Variability in Pleiades
Very Low Mass Stars and Brown Dwarfs
M. Morales – Calderón1, J. Caballero2, E. L. Martín2 , D. Barrado y Navascués1
LAEFF – INTA, IAC
ABSTRACT
We present the results of an optical photometric observation of a Pleiades field
centered around the brown dwarf Teide 2. The aim of this project is to look for
variability using differential photometry and a statistical analysis of the data.
We have found variability in intermediate timescales for 17 out of 37 targets
studied.
OBSERVATIONS AND DATA REDUCTION
The observations were carried out during five photometric nights distributed from
12 January 2004 to 2 February 2004, using the ESA 1m Optical Ground Station
(OGS) telescope installed at the Teide Observatory. The telescope has a modified
Ritchey-Chretien/Coudé configuration, with a focal ratio of F4.5. The detector
consists of a mosaic of four EEV 42-40 CCDs each one with 2048x2048 pixels
which cover a field of 0.5 square degrees in total. There is no filter so the
wavelength band is given by the response of the CCD.
We have a total of 100 images, each one with an exposure time of 300 seconds. The
raw images were reduced and aligned using the standard procedure (packages imred
and ccdred) within the IRAF environment.
COLOR-MAGNITUDE DIAGRAMS
We present the IKP, I-ZKP and Ic, (R-I)c color-magnitude
diagrams for previously identified proper-motion
members (crosses). Our targets are marked with red,
solid circles.
The dashed lines correspond the cluster Main Sequence
in Ic vs (R-I)c, and the cut-off membership line in IKP vs
I-ZKP. Objects fainter than 17.5 mag are probable
cluster brown dwarfs.
TEIDE 2
DATA ANALYSIS: DIFFERENTIAL
PHOTOMETRY
We have obtained the photometry of 16193 objects
falling within the FOV, although we have removed
those either so bright that are out of the linear
regime or so faint that have a high poissonian noise.
From those still remaining, we have selected the
Pleiades members for further analysis .
In order to convert the instrumental magnitudes into
differential magnitudes we have calculated a
reference value for each exposure using an iterative
method and taking into account only moderately
bright objects, not variable and well spread out all
over the whole frame.
LIGHT CURVES
Relative magnitudes of the targets, with
respect the reference stars, have been
obtained. This procedure cancels the
effects that are not due to the intrinsic
variability of the source. In the figures we
can see the light curves of some of our
objects in comparison with the reference
magnitude (dashed line).
STATISTICAL ANALYSIS
We have analyzed the variability according to
different timescales (Caballero et al. 1994),
distinguishing between very short timescales
-within one night- and intermediate timescales
-from one night to the others.
c2 Test was used to evaluate the probability of the
scatter in a light curve to be consistent with the
photometric errors or with intrinsic variability
(Bailer-Jones&Mundt 2000). We have not found
variability in short timescales but in the whole
period we found that 17 out of 37 targets studied
were variable with a 99% confidence level with
amplitudes in the range 23 mmag ≤ σ(m) ≤ 190
mmag and with Teide 2 having an amplitude of 49
mmag.These amplitudes are consistent with those
found in the σ Orionis cluster (in a range from less
than 10 mmag to 400 mmag).
Address for correspondence: María Morales Calderón, e-mail: mariamc@laeff.esa.es, telephone:+34918131163
1
Laboratorio de Astrofísica Espacial y Física Fundamental ( LAEFF – INTA ). Villafranca del Castillo, Madrid, Spain.
2
Instituto de Astrofísica de Canarias ( IAC ). La Laguna, Tenerife, Spain.