Interaction between the ejecta, the accretion disk, and the secondary star in the recurrent nova system U Sco

Technical Summary: Interaction between the ejecta, the accretion disk, and the secondary star in the recurrent nova system U Sco

Problem Statement
While significant effort has been dedicated to modeling the initial thermonuclear runaway and ejection phases of recurrent novae (RNe), the subsequent hydrodynamic interaction between the nova ejecta, the accretion disk orbiting the white dwarf, and the secondary stellar companion remains under-explored. This gap is critical for understanding the long-term evolution of RNe, specifically regarding the integrity of the accretion disk (which dictates recurrence periods), the mass budget of the binary system (escape vs. retention), and the potential chemical contamination of the secondary star. The paper focuses on U Sco, a prototype recurrent nova with a massive white dwarf, a subgiant companion, and a short orbital period, to address these dynamics.

Methodology
The study employs a hybrid modeling approach combining 1D and 3D hydrodynamics:

1. Initial Conditions: The explosion and initial ejection are modeled using the spherically symmetric Lagrangian code SHIVA. The resulting ejecta structure (mass, radius, density, temperature, and velocity profiles) is mapped into a 3D domain.
2. 3D Simulation: The subsequent evolution is simulated using the GADGET-2 smoothed-particle hydrodynamics (SPH) code. The system includes:
   • A point-like white dwarf ($1.38 M_\odot$).
   • A subgiant secondary ($0.88 M_\odot$), modeled with only its outermost layers ($0.0054 M_\odot$) resolved in SPH to manage computational load, with the core treated as a point mass.
   • An accretion disk orbiting the white dwarf, tested in various geometries (flared vs. V-shaped) and density profiles (Shakura-Sunyaev derived vs. constant).
   • Nova ejecta with masses ranging from $1.1 \times 10^{-6}$ to $2.1 \times 10^{-6} M_\odot$ and velocities up to $10,000$ km s$^{-1}$.
3. Experimental Design: A series of ten simulations (Models A–H) were conducted to isolate the effects of specific parameters: ejecta mass, ejecta velocity, ejecta density profile, disk mass, and disk geometry. Crucially, these simulations incorporate orbital rotation around the system's center of mass, a feature often omitted in previous studies of wider binaries.

Key Results
The simulations reveal distinct outcomes based on the ratio of ejecta mass to disk mass ($M_{ejecta}/M_{disk}$) and the disk geometry:

• Disk Disruption: For models featuring flared disks where $M_{ejecta}/M_{disk} \geq 1$ (Models A, B, C, D, G, H), the nova blast results in the complete disruption and sweeping of the accretion disk. The kinetic energy of the ejecta significantly exceeds the gravitational binding energy of the disk.
• Disk Survival: In contrast, models with a massive disk ($M_{ejecta}/M_{disk} = 0.48$, Model E) or V-shaped disks with constant high initial density (Model F) show partial survival of the disk material following the impact.
• Mass Loss and Retention:
  • High ejecta velocities (characteristic of RNe, e.g., $10,000$ km s$^{-1}$) drive a significant fraction of the combined disk and ejecta mass to exceed the system's escape velocity. In the fiducial Model A, $\sim97\%$ of the material leaves the binary system, while only $\sim3\%$ remains gravitationally bound to the white dwarf.
  • This contrasts with previous findings for classical novae (Paper I), where lower velocities resulted in higher retention.
  • Increasing the ejecta mass or velocity increases the total mass lost from the system and decreases the mass accreted by the white dwarf.
  • Conversely, denser ejecta (constant density vs. profile) and more massive disks increase the fraction of material retained by the white dwarf due to enhanced deceleration during collision.
• Secondary Star Interaction:
  • The impact of ejecta and disk material on the subgiant companion is minimal. Only a tiny fraction of particles ($\sim0.07\%$ of the total ejecta/disk mass in Model A) become incorporated into the secondary's outer layers, suggesting negligible chemical contamination.
  • A minor ejection of mass from the subgiant's outer layers is observed during the late-stage collision. Some of this secondary mass is ejected from the system, while a small fraction is accreted by the white dwarf.
• Role of Rotation: In the wide orbit of U Sco, rotation plays a marginal role in the overall mass loss or retention of the ejecta/disk mixture ($<1\%$ difference). However, rotation does reduce the amount of mass lost by the subgiant that is subsequently accreted by the white dwarf by approximately $20\%$.

Significance and Claims
The authors position this work as a necessary qualitative step to fill the gap in understanding the post-outburst evolution of recurrent novae. The study claims to provide the first systematic 3D SPH analysis of the interaction between nova ejecta, accretion disks, and companions in a rotating binary system with parameters specific to U Sco.

The paper modestly concludes that its findings are qualitative due to resolution limitations (specifically the low particle count for low-density ejecta and the disk). However, it asserts that these results offer valuable guidance for future research, particularly in:

1. Identifying the conditions (mass ratios, geometry, density) that lead to disk survival versus total disruption.
2. Highlighting the sensitivity of mass retention to ejecta velocity, a key differentiator between recurrent and classical novae.
3. Emphasizing the need for detailed disk geometry models (flared vs. V-shaped) to accurately simulate the long-term evolution of these systems.

The authors note that while their simulations suggest complete disk disruption in many standard scenarios, observational claims of disk survival (e.g., in the 2022 U Sco outburst) may imply the presence of extended disks or different physical configurations not fully captured in the current parameter space.