Astronomers have observed an unexplained shock wave around the white dwarf star RXJ0528+2838, located approximately 730 light-years from Earth. This phenomenon challenges existing astronomical theories regarding how dead stars, particularly those without accretion discs, interact with their environments and generate powerful outflows of material. The discovery, detailed in Nature Astronomy, suggests the need for revised models of binary star evolution.
Discovery and Characteristics of RXJ0528+2838
RXJ0528+2838 is a white dwarf, which is the dense remnant core of a low-mass star, orbited by a companion star. Typically, material transferred from a companion star forms an accretion disc around the white dwarf, leading to powerful outflows that can create shock waves or nebulae. However, observations indicate that RXJ0528+2838 exhibits no signs of such a disc. The star is classified as a polar white dwarf, a type generally not expected to form an accretion disc from its companion.
The Observed Shock Wave
Researchers initially identified an unusual nebulosity around the star using the Isaac Newton Telescope. Subsequent detailed observations conducted with the MUSE instrument on the European Southern Observatory's (ESO) Very Large Telescope (VLT) confirmed the presence of a bow shock originating from the binary system. This bow shock is a curved arc of material formed as the white dwarf moves supersonically through interstellar gas, causing its outflows to interact with the surrounding material.
The bow shock extends approximately 3,800 to 4,000 times the distance between Earth and the Sun. Its observed size and shape indicate that the white dwarf has been expelling a powerful outflow of material for at least 1,000 years.
Unexplained Mechanism and Magnetic Field Hypothesis
The absence of an accretion disc around RXJ0528+2838, coupled with the long-lasting powerful outflow, presents a challenge to current physical theories concerning polar white dwarfs. The mechanism by which a disc-less white dwarf could power such an outflow remains unclear.
Scientists hypothesize that a strong magnetic field, confirmed by MUSE data, may play a role in this process. This magnetic field could channel material directly from the companion star onto the white dwarf, bypassing the formation of an accretion disc. While this suggests that powerful outflows can occur without a disc, the measured strength of the magnetic field can only account for powering a bow shock for a few hundred years. This leaves a significant portion of the bow shock's observed longevity unexplained, suggesting an additional, currently unidentified, energy source may be involved.
Implications and Future Research
This discovery necessitates a re-evaluation of current understandings regarding how dead stars interact with their environments and their evolutionary paths. The presence of this unpredicted "energy leak" may also influence models of binary star evolution over cosmic timescales.
A team of researchers from the universities of Southampton and Durham contributed to this discovery. Future plans include searching for similar systems within the Milky Way to determine if this phenomenon is rare or if such objects have been previously overlooked. Further observations with advanced instruments like ESO's Extremely Large Telescope (ELT) are anticipated to provide more insights into this phenomenon.