The Formation of Brown Dwarfs

In the presented work, a population of twelve very young bona fide and candidate brown dwarfs in the ChaI star forming cloud (ChaHa1-12) was studied observationally in terms of their kinematic properties, the occurrence of multiple systems among them as well as their rotational characteristics. Based on high-resolution spectra taken for nine out of the twelve objects with UVES at the VLT, radial and rotational velocities have been measured with high accuracy. A kinematic study of the sample showed that their radial velocity dispersion is relatively small (2.2 kms). It is significantly smaller than the radial velocity dispersion of the T~Tauri stars in the field (3.6 kms) and slightly larger than that one of the surrounding molecular gas (1.2 kms). This result indicates that the studied brown dwarfs are not ejected during their formation with velocities large than ~2 kms as proposed in recent formation scenarios. The brown dwarfs may have larger 3D velocities. However, brown dwarfs ejected during the early accretion phase in directions with a significant fraction perpendicular to the line-of-sight, would have flown out of the field a long time ago. By means of time-resolved UVES spectra, a radial velocity survey for close companions to the targets was conducted. In addition, a direct imaging survey for wide companions was carried out with the WFPC camera on board the HST, with FORS at the VLT as well as with SofI at the NTT. With these two complementary search methods, a wide range of possible companion separations has been covered. For brown dwarf companions (> 13 M_Jup) to the targets, separations < 3 AU and between 50 and 1000 AU were covered. With more restricted separations (< 0.1 AU and 300-1000 AU) the surveys were sensitive also to companion masses down to 1 M_Jup. HST images of ChaHa2 hint at a binary system comprised of two approximately equal-mass companions with a separation of ~30 AU. No further indications for companions have been found in the images. Furthermore, the radial velocities of the targets turned out to be rather constant setting upper limits for the mass Msini of possible companions to 0.1 M_Jup to 2 M_Jup These findings hint at a rather low (smaller or equal 10%) multiplicity fraction of the studied brown dwarfs. Furthermore, a photometric monitoring campaign of the targets yielded the determination of rotational periods for ChaHa2, 3 and 6 in the range of 2.2 to 3.4 days. These are the first rotational periods for very young brown dwarfs and among the first for brown dwarfs at all. They are complemented by measurements of rotational velocities vsini from UVES spectra. The observations show that brown dwarfs at an age of 1-5 Myr display surface spots like T Tauri stars and are moderately fast rotators in contrast to rapidly rotating old brown dwarfs consistent with them being in an early contracting stage. A comparison with rotational periods from the literature indicates that most of the acceleration of brown dwarfs takes place in the first 30 million years or less of their lifetime.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Ludwig-Maximilians-Universität

Year

2003

Author dc:creator

• Joergens, Viki

Repository record source_url

https://edoc.ub.uni-muenchen.de/864/

OAI identifier oai:identifier

oai:edoc.ub.uni-muenchen.de:864