Peremennye Zvezdy (Variable Stars) 27, No. 1, 2007 Received 5 January; accepted 7 March.
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Article in PDF |
We present new CCD ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Introduction
Studies of activity in late-type stars have revealed enhanced solar activity in objects ranging from very young to evolved stars (e.g. Vogt, 1983). Detailed behaviour of starspots on a small number of objects has been obtained via Doppler Imaging (Vogt & Penrod, 1983, Collier-Cameron & Unruh, 1994), often in conjunction with multi-colour photometry. However, most of our understanding of the longer-term behaviour of spotted stars has come from photometry alone, revealing evidence for stellar activity cycles (Baliunas et al. 1995), presumed to be the analogues of the solar magnetic cycle, and allowing estimates to be made of spot properties (Olah et al, 1997).
Gathering such data historically required a large investment of time of small- to medium-sized telescopes, along with observers to run them. Developments in CCDs and the availabilty of cheap computing power means that a semi-automated or even a fully automated CCD photometry system can be assembled for modest cost. Two examples that illustrate what can be done are the WASP (Kane et al. 2005) and ASAS systems (Pojmanski, 1997). These instruments use short focal-length camera lenses coupled to commercial CCDs, on mounts under computer control, and can automatically gather a large number of photometric quality CCD frames containing images of many hundreds of stars each night.
We used a semi-automated CCD system to collect and
data
on the active young dwarf PZ Telescopii. Comparing these new data
to earlier observations suggests that a long-term brightening of
the mean light level of PZ Tel may have peaked around
the year 2000, and that the star is now fading.
The Brightwater Photometry system
The Brightwater Observatory is located in southern
Tasmania, Australia, about 20 km south of Hobart in a semi-rural
location. Approximate geographic co-ordinates are
E,
S, at 80 m elevation. The photometry
system, recently commissioned, has been assembled from readily
available components (Table 1).
Item | Vendor | Model/Type |
CCD | S-BIG | ST7E |
Filter Wheel | S-BIG | CFW8A |
Filters | Schuler | UBVRI |
Telescope | Televue | 70 mm, f/6.8 |
Mount | Vixen | GP-E |
Mount control | COAA | Win-CTC |
CCD control | Cyanogen | Maxim DL CCD |
The CCD size is 745512 pixels, each pixel being 9
m square. At the prime
focus of the 485 mm focal-length telescope we cover a field of about
0.80
0.55 degree. Each pixel subtends a 4
4 arc second area. We usually
perform a 2
2 pixel binning at readout to reduce both read-out time and
the size of a CCD frame on disk. We note that both the WASP and ASAS systems
use a similar resolution to this.
The CCD and filter wheel are under software control via Maxim DL CCD. Scripts
written in Visual Basic select the filter and control the exposure times. For the
observations of PZ Tel reported here, exposures were limited to 30 seconds duration to remain in the linear part of the
CCD response. The ,
and
filters have been calibrated to the Cousins system from
observations of 14 E-region standards (Table 2).
The system is housed in a small roll-off roof observatory located at a domestic
residence. It is intended to dedicate this system to intensive studies of a
small number of variable stars.
Equation | colour range | rms residual |
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0.01 mag |
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0.02 mag |
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0.02 mag |
PZ Telescopii
PZ Telescopii (HD 174429, =
18
53
05
88,
= -50
10
49
9 (J2000.0))
is a young, relatively bright (
) and nearby
(
50 pc) active K0 star. Attention was first drawn to it
via the presence of strong Ca II emission (Bidelmann & MacConnel,
1973, Weiler & Stencel, 1979). Coates et al. (1980) presented the
first optical photometry, discovering the spot-induced rotational
modulation with the short period of 0
94. It is active at a
variety of wavelengths, including detections in microwave radio
(Slee et al., 1987), UV (Walter & Neff, 2006) and X-rays
(Agrioffi et al. 2004). While two radial velocity measurements by
Stacy et al. (1980) suggested variability, there is no evidence
from the subsequent and extensive published radial velocity
studies that the star is a close binary (means and standard
deviations: Balona (1987): +4.4
6.2 kms
; Innis et al. (1988):
3.2
3.7 kms
; Barnes et al. (2000):
0.1
1.0 kms
;
Nordstrom et al. (2004):
5.2
5.7 kms
). A single later
measurement (Soderblom et al. (1998):
13.5
3.0 kms
)
appears significantly different from the other data, however as
noted by Barnes et al. (2000) spot-induced profile asymmetries,
combined with the relatively large v sini (e.g.
Barnes et al., 2000,
70 kms
), can contribute to scatter in
radial velocity measurements. Evidence of PZ Tel's youthfulness
was revealed by the strong lithium
6707 Å line
(Robinson et al. 1986), and from kinematic arguments (e.g. Innis
et al., 1986, Zuckerman et al., 2001). It has been the target of
Doppler Imaging work by Barnes et al. (2000), who found evidence
for differential rotation. The GCVS (Kholopov et al., 1985) lists
the star as type RS, with a remark it is a double-lined
spectroscopic binary. Presumably the SB2 designation has its
origin in the Stacy et al. (1980) finding, which as noted has not
been supported by subsequent studies.
A photometric compilation of several years of data and a qualitative analysis was presented by Innis et al. (1990), which demonstrated the rapid changes that take place in the light curve. Other photometry has been performed by Bopp et al. (1986), Lloyd Evans & Koen (1987), Cutispoto & Leto (1997), Cutispoto (1998), Waite (2000), and the Hipparcos mission, but in general the star does not appear to have been well observed photometrically in recent years. Some ASAS-3 data on this star exist. At present (January 2007) only a very small number of measurements are available from the ASAS-3 database.
As the star is relatively bright, and undergoes rapid changes in its
spot-induced variation, it appeared as an interesting first target for the
Brightwater photometer.
Observations
PZ Tel was observed for a total of 12 nights between 2006 May and July. Some nights were cloud-affected. We present data from 7 nights from the interval early June to early July. A small number of observations from late July (not shown here) do not fit the light curve defined by the earlier data, indicating a change in spot properties. Such behaviour is well documented in earlier photometry (see references cited earlier).
Exposures were taken in and
filters, in an alternating sequence of four 30-second
exposures in a given filter. Around 1000 individual CCD frames were taken in
both
and
, of which about 800 of each were obtained in clear-sky conditions.
Measurements from four adjacent exposures were combined into
normal points representing an effective two-minute integration. Bias, dark and
flat-field frames were also obtained. Flat-field exposures were made
of diffusely illuminated sections of the observing hut wall. The Televue
refractor telescope we use is in part designed with wide field photography in
mind. We have found the field-of-view across the CCD is relatively
unvignetted, with only a slight fall off of a few percent near the field edges.
The flat-field exposures correct for this vignetting.
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Fig. 1.
Instrumental |
A 30-second -filter exposure resulted in a peak pixel analog-to-digital-unit
(ADU) count near 15k (with a conversion rate of 2.3 e ADU
) , and a
total ADU count near 50k in the PZ Tel profile. In the
filter the
corresponding values were around 4k and 13k, due to the lower
quantum efficiency of the detector and the late-type spectrum of the star. The
wide field of view meant that comparison stars suitable for differential
photometry were also present in the field of PZ Tel. Hence once the field was
acquired and the data collection script started, the system was able to collect
data unattended, although the telescope operator periodically checked on the telescope
tracking and the weather, and also on the data quality in near real-time via
the MUNIWIN package (http://integral.sci.muni.cz/cmunipack).
We selected HD 174014 as the comparison star, and HD 176440 as the check star,
although we did in fact measure a number of stars in the frame. Aperture
photometry measurements
were performed with both IRAF and MUNIWIN. The data presented here are from
MUNIWIN analysis. We used a fixed aperture of 5 pixels (40 arc second) radius. We
measured the comparison and check star
and
magnitudes by reference to
one of the Cousins E-region standards, HD 192633, as well as measuring on one
night HD 176557, which has been used as a comparison star for PZ Tel by
Innis et al. (1990), Cutispoto & Leto (1997), Cutipoto (1998) and Waite (2000).
We found for HD 174014:
=9.10;
=1.07, and for HD 174660:
=9.56;
=1.15. We determine the
and
data for
PZ Tel relative to these derived measurements of HD 174014.
Our and
measurements of PZ Tel are available
electronically in the html version of this paper.
The instrumental -magnitude differences for PZ Tel
HD 174014, and HD 174660
HD 174014, are shown in Figure 1
(upper and lower panels respectively). Just over 200 separate
data points, each equivalent to a 120-second exposure, are shown
in each panel. The root-mean-square deviation of the data in the
lower panel (i.e. check star
comparison star) is 0
009. The
scatter in the
-filter measurements is around 2 to 3 times as
large, in reasonable agreement with the reduced ADU count.
We performed a period search on the data shown in the top panel of
Figure 1. We used both the phase dispersion minimisation method
of Stellingwerf (1978) and the string length method of Dworetsky
(1983) and found identical results. The best-fit period was
094, effectively identical, given the limited time-span of
these current data, with the value of 0
94486 obtained by
Innis et al. (1990) from several seasons of observations. Phase
plots of the transformed
and
data are shown in the top and
middle panels of Figure 2, using the period and epoch from Innis
et al. (1990).
We binned the and
data separately into phase bins of width
0.01, and looked for changes in
with rotation phase. A plot
of these data is shown in the lower panel of Figure 2. The mean
for PZ Tel was found to be 0.78 (with a standard deviation
of 0
01 mag). There is an indication of a colour change, with
the star being redder when fainter. The solid curve is a best-fit
line, and suggests a colour change of around 0
01 or 0
02
over a rotation. If we split the data into
brighter than 8.36
(39 points) and V fainter than 8.36 (31 points), we find the
corresponding
mean values and standard errors are
0.777
0.002 and 0.784
0.002 respectively, also suggestive
of a small but systematic change. Small colour changes (of order
0
02) have occasionally been seen on PZ Tel (Cutispoto, 1998,
Innis, 1986). The larger scatter in these current
data
compared to the
data tend to mask small colour changes. In
future we intend to make longer integrations in the
band to
reduce the
observational uncertainties.
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Fig. 2.
PZ Tel phase plots and
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Discussion
We plot in Figure 3 the photometric history of PZ Tel (from references
cited in
the previous section). The dominant cause of the varying amplitudes,
and changes in mean light levels, is probably starspot activity. The contribution to the
overall light-level from bright plage-like areas, and any other possible long-term
secular variation, is much more uncertain.
The smallest -range shown in Figure 3 is 0
03, from 1985.5
(near JD 2446250), from the data of Bopp et al. (1986). However
only approximately 0.5 of a rotational phase was observed at this
epoch. The smallest amplitude seen in a light curve with near
complete phase cover is 0
04, in late 1990 (Cutispoto & Leto,
1997). The range in
light of PZ Tel in the current data from
mid 2006 of 0
06 is therefore amongst the smallest seen. For
reference, the median amplitude of PZ Tel in
is 0
1.
![]() |
Fig. 3.
PZ Tel |
The photometric history of PZ Tel spans some 27 years, although
there are gaps in the record. There is no obvious indication of a
periodicity in the light levels of this star. In contrast,
the very similar star AB Dor exhibits a strong suggestion of a
25 year periodicity in mean
light, however as only about
30 years of photometric data exist only one putative cycle has
been observed (Jarvinen et al. 2005). For PZ Tel, there may be an
indication that a maximum in the long-term
magnitude occurred
near JD 2451000 (i.e. near the year 2000), with the current data
suggesting a decline has commenced. We consider it very
worthwhile to continue to monitor this star to follow the future
behaviour. It would be of interest to understand why PZ Tel and
AB Dor differ in their long-term photometric properties, given
they are otherwise
so similar.
Conclusion
and
light curves of the active late-type star
PZ Tel were obtained via a semi-automated CCD photometer system,
using a 70-mm diameter, short-focal-length telescope. The wide
field of view allowed target and comparison stars to be observed
simultaneously. The root-mean-square deviation between the check
and comparison stars (both of
) for the time
series of 120-second exposures was slightly below 0
01 in
,
and around twice this in
, which we take as the precision in
the determination of the magnitude of PZ Tel. A period analysis
of around 200
measurements for PZ Tel returned a best period
of 0
94, effectively identical with a previous determination
for this star from multi-year photometry. The amplitude of the
variation was near 0
06, which is amongst the smallest this
star has exhibited. There is an indication of a small colour
change, with the star being redder when fainter. Inspection of the
long-term
light levels of PZ Tel leads us to speculate that a
maximum may have been reached around the year 2000, with the
current data possibly showing the beginnings of a decline. Further
observations will reveal if this is in fact the case.
Acknowledgments: We thank D. Partridge, S. Norris, and T. Moon for assistance with the construction of the observatory. We thank Doug George of Diffraction Limited for data-acquisition software support. This work has made use of the SIMBAD database of the Stellar Data Centre (CDS) Strasbourg, the NASA ADS abstract database, the ASAS-3 photometric database, and the data-reduction packages IRAF (NOAA, USA) and MUNIWIN (by David Motl). John Innis thanks Petra Heil and Julian Innis for their support and patience while these data were collected.
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