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1.
D. J. Mullan 《Solar physics》1977,54(1):183-206
Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere. 相似文献
2.
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
3.
4.
P. B. Byrne 《Solar physics》1989,121(1-2):61-74
We present observational data on stellar flares from a range of wavelength regimes, many of which were obtained simultaneously. Physical parameters of these flares are derived and discussed in the frame-work of the general solar flare model. It is found that flares on dMe stars are solar-like, except in mean energy. The parameters of flares on RS CVn stars are more extreme, however, and may require new models for their interpretation. 相似文献
5.
The current Cherenkov telescopes together with GLAST are opening up a new window into the physics at work close to black holes and rapidly rotating neutron stars with great breakthrough potential. Very high energy gamma-ray emission up to 10 TeV is now established in several binaries. The radiative output of gamma-ray binaries is in fact dominated by emission above 1–10 MeV. Most are likely powered by the rotational spindown of a young neutron star that generates a highly relativistic wind. The interaction of this pulsar wind with the companion’s stellar wind is responsible for the high energy gamma-ray emission. There are hints that microquasars, accretion-powered binaries emitting relativistic jets, also emit gamma-ray flares that may be linked to the accretion–ejection process. Studying high energy gamma-ray emission from binaries offers good prospects for the study of pulsar winds physics and may bring new insights into the link between accretion and ejection close to black holes. 相似文献
6.
We have determined the dominant global modes of stellar discs by integrating the linearized collisionless Boltzmann equation. The models examined are the Kuzmin discs with two types of the distribution function. It is found that the growth rate correlates well with the central value ofQ.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
7.
S. O. Ifedili 《Solar physics》1974,39(1):233-241
The upper limit on the solar neutron flux from 1–20 MeV has been measured, by a neutron detector on the OGO-6 satellite, to be less than 5 × 10–2 n cm–2 s–1 at the 95% confidence level for several flares including two flares of importance 3B and a solar proton event of importance 3B. The measurements are consistent with the models proposed by Lingenfelter (1969) and by Lingenfelter and Ramaty (1967) for solar neutron production during solar flares. The implied upper limit on the flux of 2.2 MeV solar gamma rays is about the same as the 2.2 MeV flux observed by Chupp et al. (1973). 相似文献
8.
The distribution of monthly counts of grouped solar flares N
f
has been studied for the time period 1967–1985 and they have been compared to other solar activity index R
z
, F
2800, and F
3750 i.e. intensities of solar radio flux at 2800 MH
z
and 3750 MH
z
. Seasonal variations have been found in the monthly distribution of solar flares.We have also studied the variation of the correlation coefficient for every year between N
F
and R
z
for the time period 1967–1985. The distribution of monthly counts of grouped solar flares N
f
has also been compared to the number at high velocity solar-wind streamers for the same period. 相似文献
9.
{We investigate the conversion of the 0.5–4 and 1–8 Å soft X-ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr and Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun’s X-ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free–bound emission processes, not free–free emission, and thus is abundance-dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (~25%) lower temperatures being derived. 相似文献
10.
Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan
Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known
to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar
tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of
the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum
for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in
the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number
of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently
must be flare stars.
Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999. 相似文献
11.
The mean durations of spotless H flares, SFs were calculated as a function of their importance. Totally 3569 SFs chosen from theQuarterly Bulletin (QB) andSolar Geophysical Data (SGD) were used in the present analysis for the period 1947–1990. The detailed analysis of average life and rise times of SFs is discussed. 相似文献
12.
Rafael Cubarsí 《Astrophysics and Space Science》1992,191(2):213-221
The superposition of two stellar velocity distribution functions of Gaussian type is applied in order to study the velocity distribution of a stellar sample in the galactic plane. Assuming that the Galaxy is symmetric respect to this plane, the relationships to be fulfilled by the central velocity moments of the total stellar sample are obtained. If the total velocity distribution is consistent with this type of superposition, the partial moments of the subsamples, which are associated with the Gaussian components, can be calculated and, also, the population percentage. Our results show that first component corresponds to young disk population and the second one to intermediate disk population.Paper presented at the 12th European Astronomical Meeting of theIAU on European Astronomers Look to the Future, held 8–11 October, 1990, Davos, Switzerland. 相似文献
13.
K. Petrovay 《Astrophysics and Space Science》1987,138(2):323-332
Some analytical relations for the phase space functions of a self-consistent spherical stellar system are derived. The integral constraints on the distribution function by imposing a given (r) density distribution andN(E) fractional energy distribution are determined. For the case of radially-anisotropic velocity distribution in theE0 limit the constraint by an exponentialN(E) implies thatf(E, J
2) tends to zero in the order (–E)3/2. This lends analytical support to the use of the Stiavelli and Bertin (1985) distribution function for modeling elliptical galaxies. Maximum phase space density constraint confirms the necessity of high collapse factors to produce such a distribution function. Limits on the steepness of an exponentialN(E) for the case when (r) resembles the emissivity law of ellipticals are also derived. 相似文献
14.
Ramon J. Quiroga 《Astrophysics and Space Science》1977,50(2):281-310
The spiral structure of the inner parts of the Galaxy is studied using 21 cm line data and stellar data. To study the neutral hydrogen distribution in the galactic layer a parameter =(dV/dr) proportional to the mean densities is calculated using a first approximation for the velocity gradients due to differential rotation.The obtained distribution (R, Z) shows spiral features completely consistent with the early star distribution and with the Hii regions. The corrugation effect of the galactic layer is observed in all the studied zones in neutral hydrogen and in the distribution of the OB stars in the Carina zone.The pattern obtained indicates four spiral arms for the inner parts of the Galaxy, three of which are identified also in the stellar data (arms -I, -II, and -III) and the more distant -IV in Hii regions.The local arm according to the stellar data of Kilkennyet al. forms a feature completely similar to the arms -I and -II and there are no indications that this arm is a special material branch between two main spiral arms as has been supposed in order to conciliate the neutral hydrogen pattern with the stellar distribution.The pitch angles for the spiral arms are approximately 13°–17°.The observed wave form distribution of the hydrogen cloud layer is completely consistent with the theoretical predictions of Nelson (1976) but there are no indications of such an effect in the intercloud hydrogen. The corrugated cloud layer has a width of 100 pc, a wave amplitude of 70 pc, and a wavelength which grows with the galactic center distance (approx. 2 kpc in the zones next to the galactic nucleus and 2.6–3.0 kpc in the zones next to the Sun). To each wavelength correspond two spiral arms. The spiral features in our Galaxy show characteristics quite similar to the features in the Andromeda nebula, not only in the component materials (neutral hydrogen, Hii regions and possibly also dust and stars) but also in their kinematics. 相似文献
15.
A. A. Akopyan 《Astrophysics》1994,37(1):51-59
A scenario is suggested for the development of astrophysical objects. The equations derived are applied to the protostar—T Tauri star—flare star evolutionary sequence. The ages of the Orion, NGC 2264, and NGC 7000 stellar associations are estimated.Translated from Astrofizika, Vol. 37, No. 1, pp. 83–96, January–February, 1994.In conclusion, I wish to thank Professor L. V. Mirzoyan for very useful critical remarks, allowance for which considerably influenced the quality of this paper. 相似文献
16.
J. E. Dyson 《Astrophysics and Space Science》1977,51(1):197-204
The galactic nebula S206 contains a half shell of high excitation nebulosity which is centred on the associated exciting star. The suggestion has been made that this structure is caused by the interaction of stellar mass loss from the star with nebular gas. A steady state model of such an interaction is investigated quantitatively. The required mass loss rate from the star is about 10–7
M
yr–1 which is compatible with the observationally derived mass-loss rates from early-type stars. 相似文献
17.
A. A. Akopian 《Astrophysics》1996,39(4):331-333
A method is suggested for the determination of the distribution function for the mean frequency of supernova explosions, which is similar to the Ambartsumian method for the determination of the distribution function for the mean flare frequency of flare stars. This method has not been applied because of the lack of the required data.Translated fromAstrofizika, Vol. 39, No. 4, pp. 561–566, November, 1996. 相似文献
18.
It is widely accepted now that a significant fraction of the solar energetic particles (SEPs) observed at 1 AU after major solar flares are actually accelerated at a CME-driven shock. In addition, in the emerging new paradigm for SEP acceleration in different sources at or near the Sun, the existence of two types of flares – impulsive and gradual – is recognized. Within this concept, it is tempting also to separate SEPs into two groups – interacting and escaping – and to derive their 'source spectra' from observational data on various flare emissions (protons, gamma rays, neutrons, etc.). By different techniques, those spectra have been reconstructed for 80 solar proton events (SPE) in 1949–1991. In this paper, all available data on the source spectra of solar protons are summarized and revised. We discuss in detail existing uncertainties in the derived spectral indexes, consider other methodological problems involved in this study, and suggest several possible lines for the future investigations of solar flares and SCRs using the source spectrum data. It is noted that some peculiarities of the spectra, for instance, spectral steepening for high energies, may be characteristic of large events of the 23 February 1956 type. 相似文献
19.
Morita Satoshi Uchida Yutaka Hirose Shigenobu Uemura Shuhei Yamaguchi Tomotaka 《Solar physics》2001,200(1-2):137-156
In February 1992, three flares, which we consider constitute a homologous flare series (flares having basically the same configuration repeating in the same situation), occurred in the active region NOAA 7070 and were observed by Yohkoh SXT. In the present paper, we first discuss the homology of these three flares, and derive the 3D structure by making use of the information obtained from the three different lines of sight at common phases. The result of this analysis made clear for the first time that the so-called `cusped arcade' at the maximum phase in the well-known 21 February 1992 flare is, contrary to the general belief, an `elongated arch' created at the beginning of the flare, seen with a shallow oblique angle. It is not the `flare arcade' seen on axis as widely conceived. This elongated arch roughly coincides with a diagonal of the main body of the soft X-ray arcade that came up later. The magnetic structure responsible for the flare as a whole turned out to be a structure with quadruple magnetic sources – with the third and fourth sources also playing essential roles. The observationally derived information in our paper provides strong restrictions to the theoretical models of the process occurring in arcade flares. 相似文献
20.
B. R. Pettersen 《Solar physics》1989,121(1-2):299-312
We review the flaring activity of stars across the HR-diagram. Brightenings have been reported along the entire Main Sequence and in many stars off the Main Sequence. Some stars are decidedly young, others are in advanced stages of stellar evolution. Flares are common on stars with outer convection zones and outbursts have been reported also on other types of stars, although confirmations are needed for some of them.Analyses of flare occurrence sometimes find flares to be randomly distributed in time, and sometimes indicate a tendency for flares to come in groups. Preferred active longitudes have been suggested. Recent solar results, where the occurrence rate for flares is found to exhibit a periodicity of 152 days, suggest that stellar flare data should be reanalyzed over long time baselines to see if the present confusing situation can be resolved.The radiation from stellar flares is dominated by continuum emission and about equal amounts of energy have been recorded in the optical, UV, and X-ray regions of the spectrum. In solar flares strong continuum emission is rarely recorded and a large collection of bright emission lines takes prominence. Small flares occur more frequently than large ones and the latter have longer time-scales. Flare energies can exceed 1037 erg. The most productive flare stars are those where the convective envelopes occupy large volumes. Slow stellar rotation rates are believed to reduce the level when the star has been braked significantly from its young rotation rate. 相似文献