Eike Guenther — Page 2

arXiv2002

Rotational Periods of very Young Brown Dwarfs in ChaI

A photometric monitoring campaign of brown dwarfs in the ChaI star forming region in the i and R band revealed significant periodic variations of the three M6.5-M7 type brown dwarf candidates ChaHa2, 3 and 6 (Joergens et al. 2002). These are the first rotational periods for very young (1-5 Myr) brown dwarfs and among the first rotational periods for brown dwarfs at all. The relatively long periods of 2.2 to 3.4 days as well as vsini values (Joergens & Guenther 2001) indicate that our targets are moderately fast rotators in contrast to very rapidly rotating old brown dwarfs. The periods for the ChaI brown dwarf candidates provide valuable data points in an as yet, in terms of rotational characteristic, almost unexplored region of the age-mass diagram. A comparison with rotational properties of older brown dwarfs indicates that most of the acceleration during the contraction phase takes place within the first 30 Myr or less of the lifetime of a brown dwarf. We have also determined periods for the two M5-M5.5 type very low-mass stars B34 and CHXR78C.

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arXiv2002

Imaging and spectroscopy of DENIS-P J104814-395606

We obtained deep H- and K-band images of DENIS-P J104814-395606 using SofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRC at the Mauna Kea 2.2m telescope between December 2000 and June 2001. The target was recently discovered as nearby M9-dwarf among DENIS sources (Delfosse et al. 2001). We detect parallactic motion on our images and determine the distance to be 4.6 pm 0.3 pc, more precise than previously known. From the available colors, the distance, and the spectral type, we conclude from theoretical models that the star has a mass of ~0.075 to 0.09 M_sun and an age of ~1 to 2 Gyrs. We also obtained H- and K-band spectra of this star with ISAAC at the VLT. A faint companion candidate is detected 6 arc sec NNW of the star, which is 6.4 pm 0.5 mag fainter in H. However, according to another image taken several month later, the companion candidate is not co-moving with the M9-dwarf. Instead, it is a non-moving background object. Limits for undetected companion candidates are such that we can exclude any stellar companions outside of ~0.25 arc sec (1 AU), any brown dwarf companions (above the deuterium burning mass limit) outside of ~2 arc sec (9 AU), and also any companion down to ~40 M_jup with \ge 0.15 arc sec (0.7 AU) separation, all calculated for an age of 2 Gyrs. Our observations show that direct imaging of sub-stellar companions near the deuterium burning mass limit in orbit around nearby ultra-cool dwarfs is possible, even with separations that are smaller than the semi-major axis of the outermost planet in our solar system, namely a few tens of AU.

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