A&A 371, 233-239 (2001)
DOI: 10.1051/0004-6361:20010375
A.-Y. Zhou - Z.-L. Liu - B.-T. Du
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
Received 8 January 2001 / Accepted 15 March 2001
Abstract
The results of Johnson V time-series CCD photometry of
the
Scuti star V1821 Cygni are presented.
Our data set consists of 2431 differential magnitudes and spans the period
of July 1999 to September 2000.
We detect two best-fit pulsation frequencies representing the light
variations of the variable. V1821 Cyg is most likely pulsating in
a mixture of radial and nonradial modes.
The
data in the existing catalogues have been used to derive
the main physical parameters for the variable. We obtain:
K,
,
dex,
,
,
,
Gyr and
a distance modulus of
12.
V1821 Cyg is suggested to be a Population I
Scuti star
with metal-enriched abundances (
)
evolving on its late
main sequence stage on the basis of the colour indices and derived
properties.
Key words: stars: Scuti stars - stars: oscillations - stars: individual: V1821 Cygni
The CCD Johnson V photometry of V1821 Cyg was carried out with
the CCD light-curve survey photometer mounted on the 85-cm telescope at
the Xinglong Station of Beijing Astronomical Observatory (BAO) of China.
The CCD photometer used a red-sensitive Thomson TH7882
CCD
with a whole imaging size of
mm^{2} corresponding to a
sky field-of-view of
(square arcmin), which allows
sufficient stars to be toggled in a frame as reference.
The magnitude differences between two comparison stars observed
in the field of V1821 Cyg yielded a typical accuracy of
0
010 to 0
006.
One of the comparison stars, GSC 2683-3076, was discovered to be a new
low-amplitude multiperiodic
Scuti star
(Du et al. 1999; Zhou et al. 2001a) as a by-product of
the observations.
No variability was detected in the light curves of
GSC 2683-2994 (
,
2000.0,
6, F8) within
the accuracy of our observations. Hence this star was finally selected as
comparison to produce differential magnitudes for the variable.
Differential atmospheric and colour extinction effects were eliminated
whenever they appeared in the light curves of V1821 Cyg.
Exposure times ranged from 10 to 60 s depending on the atmospheric conditions.
Most of the V data have been merged into 120-s bins, the others 60-s bins.
The detailed features of the CCD system and the procedures of data reduction
including on-line bias subtraction, dark reduction and flatfield correction
have been described in Wei et al. (1990) and
Zhou et al. (2001a).
Date | HJD(start) | Points | Filter | Int. time |
yymmdd | 2451000+ | second | ||
990727 | 386.0910 | 216 | W | 60 |
990728 | 387.1828 | 109 | W | 60 |
990815 | 406.1062 | 70 | W | 60 |
990816 | 407.0894 | 45 | W | 60 |
990821 | 412.0260 | 113 | W | 60 |
990920 | 441.9880 | 130 | V | 60 |
991014 | 466.0621 | 137 | V | 60 |
991015 | 467.0380 | 92 | V | 120 |
991016 | 468.0584 | 65 | V | 120 |
991017 | 468.9503 | 198 | V | 60 |
991018 | 469.9410 | 115 | V | 120 |
991019 | 470.9614 | 161 | V | 60 |
991021 | 473.0471 | 94 | V | 120 |
991024 | 475.9338 | 69 | V | 120 |
991028 | 479.9474 | 165 | V | 60 |
991029 | 480.9266 | 82 | V | 120 |
991031 | 482.9335 | 116 | V | 120 |
991114 | 497.0134 | 81 | V | 60 |
000829 | 786.0540 | 123 | V | 120 |
000831 | 788.0316 | 97 | V | 120 |
000901 | 789.0101 | 109 | V | 120 |
000905 | 793.0175 | 120 | V | 120 |
000906 | 794.0056 | 133 | V | 120 |
000907 | 795.0090 | 116 | V | 120 |
000908 | 796.0618 | 91 | V | 120 |
000909 | 796.9986 | 137 | V | 120 |
Following the formulae of Montgomery & O'Donoghue (1999)
errors on the fitted frequencies, amplitudes and phases were estimated
with the root-mean-square deviation of observational noise of 0
008.
The results of frequency analysis are given in Table 2.
The power spectra and spectral window are shown in Fig. 1,
in which each power spectrum panel corresponds to the residuals with all
the previous frequencies prewhitened.
We see that the residuals in panel "Data - 2f'' are mostly noise.
The differential CCD light curves along with the fit of the 2-frequency
are presented in Fig. 2.
In addition to the V data, we also acquired 553 unfiltered measurements
(18 hours) on five nights. The unfiltered light curves can be well
fitted by the same two frequencies, allowing variations of amplitude and
phase. Figure 3 displays the results.
Figure 1: The spectral window and power spectra of V1821 Cyg | |
Open with DEXTER |
Figure 2: V differential light curves (circles) of V1821 Cyg together with the 2-frequency sinusoids represented in solid lines. Abscissa in HJD 2451000+ days, ordinate in magnitude | |
Open with DEXTER |
Figure 3: The unfiltered CCD photometric magnitude differences (filled squares) of V1821 Cyg along with the 2-frequency fitted curves. Abscissa in HJD 2451000+ days, ordinate in magnitude | |
Open with DEXTER |
The two frequencies obtained by Delgado et al. (1984)
were not fully supported in our work. A reanalysis of their data showed
two terms at 11.0674 and 8.9788 cd^{-1} with amplitudes of
0
009 and 0
0078, respectively. The standard deviation of the
residuals after prewhitening the two terms is
0085.
We further calculated S/N for the two frequencies and they are 5.1 and
4.2, respectively. If the term at
11.0674 cd^{-1} (near to
cd^{-1} by these authors) is
real, it should be an unstable mode accounting for its amplitude and
absence from the new data. The second term 8.9788 should be regarded as
uncertain. Nonetheless, the frequency
cd^{-1} given by
them roughly coincides with the daily alias of our frequency
f_{2}=8.2439 cd^{-1}.
Because of the limitation of their data (only 66 measurements in total),
any Fourier analyses based on these data are to be less reliable and
must suffer from great uncertainty.
We do not think their detection is real for the variable.
Alternatively, we fit their light curves with our two frequencies
f_{1}=8.8218 and
f_{2}=8.2439 cd^{-1} allowing variations of amplitudes
and phases. Then we arrived at the amplitudes of 0
0056 and 0
0078
with
0102, which conforms to the observational accuracy of
0
013 of the data (Delgado et al. 1984).
Freq. | Ampl. | Phase, Epoch | S/N | Conf. | |
(cd^{-1}) | (mmag) | (0-1, days) | (%) | ||
f_{1} | 8.82179 | 16.8 | 0.641, .965 | 17.2 | 100 |
2 | .2 | 14, | |||
f_{2} | 8.24389 | 7.5 | 0.875, .938 | 7.7 | 100 |
5 | .2 | 31, |
To investigate the variability in amplitude, we performed the
nonlinear least-squares fitting to the subsets of our data in 1999 and
2000 and to the data of D84 separately. The results
(Table 3) show that the main frequency f_{1} underwent
an evident amplitude change from 1983 to 1999/2000 but the amplitude of
f_{2} was constant over the period.
Further observations will be necessary and helpful to study the stability
of the pulsations of V1821 Cyg in amplitude as well as in period.
Frequency | Amplitude(mmag) | ||
(cd^{-1}) | 1983 | 1999 | 2000 |
f_{1}= 8.82179 | 5.6 | 16.6 | 17.0 |
f_{2}= 8.24389 | 7.8 | 7.5 | 7.5 |
Domingo & Figueras (1999) showed that the Crawford (1979) calibration is able to reproduce quite well the absolute magnitude of normal A-type stars. We first dereddened the indices for the variable with the dereddening formulae and calibrations given by Crawford (1979). The indices are , 164, 216, 906, 027 and 206. The results are in good agreement with those of Delgado et al. (1984) and L90.
Here we note that R00 adopted the spectral type of A5p from Reimann (1989) while Rodríguez et al. (1994, hereafter R94) used A8III for V1821 Cyg. The discrepancy in spectral type has little effect on the derived indices for the calibrations used. In addition, we also note that the two versions of the catalogues of Scuti stars (R94; R00) adopted the indices of D84, in which 720. However, this value is wrong, this is a typographical mistake of D84 (Zhou et al. 2001a). The true value is 760. This mistake was taken into account in the L90 list, but not in the catalogues of R94 nor R00.
Actually, we met a problem during derivations if 720. This value is the lowest limit and correspondingly c_{0} is larger than 0 28, the upper limit for the Crawford (1979) calibrations. In general, late A-type stars have 72 (i.e. K). 72 seems to be lower for a star of A8III or A5p type and it is preferable to an A3-type star. In addition, Zhou et al. (2001a) obtained 762 for GSC 2683-3076, a new Scuti star discovered in the field of V1821 Cyg. Both variables are foreground stars and not members of NGC 6871. Their colour indices should be very similar.
Anyway, the difference (0 04) between the two values introduced an uncertainty of 0 2 for absolute magnitude. Consequently, we derived using the Crawford (1979) calibration as well as the new Strömgren photometric calibration of absolute magnitudes from HIPPARCOS (Domingo & Figueras 1999). was utilized to account for effective temperature. This means a distance modulus of 9 12, which is consistent with that of GSC 2683-3076 (Zhou et al. 2001a).
Using the adjusted ,
c_{0} grid for Kurucz model atmospheres
(Moon & Dworetsky 1985), we obtained
K and
dex.
They agree with the estimates using the grids for
or 0.5 by
Smalley & Kupka (1997) with (b-y)_{0} and c_{0}.
With
and
and using the evolutionary tracks from
Claret (1995) for solar abundances, we obtain mass
,
Gyr and absolute bolometric
magnitude
(assuming B.C. 0
01).
Then a radius of
was calculated using the
radiation law
(1) |
Moreover, with
=
027 and using the calibrations
by Smalley (1993) for metallicity of A-type stars, we can
obtain mean metal abundance
dex, i.e. Z=0.046, adopting the
solar abundances X=0.75, Y=0.23 and Z=0.02 (
)
(Mazzitelli 1979).
According to Stellingwerf (1979) normal Sct stars have solar abundances (i.e.
or
).
Therefore the star is about 130 per cent metal-enriched compared to the Sun.
The metallicity
is also higher than
of its
neighbour star GSC 2683-3076.
But the metallicity inferred from m_{1} is a normal value
compared with
,
the metal content of
Scuti
itself, the prototype of the
Scuti class (Russell 1995).
It is also typical of normal Population I stars.
Consequently, we consider V1821 Cyg to be a metal-rich Pop I
Sct
star. Table 4 summarizes the indices, comparing with those
in the literature. The key stellar physical parameters for V1821 Cyg
are given in Table 5. According to the colour indices
and Fig. 8 of Rodríguez & Breger (2001),
the variable is well-situated in the middle of
Scuti instability
region in the Hertzsprung-Russell diagram.
V | b-y | m_{1} | c_{1} | Ref. | ||
10.122 | 0.265 | 0.099 | 0.847 | -- | HM98 | |
10.140 | 0.247 | 0.189 | 0.922 | 2.760 | L90 | |
E(b-y) | (b-y)_{0} | m_{0} | c_{0} | |||
0.082 | 0.165 | 0.215 | 0.902 | -- | -- | D84 |
0.083 | 0.164 | 0.216 | 0.906 | 0.206 | -0.027 | |
0.077 | 0.170 | 0.214 | 0.903 | 0.203 | -0.026 | L90 |
Parameter | Values | Parameter | Values |
E(b-y) | M_{V} | ||
(b-y)_{0} | |||
m_{1} | D.M. | ||
c_{1} | |||
Age(Gyr) | (K) | ||
(dex) | |||
(dex) | 0.366 |
(2) |
On the other hand, the higher value of period ratio allows us to rule out the presence of a purely radial pulsation and indicates that at least one of the two modes must be nonradial. In Sect. 1, we mentioned the estimations of modes for the oscillations in Delgado et al. (1984). Now we are clear they incorrectly resolved the pulsation in the star due to insufficient data. Ultimately, for pulsation modelling, we may use the , Z=0.02 and , Z=0.06 models of Templeton et al. (1997). The two models suggest f_{1} as a nonradial p_{2} mode with l=1 and f_{2} as the second radial overtone (2H) or a nonradial p_{2} (l=1) mode. To summarize, we list the results of mode identifications in Table 6. The attempts at mode identification at the present time are completely preliminary, due to the lack of Strömgren colour or spectroscopic data. Safe mode identification strongly needs additional colour photometry.
By means of the basic pulsation equation, the mean density and pulsation
mass of the variable can be calculated in terms of its principal period
and pulsation constant. With
and
we find
(0.062 if
and
were used).
The density agrees closely with the value of
0.059
derived from the Eq. (3) of
Viskum et al. (1998). Then we obtained the pulsation mass
with
,
which agrees
quite well with the evolutionary mass estimated above and with that
derived from the relation
(3) |
Frequency | Q | Modes | |
(cd^{-1}) | (Hz) | (days) | |
f_{1} = 8.82179 | 102.10 | 0.0282 | p_{1} or p_{2}(l=1) |
f_{2} = 8.24389 | 95.42 | 0.0302 | F;1H or p_{2}(l=1) |
Acknowledgements
We thank Prof. R.-X. Zhang and Dr. X.-B. Zhang for their joint observations. The authors are very grateful to the referee Dr. Eloy Rodríguez at IAA (Spain) for his constructive comments on the original manuscript. This research was funded by the Natural Science Foundation of China.