The light curve of asteroid 2867 Steins measured by VIRTIS-M during the Rosetta fly-by

On 5 September 2008, the Rosetta spacecraft encountered the asteroid 2867 Steins on its way to the comet 67P/Churyumov-Gerasimenko. This was the first of two planned asteroid fly-bys performed by the probe, the second fly-by being with the much larger asteroid 21 Lutetia in July 2010. The VIRTIS imaging spectrometer (IFOV 0.250 mrad, overall spectral range 0.25-5.1 μm) onboard Rosetta acquired data of Steins already before the closest approach phase, when the target was spatially unresolved, in order to obtain a light curve of the asteroid in the infrared spectral range extending up to 5 μm, that was never explored before. The VIRTIS light curve campaign started at 11:30 UTC onboard time, when the spacecraft was about 221,377 km away from the target, and ended at 17:58 UTC, at a distance of 20,741 km away from Steins. During this timeframe, the solar phase angle of the asteroid was roughly constant, ranging from 38.2° to 36.3°.Assuming the most recent value derived for the rotational period of Steins (Lamy et al., 2008), the VIRTIS observations covered slightly more than one rotation of the asteroid. In this interval, VIRTIS collected 8 hyperspectral cubes where Steins was captured 119 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source, for which the instrument was not designed, only a small subset of wavelengths turned out to be suitable to sample the light curve. Nevertheless, in both the VIS and NIR ranges we find a similar trend, with two different maxima and minima during one rotational period, and amplitudes consistent with the results in the visual range obtained in previous works, including the data set acquired by the OSIRIS camera onboard Rosetta. We also report the presence of a new broad feature centered at approximately 0.81-0.82 μm, which is seen in the visual data throughout the rotation of the asteroid.     

Combined experimental and theoretical studies on methane photolysis at 121.6 and 248 nm—implications on a program of laboratory simulations of Titan’s atmosphere

Methane is, together with N₂, the main precursor of Titan’s atmospheric chemistry. In our laboratory, we are currently developing a program of laboratory simulations of Titan’s atmosphere, where methane is intended to be dissociated by multiphotonic photolysis at 248 nm. A preliminary study has shown that multiphotonic absorption of methane at 248 nm is efficient and leads to the production of hydrocarbons such as C₂H₂ (Romanzin et al., 2008). Yet, at this wavelength, little is known about the branching ratios of the hydrocarbon radicals (CH₃, CH₂ and CH) and their following photochemistry. This paper thus aims at investigating methane photochemistry at 248 nm by comparing the chemical evolution observed after irradiation of CH₄ at 248 and at 121.6 nm (Ly-α). It is indeed important to see if the chemistry is driven the same way at both wavelengths in particular because, on Titan, methane photolysis mainly involves Ly-α photons. An approach combining experiments and theoretical analysis by means of a specifically adapted 0-D model has thus been developed and is presented in this paper. The results obtained clearly indicate that the chemistry is different depending on the wavelength. They also suggest that at 248 nm, methane dissociation is in competition with ionisation, which could occur through a three-photon absorption process. As a consequence, 248 nm photolysis appears to be unsuitable to study methane neutral photochemistry alone. The implications of this result on our laboratory simulation program and new experimental developments are discussed. Additional information on methane photochemistry at 121.6 nm are also obtained.     

菏泽市紫外线辐射变化特征及月预报方程

利用2008年4月至2012年12月菏泽市紫外线观测资料以及地面常规气象观测资料和空气质量资料,分析了该地区太阳紫外线辐射的变化特征及其与各因子的相关关系,并建立逐月预报方程。结果表明:菏泽紫外线辐射年总量达到187.98 W/m2,春夏两季明显高于秋冬两季,5月达到全年的最大值,1月为全年的最小值;3~9月紫外线辐射极大值均可达到5级,其他月份均可达到4级。不同天气条件下紫外线辐射强度存在明显差异,其中晴天紫外线辐射强且稳定,呈抛物线变化;多云天紫外线辐射波动较大,时强时弱;阴天紫外线辐射相对较弱。紫外线辐射强度与风速、能见度、气温呈正相关,与总云量、低云量、相对湿度呈负相关,与SO2、PM10、NO2、PM2.5多呈负相关。基于多元线性回归分析向后剔除变量方法得出的逐月预报方程,经检验总体预报效果较好,对当地紫外线等级预报工作具有参考意义。     

Array Configurations That Tile the Plane

Future radiotelescopes need ever increasing sensitivity. With the limits of receiver sensitivity being reached this can only be achieved by increasing collecting area. The increase in antenna numbers and the multi-resolution capabilities of these telescopes, such as the SKA, means a significant departure from existing designs is needed. In this article, some fundamental designs for antenna arrays that uniformly sample the UV plane are presented. The SKA is used to provide target specifications. The designs presented allow asymmetric configurations providing more freedom for site selection. They also allow a direct tradeoff between field of view and correlator complexity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical evolution of the high redshift galaxies

We have tried to determine the rate of chemical evolution of high redshift galaxies from the observed redshift distribution of the heavy element absorption systems in the spectra of QSOs, taking into account the evolution in the intensity of the metagalactic UV ionizing radiation background, the radius and/or the co-moving number density of, and the fraction of mass in the form of gas in, the absorbers. The data for both the Lyman limit systems and the C IV systems have been fitted simultaneously. It seems that the abundance of carbon has possibly increased by about a factor of 5 to 20 from the cosmic time corresponding to the redshift ≃ 4 to 2. The data also suggest that either the radius or the co-moving number density of the galaxies increased with redshift up to z = 2.0 and decreased slowly thereafter. The total mass of the halo gas was higher in the past, almost equal to the entire mass of the galaxy at z = 4. The hydrogen column density distribution for Lyman limit systems predicted by the model is in agreement with the observed distribution.     

巯基棉富集-分光光度法测定地球化学样品中铂钯的野外快速分析方法研究

对于地球化学样品中铂钯的测定,传统和现代分析方法均是建立在试金或树脂及活性炭富集后用分光光度法或电感耦合等离子体质谱法测定,而在野外满足不了这些条件。本文基于样品的性质及分光光度法的适应性强等特点,开发了一种适合于野外简单条件下快速测定地质样品中铂和钯的分析方法。样品用盐酸-氯酸钾-氯化钠-氟化氢铵常温常压密闭分解,巯基棉分别富集铂和钯,灰化处理后采用三氯甲烷-石油醚(1∶3)为萃取剂,DDO为显色剂萃取比色测定铂、钯的含量。巯基棉对钯的吸附率可达98%;加入氯化亚锡-水合肼(还原剂)使铂的吸附率提高到99%。方法检出限为Pt 0.05μg/g,Pd 0.02μg/g,标准物质的测定值与推荐值基本一致,野外地质样品的测定值与实验室分析结果吻合。     

R- and J-band photometry of Comets 2P/Encke and 9P/Tempel 1

Near-simultaneous R- and J-band photometric measurements of the short-period Comets 2P/Encke and the Deep Impact mission target 9P/Tempel 1 were obtained. The resulting R-J colors are +0.82±0.08 mag and +1.46±0.13 mag for Encke and Tempel 1, respectively. Tempel 1's color is redder than the solar R-J color index of +0.76. The Tempel 1 observations directly detected the nucleus while the Encke observations likely suffered from coma contamination.     

Massive stars in the UV

We emphasize in this paper the importance of the UV range for our knowledge of massive stars and the fundamental role played by past and present space-based UV capabilities (IUE, HST, FUSE and others). Based on a review of the work developed in the last years and the state of the art situation for quantitative spectroscopy of massive stars, we present crucial advances which could be addressed by hypothetical future space-based UV missions. Advantages and unique data that these missions could provide are explained in the context of our present knowledge and theories on massive stars in the Milky Way and nearby galaxies. It is argued that these studies are our key to a correct interpretation of observations of more distant objects.     

Simulation of Titan haze formation using a photochemical flow reactor: The optical constants of the polymer

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn's magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real (n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6±0.1 throughout the 0.2-2.5 μm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k(λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119-130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185-190). The slopes of the k(λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127-137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515-529) were also comparable to Titan's. These finding show that the k(λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2-2.5 microns). There is a large difference between the k(λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127-137; Coll et al., 1999, Planet. Space Sci. 47, 1331-1340; Khare et al., 2002, Icarus 160, 172-182). The C/N ratio in the haze analog prepared by discharges is in the 2-11 range while that of the photochemical analogs is in the 18-24 range. The use of discharges and UV light for initiating the chemistry in Titan's atmosphere is discussed.