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One century data of τ CMa: a (2+1)+1 system with a short-period overcontact binary and an eccentric intermediate orbit with probably no apsidal motion

Sophie Rosu, Jesús Maíz Apellániz, Luca Sciarini, Roberto C. Gamen, Juan A. Molina-Calzada, Gonzalo Holgado, Rodolfo H. Barbá

TL;DR

τ CMa is revealed to be a complex hierarchical system in which the outer Aa–Ab orbit hosts a nested structure: Aa1–Aa2 as a spectroscopic pair, with Aa2 itself comprising a short-period eclipsing binary Aa2a–Aa2b. The authors synthesize astrometry, spectroscopy, photometry, and dynamical simulations to disentangle the architecture, finding that Aa2 is the eclipsing binary and that secular changes in the Aa1–Aa2 orbit are likely driven by the outer Aa–Ab motion rather than apsidal motion. Three-body dynamics indicate that the Ab component is unlikely to induce significant apsidal motion in Aa1–Aa2, while Aa2a–Aa2b could do so under certain mutual orientations. Additional spectroscopic work is needed to precisely determine the stellar parameters and wind properties, but the study demonstrates how multi-technique, hierarchical modeling can resolve the complexity of massive-star multiplicity and inform future observations.

Abstract

τ Canis Majoris (CMa) is an intriguing system that has captured astronomers' attention for more than a century. The two main components Aa and Ab are two evolved O stars on a 350 years orbit. Aa is itself a SB1 with a 155-days period and a 0.3 eccentricity. Since Hipparcos, we know that a 1.28-days period eclipsing binary (EB) is hidden somewhere in Aa or Ab, but nowhere else. Our recent analysis finally disentangles the system. We calculated the visual Aa-Ab orbit from AstraLux imaging. We detected the SB2 nature of Aa based on STIS spectra, the companion of the O star (Aa1) being a B+B binary (Aa2 = Aa2a + Aa2b). Multiple lines of evidence point towards Aa2 being the EB: time delays in the eclipsing orbit detected by TESS, high mass for Aa2 from SB1 from constraints from the orbit of Aa1, and a lack of radial-velocity motion of Ab synchronised with the eclipsing orbit. This remains as a tentative conclusion pending further analysis. We detect secular changes in the SB1 orbit of Aa1 on a baseline longer than a century. At this stage, the effect is most likely caused by the change in velocity of the Aa center of mass due to the Aa-Ab visual orbit. Apsidal motion is most probably not the culprit.

One century data of τ CMa: a (2+1)+1 system with a short-period overcontact binary and an eccentric intermediate orbit with probably no apsidal motion

TL;DR

τ CMa is revealed to be a complex hierarchical system in which the outer Aa–Ab orbit hosts a nested structure: Aa1–Aa2 as a spectroscopic pair, with Aa2 itself comprising a short-period eclipsing binary Aa2a–Aa2b. The authors synthesize astrometry, spectroscopy, photometry, and dynamical simulations to disentangle the architecture, finding that Aa2 is the eclipsing binary and that secular changes in the Aa1–Aa2 orbit are likely driven by the outer Aa–Ab motion rather than apsidal motion. Three-body dynamics indicate that the Ab component is unlikely to induce significant apsidal motion in Aa1–Aa2, while Aa2a–Aa2b could do so under certain mutual orientations. Additional spectroscopic work is needed to precisely determine the stellar parameters and wind properties, but the study demonstrates how multi-technique, hierarchical modeling can resolve the complexity of massive-star multiplicity and inform future observations.

Abstract

τ Canis Majoris (CMa) is an intriguing system that has captured astronomers' attention for more than a century. The two main components Aa and Ab are two evolved O stars on a 350 years orbit. Aa is itself a SB1 with a 155-days period and a 0.3 eccentricity. Since Hipparcos, we know that a 1.28-days period eclipsing binary (EB) is hidden somewhere in Aa or Ab, but nowhere else. Our recent analysis finally disentangles the system. We calculated the visual Aa-Ab orbit from AstraLux imaging. We detected the SB2 nature of Aa based on STIS spectra, the companion of the O star (Aa1) being a B+B binary (Aa2 = Aa2a + Aa2b). Multiple lines of evidence point towards Aa2 being the EB: time delays in the eclipsing orbit detected by TESS, high mass for Aa2 from SB1 from constraints from the orbit of Aa1, and a lack of radial-velocity motion of Ab synchronised with the eclipsing orbit. This remains as a tentative conclusion pending further analysis. We detect secular changes in the SB1 orbit of Aa1 on a baseline longer than a century. At this stage, the effect is most likely caused by the change in velocity of the Aa center of mass due to the Aa-Ab visual orbit. Apsidal motion is most probably not the culprit.
Paper Structure (6 sections, 5 figures, 2 tables)

This paper contains 6 sections, 5 figures, 2 tables.

Table of Contents

  1. $\boldsymbol{\tau}$ CMa: Stellar multiplicity does not have to be simple
  2. Astrometry: Long visual outer orbit Aa--Ab
  3. Spectroscopy: Intermediate orbit Aa1--Aa2
  4. Photometry: Short-period orbit Aa2a--Aa2b
  5. Three-body simulations with TRES
  6. What we still do not know but will soon

Figures (5)

  • Figure 1: Hierarchy of $\tau$ CMa. Results of our analysis are in orange. First image: 3 channel DDS2 RGB image. Second image: AstraLux $3"\times3"$ cut-out of the region around $\tau$ CMa A.
  • Figure 2: Visual orbit of Aa--Ab. Colours indicate three possible orbits based on different sets of data.
  • Figure 3: STIS spectroscopy: Aa (left) and Ab (right).
  • Figure 4: TESS light curves of the eclipsing binary.
  • Figure 5: TRES simulations: $\dot\omega$ in Aa1--Aa2 if induced by Aa2a--Aa2b (left) or Ab (right) for different $i_\text{mut}$.