The VLT-FLAMES survey of massive stars

NGC 2004#115: A triple system hosting a possible short period B+BH binary

¹ Instituto de Astrofísica de Canarias, 38 200 La Laguna, Tenerife, Spain
e-mail: dlennon@iac.es
² Dpto. Astrofísica, Universidad de La Laguna, 38 205 La Laguna, Tenerife, Spain
³ Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University, Belfast BT7 1NN, UK
⁴ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Telespazio UK for ESA, European Space Astronomy Centre, Operations Department, 28691 Villanueva de la Cañada, Spain
¹⁰ South African Astronomical Observatory, PO Box 9, 7935 Cape Town, South Africa
¹¹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹² Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands

Received: 11 October 2021
Accepted: 23 November 2021

Abstract

Context. NGC 2004#115 was classified as a single lined Be spectroscopic binary in the Large Magellanic Cloud. Its Hα morphology is reminiscent of the Galactic systems LB-1 and HR 6819, both of which are proposed as either Be+BH or Be+stripped He-star systems.

Aims. Multi-epoch optical spectra of NGC 2004#115 are used to determine if this binary can be explained by either of these two scenarios, and hence shed additional light on these interesting systems.

Methods. VLT-FLAMES and SALT-HRS data covering a baseline of ∼20 years were analyzed to determine radial velocities and orbital parameters, while non-LTE model atmospheres were used to determine stellar parameters and the relative brightness of the system components. Archive MACHO, Gaia, and XMM-Newton data provide additional constraints on the system.

Results. NGC 2004#115 is found to be a triple system consisting of an inner binary with a period P = 2.92 d, eccentricity e ∼ 0.0, and mass function f = 0.07 M_⊙. The only firmly detected star in this inner binary is a B2 star, the primary, with a projected rotational velocity (v_e sin i) of 10 km s⁻¹ and a luminosity of log L/L_⊙ = 3.87. It contributes ∼60% of the total V-band light, with the tertiary contributing the other ∼40% of the light, while the secondary is not detected in the optical spectrum. The possibility that the primary is a low mass inflated stripped star is excluded since its Roche radius would be smaller than its stellar radius in such a compact system. A main sequence star of mass 8.6 M_⊙ is therefore inferred; however, the assumption of synchronous rotation leads to a secondary mass in excess of 25 M_⊙, which would therefore be a black hole. The tertiary is detected as a fainter blended component to the hydrogen and helium lines, which is consistent with a slightly less massive B-type star, though with v_e sin i ∼ 300 km s⁻¹. The data do not permit the characterization of the outer period, though it likely exceeds 120 days and is therefore in a stable configuration. The disk-like emission is variable, but may be associated with the inner binary rather than the rapidly rotating tertiary. XMM-Newton provides an upper limit of 5 × 10³³ ergs s⁻¹ in the 0.2–12 keV band which is consistent with, though not constraining, the system hosting a quiescent B+BH binary. A number of caveats to this scenario are discussed.