DA型脉动白矮星G255—2的双站联合观测和初步的星震学分析

介绍中法双方分别于１９９５年对ＤＡ型脉动白矮星Ｇ２５５－２进行的两次高精度的光电测光联合观测,在仔细的数据处理之后 进行了多重频率脉动分析,得到了该星的本征脉动模式,同时,结合ＤＡ型脉动白矮星的理论模型进行了初步的星震学分析。     

空间星震学观测—国外项目与中国方案

介绍了恒星星震学研究的内容和目的以及国际上该领域的研究和最新进展，阐述了空间星震学观测的科学意义；在对国外空间星震学观测项目进行综述之后，提出了一个中国空间星震学观测方案。     

Surface motion in the pulsating DA white dwarf G29‐38

We present time-resolved spectrophotometry of the pulsating DA white dwarf G29-38. As in previous broad-band photometry, the light curve shows the presence of a large number of periodicities. Many of these are combination frequencies, i.e. periodicities occurring at frequencies that are sums or differences of frequencies of stronger, real modes. We identify at least six real modes, and at least five combination frequencies. We measure line-of-sight velocities for our spectra and detect periodic variations at the frequencies of five of the six real modes, with amplitudes of up to 5 km s⁻¹. We argue that these variations reflect the horizontal surface motion associated with the g-mode pulsations. No velocity signals are detected at any of the combination frequencies, confirming that the flux variations at these frequencies do not reflect physical pulsation, but rather reflect mixing of frequencies owing to a non-linear transformation in the outer layers of the star. We discuss the amplitude ratios and phase differences found for the velocity and light variations, as well as those found for the real modes and their combination frequencies, both in a model-independent way and in the context of models based on the convective-driving mechanism. In a companion paper, we use the wavelength dependence of the amplitudes of the modes to infer their spherical degree.     

Asteroseismology of DAV white dwarf stars and G29-38

Asteroseismology is a powerful tool used for detecting the inner structure of stars, which is also widely used to study white dwarfs. We discuss the asteroseismology of DAV stars. The period-to-period fitting method is discussed in detail, including its reliability in detecting the inner structure of DAV stars. If we assume that all observed modes of some DAV stars are the l = I cases, the errors associated with model fitting will be always large. If we assume that the observed modes are com- posed of I = 1 and 2 modes, the errors associated with model fitting in this case will be small. However, there will be modes identified as l = 2 that do not have ob- served quintuplets. G29-38 has been observed spectroscopically and photometrically for many years. Thompson et al. made 1 modes identifications in the star through the limb darkening effect. With 11 known I modes, we also study the asteroseismology of G29-38, which reduces the blind l fittings and is a fair choice. Unfortunately, our two best-fitting models are not in line with the previous atmospheric results. Based on factors like only a few observed modes, stability and identification of eigenmodes, identification of spherical degrees, construction of physical and realistic models and so on, detecting the inner structure of DAV stars by asteroseismology needs further development.     

Evolutionary Timescale of the Pulsating White Dwarf G117-B15A: The Most Stable Optical Clock Known

We observe G117-B15A, the most precise optical clock known, to measure the rate of change of the main pulsation period of this blue-edge DAV white dwarf. Even though the obtained value is only within 1 sigma, P&d2;=&parl0;2.3+/-1.4&parr0;x10-15 s s-1, it is already constraining the evolutionary timescale of this cooling white dwarf star.     

Deciphering the pulsations of G 29-38 with optical time series spectroscopy

We present optical time series spectroscopy of the pulsating white dwarf star G 29-38 taken at the Very Large Telescope (VLT). By measuring the variations in brightness, Doppler shift and line shape of each spectrum, we explore the physics of pulsation and measure the spherical degree (ℓ) of each stellar pulsation mode. We measure the physical motion of the g modes correlated with the brightness variations for three of the eight pulsation modes in this data set. The varying line shape reveals the spherical degree of the pulsations, an important quantity for properly modelling the interior of the star with asteroseismology. Performing fits to the Hβ, Hγ and Hδ lines, we quantify the changing shape of the line and compare them to models and previous time series spectroscopy of G 29-38. These VLT data confirm several ℓ identifications and add four new values, including an additional ℓ= 2 and a possible ℓ= 4. In total, from both sets of spectroscopy of G 29-38, eleven modes now have known spherical degrees.     

Non-linear behaviour of the pulsating white dwarf G29-38 – III. Relative amplitudes of the cross-frequencies

In each season when the DA pulsating white dwarf G29-38 has been observed, its period spectrum appears very different, but it always contains a forest of harmonics and cross-frequencies. The ratio of the amplitude of these non-linear frequencies A _c to the product of the amplitudes of the corresponding parent modes A ₁ A ₂ has been measured. The results are compared with the predictions given by three existing theoretical models. Our analysis shows that the non-linear frequencies present in the period spectrum of G29-38 owe their presence mostly to the inelastic response of the stellar medium to the perturbation travelling through it, rather than to the non-linear response of the emergent luminous flux to the surface temperature variation. This analysis also confirms that most identified modes are ℓ=1, as previously asserted by Kleinman et al.     

Non-linear behaviour of the pulsating white dwarf G29-38 – I. Evolution of the temporal spectrum

An analysis of the evolution of the amplitude spectrum over many seasons of the DA pulsating white dwarf G29-38 has been performed. Neither beating nor resonant mode coupling can account for the observed appearance and disappearance of modes, although some of them clearly grow while others get damped. Therefore some unknown non-adiabatic, non-linear process has to be invoked that affects both the mode selection mechanism and the driving efficiency on a time-scale as short as a day.     

Non-linear behaviour of the pulsating white dwarf G29-38 – II. Analysis of the phase spectrum

An analysis of the phase spectrum of the DA pulsating white dwarf G29-38 over many seasons has been performed. This complements a companion paper which analyses the evolution of the temporal spectrum of G29-38.
We show that, with one exception, the relative phases of the harmonics and combination frequencies are all oscillating in phase with their parent modes. This not only suggests that these non-linear frequencies owe their presence to harmonic distortion and not to resonant mode coupling, but also explains the typical pulse shapes observed in the light curves of large-amplitude variable white dwarfs.
The one exceptional cross-frequency that does not show a phasing with its parent modes is thought to be a resonance.     

白噪声中f~(-2)随机过程的最优估计

从功率谱的角度分析了白噪声中f-2随机过程的最优估计问题，推导了线性估计方法的精度极限，并通过对若干线性估计方法的精度进行分析和比较，得到了一些有意义的结论。     

Evidence for the stratification of Fe in the photosphere of G191-B2B

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-local thermodynamic equilibrium model atmospheres with improved atomic data has allowed self-consistent analysis of the extreme ultraviolet (EUV), far UV and optical spectra of the prototypical object G191-B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below ≈190 Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191-B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions, particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass-loss in G191-B2B.