CORDIS - EU research results

Appearance and evolution of stars in the state of Propeller

Final Activity Report Summary - ASTRO-PROPELLER (Appearance and Evolution of Stars in the State of Propeller)

Recent studies on neutron stars' population with modern sensitive X-ray telescopes revealed a number of inconsistencies between the theoretically predicted and the observed properties of these objects. For instance, the spin period of some neutron stars in massive close binary systems was found to be in excess of 1 000 seconds, which was significantly larger than the maximum possible spin period predicted by the canonical spin-down scenario. Another example was the lack of success in searching for faint X-ray sources powered by accretion of interstellar material onto the surface of old isolated neutron stars. While 30 000 such sources were expected to be discovered using modern X-ray telescopes, namely Chandra and XMM-Newton, none of them has been identified so far.

After analysing these problems, we found that the mechanism of interaction between the compact star's magnetosphere and surrounding material used in the canonical model of compact stars evolution was oversimplified. The improvement of the mechanism performed within this project led us to the following results:
1. The long-period X-ray pulsars were neutron stars in massive wind-fed close binary systems, which underwent spherical accretion and whose evolutionary tracks in a previous epoch contained three instead of two states, namely the ejector, supersonic propeller and subsonic propeller. All currently known long-period pulsars could be interpreted within this scenario provided that the masses and radii of the neutron stars corresponded to the hard equation of state. The assumption about a supercritical value of the initial magnetic field of neutron stars within this scenario was not necessary. These results provided us with indirect justification of the existence of subsonic propellers.
2. Via incorporation of the subsonic propeller state into the evolutionary tracks of old isolated neutron stars we found that the number of X-ray sources associated with objects which could be observed with recent and modern X-ray missions was almost nine orders of magnitude smaller than the previously estimated one. This result provided us with a natural explanation of the lack of success in searching for the accreting isolated neutron stars with Chandra and XMM-Newton satellites.
3. As long as the heating of a spherically symmetrical accretion flow beyond the magnetospheric boundary of a neutron star dominated cooling the boundary remained stable with respect to the interchange type instabilities. These instabilities of the boundary were suppressed in both slow and fast rotating stars. This indicated that the process of plasma entry into the magnetosphere of long-period pulsars, which were persistent X-ray sources of a low or moderate luminosity, could not be explained in terms of the interchange instabilities and, therefore, the canonical accretion model developed for these sources had to be reconsidered.

Studies on the cataclysmic variable AE Aquarii showed that the basic methods of the magneto-rotational theory developed for neutron stars could also be applied to white dwarfs. In particular, we demonstrated that the rapid braking of the white dwarf in this close binary system could be explained provided its surface magnetic field was about 100 MG. This result was justified by our analysis of the Doppler H-alpha tomogram, circularly polarised optical emission and pulsing X-ray emission of the system. Under these conditions the braking of the white dwarf was governed by the pulsar-like mechanism and the white dwarf operated as a magnetic propeller with respect to the material surrounding its magnetosphere.