Brown dwarfs
Brown dwarfs:
Why search for companions:
Planets around brown dwarfs: More than 200 extrasolar planets have been detected around stars by the radial velocity (RV) technique. Among them are 6 that orbit stellar M-dwarfs (e.g. Udry et al. 2007) showing that planets can form also around primaries of substantially lower mass than our Sun. If brown dwarfs (BDs) can harbor planets is still unknown. Observations hint that basic ingredients for planet formation (disk material, grain growth) are present also for them (e.g. Apai et al. 2005), though, no planet around a BD in a few AU orbit has been found yet. The only planet detection around a BD is a very wide 55\,AU system (Chauvin et al. 2005), which is presumably formed very differently from the solar system and RV planets. The finding of a planet at a few AU around a BD would be a unique discovery in this field. Recent simulations hint at difficulties in forming giant planets around M-dwarfs by core accretion (e.g. Ida \& Lin 2005), while this seems to be not the case for disk instability models (Boss 2006a), at least in low-mass star-forming regions (Boss 2006b). % Another important step towards understanding planet formation would be the detection of {\em very young} (few Myr) planets, which would constrain time scales of involved processes (growth, migration, disk depletion time scales). However, the youngest known planet has an age of 80--120\,Myr (Setiawan et al. 2007).
Close brown dwarf binaries: A spectroscopic survey for planetary or BD companions to BDs is of primary interest also for BD formation models. The frequency of BDs in multiple or planetary systems is a fundamental parameter in these models, however, it is poorly constraint for close separations: Most of the current surveys for companions to BDs are done by direct (AO) imaging and are not sensitive to close binaries (a<1 and a<10AU for the field and clusters, respectively), and have a strong bias towards equal mass systems %making the resulting binary fractions only lower limits (e.g. Burgasser et al. 2007).
Mass determinations for brown dwarfs: BDs in gravitational bound systems are key astronomical objects also because they might allow dynamical mass measurements. Masses are the very foundation of our understanding of (sub)stellar formation and evolution (e.g. IMF, evolutionary models). The direct imaging surveys are mostly sensitive to long-period systems, which do not (easily) allow mass measurements (Burgasser et al. 2007). A means to search for close companions are spectroscopic surveys. Until very recently, there were only three spectroscopic BD binaries confirmed (Basri \& Martin 1999; Stassun et al. 2006; Simon et al 2006). One of them is a very young system, and for masses $<$0.3\,M$_\mathrm{\odot}$, evolutionary models rely only on the two masses determined for this system (Mathieu et al 2007). Based on ESO VLT DDT observations in March 2007 the detection of a fourth spectroscopic BD or very low-mass star binary was possible (Joergens 2007, in prep.; see also proceeding of talk given in May 2007 in Toronto ). This system is outstanding because of its youth, its low mass ratio and its relatively long period.
The orbital motion of orbiting companions, like planets, can be monitored by means of spectroscopic observations of their radial velocities. The figure illustrates the orbital motion of a high-mass primary due to an unseen low-mass companion. In the case of the sketch, the orbiting object is a planet and causes a periodic wobble of the host star. As the star is moving towards the observer, spectral lines are shifted to shorter wavelength (blue-shift) and as the star is moving away from the observer the spectral lines are shifted to longer wavelength (red-shift). Credit California & Carnegie Planet Search (www.exoplanets.org).
Radial Velocity (RV) Survey:
We are carrying out a RV search for planets and brown dwarf companions to very
young (1-10 Myr) brown dwarfs and very low-mass stars in the ChaI star forming region with the high-resolution Echelle spectrograph UVES attached to the 8.2m VLT Kuyen telescope.
The observations have been carried out
at the Very Large Telescope (VLT) at
Cerro Paranal in the Atacama desert in
Chile, which is operated by the European
Southern Observatory (ESO) and displayed
in the left picture. It consists of four
8.2m telescopes, Kuyen being the second
from the left.
Credit European Southern Observatory (ESO).
The targets are 12 young bona fide and candidate brown dwarfs of spectral type M6-M8, named ChaHa1-12, located in the center of the ChaI star forming cloud (displayed in the image on the right; credit European Southern Observatory (ESO)). They are at a distance of about 160pc and have been detected by Comeron et al. (1999, 2000) and Neuhäuser & Comeron (1999). Furthermore, we have searched for planets around several young very low-mass stars in the same region, with spectral types M2.5-M5 and masses of about 0.1-0.3 solar masses.
UVES spectra and RV measurements:
A small part of a UVES spectrum of one of the targets is shown. Remarkable features are the Lithium absorption line, which is an indicator of youth and/or substellar nature, the TiO molecular absorption bands, which are typical for late M-dwarfs as well as the telluric absorption lines, which are produced in the Earth' atmosphere and served as stable wavelength reference.
Results:
A fourth spectroscopic BD/VLMS binary,
Very recent results presented in May 2007 in Toronto
Viki Joergens / viki @ mpia.de