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1.
熊大闰  邓李才 《天文学报》2006,47(3):256-267
利用一种非定常的恒星非局部对流理论,对球状星团中低光度的红巨星进行了线性非绝热脉动理论计算．结果表明,对所有温度高于约5400 K模型的基音到4阶泛音都是脉动稳定的．随着恒星光度的增大,低阶泛音也变得脉动不稳定．对中低光度的红巨星,脉动稳定性非常低,接近中性稳定．因此他们将是不变星或非常小振幅的短周期变星(P<2天)．  相似文献   

2.
We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of     . The stellar models were computed by using convective overshoot distance     , 0.25 and 0.40  H P. Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at     , while the top of the instability strip extends up to     . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index     is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter d over used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for     .  相似文献   

3.
The linear stability analysis of the radial and non-radial oscillations for the evolutionary model of a star with the mass of 0.6∼3 M8 has been per- formed by using the nonlocal and time-dependent convection theory. The results show that the unstable low-temperature stars on the right side of the instabil- ity strip in the HR diagram can be divided into two groups. One is of the stars of solar-like oscillations, composed of the main-sequence dwarfs, subgiants, and the red giants with low- and intermediate-luminosity, which are unstable in the intermediate- and high-order (nr ≥ 12) p-modes, but stable in the low- order (nr ≤ 5) p-modes. Another is of the Mira-like stars, composed of the luminous red giants and AGB stars, which are just contrary to the solar-like stars, unstable in the low-order (nr ≤ 5) p-modes, but stable in the intermediate- and high-order (nr ≥ 12) p-modes. On the red edge of Cepheid (δ Scuti) insta- bility strip, the oscillations of solar-like and Mira-like stars can be explained uniformly by the coupling between convection and oscillations (CCO). For the low-temperature stars on the right side of the instability strip, the CCO is the dominant excitation and damping mechanism for the low- and intermediate-order p-modes, and the stochastic excitation of turbulence becomes important only for the high-order p-modes of solar-like oscillations.  相似文献   

4.
The thermodynamical compling between radial pulsation and convection is studied in accordance with Unno's theory of time-dependent convection. Linear, non-adiabatic calculations are made for 11 series of RR Lyrae models to illuminate the dependence of the red edge of their instability strip on mass, luminosity, helium abundance and the convective parameter C1.  相似文献   

5.
This paper concerns the calibration of the K period–luminosity relation for Mira variables using Hipparcos parallaxes. K magnitudes are available for 255 Mira-like variables which were observed by Hipparcos . Period–luminosity zero-points are evaluated for various subgroups of data. The best solution for oxygen-rich Miras, which uses 180 stars, omitting the short-period red group (which had different kinematics from the short-period blue stars) and the low-amplitude variables, provides a zero-point of     which implies a distance modulus for the Large Magellanic Cloud of     or perhaps slightly greater if a metallicity correction is required, in good agreement with the value derived from Cepheids. The zero-point of the period–luminosity relation for carbon stars is briefly discussed.
Linear diameters are derived for red variables with measured angular diameters and parallaxes, and are used to examine the long-standing question of the pulsation mode(s) of these stars. Evidence is presented to suggest that most of them are pulsating in the same mode and, if published model atmospheres are correct, this is probably the first overtone. Some discussion is given of sequences in the period–luminosity and period–colour diagrams and their bearing on the pulsation mode problem.  相似文献   

6.
The linear nonadiabatic oscillations for evolutionary models of 0.6- 3M8 stars are calculated by using a nonlocal and time-dependent convection theory. The results show that in the HR diagram the pulsation-unstable low- temperature stars on the right side of instability strip can be divided into two groups. One group indicates the Sun-like oscillation stars composed of the main- sequence dwarfs, sub-giants and red giants (RGs) of low and intermediate lu- minosities, which are unstable in the intermediate- and high-order (n ≥ 12) p- modes, and stable in the low-order (n ≤ 5) p-modes. Another group indicates the Mira-like stars composed of the bright RGs and asymptotic giant branch (AGB) stars, which are just contrary to Sun-like stars, unstable in low-order (n ≤ 5) p-modes and stable in the intermediate- and high-order (n ≥ 12) p-modes. The oscillations for the red edge of Cepheid (δ Scuti) instability strip, Sun-like and Mira-like stars can be explained uniformly by the coupling between convection and oscillation (CCO). For the low-temperature stars on the right side of in- stability strip, CCO is the dominant excitation and damping mechanism of the oscillations of low- and intermediate-order p-modes, and the turbulent stochas- tic excitation becomes important only for the high-order p-modes of Sun-like oscillations.  相似文献   

7.
Preliminary results on KUV 02464+3239, a pulsating DA white dwarf are presented. Located near the red edge of the DAV instability strip, KUV 02464+3239 shows large amplitude and long period pulsation modes. Up to now only one mode was known from a 50‐minute‐long light curve. Our more extended observations allowed the identification of three additional frequencies. The presence of previously known harmonics were confirmed and weak subharmonics are also noticeable at some parts of the light curve. This suggests the dominance of nonlinear pulsation effects from time to time. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
We investigate a model for the excitation of high-order oscillations in roAp stars. In this model we assume that the strong concentration of magnetic field about the magnetic poles is enough to suppress convection. Thus the model considered is composed of two polar regions, in which convection is presumed to be suppressed totally, and an equatorial region, where the convection is unaffected. This model is generated by building pairs of locally spherically symmetrical equilibria to represent the polar and equatorial regions of the star, which are patched together below the base of the convection zone. Gravitational settling of heavy elements is taken into account by choosing appropriate chemical composition profiles for both the polar and equatorial regions. Our results indicate that the composite model is unstable against axisymmetric non-radial high-order modes of pulsation that are aligned with the magnetic poles. The oscillations are excited by the κ mechanism acting principally in the hydrogen ionization zones of the polar regions. The effect of the lateral inhomogeneity on the second frequency differences is also investigated; we find that the perturbation to them by the inhomogeneity is of the same order as the second differences themselves, thereby hindering potential attempts to use such differences to identify the degrees of the modes in a straightforward way.  相似文献   

9.
Both linear and nonlinear calculations of the 331 day, long period variable star Mira have been undertaken to see what radial pulsation mode is naturally selected. Models are similar to those considered in the linear nonadiabatic stellar pulsation study of Ostlie and Cox (1986). Models are considered with masses near one solar mass, luminosities between 4000 and 5000 solar luminosities, and effective temperatures of approximately 3000 K. These models have fundamental mode periods that closely match the pulsation period of Mira. The equation of state for the stellar material is given by the Stellingwerf (1975ab) procedure, and the opacity is obtained from a fit by Cahn that matches the low temperature molecular absorption data for the Population I Ross-Aller 1 mixture calculated from the Los Alamos Astrophysical Opacity Library. For the linear study, the Cox, Brownlee, and Eilers (1966) approximation is used for the linear theory variation of the convection luminosity. For the nonlinear work, the method described by Ostlie (1990) and Cox (1990) is followed. Results showing internal details of the radial fundamental and first overtone modes behavior in linear theory are presented. Preliminary radial fundamental mode nonlinear calculations are discussed. The very tentative conclusion is that neither the fundamental or first overtone mode is excluded from being the actual observed one.  相似文献   

10.
We discuss visual observations spanning nearly 70 years of the nearby semiregular variable R Doradus. Using wavelet analysis, we show that the star switches back and forth between two pulsation modes having periods of 332 d and about 175 d, the latter with much smaller amplitude. Comparison with model calculations suggests that the two modes are the first and third radial overtone, with the physical diameter of the star making fundamental-mode pulsation unlikely. The mode changes occur on a time-scale of about 1000 d, which is too rapid to be related to a change in the overall thermal structure of the star and may instead be related to weak chaos.   The Hipparcos distance to R Dor is 62.4 ± 2.8 pc which, taken with its dominant 332-d period, places it exactly on the period–luminosity (P–L) relation of Miras in the Large Magellanic Cloud. Our results imply first-overtone pulsation for all Miras which fall on the P–L relation. We argue that semiregular variables with long periods may largely be a subset of Miras and should be included in studies of Mira behaviour. The semiregulars may contain the immediate evolutionary Mira progenitors, or stars may alternate between periods of semiregular and Mira behaviour.  相似文献   

11.
The star I-I-42 (=vZ1390),a cluster member in M3,located near the red edge of the instability strip of the horizontal branch,was discovered by Roberts and Sandage as a low amplitude variable,it was designated as V204 in the "second catalogue of variable stars in globular clusters",but its coordinates given in all versions of this catalogue are wrong since 1955. We argue that V204 is indeed a low amplitude HB variable star,located near to the red edge of the instability strip,with a period of 0.74785d and an amplitude of about 0.04mag in V . We also find that the red cluster member star I-I-39 is a low amplitude variable with a period of 1.16d and amplitude of about 0.03mag in V which might be pulsating at the second overtone.  相似文献   

12.
We study the possibility of the excitation of non-radial oscillations in classical pulsating stars. The stability of an RR Lyrae model is examined through non-adiabatic non-radial calculations. We also explore stability in the presence of non-linear coupling between radial and non-radial modes of nearly identical frequency.   In our model, a large number of unstable low-degree (ℓ = 1,2) modes have frequencies in the vicinity of unstable radial mode frequencies. The growth rates of such modes, however, are considerably smaller than those of the radial modes. We also recover an earlier result that at higher degrees (ℓ = 6–12) there are modes trapped in the envelope with growth rates similar to those of radial modes.   Subsequently, monomode radial pulsation of this model is considered. The destabilizing effect of the 1:1 resonance between the radial mode and nearby non-radial modes of low degrees is studied, with the assumption that the excited radial mode saturates the linear instability of all other modes. The instability depends on the radial mode amplitude, the frequency difference, the damping rate of the non-radial mode, and the strength of the non-linear coupling between the modes considered. At the pulsation amplitudes typical for RR Lyrae stars, the instability of the monomode radial pulsation and the concomitant resonant excitation of some non-radial oscillation modes is found to be very likely.  相似文献   

13.
G328 = A65 = L3314 (V = 13.83, B-V = 1.91) is a field star in the direction of the globular cluster M4. If we take E(B - V) = 0.40, then its (B - V)0 = 1.51, corresponding to a spectral type of K5III if it is a giant star; or of dM2 if it is a dwarf. Observations at both the MSSSO and Yunnan Observatory have shown that G328 is a new variable with peak to peak amplitude - 0.05 mag in V. While it is not unusual for so red a star to be a variable, special attention must be paid to its short period of about one day. If the variability is due to pulsation, the spectral type and luminosity as well as effective temperature should be determined in order to compare it with Xiong's theory.  相似文献   

14.
熊大闰  邓李才 《天文学报》2011,52(2):115-125
根据非局部和非定常恒星对流理论,计算了质量3～30 M_⊙恒星演化模型低球谐阶(l=1～4)p4～g39非径向模的线性非绝热脉动,研究了对流对非径向脉动的影响.结果表明,对流对恒星非径向脉动稳定性有不可忽略的影响.然而假若不关注单个恒星和单个振动模的细节,从总体上粗略地看,理论的脉动不稳定区的大致位置和范围并未受到对流的明显影响.  相似文献   

15.
Some theoretical calculations of linear non-adiabatic pulsations of intermediate- and low- luminosity red giants in globular clusters have been carried out using a time-dependent theory of nonlocal stellar convection. As shown by the results, for all models with temperatures higher than 5400 K the modes up to the fourth overtone are pulsationally stable. With the increase of stellar luminosity, the low-order overtones also become pulsationally unstable. For red giants of intermediate and low luminosities, the pulsational stability is exceedingly low and is close to neutral stability. Therefore, they will be either non-variables or short-period variables (P < 2 days) with extremely small amplitudes.  相似文献   

16.
We carried out a multicolour time-series photometric study of six stars claimed as 'hybrid' p and g mode pulsators in the literature. γ Peg was confirmed to show short-period oscillations of the β Cep type and simultaneous long-period pulsations typical of Slowly Pulsating B (SPB) stars. From the measured amplitude ratios in the Strömgren uvy passbands, the stronger of the two short period pulsation modes was identified as radial; the second is  ℓ= 1  . Three of the four SPB-type modes are most likely  ℓ= 1  or 2. Comparison with theoretical model calculations suggests that γ Peg is either a  ∼8.5 M⊙  radial fundamental mode pulsator or a  ∼9.6 M⊙  first radial overtone pulsator. HD 8801 was corroborated as a 'hybrid'δ Sct/γ Dor star; four pulsation modes of the γ Dor type were detected, and two modes of the δ Sct type were confirmed. Two pulsational signals between the frequency domains of these two known classes of variables were confirmed and another was newly detected. These are either previously unknown types of pulsation or do not originate from HD 8801. The O-type star HD 13745 showed small-amplitude slow variability on a time-scale of 3.2 d. This object may be related to the suspected new type of supergiant SPB stars, but a rotational origin of its light variations cannot be ruled out at this point. 53 Psc is an SPB star for which two pulsation frequencies were determined and identified with low spherical degree. Small-amplitude variability was formally detected for 53 Ari but is suspected not to be intrinsic. The behaviour of ι Her is consistent with non-variability during our observations, and we could not confirm light variations of the comparison star 34 Psc previously suspected. The use of signal-to-noise criteria in the analysis of data sets with strong aliasing is critically discussed.  相似文献   

17.
By adopting the same approach outlined by De Santis & Cassisi, we evaluate the absolute bolometric magnitude of the zero-age horizontal branch (ZAHB) at the level of the RR Lyrae variable instability strip in selected Galactic globular clusters. This allows us to estimate the ZAHB absolute visual magnitude for these clusters and to investigate its dependence on the cluster metallicity. The derived M V (ZAHB)–[Fe/H] relation, corrected in order to account for the luminosity difference between the ZAHB and the mean RR Lyrae magnitude, has been compared with some of the most recent empirical determinations in this field, such as the one provided by Baade–Wesselink analyses, RR Lyrae periods, Hipparcos data for field variables and main-sequence fitting based on Hipparcos parallaxes for field subdwarfs. As a result, our relation provides a clear support to the 'long' distance scale. We discuss also another method for measuring the distance to Galactic globular clusters. This method is quite similar to the one adopted for estimating the absolute bolometric magnitude of the ZAHB but it relies only on the pulsational properties of the Lyrae variables in each cluster. The reliability and accuracy of this method have been tested by applying it to a sample of globular clusters for which, owing to the morphology of their horizontal branch (HB), the use of the commonly adopted ZAHB fitting is a risky procedure. We notice that the two approaches for deriving the cluster distance modulus provide consistent results when applied to globular clusters, the RR Lyrae instability strip is well populated. As the adopted method relies on theoretical predictions on both the fundamental pulsational equation and the allowed mass range for fundamental pulsators, we give an estimate of the error affecting present results, owing to systematic uncertainties in the adopted theoretical framework.  相似文献   

18.
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   

19.
The S Scuti star catalogue is used to derive the observational locations of such stars on the HR diagram. The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory. The observational instability strip almost overlaps with the theoretical one, but the observed blue and red envelopes are hotter than the theoretical edges. The distribution of S Scuti stars in the pulsation strip is not uniform.  相似文献   

20.
本文采用恒星演化计算与恒星振动计算相结合的方法，对中等质量富金属恒星演化到渐近巨星分支时的振动性质进行了分析研究，从理论上得出这类恒星的振动方式是处于一阶谐频振动，而振动的激发则是在氢电离区和氦的二次电离区由多种机制共同作用造成的，同时提出很长周期的ＡＧＢ长周期变星只能是由中等质量恒星演化到ＡＧＢ阶段形成的。我们的理论计算结果还比较支持在ＡＧＢ顶端存在巨大星风物质损失的观点，且这种星风物质损失很可能与恒星振动有关。  相似文献   

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