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

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

Corresponding author: S. Jordan, jordan@astrophysik.uni-kiel.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 C₂ 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 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 ≲0.7% in the continuum and ≲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.