[Paper Review] Erosion of Icy Interstellar Objects by Cosmic Rays and Implications for `Oumuamua

Vo Hong Minh Phan, Thiem Hoang+1 more|arXiv (Cornell University)|Sep 9, 2021

Solar and Space Plasma Dynamics5 references4 citations

TL;DR

This paper investigates cosmic-ray and gas collision erosion of icy interstellar objects (ISOs), finding that impulsive cosmic-ray heating significantly shortens erosion timescales. For an N₂ fragment to survive ~0.5 Gyr in the ISM, it must have originated with an initial radius of at least 0.5 km—implying a mass budget challenge for explaining 'Oumuamua as an N₂ iceberg, especially if formed in high cosmic-ray flux regions.

ABSTRACT

We study the destruction and modification of icy interstellar objects by cosmic rays and gas collisions. Using the cosmic-ray flux measured in the local interstellar medium as well as inferred from gamma-ray observations at the different galactocentric radii, we find that cosmic-ray erosion is significant for interstellar objects made of common types of ices. Interestingly, cosmic-ray heating might destroy icy interstellar objects very efficiently such that the initial size of an N$_2$ fragment as suggested by \citet{jackson2021} to explain the composition of `Oumuamua should be at least 0.5 km in size in order to survive the journey of about 0.5 Gyr in the ISM and might be even larger if it originated from a region with an enhanced cosmic-ray flux. This implies an initial N$_2$ mass that is at least an order of magnitude larger than the final value, exacerbating the N$_2$ mass budget deficiency for explaining `Oumuamua. The erosion time due to cosmic-ray heating and gas collisions also allows us to set approximate limits on the initial size for other types of icy interstellar objects, e.g. composed of CO, CO$_2$, or CH$_4$. For a given initial size, we constrain the maximum distance to the birth site for interstellar objects with different speeds. We also find that cosmic-ray and gas heating could entirely modify the ice structure before destroying interstellar objects.

Motivation & Objective

• To assess the role of cosmic-ray impulsive spot heating and interstellar gas collisions in eroding icy interstellar objects (ISOs) during their journey through the ISM.
• To reevaluate the minimum initial size required for N₂ ice fragments to survive ~0.5 Gyr in the ISM, given updated erosion rates from cosmic-ray heating.
• To constrain the maximum distance to the birth site of ISOs based on their initial size and velocity, using erosion and modification timescales.
• To examine how cosmic-ray and gas heating modify the internal ice structure (e.g., amorphous vs. crystalline), with implications for future spectroscopic observations.
• To quantify the impact of varying cosmic-ray flux (e.g., due to galactocentric location) on erosion and survival timescales of ISOs composed of N₂, CO, CO₂, or CH₄ ice.

Proposed method

• Used cosmic-ray flux measurements and gamma-ray observations to model the local and galactocentric variations in cosmic-ray density, parameterized by ξ_CR.
• Applied the impulsive spot heating model from Hoang & Loeb (2020) to calculate the erosion rate of icy ISOs, accounting for energy deposition per cosmic-ray impact.
• Derived the erosion timescale τ by integrating the rate of radius reduction due to cosmic-ray heating and gas collisions, using a surface-to-volume enhancement factor ε.
• Calculated the modification timescale τ_m for ice structure transformation (e.g., amorphous to crystalline or vice versa) based on thermal processing from cosmic rays and gas collisions.
• Combined the erosion and modification timescales with orbital parameters (initial size R₀, velocity v_obj) to estimate the maximum distance D_max to the birth site for ISOs to survive to the Solar System.
• Used numerical simulations to track the evolution of the volume filling factor of amorphous ice over time, assessing structural modification before complete destruction.

Experimental results

Research questions

• RQ1 What is the minimum initial size required for an N₂ ice fragment to survive ~0.5 Gyr in the ISM when cosmic-ray impulsive heating is considered?
• RQ2 How does an enhanced cosmic-ray flux (ξ_CR > 1) in certain galactic regions affect the required initial size and survival probability of icy ISOs?
• RQ3 What is the maximum distance to the birth site of an ISO with a given initial size and velocity, such that it could still be observed today?
• RQ4 How do cosmic-ray and gas collision heating modify the internal ice structure of ISOs, and what are the implications for spectroscopic detection?
• RQ5 To what extent does the erosion time from cosmic-ray heating reduce the viability of the N₂ iceberg hypothesis for explaining 'Oumuamua’s properties?

Key findings

• The erosion timescale due to cosmic-ray impulsive spot heating is estimated to be 1–2 orders of magnitude shorter than previous estimates, implying faster destruction of icy ISOs.
• An N₂ fragment must have an initial radius of at least 0.5 km to survive ~0.5 Gyr in the ISM, assuming ε = 3 and local cosmic-ray flux (ξ_CR = 1), which increases the required initial N₂ mass by at least an order of magnitude.
• If the cosmic-ray flux is enhanced by a factor of 5 (ξ_CR = 5), the required initial size increases further, exacerbating the N₂ mass budget problem for explaining 'Oumuamua.
• For ISOs composed of common ices (N₂, CO, CO₂, CH₄), the minimum initial size required to survive 0.5 Gyr is between 0.1 km and 0.5 km, depending on distance from the birth site and velocity.
• The maximum distance to the birth site for an N₂ ISO with R₀ = 0.5 km and v_obj ≈ 10 km/s is constrained to D_max ≈ 4 kpc (for ξ_CR = 1) or D_max ≈ 1 kpc (for ξ_CR = 5), assuming survival to the Solar System.
• Cosmic-ray and gas heating can entirely modify the ice structure (e.g., amorphous to crystalline) before complete destruction, with modification timescales τ_m ≈ 0.17 Gyr and erosion timescales τ ≈ 0.58 Gyr for a 0.5 km N₂ fragment with ε = 3 and v_obj = 10 km/s.