[Paper Review] Kelu-1 AB - A possible brown dwarf triple system

M. B. Stumpf, W. Brandner+6 more|ArXiv.org|Nov 4, 2008

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This study presents the first dynamical mass measurement of the Kelu-1 AB brown dwarf binary system using high-resolution imaging and astrometry from HST and the VLT/NACO, revealing a total mass of 177$^{+113}_{-55}$ M$_{\rm Jup}$, and provides strong evidence for a possible third component, suggesting Kelu-1 AB may be the first confirmed brown dwarf triple system.

ABSTRACT

We have monitored the benchmark L dwarf binary Kelu-1AB over the past 3 years to derive improved spectral types and luminosities for the individual components. The astrometric measurements enable us to compute the orbital parameters and result in the first dynamical mass estimate for the system. We obtained resolved high angular resolution, near-IR images with HST and the VLT/NACO adaptive optics instrument in the J, H and Ks bands. In addition we used NACO to achieve the first spatially resolved mid-resolution spectra in the H- and K-band for a precise spectral type determination. The near-IR spectrum of Kelu-1A reveals a distinct dip in the H-band providing evidence that Kelu-1 A itself is a spectroscopic binary. We derive spectral types of L0.5+- 0.5 for Kelu- 1 Aa and T7.5+-1 for Kelu-1 Ab. Kelu-1B is classified as spectral type L3pec +-1.5. From the relative orbit, we derive an orbital period of 38 +8 -6 years and a semi-major axis of 6.4+2.4 -1.3 AU. This yields the first dynamically determined total system mass of 177 +113 -55 Mjup for the Kelu-1AB system, with the uncertainties mainly attributed to the error of the system distance measurement and the yet missing apastron passage. The derived properties of Kelu-1AB allow us to test common theoretical models. The comparison of our results with color-magnitude diagrams based on evolutionary models yields a slightly revised age estimate (0.3 - 0.5 Gyr) and a discrepancy between dynamically and theoretically derived masses, stressing the importance for further dynamical mass determinations of brown dwarf binaries.

Motivation & Objective

• To determine the orbital parameters and dynamical mass of the Kelu-1 AB brown dwarf binary system independently of theoretical models.
• To resolve and characterize the individual components of Kelu-1 AB using high angular resolution near-IR imaging and spectroscopy.
• To test the consistency of observed properties with theoretical evolutionary models and mass-luminosity relations.
• To investigate the possibility of a third, unresolved component in the system to resolve discrepancies between dynamical and theoretical masses.
• To improve spectral type and photometric measurements for each component using spatially resolved data.

Proposed method

• Acquired high angular resolution near-IR images in J, H, and K$_{\rm s}$ bands using HST/NICMOS and VLT/NACO adaptive optics.
• Obtained spatially resolved mid-resolution H- and K-band spectra with NACO to enable precise spectral type determination of each component.
• Performed eight-epoch astrometric measurements from 2005 to 2008 to track orbital motion and derive orbital elements.
• Fitted the observed astrometric data to Keplerian orbital elements to compute orbital period, semi-major axis, eccentricity, and inclination.
• Calculated the total system mass using the orbital solution and distance measurement, applying Newtonian dynamics: $ M = \frac{a^3}{P^2} \cdot \frac{4\pi^2}{G} $.
• Compared observed photometry and spectral types with theoretical DUSTY evolutionary models to estimate age and test mass-luminosity relations.

Experimental results

Research questions

• RQ1 What is the dynamical mass of the Kelu-1 AB brown dwarf binary system, and how does it compare to theoretical predictions?
• RQ2 Are the individual components of Kelu-1 AB resolved in near-IR imaging and spectroscopy, and what are their precise spectral types?
• RQ3 Is there evidence for a third, unresolved component in the system that could explain the discrepancy between dynamical and theoretical masses?
• RQ4 How do the observed photometric and spectroscopic properties of Kelu-1 AB compare with predictions from evolutionary models?
• RQ5 What is the orbital solution of the system, and how does it constrain the system's age and mass distribution?

Key findings

• The first resolved near-IR spectra of Kelu-1 A and B were obtained, revealing a distinct H-band dip in Kelu-1 A consistent with a spectroscopic binary.
• Spectral types were refined to L0.5$\pm$0.5 for Kelu-1 Aa and T7.5$\pm$1 for Kelu-1 Ab, with Kelu-1 B classified as L3 pec$\pm$1.5.
• The orbital solution yields a period of $38^{+8}_{-6}$ years and a semi-major axis of $6.4^{+2.4}_{-1.3}$ AU with eccentricity $e = 0.82 \pm 0.10$.
• The total dynamical mass is $177^{+113}_{-55}$ M$_{\rm Jup}$, significantly higher than theoretical expectations for a brown dwarf binary.
• The high mass implies the primary may be a very low-mass star if only two components exist, contradicting its early L-type classification.
• The presence of a third, unresolved component—implied by the mass discrepancy—would make Kelu-1 AB the first observed brown dwarf triple system, with all components below 75 M$_{\rm Jup}$.