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Utrecht University

Student project: the formation of double white dwarfs (for LISA)

Background

Common Envelopes (CEs) provide our best hypothesis to explain observed compact binaries. These include the CBCs that LIGO and Virgo observe, the double white dwarfs (DWDs) that LISA will observe and cause Type Ia supernovae (SNIae) used to measure distances. By default a CE is treated by energy conservation (Paczynski, 1976): orbital energy unbinds the donor envelope, which provides the post-CE orbital separation. This leads to a dramatic orbital shrinkage, very different from stable mass-transfer (MT) usually ending in a wider binary. Neither MT type can explain ten observed double-lined spectroscopic DWDs with well-constrained masses and ages. The two WDs have similar masses, which cannot be explained by a CE (second WD should be lighter) or stable MT (heavier second WD) for the first MT. Nelemans et al. (2000) proposed an alternative "gamma-CE", based on angular-momentum balance, which can "explain" these systems. An issue is the sensitive dependence on gamma, explaining why reconstruction works well but implying that forward calculations lead to a wide range of outcomes for small variations in gamma. Secondly, there is little physics behind the model.

Research questions

  • Can stable mass transfer with mass loss from L2 explain observed double white dwarfs?
  • Is stable, non-conservative mass transfer an explanation for the gamma-CE?
  • What are the properties of the ZAMS binaries that evolve into the observed DWDs?

Methods

Stable, non-conservative MT may provide a physical explanation for the gamma-CE and hence the observed DWDs. Our study shows that earlier assumptions on the stable-unstable MT boundary are incorrect (Woods et al. 2012). Also, preliminary calculations show that transferring mass to the secondary and then (partly) losing it leads to little orbital change. The secondary can lose material by a fast isotropic wind, or through the L2 Lagrangian point. While we have explored the first scenario, the second needs to be looked into in more detail. This project involves using the implementation of mass loss through L2 into the population-synthesis code BSE (Hurley et al 2002) by a previous student, running the models and comparing the DWD population to the scenarios of conservative MT and wind-mass loss from the secondary.

Interested?

If you are interested in this project, please send me an email. If I'm on holiday (e.g. the beginning of January or September), send me a WhatsApp on my mobile to expedite things. (There will be plenty of arranging and reading for you to do until I get back, see Getting started.) You can find my email address and phone number on my home page.


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