2009A&A...498..281G


C.D.S. - SIMBAD4 rel 1.7 - 2021.02.26CET09:00:30

2009A&A...498..281G - Astronomy and Astrophysics, volume 498, 281-287 (2009/4-4)

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields.

GUENTHER E.W., ZAPATERO OSORIO M.R., MEHNER A. and MARTIN E.L.

Abstract (from CDS):

There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere and of a solar-like chromosphere and corona. We carried out a spectroscopic monitoring campaign in which we observed ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. The objects were monitored over several rotation periods spectroscopically. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. From the spectra, we determined the temperature of the objects in each spectrum, and measured the equivalent width in a number of diagnostically important lines. Temperature variations would indicate the presence of warm and cold regions, variations in the equivalent widths of photospheric lines are sensitive to the structure of cloud layers, and Hα is a diagnostic for chromospheric structures. Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the Teff-variations are ≤50K, and the rms-variations in the equivalent widths of Hα small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. We find that Hα-line is in emission but the equivalent width is only -4.4±0.3Å. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

Abstract Copyright:

Journal keyword(s): stars: low-mass, brown dwarfs - stars: activity - stars: individual: LP 944-20 - stars: magnetic fields - stars: individual: 2MASSW J0036159+182110

Simbad objects: 9

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Number of rows : 9

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2021
#notes
1 * gam Peg bC* 00 13 14.1512308 +15 11 00.936759 1.75 2.61 2.84 2.94 3.13 B2IV 665 0
2 HD 955 * 00 13 55.6007172748 -17 32 43.332248066 6.52 7.203 7.361     B3/5V 47 0
3 LSPM J0036+1821 BD* 00 36 16.1119130763 +18 21 10.291990145       18.451 16.053 L3.5 189 0
4 LP 647-13 PM* 01 09 51.2023110590 -03 43 26.391153646   20.9 19.266 17.168 14.708 M9Ve 39 0
5 G 34-49 WD* 01 45 19.4226019904 +23 17 53.945042637   17.3   16.6   DC4 24 0
6 CPD-69 177 WD* 03 10 31.0201840720 -68 36 03.381441176 10.757 11.413 11.394 11.47 11.558 DA3.0 239 0
7 LP 944-20 LM* 03 39 35.2525168530 -35 25 43.628465433     18.69 16.39 13.288 M9.5Ve 266 0
8 DENIS J104814.6-395606 LM* 10 48 14.5734267094 -39 56 06.843210566     17.532 15.051 12.67 M8.5Ve: 115 0
9 NAME Castor Moving Group As* ~ ~           ~ 117 1

    Equat.    Gal    SGal    Ecl

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