Radii and luminosities of classical cepheids

The radiii of 17 classical Cepheids are determined. The linear surface brightness-colour relationS _v=b(B–V)_o+const is accepted. The present method permits the determination of the coefficientb for each star separately and the obtaining of the absolute magnitude of Cepheids. The coefficientb shows a slight dependence on the periodP of stars. The period-luminosity relation is approximately the same as the one obtained by van den Bergh (1976) for Cepheids in open clusters. The simultaneous radial velocity and photoelectric observation may show the phase shifts between motions of the continuum layer and of the level where the Fe I line is formed. The Cepheidl Car is outside the instability strip, and probably has a red companion as was suggested by Schmidt (1980a). Conclusions about the existence of overtone pulsators cannot probably be drawn only from the scattering in the period-radius relation.     

Wesselink radii of 33 classical cepheids

The radii of 33 classical cepheids are determined. We have used the simultaneous radial velocity and photoelectric observation of 15 cepheids of Gieren (1981, 1982a). We have suggested that the best phase alignment between the light and the radial velocity curves in different epochs of 18 cepheids appears when the points of the phases at equal radius on the V, (B-V) diagram are transformed from a loop into a straight line. In such way we have determined the radii of 18 cepheids.     

Distribution of classical cepheids in the galactic plane

Major spiral arms were traced from the distribution of long-period classical cepheids on the projected galactic plane. The position of these spiral features have been compared with those from other optical tracers such as H II regions and OB star groups. Also the galactic longitude distribution of classical cepheids and open clusters are compared.     

Scanner observations of the classical cepheids RT Aur and T Vul

The effective temperatures of the classical Cepheids RT Aur and T Vul have been determined by a comparison of their spectral scans with appropriate model atmospheres. The radii of the stars have been determined through the Wesselink method. Using these temperatures and the Wesselink radii, the luminosities of the stars have been determined. These radii estimates, including the radii of SU Cas (Joshi & Rautela 1980) andζ Gem (unpublished) fit better in the theoretical period-radius relationship given by Cogan (1978), as compared to earlier determinations of Wesselink radii. The pulsation masses and evolutionary masses of the stars have been calculated. The pulsation to evolutionary mass ratio is derived to be 0.85. Based on the effective temperatures obtained by us at different phases of the stars aθ _c ? (B-V)₀ relationship is found of the form, \(\begin{gathered} \theta _e = 0.274 (B - V)_0 + 0.637 \\ \pm 0.011 \pm 0.007 \\ \end{gathered} \)      

Period-luminosity relation for classical cepheids of the large magellanic cloud

Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 34, No. 2, pp. 199–204, March–April, 1991.     

A method of colour excess determination for classical cepheids

By use of the reddening free [m ₁], [c ₁], and indices data inuvby photometric system for three classical cepheids whose reddening values had been determined with the aid of photometry of field stars, three intrinsic relations of [m ₁]–(b–y), [c ₁]–(b–y), and –(b–y) have been established. It was shown that these three relations can be used to determine the colour excesses for other classical cepheids.     

The effect of synchronization on the modal selection in classical cepheids

The coupled self-exciting oscillator model is investigated in the non-resonant case and applied to classical cepheids. The modal selection in these models is explained as the result of the synchronization caused by the mutual interaction between different modes. By using linear adiabatic coupling coefficients, it is shown that the fundamental mode suppresses the first overtone mode marginally in short-period classical cepheids. The double periodicity of several cepheids is expected as the result of a small change of physical state in the outer envelopes of these stars.     

On properties of the fourier harmonics in the limit-cycle models of classical cepheids

The Fourier coefficients of the hydrodynamic variables are calculated for the limit-cycle models of classical Cepheids having periods from 7.2 to 10.9 days. In adiabatically pulsating layers of the stellar envelope, each Fourier harmonic of orderk 8 is shown to be identified with a corresponding standing wave, so that the pulsation motions of the adiabatic layers may be represented as a superposition of standing waves. Each Fourier harmonic of orderk may also be identified with the eigenfunction of orderl of the linear adiabatic wave equation when the resonance condition _l /₀ =k is fulfilled. The spectra of the oscillatory moment of inertia and the spectra of kinetic energy obey the power law for the Fourier harmonics of orderk 15, the spectrum slope being steeper for shorter pulsation periods. In the helium and hydrogen ionizing regions all of the Fourier harmonics drive the pulsation instability, whereas in the radiative damping region the mechanical work done by each Fourier harmonic is negative. In classical Cepheids having periods shorter than 10 days the period dependence of the secondary bump is due to phase changes of the second order Fourier harmonic in the outer nonadiabatic layers of the stellar envelope. At a pulsation period of II 9.7 days the second order Fourier harmonic is identified with the second overtone. At periods II > 10 days the second order Fourier harmonic tends to be attracted by the fundamental mode in such a way that their phases coincide in the outer layers of the stellar envelope.     

Optimal curve fitting procedures applied to the light curves of classical cepheids

Optimal curve fitting procedures based on the well known Baade/Wesselink methodology have been applied to Stebbinset al.'s 6-colour photometry of the classical cepheids Cep and Aql. Radial velocity data have been represented by a Fourier series, while the brightness temperature at the effective wavelength of observation forms a convenient temperature variable in the fitting function. This fitting function requires the specification of six parameters which thus play the role of unknowns in the optimization problem, though, in fact, all six parameters cannot be independently determined.The formulation involves a simple connection between colours and brightness temperatures, and model stellar atmosphere calculations can provide such a connection. The model stellar atmosphere data of Carbon and Gingerich (1969), which take careful account of line blanketing effects are, to some extent, supported by the results for Aql, though the position is less certain in the case of Cep. On the basis of the Carbon and Gingerich data, and if we take into account various estimates of the interstellar reddening, the absolute magnitudes of Cep and Aql areM _v=–3.57 andM _v=–3.79, respectively; but optimal curve fits would decrease both these values by about 0^m.09.     

Fundamental parameters and intrinsic colors of F,G, and K supergiants and classical cepheids

We used high-resolution echelle spectra with high signal-to-noise ratio to determine with a high degree of accuracy some atmospheric parameters (T _eff, log g and [Fe/H]) for 68 non-variable supergiants of types F, G, and K and 26 classical Cepheids in 302 pulsation phases. Very accurate effective temperatures, with errors of only 10–30 K, were determined by the line-depth ratio method. We found that the observed intrinsic color indices (B ? V)₀ can be related to these parameters: (B ? V)₀ = 57.984? 10.3587(log T _eff)² + 1.67572(log T _eff)³ ? 3.356 log g+ 0.321 V _t + 0.2615[Fe/H] + 0.8833log g(log T _eff). With this empirical relation, the intrinsic colors of individual supergiants and classical Cepheids of spectral types F0-K0 and of luminosity classes I and II can be estimated with an accuracy as high as 0.05 ^m , which is comparable to the accuracy of the most elaborate photometric procedures. In view of large distances to supergiants, the method we propose here allows a large-scale mapping of interstellar extinction with an accuracy of 0.1–0.2 ^m in a quite large region of the Galaxy.     

On the metallicity dependence of classical Cepheid light amplitudes

Classical Cepheids remain a cornerstone of the cosmic distance scale, and thus characterizing the dependence of their light amplitude on metallicity is important. Period-amplitude diagrams constructed for longer-period classical Cepheids in IC 1613, NGC 3109, SMC, NGC 6822, LMC, and the Milky Way imply that very metal-poor Cepheids typically exhibit smaller V-band amplitudes than their metal-rich counterparts. The results provide an alternate interpretation relative to arguments for a null and converse metallicity dependence. The empirical results can be employed to check predictions from theoretical models, to approximate mean abundances for target populations hosting numerous long-period Cepheids, and to facilitate the identification of potentially blended or peculiar objects.     

A reanalysis of the data of three classical cepheids: V381 Cen,V500 Sco,and SV Vel

Using the recently developed CORS method, applied to the photometric data of Walravenet al. (1964), we have determined improved radii for the three classical cepheids V381 Cen, V500 Sco, and SV Vel. These new determinations are in good agreement with theoretical computations for the center of the instability strip (Cogan, 1978). Some peculiar parameters are also computed, from the analysis of photometric data.     

Intermediate‐term variations in solar radius during solar cycle 23

In this study, we look for the mid‐term variations in the daily average data of solar radius measurements made at the Solar Astrolabe Station of TUBITAK National Observatory (TUG) during solar cycle 23 for a time interval from 2000 February 26 to 2006 November 15. Due to the weather conditions and seasonal effect dependent on the latitude, the data series has the temporal gaps. For spectral analysis of the data series, thus, we use the Date Compensated Discrete Fourier Transform (DCDFT) and the CLEANest algorithm, which are powerful methods for irregularly spaced data. The CLEANest spectra of the solar radius data exhibit several significant mid‐term periodicities at 393.2, 338.9, 206.5, 195.2, 172.3 and 125.4 days which are consistent with periods detected in several solar time series by several authors during different solar cycles. The knowledge relating to the origin of solar radius variations is not yet present. To see whether these variations will repeat in next cycles and to understand how the amplitudes of such variations change with different phases of the solar cycles, we need more systematic efforts and the long‐term homogeneous data. Since most of the periodicities detected in the present study are frequently seen in solar activity indicators, it is thought that the physical mechanisms driving the periodicities of solar activity may also be effective in solar radius variations (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Classical cepheids with and without bumps in the light curves

By use of the photometric catalogue of Nikolov (1968), the presence or the absence of bumps in the light curves of 116 classical cepheids with periods approximately between 6 and 20 days is established and the ratio of the variables without bumps to all the stars in this range in period is determined. In the systems of evolutionary tracks of Iben (1967) and Paczyski (1970), the times for the first three crossings of the instability strip by model stars of different masses are calculated and the ratio of the time for the first crossing to the total time spent within the instability strip (taking into account the three crossings) is determined for different stellar masses and pulsation periods. The comparison of these observational and theoretical data does not support B. F. Madore's suggestion (in connection with the hypothesis of A. N. Cox and collaborators for inhomogeneous cepheid envelopes) that the classicial cepheids without bumps in this period range might cross for the first time the instability strip: the fraction of such variables is considerably larger than the theoretically expected fraction. The absence of bumps might be a consequence of a mixing due to, e.g., a differential rotation and/or the Rayleigh-Taylor instability, or a consequence of differences in luminosity, mass, effective temperature or chemical composition among cepheids of the same periods.     

Quick matching technique to study the relationship between solar radius and luminosity variations

A simple matching technique is developed which allows us to compute the response of the solar envelope to perturbations which occur within the solar convective region, and in timescales of importance to climate. This technique is applied to perturbation of the convective efficiency (-mechanism), and of the non-gas component of the pressure in different regions of the convection zone (-mechanism). The results indicate that while either perturbation affects the solar luminosity, the -mechanism has almost no effect on the solar outer radius, regardless of the affected region, whereas the -mechanism produces radius changes which may be large if the location of perturbation is deep enough.     

Classical cepheids and the local chemical inhomogeneities in the galactic disc

Places of formation have been derived for a sample of 23 Cepheids with well-determined atmospheric abundances in an attempt to study the chemical inhomogeneities in the local interstellar medium. The abundance data available for the sample Cepheids is compiled and critically reviewed to adopt the most reliable esimates. We find that the most conspicuous irregularity in the metallicity is exhibited by stars that are born in the local arm or in the interarm region. We propose a scenario to explain these local variations in terms of supernova-induced star formation in interstellar gas enriched by massive stars formed in the density wave.     

Spectrophotometric study of the cepheids DT Cyg and SZ Tau

The effective temperatures of the cepheids DT Cyg and SZ Tau have been determined from a comparison of their spectral scans with appropriate stellar model atmospheres. Using these temperatures and an independent Wesselink radius determination, the luminosities of the stars have been determined. The pulsation masses and the evolutionary masses of the stars have been discussed.     

Distribution of B stars in the directions of some southern cepheids

The distribution of B stars in the vicinity of nine southern, long-period (P > 11 days) Cepheids is studied on the basis of U, B, V data. The parameters Q, the normal color indices (B — V)₀ and (U — B)₀, the color excesses E(B — V), the spectral types, and the distance moduli of these stars are determined. Translated from Astrofizika, Vol. 40. No. 4, pp. 573–580, October-December, 1997.     

Abundance analysis of selected cepheids and the galactic distribution of metallicity

We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming r_gc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.