共查询到20条相似文献，搜索用时 31 毫秒
1.
The coronal and chromospheric emission of several hundred late-type stars whose activity was recently detected are analyzed. This confirms the previous conclusion for stars of HK project that there exist three groups of objects: active red M dwarfs, G-K stars with cyclic activity, and stars exhibiting high but irregular activity. The X-ray fluxes, EUV-spectra, and X-ray cycles can be used to study the main property of stellar coronas—the gradual increase in the number of high-temperature (T ≥ 10 MK) regions in the transition from the Sun to cyclically active K dwarfs and more rapidly rotating F and G stars with irregular activity. The level of X-ray emission is closely related to the spottedness of the stellar surface. The correlation between the chromospheric and coronal emission is weak when the cycles are well-defined, but becomes strong when the activity is less regular. Unexpectedly, stars whose chromospheric activity is even lower than that of the Sun are fairly numerous. Common and particular features of solar activity among the activity of other cyclically active stars are discussed. Our analysis suggests a new view of the problem of heating stellar coronas: the coronas of stars with pronounced cycles are probably heated by quasistationary processes in loops, while prolonged nonstationary coronal events are responsible for heating the coronas of F and G stars with high but irregular activity. 相似文献
2.
N. I. Bondar’ 《Astronomy Reports》2002,46(6):489-496
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in the solar neighborhood due to variations in the spottedness of their surfaces is analyzed based on the behavior of their mean annual brightnesses over long time intervals. The amplitudes and characteristic time scales of the variations of the mean annual brightness are taken to be indicators of photospheric activity and were used to infer the levels of photospheric activity in the stars studied. The influence of axial rotation on the development of cyclic activity in young red dwarfs and F-M main-sequence stars is analyzed. The durations and amplitudes of the photospheric variability of rapidly rotating (dK0e-dK5e) stars testifies to a higher level of photospheric activity among red dwarfs and solar-type stars. The X-ray luminosities of these stars grow with the amplitude of the variations of the mean annual brightness. However, this is not typical of rapidly rotating M dwarfs, for which the X-ray emission varies by more than two orders of magnitude, although their degrees of spottedness are all virtually the same. A linear relationship between the X-ray and bolometric luminosities is observed for young (dKe-dMe) stars, with their ratios log(L x/L bol) being about ?3. These properties can be used to determine whether a red dwarf is a young star or is already on the main sequence. 相似文献
3.
The results of a spottedness study for twelve red dwarf stars covering several decades and based on a vast amount of photometric observations are presented. The analysis makes use of multicolor (UBV RI) photometric monitoring of ten of these stars since 1991 at the Crimean Astrophysical Observatory, as well as data from the literature. The spottedness parameters for selected active BY Dra red dwarfs have been refined using an improved zonal model for the spotted stellar atmospheres to allow for the possible presence of two active longitudes on the stars. Time variations in the spot activity of these systems are analyzed in order to look for possible cycles. Three of the stars show a drift of their spots in the latitude towards the stellar poles; however, the magnitude of this latitude drift is a factor of two to three lower than the analogous value for sunspots. All the stars except for YZ CMi display relationships between the area of the spots and their latitude, with correlation coefficients R from 0.67 to 0.97. Evidence for the presence of activity cycles with durations from 25 to 40 years is found for six stars, which are characterized by synchronous variations in the areas and latitudes of their spots, as well as of the overall photometric brightness. 相似文献
4.
Model atmospheres are fitted to spectroscopic data in order to analyze the elemental abundances in the atmospheres of three red giants in the Hyades cluster. The three stars have almost identical chemical compositions, with iron-group elements slightly overabundant compared to the solar values—a pattern that is typical of Hyades dwarfs. The overabundances of the light elements Na, Al, and Si are virtually equal to those observed for field giants. No enrichment in rare-earth elements relative to iron was found, in sharp contrast to field giants. It is concluded that these discrepancies are due to the age difference between the two groups of stars, which have resulted in different degrees of convective overshooting. 相似文献
5.
Ya. V. Pavlenko 《Astronomy Reports》2001,45(2):144-156
Mechanisms for the formation of the optical (λλ500–950 nm) spectra of L dwarfs—stars and sub-stellar objects with T eff<2200 K—are discussed. Their spectral energy distributions are determined primarily by the K I and Na I resonance-doublet absorption lines. The equivalent widths of these absorption lines formally computed using the dusty model atmospheres of Tsuji can reach several thousand angstroms. In this case, the extended wings of these lines form a pseudo-continuum for weaker absorption lines and even molecular bands. Mechanisms for the broadening of alkali-element lines in the atmospheres of late-type stars due to interactions between neutral atoms and hydrogen molecules are analyzed. The computed optical spectral energy distributions of several L dwarfs are compared with their observed spectra. 相似文献
6.
An analysis of data on chromospheric activity obtained in the framework of exoplanet-search programs is presented. Observations
of 1334 stars showing that the chromospheric activity of the Sun is clearly higher than for the vast majority of stars in
the solar vicinity are used. A comparison of chromospheric and coronal activity led to the identification of a significant
group of stars with a low level of chromospheric activity, whose coronal radiation spans wide ranges. There are reasons to
believe that the chromospheric and coronal activities of one group of stars decrease simultaneously as the rotation decelerates,
while, in stars of the other group, the chromospheric activity diminishes, but their coronas remain stronger than that of
the Sun. Features of cyclic activity of the Sun are discussed. This enables us to associate differences in the behavior of
the activity with different depths of the convective zones of stars of spectral classes earlier and later than G6. Arguments
in favor of a two-layer dynamo and different roles of the large-scale and small-scale magnetic fields in the formation and
evolution of activity are formulated. Age estimations based on activity levels (gyrochronology) must be carried out differently
for these different groups of stars. 相似文献
7.
I. S. Savanov 《Astronomy Reports》2009,53(10):941-949
We have analyzed light curves from the MOST satellite for the two active dwarfs ɛ Eri and κ Cet. Our maps of the stellar surface-temperature inhomogeneities were obtained with no a priori assumptions about the shape,
configuration, and number of spots. We find variations of the surface-temperature inhomogeneities with time, also on time
scales about equal to their rotation periods. We consider a model of a spotted star with two types of surface inhomogeneities—spots
and related plage fields—and demonstrate that the best agreement between the theoretical and observed light curves is achieved
for small ratios of the plage-field area to the area of cool spots. This conclusion indicates that long-term brightness variations
of stars younger than the Sun are mainly due to variable spots on their surfaces, while the contribution from plage fields
becomes more significant for older stars. 相似文献
8.
I. S. Savanov 《Astronomy Reports》2009,53(10):950-956
We have analyzed light curves from the MOST satellite for the two active dwarfs ? Eri and κ Cet. Our maps of the stellar surface-temperature inhomogeneities were obtained with no a priori assumptions about the shape, configuration, and number of spots. We find variations of the surface-temperature inhomogeneities with time, also on time scales about equal to their rotation periods. We consider a model of a spotted star with two types of surface inhomogeneities—spots and related plage fields—and demonstrate that the best agreement between the theoretical and observed light curves is achieved for small ratios of the plage-field area to the area of cool spots. This conclusion indicates that long-term brightness variations of stars younger than the Sun are mainly due to variable spots on their surfaces, while the contribution from plage fields becomes more significant for older stars. 相似文献
9.
We analyze the Na, Mg, Al, and Si abundances in the atmospheres of more than 40 stars, includingred giants of different spectral subgroups (normal red giants, mild and classical barium stars) and several supergiants. All these elements exhibit abundance excesses, with the overabundance increasing with the star’s luminosity. The dependence of the overabundances for each of these elements on the luminosity (or log g) is the same for all the spectral subgroups, testifying to a common origin: they are all products of hydrogen burning in the NeNa and MgAl cycles that have been dredged up from the stellar interiors to the outer atmospheric layers by convection that gradually develops during the star’s evolution from the main sequence to the red-giant stage. The sodium abundances derived for several stars are lower than for other stars with similar atmospheric parameters. The ages and kinematic characteristics of these two groups of stars suggest that they probably belong to different stellar generations. 相似文献
10.
The origin of solar-type activity for low-mass stars of late spectral types is considered. Spectroscopic data were used to study the dependence of the activity level logR′ HK on the lithium abundance logA(Li) and axial rotation rate. A close correlation between logA(Li) and logR′ HK is found for two groups of G stars, hotter and cooler than the Sun. This relation is most clearly expressed in the case of high activity, and is somewhat more strongly expressed for G6-K3 dwarfs, which includes many BY Dra variables, than for F8-G5 stars. It is confirmed that, for stars with high activity, both the lithium abundance and the activity level are determined by the rotation rate, which depends on the age. The lithium abundance exhibits different dependences on the chromospheric activity, depending on the level of this activity. Cooler stars, with detectable lithium and solar-like chromospheres, possess much stronger coronas. This change in the relationship between the relative luminosities of the chromosphere and corona can be reliably traced using larger datasets. The different ratios between the activity of the choromosphere and corona for cooler and hotter G stars may reflect the fact that their convective zones become deeper or shallower than some critical value. This is consistent with observations of parameters describing rotational modulation and the correlation and anti-correlation of chromospheric and photospheric activity indices for stars hotter and cooler than the Sun. Physically, this means that the character of the activity could be related to a changing contribution of the large-scale and local magnetic fields to the generation of the activity. The results of this study confirm the earlier idea that there may be different evolution paths associated with solar-type activity. The results can be used to refine methods for estimating ages of stars from their activity levels (gyrochronology). 相似文献
11.
Information on the latitude distribution of starspots and changes in this distribution from year to year is very important for our understanding of the nature of stellar activity and for developing dynamo theory. The concept of butterfly diagrams is introduced for highly spotted stars of late spectral types, by analogy to the Maunder diagrams for the Sun. Our approach is based on the zonal spottedness models constructed by Alekseev and Gershberg. A detailed analysis is given for the single active star LQ Hya, and a comparison is made to similar analyses for several stars with two well-separated spot belts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundary of the butterfly diagram drifts toward the equator during the activity-rise phase, i.e., during years when the relative spotted area increases. This effect is clearly expressed for LQ Hya and other stars whose orientation enables observation of both hemispheres and virtually vanishes for V833 Tau, which is viewed nearly pole-on. The upper boundary of the diagram is virtually unchanged for all the considered spotted stars except V775 Her, for which it moves toward the pole. The drift rate of the lower boundary is ?1 to ?2 deg/yr, a factor of two to three smaller in magnitude than the corresponding solar value. Our analysis provides an independent confirmation of the occurrence of high-latitude spots on stars that are younger than the Sun and whose activity is high but less regular than the solar activity; it also enables the identification of the starting times of stellar cycles. 相似文献
12.
We analyze the X-ray emission and chromospheric activity of late-type F, G, and K stars studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity L X/L bol. This indicates that the nature of processes associated with magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α-Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations found between various activity parameters and the Rossby number are consistent with our conclusions. Our analysis makes it possible to draw the first firm conclusions about the place of solar activity among analogous processes developing in active late-type stars. 相似文献
13.
Observations of the K2 continuation of Kepler Space Telescope program are used to estimate the spot coverage S (the fractional spotted area on the surface of an active star) for stars of the Pleiades cluster. The analysis is based on data on photometric variations of 759 confirmed clustermembers, together with their atmospheric parameters, masses, and rotation periods. The relationship between the activity (S) of these Pleiades stars and their effective temperatures shows considerable change in S for stars with temperatures T eff less than 6100 K (this can be considered the limiting value for which spot formation activity begins) and a monotonic increase in S for cooler objects (a change in the slope for stars with Teff ~ 3700 K). The scatter in this parameter ΔS about its mean dependence on the (V ?Ks)0 color index remains approximately the same over the entire (V?K s )0 range, including cool, fully convective dwarfs. The computated S values do not indicate differences between slowly rotating and rapidly rotating stars with color indices 1.1 < (V?K s )0 < 3.7. The main results of this study include measurements of the activity of a large number of stars having the same age (759 members of the Pleiades cluster), resulting in the first determination of the relationship between the spot-forming activity and masses of stars. For 27 stars with masses differing from the solarmass by nomore than 0.1M⊙, themean spot coverage is S = 0.031±0.003, suggesting that the activity of candidate young Suns is more pronounced than that of the present-day Sun. These stars rotate considerably faster than the Sun, with an average rotation period of 4.3d. The results of this study of cool, low-mass dwarfs of the Pleiades cluster are compared to results from an earlier study of 1570 M stars. 相似文献
14.
Late-type stars with chromospheric and coronal activities exceeding those of the Sun and other stars with well-defined cycles are considered. These rotate more rapidly than stars with well established cycles; for single stars, this appears to be due to their younger ages. The spots on such stars cover several per cent of the total area, which is an order of magnitude higher than for the Sun at its activity maximum. Our wavelet analysis of the chromospheric-emission variability, which has been observed since 1965 in the framework of the HK project, indicates that the period of the axial rotation of some of these starts varies from year to year. This is most pronounced in two “Good” stars according to the classification of Baliunas et al., HD 149661 and HD 115404, and also in a star with a more complex variability, HD 101501. No similar effect is exhibited by the “Excellent” cyclic-activity stars. Such variations in the period can be observed during epochs of appreciable rotational modulations of the chromospheric-emission fluxes, most likely, immediately after the maximum of a long-period wave (cycle?). This seems to provide evidence for the existence of huge activity complexes in the chromospheres of these stars, whose longitudes remain virtually constant over several years; they drift from fairly high latitudes to the equator at speeds close to the value typical of sunspots. The observed period variations are most likely due to differential rotation of the same sign that is known for the Sun. Our results provide independent confirmation of similar conclusions obtained by us previously using zonal models for highly spotted stars. Other activity features of a selected star group and the implications of the results for the theory of stellar and solar dynamos are discussed. 相似文献
15.
The results of spot-coverage modeling for 13 active G–K dwarf stars based on many-year photometric observations are presented. The results of UBVRI observations of eight stars performed at the Crimean Astrophysical Observatory were used together with data from the literature in this analysis. The spot-coverage parameters for 13 selected BY Dra active red dwarfs have been redetermined to improve the zonal spot-coverage model for the stellar photospheres, which currently allows for the presence of two active longitudes. Time variations of the spot-activity characteristics of these systems were analyzed with the aim of searching for possible cyclic variations. All the stars, with the exception of OU Gem and BE Cet, show fairly strong correlations between variations in the spot latitudes and spot areas, with absolute values of the correlation coefficients, R(〈?〉, S), ranging from 0.38 to 0.92. For five stars, an anti-correlation between the mean latitude and area of the spots was found (R(〈?〉, S) from–0.24 to–0.73). This behavior may reflect the drift of spots toward the equator in the course of their development. Eight stars feature positive correlations, i.e. the spots drift towards the pole as their areas increase. Nine stars demonstrate activity cycles, which are reflected in photometric variations as well as variations of the spot areas and mean latitudes. The periods of the latitude drift of the spots are found for five stars; the magnitudes of the spot-latitude drift rates are lower than the corresponding value for sunspots by a factor of 1.5–3. 相似文献
16.
N. V. Raguzova 《Astronomy Reports》2003,47(5):401-412
Usingthe “Scenario Machine” (a specialized numerical code formodeling the evolution of large ensembles of binary systems), we have studied the physical properties of rapidly rotating main-sequence binary stars (Be stars) with white-dwarf companions and their abundance in the Galaxy. The calculations are the first to take into account the cooling of the compact object and the effect of synchronization of the rotation on the evolution of Be stars in close binaries. The synchronization time scale can be shorter than the main-sequence lifetime of a Be star formed during the first mass transfer. This strongly influences the distribution of orbital periods for binary Be stars. In particular, it can explain the observed deficit of short-period Be binaries. According to our computations, the number of binary systems in the Galaxy containing a Be star and white dwarf is large: 70–80% of all Be stars in binaries should have degenerate dwarf companions. Based on our calculations, we conclude that the compact components in these systems have high surface temperatures. Despite their high surface temperatures, the detection of white dwarfs in such systems is hampered by the fact that the entire orbit of the white dwarf is embedded in the dense circumstellar envelope of the primary, and all the extreme-UV and soft X-ray emission of the compact object is absorbed by the Be star’s envelope. It may be possible to detect the white dwarfs via observations of helium emission lines of Be stars of not very early spectral types. The ultraviolet continuum energies of these stars are not sufficient to produce helium line emission. We also discuss numerical results for Be stars with other evolved companions, such as helium stars and neutron stars, and suggest an explanation for the absence of Be-black-hole binaries. 相似文献
17.
We have studied the effect of external radiation on the formation of LTE and non-LTECaII lines in the spectra of A-M-star atmospheres. Three frequency distributions were chosen for the external radiation: X-ray radiation specified by the power law \(I_v^ + = I_{0^v } ^{ - 0.6} \) at 1–16.5 keV and UV radiation specified by blackbody distributions with the temperatures Trad=50000 and 15000 K. We analyze the influence of variations in the irradiating flux and its angle of incidence on the profiles and equivalent widths of the λλ3933, 3968 Å resonance doublet and the λλ8498, 8542, 8662 Å infrared triplet. For any type of external radiation, allowing for deviations from LTE decreases the reflection effects for the CaII lines. We conclude that the CaII profiles do not display emission components in the spectra of optically thick stellar atmospheres irradiated by X-rays. Therefore, CaII emission lines observed in the radiation of cataclysmic variables must be formed in an optically thin plasma. CaII emission lines are likely to form in the spectra of stars with UV irradiation if CaII is the dominant ionization state in atmospheric layers close to the depths at which the continuum is formed. As a result, the spectra of symbiotic variables with hot primaries can contain CaII lines originating on the surfaces of the M-giants and supergiant secondaries due to reflection effects. These lines can be used to analyze the reflection effects and the temperature structure in the atmospheres of the secondaries only if non-LTE effects are included. In the spectra of close binaries with cool white dwarfs, CaII emission lines originate in the irradiated atmospheres of the secondaries under conditions close to thermalization. These lines can be used to study the reflection effects and calcium abundances even in an LTE approximation. We calculated the profiles and equivalent widths of CaII lines in the spectra of the four precataclysmic variables BE UMa, EG UMa, MS Peg, and HR Cam. The observed and theoretical reflection effects in the λλ3933, 8542 Å emission lines for the specified parameters of the systems and a solar calcium abundance in the atmospheres of the red dwarfs are in good agreement. 相似文献
18.
A new mechanism for the generation of X-ray emission in binary Be/X stars is proposed. It is shown that the mass-transfer rate through the point L1 in a model in which the optical component of a Be/X star has an expanding envelope is sufficient to generate the observed X-ray luminosities of such stars. The results of computations indicate a dependence between the orbital periods and X-ray fluxes of these systems. The relationship between the orbital perod and the mass-transfer rate during flares obtained from modeling corresponds to the observed dependence of the maximum X-ray flux on the orbital period. 相似文献
19.
M. M. Katsova L. L. Kitchatinov D. Moss K. Oláh D. D. Sokoloff 《Astronomy Reports》2018,62(8):513-519
The Kepler mission has identified huge flares on various stars, including some solar-type stars. These events are substantially more energetic than solar flares, and are referred to as superflares. Even a low probability of such a superflare occurring on the Sun would be a menace to modern society. A flare comparable in energy to that of superflares was observed on September 24 and 25, 1989 on the binary HK Lac. Unlike the Kepler stars, observations of differential rotation are available for HK Lac. This differential rotation appears to be anti-solar. In the case of anti-solar differential rotation, dynamo models can producemagnetic-activity waves with dipolare symmetry, as well as quasi-stationary magnetic configurations with quadrupolar symmetry. The magnetic energy of such stationary configurations is usually about two orders of magnitude higher than the energy associated with activity waves. We believe that this mechanism could provide sufficient energy to produce superflares on late-type stars. Some simple models in support of this idea are presented. 相似文献
20.
Ya. V. Pavlenko 《Astronomy Reports》2002,46(7):567-578
We discuss techniques and results of computations of the infrared spectra of late-type dwarfs. Our computations of the synthetic spectra and spectral energy distributions in the infrared (λλ 1–10 µm) were carried out assuming LTE, using the grids of M-and L-dwarf model atmospheres of Allard and Hauschildt (1995) and Tsuji (1998), taking into account the opacities due to H2O and HDO absorption bands. We discuss the use of HDO bands formed in the infrared spectra of cool dwarfs to realize the “deuterium test” recently proposed for the identi fication of substellar-mass objects and large planets and to refine scenarios for the evolution of young stars and substellar objects. 相似文献