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181.
It is pointed out that the total amount of microwave radiation that could possibly be generated by all the graphite needles in space would be considerably smaller than the observed total energy of the cosmic microwave background radiation. The question of relevant optical depth of the cloud containing the needles has been examined. It is found that the optical depth is not sufficiently large for the cloud to radiate like a black body. 相似文献
182.
Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation? 下载免费PDF全文
Dominik C. Hezel Graeme M. Poole Jack Hoyes Barry J. Coles Catherine Unsworth Nina Albrecht Caroline Smith Mark Rehkämper Andreas Pack Matthew Genge Sara S. Russell 《Meteoritics & planetary science》2015,50(2):229-242
Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ56Fe values up to +0.35‰ relative to the solar system mean. On average, the δ56Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ56Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ56Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ18O (0.9–1.5‰) than their host chondrites, and the same is true for Δ17O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts. 相似文献
183.
The soft X-ray emission of the solar corona is investigated by comparison of the signals of several broad band photometers carried on the Solrad 9 satellite, and sensitive to the region 0.5–20 Å. Temperature from 1.5 × 106 to 25 × 106 K have been measured with emission measure N
e
2 dV ranging between 1050 cm–3 to 1047 cm–3.By means of the observational data and assuming magnetic confinement and hydrostatic equilibrium, the model of an active region is investigated. For temperatures larger than 107K the emission is due to flare activity and two sets of emission measure are observed which appear to be related to the evolution of flares. 相似文献
184.
185.
P. F. L. Maxted C. K. J. Moran T. R. Marsh A. A. Gatti 《Monthly notices of the Royal Astronomical Society》2000,311(4):877-880
We have used the radial velocity variations of two sdB stars previously reported to be binaries to establish their orbital periods. They are PG 0940+068 ( P =8.33 d) and PG 1247+554 ( P =0.599 d). The minimum masses of the unseen companions, assuming a mass of 0.5 M⊙ for the sdB stars, are 0.090±0.003 M. for PG 1247+554 and 0.63±0.02 M⊙ for PG 0940+068. The nature of the companions is not constrained further by our data. 相似文献
186.
Temporal variability in the maternal environment and its effect on seed size and seed quality in Flourensia cernua DC. (Asteraceae). 总被引:1,自引:0,他引:1
The biological phenomenon commonly referred to as maternal effect occurs whenever the environment exerts an influence upon the mother that is later expressed in characteristics of her offsprings. For example, the environmental conditions experienced by a mother plant during flowering and fruiting can modify the quality of her descendants (seeds), and even interrupt their development. Between-year variation in minimum temperature and the classic, between-year variation in precipitation represent an unpredictable environment for winter blooming plants in arid zones. In this study, we investigated the effect of maternal environment (temperature and precipitation) on seed size and seed quality in F. cernua, over a 5-year period. Results indicated that the proportion of filled seeds, as well as their size, increased with higher absolute minimum temperatures during seed formation, and that, to a lesser extent, both the precipitation occurred during the same period and the annual precipitation also have a positive influence on seed size. In this way, environmental conditions prevailing during seed formation exerted a strong maternal effect on the size and quality of seeds produced and, probably also, in the developmental possibilities of F. cernua seedlings. This effect was subject to important, between-year variation. 相似文献
187.
C. J. Cohen E. C. Hubbard Claus Oesterwinter 《Celestial Mechanics and Dynamical Astronomy》1973,7(4):438-448
In 1950 Brouwer and van Woerkom published a secular theory of the variations of the planetary elements in analytical form. In the present paper we provide a graphical representation of this theory in the form of element plots for a time span of ten million years. 相似文献
188.
H.R.M. Magee P.L. Dufton F.P. Keenan W.R.J. Rolleston D. Kilkenny D. O'Donoghue C. Koen R.S. Stobie 《Monthly notices of the Royal Astronomical Society》2001,324(3):747-756
We present high-resolution spectroscopic observations of 21 B-type stars, selected from the Edinburgh–Cape Blue Object Survey. Model atmosphere analyses confirm that 14 of these stars are young, main-sequence B-type objects with Population I chemical compositions. The remaining seven are found to be evolved objects, including subdwarfs, horizontal branch and post-AGB objects. A kinematical analysis shows that all 14 young main-sequence stars could have formed in the disc and subsequently been ejected into the halo. These results are combined with the analysis of a previous subsample of stars taken from the Survey. Of the complete sample, 31 have been found to be young, main-sequence objects, with formation in the disc, and subsequent ejection into the halo, again being found to be a plausible scenario. 相似文献
189.
190.
A. Pirya S. Nandi D. J. Saikia C. Konar M. Singh 《Journal of Astrophysics and Astronomy》2011,32(4):471-474
We present the results of GMRT and VLA observations of five large radio sources over a wide frequency range to investigate
their structural and spectral asymmetries. The hot-spot brightness ratios suggest intrinsic source asymmetries, while the
spectral indices show evidence of re-acceleration of particles. 相似文献