Spectral signatures of the fast optical oscillations in the AM Herculis system an Ursae Majoris.

Details Spectral signatures of the fast optical oscillations in the AM Herculis system an Ursae Majoris. Spectral signatures of the fast optical oscillations in the AM Herculis system an Ursae Majoris.

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...306..199b&link_type=abstract

Astronomy and Astrophysics, v.306, p.199

Astronomy and Astrophysics

Astrophysics

12

Stars: White Dwarf, Stars: Accretion, Am Herculis, An Uma, Oscillations

Scientific paper

We report on simultaneous photometric and spectroscopic observations of AN UMa obtained in high time resolution mode at the Russian 6m telescope on March 1991 and January 1992. These observations are the first detailed spectroscopic data obtained since the discovery period of the source. A study of the Balmer and helium emission line velocities reveals a significant phase shift with time, attributed to an inaccuracy in the rotational/orbital period. A new ephemeris is derived with a refined period of (6890.6436+/-0.0035)s. With this new period all published photometric light curves are correctly phased over more than 15 years and, in particular, the phase discrepancy previously reported in X-ray light curves is eliminated. The geometry of the system appears stable. A modelling of the narrow and broad emission line velocities allows to constrain the system parameters. The best solution is found for a low mass white dwarf (M_wd_=0.4-0.6Msun_), an inclination i=40-60deg, an angle of the magnetic field axis with respect to the perpendicular to the orbital plane of θ_d_=25-45deg and the narrow component of the emission lines produced in a stream near the capture radius. Quasi-periodic oscillations are detected in the Johnson-B light curve during all parts of the observations with rms amplitudes varying from 1.6 to 3.9%. They are comparable to what already reported for this source and appear as a broad excess of 0.18 to 0.48Hz (FWHM) around mean frequencies from 0.62 to 0.72Hz. Superposed to these low amplitude background oscillations, short duration events are also found. They consist of short-lived (30s) pulse shot of high amplitude oscillations confined in a very narrow range of frequencies. We reported here on the first spectral characteristics of these oscillations in the (4100-5100A) range. Contrary to what expected by thermal instability models, there seems to be no counterpart of these peculiar narrow frequency oscillations in the continuum bands but a significant power is present in some of the emission lines bands. We suggest that these peculiar oscillations may be of different types, produced close to the capture radius, in the turbulent region where the accretion stream interacts with the magnetic field.

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