Formation of Extremely Low-Mass White Dwarfs in Wide Orbits

Abstract

Helium white dwarfs (WDs) with masses less than 0.3 $\rm M_{\sun}$ are known as extremely low-mass WDs (ELM WDs), which cannot be produced by single stellar evolution in theory. Generally, these stars are believed to form through binary interactions. Recently, two ELM WDs in unusually wide orbits were reported, i.e., KIC 8145411 and HE 0430-2457. Their orbital separations are too wide to be produced by the binary evolution scenario. In this work, we study the formation of wide-orbit ELM WD binaries from hierarchical triple systems. In this scenario, an ELM WD is formed from the inner binary and subsequently forms a wide binary system with the third object. We find that the merger of an evolved star with a brown dwarf in the inner binary fails to produce single ELM WDs, but Type Ia supernovae (SNe Ia) explosions can successfully do so. Furthermore, we investigate the impact of the supernova explosion on the orbital distribution of the surviving binary and find that this channel may have a probability of reproducing the orbital parameters of HE 0430-2457, but fails to reproduce the observed features of KIC 8145411. This supports recent observational recalibrations suggesting that KIC 8145411 resides in a triple system rather than a binary.

Figures (5)

• Figure 1: Possible formation channels for single ELM WDs. Left panel: Merger of an evolved star with a brown dwarf. Right panel: SN explosion channel.
• Figure 2: Relationship between ELM WD mass and post-CE binary separation for different models, with $\alpha_{\rm{CE}} = 1$ for all cases. The blue, green, and black curves represent models where the envelope binding energy is calculated using the helium core boundary (defined as where $X = 0.01$), the helium core mass plus 0.01 $M_\odot$, and the helium core mass plus 0.03 $M_\odot$, respectively. For each set, solid and dashed lines correspond to metallicities of $Z = 0.02$ and $Z = 0.001$. The red solid line marks the theoretical minimum binary separation.
• Figure 3: The evolutionary tracks of single ELM WDs from the SN explosion channel in the $T_{\rm {eff}}-{\rm{log}} g$ plane. The colored solid lines show the mass variation of CO WDs. The red stars indicate the end of mass transfer. The plus sign marks the data for HE 0430-2457, taken from Vos2018b.
• Figure 4: Final separation and eccentricity distributions for HE 0430-2457. Left panel: Results for initially eccentric systems with outer orbit eccentricity distribution $P(e) \approx e^{-0.42}, 0 \leq e < 1$ case. Right panel: Results for initially circular systems with $e = 0$.
• Figure 5: Similar to Figure \ref{['Fig4']}, but for KIC 8145411.