2024/02/09 by Kareem El-Badry, Joshua D. Simon, Henrique Reggiani +14 · 3 voices · 7 citations
Physics and Astronomy · #Stellar, planetary, and galactic studies #Pulsars and Gravitational Waves Research #Gamma-ray bursts and supernovae
paper · pdf · doi:10.33232/001c.116675
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.