F. Perez Hernandez

First View of the Solar Core from Golf Acoustic Modes

After 8 months of nearly continuous measurements the GOLF instrument, aboard SOHO, has detected acoustic mode frequencies of more than 100 modes, extending from 1.4 mHz to 4.9 mHz. In this paper, we compare these results with the best available predictions coming from solar models. To verify the quality of the data, we examine the asymptotic seismic parameters; this confirms the improvements achieved in solar models during the last decade.Using the GOLF set of frequencies for l=0, 1, 2, 3 combined with the LOWL second year data set for l > 3 we then carry out inversions to infer properties of the solar core.This largely confirms the previous results down to around 0.1 R⊙, while there remain differences, even closer to the centre, where the present study shows an extreme sensitivity of the inversion results to the values of the frequencies. We finally consider physical processes which may influence directly or indirectly the solar core structure.

First Results on it p Modes from GOLF Experiment

The GOLF experiment on the SOHO mission aims to study the internal structure of the Sun by measuring the spectrum of global oscillations in the frequency range 10-7 to 10-2 Hz. Here we present the results of the analysis of the first 8 months of data. Special emphasis is put into the frequency determination of the p modes, as well as the splitting in the multiplets due to rotation. For both, we show that the improvement in S/N level with respect to the ground-based networks and other experiments is essential in achieving a very low-degree frequency table with small errors ∼ 2 parts in 10-5). On the other hand, the splitting found seems to favour a solar core which does not rotate slower than its surface. The line widths do agree with theoretical expectations and other observations.

The phase function for stellar acoustic oscillations — IV. Solar-like stars

ABSTRA C T In recent years there has been some progress towards detecting solar-like oscillations in stars. The goal of this challenging project is to analyse frequency spectra similar to that observed for the Sun in integrated light. In this context it is important to investigate what can be learned about the structure and evolution of the stars from such future observations. Here we concentrate on the structure of the upper layers, as reflected in the phase function. We show that it is possible to obtain this function from low-degree p modes, at least for stars on the main sequence. We analyse its dependence on several uncertainties in the structure of the uppermost layers. We also investigate a filtered phase function, which has properties that depend on the layers around the second helium ionization zone.

Influence of the upper layers of the sun on the p-mode frequencies

We consider the effect of uncertainties in solar models on the p-mode frequencies of moderate degree (40 ≤ l ≤ 100). This is done by means of an asymptotic relation that provides a simple way of estimating differences in p-mode frequencies of two solar models. Also, we have investigated the possibility for separating different contributions to the difference between the models, on the basis of the frequencies of the modes. Analysis of observed frequencies suggests that the solar envelope helium abundance Ys is close to 0.25; however, this result may be sensitive to other uncertainties in the solar models.

Multisite Observations of δ Scuti Stars 7 Aql and 8 Aql (a New δ Scuti Variable): The Twelfth STEPHI Campaign in 2003

We present an analysis of the pulsation behavior of the δ Scuti stars 7 Aql (HD 174532) and 8 Aql (HD 174589), a new variable star, observed in the framework of the STEPHI XII campaign during 2003 June and July; 183 hr of high-precision photometry were acquired by using four-channel photometers at three sites on three continents during 21 days. The light curves and amplitude spectra were obtained following a classical scheme of multichannel photometry. Observations in different filters were also obtained and analyzed. Six and three frequencies have been unambiguously detected above a 99% confidence level in the range 190-300 μHz and 100-145 μHz in 7 Aql and 8 Aql, respectively. A comparison of observed and theoretical frequencies shows that 7 Aql and 8 Aql may oscillate with p-modes of low radial orders, typical among δ Scuti stars. In terms of radial oscillations the range of 8 Aql goes from n = 1 to 3, while for 7 Aql the range spans from n = 4 to 7. Nonradial oscillations have to be present in both stars as well. The expected range of excited modes according to a nonadiabatic analysis goes from n = 1 to 6 in both stars.

Frequency dependence of p-mode frequency shifts inducedby magnetic activity in Kepler solar-like stars

The variations of the frequencies of the low-degree acoustic oscillations in the Sun induced by magnetic activity show a dependence with radial order. The frequency shifts are observed to increase towards higher-order modes to reach a maximum of about 0.8 muHz over the 11-yr solar cycle. A comparable frequency dependence is also measured in two other main-sequence solar-like stars, the F-star HD49933, and the young 1-Gyr-old solar analog KIC10644253, although with different amplitudes of the shifts of about 2 muHz and 0.5 muHz respectively. Our objective here is to extend this analysis to stars with different masses, metallicities, and evolutionary stages. From an initial set of 87 Kepler solar-like oscillating stars with already known individual p-mode frequencies, we identify five stars showing frequency shifts that can be considered reliable using selection criteria based on Monte Carlo simulations and on the photospheric magnetic activity proxy Sph. The frequency dependence of the frequency shifts of four of these stars could be measured for the l=0 and l=1 modes individually. Given the quality of the data, the results could indicate that a different physical source of perturbation than in the Sun is dominating in this sample of solar-like stars.

The Solar Rotation Rate Ω(R, θ)

In recent years, the capability to detect and analyse solar oscillation acoustic modes has greatly improved. The development of ground based networks like GONG or BiSON and the use of space platforms like SOHO has allowed us to study the structure and dynamics of the Sun with unprecedented precision.In this work we explore the distribution of the rotation in the solar interior, specially in the core. This kind of information is essential in order to study the physical processes involved in the evolution of the Sun and many other stars.

Diurnal photometric conditions at Teide observatory and long-term solar irradiance variations

Monochromatic extinction coefficients at four wavelengths have been obtained over a period of more than two years at the Observatorio del Teide (Izaña Tenerife) using a full disc, direct sunlight, quadruple photometer devoted to the detection of integral luminosity oscillations of the Sun. The mean extinction coefficients (0.13 at 500 nm) show a seasonal variation of about 15%, the best atmospheric conditions being in winter and autumn. Moreover, in anyone day the extinction coefficient in the afternoon is always lower than the one in the morning by ∼ 7%. A one-year period fluctuation, with an amplitude of ∼ 0.035 mag, has been identified in the instrumental magnitudes outside the atmosphere, and is interpreted as the variation produced by the different Sun-Earth distance from winter to summer. Finally, the study made to detect periodic time fluctuations in both, Suns magnitude and extinction coefficients, has given null results at levels of ∼ 0.04 and ∼ 1.8%, respectively.

Asteroseismology in Praesepe: the case of EP Cnc

On the 1992 STEPHI IV campaign, the multi-periodic δ Scuti stars BN Cnc and BU Cnc of the Praesepe cluster were monitored during a three-week, three-continental run (Belmonte et al. 1994). Five and six frequency peaks, respectively, were recorded as a result of the campaign. In a subsequent paper (Perez Hernandez et al. 1995, hereafter Paper I), an attempt to identify these frequency peaks as pulsation modes was done. An important point is that both stars belong to the same cluster and, hence, more severe constraints than for isolated stars can be imposed. Recently, Breger et al. (1994) have reported results on EP Cnc, another δ Scuti in Praesepe. This star is also multi-periodic, showing three frequency peaks above the noise level. In the present work we extend our analysis to this star.

Acoustic oscillations in main-sequence stars: HD155543

High-speed photometric techniques have been found useful as a way to study the acoustic mode signature in low main sequence stars. In this work, the discovery of solar-like oscillations associated to the presence of acoustic modes of pulsation in the F2V star HD155543, located outside of the instability strip, is reported. This finding has been obtained through an analysis of a long series of data (184 hours) obtained in 20 nights of observation with two twin three channel photometers attached to two 1.5 m telescopes sited at two observatories, simultaneously: Teide (OT) at Tenerife (Spain) and San Pedro Martir (SPM) at Baja California Norte (Mexico). The major results yielded have been: the range of frequencies where p-modes signal is present (1 to 3 mHz); an upper limit of 20 µmag for the amplitude of the modes; the mean spacing between modes of equal degree l and consecutive order n, Δvo = 97.3 ± 0.6 µHz and two possible values of Do, 1.4 or 1.8 µHz. The values of these parameters agree, within the resolution, with those yielded by standard computed models of main sequence stars compatible with the luminosity and effective temperature already known for HD155543. These results open new perspectives for astero-seismology in the near future.