Millinovae – Discovering a New Class of Stellar Phenomena

By | 26/01/2025

Astronomers have identified a new type of celestial event that challenges traditional understanding of white dwarf stars and their explosive behaviour. Dubbed “millinovae,” these phenomena are faint but fascinating, marking an exciting addition to the family of stellar explosions.

What Are Millinovae?

Millinovae are transient events occurring in binary star systems where a white dwarf—a dense remnant of a once-sunlike star—accretes matter from its companion. Unlike classical novae, which involve dramatic thermonuclear explosions ejecting vast amounts of material into space, millinovae emit supersoft X-rays without the telltale signs of mass ejection. These events are also far dimmer, approximately 1,000 times fainter than classical novae, making them easier to overlook but no less intriguing.

How Were They Discovered?

The first clue emerged in 2016 with ASASSN-16oh, an unusual optical transient discovered in the Small Magellanic Cloud. This event exhibited supersoft X-ray emissions consistent with classical novae but lacked evidence of mass ejection or rapid brightness changes typical of such eruptions.

Intrigued, researchers analysed data from the Optical Gravitational Lensing Experiment (OGLE), which monitors millions of stars. They identified 29 objects in the Magellanic Clouds with light curves resembling ASASSN-16oh—symmetrical, triangle-shaped outbursts lasting weeks to months. One of these, OGLE-mNOVA-11, confirmed the connection by displaying similar optical and X-ray properties during its 2023 outburst.

Characteristics of Millinovae

Millinovae stand out in several ways:

  1. Optical Outbursts: These events have symmetrical light curves, peaking at magnitudes between those of classical novae and fainter dwarf novae.
  2. Spectral Features: Their spectra reveal narrow hydrogen and helium lines, with no evidence of material being ejected.
  3. Supersoft X-Ray Emission: These X-rays, indicative of nuclear burning on the white dwarf’s surface, are less luminous than those from classical novae but much brighter than dwarf novae.
  4. Duration and Decline: Outbursts last for weeks to months and fade symmetrically, a behaviour distinct from other cataclysmic variables.

What Causes Millinovae?

The exact mechanism behind millinovae remains uncertain. One theory suggests that a sudden increase in mass accretion triggers thermonuclear burning on the white dwarf’s surface, but without ejecting material. Another idea points to an interaction between the white dwarf and its accretion disk, where energy from the disk’s instability creates the observed emissions.

Millinovae’s moderate brightness and peculiar X-ray behaviour hint at unique conditions. The white dwarfs involved may have higher-than-usual masses, allowing steady nuclear burning without a nova flash.

Why Do Millinovae Matter?

Millinovae could reveal new pathways in the evolution of binary star systems. They provide a potential growth mechanism for white dwarfs, which might eventually lead to Type Ia supernovae—cosmic explosions critical for measuring universal expansion.

This discovery also expands our understanding of supersoft X-ray sources. By studying millinovae, astronomers hope to refine models of white dwarf accretion and uncover connections between these faint outbursts and their more dramatic cousins.

What’s Next?

The study of millinovae is just beginning. Researchers plan to monitor these objects for future outbursts, perform detailed spectroscopic and X-ray observations, and test theoretical models. With the Magellanic Clouds offering a nearby laboratory of potential millinovae, the stage is set for deeper insights into these mysterious stellar fireworks.

 

Millinova near the solar system?

White Dwarf Binary Configuration

    • For a millinova to occur, the white dwarf needs to be in a close binary system where matter from the companion can transfer onto it. The companion’s evolution toward filling its Roche lobe is key to initiating mass transfer.
    • Observations suggest that roughly 25-50% of white dwarfs are in binary systems, but not all have companions in close enough orbits to enable significant mass transfer.

Companion Evolution

    • Current State: Many of the estimated 1500-5000 binary systems feature companions that are main-sequence stars or low-mass subgiants. These stars have not yet evolved to the point where they fill their Roche lobes.
    • Future State: As the companion stars evolve into red giants or asymptotic giant branch (AGB) stars, they will expand and begin transferring material onto the white dwarf if the white dwarf is in a close enough orbit.

Accretion Rates:

    • For a millinova, the accretion rate needs to be relatively low to moderate—just enough to trigger nuclear burning on the white dwarf’s surface without causing runaway thermonuclear reactions or excessive mass ejection that would result in a classical nova.
    • Systems with too slow accretion may remain quiescent, while those with rapid accretion likely lead to classical novae or even Type Ia supernovae.

Proximity to Millinova Conditions:

    • Nearby systems like Sirius B, Procyon B, and other known white dwarf binaries are currently stable and quiescent because their companions have not yet evolved to where they fill their Roche lobe. However, they may eventually exhibit activity as the companion star expands.

Known White Dwarf Binaries within 200 Light-Years:

    1. Surveys like the Gaia mission and Sloan Digital Sky Survey (SDSS) have catalogued many white dwarfs in binaries within this range.

Given that Gaia’s survey identified 1.3 million binary systems within 3,000 light-years, we can approximate the number within 200 light-years by considering the volume ratio. The volume of a sphere scales with the cube of its radius, so the volume within 200 light-years is about 0.0045 times that within 3,000 light-years. Doing the maths on this suggests there are ~ 5,850 binary systems within 200 light-years of which around 20-30% may contain a white dwarf star. However, the exact fraction is uncertain due to factors like binary evolution, mass transfer, and detection limitations.

    1. Systems with orbital periods of a few days to a few years are the most promising candidates, as these configurations allow material transfer when the companion evolves away from the main sequence along the asymptotic giant branch.

Future Evolution:

    1. Based on stellar evolution models, a small fraction of these systems (likely <10%) could evolve into states conducive to millinovae in the next billion years. Most systems will remain detached or transition into other phases (e.g., cataclysmic variables, novae, or Type Ia progenitors).

Timeframe:

    1. The transition into a millinova-like state could take millions to billions of years, depending on the mass and evolutionary pace of the companion star. Stars like Procyon B, with a 1.5 M progenitor, might evolve on shorter timescales compared to lower-mass stars. Over this timeframe, orbital mechanics will have greatly separated the solar system from these nearby stellar systems, however, it will also mean that more will enter this sphere to replace them, potentially bringing more highly evolved systems into close proximity to the solar system.

A millinova would need to be extremely close—within ~10 light-years or less—to pose any conceivable threat to the solar system. At distances of <50 light-years, the effects would be negligible, even for sensitive atmospheric or magnetic interactions. Given the known white dwarf population near the solar system, the likelihood of a harmful millinova occurring in our vicinity is extraordinarily low.

Detection Possibilities

Current Observations:

    • No nearby systems, within around 200 light years,  currently exhibit the telltale signs of millinovae,  transient supersoft X-ray emissions, optical outbursts, or accretion-driven variability. Whilst there are soft X-ray sourced know in the milky way, the nearest is around 1500 light years but a lot of observations would be required to determine if this is a millinova or something else, although we do know the system contains a white dwarf.

Future Surveys:

    • Missions like JWST, eROSITA, and ground-based observatories might catch nearby systems (within a few thousand light years) transitioning into early accretion states that could indicate millinovae activity.
    • Monitoring light curves and X-ray emissions from white dwarfs in binary systems could reveal precursors to millinovae.

 

Potential Nearby Candidates

While most known white dwarf binaries aren’t yet accreting material, some could become millinova candidates as their companions evolve:

  1. Cataclysmic Variables: Systems transitioning from quiescence into periodic outbursts might hint at slow, stable accretion phases conducive to millinovae.
  2. Detached Binaries: Systems with relatively close separations, where the companion is nearing the giant phase, could be monitored for signs of envelope expansion and Roche lobe overflow.

Implications for Nearby Millinovae

If a nearby white dwarf binary were to undergo a millinova-like event:

  • Observation Opportunities: Its proximity would provide a rare chance for detailed study across the electromagnetic spectrum, potentially refining our understanding of these phenomena.
  • Impact on the Solar Neighbourhood: Even at close ranges (~200 light-years), a millinova would pose no danger to Earth. Its brightness would be relatively modest compared to classical novae or supernovae.

 

Whilst the eventual evolution of nearby white dwarf binaries could indeed produce millinovae, this is likely to occur on long timescales, likely exceeding the lifespan of the human race, as their companions expand and begin mass transfer. While none are currently in this phase, systems with the right configurations could provide key insights into this fascinating new class of stellar phenomena when they do. Monitoring these systems over time will be essential to catch the first nearby millinova in action.

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