排序方式: 共有5条查询结果,搜索用时 15 毫秒
1
1.
M. D. Still † D. Steeghs V. S. Dhillon D. A. H. Buckley 《Monthly notices of the Royal Astronomical Society》1999,310(1):39-42
We present a small sample of time-resolved optical spectroscopy of the dwarf nova HL CMa during an outburst state. By combining radial velocity measurements with published data we show that the previously quoted value is not the only candidate for the orbital period of this system. We reduce the significance of daily aliasing but cannot distinguish between two periods at 0.2146±0.0004 and 0.2212±0.0005 d. We show that the low-excitation emission lines are composites from an accretion disc and the companion star, and that high-excitation emission originates in the disc or outflowing material associated with the accreting white dwarf. 相似文献
2.
3.
We present results from a survey observation of circumstellar disks around protostellar sources associated with the Taurus molecular cloud. Our result shows that the 98 GHz continuum emission tends to be weaker for embedded sources than for visible T Tauri stars, which is consistent with our previous interpretation of disk formation. Direct observations of the formation of a centrifugally supported viscous accretion disk around HL Tau is discussed. 相似文献
4.
5.
Holocene climate changes in the mid-high-latitude-monsoon margin reflected by the pollen record from Hulun Lake, northeastern Inner Mongolia 总被引:6,自引:0,他引:6
Pollen-assemblage data from a sediment core from Hulun Lake in northeastern Inner Mongolia describe the changes in the vegetation and climate of the East Asian monsoon margin during the Holocene. Dry steppe dominated the lake basin from ca. 11,000 to 8000 cal yr BP, suggesting a warm and dry climate. Grasses and birch forests expanded 8000 to 6400 cal yr BP, implying a remarkable increase in the monsoon precipitation. From 6400 to 4400 cal yr BP, the climate became cooler and drier. Chenopodiaceae dominated the interval from 4400 to 3350 cal yr BP, marking extremely dry condition. Artemisia recovered 3350-2050 cal yr BP, denoting an amelioration of climatic conditions. Both temperature and precipitation decreased 2050 to 1000 cal yr BP as indicated by decreased Artemisia and the development of pine forests. During the last 1000 yr, human activities might have had a significant influence on the environment of the lake region. We suggest that the East Asian summer monsoon did not become intensified until 8000 cal yr BP due to the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoon precipitation on millennial to centennial scales would be related to ocean-atmosphere interactions in the tropical Pacific. 相似文献
1