A 1.9 solar-mass neutron star candidate in a 2-year orbit
Kareem El-Badry, Joshua D. Simon, Henrique Reggiani, Hans‐Walter Rix, David W. Latham, Allyson Bieryla, Lars A. Buchhave, S. Shahaf, T. Mazeh, Sukanya Chakrabarti, Puragra Guhathakurta, I. Ilyin, Thomas M. Tauris
Abstract
We report discovery and characterization of a main-sequence G star orbiting a dark object with mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>1.90</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≳</mml:mo> <mml:mn>12</mml:mn> </mml:mrow> </mml:math> ,Gyr-old, low-metallicity halo star near the main-sequence turnoff (,K; ; ; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>M</mml:mi> <mml:mo>≈</mml:mo> <mml:mn>0.79</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> ) with a highly enhanced lithium abundance. The RV mass function sets a minimum companion mass for an edge-on orbit of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>></mml:mo> <mml:mn>1.67</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> , well above the Chandrasekhar limit. The Gaia inclination constraint, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>68.7</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> </mml:math> ,deg, then implies a companion mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.90</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> . 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.122</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.002</mml:mn> </mml:mrow> </mml:math> ) disfavors dynamical formation channels and implies that the neutron star likely formed with little mass loss ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≲</mml:mo> <mml:mn>1</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> ) and with a weak natal kick (). Stronger kicks with more mass loss are not fully ruled out but would imply that a larger population of similar systems with higher eccentricities should exist. 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>></mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:math> . 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.