arXiv subjects

R. Silvotti

Publications and source records attributed to R. Silvotti.

58 records · Page 4Linked to original sources

arXiv1999

Time-resolved HST and IUE UV spectroscopy of the Intermediate Polar FO Aqr

Time resolved spectroscopy of the Intermediate Polar FO Aqr reveals the presence of multiple periodicities in the UV range. A strong orbital modulation dominates both continuum and emission line flux variabilities, while line velocity motions are only detected at the rotational frequency. A prominent orbital periodicity is also observed in coordinated optical photometry, where FO Aqr was previously found to be spin dominated. The spectral dependence of the main periodicities shows the presence of multi-temperature components in FO Aqr and for the first time a hot and a cool component in the rotational modulation. From a comparison with previous UV and optical data obtained in 1990, no spectral variations in the orbital and rotational variabilities are detected, indicating no significant changes in the effects of X-ray illumination but rather a shrinking of the accretion curtain accompained by an increase in size of the thickened part of the accretion disc. These observations, consistent with the recently discovered long term trend in the X-ray pulsation amplitudes, independently confirm a change in the accretion mode in FO Aqr, which switched from a disc-fed into a disc-overflow state, likely triggered by mass accretion variations.

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arXiv1998

The photometric behaviour of the peculiar PG1159 star HS2324+3944 at high frequency resolution

We present the results from 135 hours of nearly continuous time series photometry on the ``hybrid'' (H-rich) PG1159 variable star HS2324+3944, obtained in August-September 1997. The power spectrum of the data shows several frequencies (about 20 or more), concentrated in three narrow and very crowded regions near 475, 390 and 950 microHz in decreasing amplitude order. Most (if not all) of the peaks in the latter region are linear combinations of the high-amplitude frequencies between 455 and 500 microHz. If we divide the data set into two equal parts, the power spectra are different. This is probably due to a not sufficiently long (and therefore not completely resolved) light curve; nevertheless an alternative hypothesis of a single damped oscillator may not be completely ruled out. If we adopt the first hypothesis, the high concentration of peaks between 455 and 500 microHz suggests the presence of both l=1 and l=2 high-overtone nonradial g-modes. The insufficient frequency resolution of our data does not allow to obtain definite precision asteroseismology results. Nevertheless a spacing of the signals is observed, probably due to stellar rotation with a period of 2.3 days. If the signal spacing was due to the successive overtones, the period spacings would be equal to 18.8 (l=1) and 10.4 (l=2) s.

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arXiv1996

HS2324+3944: discovery of non-radial pulsations in a hydrogen-rich PG1159 star

HS\,2324+3944 is a peculiar PG\,1159 star, with a high amount of H in its atmosphere (Dreizler et al. 1995). Its location in the $\log T_{\rm eff}$ -- $\log g$ plane is well inside the GW Vir instability strip. In this paper I report the results of two photoelectric observations of HS\,2324+39, which clearly show that the luminosity of this star presents periodical variations with a period of (2140 $\pm$ 11) s. This photometric behaviour is most easily explained by the presence of non-radial oscillations. Therefore HS\,2324+39 is a new member of the GW Vir group, and is characterized by the longest pulsation period found among these stars. Moreover HS\,2324+39 appears to be the first pulsating PG\,1159 star with a high H abundance in its atmosphere (H/He = 2 by number, Dreizler et al. 1995). Were the pulsation mechanism based on the C/O cyclic ionization (Starrfield et al. 1984) at work, the H abundance should drop to zero sharply in the driving regions. Such a strong decrease of hydrogen looks quite unlikely; for this reason the presence of pulsations in HS\,2324+39 seems to be a very interesting phenomenon.

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arXiv1995

Search for new pulsating O VI central stars of planetary nebulae

A photoelectric monitoring program has been applied, during the last four years, to five central stars of planetary nebulae (PNNs) with strong O\,VI $λ$ 3811--34 Å\hspace{0.1mm} emission. NGC\,6905 and, marginally, NGC\,2452, show intrinsic luminosity variations, while NGC\,7026, IC\,2003 and NGC\,1501 have constant luminosity within a few mmag. Photometric data have been analyzed with the best available packages for power--spectra reductions. Both pulsators have periods and physical characteristics well encompassed by the theoretical pulsational models relative to these stars.

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