Expected evolution of the binary system ATLAS J1138-5139

Abstract

ATLAS J1138-5139 is a newly detected ultra-compact double white dwarf (DWD) system which is composed of a $1.02\,M_{\odot}$ carbon-oxygen white dwarf (CO WD) and a $0.24\,M_{\odot}$ helium (He) WD with an orbital period of about 27.68 min, making it one of the shortest-period DWD systems known. The future evolution and final fate of this system remain unexplored. In this work, we investigate the evolution of ATLAS J1138-5139 with the one-dimensional stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA). We find that ATLAS J1138-5139 will evolve into an AM Canum Venaticorum (AM CVn) system in about \sim 6.3 Myr. Afterwards, the transferred material from the He WD companion start to build up to form a He shell near the surface of the CO WD. This accumulated He-shell masses can be up to approximately $0.12\,M_{\odot}$, which is likely to trigger a double-detonation (DDet) explosion of the CO WD. We therefore expect that ATLAS J1138-5139 will likely explode as a type Ia supernova eventually through the DDet explosion mechanism. Moreover, our calculations show that ATLAS J1138-5139 will be a promising target for gravitational-wave (GW) detection by future detectors like LISA, Tianqin and Taiji.

Figures (6)

• Figure 1: Evolutionary track of our best initial He WD model for ATLAS J1138-5139. The red dotted line represents the construction of a $0.255\,M_{\odot}$ He WD which is composed of a $0.240\,M_{\odot}$ He core and a $0.015\,M_{\odot}$ H-rich envelope and the black solid line represents the subsequent evolution in the Hertzsprung–Russell (HR) diagram. The blue dot with error bars indicates the observed effective temperature and luminosity of the He WD donor of ATLAS J1138-5139. The red dot denotes the selected initial He-WD model for our binary evolution simulation.
• Figure 2: Left panel: the evolution of the He WD in HR diagram from our binary evolution calculation. The black line represents the evolution of temperature and luminosity of ATLAS J1138-5139. Right panel: the orbital periods of our model change over time. The black line represents the observed period of ATLAS J1138-5139. Here, the red stars and blue dots respectively represent the observed properties of the donor star in ATLAS J1138-5139 and the onset of He mass transfer of our model.
• Figure 3: Mass transfer rate as a function of time in our binary evolution calculations. The blue dots represent temporal mass transfer rate. The horizontal lines indicate the critical rates for different accretion regimes given by Piersanti_2014, which have been marked in the figure. The time interval from approximately $5\,\mathrm{Myr}$ to $11\,\mathrm{Myr}$ corresponds to the regimes in which He can't be accumulated because it is either stably burned or lost due to strong He-shell flashes (red shaded region). The subsequent interval from about $11\,\mathrm{Myr}$ to $25\,\mathrm{Myr}$ marks the phase in which the mass transfer rate falls below the He-burning threshold, allowing a He layer to build up on the surface of the CO WD (blue shaded region).
• Figure 4: Total mass of the He-shell accumulated on the CO WD as a function of time. In this work, we simply assume that the condition for the DDet explosion is satisfied once the accumulated He-shell mass onto the CO WD exceeds $0.1\,M_{\odot}$, which means that an SN Ia explosion occurs.
• Figure 5: Temporal evolution of the GW frequency (panel a), chirp mass (panel b) and SNR (panel c) from our binary evolution calculation. The donor mass vs. GW frequency is also plotted (panel d). The horizontal dotted line indicates the critical value of $\rm{SNR=7}$, above which the source will be detectable for LISA, TianQin and Taiji. The SNR is computed for a mission lifetime of 4 years, which is the nominal operational period for LISA, TianQin and Taiji