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Issue A&A
Volume 383, Number 2, February IV 2002
Page(s) 519 - 523
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011684



A&A 383, 519-523 (2002)
DOI: 10.1051/0004-6361:20011684

Search for variations in circular-polarization spectra of the magnetic white dwarf LP 790-29

S. Jordan1, 2, 3 and S. Friedrich1, 4

1  Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
2  Universitäts-Sternwarte, Geismarlandstraße 11, 37083 Göttingen, Germany,
3  Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
    e-mail: jordan@astro.uni-tuebingen.de
4  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: sfriedrich@aip.de

(Received 21 September 2001 / Accepted 22 November 2001)

Abstract
We present highly time resolved circular-polarization and flux spectra of the magnetic white dwarf LP 790-29 taken with the VLT UT1 in order to test the hypothesis that LP 790-29 is a fast rotator with a period of the order of seconds to minutes. Due to low time resolution of former observations this might have been overlooked - leading to the conclusion that LP 790-29 has a rotational period of over 100 years. The optical spectrum exhibits one prominent absorption feature with minima at about 4500, 4950, and 5350 Å, which are most likely C 2 Swan-bands shifted by about 180 Å in a magnetic field between 50 MG and 200 MG. At the position of the absorption structures the degree of circular polarization varies between $-1\%$ and +1%, whereas it amounts to +8 to +10% in the blue and red continuum. With this very high degree of polarization LP 790-29 is very well suited to a search for short time variations, since a variation of several percent in the polarization can be expected for a magnetic field oblique to the rotational axis. From our analysis we conclude that variations on time scales from 50 to 2500 s must have amplitudes $\lesssim$0.7% in the continuum and $\lesssim$2% in the strongest absorption feature at 4950 Å. While no short-term variations could be found a careful comparison of our polarization data of LP 790-29 with those in the literatures indicates significant variations on time scales of decades with a possible period of about 24-28 years.


Key words: stars: individual: LP 790-29 -- stars: rotation -- stars: white dwarfs -- stars: magnetic fields

Offprint request: S. Jordan, jordan@astrophysik.uni-kiel.de

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