34 research outputs found
A BABCOCK-LEIGHTON SOLAR DYNAMO MODEL WITH MULTI-CELLULAR MERIDIONAL CIRCULATION IN ADVECTION- AND DIFFUSION-DOMINATED REGIMES
Babcock-Leighton type solar dynamo models with single-celled meridional
circulation are successful in reproducing many solar cycle features. Recent
observations and theoretical models of meridional circulation do not indicate a
single-celled flow pattern. We examine the role of complex multi-cellular
circulation patterns in a Babcock-Leighton solar dynamo in advection- and
diffusion-dominated regimes. We show from simulations that presence of a weak,
second, high-latitude reverse cell speeds up the cycle and slightly enhances
the poleward branch in butterfly diagram, whereas the presence of a second cell
in depth reverses the tilt of butterfly wing to an anti-solar type. A butterfly
diagram constructed from middle of convection zone yields a solar-like pattern,
but this may be difficult to realize in the Sun because of magnetic buoyancy
effects. Each of the above cases behaves similarly in higher and lower magnetic
diffusivity regimes. However, our dynamo with a meridional circulation
containing four cells in latitude behaves distinctly differently in the two
regimes, producing solar-like butterfly diagrams with fast cycles in the higher
diffusivity regime, and complex branches in butterfly diagrams in the lower
diffusivity regime. We also find that dynamo solutions for a four-celled
pattern, two in radius and two in latitude, prefer to quickly relax to
quadrupolar parity if the bottom flow-speed is strong enough, of similar order
of magnitude as the surface flow-speed.Comment: 40 pages, 19 figures, accepted in Ap
An extensive photometric study of the Blazhko RR Lyrae star RZ Lyr
The analysis of recent, extended multicolour CCD and archive photoelectric,
photographic and visual observations has revealed several important properties
of RZ Lyr, an RRab-type variable exhibiting large-amplitude Blazhko modulation.
On the time-base of \sim110 yr, a strict anticorrelation between the pulsation
and modulation period changes is established. The light curve of RZ Lyr shows a
remarkable bump on the descending branch in the small-amplitude phase of the
modulation, similarly to the light curves of bump Cepheids. We speculate that
the stellar structure temporally suits a 4:1 resonance between the periods of
the fundamental and one of the higher-order radial modes in this modulation
phase. The light-curve variation of RZ Lyr can be correctly fitted with a
two-modulation-component solution; the 121 d period of the main modulation is
nearly but not exactly four times longer than the period of the secondary
modulation component. Using the inverse photometric method, the variations in
the pulsation-averaged values of the physical parameters in different phases of
both modulation components are determined.Comment: 15 pages, 14 figures, 8 tables. Published in MNRAS, 2012. [v3]: Only
change: title correcte
Dynamics of the tachocline
The solar tachocline is an internal region of the Sun possessing strong radial and latitudinal shears straddling the base of the convective envelope. Based on helioseismic inversions, the tachocline is known to be thin (less than 5% of the solar radius). Since the first theory of the solar tachocline in 1992, this thinness has not ceased to puzzle solar physicists. In this review, we lay out the grounds of our understanding of this fascinating region of the solar interior. We detail the various physical mechanisms at stake in the solar tachocline, and put a particular focus on the mechanisms that have been proposed to explain its thinness. We also examine the full range of MHD processes including waves and instabilities that are likely to occur in the tachocline, as well as their possible connection with active region patterns observed at the surface. We reflect on the most recent findings for each of them, and highlight the physical understanding that is still missing and that would allow the research community to understand, in a generic sense, how the solar tachocline and stellar tachocline are formed, are sustained, and evolve on secular timescales
The Konkoly Blazhko Survey: is light-curve modulation a common property of RRab stars?
A systematic survey to establish the true incidence rate of Blazhko modulation among short-period, fundamental-mode, Galactic field RR Lyrae stars has been carried out. 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 per cent occurrence rate of the modulation is much larger than any previous estimate. The significant increase of the detected incidence rate is mostly a result of the discovery of small-amplitude modulation. Half of the Blazhko variables in our sample show the modulation with such a small amplitude that they would definitely have been missed in previous surveys. We have found that the modulation can be very unstable in some cases; for example, RY Com showed regular modulation during only one part of the observations, and had a stable light curve with abrupt, small changes in the pulsation amplitude during two observing seasons. This type of light-curve variability is hard to detect in the data from other surveys. The higher frequency of the light-curve modulation of RRab stars makes it even more important to find an explanation for the Blazhko phenomenon. The validity of the [Fe/H](P, ϕ31) relationship using the mean light curves of Blazhko variables is checked in our sample. We 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 for large-modulation-amplitude Blazhko stars are not extended enough (e.g. fewer than 500 data points from fewer than 15 nights), the formula may give false result owing to the distorted shape of the mean light curve use
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 framework 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 inverse photometric Baade-Wesselink method, 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 per cent of those that 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. The pulsation period of DM Cyg has been increasing by a rate of β= 0.091 d Myr−1 during the hundred-year time base of the observations. Konkoly archive photographic observations indicate that when the pulsation period of the variable was shorter by Δppuls= 5 × 10−6 d the modulation period was longer by Δpmod= 0.066 d than toda
Exploring spatial and temporal patterns in the Debrecen Solar Faculae Database: Part I
Photospheric faculae are markers of the solar magnetic field, appearing as bright regions along the edges of granules on the Sun's surface. Using data from the Debrecen Solar Faculae Database, we investigated the spatiotemporal distribution of photospheric faculae between 2010 May 1 and 2014 December 31 and found the following. (i) At lower latitudes, there is an enhanced abundance of faculae appearing as stripes at given Carrington longitudes, which are interpreted as indicative of the presence of active longitudes. (ii) At higher latitudes, we identified so-called crisscross patterns of facular appearance. These patterns are likely the result of faculae in regions situated along the boundaries of supergranules. Last but not least, (iii) various periods of oscillatory phenomena were identified in this facular data set, including a longer periodic range consistent with the quasi-biennial oscillations and shorter ones with periods of 4–12 days. Our findings are supported by the visualization of a simple heuristic thought experiment and more complex dynamo simulations, strengthening the proposed interpretation of the three observed solar phenomena reported
The light-curve modulation of XY And and UZ Vir - Two Blazhko RR Lyrae stars with additional frequencies
A thorough analysis of multicolour CCD observations of two modulated
RRab-type variables, XY And and UZ Vir is presented. These Blazhko stars show
relatively simple light-curve modulation with the usual multiplet structures in
their Fourier spectra. One additional, independent frequency with
linear-combination terms of the pulsation frequency is also detected in the
residual spectrum of each of the two stars. The amplitude and phase relations
of the triplet components are studied in detail. Most of the epoch-independent
phase differences show a slight, systematic colour dependence, however, these
trends have the opposite sign in the two stars. The mean values of the global
physical parameters and their changes with Blazhko phase are determined
utilizing the Inverse Photometric Method (IPM). The modulation properties and
the IPM results are compared for the two variables. The pulsation period of XY
And is the shortest when its pulsation amplitude is the highest, while UZ Vir
has the longest pulsation period at this phase of the modulation. Despite this
opposite behaviour, the phase relations of their mean-physical-parameter
variations are similar. These results are not in accord with the predictions of
the Blazhko model of Stothers (2006, ApJ, 652, 643).Comment: Accepted for publication in MNRAS. The paper contains 7 figures and
12 tables. Tables 5, 6 and A1-A5 will be published in full online only. See
electronic data on http://konkoly.hu/24/publications/ als
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
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