河西走廊春末夏初降水的空间异常分布及年代际变化

利用河西走廊19个气象代表站建站至2002年5～6月降水量资料, 分析了河西走廊春末夏初干旱的基本气候特征; 在利用EOF和REOF方法进行降水空间异常变化分析和气候分区的基础上,讨论了第一时间系数(PC1)及各区代表站降水量的年代际变化规律. 结果表明, 河西走廊春末夏初降水量在第一空间尺度上为全区一致; 在第二空间尺度上可分为3个气候区; 在第三空间尺度上可分为5个自然气候区. 1980年代为近50 a来降水最多的10 a, 1990年代有所减少, 20世纪末至21世纪初有明显增加. 前期冬季欧亚径向环流加强, 亚洲区极涡面积扩大、强度加强, 冷空气活动频繁, 将有利于次年春末夏初河西走廊降水偏多. 欧洲青藏高原华北西太平洋的波列, 特别是东亚大槽的填塞和青藏高原低值系统频繁活动, 造成了500 hPa高空场上"东高西低"的典型多雨流型.     

Molecular lines from protoplanetary nebulae: observations with ALMA

Planetary nebulae (PNe) are formed in a very fast process. In just about 1000 years, the nebula evolves from a spherical and slowly expanding AGB envelope to a PN, with usually axial symmetry and high axial velocities. Molecular lines are known to probe most of the nebular material in young PNe and protoplanetary nebulae (PPNe), and are therefore very useful to study such an impressive evolution. Many quantitative results on these objects have been so obtained, including general structure, total mass and density distribution, kinetic temperatures, velocity fields, etc. Existing observations probe both the gas accelerated by post-AGB shocks and the quiescent components. But the study of crucial regions to understand PN formation (recently shocked shells, regions heated by the stellar UV and inner rotating disks) requires observations at higher frequency and with better spatial resolution.      

Chemistry in luminous AGN and starburst galaxies

Molecular line emission is a useful tool for probing the highly obscured inner kpc of starburst galaxies and buried AGNs. Molecular line ratios serve as diagnostic tools of the physical conditions of the gas—but also of its chemical properties. Both provide important clues to the type and evolutionary stage of the nuclear activity. While CO emission remains the main tracer for molecular distribution and dynamics, molecules such as HCN, HNC, HCO⁺, CN and HC₃N are useful for probing the properties of the denser (n≳10⁴ cm⁻³), star-forming gas. Here I discuss current views on how line emission from these species can be interpreted in luminous galaxies. HNC, HCO⁺ and CN are all species that can be associated both with photon dominated regions (PDRs) in starbursts—as well as X-ray dominated regions (XDRs) associated with AGN activity. HC₃N line emission may identify galaxies where the starburst is in the early stage of its evolution.     

Summer season variability of the north residual cap of Mars as observed by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES)

Previous observations have noted the change in albedo in a number of North Pole bright outliers and in the distribution of bright ice deposits between Mariner 9, Viking, and Mars Global Surveyor (MGS) data sets. Changes over the summer season as well as between regions at the same season (L_s) in different years have been observed. We used the bolometric albedo and brightness temperature channels of the Thermal Emission Spectrometer (TES) on the MGS spacecraft to monitor north polar residual ice cap variations between Mars years and within the summer season for three northern Martian summers between July 1999 and April 2003. Large-scale brightness variations are observed in four general areas: (1) the patchy outlying frost deposits from 90 to 270°E, 75 to 80°N; (2) the large “tail” below the Chasma Boreale and its associated plateau from 315 to 45°E, 80 to 85°N, that we call the “Boreale Tongue” and in Hyperboreae Undae; (3) the troughed terrain in the region from 0 to 120°E longitude (the lower right on a polar stereographic projection) we have called “Shackleton's Grooves” and (4) the unit mapped as residual ice in Olympia Planitia. We also note two areas which seem to persist as cool and bright throughout the summer and between Mars years. One is at the “source” of Chasma Boreale (∼15°E, 85°N) dubbed “McMurdo”, and the “Cool and Bright Anomaly (CABA)” noted by Kieffer and Titus 2001. TES Mapping of Mars’ north seasonal cap. Icarus 154, 162-180] at ∼330°E, 87°N called here “Vostok”. Overall defrosting occurs early in the summer as the temperatures rise and then after the peak temperatures are reached (L_s∼110) higher elevations and outlier bright deposits cold trap and re-accumulate new frost. Persistent bright areas are associated with either higher elevations or higher background albedos suggesting complex feedback mechanisms including cold-trapping of frost due to albedo and elevation effects, as well as influence of mesoscale atmospheric dynamics.     

Cosmic dynamics in the era of Extremely Large Telescopes

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterizing the cosmological redshift drift. We consider the Lyα forest in the redshift range  2 < z < 5  and other absorption lines in the spectra of high-redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise-limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Lyα forest as a function of signal-to-noise ratio and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ∼20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.     

Covariance of dark energy parameters and sound speed constraints from large H i surveys

An interesting probe of the nature of dark energy is the measure of its sound speed, c _s. We review the significance for constraining sound speed models of dark energy using large neutral hydrogen (H  i ) surveys with the square kilometre array (SKA). Our analysis considers the effect on the sound speed measurement that arises from the covariance of c _s with the dark energy density, Ω_de, and a time-varying equation of state,   w ( a ) = w ₀+ (1 − a ) w _a   . We find that the approximate degeneracy between dark energy parameters that arises in power spectrum observations is lifted through redshift tomography of the H  i -galaxy angular power spectrum, resulting in sound speed constraints that are not severely degraded. The cross-correlation of the galaxy and the integrated Sachs Wolfe (ISW) effect spectra contributes approximately 10 per cent of the information that is needed to distinguish variations in the dark energy parameters, and most of the discriminating signal comes from the galaxy auto-correlation spectrum. We also find that the sound speed constraints are weakly sensitive to the H  i bias model. These constraints do not improve substantially for a significantly deeper H  i survey since most of the clustering sensitivity to sound speed variations arises from   z ≲ 1.5  . A detection of models with sound speeds close to zero,   c _s≲ 0.01,  is possible for dark energy models with   w ≳−0.9  .     

Grain alignment in dense interstellar environments: spectropolarimetry of the 4.67-μm CO-ice feature in the field star Elias 16 (Taurus dark cloud)

We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ∼20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ∼5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds.