The VLT-FLAMES survey of massive stars. NGC2004#115: A triple system hosting a possible short period B+BH binary

Understand

The star NGC2004#115 in the LMC, originally classified as an (SB1) Be spectroscopic binary, bears some morphological resemblance to the Galactic systems LB-1 and HR 6819, both of which are proposed as either Be+black hole (BH) or Be+stripped He-star systems.

• Two data-sets (ESO/VLT and SALT) of multi-epoch optical spectra of NGC 2004#115, separated by baseline of $\sim$20 years, lead us conclude it is a triple system hosting an inner binary with a period of 2.92 d, eccentricity $\sim$0.0 and mass function $\sim$0.07 $M_\odot$.
• The inner binary harbours a B-type star (the primary) with projected rotational velocity of 10km/s, and luminosity $\log L/L_\odot$=3.87, contributing $\sim$60% of the V-band light to the system.
• The secondary is not detected, while the tertiary, which contributes 40% of the light, is tentatively identified as a less luminous B-type star with high projected rotational velocity.