共查询到20条相似文献，搜索用时 31 毫秒
1.
We present temporal and spectral characteristics of X-ray flares observed from six late-type G–K active dwarfs (V368 Cep, XI Boo, IM Vir, V471 Tau, CC Eri and EP Eri) using data from observations with the XMM–Newton observatory. All the stars were found to be flaring frequently and altogether a total of 17 flares were detected above the 'quiescent' state X-ray emission which varied from 0.5 to 8.3 × 1029 erg s−1 . The largest flare was observed in a low-activity dwarf XI Boo with a decay time of 10 ks and ratio of peak flare luminosity to 'quiescent' state luminosity of 2. We have studied the spectral changes during the flares by using colour–colour diagram and by detailed spectral analysis during the temporal evolution of the flares. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. We have derived the semiloop lengths of flares based on the hydrodynamic flare model. The size of the flaring loops is found to be less than the stellar radius. The hydrodynamic flare decay analysis indicates the presence of sustained heating during the decay of most flares. 相似文献
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We present new population synthesis calculations of close young neutron stars. In comparison with our previous investigation
we use a different neutron star mass spectrum and different initial spatial and velocity distributions. The results confirm
that most of ROSAT dim radioquiet isolated neutron stars had their origin in the Gould Belt. We predict that about several
tens of young neutron stars can be identified in ROSAT All Sky Survey data at low galactic latitudes. Some of these sources
also can have counterparts among EGRET unidentified sources. 相似文献
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L. Pavan R. Turolla S. Zane L. Nobili 《Monthly notices of the Royal Astronomical Society》2009,395(2):753-763
There is an increasing theoretical and observational evidence that the external magnetic field of magnetars may contain a toroidal component, likely of the same order of the poloidal one. Such 'twisted magnetospheres' are threaded by currents flowing along the closed field lines which can efficiently interact with soft thermal photons via resonant cyclotron scatterings (RCS). Actually, RCS spectral models proved quite successful in explaining the persistent ∼1–10 keV emission from the magnetar candidates, the soft γ-ray repeaters (SGRs) and the anomalous X-ray pulsars (AXPs). Moreover, it has been proposed that, in the presence of highly relativistic electrons, the same process can give rise to the observed hard X-ray spectral tails extending up to ∼200 keV . Spectral calculations have been restricted up to now to the case of a globally twisted dipolar magnetosphere, although there are indications that the twist may be confined only to a portion of the magnetosphere, and/or that the large-scale field is more complex than a simple dipole. In this paper, we investigate multipolar, force–free magnetospheres of ultramagnetized neutron stars. We first discuss a general method to generate multipolar solutions of the Grad-Schlüter-Shafranov (GSS) equation, and analyse in detail dipolar, quadrupolar and octupolar fields. The spectra and lightcurves for these multipolar, globally twisted fields are then computed using a Monte Carlo code and compared with those of a purely dipolar configuration. Finally, the phase-resolved spectra and energy-dependent lightcurves obtained with a simple model of a locally sheared field are confronted with the International Gamma-Ray Astrophysics Laboratory ( INTEGRAL ) observations of the AXPs 1RXS J1708−4009 and 4U 0142+61. Results support a picture in which the field in these two sources is not globally twisted. 相似文献
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An atlas of high resolution (R = 60 000) CCD-spectra in the wavelength range 3500-5000A is presented for four objects in metallicity range -3.0 < [Fe/H] < -0.6, temperature range 4750 < Teff < 5900 K, and surface gravity range 1.6 < lgg < 5.0. We describe the calibration of the stellar atmospheric parameters using Alonso's formula based on the method of infrared flux and outline the determination of the abundances of a total number of 25 chemical elements. An analysis of the abundance determination errors for different chemical elements is carried out, and a method is provided for the observations and reduction of spectral material. Properties of the method of producing an atlas of spectra and line identifications are described. 相似文献
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We re‐discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M⊙ are concentrated towards the centre of the main‐sequence band. In contrast, stars with masses M > 3 M⊙ seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M⊙ and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main‐sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities β is detected in both higher and lower mass stars. It is quite possible that the angle β becomes close to 0. in slower rotating stars of mass M > 3 M⊙ too, analog to the behaviour of angles β in slowly rotating stars of M < 3 M⊙. The obliquity angle distribution as inferred from the distribution of r ‐values appears random at the time magnetic stars become observable on the H‐R diagram. After quite a short time spent on the main sequence, the obliquity angle β tends to reach values close to either 90. or 0. for M < 3 M⊙. The evolution of the obliquity angle β seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of β with the elapsed time on the main sequence for stars with M > 3 M⊙, no similar trend is found for stars with M < 3 M⊙. However, the predominance of high values of β at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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P. J. Amado † J. G. Doyle P. B. Byrne‡ 《Monthly notices of the Royal Astronomical Society》2000,314(3):489-497
A fortran code which computes synthetic light and colour curves of active, spotted stars has been developed. The main feature of this code is that it can simultaneously model the V light curve and the ( V − R )c , ( V − I )c , ( V − K ) colour data. It also uses new effective temperature–colour and Barnes–Evans-like calibrations, temperature and gravity-dependent limb darkening coefficients and different effective surface gravities for the spotted and unspotted photosphere. The code allows for two-component spots, i.e. spots with umbral and penumbral components. Various problematic spot configurations were investigated and we conclude that, in order to be able to differentiate spots with various thermal structures (umbrae, penumbrae, faculae) or polar spots from equatorial bands, the modelling of the infrared colours, especially ( V − I )c and ( V − K ), is needed. 相似文献
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Filippina Caputo Marcella Marconi & Patrizia Santolamazza 《Monthly notices of the Royal Astronomical Society》1998,293(4):364-368
We analyse the periods of theoretical radial pulsators, covering the range of total masses, luminosities, effective temperatures and chemical compositions expected for RR Lyrae variables in both galactic fields and globular clusters.
We show that for fixed values of the structural parameters (mass, luminosity and effective temperature), the period of fundamental and first-overtone pulsators is independent of the helium content ( Y ), whereas it slightly increases as the amount of metals ( Z ) increases. Furthermore, we find that the period along the blue edge for first-overtone pulsation is a function of mass, luminosity and helium content, with a marginal dependence on Z .
On these grounds, new linear relations connecting the periods to stellar parameters are derived. Such new relations should allow a more accurate interpretation of the RR Lyrae observed periods and, in particular, they should help in ascertaining the calibration of the mean absolute magnitude of RR Lyrae stars in terms of metal content. 相似文献
We show that for fixed values of the structural parameters (mass, luminosity and effective temperature), the period of fundamental and first-overtone pulsators is independent of the helium content ( Y ), whereas it slightly increases as the amount of metals ( Z ) increases. Furthermore, we find that the period along the blue edge for first-overtone pulsation is a function of mass, luminosity and helium content, with a marginal dependence on Z .
On these grounds, new linear relations connecting the periods to stellar parameters are derived. Such new relations should allow a more accurate interpretation of the RR Lyrae observed periods and, in particular, they should help in ascertaining the calibration of the mean absolute magnitude of RR Lyrae stars in terms of metal content. 相似文献
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L. S. Lyubimkov S. I. Rostopchin T. M. Rachkovskaya D. B. Poklad D. L. Lambert 《Monthly notices of the Royal Astronomical Society》2005,358(1):193-201
From high-resolution spectra a non-local thermodynamic equilibrium analysis of the Mg ii 4481.2-Å feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line Al iii 4479.9-Å is considered. The magnesium abundance is determined; it is found that log ɛ(Mg) = 7.67 ± 0.21 on average. It is shown that uncertainties in the microturbulent parameter Vt are the main source of errors in log ɛ(Mg) . When using 36 stars with the most reliable Vt values derived from O ii and N ii lines, we obtain the mean abundance log ɛ(Mg) = 7.59 ± 0.15 . The latter value is precisely confirmed for several hot B stars from an analysis of the Mg ii 7877-Å weak line. The derived abundance log ɛ(Mg) = 7.59 ± 0.15 is in excellent agreement with the solar magnesium abundance log ɛ⊙ (Mg) = 7.55 ± 0.02 , as well as with the proto-Sun abundance log ɛ ps (Mg) = 7.62 ± 0.02 . Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity. 相似文献
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Leonid S. Lyubimkov Tamara M. Rachkovskaya Sergey I. Rostopchin David L. Lambert 《Monthly notices of the Royal Astronomical Society》2002,333(1):9-26
Effective temperatures T eff , surface gravities log g and interstellar extinctions A V are found for 107 B stars. Distances d of the stars, which are based on the derived T eff , log g and A V values, show good agreement with those obtained from the Hipparcos parallaxes. Comparing the T eff and log g values with evolutionary computations, we infer masses, radii, luminosities, ages and relative ages of the stars. Empirical relations between the T eff and log g parameters, on the one hand, and the photometric indices Q , [ c 1 ] and β , on the other hand, are constructed; these relations give a fast method for the T eff and log g estimation of early and medium B stars. Inclusion of the infrared J , H and K colours into the T eff , log g and A V determination shows that (i) the T eff and log g parameters are altered only slightly; (ii) the A V value is rather sensitive to these colours, so an accuracy better than 0.05 mag in the JHK data is necessary for precise A V evaluation. 相似文献
20.
To try to understand the behavior of helium variability in Chemically Peculiar stars, we continued our on‐going observational campaign started by Catanzaro, Leone & Catalano (1999). In this paper we present a new set of time resolved spectroscopic observations of the HeI5876 Å line for a sample of 10 stars in the spectral range B3 ‐ A2 and characterized by different overabundances. This line does not show variability in two stars: HD77350 and HD175156. It shows instead an equivalent width variation in phase with the Hipparcos light curve for two stars: HD79158 and HD196502. Antiphase variations have been found in 4 stars of our sample, namely: HD35502, HD124224, HD129174 and HD142990. Nothing we can say about HD115735 because of the constancy of Hipparcos photometric data, while no phase relation has been observed for HD90044. In the text we discuss the case of HD175156, according to photometric calibration and our spectroscopic observations we rule out the membership of this star to the main sequence chemically peculiar stars. We confirm the results obtained in the previous paper for which phase relations between light, spectral and magnetic variations are not dependent on stellar spectral type or peculiarity subclass. 相似文献