Title: Minimum Neutron Star Mass in Neutrino-Driven Supernova Explosions
Authors: Bernhard Müller, Alexander Heger, Jade Powell
First Author’s Institution: Monash University, Clayton, Victoria, Australia
Status: Published in Physical Review Letters [closed access]
In 2015, astronomers discovered the pulsar PSR J0453+1559, which was revealed to be a rare binary system of two neutron stars. Double neutron star systems are theorized to begin as two massive stars in orbit; the more massive star explodes in a supernova, forming a neutron star and leaving behind a high-mass companion. Over time, the neutron star steals mass from the companion star until that companion undergoes its own supernova explosion, creating another neutron star. Since surviving one supernova is already uncommon, surviving two within the same system is extraordinarily rare. This makes studying these double neutron star systems invaluable.
What made PSR J0453+1559 more surprising were the masses of the neutron stars. While the primary star within the binary has a mass of 1.559 solar masses (M☉), the companion star came in at 1.174 M☉. The companion neutron star is believed to have one of the lowest–if not the lowest–masses of any observed neutron star to date. Not only is this mass unusual, but scientists were also surprised at how different the two stars’ masses were since neutron star binaries have historically been observed to have quite similar masses. Neutron stars with low masses challenge our current understanding of how stars live, die, and explode. Today’s paper takes a computational approach to how a supernova may have formed an unexpectedly low-mass neutron star.
Need to Make a Neutron Star? Just Add Some Collapse
The initial mass of a star determines its fate. The overarching theory for stars with masses between ~8 M☉ to ~20 M☉ is that throughout their lives, they undergo multiple cycles of burning progressively heavier elements, first starting with their initial stores of hydrogen and helium and eventually forming an iron core and onion-like structure of different elemental layers. Once the core reaches a particular mass–called the Chandrasekhar mass limit of ~1.4 M☉–the core collapses and produces an aptly-named core collapse supernova (CCSN). Neutrinos–extremely light, difficult to detect, and weakly interacting particles–carry lots of energy away from the core very quickly, further driving the explosion.
In the aftermath of this supernova, a neutron star is often “born from the ashes” of the core and is generally thought to have a mass of no more than ~2 M☉. (This is because neutron star masses are believed to hover around the Chandrasekhar mass.) Estimating a minimum neutron star mass is complicated because we don’t fully understand how a neutron star forms. The details of the supernova explosion–such as how much mass is ejected versus how much remains–that creates a neutron star are unclear. We also don’t understand much about the neutron star equation of state, which is a relationship that describes how pressure and density behave under extreme conditions, or about the role of magnetic fields in their formation. (It also turns out that trying to replicate the properties of a neutron star on Earth in a lab is quite difficult!) With so much uncertainty regarding the theory behind neutron stars and CCSNe, it’s no surprise that our observations might not align with current theory.
Simulate It ‘Til You Make It
Simulations of supernova explosions with accurate physics can help astronomers figure out what type of progenitor and what type of explosion scenario might produce a neutron star this light. Fortunately, modern technology lets us run incredibly complex simulations of highly chaotic (and explosive!) phenomena like supernovae without replicating these explosions in a lab. To test what type and what mass of progenitor produces a low-mass neutron star, the authors ran three-dimensional core-collapse supernovae simulations on a range of potential progenitors with masses from 9.45 M☉ to 9.95 M☉. After initial testing, they selected 5 promising candidates from this range to focus on.
After replicating a supernova with each of these progenitors, they determined the baryonic neutron star mass of the resulting neutron star, which describes the mass of the star’s protons and neutrons. Of their 5 models, their 9.9 M☉ looked the most promising by producing a baryonic mass of 1.313 M☉ (see bottom panel of Figure 1). Converting this to gravitational mass–the mass that the initial 2015 observations describe, which is lower due to the energy lost to binding during the star’s formation–results in a mass of 1.192 M☉. It’s not quite the 2015 observation’s mass of 1.174 M☉, but it’s a lot closer than initially thought.
One of the benefits of running 3D simulations instead of 2D is that there can be asymmetry to an explosion that is better described in three dimensions. When a core collapse supernova explodes, the neutron star produced in the explosion is “kicked” in a direction with high speed. Along with a lowest mass of 1.192 M☉, the authors obtained a kick for the neutron star of around 100 km/s, which is roughly the scale of kick they would expect for a supernova like this (Figure 2).
So…Mystery Solved? Not Quite.
The authors set a new record for the lowest neutron star mass obtained through 3D supernova simulations incorporating accurate neutrino physics. While the results are pretty promising, it is still plausible that the companion star in J0453+1559 is a white dwarf, as white dwarfs have masses around 1 M☉. Because they have used up most of their fuel and are quite small, white dwarfs are faint, not very hot, and old, making them difficult to observe. This neutron star binary system is estimated to be around 4 billion years old, which makes it possible for the companion star to be a white dwarf.
Regardless of whether this companion star ends up being a white dwarf instead of a neutron star, this work is important because it helps relieve some of the tensions between theory and observation. Iron-core collapse supernovae likely produce these low-mass neutron stars, either instead of or in addition to electron-capture supernovae–a particular type of supernovae often produced by binary systems that have been thought to produce low-mass neutron stars. This work also presents a step forward in better understanding supernova physics and stellar evolution using improved simulations.
Astrobite edited by Kylee Carden
Featured image credit: Mckenzie Ferrari (made in Canva)
