The progenitors of magnetic white dwarfs in open clusters

¹ Institut de Ciències de l’Espai (CSIC–IEEC), Facultat de Ciències, Campus UAB, Torre C5-parell, 2a planta, 08193 Bellaterra, Spain
e-mail: bkulebi@ieec.cat
² Institute for Space Studies of Catalonia (ICE), c/Gran Capità 2–4, Edif. Nexus 104, 08034 Barcelona, Spain
³ Astronomisches Rechen–Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21042, USA
⁵ Center for Astrophysical Sciences, Johns Hopkins University, Baltimore, MD 21218, USA
⁶ Swiss Seismological Service, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland

Received: 18 January 2012
Accepted: 24 April 2013

Abstract

Context. White dwarfs are the final stages of stellar evolution for most stars in the galaxy and magnetic white dwarfs (MWDs) represent at least ten percent of the whole sample. According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the MWD sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields.

Aims. One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. This enables an independent way of determining the mass of their progenitors. The discovery of the magnetic WD 0836+201 in the Praesepe cluster prompted the question whether magnetic fields affect the IFMR. In this work we investigate this suggestion through investigations of all three known MWDs in open clusters.

Methods. We assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. Furthermore, we use mass-radius relations and evolutionary models to constrain the histories of the probable cluster members.

Results. We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 ± 0.11 M_⊙.

Conclusions. We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.