CCD photometry was carried out from 1993 to 1995 at the 0.6 m telescopes of OPD
with a front illuminated GEC P8603A CCD detector with
resolution of about (385
578 pixels). No attempt
was made to transform the computed magnitudes to a standard system
since our main interest is to study time variations. This procedure
does not require photometric sky conditions and successfully overcomes the
problematic effects of variable transparency.
In deciding between observing a large sample of diverse objects or fewer
objects at more colors, we chose the former. Therefore, our images were
taken with the V and I filters, and exposures were all 3 minutes long.
Photoelectric differential photometry in were also taken with the
same telescopes using the FOTRAP (Jablonski et al. 1994). This
sample includes recently identified TTS sorted by their IRAS colors and
conspicuous Li lines (Pico dos Dias Survey - henceforth PDS;
Gregorio-Hetem et
al. 1992, henceforth GHETAL92). Special attention
is given to AS 216 and AS 218, monitored since 1985 to
establish the stability of their rotational periods.
In general the objects are brighter than and
cooler than K5. T Cha (G8) is included because of its
reportedly extreme variability.
We adopt the standard method to perform the differential photometry. The stars in each field - excluding the variable target - are examined for stability and those showing no trace of variability are chosen as comparisons. The uncertainty in the photometry is estimated from the dispersion in the magnitude differences and ranges from 0.01 to 0.02 mag. The TTS observed with the CCD are SY Cha, TW Cha, CT Cha, VZ Cha, WY Cha, SZ 45, T Cha, SZ 77, SZ 82 and SZ 108.
Data reduction is carried out with the Image Reduction and Analysis Facility (IRAF) software. Images are corrected for electronic bias, pixel-to-pixel gain variations (with dome flats), and cosmic rays are removed. The routines of the APPHOT package are used to obtain relative magnitudes for the objects selected in each field. The sky contribution is the modal value inside a specified annulus whose internal radius and width are automatically determined for each image by the RADPROF routine. This procedure avoids possible effects of nearby stars or defective pixels. Typical values of the aperture range from four to seven pixels, corresponding to 2.3'' to 4'' respectively.
Photoelectric photometry was performed using the FOTRAP
(Jablonski et al.
1994).
This device has a rotating filter wheel (1200 rpm) that enables
measurements in 5 colors in a quasi-simultaneous way. The photometric system
is matched to the Johnson/Cousins
standard system. We achieve differential
magnitudes by observing the stars in the
order: TTS, comparison 1, comparison 2 and TTS. The two arrays
of differential values per season (TTS - Comp) are
then combined and used as input to the periodogram
analysis.
The final set of observed TTS include all the listed PDS objects and
AS 216, AS 218, FK Ser.
Table 1 summarizes our observation logs. The first and second columns list the
names of the objects and are followed by
the year of the observation. Columns 4 through 8 list the of the
differential magnitudes. The number of observations in each of the observed
seasons
are indicated next. The spectral types are taken from the Herbig & Bell
catalogue (1988 - henceforth HBC) or from a spectral type classification method
developed to utilize the medium resolution data of the PDS survey. This method
essentially compares the unknown TTS spectrum to those of standards of different
spectral class observed with the same instrument and spectral format. The
spectral type of T Cha is taken from
Alcalá et al. (1993). Complete electronic
tables with the photoelectric magnitudes can be accessed at the CDS, via
anonymous ftp to cdsarc.u-strasbg.fr or via
http://cdsweb.u-strasbg.fr/Abstract.html.
Star | Other name | Year | ![]() | ![]() | ![]() | ![]() | ![]() | NP | SpT | |||||||
PDS 01 | Hen 1 | 1989 | 0.01 | 0.02 | 0.03 | 0.03 | 0.02 | 4 | K1 | |||||||
45 | CoD-29 8887 | 1989/90 | 0.15 | 0.06 | 0.04 | 0.04 | 0.04 | 10 | M3 | |||||||
50 | Hen 600 | 1989/90 | 0.17 | 0.05 | 0.02 | 0.02 | 0.02 | 13 | M3+M3 | |||||||
59 | GSG 9419-1065 | 1991 | 0.34 | 0.11 | 0.11 | 0.10 | 0.07 | 9 | M1 | |||||||
66 | Hen 892 | 1990 | 0.12 | 0.05 | 0.03 | 0.03 | 0.02 | 10 | K2 | |||||||
70 | CoD-40 8434 | 1990 | 0.06 | 0.04 | 0.03 | 0.03 | 0.02 | 9 | K7 | |||||||
77 | CoD-41 10484 | 1990 | 0.47 | 0.43 | 0.37 | 0.33 | 0.29 | 7 | K0 | |||||||
81 | GSC 6209-923 | 1989/90 | 0.05 | 0.05 | 0.04 | 0.04 | 0.03 | 11 | K0 | |||||||
82 | VV Sco | 1990 | 0.17 | 0.15 | 0.10 | 0.08 | 0.06 | 7 | K0 + M1 | |||||||
83 | V896 Sco | 1990 | 0.35 | 0.24 | 0.19 | 0.15 | 0.12 | 7 | K5 | |||||||
89 | GSC 6217-126 | 1990 | 0.12 | 0.08 | 0.07 | 0.06 | 0.04 | 4 | M0 | |||||||
99 | 1989 | 0.31 | 0.19 | 0.16 | 0.14 | 0.13 | 4 | M0 | ||||||||
101 | BZ Sgr | 1989 | 0.91 | 0.66 | 0.53 | 0.42 | 0.34 | 7 | K0 | |||||||
HBC 565 | SY Cha | 1994/ 1995 | -- | -- | 0.23 | -- | 0.14 | 22 | M0 | |||||||
567 | TW Cha | 1994/1995 | -- | -- | 0.29 | -- | 0.14 | 18 | M0 | |||||||
570 | CT Cha | 1995 | -- | -- | 0.08 | -- | 0.04 | 10 | M0 | |||||||
578 | VZ Cha | 1994/1995 | -- | -- | 0.46 | -- | 0.23 | 15 | K6 | |||||||
583 | WY Cha | 1994/1995 | -- | -- | 0.08 | -- | 0.07 | 11 | K7 | |||||||
590 | Sz 45 | 1994/1995 | -- | -- | 0.15 | -- | 0.05 | 10 | M0.5 | |||||||
591 | T Cha | 1995 | -- | -- | 0.90 | -- | 0.67 | 7 | G8 | |||||||
603 | Sz 77 | 1994/1995 | -- | -- | 0.13 | -- | 0.08 | 10 | M0 | |||||||
605 | Sz 82 | 1995/1994 | -- | -- | 0.07 | -- | 0.05 | 16 | M0 | |||||||
620 | Sz 108 | 1994/1995 | -- | -- | 0.04 | -- | 0.06 | 10 | M0 | |||||||
656 | AS 216 | 1985 | 0.21 | 0.09 | 0.05 | -- | -- | 21 | K2 | |||||||
1986 | 0.24 | 0.11 | 0.06 | -- | -- | 67 | ||||||||||
1987 | 0.23 | 0.09 | 0.05 | -- | -- | 41 | ||||||||||
1989 | 0.44 | 0.14 | 0.08 | 0.05 | 0.04 | 7 | ||||||||||
1990 | 0.45 | 0.16 | 0.09 | 0.07 | 0.06 | 11 | ||||||||||
657 | AS 218 | 1986 | 0.93 | 0.30 | 0.21 | 56 | K7 | |||||||||
1989 | 1.47 | 0.70 | 0.40 | 0.24 | 0.12 | 5 | ||||||||||
1990 | 0.49 | 0.13 | 0.08 | 0.07 | 0.07 | 11 | ||||||||||
663 | FK Ser | 1986 | 0.89 | 0.32 | 0.18 | 30 | K5/K7 | |||||||||
1989/1990 | 0.60 | 0.33 | 0.31 | 0.31 | 0.24 | 11 | ||||||||||
|
For all the stars but Hen 1, V896 Sco and PDS 89, we gathered enough observations to search for photometric periodicity. Since the observations were generally taken near meridian transit, our frequency search is limited to the interval 0.0 to 0.5 day-1, the quasi-Nyquist frequency. We cannot discriminate these from the aliases which occur in the frequency interval between 0.5 and 1.0 day-1.
We use the two following methods to access frequencies:
1) The Date Compensated Discrete Fourier Transform (DCDFT, Ferraz-Mello 1981) that computes the correlation between a sinusoid-plus-constant curve with the photometric data. The significance of the candidate peak is assessed with the method outlined by Ferraz Mello & Quast (1987), and gives the probability that a peak in the distribution is not formed by chance (the acceptance of a given period). The method also gives the error bar of the computed period.
2) The method developed by
Akerlof et al. (1994, henceforth BSP) that computes
the least-square fit of a sum of cubic
B-spline functions to the period-folded light curve. The chosen
frequency is that which provides the minimum statistic.
A characteristic of this method is that the computed frequency
is not affected by any predefined light curve shape (e.g. sinusoidal).
This degree of freedom allows a range of possible solutions for time
series with less than 10 entries. We
tested the efficiency of this program using the photometry of
several TTS available in electronic format
(Herbst et al. 1994). The
program provides firm solutions for stars with large numbers of entries
(though sometimes disagreeing with the published value) and handles data sets
with fewer entries (down to 7), though not claiming a unique solution for
those cases. The final range of solutions can be narrowed down to a few
options if the shape of the folded light
curve is fixed to a sinusoidal one.
Our basic procedure which leads to a final rotational period is as follows.
First, we utilize the BSP code for the V, R and I colors and select the 15
frequencies, in each color, yielding light curves of low .
The final solution tends to be unique if the inputed time series has
a large number of entries. In this case, that frequency is easily identified
in every
color and distinguishable from the others by its low
(see
example in Akerlof et al. 1994).
In some cases, we
have a modest data set for which, as discussed above, the BSP code
ascribes several possible solutions. In these cases, we select as the
final BSP
solutions those where the shape of the U, B, V, R and I folded light curves
are correlated in phase. In addition, the curve shall
present an even distribution of data-points in phase.
In some cases, we are satisfied with more than one final BSP
solution. Secondly, we run the DCDFT code and search for the best
frequency along the steps indicated by this method. We analyze individual
color bands, selecting the solutions yielding good phase coverage, large
acceptance in all colors (U excluded),
and consistency among the period-folded light curves for each color band.
The chosen DCDFT period is then compared with that of the BSP method and usually
the sinusoidal-light curve
solution of the former method is adopted.