V. Costa

Detection of 75+ pulsation frequencies in the delta Scuti star FG Virginis

Extensive photometric multisite campaigns of the 6 Scuti variable FG Vir are presented. For the years 2003 and 2004, 926 h of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed. quencies. This represents a new Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the missing modes may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals confirms the theoretical prediction of a large number of excited modes. FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d (m = 0) with other modes of relatively high photometric amplitude. The amplitudes of the frequency sums are higher than those of the difference;. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order m = -1 is coupled with two other modes of m = + 1.

Detection of 75+ pulsation frequencies in the Delta Scuti star FG Vir

Extensive photometric multisite campaigns of the 6 Scuti variable FG Vir are presented. For the years 2003 and 2004, 926 h of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed. quencies. This represents a new Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the missing modes may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals confirms the theoretical prediction of a large number of excited modes. FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d (m = 0) with other modes of relatively high photometric amplitude. The amplitudes of the frequency sums are higher than those of the difference;. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order m = -1 is coupled with two other modes of m = + 1.

The frequency ratio method and the new multiperiodic gamma Doradus star HD 218427

gamma Dor-type oscillations have been discovered in the star HD218427 through simultaneous uvby photometric observations carried out in the year 2003. A few Hbeta-Crawford measurements were also collected for calibration purposes which place this star well-located inside the gamma Dor instability region. Deficiency in metal content, similar to other well-defined gamma Dor stars, is found in HD218427 and the possibility of a lambda Boo nature is discussed. Frequency analysis was carried out for different filters, the combined vby filter was also used and five frequencies were found as significant with periods ranging between 0.3 and 0.8 days. The recently developed Frequency Ratio Method is used in order to perform an identification of the excited modes. The results are consistent with an l=2 identification for all the modes and high radial quantum numbers (n~40) for the three main observed periodicities. The possibility of multiplet structures is also discussed. However, no consistency is found when the Time-Dependent Convection treatment is used for modes discrimination. The disagreement can be due to the large rotation velocity taking place in HD218427 and, consequently, the significant coupling between the modes.Oscillations of γ Dor-type were discovered in the star HD 218427 through simultaneous uvby photometric observations carried out in the year 2003. A few Hβ-Crawford measurements were also collected for calibration purposes and they locate this star well inside the γ Dor instability region. We find HD 218427 to be deficient in metals, similar to other well-defined γ Dor stars, and discuss the possibility that it has a λ Boo nature. We carried out frequency analysis for different filters, including the combined “vby” filter, and five frequencies were found as significant with periods ranging between 0.3 and 0.8 days. The recently-developed frequency ratio method is used in order to identify the excited modes. The results are consistent with an l = 2 identification for all the modes and with high radial quantum numbers (n ∼ 40) for the three main observed periodicities. The possibility of multiplet structures is also discussed. However, no consistency is found when using the time-dependent convection treatment to discriminate modes. This disagreement can be due to the large rotation velocity taking place in HD 218427 and, consequently, the significant coupling between the modes.

Asteroseismology of the new multiperiodic

The variability of HDu2009239276 was suspected photometrically nearly twenty years ago, but was confirmed with new observations obtained in 2001 during a two-site photometric campaign carried out from Spain, in uvby β Stromgren-Crawford photometry, and China, using the Johnsonxa0 V xa0filter. Two low-dispersion spectra were also collected. The results establish this star as a new multiperiodic γ xa0Dor-type pulsator with deficiency in metallicity. Its possible λ xa0Boo nature is discussed. The frequency analysis shows three pulsational frequencies as significant, but some more are probably present among the residuals. The method based on phase shifts and amplitude ratios in multicolour photometry is used to identify the excited modes with non-adiabatic time-dependent convection models. A very good agreement between the theoretical and observed amplitude ratios is obtained and the two main modes are identified as

\gamma

l=1

Dor variable HD 239276

modes. Nevertheless, our results do not allow us to discriminate between a solar abundance and a metal deficient nature for this star. The frequency ratio method is further used for the identification of the modes. The results suggest low metallicity for this star, but a λ xa0Boo nature may be not ruled out.

The delta Scuti star FG Vir. V. The 2002 photometric multisite campaign

A high-accuracy multisite campaign was carried out from 2002 January to May with a photometric coverage of 398 h at five observatories. The concentration on a few selected sites gives better consistency and accuracy than collecting smaller amounts from a larger number of sites. 23 frequencies were detected with a high statistical significance. 6 of these are new. The 17 frequencies found in common with the 1992-1995 data are the modes with highest amplitudes. This indicates that the pulsation spectrum of FG Vir is relatively stable over the ten-year period. Two frequencies have variable amplitudes and phases from year to year as well as during 2002. These were both found to be double modes with close frequencies. For the mode at 12.15 c/d this leads to an apparent modulation with a time scale of ∼ 129 d. The close frequencies at 12.15 c/d are composed of a radial and a nonradial mode, suggesting a similarity with the Blazhko Effect seen in RR Lyrae stars.

Asteroseismology with the WIRE satellite - I. Combining ground- and space-based photometry of the δ Scuti star ϵ Cephei

Aims. We have analysed ground-based multi-colour Stromgren photometry and single-filter photometry from the star tracker on the WIRE satellite of the 6 Scuti star ∈ Cephei. Methods. The ground-based data set consists of 16 nights of data collected over 164 days, while the satellite data are nearly continuous coverage of the star during 14 days. The spectral window and noise level of the satellite data are superior to the ground-based data and this data set is used to locate the frequencies. However, we can use the ground-based data to improve the accuracy of the frequencies due to the much longer time baseline. Results. We detect 26 oscillation frequencies in the WIRE data set, but only some of these can be seen clearly in the ground-based data. We have used the multi-colour ground-based photometry to determine amplitude and phase differences in the Stromgren b - y colour and the y filter in an attempt to identify the radial degree of the oscillation frequencies. We conclude that the accuracies of the amplitudes and phases are not sufficient to constrain theoretical models of e Cep. We find no evidence for rotational splitting or the large separation among the frequencies detected in the WIRE data set. Conclusions. To be able to identify oscillation frequencies in 6 Scuti stars with the method we have applied, it is crucial to obtain more complete coverage from multi-site campaigns with a long time baseline and in multiple filters. This is important when planning photometric and spectroscopic ground-based support for future satellite missions like COROT and KEPLER.

Multiperiodicity in the newly discovered mid-late Be star V2104 Cygni

Aims. We obtained the first long, homogenous time-series of V2104 Cyg, consisting of 679 datapoints, with the uvbyβ photometers of the Sierra Nevada and San Pedro Martir Observatories. Our aim was to detect and subsequently interpret the intrinsic frequencies of this previously unstudied variable star, which turned out to be a Be star. We evaluate its place among the variable B stars on the upper Main Sequence. To obtain additional information on physical parameters we collected a few spectra with the Elodie and FIES instruments. Methods. We searched for frequencies in the uvby passbands using 2 different frequency analysis methods and used the S /N > 4 criterion to select the significant periodicities. We obtained an estimate of the physical parameters of the underlying B star of spectral type between B5 and B7, by correcting for the presence of a circumstellar disk using a formalism based on the strength of the Hα line emission. Results. We detected 3 independent frequencies with amplitudes below 0.01 mag, ν1 = 4.7126 d −1 , ν2 = 2.2342 d −1 and ν3 = 4.671 d −1 , and discovered that V2104 Cyg is a Be star. The fast rotation (v sini = 290 ± 10 km s −1 , and 27 ◦ < i < 45 ◦ ) hampered the investigation of the associated pulsational parameters � . Conclusions. The most plausible explanation for the observed variability of this mid-late type Be star is a non-radial pulsation model.

Searching for signatures of stochastic excitation in stellar pulsations : a look at γ doradus stars

In stellar oscillations, the temporal variation in the amplitude of a given mode can yield some information about the interior of the star. In what concerns the excitation mechanism, it has been shown that for oscillations that are excited and damped by a physical process in stochastic equilibrium, the ratio of the standard deviation of the amplitude σ A over the amplitude mean value μ A is of the order of 0.52. This statistical signature is a general property of stochastically excited oscillations, so it can be used with any type of star - making it a powerful tool. Although the method is simple, its reliability and robustness have yet to be proven. With the help of simulations, these points are addressed in the present work, and the observational requirements for this method to work are more clearly defined. We show that a number of amplitude measurements of about 10 2 are required to get a good estimate of (σ A /μ A . The method is also tested against the complication of having a time series crowded with many modes (unresolvable in short time series), and results show that the theoretical relation of σ A /Pμ A still holds true. Some of the motivation behind the new tests of the method involves applying it to y Doradus stars. The open question of what drives oscillations in these stars makes this statistical method very interesting when applied to y Doradus stars. A test of the method is performed using y Doradus observations. As expected, the results are inconclusive and demonstrate how ground-based observations of y Doradus stars hardly fulfill the applicability requirements of the method. We show, however, how a possible scenario of stochastic excitation in these stars could be detected with the COROT space mission.