A population of neutron star candidates in wide orbits from Gaia astrometry (2405.00089v2)

Abstract: We report discovery and spectroscopic follow-up of 21 astrometric binaries containing solar-type stars and dark companions with masses near 1.4 $M_{\odot}$. The simplest interpretation is that the companions are dormant neutron stars (NSs), though ultramassive white dwarfs (WDs) and tight WD+WD binaries cannot be fully excluded. We selected targets from Gaia DR3 astrometric binary solutions in which the luminous star is on the main sequence and the dynamically-implied mass of the unseen companion is (a) more than $1.25\,M_{\odot}$ and (b) too high to be any non-degenerate star or close binary. We obtained multi-epoch radial velocities (RVs) over a period of 700 days, spanning a majority of the orbits' dynamic range in RV. The RVs broadly validate the astrometric solutions and significantly tighten constraints on companion masses. Several systems have companion masses that are unambiguously above the Chandrasekhar limit, while the rest have masses between 1.25 and 1.4 $M_{\odot}$. The orbits are significantly more eccentric at fixed period than those of typical WD + MS binaries, perhaps due to natal kicks. Metal-poor stars are overrepresented in the sample: 3 out of 21 objects (14%) have [Fe/H]$\sim-1.5$ and are on halo orbits, compared to $\sim$0.5% of the parent Gaia binary sample. The metal-poor stars are all strongly enhanced in lithium. The formation history of these objects is puzzling: it is unclear both how the binaries escaped a merger or dramatic orbital shrinkage when the NS progenitors were red supergiants, and how they remained bound when the NSs formed. Gaia has now discovered 3 black holes (BHs) in astrometric binaries with masses above 9 $M_{\odot}$, and 21 NSs with masses near $1.4\,M_{\odot}$. The lack of intermediate-mass objects in this sample is striking, supporting the existence of a BH/NS mass bimodality over 4 orders of magnitude in orbital period.

• The paper presents a method using Gaia DR3 astrometry and 700-day radial velocity monitoring to identify neutron star candidates in wide binaries.
• The research finds systems with companion masses above 1.25 solar masses, indicating possible dormant neutron stars with high orbital eccentricities.
• The study reveals an overrepresentation of metal-poor stars with enhanced lithium, offering new insights into binary evolution and the mass distribution of compact objects.

Overview of "A Population of Neutron Star Candidates in Wide Orbits from Gaia Astrometry"

The paper titled "A Population of Neutron Star Candidates in Wide Orbits from Gaia Astrometry" presents an analysis of 21 astrometric binaries containing solar-type stars and dark companions with masses near 1.4 solar masses. The research utilizes data from Gaia's third data release (Gaia DR3) to identify potential candidates for binaries where the unseen companion might be a dormant neutron star (NS). This work illustrates a methodical approach to uncover a population of neutron stars in wide binary systems, offering new insights into the Galactic distribution and properties of these compact objects.

Methodology and Dataset

The research team selected targets using astrometric binary solutions from Gaia DR3, focusing on systems where the visible star remains on the main sequence and the dynamically implied mass of the companion exceeds 1.25 solar masses, a value too high to be attributable to a non-degenerate stellar companion or a close binary. To validate their selections, they acquired multi-epoch radial velocity (RV) data over a span of 700 days, which helped refine the mass constraints on these companions.

Results and Interpretation

The RVs largely corroborated the astrometric solutions and significantly narrowed down the companion mass range. Notably, several systems presented companion masses well over the Chandrasekhar limit, hinting at the presence of neutron stars. Meanwhile, others remained within the range of 1.25 to 1.4 solar masses. The paper highlights that the identified binary orbits exhibit higher eccentricity compared to typical WD + MS binaries, a feature potentially indicative of natal kicks that neutron stars might have received during supernova explosions.

The sample revealed an overrepresentation of metal-poor stars, particularly with [Fe/H] ~ -1.5, residing in halo orbits. This is intriguing because such stars constitute merely 0.5% of Gaia's binary sample. The detection of significantly enhanced lithium in these metal-poor stars adds a puzzling dimension regarding their formation history, especially concerning how these binaries avoided merger or severe orbital contraction during stellar evolution phases.

Implications and Future Directions

The identification of these neutron star candidates via astrometry offers a new observational perspective on the binary evolution of massive stars and the remnants they leave behind. These findings suggest a potential bimodal distribution in the compact object mass spectrum, consistent with emerging results across different observational regimes. The absence of intermediate-mass black hole or neutron star candidates supports the notion of a mass gap, bolstering theories around the mass distribution of these exotic objects.

Further observational campaigns, particularly those that can refine orbital parameters and detect electromagnetic signatures from the dark companions, would be crucial in confirming the nature of these objects. Additionally, future Gaia data releases with improved precision and complementary observational efforts using different astrophysical methods hold the promise of enhancing the constraints on the mass and nature of these companions.

This research provides foundational data pointing towards the existence of wide neutron star binaries, enriching our understanding of binary star evolution, stellar remnants, and the Galactic population of compact objects.