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Issue A&A Volume 389, Number 3, July III 2002 Page(s) 963 - 976 Section Stellar atmospheres DOI 10.1051/0004-6361:20020730

A&A 389, 963-976 (2002)
DOI: 10.1051/0004-6361:20020730

Dust clouds or magnetic spots? Exploring the atmospheres of L dwarfs with time-resolved spectrophotometry

C. A. L. Bailer-Jones

Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

(Received 27 March 2002 / Accepted 15 May 2002 )

Abstract
I present the results of a program to spectrophotometrically monitor the L1.5 dwarf to identify the cause of photospheric variability in ultra cool dwarfs. Plausible candidates are magnetically-induced star spots and inhomogeneous photospheric dust clouds. Based on the atmospheric models and synthetic spectra of Allard et al. (2001), the expected signatures of these phenomena in the 0.5-2.5 m wavelength region are presented and discussed. Near infrared spectra of 2M1145 were obtained along with a nearby reference star observed simultaneously in the spectrograph slit. No convincing variability over a 54 hour interval is found in any one of several colour indices designed to be most sensitive to dust- and spot-related variability. Nonetheless, a significant correlation between the variability of two colour indices is found. This is slightly more consistent with the dust-related variability model than the cool spot one considered. Based on the theoretically predicted signatures and the median errors in the colour indices (0.03-0.05 mag), upper limits are placed on the coverage of possible spots and clouds. Assuming the L dwarf to be best modelled by a dusty atmosphere at 1900 K, coherent clear clouds are limited to a coverage of 10-15% of the projected surface area and 200 K cooler spots to a 20% coverage. A larger coverage of many small features varying incoherently cannot be ruled out with this method. A lower effective temperature restricts coherent clear clouds to be much smaller; a higher temperature allows both clouds and spots to be larger. These upper limits are consistent with the two separate variability detections in the I-band reported by Bailer-Jones & Mundt (2001).

Key words: stars: atmospheres -- stars: low-mass, brown dwarfs -- stars: starspots -- stars: variables: general -- stars: individual: 2MASSW J1145572+231730 -- methods: data analysis

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