A phenomenological interpretation of stellar chromospheres

An attempt is made to develop a phenomenological interpretation of stellar chromospheres. The following problems are examined: observed emission powers of magnesium chromospheres on stars based on the ultraviolet doublet, 2800 Mgii, observations; dependence of chromosphere emission on spectral and luminosity classes; stellar chromospheres as an accidental event; chromospheres of stars-components of binary systems; stars with the chromospheres of solar type (S) and nonsolar (NS) type; distribution of stars by means of the type of their chromosphere on luminosity class; stars with superpower magnesium emission; emission measures for both the magnesium and calcium chromospheres; interrelation between chromosphere, transition zone and corona; chromospheric activity and rotation of stars; possibility of the existence of chromospheres on hot stars; phenomenological picture of stellar chromospheres; stars without the line 2800 Mgii, in emission or in absorption; syndrome of red giant HD 4174. At the end, the problem of heating of stellar chromospheres is discussed.     

Demonstration of photon-noise limit in stellar radial velocities

We have measured apparent fluctuations in stellar radial velocities with the ELODIE fiber-fed crossed-dispersion spectrograph and the 193-cm telescope of Observatoire de Haute-Provence. Within one given night, the fluctuations consist of two terms which may be sorted out. The first comes from imperfect scrambling of the stellar beam; the second arises from photon noise and agrees closely with our published calculations. So far, scrambler noise dominates for bright stars, but a perfect scrambler could be built by combining adatative optics and a single-mode fiber. The photon-noise results confirm that extrasolar planetary searching by the radial-velocity technique may be implemented with relatively small telescopes for a large number of stars. Consequences for the detection of astrophysical noise are discussed.     

Stellarv sini and optical emission line widths in Be stars

I re-investigate empirical correlations between line widths of photospheric absorption lines — as a measure of the stellar projected rotational velocityv sini- and of circumstellar emission lines in Be stars. The degree of this correlation is a measure for the kinematic, i.e., rotational, contribution to the total emission line broadening.From the literature and from additional data, I have collected a sample of photoelectric measurements of Balmer and Feii emission lines of the 115 brightest Be stars in the sky, most of them observed several times during the past decade. I investigate the following emission line widths parameters: full width at half maximum, total width at the base, and double-peak separation. I find from the FWHM that the basic origin of emission line broadening in Be stars is undoubtedly rotational, the correlations withv sini being best for weaker emission lines while H shows considerable scatter. Total width at the base is uncorrelated withv sini except for the Feii lines. Peak separation is found to be a function both ofv sini and of equivalent width. The latter dependence can straightforwardly be understood by the fact that within Huang's disk model peak separation, normalized tov sini, is governed by the outer envelope radius.I conclude that observational evidence leaves no doubt that Be star envelopes as visible in optical emission lines are rotationally supported and have axisymmetric geometry.Partly based on observations obtained at the European Southern Observatory, La Silla, Chile.     

X-ray stars observed in LAMOST spectral survey

X-ray stars have been studied since the beginning of X-ray astronomy. Investigating and studying the chromospheric activity from X-ray stellar optical spectra is highly significant in providing insights into stellar magnetic activity. The big data of LAMOST survey provides an opportunity for researching stellar optical spectroscopic properties of X-ray stars. We inferred the physical properties of X-ray stellar sources from the analysis of LAMOST spectra. First, we cross-matched the X-ray stellar catalogue (12254 X-ray stars) from ARXA with LAMOST data release 3 (DR3), and obtained 984 good spectra from 713 X-ray sources. We then visually inspected and assigned spectral type to each spectrum and calculated the equivalent width (EW) of H\(\alpha\) line using the Hammer spectral typing facility. Based on the EW of H\(\alpha\) line, we found 203 spectra of 145 X-ray sources with H\(\alpha\) emission above the continuum. For these spectra we also measured the EWs of H\(\beta\), H\(\gamma\), H\(\delta\) and Ca ii IRT lines of these spectra. After removing novae, planetary nebulae and OB-type stars, we found there are 127 X-ray late-type stars with H\(\alpha\) line emission. By using our spectra and results from the literature, we found 53 X-ray stars showing H\(\alpha\) variability; these objects are Classical T Tauri stars (CTTs), cataclysmic variables (CVs) or chromospheric activity stars. We also found 18 X-ray stars showing obvious emissions in the Ca ii IRT lines. Of the 18 X-ray stars, 16 are CTTs and 2 are CVs. Finally, we discussed the relationships between the EW of H\(\alpha\) line and X-ray flux.     

Solar-type stars in Cepheus OB 2: Hα emission stars and IRAS point sources

A survey for emission line stars was carried out in 1980 with the Schmidt telescope of Konkoly Observatory in the region of IC 1396 (Kun, 1986a, hereafter referred to as Paper I). This work was aimed at a preliminary mapping of the probable medium mass members of the association Cepheus OB 2. The stars of the region have been followed since then with the same telescope in order to obtain a more complete view on the stellar content of this giant star-forming region. The common variability of the H emission strength makes the sense of repeating such surveys. The new survey resulted in the discovery of 65 further emission stars. Equatorial coordinates and finding charts are given for them.PhotographicBVRI photometry was carried out for all known emission stars of the field. The colourmagnitude and two-colour diagrams derived from theB, V, R, andI magnitudes show that these stars are probably F-G-type (1.5M <M<3M ), partly pre-Main-Sequence members of Cep OB 2, whereas a few of them may be distant red giants.The youngest objects of the region can be found among the IRAS point sources. Their properties are discussed on the basis of IRAS data alone.     

Stellar flare statistics — Physical consequences

The observational data permit us to establish clear statistical correlations between different parameters of stellar flare activity and the characteristics of quiet stars. These relations are:

1. between energies and frequencies of flares on stars of different luminosities;
2. between total radiation energies of flares and quiet stars both in X-ray and Balmer emission lines;
3. between flare decay rates just after the maxima and flare luminosities at maxima.

     

Circumstellar clouds derived from ultraviolet stellar spectra

It is shown that the observed variety in macrostructures of continuous spectra in the ultraviolet (2000–3000 Å) of hot stars is a result of the presence of circumstellar clouds around such stars. A method for calculations of synthetic spectra, originating as a result of passage of central star photospheric radiation through its own circumstellar cloud, is developed. It introduces a new idea of spectral class for circumstellar cloud, and a recommended method for its determination depending from the spectral class of central star and cloud's parameters (Figure 2). The results of calculations of synthetic spectra for the four combinations of system star+cloud are presented (Figures 7-10). The strongest influence of circumstellar cloud in ultraviolet is discovered on A-class stars (Figure 13). Graphic relations are introduced for determination of cloud power by observed parameters of synthetic spectra (Figures 14 and 15).It establishes an important fact for an understanding of the nature of circumstellar clouds and processes occurring in them, according to which the selective absorption in such clouds stimulatesresonance lines only, the largest number of which lies in the ultraviolet in the region of 2100–2600 Å (Figure 1). An absence of visible signs of the effect of circumstellar clouds on continuous spectra of stars in visual region can be explained by a very small number of resonance lines in this region.Lastly, the possibility of determination of physical and geometric parameters of circumstellar clouds from stellar continuous spectra in the ultraviolet is analysed.     

Progress in ultraviolet stellar spectrophotometry with S59

A progress report is given of investigations related to observations of stellar spectra obtained with the ultraviolet stellar spectrophotometer S59 aboard the ESRO TD-1A satellite. We describe first the processing of the observations: intensity and wavelength calibration, identification of lines, classification of spectra. Thereafter some important groups of lines are dealt with: non-LTE computation of the Mgii lines are presented: apart from the peculiar emission line and shell stars they compare well with the observations; intensity ratios, of Feii and Feiii lines are an important temperature classification criterion. Interstellar UV lines indicate large deficiencies of some metals, as compared to solar values. A new ultraviolet continuous extinction curve is determined. We discuss the strong outstreaming motions observed in α Cygni, and the consequent mass loss of this star (< 3 × 10^?10 \(\mathfrak{M}_ \odot \) yr^?1and finally we describe the composite spectrum of the (WC8+09I) bnary γ² Velorum; the ultraviolet continuous spectrum of the WC star is about one magnitude brighter than any theory predicts.     

Chromospheres of flare stars

The present paper contains an attempt to formulate a theory, based on fast electrons hypothesis, of the chromospheres of flare stars. At the same time we shall undertake a survey of observations on the emission lines of flare stars and comparisons with the theory. The general discussion in the introduction concerning the civilian right of fast electron hypothesis is followed by sections in which the following problems are tackled: the anatomy of the light curve of the flare in UV Cet-type stars or its division into two components; the sources of radiation, ionizing hydrogen and other elements, and the estimation of their power in cold dwarf stars with emission lines; conditions for the excitation of emission lines in the chromospheres of those stars; the problem of duration of luminescence of flare stars in the emission lines (observations and theory); the electron temperature and electron concentration in the chromospheres of flare stars; the problem of the luminescence of emission lines in the quiescent star; the degree of ionization and the role of inelastic collisions of the electrons in the chromosphere of flare stars; the profiles of emission lines, their broadening and intensification during the flare; the dependence of the infensity of emission lines on the flare amplitude; the nature and peculiarities of two types of the Haro flares; the impact of radiation dilution and the spectral class of the star on the equivalent width of the emission lines; the possibility of exciting the forbidden lines; the problem of generation of the emission lines of neutral and ionized helium; the possibility of the Lyman-alpha emission in flare stars, the expected parameters of such emission — the radiation power, the equivalent width and profile of the Lyman-alpha line; the possibility of the presence of the strongest (after the -line) emission line — of doublet 2800 Mgii in the spectra of flare stars, the expected value of the intensity and equivalent width of that line.     

Stellar analogs of solar magnetic activity

The techniques and principal results of observational studies of stellar activity are summarized. Both chromospheric and coronal emission clearly track surface magnetic field properties, but it is not well known how the detailed relation between the emission and surface magnetic fields varies with spectral type. For lower Main-Sequence stars of the same spectral type, there is clear evidence of a close relationship between mean activity level and rotation period P _rot. There is also less definitive evidence for a similar dependence on convective overturn time _c , such that activity depends on the single parameter Ro = P _rot/ _c . For single stars, stellar rotation, and magnetic activity both decline smoothly with age. This implies a feedback between angular momentum loss rate and activity level. Temporal variations in mean stellar activity level mimic the solar cycle only for old stars like the Sun, being much more irregular for younger stars. The characteristic timescale of the variations (the cycle period) appears to depend on Ro for old stars, but shows no clear dependence on either rotation rate or spectral type for younger stars. Further data on mean activity and its variation for a large number of lower Main-Sequence stars should contribute significantly to our understanding of the causes of stellar magnetic activity.     

O VI in the local interstellar medium

We report preliminary results of a search for O VI absorption in the spectra of ～100 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In many cases, the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs. However, in a significant number of cases the interstellar and photospheric contributions can be separated. In the majority of stars where we find O VI absorption lines, the material is clearly associated with the stellar photosphere and not the interstellar medium. There are a small number of lines-of-sight where the gas is interstellar in nature but the stars are located beyond the boundaries of the local cavity.     

Parameters of superflares on G-Type stars observed with the Kepler space telescope

We continued the analysis of 279 G-type stars with superflares (energies in the range of 10³³–10³⁶ erg). We calculated the SFL parameter (part of the stellar surface which emits in the flare). The SFL estimates were derived from the relation connecting this value with the amplitude of the flare and its radiation on the assumption of the blackbody character of the emission at times close to its maximum. Most SFL values are in the range of 0–0.1, with values of 0.2–0.4 for some strong flares. Dependence of SFL on effective temperature for stars with superflares is similar to that found earlier for the spottedness parameter S. The SFL distribution reaches its maximum in the temperature range of about 5100–5250 K and decreases with the effective temperature increase. We suggested an assumption on the presence of bimodal distribution in the “SFL–rotation period” relation with a gap for objects with rotation periods P of about 10 days. For stars with P less than 10 days, the given data can indicate a decrease in flare areas with the P increase. Our analysis showed that significant changes both in flare energy and in flare areas can be achieved with small changes in spottedness S for one and the same star.     

3-D radiative line transfer for be star envelopes

Based on numerical three-dimensional radiative line transfer calculations H emission line profiles of circumstellar Be star envelopes have been derived. The results show that the socalled winebottle-type emission line profiles can be explained by the combination of rotational broadening and non-coherent scattering in optically thick Keplerian disks. In a further calculation the stellar wind model of Be star envelopes has been re-investigated assuming an additional expansion component in the velocity field. The resulting asymmetric winebottle-type profiles and asymmetric shell-type emission lines with blue-shifted central depressions are in contradiction with the observed line shapes. It is concluded that isotropic stationary outflows are not suitable to explain observed asymmetric emission line profiles of Be stars.     

An explanation of the ‘granulation boundary’ in the HR diagram

The granulation boundary is a line running in the HR diagram from about F0V stars to G1Ib. It divides the HR diagram in two regions in which the spectral line bisector behaves differently. To the right (cooler stars) the Fraunhofer lines in stellar spectra show the characteristics typical for convective motions. For hot stars it indicates the presence of large velocities involving a large part of the line-forming region. We give evidence that the opposite behaviour shown by spectra of hotter stars can be explained by gravity waves.     

Robotic telescopes and networks: New tools for education and science

Nowadays many telescopes around the world are automated and some networks of robotic telescopes are active or planned. Such equipment could be used for thetraining of students and for science in the Universities of DCs and of newastronomical countries, by sending them observational data via Internetor through remotely controlled telescope. It seems that it is thetime to open up for discussion with UN and ESA organizations and also withIAU, how to implement links between robotic telescopes and suchUniversities applying for collaborations. Many scientific fields couldthus be accessible to them, for example on stellar variability, near-earthobject follow-up, -ray burst counterpart tracking, and so on.     

Symbiotic stars

I review our current knowledge of symbiotic stars. A great many papers have graced the literature in the fifty years of their study, and many data are available on the spectral variations at optical wavelengths these stars undergo. I do not give extensive references to those data, for previous reviews have done so quite adequately. Rather, I concentrate on the extensive widening of the wavebands within which symbiotic stars have been studied over the past few years, and attempt to synthesise the data into a coherent picture.Symbiotic stars are most readily explained as interacting binaries, though single star models may still be tenable for some systems. They are made much more complex than most other interacting binaries by the variety of accreting stars, and because gas flows may be highly structured. Moreover, their study is more difficult than that of dwarf novae because the orbital periods are long compared to the activity cycles of the accretion phenomena.Our data base has expanded enormously with our present spectral catholicism. But there remains much valuable work to be done with even simple equipment on small telescopes. I suggest in a final section areas for future work.Invited paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia 3–7 June, 1983.     

Ground-based interferometry

This paper summarizes the limits of ground-based interferometry for differential astrometry as well as ground-based interferometry for direct detection of exo-planets and exo-zodi dust levels. For direct detection, ground-based interferometry at near IR wavelengths using large telescopes with adaptive optics offers a significant advantage over single telescopes with adaptive optics. Ground-based differential astrometry for exo-planet detection is extremely accurate with sufficient accuracy to detect Neptune mass planets around 400–600 nearby stars. Ground-based interferometry using large (>6m) telescopes is also capable of detecting the 10 m emission of the zodiacal light around nearby stars with zodi levels similar to our solar system     

Magnetic activity optimal tracers: from radio to X-ray; the relevance of UV astronomy

Observations and analysis of magnetic activity phenomena in the atmospheres of cool stars—e.g., active regions, flares, stellar cycles—give insight into the fundamental processes in the heating of chromospheres, transition regions (TRs), and coronae. Diagnostics of magnetic activity can be found throughout the whole electromagnetic spectrum; from radio wavelengths, where gyrosynchrotron radiation arises from the quiescent and flaring corona, to optical, where important signatures are the Balmer lines and the Ca ii IRT and H&K lines, eventually to UV and X-rays, the latter mainly due to coronal thermal plasma. The UV and EUV ranges contains a plethora of emission lines that are powerful diagnostics for the warm (10?000 K) chromospheres, hot (100?000–800?000 K) TRs and very hot (1–10 MK) coronae. Also very weak coronal winds from cool stars have been identified and characterized thanks to high resolution UV spectra. Here I review the main results from UV observations of cool stars atmospheres and outline what can be expected from future UV imaging and spectroscopy measurements.     

Infrared hydrogen emission lines in the spectra of Be stars

For the interpretation of observed hydrogen emission lines in the spectrum of Cas calculations of the relative intensities of spectral lines of the Balmer, Paschen, Brackett and Pfund series are carried out, by using the theory of moving stellar envelopes.It is shown that in the spectra of Be stars, which have opaque envelopes and therefore a slow Balmer decrement, infrared hydrogen lines must be relatively strong.It is also found that the observed relative line intensities of Cas are in qualitative agreement with those calculated for the case of a simple model-envelope which consists of low and high opacity regions.     

The Nobeyama Millimeter Array Survey for protoplanetary disks around protostar candidates and T Tauri stars in Taurus

The Nobeyama Millimeter Array Survey for protoplanetary disks has been made for 19 protostellar IRAS sources in Taurus; 13 of them were optically invisible protostars and 6 were young T Tauri stars. We observed 98-GHz continuum and CS(J = 2 – 1) line emissions simultaneously with spatial resolutions of 2 _. 8-8 _. 8 (360-1,200 AU). The continuum emission was detected from 5 out of 6 T Tauri stars and 2 out of 13 protostar candidates: the emission was not spatially resolved and was consistent with being originated from compact circumstellar disks. Extended CS emission was detected around 2 T Tauri stars and 11 protostar candidates. There is a remarkable tendency for the detectability of the 98-GHz continuum emission to be small for protostar candidates. This tendency is explained if the mass of protoplanetary disks around protostars is not as large as that around T Tauri stars; the disk mass may increase with the increase of central stellar mass by dynamical accretion in the course of evolution from protostars to T Tauri stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.