arXiv subjects

Ennio Poretti

Publications and source records attributed to Ennio Poretti.

62 records · Page 4Linked to original sources

arXiv1998

The light curves of the short-period variable stars in the Carina dwarf Spheroidal galaxy

The re-analysis of the data reported by Mateo et al. (1998) allowed us to obtain a partially different set of frequencies for the 20 short period variable stars discovered in the Carina dSph galaxy. They are subdivided into 6 overtone-pulsators, 13 fundamental-pulsators; 1 case remains uncertain. The analysis of the light curves was conducted star by star and the first Fourier parameters were obtained for 17 variables. No clear separation between pulsation modes is discernible in the F21-P plot, but the very narrow range of the observed periods hampered us to consider that statement as a definitive one. A more extensive observational study is recommended, since a preliminary comparison with galactic variable stars displays small differences in the F21 values. Owing to the high standard deviation of the fit, the possibility that some stars are double-mode pulsators is also suggested.

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arXiv1998

The measurement of the sky brightness at Merate Observatory

We describe the problems met using the telescopes of Merate Observatory, the branch of Brera Astronomical Observatory. From the point of view of the data analysis, we discuss how to introduce a satisfactory correction of the variable extinction coefficient, giving also some examples. We also measured the sky brightness, obtaining $V$=18.2 mag/arcsec$^2$. The light pollution is responsible for such a bright background and we present pictures showing its effects. We are limiting it trying to persuade public administrations to reduce the light scattered toward the sky by using cutoff lamps and putting out commercial searchlights. The need of a law which safeguards astronomical activities is stressed.

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arXiv1998

CCD Photometry of the High Amplitude Delta Scuti Stars V798 Cyg and V831 Tau

The possibility to perform the frequency analysis of the light variation of V798 Cyg on the basis of a more accurate set of data and the Fourier decomposition of the new light curve of V831 Tau allowed us to improve the knowledge of the HADS. The following points are now better defined: 1) V798 Cyg is confirmed to be a double-mode pulsator and its similarity with V1719 Cyg definitely established: the peculiar light curve on f1 (descending branch steeper than the observed one) and the double-mode pulsation with ratio 0.800 have to be considered together in modelling these stars. 2) The detection of additional periodicities in light curves is confirmed to be a delicate matter. A thorough discussion would be needed to invalidate previous results estalished on the basis of better time--series, as otherwise confusing results are obtained, as in the case of V798 Cyg (Hintz and Joner 1997, PASP 109, 639) 3) The short-period variable V831 Tau shows a light curve very similar to other stars having the same period; the previously reported sinusoidal light curve is not confirmed. The light curves of HADS towards short periods have a very regular progression in the Fourier parameters space. 4) The gap around 0.25-0.30 in the R21 distribution of HADS light curves is confirmed. Hintz and Joner (1997) claimed that it can be filled by the values obtained from double-mode pulsators, but the available photometry does not support that claim which seems the result from improper handling of the data.

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arXiv1998

YY CMi: contact or near contact system?

New V photoelectric observations of the eclipsing system YY CMi, obtained at La Silla, Chile, and Merate Observatory, Italy, are presented. New times of minima and ephemeris based on our observations are also given. The V light curve was analysed by using the WD code to derive the geometrical and physical parameters of the system. Since no spectroscopic mass ratio is available, the q-search method was applied to yield the preliminary range of the mass ratio in order to search for the final solution. First the unspotted solution was carried out by using the unperturbed parts of the light curve and applying the DC program of the WD code. The solution was performed by assuming contact (mode 3) and semi-detached (mode 4) configuration, since no classification of the system is possible from the shape of the light curve. The solution in mode 4 does not lead to an acceptable model, since the secondary was found to be slightly overcontact. Therefore the contact solution was finally adopted. Moreover the light curve peculiarities (Max II fainter than Max I and excess of light around the phase 0.32) were explained by assuming a cool and a hot spot on the surface of the secondary (cooler) component. The degree of contact is very small (f about 3%) and the thermal contactis poor (T1-T2) about 650 K. These results together with the high photometric mass ratio q about 0.89 indicate that YY CMi is very probably a system at the beginning or the end of the contact phase.

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arXiv1997

The frequency content of Double-Mode Cepheids light curves and the importance of the cross-coupling terms

The recent results (Pardo & Poretti 1997, A&A 324, 121; Poretti & Pardo 1997, A&A 324, 133) obtained on the frequency content of Double-Mode Cepheids light curves and the properties of their Fourier parameters are reviewed. Some points briefly discussed in previous papers (no third periodicity, methodological aspects on the true peaks detection, the action of the cross coupling terms and the impact on theoretical models) are described.

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arXiv1997

Stellar Variability in the Lower Part of the Instability Strip

The photometric properties of the variable stars located in the lower part of the classical instability strip are discussed. The importance of the determination of some light curve parameters and their connection with the stellar models are stressed, with a particular emphasis on large amplitude Delta Sct (DSCT) stars.

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arXiv1996

The galactic double-mode Cepheids I. Frequency analysis of the light curves and comparison with single-mode Cepheids

We submitted the available photometric V data of all the known galactic Double Mode Cepheids (DMCs) to a careful frequency analysis with the aim of detecting in each case the importance of the harmonics and of the cross coupling terms. For each object, starting from different data subsets, we progressively built a homogenous set of data, checking the consistency of the results step by step. It was demonstrated that each star displays a different content, showing that no a priori fit can be applied. Up to 4 harmonics were found for the fundamental radial mode (F); in every case, 2 harmonics were found for the first overtone radial mode (1 O). We also proceeded to a preliminar analysis of the Fourier parameters of the DMC light curves and we found a very close similarity between i) the light curves of the Classical Cepheids and those of the F-mode of the DMCs; ii) the light curves of the s-Cepheids and those of the 1 O-mode of the DMCs. The analysis of DMC light curves offers the possibility of unifying the light curves of Classical and s-Cepheids. The case of the unique DMC CO Aur is also discussed.

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arXiv1996

The galactic double-mode Cepheids II. Properties of the generalized phase differences

By considering the least-squares fits of the double-mode Cepheid light curves discussed in Paper I we defined their properties by their Fourier parameters and generalized phase differences $G_{i,j}$. When plotting the latter quantities as a function of the respective order, the second order terms are confined in the region just below 3/2$π$; the third order terms have $π/2 < G_{i,j} < π$, the fourth order ones cluster around 2$π$ (or 0), the fifth order ones seem to have $π< G_{i,j} < 3/2 π$. The mean $G_{i,j}$ values are also regularly spaced. The progression of the $G_{i,j}$ values as a function of the period was investigated and the signature of a possible resonance near 6.0 d was found.

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