Direct continuation of our work

As mentioned in the previous chapter, the conclusions we draw regarding the multiplicity of ultracool dwarfs in the field and their consequences on the models of formation and evolution are only tentative because of the limitations of the samples we used (magnitude limited sample). New investigations over a statistically well defined sample should be done in order to confirm the results we obtained. Such a study is already on-going with HST on a volume-limited complete sample of L dwarfs (K. Cruz, private communication) from the 2MASS survey, and a similar survey is planned with VLT/NACO on an even larger volume-limited complete sample of DENIS L-dwarfs (X. Delfosse, private communication).

As for the physical properties, an effort should be made in order to carry on with the study of individual properties via their spatially resolved spectra. In order to be able to derive statistical properties, this work should be extended to even more objects over a larger range of spectral types. A complementary study is on-going in the near-infrared: using VLT-NACO, we obtained

K band resolved spectra of two of the four binaries presented in Chapter 1 of Part II. These spectra cover the spectral region where the following major spectral features can be identified: the CO bands (2.295 µm) which are dominating through L8, the CH4 bands for late L and T dwarfs (2.2 µm), and the Na I doublet (2.206/2.209 µm) which becomes more pronounced in later spectral types.

Ultimately, it would be very important to follow more very low mass binaries on their orbits, with the aim of measuring dynamical masses. Only two such orbits have been covered nowadays (Lane et al. 2001, , and Chapter 2 of Part II). A “good candidate” for such a study should have

- it should rotate fast enough to allow the determination of its orbital parameters in a reasonable time-scale (of let say 3 to 5 years). The uncertainties on the derived orbital parameters indeed depend strongly on the fraction of the orbit covered by the observations. Objects with periods larger than 15 years are unlikely to be followed on more than 50% of their orbit, depending on the eccentricity.

- its distance should be known with a high precision. The distance indeed appears to the cube in Kepler’s law (through the semi-major axis), and a small uncertainty on the distance can translate into large uncertainties on the derived orbital parameters.

- its age should be known or at least sufficiently constrained. We have indeed mentioned that the impacts of the current orbital measurements on the models are relatively small, because the age of the two corresponding systems are unknown. A calibration of the models requires to know the age of the object, or at least constraints on its age.

- its separation should make it easily observable. If the separation is too close to the limit of resolution of the currently available instruments, the uncertainties on the relative as- trometry will be relatively high, translating again into large uncertainties on the inferred dynamical masses.

- its inclination should not be close to edge-on: in such case, the orbital solutions cover a much broader range of eccentricities, translating also into large uncertainties on the dynamical mass of the system.

- the system should be bright enough to enable the use of ground-based adaptive optics instruments. HST is approaching the end of its “career”, and the only high angular res- olution instruments remaining for the near future will be ground based telescopes with adaptive optics. The imminent installation of laser guide stars should soften this condi- tion. In the long term, NASA/ESA/CSA James Webb Space Telescope should allow to reach the required resolution in the near-infrared, thus down to even smaller mass ratios and separations.

- astrometric or spectroscopic orbits should be measurable as well. The visual orbit is indeed not enough to derive the dynamical masses of the individual components since it yields only the angular separation between the stars, while Kepler’s Third law requires the linear separation. In order to derive the dynamical masses of the individual components, one thus needs to measure the linear separation either by observing the spectroscopic or absolute

visual orbit16

Fortunately half a dozen of candidates meeting most of these conditions can be distinguished from the sample of currently known binaries. Most of them are binary brown dwarfs orbiting a G, K or M dwarf, which age and distance can therefore relatively easily well constrained. Moreover, the presence of this bright primary offers two major advantages: 1) it provides a good and bright reference star close to the object of interest, which is of first importance for adaptive optics observations; 2) it enables precise measurements of the absolute motion of each components of the binary, leading to the determination of the centre of mass and consequently to

16_{The absolute visual orbit refers to the motion of each components with respect to one or more other stars in} the field, leading to the determination of the centre of mass, and therefore to the individual masses.

Proposals will be submitted to the relevant observatories in order to start this study.

In collaboration with Dr. W. Brandner, we are currently working on the orbital fit of DENIS- P J122813.8-154711. Seven observations spread over 6 years already allow us to constrain the orbital period to 42.7 yr for a total mass of 0.110_±0.013 M (Brandner et al., in prep.). A longer time will thus be needed before we can improve significantly these values.

As explained above, astrometric binaries provides a very straightforward method to measure the masses of the individual components. In a collaboration with prof. E. L. Mart´ın and Dr. W. Brandner, I will work on the determination of the absolute astrometric orbits of a sample of 4 ultracool and brown dwarfs from the field. Six epochs images spread over 4 years have been obtained with HST/WFPC2 for each object, and the presence in the field of one (more in some cases) object will allow us to measure precisely the motion of the centre of mass and the individual masses.