اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدEffect of NLTE model atmospheres on photometric amplitudes and phases of early B-type pulsating stars
100
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We study all possible sources of inaccuracy in theoretical values of the photometric observables, i.e. amplitude ratios and phase differences, of early B-type main sequence pulsators. Here, we discuss effects of parameters coming from both models of stellar atmospheres and linear nonadiabatic theory of stellar pulsation. In particular, we evaluate for the first time the effect of the departure from the LTE approximation. The atmospheric input comes from line-blanketed, LTE and NLTE plane-parallel, hydrostatic models. To compute the limb-darkening coefficients for NLTE models, we use the Least-Square Method taking into account the accuracy of the flux conservation. We present effects of NLTE atmospheres, chemical composition and opacities on theoretical values of the photometric observables of early B-type pulsators. To this end, we compute tables with the passband fluxes, flux derivatives over effective temperature and gravity as well as the non-linear limb-darkening coefficients in 12 most often used passbands, i.e. in the Stromgern system, $uvby$, and in the Johnson-Cousins-Glass system, $UBVRIJHK$. We make these tables public available at the Wroc{l}aw HELAS Web page, http://helas.astro.uni.wroc.pl.
قيم البحث
اقرأ أيضاً
We present a model atom for C I - C II - C III - C IV using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium (NLTE) line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emiss
ion lines of C II 9903~AA and 18535~AA to appear at effective temperature Teff~$geq$~17,500~K, those of C II 6151~AA and 6461~AA to appear at Teff~$>$~25,000~K, and those of C III 5695, 6728--44, 9701--17~AA to appear at Teff~$geq$~35,000~K (log~$g$=4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C II 9903 and 18535~AA, the upper levels are mainly populated from C III reservoir through the Rydberg states. For C III 5695 and 6728--44~AA, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426--28~AA which become optically thin in the photosphere. We analysed the lines of C I, C II, C III, and C IV for twenty-two O-B-type stars with temperature range between 15,800 $leq$~Teff~$leq$ 38,000~K. Abundances from emission lines of C I, C II and C III are in agreement with those from absorption ones for most of the stars. We obtained log~$epsilon_{rm C}$=8.36$pm$0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15~Mon and HD~42088 from emission and absorption lines are 8.27$pm$0.11 and 8.31$pm$0.11, respectively.
Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsa
ting stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $beta$ Cep stars, including three that are hybrid $beta$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $beta$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $chi$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars.
The powerful radiative winds of hot stars with strong magnetic fields are magnetically confined into large, corotating magnetospheres, which exert important influences on stellar evolution via rotational spindown and mass-loss quenching. They are det
ectable via diagnostics across the electromagnetic spectrum. Since the fossil magnetic fields of early-type stars are stable over long timescales, and the ion source is internal and isotropic, hot star magnetospheres are also remarkably stable. This stability, the relative ease with which they can be studied at multiple wavelengths, and the growing population of such objects, makes them powerful laboratories for plasma astrophysics. The magnetospheres of the magnetic early B-type stars stand out for being detectable in every one of the available diagnostics. In this contribution I review the basic methods by which surface magnetic fields are constrained; the theoretical tools that have been developed in order to reveal the key physical processes governing hot star magnetospheres; and some important recent results and open-ended questions regarding the properties of surface magnetic fields and the behaviour of magnetospheric plasma.
We summarize the properties of the new periodic, small amplitude, variable stars recently discovered in the open cluster NGC 3766. They are located in the region of the Hertzsprung-Russell diagram between delta Sct and slowly pulsating B stars, a reg
ion where no sustained pulsation is predicted by standard models. The origin of their periodic variability is currently unknown. We also discuss how the Gaia mission, to be launched at the end of 2013, can contribute to our knowledge of those stars.
We constructed a comprehensive model atom for Cione -- Cii using the most up-to-date atomic data available and evaluated the non-local thermodynamic equilibrium (NLTE) line formation for Cione and Cii in classical 1D models representing the atmospher
es of A and late B-type stars. Our NLTE calculations predict the emission that appears at effective temperature of 9250 to 10,500~K depending on log~$g$ in the Cione 8335, 9405,AA singlet lines and at Teff~$>$~15,000~K (log~$g$ = 4) in the Cione 9061 -- 9111,AA,, 9603 -- 9658,AA, triplet lines. A prerequisite of the emission phenomenon is the overionization-recombination mechanism resulting in a depopulation of the lower levels of Cione to a greater extent than the upper levels. Extra depopulation of the lower levels of the transitions corresponding to the near-infrared lines, is caused by photon loss in the UV lines Cione 2479, 1930, and 1657,AA. We analysed the lines of Cione and Cii in Vega, HD~73666, Sirius, 21~Peg, $pi$~Cet, HD~22136, and $iota$ Her taking advantage of their observed high-resolution spectra. The Cione emission lines were detected in the four hottest stars, and they were well reproduced in our NLTE calculations. For each star, the mean NLTE abundances from lines of the two ionization stages, Cione and Cii, including the Cione emission lines, were found to be consistent. We show that the predicted Cione emission phenomenon depends strongly on whether accurate or approximate electron-impact excitation rates are applied.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
