91 research outputs found
The Konkoly Blazhko Survey: Is light-curve modulation a common property of RRab stars?
A systematic survey to establish the true incidence rate of the Blazhko
modulation among short-period, fundamental-mode, Galactic field RR Lyrae stars
has been accomplished. The Konkoly Blazhko Survey (KBS) was initiated in 2004.
Since then more than 750 nights of observation have been devoted to this
project. A sample of 30 RRab stars was extensively observed, and light-curve
modulation was detected in 14 cases. The 47% occurrence rate of the modulation
is much larger than any previous estimate. The significant increase of the
detected incidence rate is mostly due to the discovery of small-amplitude
modulation. Half of the Blazhko variables in our sample show modulation with so
small amplitude that definitely have been missed in the previous surveys. We
have found that the modulation can be very unstable in some cases, e.g. RY Com
showed regular modulation only during one part of the observations while during
two seasons it had stable light curve with abrupt, small changes in the
pulsation amplitude. This type of light-curve variability is also hard to
detect in other Survey's data. The larger frequency of the light-curve
modulation of RRab stars makes it even more important to find the still lacking
explanation of the Blazhko phenomenon. The validity of the [Fe/H](P,phi_{31})
relation using the mean light curves of Blazhko variables is checked in our
sample. We have found that the formula gives accurate result for
small-modulation-amplitude Blazhko stars, and this is also the case for
large-modulation-amplitude stars if the light curve has complete phase
coverage. However, if the data of large-modulation-amplitude Blazhko stars are
not extended enough (e.g. < 500 data points from < 15 nights), the formula may
give false result due to the distorted shape of the mean light curve used.Comment: Accepted for publication in MNRAS, 14 pages, 7 Figure
An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: II. Changes in the physical parameters
The analysis of the multicolour photometric observations of MW Lyr, a large
modulation amplitude Blazhko variable, shows for the first time how the mean
global physical parameters vary during the Blazhko cycle. About 1-2 percent
changes in the mean radius, luminosity and surface effective temperature are
detected. The mean radius and temperature changes are in good accordance with
pulsation model results, which show that these parameters do indeed vary within
this order of magnitude if the amplitude of the pulsation changes
significantly. We interpret the phase modulation of the pulsation to be a
consequence of period changes. Its magnitude corresponds exactly what one
expects from the detected changes of the mean radius assuming that the
pulsation constant remains the same during the modulation. Our results indicate
that during the modulation the pulsation remains purely radial, and the
underlying mechanism is most probably a periodic perturbation of the stellar
luminosity with the modulation period.Comment: 10 figures, 2 tables. Accepted for publication in MNRA
An extensive photometric study of the Blazhko RR Lyrae star DM Cyg
DM Cyg, a fundamental mode RRab star was observed in the 2007 and 2008
seasons in the frame of the Konkoly Blazhko Survey. Very small amplitude light
curve modulation was detected with 10.57 d modulation period. The maximum
brightness and phase variations do not exceed 0.07 mag and 7 min, respectively.
In spite of the very small amplitude of the modulation, beside the frequency
triplets characterizing the Fourier spectrum of the light curve two quintuplet
components were also identified. The accuracy and the good phase coverage of
our observations made it possible to analyse the light curves at different
phases of the modulation separately. Utilizing the IP method (S\'odor, Jurcsik
and Szeidl, 2009) we could detect very small systematic changes in the global
mean physical parameters of DM Cyg during its Blazhko cycle. The detected
changes are similar to what we have already found for a large modulation
amplitude Blazhko variable MW Lyrae. The amplitudes of the detected changes in
the physical parameters of DM Cyg are only about 10% of that what have been
found in MW Lyr. This is in accordance with its small modulation amplitude
being about one tenth of the modulation amplitude of MW Lyr.Comment: 12 pages, 15 figures, 4 tables. Table 1 is available electronically
eg., from our website: http://www.konkoly.hu/24/publications/. Accepted for
publication in MNRA
RR Lyrae Variables in the Globular Cluster M55. The First Evidence for Non Radial Pulsations in RR Lyr Stars
We present the results of a photometric study of RR Lyrae variables in the
field of the globular cluster M55. We have discovered nine new RR Lyrae stars,
increasing the number of known variables in this cluster to 15 objects. Five of
the newly discovered variables belong to Bailey type RRc and two to type RRab.
Two background RRab stars are probable members of the Sagittarius dwarf galaxy.
Fourier decomposition of the light curves was used to derive basic properties
of the present sample of RR Lyrae variables. From an analysis of the RRc
variables we obtain a mean mass of , luminosity , effective temperature K, and helium
abundance . Based on the colors, periods and metallicities
of the RRab stars we estimate the value of the color excess for M55 to be equal
to . Using this value we derive the colors of the blue and
red edges of the instability strip in M55. The blue edge lies at
mag and the red edge lies at mag. We estimate the values of the
visual apparent and dereddened distance moduli to be and
, respectively. The light curves of three of the RRc variables
exhibit changes in amplitude of over 0.1 mag on the time scale of less than a
week, rather short for the Blazhko effect, but with no evidence for another
radial pulsational frequency. However we do detect other periodicities which
are clearly visible in the light curve after removing variations with the first
overtone radial frequency. This is strong evidence for the presence of
non-radial pulsations, a behavior common for Scuti stars but not yet
observed among RR Lyr variables.Comment: submitted to Astronomical Journal, 33 pages with 11 figure
New RR Lyrae variables in binary systems
Despite their importance, very few RR Lyrae (RRL) stars have been known to
reside in binary systems. We report on a search for binary RRL in the OGLE-III
Galactic bulge data. Our approach consists in the search for evidence of the
light-travel time effect in so-called observed minus calculated ()
diagrams. Analysis of 1952 well-observed fundamental-mode RRL in the OGLE-III
data revealed an initial sample of 29 candidates. We used the recently released
OGLE-IV data to extend the baselines up to 17 years, leading to a final sample
of 12 firm binary candidates. We provide diagrams and binary parameters
for this final sample, and also discuss the properties of 8 additional
candidate binaries whose parameters cannot be firmly determined at present. We
also estimate that per cent of the RRL reside in binary systems.Comment: MNRAS Letters, in pres
Blazhko modulation in the infrared
We present first direct evidence of modulation in the K band of Blazhko-type RR Lyrae stars that are identified by their secular modulations in the I-band data of Optical Gravitational Lensing Experiment-IV. A method has been developed to decompose the K-band light variation into two parts originating from the temperature and the radius changes using synthetic data of atmosphere-model grids. The amplitudes of the temperature and the radius variations derived from the method for non-Blazhko RRab stars are in very good agreement with the results of the Baade-Wesselink analysis of RRab stars in the M3 globular cluster confirming the applicability and correctness of the method. It has been found that the Blazhko modulation is primarily driven by the change in the temperature variation. The radius variation plays a marginal part, moreover it has an opposite sign as if the Blazhko effect was caused by the radii variations. This result reinforces the previous finding based on the Baade-Wesselink analysis of M3 (NGC 5272) RR Lyrae, that significant modulation of the radius variations can only be detected in radial-velocity measurements, which relies on spectral lines that form in the uppermost atmospheric layers. Our result gives the first insight into the energetics and dynamics of the Blazhko phenomenon, hence it puts strong constraints on its possible physical explanations
Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window
We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color
diagrams for Baade's Window and local RRab Lyrae stars. We find that for a
fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in
(V-I)_0 than the local RR Lyrae stars. We also show that there is no such
effect observed in (V-K)_0. We argue that an extinction misestimate towards
Baade's Window is not a plausible explanation of the discrepancy. Unlike
Baade's Window RR Lyrae stars, the local ones follow a black-body color-color
relation and are well approximated by theoretical models. We test two
parameters, metallicity and surface gravity, and find that their effects are
too small to explain the (V-I)_0 discrepancy between the two groups of stars.
We do not provide any explanation for the anomalous (V-I)_0 behavior of the
Baade's Window RR Lyrae stars. We note that a similar effect for clump giant
stars has been recently reported by Paczynski and we caution that RR Lyrae
stars and clump giants, often used as standard candles, can be subject to the
same type of systematics.Comment: 10 pages, 7 figures, submitted to Ap
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
An extensive photometric study of the Blazhko RR Lyrae star MW Lyr: I. Light curve solution
We have obtained the most extensive and most accurate photometric data of a
Blazhko variable MW Lyr during the 2006-2007 observing seasons. The data within
each 0.05 phase bin of the modulation period (P_m=1/f_m) cover the entire light
cycle of the primary pulsation period (P_0=1/f_0), making possible a very
rigorous and complete analysis. The modulation period is found to be 16.5462 d,
which is about half of that was reported earlier from visual observations.
Previously unknown features of the modulation have been detected. Besides the
main modulation frequency f_m, sidelobe modulation frequencies around the
pulsation frequency and its harmonics appear at +/- 2 f_m, +/- 4 f_m, and +/-
12.5 f_m separations as well. Residual signals in the prewhitened light curve
larger than the observational noise appear at the minimum-rising branch-maximum
phase of the pulsation, which most probably arise from some stochastic/chaotic
behaviour of the pulsation/modulation. The Fourier parameters of the mean light
curve differ significantly from the averages of the Fourier parameters of the
observed light curves in the different phases of the Blazhko cycle.
Consequently, the mean light curve of MW Lyrae never matches its actual light
variation. The Phi_21, Phi_31 phase differences in different phases of the
modulation show unexpected stability during the Blazhko cycle. A new
phenomenological description of the light curve variation is defined that
separates the amplitude and phase (period) modulations utilising the phase
coherency of the lower order Fourier phases.Comment: Accepted for publication in MNRAS. 15 pages, 14 figures, and 7
printed tables (2 of them available in electronic form
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