A&A 398, 213-222 (2003)
DOI: 10.1051/0004-6361:20021595
P. Moskalik1 - E. Poretti2
1 - Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
2 -
INAF-Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
Received 10 September 2002 / Accepted 30 October 2002
Abstract
We present the results of a systematic search for multiperiodic pulsators
among the Galactic Bulge RR Lyrae stars of the OGLE-1 sample. We identify
one "canonical'' double-mode variable (RRd star) pulsating in two radial
modes. In 38 stars we detect secondary periodicities very close to the
primary pulsation frequency. This type of multiperiodic variables
constitute 23% of RRab and
5% of RRc population of the
Bulge. With the observed period ratios of
0.95-1.02 the secondary
periods must correspond to nonradial modes of oscillation. Their beating
with the primary (radial) pulsation leads to a long-term amplitude and
phase modulation, known as the Blazhko effect. The Blazhko RRab variables
occur more frequently in the Galactic Bulge than in the LMC. The opposite
tendency is seen in case of the RRd stars. The differences of incidence
rates are most likely caused by different metallicity of the two
populations. We discuss pulsation properties of the OGLE-1 Blazhko stars
and compare them with predictions of theoretical models. We argue, that the
oblique magnetic pulsator model of Shibahashi (2000) cannot account for the
observations and should be ruled out.
Key words: stars: horizontal branch - stars: oscillations - stars: variables: RR Lyr
The Optical Gravitational Lensing Experiment (OGLE;
Udalski et al. 1992) is
devoted to search for dark matter in our galaxy through the detection of
microlensing events. As a by-product of this program a large amount of
photometric data has been collected for stars in the Galactic Bulge and
Magellanic Clouds. In this series of papers we examine short period
pulsating variables discovered in the OGLE-1 Galactic Bulge sample (Udalski
et al. 1994, 1995, 1996, 1997).
Paper I (Poretti 2001) was devoted to
monoperiodic Scuti and RR Lyrae variables. In the current
paper, second in the series, we discuss multiperiodic RR Lyrae
stars.
The existence of multimode RR Lyrae variables has been known since the
discovery of double-mode pulsations in AQ Leonis
(Jerzykiewicz & Wenzel 1977).
In following years about 300 similar variables have been identified
in several stellar systems, including 181 stars in the Large Magellanic
Cloud (see Kovács 2001 for comprehensive review). All these variables
share common properties: they pulsate in two radial modes, namely
fundamental and the first overtone, with the period ratio of
.
These stars are commonly
referred to as RRd variables.
In recent years a new class of multiperiodic RR Lyrae variables has been identified (Olech et al. 1999). They are characterised by the presence of two or more very closely spaced frequencies, with period ratios in the range of 0.95-1.05. Such period ratios are incompatible with the radial mode oscillations and point strongly towards the presence of nonradial modes. This new form of multiperiodicity was first discovered in the RRc variables, but later was shown to occur in the RRab variables as well (Moskalik 2000). For the most complete summary of recent observations see Kovács (2002).
Massive photometry collected during microlensing surveys is particularly well suited for a systematic search for multiperiodic RR Lyrae stars. Such a search has already been performed in the Large Magellanic Cloud, using MACHO data (Alcock et al. 2000; Welch et al. 2002). In this paper we present a complete inventory of multiperiodic RR Lyrae stars in the OGLE-1 Galactic Bulge sample. Our primary goal is to establish the incidence rates of different types of multimode oscillations in the Galactic Bulge population. Comparison between the rates in the Bulge and in the LMC can shed new light on the physical conditions favouring such oscillations. We also discuss pulsation properties of the identified multiperiodic RR Lyrae stars. Preliminary results of this work have already been published by Moskalik & Poretti (2002). Here we present the complete discussion of our findings.
The OGLE-1 database contains 215 RR Lyrae stars, 150 of them have been
classified as RRab (fundamental-mode pulsators) and 65 as RRc (first
overtone pulsators). The photometric data span 900 days, with
typically 130-150 I-band measurements published for each star. As a first
step, the lightcurve is fitted with the Fourier sum of the form
![]() |
(1) |
In the next step, the search for additional periodicities is performed. This is done with two different, independently applied methods:
The two procedures give the same results for all the stars except one, strengthening our confidence in the frequency identifications. In the following, we accept as multiperiodic only those variables in which a secondary period is detected with both methods.
As a third step, the Fourier fit with two identified frequencies and
their linear combinations is performed. To this effect, we fit the
lightcurve with the following formula:
![]() |
(2) |
![]() |
Figure 1: Spectral window for BW6 V20 dataset. |
Open with DEXTER |
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Figure 2: Frequency analysis of BW6 V20. Subsequent panels show the power spectrum of the original data and the results of consecutive prewhitening steps. The right column of the plot displays fine structure around the main peak. The spectra are normalised separately in each panel. |
Open with DEXTER |
![]() |
Figure 3: Phased lightcurve (I-magnitude) of BW6 V20. Top: original data phased with the primary period. Bottom: lightcurves of the three detected frequencies, f0 = 2.539947 c/d, f1 = 2.554618 c/d and f2 = 2.525276 c/d, respectively. |
Open with DEXTER |
As an example, we show the application of the first procedure described above to the case of BW6 V20. In Fig. 1 we present the spectral window of the data. This window is typical for OGLE-1 photometry. Figure 2 shows consecutive steps of frequency analysis. The top panel displays the Fourier power spectrum of the original data. The blow-up of the neighboarhood of the dominant peak is shown in the right column. The second panel displays the power spectrum of data prewhitened by the first frequency and its harmonics. A new frequency has clearly emerged. The next prewhitening (panel three) reveals another frequency on the opposite side of the dominant peak. Finally, after prewhitening by three frequencies no more periodicities are detected (panel four). The analysis is stopped. In Fig. 3 we display the phased lightcurve of BW6 V20, first for the original data (top panel), then separately for the three detected periods (three bottom panels).
Most of the RR Lyrae stars in the OGLE-1 sample are strictly periodic. Those stars are discussed in Paper I. Departures from a monoperiodic behaviour have been detected in 49 RR Lyrae variables. They can be divided in several distinctive types.
OGLE No. | P0 | ![]() |
A0 | A1 |
[d] | [c/d] | [mag] | [mag] | |
RR1-![]() |
||||
BWC V47 | 0.256926 | -0.02439 | 0.067 | 0.050 |
MM5A V20a | 0.391192 | 0.04546 | 0.126 | 0.009 |
RR0-![]() |
||||
MM5A V46a | 0.370187 | 0.02530 | 0.202 | 0.041 |
BW3 V17 | 0.403331 | 0.12005 | 0.258 | 0.013 |
BW11 V10 | 0.450658 | 0.01864 | 0.252 | 0.049 |
BW3 V13 | 0.457517 | 0.03255 | 0.230 | 0.042 |
BWC V41 | 0.462144 | 0.01767 | 0.221 | 0.041 |
BW11 V44 | 0.482493 | 0.03329 | 0.204 | 0.028 |
BW5 V34 | 0.490265 | -0.00873 | 0.223 | 0.028 |
BW2 V23 | 0.492764 | 0.01198 | 0.244 | 0.044 |
BW7 V15b | 0.497071 | 0.00941 | 0.226 | 0.035 |
BW7 V8 | 0.507103 | -0.03312 | 0.244 | 0.024 |
BW7 V33 | 0.511226 | -0.00689 | 0.261 | 0.030 |
MM5A V9 | 0.516204 | 0.03580 | 0.251 | 0.016 |
BW10 V21 | 0.521996 | 0.01740 | 0.136 | 0.042 |
MM5B V4 | 0.524886 | 0.02237 | 0.184 | 0.024 |
BW10 V20 | 0.527123 | -0.02450 | 0.244 | 0.023 |
BW10 V44 | 0.527263 | -0.01206 | 0.191 | 0.036 |
BW8 V15 | 0.577234 | 0.02543 | 0.167 | 0.028 |
BW5 V36c | 0.594501 | 0.01445 | 0.139 | 0.015 |
BW2 V24 | 0.597426 | 0.01641 | 0.118 | 0.018 |
BWC V61 | 0.615946 | 0.02145 | 0.096 | 0.022 |
BW10 V41 | 0.626882 | 0.00855 | 0.215 | 0.049 |
BW1 V34 | 0.632719 | 0.01434 | 0.154 | 0.031 |
BWC V51 | 0.649494 | 0.01618 | 0.071 | 0.031 |
BW6 V17 | 0.651601 | 0.00545 | 0.146 | 0.022 |
BW10 V40 | 0.682741 | 0.01393 | 0.117 | 0.021 |
Frequency | BWC V47 | MM5A V46 | BW11 V10 | BWC V41 | BW3 V13 | BW5 V34 | MM5A V9 |
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0.005 | -- | 0.014 | -- | -- | -- | -- |
f0 | 0.067 | 0.202 | 0.252 | 0.221 | 0.230 | 0.223 | 0.251 |
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0.050 | 0.041 | 0.049 | 0.041 | 0.042 | 0.028 | 0.016 |
2f0 | 0.008 | 0.068 | 0.115 | 0.087 | 0.103 | 0.107 | 0.123 |
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-- | 0.028 | 0.040 | 0.032 | 0.027 | 0.025 | 0.010 |
3f0 | 0.021 | 0.072 | 0.044 | 0.062 | 0.072 | 0.080 | |
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0.011 | 0.033 | 0.022 | 0.022 | 0.024 | -- | |
4f0 | 0.043 | 0.028 | 0.031 | 0.038 | 0.052 | ||
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0.028 | -- | -- | -- | -- | ||
5f0 | 0.027 | 0.021 | 0.031 | ||||
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0.024 | -- | -- | ||||
6f0 | 0.018 | 0.016 | 0.017 | ||||
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0.018 | -- | -- | ||||
7f0 | 0.010 | ||||||
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-- | ||||||
BW10 V21 | BW10 V20 | BW10 V44 | BWC V61 | BW10 V41 | BWC V51 | BW6 V17 | |
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-- | -- | -- | -- | -- | -- | -- |
f0 | 0.136 | 0.244 | 0.191 | 0.096 | 0.215 | 0.071 | 0.146 |
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0.042 | 0.023 | 0.036 | 0.022 | 0.049 | 0.031 | 0.022 |
2f0 | 0.066 | 0.118 | 0.083 | 0.034 | 0.100 | 0.019 | 0.068 |
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0.029 | 0.022 | 0.024 | 0.014 | 0.032 | 0.009 | 0.010 |
3f0 | 0.033 | 0.062 | 0.052 | 0.015 | 0.052 | 0.006 | 0.041 |
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0.023 | 0.024 | 0.008 | 0.016 | 0.029 | -- | 0.017 |
4f0 | 0.013 | 0.036 | 0.028 | 0.035 | 0.019 | ||
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-- | 0.016 | 0.021 | -- | 0.010 | ||
5f0 | 0.017 | 0.013 | |||||
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0.015 | -- | |||||
6f0 | 0.007 | ||||||
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-- |
This type of oscillations has been detected in one object of the sample:
BW7 V30. The star pulsates with
and
.
The resultant period
ratio of
is typical for the RRd
variables. The first overtone strongly dominates over the fundamental mode,
with
A0/A1 = 0.142. Two harmonics of the first
overtone are detected, but no harmonics of the fundamental and no frequency
combinations can be identified. This is perhaps not surprising, considering the
extremely low amplitude of the fundamental mode (
A0 = 0.018 mag).
In 38 RR Lyrae variables secondary peaks close to the dominant pulsation frequency are detected. They are well-resolved within our dataset and are not due to a secular period variability. Their beating with the primary (radial) pulsation results in an apparent long-term amplitude and phase modulation, a phenomenon referred to as Blazhko effect.
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A- | |||
OGLE No. |
P0 |
![]() |
A0 | A+ |
[d] | [c/d] | [mag] | [mag] | |
RR1-BL | ||||
BW8 V34 | 0.320365 | 0.01544 | 0.127 | 0.030 |
0.016 | ||||
RR0-BL | ||||
BW6 V20 | 0.393709 | 0.01467 | 0.244 | 0.031 |
0.065 | ||||
BW9 V34 | 0.449297 | 0.00896 | 0.254 | 0.042 |
0.063 | ||||
BWC V15 | 0.458717 | 0.00376 | 0.228 | 0.072 |
0.056 | ||||
BW9 V24 | 0.476339 | -0.00185 | 0.206 | 0.021 |
0.00239 | 0.103 | |||
BW6 V7 | 0.525015 | 0.03363 | 0.257 | 0.035 |
0.042 | ||||
BW1 V18 | 0.529560 | 0.01876 | 0.250 | 0.032 |
0.034 | ||||
BWC V33 | 0.550304 | 0.00865 | 0.175 | 0.020 |
0.034 | ||||
BW6 V29 | 0.562906 | 0.02283 | 0.164 | 0.019 |
0.022 | ||||
BW10 V66 | 0.581234 | 0.01410 | 0.156 | 0.030 |
0.053 |
Frequency | BW8 V34 | BW6 V20 | BW9 V34 | BWC V15 | BW9 V24 | BW6 V7 | BW1 V18 | BWC V33 | BW10 V66 |
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-- | -- | -- | -- | -- | -- | 0.020 | -- | -- |
![]() |
0.030 | 0.031 | 0.042 | 0.072 | 0.020 | 0.035 | 0.032 | 0.020 | 0.030 |
f0 | 0.127 | 0.244 | 0.254 | 0.228 | 0.206 | 0.257 | 0.250 | 0.175 | 0.156 |
![]() |
0.016 | 0.065 | 0.063 | 0.056 | 0.103 | 0.042 | 0.034 | 0.034 | 0.053 |
![]() |
0.008 | 0.028 | 0.039 | 0.059 | -- | 0.034 | 0.028 | 0.019 | 0.027 |
2f0 | 0.014 | 0.104 | 0.103 | 0.081 | 0.068 | 0.124 | 0.127 | 0.087 | 0.062 |
![]() |
-- | 0.042 | 0.051 | 0.044 | 0.066 | 0.038 | 0.020 | 0.033 | 0.035 |
![]() |
0.021 | 0.030 | 0.035 | -- | 0.029 | 0.019 | -- | 0.016 | |
3f0 | 0.040 | 0.052 | 0.036 | 0.018 | 0.065 | 0.080 | 0.051 | 0.036 | |
![]() |
0.030 | 0.050 | 0.024 | 0.051 | 0.038 | -- | 0.030 | 0.028 | |
![]() |
0.012 | 0.022 | 0.016 | -- | 0.024 | 0.018 | -- | -- | |
4f0 | 0.022 | 0.020 | 0.022 | -- | 0.038 | 0.049 | 0.033 | 0.017 | |
![]() |
0.029 | 0.027 | 0.014 | 0.026 | 0.030 | -- | 0.023 | 0.019 | |
![]() |
-- | 0.011 | 0.015 | -- | -- | ||||
5f0 | 0.013 | 0.017 | 0.018 | 0.020 | -- | ||||
![]() |
0.015 | 0.013 | 0.022 | -- | 0.014 |
In most cases (27 stars) only one secondary peak is present, forming a
close doublet with the primary peak. Such a frequency pattern occurs in
2 RRc and in 25 RRab stars. Following Alcock et al. (2000), we denote
these variables as RR1-1 and RR0-
1, respectively. Their main
properties (period
and amplitude A0 of the
primary peak, frequency separation
and amplitude of the secondary peak A1) are listed in
Table 1. In most stars the frequency separation is between
0.008 c/d and 0.036 c/d, corresponding to a beat (Blazhko) period of
28- 125 days. The amplitude of the secondary peak is always very small,
never exceeding 0.05 mag. Contrary to the case of the LMC stars (Alcock
et al. 2000; Welch et al. 2002), combination frequencies
can be detected in
about half of the RR-
1 stars of the OGLE-1 sample. The amplitudes of
all components identified in these variables are given in Table 2.
The amplitudes and frequencies listed in Tables 1 and 2 are
determined through a least-squares fit of Eq. (2) to the data.
In several RR Lyrae variables two secondary frequencies, located on
opposite sides of the primary peak, are present. Together with the primary
frequency they form a close equally spaced triplet. Such a frequency
pattern is found in 1 RRc and in 9 RRab stars. Following Alcock et al.
(2000), we denote these variables as RR1-BL and RR0-BL, respectively. Their
properties are listed in Table 3. The amplitudes of secondary peaks are
somewhat larger than in the RR-1 stars, reaching up to 0.1 mag. The
frequency separations
are similar to those observed in
RR-
1 stars. With the exception of one star, the triplets are
exactly equidistant. This has been verified by fitting the data with
Eq. (2), with all 3 frequencies of a triplet treated as independent
parameters. The resulting separations
and
differ by
less than 0.00005 c/d, which within our dataset is indistinguishable from
zero. Thus, in the final solution
is
assumed. The one deviating case is BW9 V24, where
c/d. We note, that in this particular star the
triplet is barely resolved. Since the A- component in BW9 V24 is
very weak, the insufficient resolution can have a significant effect on its
frequency determination. The deviation from equidistant spacing seen in
BW9 V24 needs to be confirmed with a longer dataset.
In all but one RR-BL stars we detect combination frequencies
.
The amplitudes of all identified components
are given in Table 4. The combination peaks, together with harmonics of
f0, form a sequence of equidistant triplets. This structure can be
followed up to the third or fourth harmonic. We note in passing, that the
observed frequency pattern is the same as found in several well-studied
field RR Lyrae Blazhko variables (Borkowski 1980;
Smith et al. 1994, 1999;
Kovács 1995; Nagy 1998; Szeidl & Kolláth 2000).
Yet another pulsation patter is found in BWC V106. In this star, two
secondary frequencies are detected, but both located on the same side
of the primary peak. Together with the primary frequency they form a close
triplet, which is neither equally spaced, nor centered on the primary.
Following Alcock et al. (2000) we classify BWC V106 as an RR0-2-type
variable. This type of pulsation pattern is extremely rare, only 3 similar
stars have been identified among 1327 first overtone RR Lyrae pulsators in
the LMC (Alcock et al. 2000). The properties of BWC V106 are listed in
Table 5. No combination peaks are detected in this star.
In 10 RR Lyrae stars we find after prewhitening a significant residual
power at a frequency almost identical to that of the (just removed) primary
component. Such behaviour is a signature of a slow phase and/or amplitude
variability, not resolved within the length of available data
(900 days for OGLE-1).
For a closer examination of the long-term variations in these objects we have
performed Fourier fits of their lightcurves separately for each
observing season. In 6 stars, all pulsating in the first overtone, we find that
the pulsation phase varies from year to year, but the amplitude of the
pulsation remains constant. These stars are listed in Table 6.
Clearly, they undergo a secular period change. Assuming that the period
varies linearly in time, we can estimate for each object the slowest
consistent with the data. We show them in the last two columns of
Table 6. We stress that the numbers derived here are based on three
observing seasons only and therefore should be treated as preliminary.
Nevertheless, we can conclude that in all 6 cases the pulsation period changes
on a timescale of no more than
104 yr, which is much shorter
than the timescale predicted by the theory of stellar evolution.
In the remaining 4 stars (1 RRc and 3 RRab-type) both amplitude
and phase vary from year to year. We list these objects in Table 7,
where in the last two columns we give the values of the Fourier amplitude
A1 and phase
(see Eq. (1)) for each observing season.
Seasonal lightcurves of the stars are displayed in Fig. 4. The
amplitude variability is hardly noticeable in the RRc star BW11 V34, but in
the RRab stars it is rather obvious. In the extreme case of BW5 V13 the
amplitude increases by a factor of five within one year. The change of the
pulsation amplitude is accompanied by a change of phase and change of shape
of the lightcurve. This last effect is most noticeable in BW4 V5 and
BW6 V18. All 4 stars are most likely Blazhko variables with modulation
period (beat period between the modes) longer than the span of available
data. However, with the data in hand we are not able to make any firm
statement about the nature of the observed long term behaviour. In the
following discussion we will treat all 4 variables as "nonclassified''.
OGLE No. | P0 | ![]() |
A0 | A1 |
[d] | [c/d] | [mag] | [mag] | |
BWC V106 | 0.464967 | 0.00708 | 0.188 | 0.020 |
0.02675 | 0.020 |
OGLE No. | P0 | A0 | ![]() |
![]() |
[d] | [mag] | [s/s] | [yr] | |
BWC V37 | 0.380122 | 0.140 | 1.7
![]() |
6.1
![]() |
BW2 V10 | 0.507782 | 0.118 | 3.1
![]() |
4.5
![]() |
BW7 V51 | 0.272148 | 0.154 | 2.9
![]() |
2.5
![]() |
BW9 V38 | 0.305731 | 0.117 | -4.0
![]() |
-2.1
![]() |
BW11 V55 | 0.286179 | 0.123 | -5.1
![]() |
-1.5
![]() |
MM5A V24 | 0.379396 | 0.093 | -5.4
![]() |
-1.9
![]() |
OGLE No. | P0 | A1 | ![]() |
[d] | [mag] | [rad] | |
RRc | |||
BW11 V34 | 0.341674 |
![]() |
![]() |
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||
![]() |
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||
RRab | |||
BW4 V5 | 0.474673 |
![]() |
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||
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-
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||
0.494952 |
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|
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-
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||
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-
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||
0.541402 |
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|
![]() |
-
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||
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-
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![]() |
Figure 4: Seasonal lightcurves (I-magnitude) of suspected Blazhko variables. Panels from top to bottom correspond to observing seasons of 1992, 1993 and 1994, respectively. |
Open with DEXTER |
Type | Description | Number | % | LMC % |
RRc ............ | single-mode | 54 |
![]() |
![]() |
RRd ............ | F/1H double mode | 1 |
![]() |
![]() |
RR1-![]() |
two close components | 2 |
![]() |
![]() |
RR1-BL ...... | symmetric triplet | 1 |
![]() |
![]() |
RRc-PC ...... | period change | 6 |
![]() |
![]() |
RRc-NC ...... | nonclassified | 1 |
![]() |
![]() |
Type | Description | Number | % | LMC % |
RRab ................ | single-mode | 112 |
![]() |
? |
RR0-![]() |
two close components | 25 |
![]() |
![]() |
RR0-![]() |
three close components | 1 |
![]() |
? |
RR0-BL .......... | symmetric triplet | 9 |
![]() |
![]() |
RRab-PC ...... | period change | - | --- | ? |
RRab-NC ...... | nonclassified | 3 |
![]() |
? |
Tables 8 and 9 present the inventory of different types of RR Lyrae variables in the OGLE-1 Galactic Bulge sample, separately for the overtone and for the fundamental mode pulsators. The canonical double mode variables (RRd stars) are somewhat arbitrarily included into the table for overtone variables. This is justified by the strong dominance of the overtone mode in these stars - as a result they are usually disguised, until full frequency analysis, as RRc stars. For each type of variability we list the number of stars found (third column ) and the estimated incidence rate in the population (fourth column). The standard deviations of the incidence rates are calculated assuming Poisson distribution. For the purpose of comparison, we also show the corresponding rates for the LMC sample (Alcock et al. 2000; Welch et al. 2002), these are listed in the last column of the tables. The LMC rates have not been published for all the subclasses; the missing entries in the tables are raplaced by question marks.
Several important conclusions can be drawn from data of Tables 8 and 9:
This difference does not result from the lower noise
level of the Galactic Bulge data. The weakest secondary peaks
detected in the LMC stars have amplitudes of
0.020 mag. Adopting the same threshold for our
sample, we are still left with 31 RRab Blazhko stars, which
is
of the RRab population. We conclude, that
the higher incidence rate of RRab Blazhko stars in the
Galactic Bulge, as compared to the LMC, is real. It is
tempting to speculate that this effect is caused by a
difference in metallicity of the two populations.
With the sample of 35 fundamental mode Blazhko stars, we are in a position to
discuss the group properties of this type of variables. In Fig. 5 we
display the Period-Amplitude diagram for the Galactic Bulge RRab stars.
In the upper panel we plot the peak-to-peak amplitude, as given by OGLE-1
catalog. In the lower panel we plot the Fourier amplitude of the radial mode,
as determined by the Fourier fit Eq. (1) or Eq. (2).
Monoperiodic variables are marked by asterisks, the Blazhko variables by
open circles (RR0-1 and RR0-
2) and by filled circles (RR0-BL).
It is immediately apparent that the presence of secondary frequencies
neither depends on nor affects the amplitude of the primary (radial)
pulsation. It does not depend on the pulsation period, either. The stars with
close frequency doublets (RR0-
1 variables) are detected with roughly
the same probability at all periods represented in the sample. The
occurrence of RR0-BL variables, however, seems to be limited to
.
Nevertheless, the overall fraction of Blazhko variables is
essentially period independent. This conclusion is also supported by the
histogram shown in Fig. 6. The fraction of Blazhko variables in each period
bin is within 1
consistent with the average value of 23.3%.
In Fig. 7 we show the frequency separation
for RRab Blazhko variables of our sample.
displays
no trend with the radial mode period
.
For 80% of
RR0-
1 stars frequency separation is positive, corresponding to
the secondary frequency being higher than the primary one. An identical
distribution of
has also been found for RRab stars in the LMC
(Welch et al. 2002). The negative values of
occur only in
the narrow range of periods between 0
49 and 0
53. Interestingly,
this particular period range is avoided by RR0-BL stars. The values of
for triplets (RR0-BL stars) are on average slightly smaller
than for the doublets (RR0-
1 stars). In 8 out of 9 RR0-BL stars the
higher amplitude secondary peak has also higher frequency. Again, very
similar statistics have been found for the RRab stars in the LMC.
![]() |
Figure 5:
Period-amplitude diagram for OGLE-1 RRab stars. Top:
peak-to-peak amplitudes from OGLE-1 catalog, bottom: Fourier
amplitudes of the radial mode. Monoperiodic variables marked by
asterisks, Blazhko variables by open circles (RR0-![]() ![]() |
Open with DEXTER |
![]() |
Figure 6: Period distribution of OGLE-1 RRab stars. Black area corresponds to Blazhko variables. |
Open with DEXTER |
![]() |
Figure 7:
Frequency difference, ![]() |
Open with DEXTER |
While the
distribution for the RRab stars is very similar in both
galaxies, it is distinctively different from that for the RRc stars of the LMC
(Alcock et al. 2000). In the latter case, the frequency separation is
negative in more than half of the doublets (RR1-
1 stars) and the
higher amplitude secondary peak in the triplets (RR1-BL stars) can occur on
either side of the primary peak, with about equal probability. The values
of
in the LMC RRc stars are also noticably larger
than in the RRab stars. Unfortunately, the number of RRc Blazhko variables
in the Galactic Bulge is too small for a meaningfull discussion.
Nevertheless, even for this very limited sample we find the same result: the
average
is larger than for the RRab stars. Apparently,
whatever mechanism is responsible for the excitation of the secondary
frequencies, it works differently in the overtone and in the fundamental
mode RR Lyrae pulsators.
In Fig. 8 we display the Fourier phases
and
for all
the Galactic Bulge RRab stars. This plot for the monoperiodic pulsators
has already been presented in Paper I (their Fig. 12), here we have added
the multiperiodic (Blazhko) stars. In the case of multiperiodic stars, the
Fourier phases are derived for the primary (radial) component of the
pulsation. Specifically, they are constructed with the phases of the
kf0 components of the Fourier fit Eq. (2). We stress that this is
a different procedure from that used by Jurcsik et al. (2002), who
have evaluated the Fourier parameters of the snapshot lightcurves at various
phases of the beat (Blazhko) period. As a result, their parameters are time
dependent. The phases derived in the current paper do not vary in time.
They describe the shape of the primary (radial) pulsation after
secondary frequencies have been filtered out.
In the case of monoperiodic stars the Fourier phases
and
follow a single progression up to
.
At
longer periods the progression splits into three separate tails. As has
been shown in Paper I, these tails are formed by stars of different
metallicity, with the upper tail corresponding to the highest and the lower
tail to the lowest value of [Fe/H].
![]() |
Figure 8:
Fourier phases ![]() ![]() |
Open with DEXTER |
Figure 8 shows that the Blazhko variables of our sample choose preferentially
the upper tail of the progression. This is best visible in the plot
of .
Out of 11 Blazhko stars with
,
7 fall
on the upper tail, while only 2 belong to the central and to the lower
tails, each. In the
plot, these variables are split between
the tails in the same way. The distribution of Fourier phases
implies that the incidence rate of Blazhko variables increases with
metallicity. We note, that the same dependence has already been suggested
by comparison between the Galactic Bulge and the LMC (see Sect. 4.1).
We have conducted a systematic search for multiperiodic pulsators in the OGLE-1 sample of Galactic Bulge RR Lyrae stars. Multiperiodicity has been established for 39 variables. In addition, we have identified 6 RRc stars with changing pulsation period and 4 stars exhibiting long-term change of pulsation amplitude.
Among 39 multiperiodic variables only one is a "canonical" double-mode
pulsator (RRd stars), with two radial modes - fundamental and first
overtone - excited. In the remaining 38 variables a different type of
multiperiodicity is found - additional peaks are present very close to the
primary pulsation frequency. Their beating with the primary (radial)
pulsation results in an apparent long term amplitude and phase modulation.
This phenomenon is referred to as Blazhko effect. The majority of the Blazhko
variables come in one of two flavours: we detect either a single secondary
peak, forming a doublet with the primary frequency (RR-1 stars),
or a pair of secondary peaks, which together with the primary frequency
form an equidistant triplet centered on the primary peak (RR-BL
stars). While the frequency triplet can result from periodic amplitude
and/or phase modulation of a purely radial pulsation, such a process cannot
produce a doublet. The observed period ratios of
are not compatible with the excitation of two radial
modes. This implies that a secondary component of the doublet must
correspond to a nonradial mode of oscillation.
The Blazhko variables occur among both RRc (overtone) and RRab (fundamental-mode) stars, but the incidence rate is not the same. In the case of the Galactic Bulge it is 5% for the RRc stars and 23% for the RRab stars.
The incidence rates of multiperiodic RR Lyrae variables differ between the Galactic Bulge and the Large Magellanic Cloud. Specifically, the double-mode (RRd) stars occur much more frequently in the LMC, while the opposite is true for the RRab Blazhko stars. We believe that the effect is caused by a different metallicity of the two populations. Such an interpretation is supported by the Fourier analysis of the Bulge RRab stars. The hypothesis of metallicity influence can be further tested when statistics of multiperiodic variables in the SMC are established.
The occurrence of Blazhko variability among RRab stars correlates neither
with amplitude nor with period of the primary pulsation. In the vast majority
of doublets (RR0-1 stars) the secondary frequency is higher than the
primary one. The same is also true for the higher of the two secondary peaks
in the triplets (RR0-BL stars). This is a different distribution than is
observed in the case of the RRc stars (Alcock et al. 2000). The values of the
frequency separation
are also different, being systematically
smaller than in the RRc stars. Clearly, the detailed picture of the Blazhko
variability depends on the primary mode of pulsations.
The observational properties discussed in the current paper put constraints on any proposed explanation of the Blazhko effect. Two models are most popular nowadays: the oblique magnetic pulsator model (Shibahashi 1995, 2000) and the 1:1 resonance model (Nowakowski & Dziembowski 2001). The results presented here seem to rule out the first of the two. The oblique pulsator model predicts splitting the pulsation frequency of a mode into an equally spaced quintuplet. We never detect such a structure, despite specifically looking for it. We see only triplets or doublets. The model cannot explain a striking difference of the Blazhko incidence rate between RRc and RRab stars. If the modulation is indeed due to presence of the magnetic field, then it should occur equally likely independently of the pulsation mode. Finally, as the pulsation amplitude is supposed to vary with the rotation period of the stars, it is hard to understand why Blazhko periods of RRc stars are systematically shorter than those of the RRab stars.
The resonant model avoids all of the above difficulties. It generates equally split triplets, not quintuplets. It can explain naturally the asymmetry of amplitudes in the triplet. The model predicts a higher incidence rate of Blazhko variables among RRab than among RRc stars (Dziembowski & Cassisi 1999). This is in qualitative (but not quantitative!) agreement with the observations. The modulation period is not directly related to the rotation period of the stars, but is determined by the interplay between the frequency spacing and rotational splitting of the nonradial modes involved in the interaction. As such, it can be different for the overtone and for the fundamental-mode pulsators.
The outstanding difficulty of the resonant model is explaining the
existence of frequency doublets. In some cases, the observed doublet can
actually be a triplet, with one sidepeak too weak to be detected. For
several stars, however, such an explanation would require an extreme
amplitude asymmetry in order to be valid. For example, we find upper limits of
A+/A- < 0.08 for BWC V47 (RRc star) and
A-/A+ < 0.19 for BWC V51 (RRab star). It is not clear at present
if so strongly asymmetric triplets (or pure doublets) can be reproduced by
the resonant mode coupling theory.
Acknowledgements
This work has been supported in part by Polish KBN grants 2 P03D 002 20 and 5 P03D 012 20.