Population synthesis of double white dwarfs: evolutionary effects on system properties

TL;DR

This study uses the rapid binary population synthesis code COMPAS to map how two dominant DWD formation channels—stable RLOF-only and CE-involving histories—shape the WD masses, core compositions, and orbital separations. A robust bimodal separation distribution with a gap near $\sim$ $100-500\,R_\odot$ arises from the contrasting effects of stable mass transfer versus CE phases; He-core WDs preferentially occupy close systems while CO- and ONe-core WDs span a broad range. The work also links accretion history and angular-momentum loss to final separations, and demonstrates that the $\lambda_\text{CE}$ parameter modulates the gap's depth but not its existence. These results offer a physically interpretable framework to interpret Gaia DWD samples and forecast the joint EM and GW populations accessible to LISA, with implications for calibrating CE physics and mass-transfer efficiency in binary evolution.

Abstract

Double white dwarf (DWD) binaries are natural outcomes of binary stellar evolution and key sources for future space-based gravitational wave (GW) observatories such as Laser Interferometer Space Antenna (LISA). We investigate how different binary interaction channels shape the physical and orbital properties of DWD systems, focusing on component masses, orbital separations, core compositions, and mass transfer rates. Using the binary population synthesis code COMPAS, we evolve $10^7$ binaries with physically motivated initial distributions of binary parameters. Our simulations reproduce the strong bimodality in the final orbital separations, including a pronounced deficit of systems around $100-500 \rm\,R_\odot$, arising from distinct evolutionary pathways: wide DWDs predominantly originate from stable Roche lobe overflow (RLOF), while close DWDs form through unstable RLOF leading to at least one common envelope (CE) phase. Moreover, we show that the core compositions of WDs provide a powerful tracer of evolutionary history: He-core WDs are strongly concentrated in close systems, whereas CO-core WDs span the full separation range and exhibit a small mass gap in wide binaries. We further identify a correlation between the accreted mass and the final orbital separation, highlighting the impact of non-conservative mass transfer on the resulting orbital configuration of DWD systems. These results underscore the links among evolutionary channels, chemical composition, and mass transfer rates; thereby provide a unique framework for interpreting Gaia DWD samples and forecasting the joint electromagnetic and GW population accessible to LISA.

Figures (4)

• Figure 1: Final WD mass as a function of orbital separation for DWDs for $\alpha_\text{CE}=1.0$ and $\lambda_\text{CE} = 0.1$. Systems evolving solely through stable RLOF are shown in blue, while those involving at least one CE phase are shown in orange. Panel (a) represents the more massive WD and panel (b) represents the less massive companion in each binary system.
• Figure 2: Same as Figure \ref{['Fig: CE vs. RLOF']} expect each WD is marked according to their core composition.
• Figure 3: Same as Figure \ref{['Fig: CE vs. RLOF']} except that the accretion rates onto WDs during the last stable mass transfer episode is shown in the colour bar.
• Figure 4: Same as Figure \ref{['Fig: CE vs. RLOF']} except for $\alpha_\text{CE}=1$ and $\lambda_\text{CE}=0.5$.