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1.
We have used time-resolved spectroscopy to measure the colour dependence of pulsation amplitudes in the DAV white dwarf G29-38. Model atmospheres predict that mode amplitudes should change with wavelength in a manner that depends on the spherical harmonic degree ℓ of the mode. This dependence arises from the convolution of mode geometry with wavelength-dependent limb darkening. Our analysis of the six largest normal modes detected in Keck observations of G29-38 reveals one mode with a colour dependence different from the other five, permitting us to identify the ℓ-value of all six modes and to test the model predictions. The Keck observations also show pulsation amplitudes that are unexpectedly asymmetric within absorption lines. We show that these asymmetries arise from surface motions associated with the non-radial pulsations (which are discussed in detail in a companion paper). By incorporating surface velocity fields into line profile calculations, we are able to produce models that more closely resemble the observations.  相似文献   

2.
We analyse a series of line profile observations of the He  i 6678 line in ζ Oph. A period analysis on these data using the mode and moments of the line profile confirms the two previously known periods. We describe a new method of mode identification for pulsating stars in which the calculated profiles are directly fitted to observed profiles. The method yields the full set of pulsational parameters including the spherical harmonic degree, ℓ, and azimuthal number, m . Application of the method to these data confirms the mode identifications previously suggested for the two periodicities. We find that the derived pulsational parameters are physically realistic and conclude that non-radial pulsation is the most likely explanation for the travelling subfeatures. However, a unique mode identification is still not possible – several non-sectorial modes fit the data as well as the usually adopted sectorial identifications. The predicted photometric amplitudes are in good accord with upper limits derived from photometric observations. We conclude that ζ Oph is a star in the β Cep instability strip in which two modes of high degree (probably ℓ=4 and ℓ=8) are excited. We present an interpretation of these findings in which the cause of the low-order line profile and light variations in periodic Be stars is corotating photospheric clouds, while the travelling subfeatures are incidental to the Be phenomenon and are a result of non-radial pulsation.  相似文献   

3.
We undertook a time-series photometric multisite campaign for the rapidly oscillating Ap (roAp) star HD 99563 and also acquired mean light observations over four seasons. The pulsations of the star, which show flatter light maxima than minima, can be described with a frequency quintuplet centred on 1557.653 μHz and some first harmonics of it. The amplitude of the pulsation is modulated with the rotation period of the star that we determine with 2.91179 ± 0.00007 d from the analysis of the stellar pulsation spectrum and of the mean light data. We break up the distorted oscillation mode into its pure spherical harmonic components and find it is dominated by the ℓ= 1 pulsation, and also has a notable ℓ= 3 contribution, with weak ℓ= 0 and 2 components. The geometrical configuration of the star allows us to see both pulsation poles for about the same amount of time; HD 99563 is only the fourth roAp star for which both pulsation poles are seen and only the third where the distortion of the pulsation modes has been modelled. We point out that HD 99563 is very similar to the well-studied roAp star HR 3831. Finally, we note that the visual companion of HD 99563 is located in the δ Scuti instability strip and may thus show pulsation. We show that if the companion was physical, the roAp star would be a 2.03-M⊙, object, seen at a rotational inclination of 44°, which then predicts a magnetic obliquity     .  相似文献   

4.
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   

5.
We present simultaneous UBVRI photometry and high-dispersion spectroscopy of the δ Scuti star QQ Tel. At least seven periodicities are detected in the light curve, but there are likely to be many more. The line profile variations suggest that some of the observed frequencies may be due to modes of moderately high degree  (ℓ≳4)  .  相似文献   

6.
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.  相似文献   

7.
We have carried out a three-site photometric campaign for the β Cephei star θ Oph from 2003 April to August. 245 h of differential photoelectric u v y photometry were obtained during 77 clear nights. The frequency analysis of our measurements has resulted in the detection of seven pulsation modes within a narrow frequency interval between 7.116 and 7.973 c d−1. No combination or harmonic frequencies have been found. We have performed a mode identification of the individual pulsations from our colour photometry that shows the presence of one radial mode, one rotationally split  ℓ= 1  triplet and possibly three components of a rotationally split  ℓ= 2  quintuplet. We discuss the implications of our findings and point out the similarity of the pulsation spectrum of θ Oph to that of another β Cephei star, V836 Cen.  相似文献   

8.
The stellar surface imaging technique is used for studying stellar non‐radial pulsations on the basis of inversions of time series of variable line profiles without making assumptions on the specific shape of the pulsations. The inversion results in an image of the stellar surface in which sectoral and tesseral modes can be distinguished in many cases and the pulsational degree and the azimuthal order can be determined. The capability of the technique is studied with simulated data. Then, the surface imaging technique is applied to high‐resolution spectra of the rapidly rotating Beta Cep‐type star ω1 Sco, which shows strong line‐profile variations. Stellar surface imaging is concluded to be a useful technique for pulsation‐mode identification. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree,     the azimuthal order, m , of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of     for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably         We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the     mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star.  相似文献   

10.
We present time-series spectroscopy of the rapidly oscillating Ap (roAp) star HR 3831. This star has a dominant pulsation period of 11.7 min and a rotation period of 2.85 d. We have analysed 1400 intermediate-resolution spectra of the wavelength region 6100–7100 Å obtained over one week, using techniques similar to those we applied to another roAp star, α  Cir.
We confirm that the H α velocity amplitude of HR 3831 is modulated with rotation phase. Such a modulation was predicted by the oblique pulsator model, and rules out the spotted pulsator model. However, further analysis of H α and other lines reveals rotational modulations that cannot easily be explained using the oblique pulsator model. In particular, the phase of the pulsation as measured by the width of the H α line varies with height in the line.
The variation of the H α bisector shows a very similar pattern to that observed in α Cir, which we have previously attributed to a radial node in the stellar atmosphere. However, the striking similarities between the two stars, despite the much shorter period of α Cir (6.8 min), argues against this interpretation unless the structure of the atmosphere is somewhat different between the two stars. Alternatively, the bisector variation is a signature of the degree ℓ of the mode and not the overtone value n .
High-resolution studies of the metal lines in roAp stars are needed to understand fully the form of the pulsation in the atmosphere.  相似文献   

11.
HD 23194, a member of the Pleiades, was found to pulsate with a period of about 30 min. The literature on the star is reviewed, and it is concluded that it may be a marginal Am star in a binary system. HD 95321 is an evolved Am ( ρ Puppis) star with a 5.1-h periodicity. Mode identification of its pulsation, based on multicolour photometry, suggests that the oscillation is probably non-radial with ℓ=2. We also report on the discovery of six other new δ Scuti stars, some of which may be pulsating in gravity modes.  相似文献   

12.
The pulsating DA white dwarfs are the coolest degenerate stars that undergo self-driven oscillations. Understanding their interior structure will help us to understand the previous evolution of the star. To this end, we report the analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA white dwarf star EC 14012−1446 acquired during four observing epochs in three different years, including a coordinated three-site campaign. A total of 19 independent frequencies in the star's light variations together with 148 combination signals up to fifth order could be detected. We are unable to obtain the period spacing of the normal modes and therefore a mass estimate of the star, but we infer a fairly short rotation period of  0.61 ±0.03 d  , assuming the rotationally split modes are  ℓ= 1  . The pulsation modes of the star undergo amplitude and frequency variations, in the sense that modes with higher radial overtone show more pronounced variability and that amplitude changes are always accompanied by frequency variations. Most of the second-order combination frequencies detected have amplitudes that are a function of their parent mode amplitudes, but we found a few cases of possible resonantly excited modes. We point out the complications in the analysis and interpretation of data sets of pulsating white dwarfs that are affected by combination frequencies of the form   f A + f B − f C   intruding into the frequency range of the independent modes.  相似文献   

13.
We present and discuss 247 high dispersion échelle spectra of the δ Scuti star 38 o 1 Eri. We find at least three periods, but all are affected by aliasing problems. We attempt to identify the modes by a technique involving both temporal and spatial pre-whitening. All modes are of high degree, probably ℓ≈4 and ℓ≈7.  相似文献   

14.
A multisite campaign of BI CMi was carried out with excellent frequency resolution and high photometric accuracy from 1997 to 2000, including two long observing seasons. 29 pulsation frequencies could be extracted from the 1024 h (177 nights) of photometry used. The detected frequencies include 20 pulsation modes in the main pulsation frequency range from 4.8 to 13.0 cycle d−1 (55 to 150 μHz), eight linear combinations of these frequencies, and a very low frequency at 1.66 cycle d−1. Since the value of the low frequency at 1.66 cycle d−1 cannot be identified with a linear combination of other frequencies, g-mode pulsation is suspected, but rotational modulation of abundance spots cannot be ruled out. BI CMi, which is situated near the cool edge of the classical instability strip, may be both a δ Scuti and a γ Doradus star. Another outstanding property of BI CMi is the presence of a number of close frequency pairs in the power spectrum with separations as small as 0.01 cycle d−1.
A rotational velocity of     was determined from a high-dispersion spectrum. From phase differences, the dominant modes can be identified with ℓ values from 0 to 2. The spectral type and evolutionary status of BI CMi are examined.  相似文献   

15.
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.  相似文献   

16.
We present an analysis of high-speed spectroscopy of the pulsating subdwarf B star PG 1605+072. Periodic radial motions are detected at frequencies similar to those reported for photometric variations in the star, with amplitudes of up to 6 km s−1. Differences between relative strengths for given frequency peaks for our velocity data and previously measured photometry are probably a result of shifting of power between modes over time. Small differences in the detected frequencies may also indicate mode-shifting. We report the detection of line-shape variations using the moments of the cross-correlation function profiles. It may be possible to use the moments to identify the pulsation modes of the star.  相似文献   

17.
Only a fraction of the theoretically predicted non-radial pulsation modes have so far been observed in δ Scuti stars. Nevertheless, the large number of frequencies detected in recent photometric studies of selected δ Scuti stars allow us to look for regularities in the frequency spacing of modes. Mode identifications are used to interpret these results.
Statistical analyses of several δ Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited non-radial modes cluster around the frequencies of the radial modes over many radial orders.
The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Królikowska and shown to be efficient for  ℓ= 1  modes. New pulsation model calculations confirm the observed regularities.
We present the s – f diagram, which compares the average separation of the radial frequencies ( s ) with the frequency of the lowest frequency unstable radial mode ( f ). This provides an estimate for the  log  g   value of the observed star, if we assume that the centres of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied δ Scuti variables in which radial modes were definitely identified.  相似文献   

18.
We have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure phase with the Integrated Optics Near-Infrared Camera (IONIC) beam combiner, at the infrared optical telescope array interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-year period found in the radial velocity measurements for this star. We can model the time-dependent asymmetry either as the orbit of a low-mass companion around the M giant or as an asymmetric, 20 per cent change in brightness across the M giant. We do not detect a change in the size of the star during a 3-year monitoring period neither with respect to time nor with respect to wavelength. We find a spherical dust shell with an emission size of 2.2 ± 0.1 D * full width at half-maximum around the M giant star. The star to dust flux ratio is estimated to be 11.63 ± 0.3. While the most likely explanation for the 20 per cent change in brightness is non-radial pulsation, we argue that a low-mass companion in close orbit could be the physical cause of the pulsation. The combined effect of pulsation and low-mass companion could explain the behaviour revealed by the radial velocity curves and the time-dependent asymmetry detected in the closure-phase data. If CH Cyg is a typical long secondary period variable then these variations could be explained by the effect of an orbiting low-mass companion on the primary star.  相似文献   

19.
A global analysis of the surface trapping of low-frequency non-radial g modes in rotating early-type stars is undertaken within the Cowling, adiabatic and traditional approximations. The dimensionless pulsation equations governing these modes are reviewed, and the boundary conditions necessary for solution of the equations are considered; in particular, an outer mechanical boundary condition, which does not enforce complete wave trapping at the stellar surface, is derived and discussed in detail. The pulsation equations are solved for a 7-M⊙ model star over a range of rotation rates, using a numerical approach.
The results of the calculations confirm the findings of the preceding paper in the series: modes with eigenfrequencies below a cut-off cannot be fully trapped within the star, and exhibit leakage in the form of outwardly propagating waves at the surface. The damping rates resulting from leakage are calculated for such 'virtual' modes, and found to be appreciably larger than typical growth rates associated with opacity-driven pulsation. Furthermore, it is demonstrated that the surface perturbations generated by virtual modes are significantly changed from those caused by fully trapped modes; the latter result suggests differences in the line-profile variations exhibited by these two types of mode.
The findings are discussed in the context of the 53 Per, SPB and pulsating Be classes of variable star. Whilst wave leakage will probably not occur for overstable g modes in the 53 Per and slowly rotating SPB stars, the adoption of the new outer mechanical boundary condition may still affect the pulsational stability of these systems. Wave leakage for overstable modes remains a possibility in Be stars and the more rapidly rotating SPB stars.  相似文献   

20.
The central part of the Cygnus OB2 association was searched for the presence of short-period hot pulsators. Out of 16 O-type stars in the field, five turned out to be variable. None of the O-type variables showed short-period variations characteristic for the β Cephei-type stars. However, two β Cephei stars were found among the early B-type members of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discrepancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour–magnitude diagrams of the association.   We found 29 new variables in all fields searched by us. About half of these are Cygnus OB2 members. By means of Hα photometry, we confirmed the presence of Hα emission in two stars and found one new Be star. A weak Hα emission is probably present in all observed O-type supergiants and giants. We also give the new BVRI Hα photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour–magnitude diagram for the association and make a reddening map of the observed field. The extinction in the field, expressed as the total absorption in the Johnson V filter, A v , ranges from 4.1 to 7.0 mag; the average A V amounts to 5.8 mag. One O-type star is probably a background object.  相似文献   

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