High-resolution smoothed particle hydrodynamics simulations of the merger of binary white dwarfs

¹ Departament de Física Aplicada, Escola Politécnica Superior de Castelldefels, Universitat Politècnica de Catalunya, Avda. del Canal Olímpic 15, 08860 Castelldefels, Spain e-mail: garcia@fa.upc.es
² Institute for Space Studies of Catalonia, c/Gran Capità 2–4, Edif. Nexus 104, 08034 Barcelona, Spain
³ Institut de Ciències de l'Espai, CSIC, Campus UAB, Facultat de Ciències, Torre C-5, 08193 Bellaterra, Spain

Received: 30 September 2008
Accepted: 15 March 2009

Abstract

Context. The coalescence of two white dwarfs is the final outcome of a sizeable fraction of binary stellar systems. Moreover, this process has been proposed to explain several interesting astrophysical phenomena.

Aims. We present the results of a set of high-resolution simulations of the merging process of two white dwarfs.

Methods. We use an up-to-date smoothed particle hydrodynamics code that incorporates very detailed input physics and an improved treatment of the artificial viscosity. Our simulations have been done using a large number of particles (~410⁵) and covering the full range of masses and chemical compositions of the coalescing white dwarfs. We also compare the time evolution of the system during the first phases of the coalescence with what is obtained using a simplified treatment of mass transfer; we discuss in detail the characteristics of the final configuration; we assess the possible observational signatures of the merger, such as the associated gravitational waveforms and the fallback X-ray flares; and we study the long-term evolution of the coalescence.

Results. The mass transfer rates obtained during the first phases of the merger episode agree with the theoretical expectations. In all the cases studied, the merged configuration is a central compact object surrounded by a self-gravitating Keplerian disk, except in the case where two equal-mass white dwarfs coalesce.

Conclusions. We find that the overall evolution the system and the main characteristics of the of the final object agree with other previous studies in which lower resolutions were used. We also find that the fallback X-ray luminosities are close to 10⁴⁷ erg/s. The gravitational waveforms are characterized by the sudden disappearance of the signal in a few orbital periods.