A $1.9\,M_{\odot}$ neutron star candidate in a 2-year orbit

Abstract: We report discovery and characterization of a main-sequence G star orbiting a dark object with mass $1.90\pm 0.04 M_{\odot}$. The system was discovered via Gaia astrometry and has an orbital period of 731 days. We obtained multi-epoch RV follow-up over a period of 639 days, allowing us to refine the Gaia orbital solution and precisely constrain the masses of both components. The luminous star is a $\gtrsim 12$ Gyr-old, low-metallicity halo star near the main-sequence turnoff ($T_{\rm eff}\approx 6000$ K; $\log(g/\left[{\rm cm\,s^{-2}}\right])\approx 4.0$; $\rm [Fe/H]\approx-1.25$; $M\approx0.79 M_{\odot}$) with a highly enhanced lithium abundance. The RV mass function sets a minimum companion mass for an edge-on orbit of $M_2 > 1.67 M_{\odot}$, well above the Chandrasekhar limit. The Gaia inclination constraint, $i=68.7\pm 1.4$ deg, then implies a companion mass of $M_2=1.90\pm0.04 M_{\odot}$. The companion is most likely a massive neutron star: the only viable alternative is two massive white dwarfs in a close binary, but this scenario is disfavored on evolutionary grounds. The system's low eccentricity ($e=0.122\pm 0.002$) disfavors dynamical formation channels and implies that the neutron star likely formed with little mass loss ($\lesssim1\,M_{\odot}$) and with a weak natal kick ($v_{\rm kick}\lesssim 20\,\rm km\,s^{-1}$). The current orbit is too small to have accommodated the neutron star progenitor as a red supergiant or super-AGB star. The simplest formation scenario -- isolated binary evolution -- requires the system to have survived unstable mass transfer and common envelope evolution with a donor-to-accretor mass ratio $>10$. The system, which we call Gaia NS1, is likely a progenitor of symbiotic X-ray binaries and long-period millisecond pulsars. Its discovery challenges binary evolution models and bodes well for Gaia's census of compact objects in wide binaries.

• The paper presents a detailed analysis of Gaia NS1, revealing a neutron star candidate with a mass of 1.9 M⊙ using combined radial velocity and Gaia astrometric data.
• The methodology leverages 639 days of RV follow-up to constrain a low-eccentricity (e ≈ 0.122) 731-day orbit, implying a weak natal kick during formation.
• The findings challenge conventional NS formation models, highlighting the significance of low-metallicity binary evolution in ancient halo stars.

Analyzing A $1.9\,M_{\odot}$ Neutron Star Candidate in a 2-Year Orbit

The paper presents a detailed analysis of a newly identified neutron star (NS) candidate, labeled as Gaia NS1, discovered in data from the Gaia satellite's astrometric observations. This report explores the characteristics, mass constraints, and potential evolutionary history of this astrophysical object. Situated at a distance of approximately 735 parsecs, Gaia NS1 features a low-metallicity, ancient halo main-sequence star orbiting an unexpectedly massive and unseen companion. The orbital arrangement and constraints surrounding this binary system provide intriguing insights into NS formation and binary evolution models.

Properties of the Observed System

Gaia NS1 comprises a main-sequence G star with an effective temperature near 6000 K, surface gravity of around $\log g \approx 4$, metallicity of [Fe/H] $\approx -1.25$, and a lithium-enhanced abundance—a peculiarity given its aged status of approximately 12 Gyr. These spectral and chemical diagnostics, coupled with a Galactic orbit typifying halo stars, suggest potential significance to the study of NS formation at low metallicity.

Orbit and Mass Constraints

Leveraging radial velocity (RV) follow-up observations over 639 days, the analysis constrains the unseen companion's mass primarily through the radial velocity mass function and Gaia's astrometric parameters. These measurements yield a surprisingly large NS mass estimate of $1.90\pm 0.04\,M_{\odot}$, considerably exceeding the Chandrasekhar limit and approaching the theoretical upper mass bounds for neutron stars. The NS is found in a relatively wide, low-eccentricity ($e \approx 0.122$) 731-day orbit, suggesting an unusual formation channel or dynamics uncommon in similar systems.

Formation and Evolutionary Considerations

Given the wide separation and low eccentricity, along with the system's old age and low metallicity, the conventional scenario of star evolution, followed by supernova and potential associated kicks, is critically examined. For Gaia NS1, it appears that minimal mass loss occurred during the companion's transition to a compact object, as evidenced by the orbit's low eccentricity and suggests a weak natal kick ($v_{\rm kick}\lesssim 20\,\rm km\,s^{-1}$). This lends credence to models that propose NS formation with limited angular momentum loss, potentially from lower-mass helium progenitors.

Implications and Future Prospects

Gaia NS1 exemplifies a significant challenge to the canonical understanding of NS formation at low metallicity, particularly regarding common envelope evolution and the associated mass transfer required to produce such a system without exacerbating orbital eccentricity. The lithium abundance peculiarity points to potentially novel polluting mechanisms, possibly linked to the initial conditions of the binary formation or subsequent mass transfer processes. The precise orbital dynamics inferred and the robust nature of Gaia's astrometric measurements set a strong precedent for further investigations into similar low-metallicity, massive, non-recycled NSs.

This paper opens avenues for investigating symbiotic X-ray binary progenitors, given that Gaia NS1 could analogously evolve into systems akin to long-period millisecond pulsars. Additionally, the implications for binary evolution extend to refining the star's mass—integral to understanding isolated binary evolution within the Milky Way's halo and potentially providing clues to unobserved heavy companions in ancient stars, with Gaia's deliverables serving as a pivotal statistical lens.

In sum, while the findings defy certain long-held assumptions of late-stage stellar evolution and mass loss, they also expand the frontier of potential discoveries of peculiar compact object configurations and their respective dynamical histories. The study exemplifies the profound potential of combining astrometric and spectroscopic techniques in uncovering and characterizing elusive cosmic phenomena.