Stellar spectroscopy in ground-based ultraviolet. III: Chromospheres and envelopes of cool stars

In historical perspective we discuss selected tasks in the studies of stellar chromospheres and circumstellar envelopes using spectroscopic methods in the ground-based ultraviolet. We illustrate our discussion by fragments of spectra that we took with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences.     

Neutrino luminosity of stars with different masses

Neutrinos play an important role in stellar evolution. They are produced by nuclear reactions or thermal processes. Using the stellar evolution code Modules for Experiments in Stellar Astrophysics(MESA), we study stellar neutrino luminosity with different masses. The neutrino luminosities of stars with different initial masses at different evolutionary stages are simulated. We find that the neutrino flux of a star with 1 M⊙mass at an evolutionary age of 4.61 × 10~9 yr is consistent with that of the Sun. In general, neutrinos are produced by nuclear reactions, and the neutrino luminosity of stars is about one or two magnitudes lower than the photo luminosity. However, neutrino luminosity can exceed photo luminosity during the helium flash which can occur for stars with a mass lower than 8 M_⊙. Although the helium flash does not produce neutrinos, plasma decay, one of the thermal processes, can efficiently make neutrinos during this stage. Due to the high mass-loss rate, a star with a mass of 9 M_⊙does not undergo the helium flash. Its neutrinos mainly originate from nuclear reactions until the end of the AGB stage. At the end of the AGB stage, its neutrino luminosity results from plasma decay which is triggered by the gravitational energy release because of the stellar core contracting.     

Measuring Stellar Differential rotation with asteroseismology

The variation of rotation with latitude is poorly known on stars other than the Sun. Several indirect techniques, photometric and spectroscopic, have been used to search for departure from rigid rotation for sufficiently fast rotators. Here we investigate the possibility of measuring stellar differential rotation for solar-type stars through asteroseismology. Rotationally split frequencies of global oscillation provide information about rotation at different latitudes depending on the azimuthal order, m, of the mode of pulsation. We present a method to estimate differential rotation based on the realization that the m = ±1 and m = ±2 components of quadrupole oscillations can be observed simultaneously in asteroseismology. Rotational frequency splittings can be inverted to provide an estimate of the difference in stellar angular velocity between the equator and 45° latitude. The precision of the method, assessed through Monte Carlo simulations, depends on the value of the mean rotation and on the inclination angle between the rotation axis and the line of sight.     

Determination of Stellar Parameters with GAIA

The GAIA Galactic survey satellite will obtain photometry in 15 filters of over 10⁹ stars in our Galaxy across a very wide range of stellar types. No other planned survey will provide so much photometric information on so many stars. I examine the problem of how to determine fundamental physical parameters (T _eff, log g, [Fe/H] etc.) from these data. Given the size, multidimensionality and diversity of this dataset, this is a challenging task beyond any encountered so far in large-scale stellar parametrization. I describe the problems faced (initial object identification, interstellar extinction, multiplicity, missing data etc.) and present a framework in which they can be addressed. A probabilistic approach is advocated on the grounds that it can take advantage of additional information (e.g. priors and data uncertainties) in a consistent and useful manner, as well as give meaningful results in the presence of poor or degenerate data. Furthermore, I suggest an approach to parametrization which can use the other information GAIA will acquire, in particular the parallax, which has not previously been available for large-scale multidimensional parametrization. Several of the problems identified and ideas suggested will be relevant to other large surveys, such as SDSS, DIVA,FAME, VISTA and LSST, as well as stellar parametrization in a virtual observatory. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stellar dynamics

This review attempts to place stellar dynamics in relation to other dynamical fields and to describe some of its important techniques and present-day problems. Stellar dynamics has some parallels, in increasing order of closeness, with celestial mechanics, statistical mechanics, kinetic theory, and plasma theory; but even in the last case the parallels are not very close. Stellar dynamics describes, usually through distribution functions, the motions of a large number of bodies as they all act on each other gravitationally. To a good approximation each star can be considered to move in the smoothed-out field of all the others, with random encounters between pairs of stars adding a slow statistical change to these smooth motions. Smooth-field dynamics has a well-developed theory, and the state of smooth stellar systems can be described in some detail. The ‘third integral’ presents an outstanding problem, however. Stellar encounters also have a well-developed theory, but close encounters and encounters of a single star with a binary pose serious problems for the statistical treatment. Star-cluster dynamics can be approached through a theory of smooth-field dynamics plus changes due to encounters, or alternatively through numerical simulations. The relation between the two methods is not yet close enough. The dynamical evolution of star clusters is still not fully understood.     

Stellar associations

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).     

Stellar flares

Different aspects of stellar flares as one of the events of stellar activity with different levels of similarity to solar-activity phenomena are briefly discussed: those which are understandable by means of scaling of solar events, those that do not occur on the Sun but general solar features allow us to understand them, and such stellar events that have not been understood as yet.     

Stellar Inversions

Inversions of oscillation frequencies have proved to be extremelypowerful in studying the structure of the Sun. We examine theconditions under which we can invert oscillation frequenciesof other stars to study their structure. We show that despite thevery limited number of modes that are expected to be observed in otherstars, we can perform inversions that will give localized informationof the stellar core. These results can limit the space of admissiblemodels of a given star.     

Stellar Convection

Seismology provides powerful tests of convection deep instellar interiors. First, the role of convective overshoot and lowefficiency convection, two areas of uncertainty with importantastrophysical implications, are reviewed briefly. In the rest of thetalk, a critical introduction to numerical simulations of radiativehydrodynamics will be given. The basic underlyingassumptions and challenges are explained, and some recent resultsare presented.     

Stellar Tomography

I review recent progress in the field of stellar surface imaging, with particular reference to advanced methods for mapping surface-brightness inhomogeneities and the surface vector magnetic field on magnetically active late-type stars. New signal enhancement techniques, utilising profile information from hundreds or thousands of photospheric lines simultaneously, allow images to be derived for stars several magnitudes fainter than was previously possible. For brighter stars, the same techniques make it possible to map features as small as two or three degrees in extent on the stellar surface. This opens up whole new areas of research, such as the ability to use starspot tracking to study surface differential rotation patterns on single and binary stars, and to follow the secular evolution of the magnetic field itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multilayer Spherical Stellar Clusters with Uniform Density

Astrophysics - Various solutions of the kinetic equation for the equilibrium of a gravitating sphere of uniform density with a quadratic gravitational potential and a linear dependence of...     

Stellar spectra classification with twin hypersphere model

With the increase of stellar spectra, how to automatically classify these spectra have attracted astronomer's attention. Support Vector Machine (SVM), as a typical classifier, has widely used in stellar spectra classification. Due to its limited performance in various classification problems and higher training time, a model with a pair of hyperspheres named Twin Hypersphere Model (THM), proposed by Peng and Xu, is utilized for stellar spectra classification in this paper. In THM, the samples in one hypersphere is far from another according to the Euclidean distance. The comparative experiments with SVM and Twin Support Vector Machine (TWSVM) on the SDSS datasets shows that the THM model gives the best classification accuracy of 0.8836 for type F, 0.9446 for type G, and 0.9509 for type K, which are better than the classification accuracies of 0.8000, 0.8484, 0.8911 obtained by SVM and 0.8413, 0.8699, 0.9109 obtained by TWSVM. It can be concluded that THM perform better than traditional techniques such as SVM and TWSVM on the K-, F-, G- type stellar spectra classification.     

Stellar dynamic spectroscopy

The dynamic spectrum, a three dimensional record of the radio intensity as a function both of time and frequency, has long been used as a probe of plasma processes in the solar corona. Beginning with the work of Wild and McCready (1950) dynamic spectroscopy has been used to distinguish between the multitude of radio wave emitting phenomena which occur in the solar corona and to infer the physical mechanisms responsible.Stellar dynamic spectroscopy has always been a tantalizing prospect. The vast body of experience with solar dynamic spectroscopy would prove invaluable in interpreting stellar dynamic spectra. Further, the new parameter regimes presented by stellar coronas would allow further insight to be gained in the physical processes at work in stellar coronas.Recently, Bastian and Bookbinder (1987) used the Very Large Array in spectral line mode at 1.4 GHz with a bandwidth of 50 MHz to obtain the first dynamic spectra of nearby flare stars. The spectral resolution was 3.125 MHz and the temporal resolution was 5 s. While the relative bandwidth was less than ideal (/ 5%), the spectra so obtained were sufficient to show the presence of narrowband structure in a radio outburst from the well-known dMe flare star UV Ceti.Several efforts are now underway to obtain stellar dynamic spectra, of both RS CVn binaries and dMe flare stars, with higher degrees of spectral and temporal resolution. Among these are use of a 1024 channel correlator with the 1000' telescope at Arecibo and use of the Berkeley Fast Pulsar Search Machine (Kulkarni et al. 1984) with the Green Bank 140' telescope.

---

     

Stellar Wind Theories

In this lecture I describe the basic theory of stellar winds with momentum input due to a force or with energy input and I formulate the five laws of stellar winds. I review the different wind mechanisms and discuss the line driven wind model and the dust driven wind models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stellar chromospheric temperatures

The emission doublet 2800 Mgii of stellar chromospheric origin is always stronger than the emission in the Lα line of hydrogen. At the same time, the ratio of their fluxes,Q=F(Mgii)/F(Lα), varies over a wide range— from 2 up to 20 in stars of one and the same type. Assuming that both emission structures, 2800 Mgii and Lα, are excited in one and the same region of the chromosphere, an attempt is made to represent the ratioQ as a function from one physical parameter only—chromospheric electron temperature. Such an approach explains easily the observed behaviour of the ratioQ in different stars.     

星族合成

以简单星族为例介绍了演化星族合成的算法 ,并总结了在演化星族合成中常用的恒星演化库 ,光谱库 ,初始质量函数和合成判据 ,最后简要讨论了目前星族合成中仍存在的问题。