The Masses of the Millisecond Pulsar J1012+5307 and Its White Dwarf Companion^*

We report on spectroscopy of the white dwarf companion of the millisecond radio pulsar PSR J1012+5307. We find strong Balmer absorption lines, as would be expected for a cool DA white dwarf. The profiles are much narrower than usual, however, and lines are seen up to H12, indicating that the companion has a low gravity and hence a low mass. This is consistent with the expectation—based on evolutionary considerations and on the mass function—that it is a low-mass white dwarf with a helium core. By comparing the spectra to model atmospheres, we derive an effective temperature T_eff = 8550 ± 25 K and a surface gravity log g = 6.75 ± 0.07 cgs. Using the Hamada-Salpeter mass-radius relation for helium white dwarfs, with an approximate correction for finite-temperature effects, we infer a mass M_WD = 0.16 ± 0.02 M_☉. This is the lowest mass among all spectroscopically identified white dwarfs. We determine radial velocities from our spectra, and we find a radial-velocity amplitude of 280 ± 15 km s^-1. With the pulsar's radial-velocity amplitude, the mass ratio M_PSR/M_WD = 13.3 ± 0.7. From all constraints, we find that with 95% confidence, 1.5 < M_PSR/M_☉ < 3.2.