首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 171 毫秒
1.
许多Mira变星具有轴对称转动的膨胀拱星包层。因此研究有轴对称转动的膨胀拱星包层的速度流场是十分必要的。我们研究了Mira变星的有轴对称转动的膨胀的拱星包层的速度流场,得到了它的严格形式解。所得到的结果与观测给出的事实是相一致的。最后,还讨论了恒星的质量损失率。  相似文献   

2.
Although primarily intended as a test and alignment instrument in order to commission the VLTI, VINCI has taken useful scientific data in its first year and a half of operation. Our results employ coherent integration of fringe visibility in which the actual amplitudes of the raw scans are combined linearly after correcting for the position of the fringe within each scan. In addition to reducing the effect of noise compared to incoherent integration, the result contains a broader range of information, including an estimate of the complex visibility spectrum. Such an estimator is thus sensitive to instrumental phase and spectral characteristics, including the variable component of dispersion introduced by the excess air paths in the delay lines. Calibration of such instrumental effects demonstrates the ability to detect source phase at a fine level as will be required for direct interferometric detection of extra solar planets. We present diameters for five stars obtained by observing the visibility null in their correlated spectra. Using coherent integration we have also observed the peculiar correlated spectra seen in many Mira variables, possibly due to changes in the apparent diameter with wavelength. Calibration of the zero-baseline power from o Ceti is used with other interferometric observations of this star over a period of 90 days to plot diameter variations associated with its pulsation cycle. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

3.
本文给出十颗Me星的低色散光谱资料,其中有六颗星以前没有人给出发射线资料,四颗前人没有给出光谱资料;还有一颗是新发现的Me星。这里我们均给出发射线资料及光谱型。另外还给出了每颗星的光谱描迹图及其中三颗星的证认图。  相似文献   

4.
With the aim of investigating the possible particular behavior of carbon in a sample of chemically peculiar stars of the main sequence without turning to modeling, we performed spectroscopic observations of three important and usually prominent single ionized carbon lines: 4267.261, 6578.052 and 6582.882 Å. In addition, we observed a large number of standard stars in order to define a kind of normality strip, useful for comparing the observed trend for the peculiar stars. We paid particular attention to the problem of the determination of fundamental atmospheric parameters, especially for the chemically peculiar stars for which the abundance anomalies change the flux distribution in such a way that the classical photometric methods to infer effective temperatures and gravities parameter cannot be applied. Regarding CP stars, we found a normal carbon abundance in Hg–Mn, Si (with some exceptions) and He strong stars. He weak stars are normal too, but with a large spread out of the data around the mean value. A more complicated behavior has been noted in the group of SrCrEu stars: four out of seven show a strong overabundance, being the others normal.  相似文献   

5.
We present a program for observations of spectroscopic binaries with VLTI. The aim of such observations is to obtain high accuracy measurements of stellar parameters, in particular masses, combining the visual orbit with spectroscopic orbital elements. We selected a sample of spectroscopic binaries, including pre-main sequence stars, using the estimated angular nodal separation as a criterion for feasibility. A cross-check with the 4th catalog of measurements of binary stars (Hartkopf, 2001) was carried out and we discuss the statistical properties of the sample. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
VLTI interferometry will allow imaging of galactic and extragalactic sources with milliarcsecond angular resolution. For moderately bright sources the spectral resolution will be of the order of 10000. These capabilities will allow detailed studies of solar system objects, stars, proto-planetary systems and the detection of hot extra-solar planets. The observations of galactic nuclei will allow unprecedented measurements of physical parameters in these systems. VLTI will be a prime instrument to study the immediate environment of the massive black hole at the center of the Milky Way. With the exception of a few `self-referencing' sources the observations of extragalactic nuclei will benefit from an extended capability for simultaneous measurements of nearby reference sources for fringe tracking. With beam combination instruments like AMBER, MIDI, PRIMA, and GENIE the VLTI will reach full maturity at a time when other interferometric instruments at different wavelengths will be fully operational. Most important are ALMA (in the mm- and sub-mm-domain), LOFAR and SKA (in the radio meter to centimeter domain) and of course VLB-networks in the radio, and other – at that time –well developed interferometers in the optical. A major scientific potential of future scientific VLTI programs will lie in an efficient combination of these high angular resolution capabilities. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
E01 VLTI‐PRIMA Fringe Tracking Testbed E02 The Fringe and Flexure Tracking System for the LBT interferometric camera LINC‐NIRVANA E03 GRAVITY: The AO assisted, two object beam combiner instrument for the VLTI E04 (Re)Solving the dynamic atmospheres of carbon‐rich giants E05 Do's and Don'ts of MIDI Data‐reduction E06 Bispectrum speckle imaging of the ultracompact HII region K3‐50A E07 Differential astrometry and astrometric planet searches with the VLTI E08 First AU‐scale observations of V1647 Ori: The outbursting young star in the McNeil's Nebula E09 Mid‐infrared imaging at the VLTI: An APreS‐MIDI image reconstruction study E10 VLTI/MIDI observation of the silicate carbon star Hen 38 (IRAS08002‐3803): Silicate dust reservoir spatially resolved for the first time E11 Mid‐infrared spectro‐interferometric observation of the Mira variable RR Sco with the VLTI/MIDI instrument E12 Scientific Prospects for VLTI in the Galactic Centre: Getting to the Schwarzschild Radius E13 VLTI near‐infrared observations of the stellar multiple system δ Velorum E14 The first VLTI observations of the Galactic Center E15 Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912 E16 Interferometric Observations of Infrared Companions with MIDI E17 Evolution and radial distribution of dust in the inner 1‐10A.U. of circumstellar disks around low‐mass young stellar objects. E18 Near‐infrared Interferometry with the AMBER Instrument of the VLTI E19 APreS‐MIDI ‐ Interferometic Imaging in the Mid‐Infrared E20 Mining the sky: Selection of extragalactic targets for interferometric observations  相似文献   

8.
We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. These observations are aimed at determining the differences in X-ray properties between massive WR + OB binary systems and putatively single WR stars. A new XMM spectrum of the nearby WN8 + OB binary WR 147 shows hard absorbed X-ray emission (including the Fe Kα line complex), characteristic of colliding wind shock sources. In contrast, sensitive observations of four of the closest known single WC (carbon-rich) WR stars have yielded only non-detections. These results tentatively suggest that single WC stars are X-ray quiet. The presence of a companion may thus be an essential factor in elevating the X-ray emission of WC + OB stars to detectable levels.  相似文献   

9.
The goal of this article is to show that, if one avoids star forming regions, mass-losing AGB stars can generally be selected from the data that will be provided by the near-infrared surveys which are presently considered. Also, if IRAS data are available, the separation between carbon-rich and oxygen-rich stars can be operated by their positions in infrared colour diagrams. In extragalactic systems, red supergiants can be discriminated from AGB stars by their luminosities.based on observations obtained with the ESO 1-m telescope  相似文献   

10.
We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models.
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch.  相似文献   

11.
The space experiment Gaia, the approved cornerstone 6 ESA mission, will observe up to a billion stars in our Galaxy and obtain their astrometric positions on a micro-arcsec level, multi-band photometry as well as spectroscopic observations. It is expected that about one million Eclipsing Binaries (EBs) (with V ≤ 16 mag) will be discovered and the observing fashion will be quite similar to Hipparcos/Tycho mission operational mode. The combined astrometric, photometric and spectroscopic data will be used to compute the physical parameters of the observed EBs. From a study of a small sample of EBs, it is shown that the agreement between the fundamental stellar parameters, derived from ground-based and Hipparcos (Gaia-like) observations, is more than satisfactory and the Gaia data will be suitable to obtain accurate binary solutions.  相似文献   

12.
We discuss the wavelength dependence of angular diameters of M giants from an observational perspective. Observers cannot directly measure an optical-depth radius for a star, despite this being a common theoretical definition. Instead, they can use an interferometer to measure the square of the fringe visibility. We present new plots of the wavelength-dependent centre-to-limb variation (CLV) of intensity of the stellar disc as well as visibility for Mira and non-Mira M giant models. We use the terms 'CLV spectra' and 'visibility spectra' for these plots. We discuss a model-predicted extreme limb-darkening effect (also called the narrow-bright-core effect) in very strong TiO bands which can lead to a misinterpretation of the size of a star in these bands. We find no evidence as yet that this effect occurs in real stars. Our CLV spectra can explain the similarity in visibilities of R Dor (M8IIIe) that have been observed recently, despite the use of two different passbands. We compare several observations with models, and find that the models generally underestimate the observed variation in visibility with wavelength. We present CLV and visibility spectra for a model that is applicable to the M supergiant α Ori.  相似文献   

13.
Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated as the precursors of extreme helium stars (EHes). The question arises to whether Sakurai's Object will evolve down the [WC]-PG1159 evolution track, or the RCB-EHe-HesdO track. From a number of IUE observations, we have measured thesecular contraction rates and pulsation masses of several EHes. These are inconsistent with the predicted contraction rates for helium-shell burning giants produced by a final helium-shell flash (the [WC]-PG1159 track). Although there may be some similarities between Sakurai's Object and some RCBs, if the RCB-EHe conection is valid then these must be superficial rather than structural.  相似文献   

14.
The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998  相似文献   

15.
In recent years there has been much debate, both observational and theoretical, about the nature of star formation at high redshift. In particular, there seems to be strong evidence of a greatly enhanced star formation rate early in the Universe’s evolution. Simulations investigating the nature of the first stars indicate that these were large, with masses in excess of 100 solar masses. By the use of a chemical model, we have simulated the molecular signature of massive star formation for a range of redshifts, using different input models of metallicity in the early Universe. We find that, as long as the number of massive stars exceeds that in the Milky Way by factor of at least 1000, then several ‘hot-core’ like molecules should have detectable emission. Although we predict that such signatures should already be partly detectable with current instruments (e.g. with the VLA), facilities such as ALMA will make this kind of observation possible at the highest redshifts.  相似文献   

16.
Using different sky surveys, we studied IRAS sources and embedded clusters located in the G345.5+1.5 region. Our analysis comprises multi-wavelength (optical to radio) data, exploited using various methods: photometry, near-infrared spectroscopy for ten stars in the embedded cluster DBS-114, and astrometry. We estimated the main parameters of the embedded stellar populations in the G345.5+1.5 molecular cloud, such as their extent, reddening, age, and mass. We also found a consistent distance value using different approaches. For each studied population, we classified several point objects as early main sequence stars, stars with infrared excess, and class I/II YSOs. For the particular case of DBS 114, our spectral classification revealed four B-type stars, and we used astrometric information from GAIA EDR3. The combination of optical and infrared information revealed an abnormal reddening law in some embedded clusters. Our analysis favored a scenario with a Lyman continuum emission excess at some of the studied stellar populations.  相似文献   

17.
  相似文献   

18.
Fast rotation of compact stars (at sub-millisecond period) and, in particular, their stability, are sensitive to the equation of state (EOS) of dense matter. Recent observations of XTE J1739-285 suggest that it contains a neutron star rotating at 1122 Hz. At such rotational frequency the effects of rotation on star’s structure are significant. We study the interplay of fast rotation, EOS, and gravitational mass of a sub-millisecond pulsar. We discuss the EOS dependence of spin-up to a sub-millisecond period, via mass accretion from a disk in a low-mass X-ray binary.  相似文献   

19.
仲佳勇  赵刚  邱红梅 《天文学报》2002,43(2):113-120
利用北京天文台兴隆观测基地2.16米望远镜的折轴阶梯光栅摄谱仪，获得了一批中等贫金属星的高分辨率，高信噪比光谱，通过MIDAS软件包对其中7颗中等贫金属星进行了光谱处理，得到了它们的等值宽度，最后给出院 样本恒星的等值宽度及误差分析。  相似文献   

20.
The rapidly oscillating Ap (roAp) stars currently represent the only main sequence stars other than the Sun which exhibit non-radial acoustic pulsations of high overtone. This makes them excellent subjects for asteroseismology, an approach which promises to yield accurate knowledge of the interior structures of stars. Of the 27 known roAp stars, 24 were discovered in Sutherland despite extensive searches conducted elsewhere. This paper reviews the discovery of the roAp phenomenon and describes the factors that contribute to the high discovery rate for these stars at Sutherland. Two long-term observational projects in progress at Sutherland are discussed,viz. the Cape roAp Star Survey and long-term monitoring of frequency variations in roAp stars.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号