rising temperatures in Europe, we forecast an increase in directional natural selection moving northwards from the southern end of Europe, putting many native A. thaliana populations at evolutionary risk.合作事宜：market@stimes.cn
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