Year-Long Transients as Precursors to Mergers of Low-Mass Helium Stars with Neutron Stars

Core Concepts

Low-mass helium stars in binary systems with neutron stars can produce luminous, year-long transients in the optical and ultraviolet wavelengths, serving as precursors to their eventual merger or core-collapse.

Abstract

• Bibliographic Information: Tsuna, D., Wu, S. C., Fuller, J., Dong, Y., & Piro, A. L. (2024). Merger Precursor: Year-long Transients Preceding Mergers of Low-mass Stripped Stars with Compact Objects. arXiv preprint arXiv:2406.12472v3.
• Research Objective: This study investigates the observational signatures of low-mass helium stars (2.5-3 solar masses) in binary systems with neutron stars, focusing on the period leading up to their merger or core-collapse.
• Methodology: The authors utilize a combination of stellar evolution models (MESA) and a binary interaction code (RLOF) to simulate the mass transfer process and orbital evolution of these systems. They develop a time-dependent emission model to predict the luminosity and temperature of the resulting transients, considering both the outflow from the neutron star's accretion disk and the circumbinary outflow.
• Key Findings: The models predict that these systems can power slowly rising transients with timescales of up to several years, emitting luminosities of 10^40 - 10^41 erg/s in the optical and ultraviolet wavelengths. The emission is driven by super-Eddington accretion onto the neutron star and its interaction with the dense circumstellar material. The study highlights the potential for observing these transients as precursors to either the merger of the two objects or the core-collapse of the helium star.
• Main Conclusions: The authors propose a new class of astronomical transients, termed "merger precursors," which could provide valuable insights into the late stages of stellar evolution and the dynamics of binary systems containing compact objects. They suggest that these precursors could be detectable with current and future optical and ultraviolet surveys.
• Significance: This research advances our understanding of the evolution of binary stars with compact companions and the mechanisms behind supernova precursors. It predicts a new observational signature that can be used to identify and study these systems before their final fate.
• Limitations and Future Research: The study acknowledges simplifications in their model, such as the assumption of a fixed donor radius during mass transfer and a simplified treatment of the circumbinary outflow. Future research incorporating more detailed hydrodynamical simulations and a wider range of binary parameters could refine these predictions and provide a more comprehensive picture of these events.

Stats

The helium stars have initial masses of 2.467 solar masses and 2.754 solar masses.
The neutron star companion has a mass of 1.4 solar masses.
The initial orbital periods considered are 100, 10, and 1 day.
The predicted luminosities of the transients range from 10^40 to 10^41 erg/s.
The timescales of these transients can extend up to several years.
The color temperatures of the emission range from ~10,000 K to over 20,000 K, depending on the geometry of the outflow.

Quotes

Key Insights Distilled From

Merger Precursor: Year-long Transients Preceding Mergers of Low-mass Stripped Stars with Compact Objects

by Daichi Tsuna... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2406.12472.pdf

Deeper Inquiries

How might the presence of a strong magnetic field around the neutron star affect the observational properties of these transients?

The presence of a strong magnetic field around the neutron star can significantly alter the observational properties of these merger precursors. Here's how:

Channeling of Accretion: A strong magnetic field can channel the accretion flow onto the neutron star's magnetic poles, rather than allowing it to form a radiatively efficient accretion disk. This can lead to:

Reduced Disk Wind: With less material accreting via a disk, the disk wind, which is responsible for much of the UV emission in the model, would be weaker or absent.
X-ray Emission: Channeled accretion onto the neutron star's magnetic poles can produce strong X-ray emission, making the precursor detectable in X-rays.
Different Variability: The interaction between the magnetic field and the accreting material can lead to different variability patterns in the precursor emission, potentially with more pronounced pulsations or flares.

Altered Outflow Geometry: The magnetic field can influence the geometry of the outflow, potentially collimating it into jets. This can lead to:

Anisotropic Emission: The precursor emission might become anisotropic, with different luminosities and temperatures observed depending on the viewing angle.
Polarization: The presence of jets can introduce polarization into the precursor emission, providing further clues about the magnetic field geometry.

Modified Shock Dynamics: The interaction between the magnetic field and the outflowing material can modify the shock dynamics, potentially leading to:

Different Shock Temperatures: The shock temperatures, and hence the observed colors of the precursor, could be different.
Non-thermal Emission: The presence of strong magnetic fields can accelerate particles to relativistic speeds, leading to non-thermal emission components in the precursor spectrum, potentially observable in radio or even gamma-rays.
Overall, a strong magnetic field around the neutron star can introduce a wide range of observational signatures, making the precursor emission more complex and potentially providing valuable insights into the neutron star's properties.

Could some of the observed Type Ibn supernovae without identified precursors be explained by systems with shorter-lived or less luminous precursor events?

Yes, it is plausible that some Type Ibn supernovae without identified precursors could be explained by systems with shorter-lived or less luminous precursor events. Here's why:

Limited Observational Cadence: Current supernova surveys have a limited observational cadence, meaning they observe the same patch of sky only every few days or weeks. This means that short-lived precursors, lasting for only a few days or even hours, could easily be missed.
Sensitivity Limits: Even for longer-lived precursors, their luminosity might be below the detection threshold of current surveys, especially at larger distances. This is particularly true for precursors that are not extremely bright or those whose emission peaks in the UV, where many surveys have lower sensitivity.
Dust Extinction: Dust along the line of sight to the supernova can significantly attenuate the precursor emission, making it fainter and potentially undetectable. This is particularly relevant for supernovae located in dusty environments, such as the disks of spiral galaxies.
Therefore, it is entirely possible that a population of Type Ibn supernovae exists that exhibit precursor events, but these events remain undetected due to the limitations of current observations. Future surveys with higher cadence, increased sensitivity, and better dust extinction corrections will be crucial to uncover these hidden precursors and obtain a more complete picture of the late stages of massive stellar evolution in binary systems.

If these merger precursors are successfully identified, what new information could we learn about the physics of accretion onto neutron stars and the dynamics of stellar mergers?

The successful identification of merger precursors would provide a wealth of new information about the physics of accretion onto neutron stars and the dynamics of stellar mergers. Here are some key areas where we could gain significant insights:

Super-Eddington Accretion: These precursors offer a unique opportunity to study super-Eddington accretion onto neutron stars in detail. By observing the luminosity, temperature, and variability of the precursor emission, we can constrain the accretion rate, the geometry of the accretion flow, and the mechanisms by which the excess energy is released. This can help us understand how neutron stars accrete matter at such extreme rates, a process that is still not fully understood.
Disk Wind Launching Mechanisms: The presence of powerful disk winds in these systems provides a testing ground for different wind launching mechanisms. By studying the wind velocity, mass-loss rate, and geometry, we can differentiate between various proposed mechanisms, such as radiation pressure, magnetic fields, or thermal pressure gradients. This can shed light on the complex interplay between accretion and outflow in these extreme environments.
Common Envelope Evolution: The orbital evolution of the binary system leading up to the merger is governed by the poorly understood process of common envelope evolution. Observing the precursor emission over time allows us to track the orbital decay and mass transfer rate, providing crucial constraints on the efficiency of angular momentum transport and energy dissipation during this phase. This can help us refine our models of common envelope evolution and its role in shaping the final fate of binary stars.
Pre-Merger Dynamics: The final stages of the merger process, just before the stars coalesce, are extremely difficult to model theoretically. However, the precursor emission can provide valuable clues about the dynamics of the system in these final moments. For example, changes in the luminosity, temperature, or variability of the emission could signal the onset of tidal disruption, the formation of a contact binary, or the ejection of a common envelope.
Neutron Star Equation of State: The properties of the merger remnant, such as its mass, spin, and potential for forming a black hole, depend sensitively on the neutron star equation of state, which describes the relationship between pressure and density in dense nuclear matter. By observing the merger process and its aftermath, including the precursor emission, we can potentially constrain the neutron star equation of state and gain insights into the fundamental properties of ultra-dense matter.
In summary, the discovery and study of merger precursors would open up a new window into the physics of compact objects and stellar mergers, providing invaluable data to test and refine our theoretical models and deepen our understanding of these fascinating astrophysical phenomena.