Archival photographic light curves of the red semiregular star CPD –80o966 [1964-1976], and modern CCD multicolor photometry
J.L. Innis1, D.W. Coates2, T.G. Kaye3, A.P. Borisova4, and M.K. Tsvetkov4
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Peremennye Zvezdy (Variable Stars) 27, No. 4, 2007 Received 4 July; accepted 9 October.
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Article in PDF |
1. Introduction
The star CPD
was included in the ASAS catalogue of
variable stars, being classified as `miscellaneous' with a
characteristic period of 87.6 d (Pojmanski and Maciejewski, 2005)
and amplitude 
0.3 mag in 
. The variation was also
independently noted by some of the present authors,
serendipitously, when performing CCD observations of the known
variable CF Oct in 2006 (Innis et al., 2006) - CF Oct and
CPD are separated by less than 30 arc min on the
sky. These CCD data confirmed the semiregular nature of the
CPD. Inspection of the ASAS database and our CCD
data (which is now more extensive than that which appears in Innis
et al., 2006) show that the interval between successive maxima
typically varies from 70 to 120 d, and possibly can be
as short as 50 d as shown in our most recent data.
The full range of the long-term variation (i.e. over many cycles
of the 90 d period) of CPD in the ASAS database
approaches one magnitude. This is large enough to be detectable
on photographic surveys. We recently obtained scanned digitised
images of the Bamberg Southern Photographic Patrol Survey (BSPPS)
plates from 1964-1976 for a study of CF Oct (Innis et al., 2004).
We reanalysed these scans to extract the historical light curves
of CPD. We present evidence indicating that
CPD was varying during the course of the BSPPS,
and conclude that the variation was overlooked at the time.
2. The Bamberg Southern Photographic Patrol Survey (BSPPS)
The BSPPS was conducted by the Dr Remeis-Sternwarte, Bamberg
(Bamberg Observatory) at three southern hemisphere sites (South
Africa, New Zealand, and Argentina) between 1962 and 1976, with
the specific aim of discovering new variable stars. A description
of the program can be found in Strohmeier (1965). A modern
description and summary of the BSPPS is given by Tsvetkov et al.
(2005). The main work was carried out by a bank of six cameras
(of 10 cm aperture) on a common mount, which photographed a swath
of declination near meridian transit. Typically the plates reached
14th magnitude for a 1 h exposure, and covered
.
We digitised a
area of the BSPPS
plates of the field of CF Oct for a study of that star (Innis et
al., 2004). A total of 375 plates were scanned for that study.
The scanned images included CPD, hence we have
remeasured these plates for the present study.
3. Analysis and Results
We follow the procedures described in detail in Innis et al.
(2004). In outline, we determined plate magnitudes of the target
star (CPD) and five nearby field stars, of
comparable magnitude but with a range of 
, using aperture
photometry in IRAF. We use the field stars to form, for each
plate, a transformation between plate magnitude and 
magnitude.
( and for the field stars were found by transforming Tycho
and 
magnitudes.) For each set of 5 field stars for each
plate we solve, via a least squares Singular Value Decomposition
(SVD) method, for the 
parameters in the equation
 |
(1) |
is the plate magnitude.
We then solve for for CPD given the
-terms, the plate magnitude, 
, and the star's . Our
recent photometry indicates the change in
CPD during a pulsation cycle is only a few
hundredths of a magnitude, so for the purposes of the work
presented here it can be treated as a constant.
We were able to measure CPD on 344 of the
375 scanned plates. In the other cases cloud, tracking errors
(e.g. from wind) or other effects (such as plate scratches)
interfered. We also measured CPD
as a control
star to check our method. These data are not used in the
derivation of the values. We determine the magnitudes
of the control star in exactly the same way as for
CPD.
The values of the parameters for the 344 plates we
measured are shown in Fig. 1. There is generally good consistency
in the value of a given term within a season. The step
near JD 24539500, particularly obvious in the 
data,
corresponds to the shift of the programme from South Africa to New
Zealand (see Innis et al., 2004 for further discussion). The
colour term, 
, (i.e. term) is discussed below.
 |
Fig. 1. β parameters derived for the 344 plates with useable images of CPD–80o966, plotted versus JD-2430000. Top left: β1, the term linear in plate magnitude, Mp; top right: β2, the Mp2 term; bottom left: β3, the B–V term; bottom right: β4, the zero-point. |
We use a for CPD of 1.7, found from
transforming and . Our recent CCD measurements, which
cover several pulsation cycles, give a mean . We
calibrated our CCD filter set from observations of Cousins
E-region standards, but we were not able to observe any standard
stars redder than . Hence our determination
relies on a slight extrapolation of the calibration equation. In
a similar way the reddest field star we used in deriving the
parameters is (for CPD). Hence
we require an extrapolation of the dependence of the plate
transformation equation (equation 1) in determining the
magnitude for CPD.
The high declination of the field often meant several plates that
included CPD were available on a given night,
centred at different right ascension. Hence we have averaged all
measurements available on given night to reduce observational
scatter.
Figure 2 shows the resulting transformed data for
CPD (circles) and the control star
CPD (upright crosses) for data from 1964 to 1976.
The control star measurements show no overall trend, and
relatively little scatter. In contrast, the data for
CPD show a dimming then brightening, as well as an
increased noise level. An initial conclusion that could be drawn
from Fig. 2 is that CPD shows a long-term
variation. Such a long-term change in mean magnitude is consistent
with the behaviour of the star as seen in the ASAS-3 database. It
is important to attempt to rule out artifacts as the cause of the
long-term variation in the photographic data.
For example, the colour-term, , shows a possible small
decline for the first three seasons, a further, possible step-like
decrease between the third and fourth seasons, a small decrease in
the fifth season, then an increasing trend thereafter (Fig. 1).
However, the derived magnitudes for CPD
(Fig. 2) do not show a similar form, but appear to reach a minimum
in the second and third seasons, then brighten each season after
this. Further, it is difficult to attempt to account for the
long-term brightness change of CPD, which is
around 0.5 mag in the seasonal means, purely in terms of a
calibration error in the term. The term is
in the range to , and is simply not large enough to
bring about such a result. Based on this analysis, and the fact
that the control star CPD shows no similar
long-term trend, we believe it is likely that CPD
did indeed show a long-term variation in brightness during
1964-1976.
 |
Fig. 2. Light curves for CPD–80o966 (circles) and the control star CPD–80o980 (upright crosses) from the analysis of the digitised Bamberg plates, for 1964 to 1976. The data are transformed to standard B magnitudes. Each data point represents an average from all plates taken on that night. Although the light curve for CPD–80o966 shows more scatter than for the control star, probably due to it being fainter, there are also long-term systematic changes that we believe arise from a real variation in the magnitude of this known semiregular variable. |
4. Current CCD photometry
CCD photometry obtained at the Brightwater Observatory from 2006
July to 2007 March are shown in Fig. 4. Details of the equipment
and observing method are available in Innis et al. (2007).
Typically, four exposures were made in a given filter (, ,
and 
) and were then combined into normal points, with at least
4 such normal points being collected on a given night. As
CPD varies only slowly, we present nightly
averaged points in this figure and in the tabulated data. The
figure shows and versus HJD, and versus in the
upper, middle and lower panels respectively. The total range in
is near 0.5 mag. Even in this short interval of data, the time
between successive maxima is seen to be very variable, being
95 d, 75 d, and 50 d for the four maxima we have
observed. is largest when the star is brightest. This
effect is the reverse of that expected from a simple pulsation
theory, and was noted by Wisse (1981) to occur in SRb stars of
spectral type M. The explanation lies in extreme line blanketing
by TiO in spectra of stars of this type, so that is no
longer a measure of the star's effective temperature (Smak, 1964;
Torres et al., 1993).
 |
Fig. 3. CCD V and B light curves for CPD–80o966 from the Brightwater Observatory, 2006 July to 2007 March (upper and middle panels), and B–V versus V (lower panel). |
5. Discussion
Our analysis shows there is little indication of a characteristic
90 d periodicity for CPD in the Bamberg
data, which is however seen in the ASAS-3 database and is
supported by our recent CCD photometry. In two seasons, 1964 and
1966, there is evidence for a monotonic decrease and increase
respectively over at least 120 to 150 d. There may be
slight evidence for a 100 d periodicity in the 1967 data. In
general, the scatter in the data (of order 0.05 to 0.1 mag per
averaged point) makes it difficult to see a 0.3 mag
peak-to-peak variation, which is fairly typical of a given
pulsation cycle in the ASAS database and our CCD observations.
However, the long-term variations of CPD are
larger, up to 1 magnitude (ASAS-3 data). We believe that we have
good evidence that we have detected long-term changes of similar
character in this star in the 1964-1976 interval (Fig. 2).
We have performed a period analysis of all the transformed
data for CPD we obtained from the Bamberg plates
using both the Phase Dispersion Minimum (PDM) method of
Stellingwerf (1978) and the string length method of Dworetsky
(1983), with very similar results. Searching in the range from 50
to 1000 d we find the most significant period is near 305 d.
Evidence of periodicity from many seasons for a semiregular star
of this type, whose light curve varies greatly (e.g. as shown by
the ASAS-3 data and our CCD observations), does not necessarily
mean that the period has any physical significance: when searching
over a large enough period range, some periods will fit the data
better than others by random chance. The 305 d period appears to
arise mostly from a combination of the monotonic decline in 1964
and the increase seen in 1966, but the 1965 data do not obviously
support this. A period search on the post-1966 data reveals only
weak evidence for a period near 300 d.
We have also performed a period search on the 2000 d of data
from ASAS-3 CCD photometry for CPD. There is
slight evidence for a period near 285 d, but the resulting phase
plot is very scattered. Hence we cannot confirm the existence of
a 300 d periodicity in CPD in these modern
data, although the presence of the dominating d period
complicates the analysis. If the 300 d period for
CPD is not an artifact, the period ratio 305 d to
90 d is about 3.4, which is close to the period ratios of a
possible sequence of semiregulars considered by Kiss et al.
(1999). Continued observations of CPD would be
required to study this, and to see if the star exhibits mode
switching (e.g. Kiss et al., 2000).
6. Conclusion
Our analysis of the archival Bamberg photographic plates, via
aperture photometry of digitised scans, indicates that
CPD was varying in the interval 1964-1976. We
are unable to clearly identify any seasons where the star shows a
90 d periodicity, which it typically exhibits at the present
time. There is marginal evidence for a 300 d periodicity
from the Bamberg data, but this does not appear to be confirmed
from modern CCD photometry. Further observations are planned.
Acknowledgments
This work has made use of the SIMBAD database of the Centre of Astronomical Data, Strasbourg (CDS), the NASA ADS abstract database, the ASAS-3 photometric database, the data-reduction package IRAF (NOAA, USA), the numerical analysis program OCTAVE (J. Eaton and colleagues), and the Sofia Wide Field Plate Database (WFPDB). It is a pleasure to thank the Bamberg Observatory staff for their hospitality and for making the plate archive available. We acknowledge the work of the Bamberg staff who collected the data, and archived it for later use. MKT and APB were supported by the Alexander von Humboldt Foundation under the ‘Pact of Stability of South-East Europe’ programme, and grants from BAS/DFG 436-BUL110/120/0-2 and the Bulgarian National Science Fund (NFS I-1103/2001). D. Coates thanks the Faculty of Science, Monash University, for the provision of an Honorary Research Fellowship.
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