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.
The mean density of the UV Cet-type flare stars in the solar neighbourhood is estimated. If differences of activity levels on different flare stars are taken into account, their summary flare activity is equivalent to 0.03 YZ CMi's flare activity per cubic parsec or to 4×1026 erg s–1 pc–3 in U-passband. From the X-ray flare observation on YZ CMi of 19.10.74 we estimate the luminosity of stellar flares in soft and intermediate X-ray. The ratio of X-ray to optical radiation for stellar flares is close to the respective ratio for strong solar chromospheric flares. It is shown the set of red-dwarf flare stars has all essential features of an ensemble of discrete X-ray sources to represent the galactic diffuse X-ray background.  相似文献   

2.
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.  相似文献   

3.
It is shown that the far-ultraviolet radiation (shorter than 2000 Å) discovered by ANS observations in the few T Tauri-type stars does not have any relation to the two-photon emission of hydrogen, as suggested by some authors. This is obtained from the observational data of the numerical values of the ratioQ *(2q)/Q *() for these stars, whereQ *(2q) is the complete number of the observed 2q-photons andQ *() is the number of observedH-photons. The observational values ofQ *(2q)/Q *() for four T Tauri-type stars turned out to be in the region of 20–90, while the theoretical value of this relation is 6. Hence, the observed fluxes in the region <2000 Å are 3–15 times larger than the theoretically expected values.The emission discovered in the region <2000 Å is of non-thermal origin, and can be identified with high probability with thetransition radiation. The latter originates as a result of the electromagnetic interaction of so-called fast electrons (E1.5 MeV) with dust particles in the gas-dust clouds surrounding these stars. The theoretical spectral curves of the transition radiation, for a few values of the plasma frequency 0 for the dust particles, are calculated taking into account also the self-absorption effect of the radiation in the cloud and the absorption in the interstellar medium. Qualitatively, these curves (Figures 2, 3 and 4) are in good accord with the observed spectral distribution curves for the T Tauri-type stars (Figure 1). In particular, in both cases a minimum of radiation flux occurs near to 2200 Å, and a maximum near 1800 Å.The starting point of our analysis has been the concept of the identity of the processes, non-thermal and non-stationary in character, taking place at the time of the flare phenomenon of UV Cet-type stars in one case, and at the generation of continuous emission and the excitation of the emission lines in T Tauri-type stars on the other. In the latter case, the T Tauri-type stars can be regarded aspermanently flaring stars, with a very high frequency of flare events.  相似文献   

4.
We present the results of our photometric and spectroscopic studies of the new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum exhibits double-peaked hydrogen and helium emission lines. The Doppler maps constructed from hydrogen lines show a nonuniform distribution of emission in the disk similar to that observed in IP Peg. This suggests that the object can be a cataclysmic variable with tidal density waves in the disk. We have determined the component masses (M WD = 0.8 ± 0.22M ⊙ and M RD = 0.14 ± 0.02M ⊙) and the binary inclination (i = 78° ± 1.5°) based on well-known relations between parameters for cataclysmic variable stars. We have modeled the binary light curves and showed that the model of a disk with two spots is capable of explaining the main observed features of the light curves.  相似文献   

5.
The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called slow flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.  相似文献   

6.
A continued search for emission objects in three new fields within the association Cyg OB7 yielded eight new emission stars in one of these fields. No emission stars were detected in the other two fields, apparently because we have come to the boundary of the T association on this side. Two UV Cet-type flare stars have been detected in this region for the first time.  相似文献   

7.
We have obtained IUE ultraviolet spectra of the low-excitation planetary nebula CN 3-1. The recordings show the well-known doublet 2800 MgII and resonance line 2852 MgI as strong absorption lines. We show that these lines cannot be of interstellar origin and that they may be formed in two envelopes surrounding the main nebula: in the transition zone (doublet 2800 MgII) and in the neutral envelope (line 2852 MgI). These envelopes possess an important property: they contain dust particles, and even a moderate amount of such particles may influence the strength of the absorption lines of MgII and MgI.The emission line 4686 HeII has no relation to the nebula CN 3-1 and belongs to its nucleus, a star of type WR. It is very probable that the nucleus of this nebula is a binary system with a WR component (T *=50 000 K), exciting the helium lines, and a star of B0 or B2 type (T *=26 000 K) exciting the nebular linesN 1+N 2 [OIII], 3727 [OII], hydrogen lines, etc.  相似文献   

8.
Multi-channel CCD receivers are available for solar spectral research in the Astronomical Observatory of Shevchenko University of Kiev. The observational material considered here are 24 echelle spectrograms of the flare of July 15, 1981 and certain other disk and limb flares. Some temporal aspects of flare development are discussed, as well as properties of metal lines and the structure of the flare region. Disk and limb flare photometry indicate the full spatial coincidence of the emission volumes for all emission lines. No signs of altitude stratification for the lines of H and K Call, hydrogen, and other metals are detected. Metal flare emission is revealed in the cores of Fraunhofer lines, providing that a compact emission source is present in the active region and that broad wings of H and K CaII and hydrogen have formed. No broad wings are observed for metal lines. Two types of lower-intensity strips in the photospheric emission spectrum are detected; the first is the result of the lower temperature in sunspots, while the second is due to the weakening of photospheric radiation after it passes through the flare volume. No new spectral lines are observed in the spectrum of this second type of strip, which may be associated with a condensed layer of relatively cold plasma at the chromospheric level. Four components are always spatially coincident in the spectrum: (1) a compact source of higher intensity with broadened hydrogen and H and K Call line wings; (2) higher intensity in metal lines, but without broadened wings; (3) emission or absorption in helium lines; (4) a lower-intensity strip of photospheric emission due to its weakening after it passes through the flare volume.  相似文献   

9.
The list of the cooperative photometric observations of the UV Cet-type flare stars that have been organized during the years 1967 to 1971 by the Working Group on Flare Stars of the IAU Commission 27 is given. The completeness and reliability of the data obtained are evaluated by comparing simultaneous observations at different observatories. the statistical analysis of the UV Cet, YZ CMi, EV Lac and AD Leo flares observed in the B-band is carried out. The flare energy spectrum in the energy range where observational selection effects are small is found to be d lnv/d lnE B=–1.4 to –1.9,v is an occurrence of flares with radiation energy ofE B. The total flares' radiation is equal to 1.7%, 1.2%, 0.3% and 0.4% of the quiet radiation in the B-band of the stars listed, and the main part of this total radiation is due to the strongest flares. Distributions of flare rise times (t r) and of rates of flare absolute luminosity increase (d2 E B/dt r 2Ë r) are considered; these parameters of flare are independent statistically for all stars studied. Correlation coefficientsr (E B,t r) andr (E B,r(E B,Ë r)) are rather small except r (E B,t r)=0.86 for the AD Leo flares. Contradictory conclusions on temporal distribution of flares infered by different investigators are noted.  相似文献   

10.
We present the first infrared light curves of the binary V1430 Aql, in the bands J and K, plus V, R and I light curves and spectra covering the ranges of Hβ, Hα and Ca II-IRT lines. Our VRIJK data, together with published radial velocity curves, are analyzed to determine the orbital and stellar parameters of the system. Both stellar components present spectroscopic evidence of chromospheric activity, with emission excesses in the Hβ, Hα and Ca II-IRT lines. The measured ratio of the lines Hβ/Hα emission excesses can be interpreted as originated in plages. Our light curves also show photometric evidence of cool spots at least on one of the stars.  相似文献   

11.
A quantitative comparison of energetics of different manifestations of the activity of the UV Cet-type flare stars — sporadic flares, quiet chromospheres and coronae and stellar spots — have been carried out. On the basis of a statistical analysis of about 1800 flares registered for 23 flare stars, the energy spectra of flares have been constructed and time-averaged powers of optical radiation of flares —L flares — have been estimated. By use of spectroscopic observations of 26 flare stars between flares, the total radiation in Balmer lines of quiet chromospheres —L Bal — have been found. On the basis of these data and the soft X-ray observations of 29 flare stars, the mean of the ratiosL flares/L bol,L Bal/L bol andL X /L bol have been found to be equal to (1–2)×10?4. By use of photometric observations for 9 stars, the total starspot radiation deficits —L spots — have been estimated as (4×10?3?6×10?2)×L bol. Discussions of probable underestimates in the valuesL flares,L Bal,L X , andL spots leads to the conclusion that the total power of non-equilibrium processes that take place in flare star atmospheres is close to the total starspot radiation deficit in these stars.  相似文献   

12.
We have investigated the effect of ionizing radiation from the UV stars (hot prewhite dwarfs) on the intergalactic medium (IGM). If the UV stars are powered only by gravitational contraction they radiate most of their energy at a typical surface temperature of 1.5×105 K which produces a very highly ionized IGM in which the elements carbon, nitrogen and oxygen are left with only one or two electrons. This results in these elements being very inefficient coolants. The gas is cooled principally by free-free emission and the collisional ionization of hydrogen and helium. For a typical UV star temperature ofT=1.5×105 K, the temperature of the ionized gas in the IGM isT g =1.2×105 K for a Hubble constantH o=75 km s–1 Mpc–1 and a hydrogen densityn H =10–6 cm–3. Heating by cosmic rays and X-rays is insignificant in the IGM except perhaps inHi clouds because when a hydrogen atom recombines in the IGM it is far more likely to be re-ionized by a UV-star photon than by of the other two types of particles due to the greater space density of UV-star photons and their appreciably larger ionization cross-sections. If the UV stars radiate a substantial fraction of their energy in a helium-burning stage in which they have surface temperatures of about 5×104 K, the temperature of the IGM could be lowered to about 5×104 K.  相似文献   

13.
An extensive analysis is made of the theory of flare stars based on the fast electron hypothesis, in the light of the latest observational evidence. It is shown that an adequate agreement of theory with the observations obtains regarding the internal regular features in the flare amplitude data inUBV rays, as well as the changes of the colour characteristics of stars during the flares; in the latter case the analysis is made not only in respect of the UV Cet-type stars, but flare stars as well, forming a part of the Orion association. Problems bearing on the negative flare and the screening effect are dealt with. New properties of the light curves of flares are revealed, based on the above theory.Particular emphasis is laid on the X-ray radiation from flare stars. It is shown that the observed spectrum of X-ray radiation of flare stars differs sharply from that of X-ray radiation both of the stellar corona and solar X-ray flares. At the same time, the observed X-ray spectrum of flares is in complete harmony with the previously calculated theoretical spectrum corresponding to nonthermal bremsstrahlung with the energy of monoenergetic fast electrons 1.5 MeV. The durations of X-ray flares should be essentially shorter than that of the optical flares. The very high momentary intensities of the X-ray brightness with the exceedingly small duration at the curve maximum is predicted. It is shown that the gamma-ray bursts recorded so far have no relation whatever to flare stars.  相似文献   

14.
Big data obtained from a stellar spectroscopic survey carried out using the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST, also known as Guo Shou Jing telescope) provide important information for studying chromospheric activity, variability of chromospheric activity, and chromospheric statistical properties, and for searching subdwarfs and extreme subdwarfs. Using as chromospheric activity indicator the Hα line, we detected 6391 active M stars among the 99741 stars in the M-star catalogue of the LAMOST survey. We also obtained the relationship between the fraction of active stars and the spectral types, which is consistent to previous results. We also studied the effects of activity on broadband photometric colors, and we did not see significant differences between active and inactive M stars. Using as spectroscopic molecular indicators the CaH123 and TiO5 lines, we found 1288 subdwarfs (including 120 active subdwarfs). We also found 15 extreme subdwarf (2 active extreme subdwarf) candidates. Our subdwarf candidates are slightly redder by about 0.05 mag in g-r compared with dwarfs using the g-r and r-i, and g-r and i-z color diagrams. Using our active M-star catalogue, we found that 898 stars exhibited Hα emission in at least two exposures (170 of them in at least three exposures). Among these 170 stars, 163 of them show variability in Hα emission on long timescales (more than 2.5 h). Furthermore, 34 stars show variability over short timescales (less than 2.5 h), and 29 actives show variability over both short and long time scales.  相似文献   

15.
We have built different models for stars of the same spectral type than the Sun but with different levels of chromospheric activity, to study the response of the S index of activity built from the emission of the Ca II H and K lines to changes in the chromospheric structure. We found that the fact that there are many stars with either strong or weak emission, but few with intermediate values of S, the so-called Vaughan–Preston gap, can be due to a discontinuity in the response of the Ca II lines to chromospheric heating. In fact, we are able to reproduce the observed distribution of the number of stars as a function of S.  相似文献   

16.
We present six \(\mbox{V}R_{c}I_{c}\) light curves of UX CrB from observations carried out from 2011–2015. We also obtained three optical spectra using the 2.16-m optical telescope and LAMOST survey at the national astronomical observatories. We classified its spectral type as \(\mbox{G2}\pm\mbox{2V}\). We noticed that there are strong absorptions in the Hα, Hβ, Ca ii H&K and infrared triplet lines in the observed spectra. By subtracting away the photospheric contribution, we also noticed that there are small excess emissions in these chromospheric active lines, which indicate there are weak chromospheric activities. We tried and obtained four photometric solutions with different spot positions from our full and high time-resolution light curves in 2012, using the updated Wilson-Devinney code. The model with two spots on the primary produced the best result for explaining the observed light curves from 2012. Moreover, we explained all other light curves based on our photometric solution using our 2012 light curves. There are two active longitudes at about 68° and 255°. We noticed that the starspots have both long-time (years) and short-time (about two months) variation by analyzing the light curves and its starspot parameters. There is also an obvious oscillation of light curve maximum between 0.25 and 0.75 phases by analyzing the values of \(\mbox{Max.~I} - \mbox{Max.~II}\). We conclude UX CrB are the evolved main-sequence stars with strong photometric and chromospheric activities.  相似文献   

17.
Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ5000 = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n e, N 2, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.  相似文献   

18.
The weakening of EUV line emission due to continuum absorptions of neutral hydrogen and neutral helium is investigated to examine its wavelength-dependence. After convolving the intensities predicted from multilevel calculations over an instrumental profile, we found a systematic, linear weakening for lines shortward of 912 Å, which can be attributed to Lyman continuum absorption in the cool chromospheric cloud. The degree of the weakening at the quiet Sun seems to be constant in the temperature range of 4.3 < log T < 5.4. We also find that the lines shortward of 504 Å are somewhat weakened by He i continuum absorption. From the comparison of both weakenings the temperature of the absorber is estimated to be rather low (T e 7 × 103 K).  相似文献   

19.
We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H– population, and absorption of photospheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H– continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams.  相似文献   

20.
V. A. Krat 《Solar physics》1967,1(2):191-203
A new model is proposed for the solar chromosphere, which is assumed to be an instable inhomogeneous formation, consisting of numerous elements (filaments), each with different temperature and density. Fluctuations of the magnetic field may give rise to chromospheric turbulence and may also cause the chromospheric inhomogeneities.The chromosphere is suggested to consist of four discrete groups of filaments: (1) metallic filaments where the conditions for the emission of lines of neutral metals are optimal, (2) hydrogenic elements, with optimum conditions for the emission of the Balmer series of hydrogen, (3) helium filaments, with optimum conditions for the appearence of the neutral helium lines, (4) the subcoronal filaments, representing a transition from chromospheric to coronal formations.The metallic filaments may be further subdivided, first into filaments where the emission arises from scattering of photospheric radiation - these emit lines of neutral metals and of some metallic ions (CaII, SrII, and others), and further into filaments where the emission is farther from LTE conditions; the latter filaments are characterized by a somewhat higher electron temperature and by an electron density at least exceeding that of the other elements by an order of 10. Computations of the optimum conditions for the emission of the neutral helium lines were made with the aid of new tables of Sobolev. The helium filaments in the low chromosphere have lower temperatures and are denser than those in the upper chromosphere; for a part they may also be considered as hydrogen filaments. The derivation of the physical parameters of the subcoronal filaments was based on data on the Heii4686 chromospheric line emission and also on rocket observations of the ultraviolet solar spectrum. In order to evaluate the relative distribution of the various filaments between heights of 0 and 5000 km, data on the radio emission of the Sun at 8 mm are also used. Characteristics of the proposed model of the chromosphere are given in Table III and Figure 1.  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