A moon-sized, highly magnetised and rapidly rotating white dwarf may be headed toward collapse
Submitted to Nature, see paper here.
White dwarfs represent the last stage of evolution for low and intermediate-mass stars (below about 8 times the mass of our Sun), and like their stellar progenitors, they are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the masses of the coalescing white dwarfs, the merger can lead to a supernova of type Ia, or it can give birth to a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised due to the strong dynamo that may arise during the merger, and rapidly rotating due to conservation of the orbital angular momentum of the binary.
Here we report the discovery of a white dwarf, ZTF J190132.9+145808.7, which presents all these properties, but to an extreme: a rotation period of 6.94 minutes, one of the shortest measured for an isolated white dwarf, a magnetic field ranging between 600 MG and 900 MG over its surface, one of the highest fields ever detected on a white dwarf, and a stellar radius of 1810 km, slightly larger than the radius of the Moon. Such a small radius implies the star's mass is the closest ever detected to the white dwarf maximum mass, or Chandrasekhar mass. In fact, as the white dwarf cools and its composition stratifies, it may become unstable and collapse due to electron capture, exploding into a thermonuclear supernova or collapsing into a neutron star. Neutron stars born in this fashion could account for 10% of their total population.
Intermediate-mass Stars Become Magnetic White Dwarfs
The Astrophysical Journal Letters, Volume 901, Issue 1, id.L14, 9 pp.1
When a star exhausts its nuclear fuel, it either explodes as a supernova or more quiescently becomes a white dwarf, an object about half the mass of our Sun with a radius of about that of the Earth. About one-fifth of white dwarfs exhibit the presence of magnetic fields, whose origin has long been debated as either the product of previous stages of evolution or of binary interactions. We here report the discovery of two massive and magnetic white-dwarf members of young star clusters in the Gaia second data release (DR2) database, while a third massive and magnetic cluster white dwarf was already reported in a previous paper. These stars are most likely the product of single-star evolution and therefore challenge the merger scenario as the only way to produce magnetic white dwarfs. The progenitor masses of these stars are all above 5 solar masses, and there are only two other cluster white dwarfs whose distances have been unambiguously measured with Gaia and whose progenitors' masses fall in this range. This high incidence of magnetic white dwarfs indicates that intermediate-mass progenitors are more likely to produce magnetic remnants and that a fraction of magnetic white dwarfs forms from intermediate-mass stars.
Polarisation of Accreting X-ray Pulsars. I. A New Model
Polarisation of Accreting X-ray Pulsars. II. Hercules X-1
Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 1, pp.109-128
Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 1, pp.129-136
A new window is opening in high-energy astronomy: X-ray polarimetry. With many missions currently under development and scheduled to launch as early as 2021, observations of the X-ray polarisation of accreting X-ray pulsars will soon be available. As polarisation is particularly sensitive to the geometry of the emission region, the upcoming polarimeters will shed new light on the emission mechanism of these objects, provided that we have sound theoretical models that agree with current spectroscopic and timing observation and that can make predictions of the polarisation parameters of the emission. We here present a new model for the polarised emission of accreting X-ray pulsars in the accretion column scenario that for the first time takes into account the macroscopic structure and dynamics of the accretion region and the propagation of the radiation toward the observer, including relativistic beaming, gravitational lensing and quantum electrodynamics. In Paper I, we present all the details of the model, while in Paper II, we apply our model to predict the polarisation parameters of the bright X-ray pulsar Hercules X-1.
Hunting for ancient brown dwarfs: the developing field of brown dwarfs in globular clusters
Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers, no. 521; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 521 (2019)
The detection of brown dwarfs in globular star clusters will allow us to break the degeneracies in age, mass and composition that affect our current models, and therefore to constrain the physics of their atmospheres and interiors. Furthermore, detecting brown dwarfs will help us constraining the properties of the clusters themselves, as they carry information about the clusters’ age and dynamics. Great advancements in this field are to be expected in the next ten years, thanks to the extraordinary sensitivity in the infrared of upcoming telescopes like JWST and the ELTs.
Testing general relativity with accretion onto compact objects
Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers, no. 516; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 516 (2019)
The X-ray emission of neutron stars and black holes presents a rich phenomenology that can lead us to a better understanding of their nature and to address more general physics questions: Does general relativity (GR) apply in the strong gravity regime? Is spacetime around black holes described by the Kerr metric? This white paper considers how we can investigate these questions by studying reverberation mapping and quasi-periodic oscillations in accreting systems with a combination of high-spectral and high-timing resolution. In the near future, we will be able to study compact objects in the X-rays in a new way: advancements in transition-edge sensors (TES) technology will allow for electron-volt-resolution spectroscopy combined with nanoseconds-precision timing.
Polarimetry of Magnetars and Isolated Neutron Stars
Book Chapter. Astronomical Polarisation from the Infrared to Gamma Rays, Astrophysics and Space Science Library, Volume 460. ISBN 978-3-030-19714-8. Springer Nature Switzerland AG, 2019, p. 301.
Polarised radiation from isolated neutron stars provides key diagnostics on the structure of the neutron-star magnetosphere and the properties of its atmosphere. Furthermore, the detection of a large degree of polarisation is strong evidence for the presence of vacuum birefringence, which implies that photons of different polarisations travel at different speeds through the magnetised vacuum. We outline how polarisation is generated in neutron-star atmospheres and magnetospheres, how the polarisation is preserved as the radiation travels to us, what the current observations tell us and what are the prospects for future observations.
Vacuum birefringence and the x-ray polarization from black-hole accretion disks
Physical Review D, Volume 97, Issue 8, id.083001
In the next decade, x-ray polarimetry will open a new window on the high-energy Universe, as several missions that include an x-ray polarimeter are currently under development. Observations of the polarization of x rays coming from the accretion disks of stellar-mass and supermassive black holes are among the new polarimeters’ major objectives. In this paper, we show that these observations can be affected by the quantum electrodynamic (QED) effect of vacuum birefringence: after an x-ray photon is emitted from the accretion disk, its polarization changes as the photon travels through the accretion disk’s magnetosphere, as a result of the vacuum becoming birefringent in the presence of a magnetic field. We show that this effect can be important for black holes in the energy band of the upcoming polarimeters and has to be taken into account in a complete model of the x-ray polarization that we expect to detect from black-hole accretion disks, both for stellar mass and for supermassive black holes. We find that, for a chaotic magnetic field in the disk, QED can significantly decrease the linear polarization fraction of edge-on photons, depending on the spin of the hole and on the strength of the magnetic field. This effect can provide, for the first time, a direct way to probe the magnetic field strength close to the innermost stable orbit of black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general.
Polluting white dwarfs with perturbed exo-comets
Monthly Notices of the Royal Astronomical Society, Volume 469, Issue 3, August 2017, Pages 2750–2759
We present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun’s Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere.