共查询到20条相似文献，搜索用时 46 毫秒
1.
This study is part of an investigation of the possibility of using chemically peculiar (CP) stars to map local galactic structure.
Correct luminosities of these stars are therefore crucial. CP stars are generally regarded as main-sequence or near-main-sequence
objects. However, some CP stars have been classified as giants.
A selection of stars, classified in literature as CP giants, are compared to normal stars in the same effective temperature
interval and to ordinary ‘non giant’ CP stars. We find no clear confirmation of a higher luminosity for ‘CP giants’, than
for CP stars in general. In addition, CP characteristics seem to be individual properties not repeated in a component star
or other cluster members. 相似文献
2.
Graeme H. Smith & Christopher W. Churchill 《Monthly notices of the Royal Astronomical Society》1998,297(2):388-398
Echelle spectra have been obtained of the Ca II H and K lines for a sample of metal-poor subdwarf stars as well as for a number of nearby Population I dwarfs selected from among those included in the Mount Wilson HK survey. The main conclusion of this paper is that Ca II H- and K-line emission does occur among subdwarfs. It is particularly notable among those subdwarfs with colours of B − V ≥0.75; all such stars observed exhibit chromospheric emission, although emission is observed among some subdwarfs bluer than this colour. The Ca II K emission profile in most subdwarfs exhibits an asymmetry of V / R >1, similar to that seen in the integrated light of the solar disc. Two quantitative indicators of the contrast between the peaks in the emission profile and the neighbouring photospheric line profile are introduced. Measurements of these indicators show that the level of Ca II emission among the subdwarfs is similar to that among low-activity Population I dwarfs. 相似文献
3.
G. A. Gurzadyan 《Astrophysics and Space Science》1986,123(1):67-101
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. 相似文献
4.
半个世纪以前,Ｒｏｂｅｒｔｓ 和Ｓａｎｄａｇｅ 在验证Ｓｃｈｗａｒｚｓｃｈｉｌｄ 的理论发现时,在水平支ＲＲＬｙｒａｅ 变星缺口段的红端发现了两颗小变幅的变星。当时定不出类型,标以“？”。以后Ｓａｎｄａｇｅ 在做ＲＲ Ｌｙｒａｅ 变星统计研究时,竟不加解释地当作ＲＲａｂ 型星处理。半个世纪之间,没有人对这颗星的反常加以注意。我们用国内的ＣＣＤ 观测,又发现了一颗类似的小变幅变星,但变幅更小,周期更长。这颗星与以前发现的两颗一起,似乎构成ＲＲ Ｌｙｒａｅ 星的又一个次型。现有的变星脉动理论还不能对之加以解释。我们预计,这将再一次推动脉动理论发展。 相似文献
5.
We calculated 240 model atmospheres of carbon giants with high carbon abundance (2800 K ≤ T
eff ≤ 3400 K, 0.06 ≤ log(C/O) ≤ 2.7). This set of models was used for modelling of the energy distribution in the spectrum of
an evolved carbon star DY Per demonstrating photometric features of the R CrB type stars. Most of the models are metal-poor
(−3.5 ≤ [Fe/H] < 0) and a portion of them was calculated with the hydrogen deficiency (1/9 ≤ H/He < 9/1). The calculations
of the models were carried out taking into account specific for carbon stars sources of opacity in the frame of the classic
approaches. The opacity sampling method was used to calculate the opacity due to the atomic and molecular line absorption. 相似文献
6.
G.A. Gurzadyan 《Astrophysics and Space Science》1997,257(2):223-236
The IUE observations do not favor the chromospheric origin of typical chromospheric emission lines, of the doublet 2800 MgII
first of all, in close binary systems SX Cas and 22 Vul, both with a cool giant-supergiant as primary, and a hot B7–B8 star
as secondary. Here, it is shown that the concept of common chromospheres – roundchroms can explain the unusual fluxes of chromospheric
emission lines in these systems. The hot components of these binaries – B type stars, themselves are assumed to represent
local binary systems with a white dwarf and an accretion disc: the latter being the source of generation of transition zone
type high excitation emission lines 1240 NV, 1400 SiIV, 1550 CIV, etc. It is concluded, that roundchroms of giant sizes, of
order of 100 R⊙ and larger, are not rare events for the RS CVn type binary systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
In this work, based on the analytical model with delayed production approximation developed by Pagel & Tautvaišienė (1995)
for the Galaxy, the analytic solutions of the distribution of neutron exposures of the Galaxy (hereafter NEG) are obtained.
The present results appear to reasonably reproduce the distribution of neutron exposures of the solar system (hereafter NES).
The strong component and the main component of the NES are built up in different epochs. Firstly, the strong component is
produced by the s-process nucleosynthesis in the metal-poor AGB stars, starting from [Fe/H] ≈ −1.16 to [Fe/H] ≈ −0.66, corresponding
to the time interval 1.06 < t < 2.6 Gyr. Secondly, the main component is produced by the s-process in the galactic disk AGB stars, starting from [Fe/H]
≈ −0.66 to [Fe/H] ≈ 0, corresponding to the time interval t > 2.6 Gyr. The analytic solutions have the advantage of an understanding of the structure and the properties of the NEG.
The NEG is believed to be an effective tool to study the s-process element abundance distributions in the Galaxy at different
epochs and the galactic chemical evolution of the neutron-capture elements. 相似文献
8.
G. V. Zajtseva 《Astrophysics》2010,53(4):573-574
The V magnitude and B-V and U-B color indices of comparison stars within distances of about 2′ from the variable Ry Tauri
are given. 相似文献
9.
We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which
the chromospheric evaporation process takes place either in a single ‘monolithic’ loop or in a loop consisting of several
filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric
evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions
are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops
as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high
beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are
derived by using the Rosner – Tucker – Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for
this discrepancy are discussed. 相似文献
10.
A. Takeda 《Solar physics》2011,273(1):295-306
The temperature response functions of the Yohkoh/SXT are re-calculated based on the most recent elemental abundances and ionization balance available in the CHIANTI atomic
database version 6.0.1. The new standard responses are calculated for three types of abundance; i.e., ‘coronal’, ‘hybrid’, and ‘photospheric’ abundances included in the CHIANTI database, and are available in SolarSoft since
2010. Comparison plots of the new and old response functions and filter ratios are available at the Yohkoh Legacy data Archive
(). The three new responses generally peak at higher temperatures (at ≈ 10 MK) than the former standard response (at ≈ 5.6 MK)
based on Mewe’s spectral model. The new responses with coronal and hybrid abundances have higher peak counts by a factor of
3 and 2, respectively, than those with the photospheric abundances and the former response based on Mewe’s model. The correction
of the filter ratios depends on the type of filter and the range of the ratios to be used. In the significant cases, the new
filter ratio produces 20 to 30% higher temperatures than the previous calibration. The choice of elemental abundance has a
strong influence on the derived temperatures and emission measures, and often produces a variation significantly larger than
the statistical and systematic errors considered so far. 相似文献
11.
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
12.
G. A. Gurzadyan 《Astrophysics and Space Science》1981,80(1):197-209
A new method for the detection of the circumstellar clouds around the hot stars of O-B classes is developed. The method is based on the fact, connected with the large dispersion in the observed equivalent widths,W
*(2800 MgII) of non-stellar origin, for a selected group of stars withE(B-V)=0. The separation of observed magnitudes ofW(2800) in two components interstellarW
i, and circumstellarW
c is realised. It is shown that the circumstellar clouds really existed around 90% of hot stars analysed in the present paper (total number of stars 46, Table II). In 30% cases of hot stars circumstellar clouds are very powerful (the radii are less than 1pc, the masses less than 1 solar mass). The usual model for interstellar medium seems to be unacceptable for the system hot star+circumstellar cloud. 相似文献
13.
D. Montes 《Astrophysics and Space Science》1998,263(1-4):231-234
A summary of our ongoing high and medium resolution optical observational program, devoted to the study of chromospheric activity
of pre-main sequence stars, is reported here. The study is centered in bona-fid weak-lined T Tauri stars (WTTS) in Taurus
with known rotational period, and in WTTS recently discovered by the ROSAT All-Sky Survey. It is our purpose to quantify the
phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric
activity indicators (Ca II H & K, Hβ, Hα, Ca II IRT) as well as to obtain the Li I abundance, a better determination of the
stellar parameters, spectral type, and possible binarity. By using these chromospheric excess emissions we will study the
flux-flux and rotation-activity relationships in order to get insights into the mechanisms which drive solar-like stellar
activity in this kind of low mass pre-main sequence stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
J. H. Piddington 《Astrophysics and Space Science》1976,41(2):371-385
Using a combination of solar and interplanetary measurements, a topological model is developed of the overall magnetic and plasma structures.
相似文献
| (i) | The basic framework is the magnetic field, whose structure is found by combining measurements made at the photosphere, the transition region, and at 1 AU. It divides the atmosphere into three physically isolated regions having quite different processes of energy and plasma transfer, and very different properties. |
| (ii) | A magnetically open atmosphere is confined within 10% of the surface magnetic flux in the form of tiny nozzles. It expands from 0.1% of the photospheric area to 10% of the low corona and 100% of the solar wind. Energy absorption and resulting expansion is traced from chromospheric levels. |
| (iii) | A model of M-regions, high-speed plasma streams and interplanetary sector structure is based on refraction of acoustic waves and their focusing into the centres of sectors. |
| (iv) | The average magnetically enclosed atmosphere occupies1% of the photosphere, spreading to 90% of the low corona. Surface flux is concentrated into strands of 4×1018 Mx, with 30 per supergranule cell. The strands spread and also divide into smaller flux tubes to accout for chromospheric fine structure in which magnetic forces dominate. It is questionable that this complex of plasma elements should be called an atmosphere. |
| (v) | The third, non-magnetic part of the solar atmosphere comprises on averae 99% of the photosphere and a large part of the chromosphere (the network interior), but little if any of the corona. It is stressed that measurements or models of the solar atmosphere have little meaning unless they relate to a particular one of the three regions described here. |
| (vi) | It is confirmed that most of the energy needed to heat the solar atmosphere traverses the photosphere as Alfvén waves. Some energy is converted to acoustic waves at the boundaries of the magnetic fields, some is dissipated when the Alfvén waves become non-linear. |
15.
S. C. Marsden B. D. Carter J.-F. Donati 《Monthly notices of the Royal Astronomical Society》2009,399(2):888-905
In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High-resolution spectroscopic data were obtained for over 50 F, G and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid rotators ( v sin i > 20 km s−1 ) with five of the stars being classified as ultra-rapid rotators ( v sin i > 100 km s−1 ) . The emission levels in the calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against the Rossby number ( N R ), the star's chromospheric emission levels show a plateau in the emission for log( N R ) ≲−1.1 indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars shows little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators. 相似文献
16.
The evolutionary and spatial characteristics of the motions in the flaring chromosphere of a 2B/M2.3 flare are investigated
by analyzing the asymmetry in the Hα profiles. The possibility of reconciling the results of observations with the theory of chromospheric evaporation is considered.
The spectroscopic Hα observations of the flare performed with the KG-2 CrAO coronagraph with a temporal resolution of 5–10 s and a spatial resolution
as high as 1 arcsec cover all stages of flare development. The following results have been obtained: (1) The Hα profile asymmetry is a general characteristic of the flare emission irrespective of its intensity and its belonging to different
structural features and phases of flare development. (2) Most of the Hα emission profiles in flare regions exhibit a red asymmetry. However, a blue asymmetry was observed in small local regions
at all stages of flare development. (3) A red asymmetry that appeared before the onset of the impulsive phase and persisted
after its end was observed at the sites of main energy release, i.e., the energy source responsible for the dynamical processes
in the flare came into operation earlier and existed longer than the HXR emission. (4) The asymmetry pattern changed with
flare phase: the red wing intensity dominated in the pre-impulsive phase and at the onset of the impulsive and gradual phases
(while the line core was unshifted or slightly shifted). At the maximum of the impulsive phase, the nearly symmetric profiles
with extended wings were redshifted as a whole, i.e., the entire emitting volume moved down with a velocity of several tens
of km/s. This type of asymmetry cannot be explained by the dynamical model of chromospheric condensation (Canfield and Gayley
1987). (5) The Hα profiles show no evidence of chromospheric heating by a beam of nonthermal electrons during the impulsive phase (Canfield
et al. 1984). (6) The lifetime of the downflows and the change in their velocities with time are inconsistent with the dynamical
model of chromospheric condensation (Fisher 1989). (7) The morphological features of the velocity field are also inconsistent
with the theory of chromospheric evaporation, because the highest differently directed velocities were detected at the flare
loop tops, not at the sites of main energy release. We conclude that the investigated flare shows spectral features that are
inconsistent with the standard chromospheric evaporation model. 相似文献
17.
This review presents a selection of recent highlights of observations of R Coronae Borealis variables. Emphasis is placed
on an abundance analysis of a complete sample (18 stars) of the warm galactic RCBs. It is shown that 14 of the 18 have very
similar compositions: the iron mass fraction ranges about a factor of 3 around the solar value (assuming C/He = 3%) but abundance
ratios X/Fe for elements from Na to Ba show little variation. By contrast, the other 4 stars are deficient in iron but not
in Na, Si, S and some other elements. With for example, [Si/Fe] ≃ 2, the quartet is indeed ‘peculiar’. One of the quartet,
V854 Cen shows depletions of elements (other than CNO) similar to the depletions seen in interstellar medium corresponding
to average logn(H
tot) = − 1.5. Scenarios for creating RCB from normal single and double stars are summarised.
Invited review talk presented at the Asia-Pacific regional IAU meeting held at Pune from 16–20 August 1993. 相似文献
18.
David L. Lambert 《Journal of Astrophysics and Astronomy》1987,8(2):103-122
The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements
considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also25Mg and26Mg).
The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3
with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is
small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical.
For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show
[Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and
[Fe/H] remains uncertain. 相似文献
19.
D. Montes 《Astrophysics and Space Science》1998,263(1-4):275-278
I have compiled here the three libraries of high and mid-resolution optical spectra of late-type stars I have recently published.
The libraries include F, G, K and M field stars, from dwarfs to giants. The spectral coverage is from 3800 to 1000 Å, with
spectral resolution ranging from 0.09 to 3.0 Å. These spectra include many of the spectral lines most widely used as optical
and near-infrared indicators of chromospheric activity. The spectra have been obtained with the aim of providing a library
of high and mid-resolution spectra to be used in the study of active chromosphere stars by applying a spectral subtraction
technique. However, the data set presented here can also be utilized in a wide variety of ways. A digital version of all the
fully reduced spectra is available via FTP and the World Wide Web (WWW) in FITS format.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
A. Pigulski & Z. Ko&#;aczkowski 《Monthly notices of the Royal Astronomical Society》1998,298(3):753-762
The central part of the Cygnus OB2 association was searched for the presence of short-period hot pulsators. Out of 16 O-type stars in the field, five turned out to be variable. None of the O-type variables showed short-period variations characteristic for the β Cephei-type stars. However, two β Cephei stars were found among the early B-type members of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discrepancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour–magnitude diagrams of the association. We found 29 new variables in all fields searched by us. About half of these are Cygnus OB2 members. By means of Hα photometry, we confirmed the presence of Hα emission in two stars and found one new Be star. A weak Hα emission is probably present in all observed O-type supergiants and giants. We also give the new BVRI Hα photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour–magnitude diagram for the association and make a reddening map of the observed field. The extinction in the field, expressed as the total absorption in the Johnson V filter, A v , ranges from 4.1 to 7.0 mag; the average A V amounts to 5.8 mag. One O-type star is probably a background object. 相似文献