The tightest orbiting brown dwarf companion ever seen has been discovered orbiting the low mass star LHS 2397a, using adaptive optics on Gemini North. The faint companion completes its orbit around the parent star at a distance of about 3 astronomical units (AUs) every 20 years.

The images above show LHS 2397a in four bandpasses. The first image on the left is an optical image in the I Band obtained with the Hubble Space Telescope (HST) on April 12, 1997. The brown dwarf is seen as the very faint object at nine o'clock and is very difficult to detect in this unenhanced image. The next three images were obtained using adaptive optics in the near infrared (J, H and K' bands) on Gemini North five years later (February 7, 2002). The Gemini images use a Fourier filter to enhance the visibility of the companion, hence the presence of the companion is much clearer - especially at K'.

Using the adaptive optics system Hokupa`a/QUIRC on the Frederick C. Gillett Telescope (Gemini North), Melanie Freed, Laird Close and Nick Siegler of the University of Arizona have imaged the tightest brown dwarf companion known to a low mass star in the infrared spectral domain. Brown dwarfs are celestial objects that have masses of less than 7 percent that of the Sun and 100 times that of giant planets like Jupiter. However their masses are too small to trigger nuclear reactions capable of burning hydrogen in their centers; hence, their luminosity is derived from the heat generated by their very slow gravitational contraction that takes place over several tens of billions of years.

The 0.068 solar mass brown dwarf was discovered to orbit around the faint M8 star LHS 2397a. This binary system represents the first clear example of a brown dwarf within 4 AUs of its parent star; this corresponds to a smaller distance than the orbit of Jupiter around the Sun. This discovery indicates that there is no brown dwarf desert around low mass primaries. (See our earlier story: "Closest Brown Dwarf Companion Ever Spotted Around a Star Provokes New Perspective on Formation of Low Mass Objects"). The system is at about 14 parsecs (50 light-years) from our Sun.

The angular separations of this pair of very low mass stars correspond to only 0.2 arcsec. Only with adaptive optics or space-based imaging (e.g., with the Hubble Space Telescope) can separations significantly smaller than 20 AUs be investigated.

The Freed et al. discovery is interesting from two viewpoints. First, there exists a distinct lack of brown dwarf companion detections at small separation in nearby Sun-like stars. From giant planets searches, it is estimated that only about 0.5 percent if the stars have brown dwarf companions within 3 AUs. In contrast approximately 13+/-3 percent of G-type stars and 8 percent of M stars have stellar companion within 3 AUs. This paucity of brown dwarfs in "normal" stars is referred to as the "brown dwarf desert". On the contrary, the brown dwarf frequency around very low mass stars is 5- to 21 times the brown dwarf frequency observed around Sun-like stars within 3 AUs. The dramatic dichotomy, if further confirmed, suggests that the formation of brown dwarfs may involve a very different mechanism than for higher mass stellar companions.

Secondly, the small separation of the LHS 2397a system implies that the orbital motion can be followed by taking images over several years. In the case of LHS 2397a, this means that the dynamical mass of the components will be determined accurately within a relatively short period of time (e.g., in about five years). This will help greatly in calibrating the mass-luminosity models and other properties for stars at the bottom of the main sequence. Low mass stars represent the majority of the stars in the universe.

The investigators searched the Hubble Space Telescope archive. They found that HST had actually been used to image LHS 2397a on April 12, 1997, in the optical. Because it was so faint, the companion had not been noticed. With the Gemini observations taken five years after the HST images, the significant orbital motion is obvious (See Figure- 1.). Freed et al estimate that the orbital period is about 20 years. Further observation in about another five years will allow us to derive a precise orbit and masses for LHS 2397aA and 2397aB. Temperatures of the two stars were derived from models. A has T = 2630 K, and B has T = 1470 K; our Sun has an effective temperature of 5,780 K.

More details can be found in the paper by Freed et al.: "Discovery of a Tight Dwarf Companion to the low Mass Star LHS 2397a,", The Astrophysical Journal, February 10, 2003.

The Hokupa'a/QUIRC adaptive optics system was developed and operated by the University of Hawai`i Adaptive Optics Group, with support from the National Science Foundation.