Activity Cycle Period Research Articles

Long-term starspot evolution, activity cycle, and orbital period variation of V711 Tauri (HR 1099)

The long-term optical photometry of V711 Tau (HR 1099) from 1975 to 2001 is analysed with different techniques to provide information on the activity of this widely studied RS CVn-type binary. Pooled variance analysis gives a characteristic timescale of evolution for the photospheric pattern of starspots of the order of 100–200 days, while periodogram analysis of seasonal photometric data reveals that the rotational modulation period is d, without significant changes during the 26 analysed years. Considering subsets of the entire data sequence along which the rotational modulation of the optical flux stays stable, a set of 66 light curves is obtained. They are modelled by means of inversion techniques based on Maximum Entropy and Tikhonov regularizations to derive the distributions in longitude and the total amount of the spotted area on the more luminous K1 IV component that dominates the optical variability of the system. An extensive comparison with published Doppler Imaging maps shows the advantages and drawbacks of inversion techniques based solely on optical photometry. The large-scale structure of the longitudinal distribution of the spotted area is correctly reproduced, although individual starspots revealed by Doppler Imaging cannot be detected because of the limited spatial resolution of the photometric imaging. On the other hand, the variation of the total spotted area is best reconstructed from optical photometry and shows an activity cycle with a period of yr, upon which a short-term modulation might be superposed having variable amplitude and phase and a mean duration of approximately 3–5 years. The distributions of the spotted area versus longitude show the presence of one dominant preferential longitude from 1975 to 1989; afterwards, two preferential longitudes with comparable spotted areas are detected. Their migration rates with respect to the orbital reference frame are variable versus time, possibly as a consequence of a variable drift rate of the respective non-axisymmetric dynamo modes. The variation of the orbital period of the system appears to be correlated with the magnetic activity of the K1 IV component. Specifically, the decrease of the orbital period that occurred around 1990 coincides with a re-configuration of the stellar magnetic field, as indicated by the simultaneous variation of its different proxies. In combination with previous studies, these results allow us to discriminate among different theoretical models proposed to explain the connection between magnetic activity and orbital dynamics.

Systematically Asymmetric Heliospheric Magnetic Field: Evidence for a Quadrupole Mode and Non-Axisymmetry with Polarity Flip-Flops

Recent studies of the heliospheric magnetic field (HMF) have detected interesting, systematic hemispherical and longitudinal asymmetries which have a profound significance for the understanding of solar magnetic fields. The in situ HMF measurements since the 1960s show that the heliospheric current sheet (HCS) is systematically shifted (coned) southward during solar minimum times, leading to the concept of a bashful ballerina. While temporary shifts can be considerably larger, the average HCS shift (coning) angle is a few degrees, less than the 7.2∘ tilt of the solar rotation axis. Recent solar observations during the last two solar cycles verify these results and show that the magnetic areas in the northern solar hemisphere are larger and their intensity weaker than in the south during long intervals in the late declining to minimum phase. The multipole expansion reveals a strong quadrupole term which is oppositely directed to the dipole term. These results imply that the Sun has a symmetric quadrupole S0 dynamo mode that oscillates in phase with the dominant dipole A0 mode. Moreover, the heliospheric magnetic field has a strong tendency to produce solar tilts that are roughly opposite in longitudinal phase. This implies is a systematic longitudinal asymmetry and leads to a “flip-flop” type behaviour in the dominant HMF sector whose period is about 3.2 years. This agrees very well with the similar flip-flop period found recently in sunspots, as well as with the observed ratio of three between the activity cycle period and the flip-flop period of sun-like stars. Accordingly, these results require that the solar dynamo includes three modes, A0, S0 and a non-axisymmetric mode. Obviously, these results have a great impact on solar modelling.

Quantification of cardiac sac network effects on a movement-related parameter of pyloric network output in the lobster.

Cardiac sac network activity (cycle period tens of seconds to minutes) has long been known to alter pyloric network activity (cycle period approximately 1 s), but these effects have not been quantified. Some pyloric muscles extract cardiac sac timed variations in pyloric motor neuron firing, and consequently produce cardiac sac timed movements even though no cardiac sac neurons innervate them. Determining pyloric behavior therefore requires detailed description of cardiac sac effects on pyloric neural output. Pyloric muscle activity correlates well with motor neuron overall spike frequency (OSF, number of spikes per burst divided by cycle period). We therefore quantified the effects of cardiac sac activity on the OSF of all pyloric neurons in the lobster, Panulirus interruptus. The ventricular dilator (VD) neuron had a biphasic response, with its OSF first increasing and then decreasing during cardiac sac bursts. Lateral pyloric (LP) neuron OSF decreased during cardiac sac activity. The pyloric (PY) neurons had two responses, with OSF either decreasing or increasing just after the beginning of cardiac sac activity. The pyloric dilator (PD) neurons had a triphasic response, with OSF increasing slightly at the beginning of cardiac sac activity, decreasing during the cardiac sac burst, and strongly increasing after cardiac sac activity ended. The inferior cardiac (IC) neuron had a biphasic response, with OSF decreasing at the beginning of cardiac sac activity and strongly increasing when cardiac sac activity ceased. These data provide the quantitative description of cardiac sac effects on pyloric activity necessary to predict pyloric movement from pyloric neural output.

Quantification of gastric mill network effects on a movement related parameter of pyloric network output in the lobster.

It has long been known that gastric mill network activity (cycle period 5-10 s) alters pyloric network output (cycle period approximately 1 s), but these effects have not been quantified. Many pyloric muscles extract gastric mill timed variations in pyloric motor neuron firing, and consequently produce gastric mill timed movements even though no gastric mill neurons innervate them. Determining pyloric behavior therefore requires detailed description of gastric mill effects on pyloric neural output. Pyloric muscle activity correlates well with motor neuron overall spike frequency (OSF, burst spike number divided by cycle period). We quantified OSF variation of all pyloric neurons as a function of time into the gastric mill cycle [as measured from the beginning of Gastric Mill (GM) neuron bursts] in the lobster, Panulirus interruptus. No repeating pattern within individual gastric mill cycles of Lateral Pyloric (LP) and Ventricular Dilator (VD) neuron OSF was visually apparent. Averaged data showed that VD and LP neuron OSF decreased (approximately 0.5 and 1.5 Hz, respectively) at the beginning of each gastric mill cycle. Visually apparent patterns of OSF waxing and waning within each gastric mill cycle were present for the Inferior Cardiac (IC), Pyloric Dilator (PD), and Pyloric (PY) neurons. However, when averaged as a function of phase or delay in the gastric mill cycle, the average changes were smaller than those in individual gastric mill cycles because when the OSF variations occurred varied considerably in different gastric mill cycles. We therefore used a "pattern-based" analysis in which an identifying characteristic of each neuron's repeating OSF variation pattern was defined as pattern pyloric cycle zero. The pyloric cycles in each repetition of the OSF variation pattern were numbered relative to the zero cycle, and averaged to create an average OSF variation profile. The zero cycle delays relative to GM neuron burst beginning were then averaged to determine when in the gastric mill cycle the profile occurred. This technique preserved the full extent of pyloric neuron OSF changes. Maximum PY neuron OSF occurred within the GM neuron burst, whereas maximum IC and PD neuron OSF occurred during the GM neuron interburst interval. Despite these changes, pyloric cycling did not phase lock with gastric mill activity, nor were an integer number of pyloric cycles present in each gastric mill cycle. In addition to providing data necessary to predict pyloric movement, this work shows how pattern-based analysis can successfully quantify interactions between nonphase-locked networks.

Long-term starspot evolution, activity cycle and orbital period variation of RT Lacertae

A sequence of V -band light curves of the active close binary RT Lacertae (G5+G9 IV), extending from 1965 to 2000, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach, the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: LG5=LG9 IV =0 :65 0:05. The more massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light curve distortions. The yearly spot distributions on both components indicate that their spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. In particular, spots are concentrated around the substellar points and their antipodes on both stars. The eclipse scanning reveals spots with diameters of40, or possibly smaller, on the hemisphere of the primary star being occulted. The primary shows clear evidence for a short-term activity cycle with a period of8: 5y r and a possible long-term cycle with a period of approximately 35 yr. The variation of the spot migration rate may be related with surface dierential rotation, with a lower limit of = 3:2 10 3 . The G9 IV secondary does not show evidence for an activity cycle, its spot coverage appearing rather constant at15 20% of its surface. The relative amplitude of its surface dierential rotation, as indicated by the variation of the spot migration rate, is = 2:7 10 3 . The variation of the orbital period shows a correlation with the activity level of the primary component. Specically, the decreases of the orbital period appear to be associated with minimum spottedness and sizeable changes of the surface spot distribution that may be related to increases of the rotation rate of the spot pattern. Conversely, an episode of increase of the orbital period was related to an increase of the spotted area on the primary star. Such results support the recently proposed models that connect the perturbations of the orbital dynamics with the variation of the gure of equilibrium of the active components, due to the operation of non-linear hydromagnetic dynamos in their extended convective envelopes.

Long-term starspot evolution, activity cycle and orbital period variation of SZ Piscium

A sequence of V -band light curves of the active close binary SZ Piscium (F8 V-IV+ K1 IV), extending from 1957 to 1998, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: L1=L2 =0 :35 0:035. The derived yearly spot distributions indicate that the spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. Starspots at latitudes higher than75 were not required to reproduce the photometric modulation. For the spot pattern on the less luminous F8 V primary component there are only some hints of the possible presence of spots around the substellar point facing the K1 IV secondary component. On the more luminous and larger secondary component there is clear evidence for the presence of three active longitudes. A stable active longitude around the substellar point facing the primary star appears to be quite compact with an extension of40 on several maps. The other two active longitudes display irregular changes of the spot area and location. The variation of the total spotted area on the secondary component shows sizeable fluctuations and a possible short-term cycle of about 13 yr, which is more clearly apparent in the variation of the spotted area unevenly distributed in longitude. A longer-term cycle of about 30 yr with a markedly non-sinusoidal variation might also be present. The spatial association between photospheric spots and chromospheric plages (as detected in the UV lines) appears signicant for the active region around the substellar point in the K1 IV component. A possible connection between magnetic activity and orbital period variation is suggested by comparing the variation of the total spotted area with the O C diagram of the primary eclipse times. The time span covered by the available data is, however, not long enough for us to draw any denite conclusions.

Stellar Active Region Evolution - II. Identification and Evolution of Variance Morphologies in CA II H+K Time Series

The relative distribution of pooled variance computed at various time scales for records of chromospheric activity has been calculated for approximately 100 stars observed at Mount Wilson Observatory. As shown in Paper I, analysis of the pooled variance provides a technique for estimating the lifetimes of stellar active regions and their influence on chromospheric time series used for determining rotation and activity cycle periods. Pooled variance diagrams may be divided into three morphological types which depend to a large extent on a star's mean level of chromospheric activity (i.e., age) and B-V color (i.e., mass), and possibly depend on star's evolutionary state.

The relation between stellar rotation rate and activity cycle periods

The empirical relation between rotation period, spectral type, and cycle activity period in 13 slowly rotating main-sequence stars is investigated, on the basis of available spectrometric data. It is shown that for slowly rotating stars with similar spectral types, the cycle period P(cyc), and rotation period P(rot) were related such that P(cyc) varies as P(rot) to the nth, where n equals 1.25. In a group of stars with individual spectral types from G2 to K7, the cycle periods were consistent with the relation P(cyc) is approximately equal to (P rot/tau<SUB>c)</SUB> exp n, where tau<SUB>c</SUB> is the convective turnover time near the bottom of the convection zone. On the basis of the above relations, it is suggested that an increase of P(cyc) with increasing P(rot) does not agree with conventional estimates from nonlinear dynamo models, and is limited by two factors: the quenching of the alpha effect; and differential rotation.

Activity cycle periods in late-type stars

Period of magnetic activity versus the stellar angular velocity for stars of given spectral type having extended convective shells is estimated within the framework of mean field dynamo theory. The dependence appears to be not monotonous, and can be checked by observations.

Experiments on the central pattern generator for swimming in amphibian embryos

The central nervous system of paralysed Xenopus laevis embryos can generate a motor output pattern suitable for swimming locomotion. By recording motor root activity in paralysed embryos with transected nervous systems we have shown that: (a) the spinal cord is capable of swimming pattern generation; (b) swimming pattern generator capability in the hindbrain and spinal cord is distributed; (c) caudal hindbrain is necessary for sustained swimming output after discrete stimulation. By recording similarly from embryos whose central nervous system was divided longitudinally into left and right sides, we have shown that: (a) each side can generate rhythmic motor output with cycle periods like those in swimming; (b) during this activity cycle period increases within an episode, and there is the usual rostrocaudal delay found in swimming; (c) this activity is influenced by sensory stimuli in the same way as swimming activity; (d) normal phase coupling of the left and right sides can be established by the ventral commissure in the spinal cord. We conclude that interactions between the antagonistic (left and right) motor systems are not necessary for swimming rhythm generation and present a model for swimming pattern generation where autonomous rhythm generators on each side of the nervous system drive the motoneurons. Alternation is achieved by reciprocal inhibition, and activity is initiated and maintained by tonic excitation from the hindbrain.