Nearby neutron stars
Neutron stars are city-sized stars, of just 20-30 kilometres in diameter. These have about one to two times the mass of the Sun and are compact remnants of supernova explosions. Their gravitational and magnetic fields are extremely strong. Some neutron stars emit sweeping beams of particles and light and are detected as pulsars.
Only a small fraction of the vast population of these objects in our Milky Way has been discovered so far. Until now, these were found at distances beyond 125 parsecs. However, a few thousands are expected within that boundary.
Most recently, I searched for nearer sources that may hold neutron stars.
I used broad-optical data from the Gaia astrometric space mission, complemented with multiwavelength data from other space and ground-based telescopes.
Artist’s impression of a neutron star - white dwarf binary, of which the light is observed with Gaia and other ground and space based telescopes. The neutron star could also emit gravitational waves. Credit: ESO/L. Calçada
Some of the candidates that I found could be white dwarf - neutron star binaries. White dwarfs are smaller than a few times the size of the Earth and are remnants of low to intermediate mass stars.
Neutron stars are important for gravitational wave searches with ground-based interferometers. When neutron stars rotate and are axially asymmetric or precess, these can emit gravitational waves. The signals could allow us to examine their still mysterious interior properties and their systems. However, these signals would be weaker than the ultra-short ones that have been measured from coalescing compact binaries. Targets should thus be either nearby or strong emitters.
Finding nearer neutron stars would increase the chance of detecting gravitational waves from them. Independently, it would offer ample opportunities to study these fascinating objects.
Bihain, G.: “Search of nearby resolved neutron stars among optical sources”, 2023, Monthly Notices of the Royal Astronomical Society, Volume 524, Issue 4, pp. 5658–5707. (Preprint merged PDF)
Substellar objects
Brown dwarfs and planetary-mass objects are smaller than several times the size of Jupiter. Their masses are of about 80-13 and less than 13 times the mass of Jupiter, respectively. These substellar objects form similarly as stars or as by-products of stellar formation.
When projecting the nearest known brown dwarfs and stars on the Galactic plane, I found that the brown dwarfs are distributed very asymmetrically relative to the Sun. Thereby, the star to brown dwarf number ratio resulted to be of 5:1, or 4:1 depending on assumptions of completeness.
Bihain, G., Scholz, R.-D.: “A non-uniform distribution of the nearest brown dwarfs”, 2016, Astronomy & Astrophysics, Volume 589, id.A26, 6 pp.
Update 2023/12 v4 of the discovery and associated results (PDF, PDF).
Substellar objects are not dense and hot enough in their cores to ignite or sustain nuclear fusion of hydrogen. Therefore these mostly dim, cool down, and emit infrared light. To be directly detected from Earth, these have to be nearby or young.
Nearby brown dwarfs were found in interstellar space for example in
Bihain, G., Scholz, R.-D., Storm, J., Schnurr, O.: “An overlooked brown dwarf neighbour (T7.5 at d ~ 5 pc) of the Sun and two additional T dwarfs at about 10 pc”, 2013, Astronomy & Astrophysics, Volume 557, id.A43, 6 pp.
Scholz, R.D., Bihain, G., Schnurr, O., Storm, J.: “Two very nearby (d~5 pc) ultracool brown dwarfs detected by their large proper motions from WISE, 2MASS, and SDSS data”, 2011, Astronomy & Astrophysics Letters, Volume 532, id.L5, 4 pp.
Or searched for closely around the nearest northern stars in
Gauza, B., Béjar, V. J. S., Rebolo, R., Álvarez, C., Zapatero Osorio, M. R., Bihain, G., Caballero, J. A., Pinfield, D. J., Telesco, C. M., Packham, C.: “GTC/CanariCam deep mid-infrared imaging survey of northern stars within 5 pc”, 2021, Astrophysical Journal, Volume 923, Issue 1, id.119, 24 pp.
Gauza, B., Béjar, V. J. S., Rebolo, R., Álvarez, C., Bihain, G., Zapatero Osorio, M. R., Caballero, J. A., Telesco, C. M., Packham, C.: “Constraints on the substellar companions in wide orbits around the Barnard's Star from CanariCam mid-infrared imaging”, 2015, Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 2, pp.1677-1683
Besides, I contributed to the search and characterization of substellar objects in the young open clusters of the Pleiades and σ Orionis (e.g. a, b, c; all).
The full list of my publications can be found at NASA ADS.