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1.
We have studied a set of equations with nonlinear and nonadiabatic terms which describes a simple oscillator. The equations have only one fixed point located at the origin. It is found that the oscillator shows the sequence of the period-doubling for the change of a parameter and results in chaotic oscillation. We illustrated the behaviour of the oscillator for several set of parameters and showed that the equations of the oscillator can be reduced to the one-zone model of stellar pulsation with simple nonlinear terms. It is suggested that the stellar irregular variability is resulted from the chaotic motion due to the nonlinear effect.  相似文献   

2.
A new crossing method for the study of turbulent velocities in solar and stellar photospheres is considered. The method does not need knowledge of the abundance and oscillator strengths for determining the microturbulent velocity, if the macroturbulent velocity is adopted; or it allows investigation of the micro- and macro-velocities simultaneously, if the abundance and oscillator strengths are known. Using the crossing method for 200 lines of neutral iron we obtain microturbulent velocities for a large range of depths in the solar photosphere. The distribution of macroturbulent velocities with depth is also investigated. The total velocity field calculated from the obtained micro- and macro-velocities agrees with previous results from independent methods. This demonstrates the reliability of using the crossing method for separate determination of the micro- and macroturbulent velocities in solar and stellar atmospheres.  相似文献   

3.
This paper presents the solutions of the Schrödinger equation for hydrogen with the application of various boundary conditions. These solutions have important consequences as regards the high pressure behaviour of the materials, i.e. for the pressure ionization, for the pressure modification of optical features. The results are of some significance in the pressure range of stellar atmospheres or even of laboratory plasmas. The number of bound levels coming into being versus density function has been derived and compared with the Inglis-Teller formula. The modification of some oscillator strengths as a function of density is calculated too.  相似文献   

4.
The large stellar/planetary flux ratio (>106) and small angular separation (0.1 arcsec when observed from 10 parsecs) make it difficult to study Earthlike extrasolar planets. Hybrid coronographs with apodized masks and nulling by Earth based interferometric techniques could reduce the flux ratio by 3 orders of magnitude. Further reduction of starlight is possible with frequency filters. Due to large (upto 30 km/s) differences in radial velocities the specific spectral line for a particular molecule will be Doppler shifted by different amounts depending on from where, the star or the planet, the emission originates. The stellar spectrum itself could be used as a dynamic reference to determine the differential Doppler shift and define the frequency search space for the sought after planetary spectral line. The Differential Radial Velocity Spectrometer (DRVS) could use a heterodyne receiver with steep skirted filters and a laser local oscillator tracking the stellar spectrum. Several planetary spectral line windows should be searched and correlation/code gain techniques used to enhance detection capabilities.  相似文献   

5.
Nonstandard theories of fundamental interactions typically predict the existence of new kinds of weakly interacting particles. These can escape freely from stellar interiors and act as additional source of cooling. Considerable agreement of a variety of astrophysical observations with standard physics can serve as a source of constraints on non-standard ideas. In this paper we consider G117-B15A pulsating white dwarf for which the secular rate, at which the period of its fundamental mode increases, has been accurately measured. This star has been claimed the most stable oscillator ever recorded in the optical band. Because an additional channel of energy loss would speedup the cooling rate, one is able to use this stability to derive a bound on axion mass and on theories with large extra dimensions. We also point to the possibility of using similar arguments to constrain supersymmetric paticles. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

6.
Radio emission from extrasolar giant planets in close orbits around their host star is an active field of research, including both observational efforts and theoretical work aiming at reasonable predictions for different target planets. So far, most theoretical work assumed a distance-independent, constant stellar wind velocity. This approach is improved and expanded in two respects: first, from stellar wind models, it is known that at close distances the stellar wind is still slow and has not yet reached the velocity it has at larger distances. For this reason, less energy is available for the generation of planetary radio emission than predicted by simplified models. This correspondingly reduces the intensity of stellar wind-driven planetary radio emission, which is calculated taking into account the stellar age. Second, it can be shown that under certain conditions the steady stellar wind has to be replaced by stellar coronal mass ejections. In those cases, the planetary radio flux is strongly increased. The different flux levels expected for planets subject to different stellar wind conditions are analyzed and compared. In addition, different uncertainties in this radio flux estimation are calculated and discussed.  相似文献   

7.
新一代大规模光谱巡天项目产生了近千万条低分辨率恒星光谱,基于这些光谱数据,介绍一种名为The Cannon的机器学习方法。该方法完全基于已知恒星大气参数(有效温度、表面重力加速度和金属丰度等)的光谱数据,通过数据驱动来构建特征向量,建立光谱流量特征和恒星参数的函数对应关系,进而应用到观测光谱数据中,实现对恒星光谱的大气参数求解。The Cannon的主要优势为不直接基于任何恒星物理模型,适用性更广;由于使用了全谱信息,即便对于低信噪比光谱也能得到较高可信度的参数结果,该算法在大规模恒星光谱的数据处理和参数求解方面具有明显的优势。此外,还利用The Cannon得到LAMOST光谱数据中K巨星和M巨星的恒星参数。  相似文献   

8.
The new generation of large sky area spectroscopic survey project has produced nearly 10 million low-resolution stellar spectra. Based on these spectroscopic data, this paper introduces a machine learning algorithm named The Cannon. This algorithm is completely based on the known spectroscopic data of stellar atmospheric parameters (effective temperature, surface gravity, and metal abundance, etc.), this algorithm builds the characteristic vector by means of data driving, and establishes the functional relation between spectral flux characteristics and stellar parameters. Then it is applied to the observed spectral data to calculate the atmospheric parameters. The main advantage of The Cannon is that it is not directly based on any stellar physical models, it has an even higher applicability. Moreover, because of the use of full-spectrum information, even for the spectra with a low signal-to-noise ratio (SNR), it still can obtain the parameter solutions of high reliability. This algorithm has significant advantages in the data processing and parameter determination of large-scale stellar spectra. In addition, this paper presents two examples of using The Cannon to obtain the stellar parameters of K and M giants from the LAMOST spectral data.  相似文献   

9.
With the help of computer tools and algorithms, automatic stellar spectral classification has become an area of current interest. The process of stellar spectral classification mainly includes two steps: dimension reduction and classification. As a popular dimensionality reduction technique, Principal Component Analysis (PCA) is widely used in stellar spectra classification. Another dimensionality reduction technique, Locality Preserving Projections (LPP) has not been widely used in astronomy. The advantage of LPP is that it can preserve the local structure of the data after dimensionality reduction. In view of this, we investigate how to apply LPP+SVM in classifying the stellar spectral subclasses. In the comparative experiment, the performance of LPP is compared with PCA. The stellar spectral classification process is composed of the following steps. Firstly, PCA and LPP are respectively applied to reduce the dimension of spectra data. Then, Support Vector Machine (SVM) is used to classify the 4 subclasses of K-type and 3 subclasses of F-type spectra from Sloan Digital Sky Survey (SDSS). Lastly, the performance of LPP+SVM is compared with that of PCA+SVM in stellar spectral classification, and we found that LPP does better than PCA.  相似文献   

10.
A new approach is proposed and developed to handle pre-processed CCD frames in order to identify stellar images and derive their relevant parameters. The present method relies on: 1) identifying stellar images and assigning approximate positions of their centres using artificial intelligence techniques, 2) accurate determination of the centre coordinates applying an elementary statistical concept and 3) estimating the image peak intensity as a stellar magnitude measure employing a simple numerical analysis approach. The method has been coded for personal computer users. A CCD frame of the star cluster M67 was adopted as a test case. The results obtained are discussed in comparison with the DAOPHOTII ones and the corresponding published data. Exact coincidence has been found between both results except very few cases. These exceptions have been discussed in view of both methods' bases and the cluster plates. It has been realized that the method suggested exhibits very simple, extremely fast and high precise approach in stellar CCD photometry domain. Moreover, it is more capable for handling blended and distorted stellar images than the DAOPHOTII. These characteristics show the usefulness of the present method in some astronomical applications such as auto-focusing and auto-guiding sensing approaches beside the main purpose, viz. stellar photometry. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
This article is intended to remind us of the history of the origins of one of the fundamental discoveries of the twentieth century, stellar associations, which are connected with the name of V. A. Ambartsumyan. It is not a complete discussion of all the published work, for this is, of course, impossible. Research on stellar associations began immediately after their discovery and continues to this day. Observations are now also carried out by automatic satellites and a vast amount of observational data has been obtained. Many stellar associations have been discovered, both in our galaxy and in other galaxies. Catalogs of stellar associations have been compiled and many problems relating to stellar associations have been solved. Translated from Astrofizika, Vol. 52, No. 2, pp. 171–184 (May 2009).  相似文献   

12.
The exact geometry of the interface region between the accretion disk and the stellar surface in young stars has crucial implications for both the origin of mass-loss and the regulation of stellar angular momentum. We discuss proposed AMBER/VLTI observations of the Paβ line emitting region that will put the first direct constraints on the magnetospheric accretion flow scenario in active young stellar systems. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
We have developed a radiative transfer code, cmfgen, which allows us to model the spectra of massive stars and supernovae. Using cmfgen we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended cmfgen to allow us to undertake time-dependent radiative transfer calculations of supernovae. Such calculations will be used to place constraints on the supernova progenitor, to place constraints on the supernova explosion and nucleosynthesis, and to derive distances using a physical approach called the “Expanding Photosphere Method”. We describe the assumptions underlying the code and the atomic processes involved. A crucial ingredient in the code is the atomic data. For the modeling we require accurate transition wavelengths, oscillator strengths, photoionization cross-sections, collision strengths, autoionization rates, and charge exchange rates for virtually all species up to, and including, cobalt. Presently, the available atomic data varies substantially in both quantity and quality.  相似文献   

14.
Dibyendu Nandy 《Solar physics》2004,224(1-2):161-169
Sun-like stars are known to display a wide variety of magnetic activity which is likely to be the signature of a hydromagnetic dynamo mechanism working in stellar interiors. This dynamo mechanism has been studied extensively in the context of the Sun. Here we take ideas and experiences gained from solar dynamo modeling and build upon it to study the inferred scaling laws, involving stellar parameters, from observations of stellar magnetic activity. We also discuss how such a synthesis of theoretical dynamo modeling of Sun-like stars and stellar cycle observations may help us reconstruct the long-term variability of the Sun – an important ingredient for understanding the effects of solar forcing on space and global climate.  相似文献   

15.
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16.
High spatial resolution visible and NIR observations of the bar of NGC 5850, a prototype of double barred spirals, together with visible intermediate and high dispersion spectra along the primary bar, are being used, with the aid of simple stellar population synthesis models, to investigate the mean age and metallicity of the different stellar components of the central part of the galaxy. The determination of stellar ages and metallicities would constrain theoretical scenarios for secondary bar formation and the evolution of barred spirals. Unfortunately, we cannot obtain a good fit with simple stellar populations (SSPs) to the spectral indices, so it can not give us, by now, insight into the mean stellar age and metallicity of the real populations in the central region of the galaxy. These preliminary results show a relatively old primary bar with metallicity about solar, although absolute values must be taken with care. The nucleus has a young stellar component, and is very dusty. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

17.
We present new X-ray data (obtained with the Chandra telescope) as well as FUV data of the supergiant shell (SGS) in IC 2574, a dwarf galaxy in the M 81 group. This region has proven to be a unique target for studying the interplay of massive star formation with the surrounding interstellar medium. The Chandra data confirm the presence of soft X-ray emission, indicative of the presence of hot gas. Using optical and FUV data, we estimate the age and the energy input of the stellar cluster inside the SGS. The age estimate agrees well with the age estimated based on the HI data alone – providing further supportfor the view that the stellar cluster interior the SGS is powering its expansion. We conclude that indeed massive stellar clusters can create supergiant shells in galaxies (even at large galactocentric distance) as predicted by the `standard' picture (creation by SN explosions and strong stellar winds), a scenario which has recently been questioned by some authors.  相似文献   

18.
The structure of critically rotating polytropes is calculated using two (reated) Roche-type approximation schemes, one of which has been developed for stellar models. Comparisons with other results are made as a validation of these methods in stellar structure calculations.  相似文献   

19.
Using two volume-limited Main galaxy samples of the Sloan Digital Sky Survey Data Release 10 (SDSS DR10), we investigate the dependence of the clustering properties of galaxies on stellar velocity dispersion by cluster analysis. It is found that in the luminous volume-limited Main galaxy sample, except at r=1.2, richer and larger systems can be more easily formed in the large stellar velocity dispersion subsample, while in the faint volume-limited Main galaxy sample, at r≥0.9, an opposite trend is observed. According to statistical analyses of the multiplicity functions, we conclude in two volume-limited Main galaxy samples: small stellar velocity dispersion galaxies preferentially form isolated galaxies, close pairs and small group, while large stellar velocity dispersion galaxies preferentially inhabit the dense groups and clusters. However, we note the difference between two volume-limited Main galaxy samples: in the faint volume-limited Main galaxy sample, at r≥0.9, the small stellar velocity dispersion subsample has a higher proportion of galaxies in superclusters (n≥200) than the large stellar velocity dispersion subsample.  相似文献   

20.
Several mechanisms of bar mode formation in stellar galactic disks, including the Toomre swing amplification mechanism and modal approaches, are considered. Using the well-known Kuzmin-Toomre stellar disk model as an example, it has been shown through numerical simulations that the stellar bar results from the development of an unstable normal mode. The pattern speed and the spiral wave growth rate found from a numerical experiment agree well with the linear perturbation theory. The nonlinear evolution of the bar is traced. The possible role of growing transient spirals in the formation of bars is discussed.  相似文献   

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