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1.
We have analyzed a master light curve of the classical T Tauri star RY Tau. The brightness variations observed in 1983–2004 were found to be periodic. A period of 377 ± 10 d is derived using a frequency analysis. A statistical analysis shows that the brightness variability cannot be described by a normal distribution. The brightness variations could be due to the presence of some bodies being formed in the circumstellar environment.  相似文献   

2.
The results of numerical modeling of lithium lines in the spectra of M dwarfs are discussed. The behavior of the lithium lines relative to the local pseudocontinuum formed by molecular bands is analyzed as a function of model atmosphere parameters: effective temperature T eff, gravity logg, and metallicity μ The molecular opacity was computed using the just overlapping line approximation (JOLA) and “line-by-line” methods. The pseudo-equivalent widths of lithium lines depend appreciably on metallicity μ and weakly on T{nteff}. The lithium abundance in the atmosphere of UX Tau C is redetermined. Previous studies underestimated the lithium abundance in this star as a result of the use of insufficiently accurate molecular-line lists. The new lithium abundance log N (Li)=3.2±0.3was derived by comparing the observed profiles of the 670.8 nm resonance doublet lines with profiles calculated using the new TiO line list of Plez.1 The new abundance agrees with the atmospheric lithium abundances of the other two components in the stellar system, providing further evidence that the three stars in the UX Tau system have the same age. A comparison of the observed spectra of UX Tau C near the lithium resonance doublet (665–680 nm) with spectra computed using JOLA and line-by-line methods suggests that the list of Plez is the best currently available.  相似文献   

3.
We have searched for protostellar or protoplanetary structures in the vicinity of young T Tauri stars using a technique that is able to distinguish long-period components of the observed light curves. We perform a statistical spectral analysis of the one-year mean light curves of three selected stars (T Tau, DI Cep, and SU Aur) and plot synthetic light curves for the most reliable period of each star. In a first approximation, the results show a good coincidence between our synthetic light curves and the original light curves, supporting the hypothesis that protostellar or protoplanetary structures are present in the studied systems. An analysis of the 0.36–20 µm spectral energy distributions of the program stars also leads to the conclusion that infrared anomalies for young stars are most likely due to thermal radiation from not-yet-formed companions in the vicinity of the star.  相似文献   

4.
We have calculated profiles of the CIV 1550, NV 1240, OVI 1035, and SiIV 1400 resonance doublets for a plane-parallel shock viewed at various angles. Calculations were performed for the range of preshock gas velocities V0 and gas densities ρ0 appropriate for classical T Tauri stars. The parameters of accretion shocks in young stars can be determined by comparing the calculated and observed profiles of the studied lines and their relative intensities. It is not possible to derive the parameters of the accreting gas from the line profiles without knowing the geometry of the accretion zone. The relation I v (µ,V0,ρ0) for a plane shock, where I v is the intensity μ=cosθ, can be used to determine the accretion parameters by either choosing a geometry for the radiating region or using a technique similar to Doppler tomography. The results obtained for DR Tau, T Tau, and RY Tau indicate that, in contrast to current concepts, the inner regions of the accretion disk are not disrupted by the magnetic field of the star, and the disk reaches the stellar surface. As a result, only a small fraction of the accreted matter passes through the shock and falls onto the star.  相似文献   

5.
A detailed study of the orbital-period variations of the Algol-type eclipsing binaries with earlyspectral- type primary components U CrB and RW Tau has been performed. The period variations in both systems can be described as a superposition of secular and cyclic variations of the period. A secular period increase at a rate of 2.58d × 10?7/year is observed for U CrB, which can be explained if there is a uniform flow of matter from the lower-mass to the higher-mass component, with the total angular momentum conserved. RW Tau features a secular period decrease at a rate of ?8.6d × 10?7/year; this could be due to a loss of angular momentum by the binary due to magnetic braking. The cyclic orbital-period variations of U CrB and RWTau can be explained by the motion of the eclipsing binary systems along their long-period orbits. In U CrB, this implies that the eclipsing binary moves with a period of 91.3 years around a third body with mass M3 > 1.13M⊙; in RW Tau, the period of the motion around the third body is 66.6 years, and the mass of the third body is M3 > 1.24M⊙. It also cannot be ruled out that the variations are due to the magnetic cycles of the late-type secondaries. The residual period variations could be a superposition of variations due to non-stationary ejection of matter and effects due to magnetic cycles.  相似文献   

6.
Astronomy Reports - Variations of the orbital period of the eclipsing binaries TT Del, EU Hya, and SV Tau have been studied. It has been shown that they can be represented by cyclic fluctuations...  相似文献   

7.
We have analyzed for the first time profiles of the SiIII 1892 Å and CIII 1909 Å intercombinational lines in HST spectra of the stars RY Tau and RU Lup. The widths of these optically thin lines exceeded 400 km/s, ruling out formation in the stellar chromosphere. Since the intensity of the Si line exceeds that of the C line, it is unlikely that a large fraction of the observed line flux is formed in a stellar wind. The observed profiles can be reproduced in the framework of an accretion shock model if the velocity field in the accretion zone is appreciably nonaxisymmetric. In this case, the line profiles should display periodic variations, which can be used to determine the accretion zone geometry and the topology of the magnetic field near the stellar surface; corresponding formulas are presented. In addition, periodic variations of the 0.3–0.7 keV X-ray flux should be observed.  相似文献   

8.
We compare U BV R photometry of CI Cam obtained using facilities at the Sternberg Astronomical Institute in 1998–2001 and medium-resolution spectroscopy obtained at the Special Astrophysical Observatory in the same period. The photometric measurements and fluxes in the Balmer and some Fe II emission lines in the post-outburst quiescent spectrum of CI Cam display cyclic variations with a period of 1100±50d. These variations could be due to orbital motion in a widely separated binary; one component, a giant, produces a reflection effect on the surface facing the other, compact component. Our V-band photometry confirms the \(11\mathop d\limits_. 7\) period found by Miroshnichenko before the outburst but shows a lower amplitude (3%). We investigate the possibility that this photometric period is the same as the rotational period of the radio jets measured using the VLA. Based on modeling of the radio map, we estimate the inclination of the jet rotation axis to the line of sight, i=35°–40°; the angle between the rotational axis and jet ejection direction, ?=7°–10°; and the jet spatial velocity, 0.23–0.26 c. The equivalent widths and fluxes of various spectral lines show different variation amplitudes during outburst and significantly different behavior during quiescence. We have detected five patterns of behavior, which provide evidence for stratification of the gas and dust envelope surrounding the system. The brightening of the [N II] forbidden line is delayed by 50–250d relative to the X-ray outburst maximum.  相似文献   

9.
The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite-rich zones associated with biotite-chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold-thiocomplexes in the ore-forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two-stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold-bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi-processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.  相似文献   

10.
Sytov  A. Yu.  Fateeva  A. M. 《Astronomy Reports》2019,63(12):1045-1055

Results of three-dimensional numerical simulations of the gas dynamics of the envelope of the young T Tauri binary star UZ Tau E are considered. The flow structure in the circumstellar envelope of the system is analyzed. It is shown that a regime with the impulsive accretion of matter from the circumstellar disk is realized in the binary system, in which there is a periodic transfer of matter to the accretion disk of the primary component through the accretion disk of the secondary.

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11.
Information on the latitude distribution of starspots and changes in this distribution from year to year is very important for our understanding of the nature of stellar activity and for developing dynamo theory. The concept of butterfly diagrams is introduced for highly spotted stars of late spectral types, by analogy to the Maunder diagrams for the Sun. Our approach is based on the zonal spottedness models constructed by Alekseev and Gershberg. A detailed analysis is given for the single active star LQ Hya, and a comparison is made to similar analyses for several stars with two well-separated spot belts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundary of the butterfly diagram drifts toward the equator during the activity-rise phase, i.e., during years when the relative spotted area increases. This effect is clearly expressed for LQ Hya and other stars whose orientation enables observation of both hemispheres and virtually vanishes for V833 Tau, which is viewed nearly pole-on. The upper boundary of the diagram is virtually unchanged for all the considered spotted stars except V775 Her, for which it moves toward the pole. The drift rate of the lower boundary is ?1 to ?2 deg/yr, a factor of two to three smaller in magnitude than the corresponding solar value. Our analysis provides an independent confirmation of the occurrence of high-latitude spots on stars that are younger than the Sun and whose activity is high but less regular than the solar activity; it also enables the identification of the starting times of stellar cycles.  相似文献   

12.
We have classified and determined the parameters of the evolved close binary MT Ser. Our moderate-resolution spectra covering various phases of the orbital period were taken with the 6-m telescope of the Special Astrophysical Observatory. The spectra of MT Ser freed from the contribution of the surrounding nebula Abell 41 contained no emission lines due to the reflection effect. The radial velocities measured from lines of different elements showed them to be constant on a time scale corresponding to the orbital period. At the same time, we find effects of broadening for the HeII absorption lines, due to the orbital motion of two hot stars of similar types. As a result, we classify MT Ser as a system with two blue subdwarfs after the common-envelope stage. We estimate the component masses and the distance to the object from the Doppler broadening of the HeII lines. We demonstrate that the age of the ambient nebula, Abell 41, is about 35 000 years.  相似文献   

13.
Results of spectral observations of the isolated Ae Herbig star CQ Tau obtained in 1995–1998 in the Hα line and near the sodium resonance doublet are presented, together with simultaneous photometric monitoring of the star. CQ Tau is a member of the family of young UX Ori stars with nonperiodic Algol-like brightness decreases. The star is surrounded by an accretion disk, in which its emission-line spectrum and part of its absorption-line spectrum are formed. The strong variability of the Hα, D NaI, and HeI 5876 Å lines testifies that the gaseous disk is appreciably inhomogeneous in both the radial and azimuthal directions. This inhomogeneity probably results from uneven feeding by the circumstellar material from the peripheral regions of the circumstellar disk. Over the four years of observations, we observed the star in deep minima three times (ΔV～2). On these nights, an increase in the Hα equivalent width followed the decrease in radiation flux. In the two deepest minima, the normally two-component line profile had only a single component with a nearly symmetrical profile. This behavior of the Hα line is in good agreement with the results of numerical modeling of Algol-like minima and can be used to estimate the parameters of the dust clouds eclipsing the star and inner accretion disk. These estimates suggest that the circumstellar dust clouds can approach very close to the star and be sublimated there.  相似文献   

14.
米兰科维奇冰期旋回理论:挑战与机遇   总被引：13，自引：6，他引：7       下载免费PDF全文
丁仲礼 《第四纪研究》2006,26(5):710-717
米兰科维奇理论认为,北半球高纬夏季太阳辐射变化是驱动第四纪冰期旋回的主因。这个理论的核心是单一敏感区的触发驱动机制,即北半球高纬气候变化信号被放大、传输进而影响全球。最近,由于大量高分辨率及精确定年的气候变化记录的获得,从以下4个方面构成了对米氏理论的挑战:1)一些低纬地区并没有明显的10万年冰量周期,而是以2万年岁差周期为主,表明北半球冰盖的扩张、收缩变化并没有完全控制低纬区的气候变化;2)在最近几次冰消期时,南半球和低纬区的温度增高,要早于北半球冰盖的融化,表明冰消期的触发机制并非是北半球高纬夏季太阳辐射;3)大气CO2浓度在第2冰消期的增加同南极升温相一致,表明该时大气CO2浓度增加亦有可能早于北半球冰盖消融;4)南半球的末次冰盛期有可能早于北半球。这就说明单一敏感区触发驱动机制已难以圆满解释所有观察事实,天文因素控制下轨道尺度气候变化机制研究正面临理论突破的新需求和新机遇。  相似文献   

15.
以黑河流域中游绿洲区域为例,近10年来,在干旱内陆流域受人类活动强烈影响的区域,景观的破碎度降低,景观异质性减弱,显著的景观变化表现在:耕种景观与荒漠化景观的扩张和沼泽草甸景观与水域景观的萎缩,其中荒漠草原景观面积的10%、山地草原景观的49.8%以及沼泽草甸(包括盐化草甸)景观面积的16.8%演变为耕种景观类型;相反,17.4%的荒漠草原景观面积和20.2%的沼泽草甸退化为沙漠与戈壁景观,另有13.02%的灌耕地面积演变为荒漠草原.景观空间格局演变驱动景观土壤N、C的源汇变化,近10年来,黑河流域中游地区由于景观空间格局变化形成的景观N净排放和有机C净释放分别达到50.65×104T和530.17×104T.山地草原景观的荒漠化与耕种利用、耕种景观的荒漠草原化、沼泽草甸与山地草甸景观以及荒漠草原景观的耕种利用和这些景观的沙漠化发展是造成区域N、C损失的主要途径.  相似文献   

16.
Bat guano cores have been used as a source of palaeoenvironmental information to aid in the reconstruction of past climates and vegetation. We collected a 104‐cm‐long (43 cm compacted) guano core from Fern Cave, Alabama, USA, that provided a c. 6000‐year record of guano accumulation. Pollen, nutrients (C, N, P) and stable isotopes (δ13C, δ15N) were measured on the guano core with the objective of reconstructing the environmental history of the area from the mid‐Holocene to present. Our data indicate that bats have utilized Fern Cave for at least 6000 years and that Woodland Indians also utilized the cave for a short period. A 3‐cm charcoal layer was dated to 2720±30 cal. a BP and inferred to be Woodland Indian in origin from microscopic inspection and thickness. Pollen and geochemical data showed that bat diets changed in the late Holocene possibly linked to food supply and climate changes. These results demonstrate that guano cores are a useful tool of palaeoenvironmental reconstruction when other forms of palaeorecords do not exist and can add to local archaeological information.  相似文献   

17.
Crustal-scale seismic profiles across Taiwan and the western Philippine Sea   总被引：13，自引：2，他引：13  
We have used combined onshore and offshore wide-angle seismic data sets to model the velocity structure of the Taiwan arc–continent collision along three cross-island transects. Although Taiwan is well known as a collisional orogen, relatively few data have been collected that reveal the deeper structure resulting from this lithospheric-scale process. Our southern transect crosses the Hengchun Peninsula of southernmost Taiwan and demonstrates characteristics of incipient collision. Here, 11-km-thick, transitional crust of the Eurasian plate (EUP) subducts beneath a large, rapidly growing accretionary prism. This prism also overrides the N. Luzon forearc to the east as it grows. Just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure. Because this break is accompanied by intense seismicity, we interpret that the forearc block is being detached from the N. Luzon arc and Philippine Sea plate (PSP) at this point. Our middle transect illustrates the structure of the developing collision. Steep and overturned velocity contours indicate probable large-scale thrust boundaries across the orogen. The leading edge of the coherent PSP appears to extend to beneath the east coast of Taiwan. Deformation of the PSP is largely limited to the remnant N. Luzon arc with no evidence of crustal thickening to the east in the Huatung basin. Our northern transect illustrates slab–continent collision—the continuing collision of the PSP and EUP as the PSP subducts. The collisional contact is below 20 km depths along this transect NE of Hualien. This transect shows elements of the transition from arc–continent collision to Ryukyu arc subduction. Both of our models across the Central Range suggest that the Paleozoic to Mesozoic basement rocks there may have been emplaced as thick, coherent thrust sheets. This suggests a process of partial continental subduction followed by intra-crustal detachment and buoyancy-aided exhumation. Although our models provide previously unknown structural information about the Taiwan orogen, our data do not define the deepest orogenic structure nor the structure of western Taiwan. Additional seismic (active and passive), geologic, and geodynamic modeling work must be done to fully define the structure, the active deformation zones, and the key geodynamic process of the Taiwan arc–continent collision.  相似文献   

18.
The Bhuj region, Kutch, India, is included in the highly seismic zonation map of India. The Kutch is a rift basin and so far has experienced three major earthquakes that are due to reverse fault mechanism, which in turn have been ascribed to compressive stresses. Origin of these stresses is considered to be due to north–south convergence of the Indian Plate with the Tibetan plate, and this has placed the entire Indian Plate under the compressive stress regime. Analysis of the stress pattern in the Bhuj region, therefore, has been carried out by extracting lineaments with the help of remote sensing data for the pre- and post-earthquake periods of 26 January 2001 earthquake. For this purpose, the area has been segmented into four sectors. The lineament frequency and the percent frequency from each sector and also for the whole area have been worked out. Resolution of stress on the principle of triaxial ellipsoid has been worked out for each sector and also for the whole area. There results a temporal change in the stress pattern in each sector and also for the whole area. However, the direction of horizontal maximum compressive stress for the whole area appears to be in N 10°E in the pre-earthquake period that has changed to N 10°W in the post-earthquake period. Thus, temporal change in the horizontal maximum compressive stress direction as N 23°E, inferred by Gowd et al. (J Geophy Res 97:11879–11888, 1992) to N 10°E prior to and N 10°W in the post-earthquake period, as inferred from lineament analysis and near parallelism of the lineament maxima with that of the North Kathiawar Fault and the Chambal Jamnagar Lineament along with the longer axis of the isoseismals of the Bhuj 2001 earthquake indicates a modification in the structural fabric of the region as well as a possibility of development of a major plane of weakness.  相似文献   

19.
In this review, the shifts in organic matter (OM) accumulation and C:N ratios in lake sediments to reconstruct paleoclimate and paleo-environmental changes since the early Holocene period are presented. The C:N proxy data of total OM reflect wet climatic conditions during early Holocene (10 to 8.2 kyrs BP) due to enhanced southwest monsoon. This was followed by intermittent arid conditions during the mid and late Holocene period (8.2 to 2.8 kyr BP). Enhanced values of C:N ratio during middle to late Holocene (7.8–2.3 kyrs B.P) indicate periods with lower lake levels and minimum precipitation, while decreased C:N ratio point to stronger SW monsoon and expansion of the lakes. Further, C:N and δ13C results from the lake sediments reveal a detailed and continuous paleo-environmental changes in the relative sources of OM (allochthonous vs autochthonous). Proxy records using such natural archives have also been utilized to reconstruct past extreme events and environmental changes around the lake systems, such as causes for lake desiccation, hydrographic changes, alternations between C3 and C4 vegetation and historical disturbances in the catchment area since the early-late Holocene period coupled with the Indian summer monsoon.  相似文献   

20.
The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli.  相似文献   

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