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New Insights into the HD 49798/RX J0648.0–4418 Binary System
A recent investigation led by an Italian astronomer has provided significant revelations regarding the binary system HD 49798/RX J0648.0–4418. Findings from this research, released on December 24 via arXiv, shed light on the characteristics of this system, confirming the presence of a substantial and rapidly rotating white dwarf.
Located approximately 1,700 light years from Earth, HD 49798 is a luminous blue subdwarf star classified as spectral type O (sdO). It holds the distinction of being the brightest known star of its kind, with an absolute magnitude of −0.24 and a bolometric luminosity measuring 4,400 times that of our Sun. The estimated effective temperature of HD 49798 is around 47,500 K, placing it among the hottest stars observable.
Throughout the years, extensive observations of HD 49798 revealed its unusual spectrum, which showcases notable helium and nitrogen lines. Additionally, the star has demonstrated radial velocity fluctuations, indicating the influence of a companion object. Despite previous observations hinting at this relationship, the exact nature of the companion remained uncertain.
Recent studies have documented soft X-ray emissions from a source identified as RX J0648.0–4418, which corresponds directly to the location of HD 49798. The observed periodicity of these emissions pointed toward the possibility of a rotating neutron star or white dwarf in the system. A thorough analysis by Sandro Mereghetti of the Institute of Space Astrophysics and Cosmic Physics in Milano has now strengthened the argument for the presence of a white dwarf.
“Multiple factors suggest that the companion to HD 49798 is indeed a massive white dwarf, leading to intriguing implications for the future development of this binary system,” Mereghetti articulated in his publication.
The research determined that the companion white dwarf possesses a mass of 1.22 solar masses and a remarkably short spin period of about 13.2 seconds, one of the briefest periods recorded in white dwarfs. While these characteristics could align with a neutron star classification, the X-ray luminosity, spin period dynamics, and the dimensions of the X-ray emission region strongly favor the white dwarf hypothesis.
As outlined in the study, the white dwarf generates pulsed X-rays characterized by a soft spectrum, driven by the accretion of material from the stellar wind of its companion subdwarf, which has a radius near 1.08 solar radii and a mass of approximately 1.41 solar masses. The orbital period of this binary system is estimated to be roughly 1.55 days.
Mereghetti pointed out that HD 49798/RX J0648.0–4418 represents the only known X-ray binary that relies on accretion from a hot subdwarf. The considerable mass and swift spin rate of the white dwarf, combined with the subdwarf’s high luminosity, comparatively low surface gravity, and unique chemical compositions, render this binary system especially distinctive in contrast to other similar systems.
“These attributes of HD 49798/RX J0648.0–4418 make it particularly compelling, contributing valuable insights into stellar evolution models and enhancing our understanding of both hot subdwarfs and white dwarfs,” Mereghetti concluded.
More information: Sandro Mereghetti, The massive fast spinning white dwarf in the HD 49798/RX J0648.0-4418 binary, arXiv (2024). DOI: 10.48550/arxiv.2412.18546
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phys.org