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991.
J. M. Oliveira J. Th. van Loon S. Stanimirovi A. A. Zijlstra 《Monthly notices of the Royal Astronomical Society》2006,372(4):1509-1524
We investigate the conditions of star formation in the Large Magellanic Cloud (LMC). We have conducted a survey for water maser emission arising from massive young stellar objects in the 30 Doradus region (N 157) and several other H ii regions in the LMC (N 105A, N 113 and N 160A). We have identified a new maser source in 30 Dor at the systemic velocity of the LMC. We have obtained 3–4 μm spectra, with the European Southern Observatory (ESO)-Very Large Telescope (VLT), of two candidate young stellar objects. N 105A IRS1 shows H recombination line emission, and its Spectral Energy Distribution (SED) and mid-infrared colours are consistent with a massive young star ionizing the molecular cloud. N 157B IRS1 is identified as an embedded young object, based on its SED and a tentative detection of water ice. The data on these four H ii regions are combined with mid-infrared archival images from the Spitzer Space Telescope to study the location and nature of the embedded massive young stellar objects and signatures of stellar feedback. Our analysis of 30 Dor, N 113 and N 160A confirms the picture that the feedback from the massive O- and B-type stars, which creates the H ii regions, also triggers further star formation on the interfaces of the ionized gas and the surrounding molecular cloud. Although in the dense cloud N 105A star formation seems to occur without evidence of massive star feedback, the general conditions in the LMC seem favourable for sequential star formation as a result of feedback. In an Appendix , we present water maser observations of the galactic red giants R Doradus and W Hydrae. 相似文献
992.
993.
James R. Shortridge 《The Professional geographer》1991,43(3):280-291
In the period between 1800 and 1950, 42 authors wrote American regional novels and poetry popular enough to be tentatively labeled place defining. These books provide concise statements of perceived regional values. As a group they suggest that the West has been dominated by the single, enduring image of youthful self-reliance; the Northeast by a set of small-scale characterizations; and the Midwest and South by more complex depictions of egalitarian pastoralism and cavalier society, respectively, each modified and challenged over time. 相似文献
994.
We report new echelle observations of the kinematics of 30 HII regions in the LMC, including the 30 Doradus giant HII region. All of the HII regions possess supersonic velocity dispersions, which can be attributed to a combination of turbulent motions and discrete velocity splitting produced by stellar winds and/or embedded supernova remnants (SNRs). The core of 30 Dor is unique, with a complex velocity structure that parallels its chaotic optical morphology. We use our calibrated echelle data to measure the physical properties and energetic requirements of these velocity structures. The most spectacular structures in 30 Dor are several fast expanding shells, which appear to be produced at least partially by SNRs. 相似文献
995.
996.
P. T. Harris 《Australian Journal of Earth Sciences》2013,60(3):241-254
Surficial deposits of the tidally influenced Australian shelf seas exhibit a variation in fades related to energy gradient. These deposits comprise a high energy gravelly facies, a mobile sand sheet facies and a low energy muddy sand facies. Such a facies distribution conforms generally with the existing model of continental shelf tidal sedimentation, derived for the west European tidal seas. However, the carbonate rich and mainly warm water deposits of the Australian shelf differ from the mainly quartzose and temperate cold‐water deposits of the European type case in terms of: (i) the role of seagrasses in trapping fine‐grained sediment; and (ii) the relative importance of the production of carbonate mud by mechanical erosion of carbonate grains. Seagrasses in Spencer Gulf, Gulf of St Vincent and Torres Strait are located in regions of strong tidal currents, associated with bedforms and gravel lag deposits. Thus, in the case of tropical carbonate shelves, seagrass deposits containing fine‐grained and poorly sorted sediments are located in close proximity to high energy gravel and mobile sand facies. In contrast, the European model (for temperate, siliciclastic shelves) places facies in a regional gradient with a wide separation (in the order of 50–100 km). Of the locations reviewed, the Gulf of St Vincent, Bass Strait, southern Great Barrier Reef, Torres Strait and Gulf of Carpentaria exhibit zones of carbonate mud accumulation. The production and winnowing of carbonate mud from the mobile sand facies is a factor that must be taken into account in the assessment of a sediment budget for this facies, and which is of relatively greater importance for carbonate shelves. Insufficient data are presently available from the macrotidal North West Shelf to test the applicability of the model to this region. 相似文献
997.
998.
Human activities have led to the serious decline and degradation of riparian areas in the Southwestern United States. Areas in Arizona, such as the Gila Box Riparian National Conservation Area, are protected to conserve and restore these unique ecosystems. This study investigated temporal change in the channel form and sinuosity of the Gila River in the Gila Box Riparian National Conservation Area. Specifically, cross sections were measured in 1994, 2001, and 2008 to evaluate changes in channel form. Aerial photographs between 1935 and 2007 were also compared to assess changes in channel sinuosity. Based on cross-sectional measurements, the thalweg was significantly deeper in 1994 than in 2001 and 2008, while width-depth ratios were significantly lower in 2008 than in 1994 and 2001. These measurements indicate that the channel is stabilizing. Sinuosity changed only slightly. Overall, climate patterns that cause large flood events appear to have been the major influence on the channel form. Still, exclusion of livestock and all-terrain vehicles, which increased vegetation in the riparian areas, also appeared to mitigate flood impacts. The stream channel appears to have been more resilient to the impacts of the flood in 2005, when vegetation was better established, than to the flood of 1993, when past management influences were still lingering. 相似文献
999.
省际边界地区因其较少受人类活动干扰,保存了较为丰富的旅游资源,但很难得到有效开发.如何化解这一矛盾,成为边界地区行政主体发展旅游业的难题.本研究以省际边界地区——湖南省新宁县为例,分析边界区位给其带来的负面影响,针对其自身发展基础和旅游资源状况,提出时间和空间开发模式,进而从建立边界旅游联动开发机制、培育边界旅游中心城市、强化对外交通易达性等方面着手,带动湘桂边界地区的旅游发展,以期为同类旅游地开发提供参考. 相似文献
1000.
《The Professional geographer》2013,65(3):249-254
Western investment and trade in East-Central Europe is likely to play a key role in the economic fortunes of the region over the next decade. Currently, the capital cities and the western parts of Poland, Czechoslovakia, and Hungary are the primary beneficiaries of economic interaction with the West. As such, existing regional inequalities are being reinforced. Deepening regional socioeconomic disparities within individual states could complicate the economic restructuring process and jeopardize state stability in the years ahead. 相似文献