The Binarity of Early-type Stars from LAMOST medium-resolution Spectroscopic Survey

Massive binaries play significant roles in many fields. Identifying massive stars, particularly massive binaries, is of great importance. In this paper, by adopting the technique of measuring the equivalent widths of several spectral lines, we identified 9382 early-type stars from the LAMOST medium-resolution survey and divided the sample into four groups, T1 (∼O-B4), T2 (∼B5), T3 (∼B7) and T4 (∼B8-A). The relative radial velocities RVrel were calculated using Maximum Likelihood Estimation. The stars with significant changes of RVrel and at least larger than 15.57 km s-1 were identified as spectroscopic binaries. We found that the observed spectroscopic binary fractions for the four groups are 24.6% ± 0.5%, 20.8% ± 0.6%, 13.7% ± 0.3% and 7.4% ± 0.3%, respectively. Assuming that orbital period (P) and mass ratio (q) have intrinsic distributions as f(P) P π (1 < P < 1000 days) and f(q) q κ (0.1 < q < 1), respectively, we conducted a series of Monte-Carlo simulations to correct observational biases for estimating the intrinsic multiplicity properties. The results show that the intrinsic binary fractions for the four groups are 68% ± 8%, 52% ± 3%, 44% ± 6% and 44% ± 6%, respectively. The best estimated values for π are -1 ± 0.1, -1.1 ± 0.05, -1.1 ± 0.1 and -0.6 ± 0.05, respectively. The κ cannot be constrained for groups T1 and T2 and is -2.4 ± 0.3 for group T3 and -1.6 ± 0.3 for group T4. We confirmed the relationship of a decreasing trend in binary fractions toward late-type stars. No correlation between the spectral type and orbital period distribution has been found yet, possibly due to the limitation of observational cadence.