Solar Analogy Research Articles

Stellar flare oscillations: evidence for oscillatory reconnection and evolution of MHD modes

Here, we report on the detection of a range of quasi-periodic pulsations (20–120 s; QPPs) observed during flaring activity of several magnetically active dMe stars, namely AF Psc, CR Dra, GJ 3685A, Gl 65, SDSS J084425.9+513830, and SDSS J144738.47+035312.1 in the GALEX NUV filter. Based on a solar analogy, this work suggests that many of these flares may be triggered by external drivers creating a periodic reconnection in the flare current sheet or an impulsive energy release giving rise to an avalanche of periodic bursts that occur at time intervals that correspond to the detected periods, thus generating QPPs in their rising and peak phases. Some of these flares also show fast QPPs in their decay phase, indicating the presence of fast sausage mode oscillations either driven externally by periodic reconnection or intrinsically in the post-flare loop system during the flare energy release.

Angle differential‐QAM scheme for resolving phase ambiguity in continuous transmission system

AbstractAn angle differential quadrature amplitude modulation (ADQAM) scheme is proposed to solve phase ambiguity problem in non‐data‐aided continuous transmission system with square QAM constellation. Starting from the 16‐ADQAM case, we derive differential encoding and decoding schemes in terms of two differential angles and use a solar system analogy for explanation. The 16‐ADQAM system incurs only about 0.5‐dB performance degradation compared with the coherent 16‐QAM system under AWGN channel. Generalization of flat fading channel and higher‐level ADQAM is straightforward. Copyright © 2007 John Wiley & Sons, Ltd.

The Solar-Stellar Connection and Disconnection

The study of stellar activity is now an almost classical astronomical topic. The first Ca ii-H&K observations were made a hundred years ago by Eberhard & Schwarzschild1 and many thousand papers were published after its rediscovery some three decades ago by O. C. Wilson. The complexity of the atmospheric and interior magnetic activity as observed on the Sun is hard, if not impossible, to extrapolate to solar-type stars. So far there is no solar twin found, despite that it appears that just a single process acts as the driving mechanism for activity in all atmospheric layers and partially even in the convective envelope: the dynamodriven magnetic field. In this paper, I will try to give examples where the solar analogy holds and where it is clearly not appropriate, putting some emphases on differential surface rotation and meridional circulation. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action and directly measure surface magnetic fields.

Modeling solar and stellar flares

The thermal phase of solar X-ray flares has been described as heating-triggered evolution of plasma confined in coronal loops. This paper describes how the modeling of the thermal phase of spatially resolved solar X-ray flares has been extended to investigate spatially unresolved stellar X-ray flares with different scopes, aims and perspectives. Hydrodynamic models are able to describe the evolution of global flare features, such as the X-ray light curves, and to put constraints on heating location. Based on the solar analogy, either detailed hydrodynamic models or approximate analytical descriptions of the decay of flaring coronal loops or loop systems have been extensively applied to infer the size of stellar flaring regions. Approximate approaches tend to overestimate the size of the flaring regions, in particular in the presence of significant heating during the decay. Such heating is consistently diagnosed and included in a model based on extensive hydrodynamic modeling of decaying loops. Applications to recent data from Chandra and XMM-Newton missions, and present and future perspectives are discussed.

Asteroseismic tests for models of magnetically active close binaries with orbital period modulation

Magnetic activity and orbital period modulation in close binaries are considered in order to identify possible aster- oseismic tests for models that have been proposed to understand their relationship. These models are based on the variation of the gravitational quadrupole moment of the active star, as a consequence of the modification of its internal rotation and magnetic field along its activity cycle (Applegate 1992; Lanza et al. 1998a). An analysis based on the solar analogy shows that surface magnetic activity may produce shifts, splitting and broadening of the p-mode peaks in the oscillation spectrum of a star. Such eects make the direct detection of the internal structural changes predicted by the models very dicult. However, a time variation of the internal rotation can be detectable because the p-mode rotational splittings are proportional to the azimuthal quantum number m, whereas structural and surface eects controlled by the magnetic fields arejmj dependent. Therefore, we propose a method that correlates the variation of the a1 splitting coecients, that sample the internal rotation, with the orbital period variation in order to test the predictions of the models. The RS CVn short-period systems, such as V 471 Tau, RT And or CG Cyg, are the most suitable objects for such a test because surface magnetic eects are likely to be less prominent than in classical or long-period RS CVn binaries.

The Sun as an X‐Ray Star. IV. The Contribution of Different Regions of the Corona to Its X‐Ray Spectrum

We study X-ray-synthesized spectra of solar regions as templates to interpret analogous stellar spectra. We define three classes of coronal structures of different brightness, low (background quiet corona), medium (active regions), and high (active region cores), and determine their contribution to the solar X-ray emission measure versus temperature, EM(T), luminosity, and spectrum. This study defines the extent of the solar analogy quantitatively and accurately. To this end, we have selected a large sample of full-disk Yohkoh soft X-ray telescope observations taken between the maximum and the minimum of solar cycle 22, obtaining the contribution of each class to the whole Sun's EM(T). From the EM(T) distributions, we synthesize the X-ray spectra of the Sun and of the single classes of solar coronal regions as they would be collected with the ROSAT Position Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging Spectrometer. We find that the Sun during the cycle fits well in the stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio (HR) and surface X-ray flux, FPSPC, both increase going from the background corona to the active regions and the cores of the active regions, and range between the values of low and intermediate activity stars. We suggest that the coronae of these stars may be explained as the effect of structures similar to those present on the Sun and that the various levels of X-ray luminosity, HR, and FPSPC are achieved by changing the surface coverage of the different classes of coronal regions.

Rotation and X‐Ray Emission from Protostars

The ASCA satellite has recently detected variable hard X-ray emission from two class I protostars in the o Oph cloud, YLW 15 (IRS 43) and WL 6, with a characteristic timescale of D20 hr. In YLW 15, the X-ray emission is in the form of quasi-periodic energetic Nares, which we explain in terms of strong magnetic shearing and reconnection between the central star and the accretion disk. The Nare modeling, based on the solar analogy, gives us access to the size of the magnetic structures, which in turn allows us to calculate the rotation parameters of the star and the disk. In WL 6, X-ray Naring is rotationally modulated and appears to be more like the solar-type magnetic activity ubiquitous on T Tauri stars. On the basis of these observations, we Ðnd that YLW 15 is a fast rotator (near break-up), while WL 6 rotates with a signiÐcantly longer period. We thus use X-ray Naring as a clock II to measure the rota- tion of protostars. With the help of the mass-radius relation on the stellar birthline,II we derive masses of and for the central stars of YLW 15 and WL 6, respectively. YLW 15 thus M * D 2 M _ (0.4 M _ appears to be a future A star. In the long term, the magnetic interactions between the star and the disk result in magnetic braking and angular momentum loss of the star. A comparison of the rotation behav- ior of YLW 15 and WL 6 conÐrms that for solar-mass stars magnetic braking takes place on timescales few ) 105 yr, i.e., of the same order as the estimated duration of the class I protostar stage. The t br D a main parameter determining turns out to be the stellar mass, so that close to the birthline there must t br be a mass-rotation relation, such that stars with are fast rotators, while t br D P M * , M * Z 1E2 M _ their lower mass counterparts have had the time to spin down and reach synchronous rotation with the inner surrounding accretion disk. The rapid rotation and strong star-disk magnetic interactions of YLW 15 also naturally explain the observation of superNares II of X-ray luminosities as high as L X Z ergs s~1 during a few hours, while at the WL 6 stage the lower X-ray luminosities are likely 1033E1034 to be of purely stellar origin. The mass-rotation relation through magnetic braking may also explain why so few class I protostars have been detected in X-rays to date, and why they all lie in clusters. In the case of YLW 15, and perhaps also in other protostars, a hot coronal wind (T D 106 K) may be responsible for the VLA thermal radio emission. This paper thus proposes the Ðrst clues to the magnetic properties of protostars, which govern their rotation status and evolution. Subject headings : stars : Nare E stars : individual (YLW 15, WL 6) E stars : magnetic Ðelds E stars : preEmain-sequence E stars : rotation E X-rays : stars

Photometric Imaging with Distributive Solar Analogy

The multicolor light curves (UBVRI) of HU Vir have been analyzed to study starspots for epochs 1987 and 1989. The term 'photometric imaging' has been used for predicting cool surface spots by photometric data. Assuming the number of spots and the area are about the same for the two hemispheres, the distributive solar analogy is introduced as a tool to constrain free values of spot parameters into unique values. Therefore, final fitting parameters are claimed unique in a sense that they are independent of free initial guesses. The light and color variations of HU Vir are better explained without polar spots. Finally, the spots are found to be ∼ 1600 K cooler than the T∼ 5000 K of this cool subgiant and cover ∼ %13.9 of the total surface at ±8.0 degrees from the equator for the epoch 1987. The spots at the epoch 1989 are found to be ∼ 2040 K cooler covering ∼ % 12.7 area at ± 27°.7 latitudes. Problems of photometric imaging and possible solutions with distributive solar analogy have been discussed.

The Neupert Effect in Active Stellar Coronae: Chromospheric Evaporation and Coronal Heating in the dMe Flare Star Binary UV Ceti

Evidence for coronal heating by chromospheric evaporation in flares of active dMe stars is presented through observations of the Neupert effect in high-frequency microwaves and soft X-rays. The Neupert effect, as originally found in solar flares, manifests itself in a close similarity between the soft X-ray light curve and the time integral of the simultaneous microwave light curve. It is interpreted as the signature of the accumulation of hot plasma due to heating by accelerated electrons in the chromosphere.We used the ROSAT and ASCA soft X-ray observatories and the Very Large Array (VLA) radio telescope (at 6 cm and 3.6 cm wavelengths) to monitor simultaneously the nearby dMe flare star binary Gliese 65 A+ B = UV Ceti during 9 hours on each of two consecutive days. We find several weakly polarized radio events that start contemporaneously (within a few minutes) with X-ray flares and then peak and decay as the X-ray flares develop gradually. A striking similarity to the temporal evolution of solar gradual events is found. We argue that the Neupert effect is best observed in relatively hard bands of the soft X-ray emission, but that its presence can be inferred from the much softer bands commonly used for stellar observations by use of the solar analogy. Together with spectral hardness observations of soft X-rays, the data suggest the operation of chromospheric evaporation on UV Cet. The observations thus indicate a causal relation between the nonthermal and thermal energies of the underlying electron populations. We find that stellar flares are, relative to solar flares, X-ray-weak. The ratio between the total energy radiated into the radio and the soft X-ray bands closely matches the corresponding ratio between the quiescent luminosities of active stars, perhaps implying similar mechanisms and similar efficiencies for the quiescent emission and for larger, single flares. Estimating the total kinetic energy in the electrons from the radio flux, we find that only a part is observed in soft X-rays, a discrepancy well known from solar flares.

A ?magnetospheric? scenario of the solar flare in a quadrupole magnetic configuration

An attempt is made to impart a constructive character to the concept of the solar flaremagnetospheric substorm analogy. An idealized scheme for a two-ribbon solar flare in the originally closed magnetosphere of the active region is discussed. The basis is formed by a terrestrial substorm scenario with two active phases (Mishin et al., 1992). While a quadrupole magnetic configuration turns out to be a ‘solar’ analog of the Earth's magnetosphere. A physical mechanism that sustains the preflare ‘storage’ phase, is provided by an instability like a ‘stretching instability’ of the closed geomagnetotail. The ‘storage’ process is attributed to the emergence into the corona of closed magnetic flux lines in adjacent (to the location of the would-be flare) regions. The flare flash-phase is determined by the change-over of the ‘stretching instability’ to a disruption instability of a nonstationary (not neutral) current sheet inside the ‘storage’ zone. The final recovery phase corresponds to the wellknown Pneuman-Kopp model.

Stellar plages

The solar surface contains of bright regions (plages) and dark regions (sunspots) superposed on the photosphere. If the solar analogy is valid, then active late-type stars should also exhibit bright, spatially distinct plages. These plages can be detected by rotational modulation of chromospheric flux, or by Doppler imaging in chromospheric/transition region lines. I review the evidence for the existence of plages, with particular emphasis on two very active systems: AR Lacertae, for which we have sequences of Doppler imaging observations since 1984, and AB Doradus, which we observed extensively in 1994. I conclude that we need to be cautious when relying on the solar analogy to interpret observations of the most active late-type stars.

Multiple starspots and the solar analogy: A new approach and applications to HD 12545

view Abstract Citations (15) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multiple Starspots and the Solar Analogy: A New Approach and Applications to HD 12545 Eker, Zeki Abstract An improved method for the modeling of light curves of spotted stars is presented. This method still operates by trial and error but assumes numerous spots, unlike the other classical light-curve solutions of one-spot or standard two-spot models. The symmetry implied by the solar analogy, which describes the distribution of spots, helps reduce the number of free parameters. Automatic iterations converge to a unique solution of spot parameters (temperature, sizes, and locations) to produce theoretical light curves fitting the five-color photometry (UBVRI) of HD 12545. The spot temperature is treated as a free parameter simultaneously with the sizes and locations, unlike other models which usually predict it from V-I or V-R color curves prior to the trials. The spots are found to be 1280 K cooler than the Teff = 4820 K of this cool giant and to cover approximately 27% of the total surface at +/- 18 deg away from the equator. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175718 Bibcode: 1995ApJ...445..526E Keywords: Light Curve; Solar Activity; Spatial Distribution; Starspots; Stellar Models; Stellar Temperature; Color Photography; Iteration; Luminous Intensity; Stellar Color; Stellar Radiation; Astrophysics; STARS: ACTIVITY; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 12545 full text sources ADS | data products SIMBAD (5)

Observations of stellar coronae

We present an overview of recent stellar X-ray observations, with some discussion of the requirements for future observations. We argue that solar observations indicate that coronal X-ray emission is strongly related to surface magnetic field activity; we show that the interpretation of X-ray stellar coronal emission from late-type stars within the framework of models analogous to those developed for the solar corona is viable, and it is supported by many experimental results. The extension of this solar analogy to the early-type stars is quite questionable and remains an unsolved problem, while the working hypothesis of an X-ray phase, related to phenomena of magnetic field-related activity, as contrasted to a wind phase during the PMS evolutionary stage is suggested by the present status of observations.

X-ray activity

We review recent observational results related to phenomena of stellar X-ray activity, and extensively discuss the experimental data for the activity from late-type stars in the context of solar coronal modelling analogy. The evidence for a relation between X-ray activity and stellar surface rotational rate, stellar age and the onset of convection along the main sequence are extensively discussed, with some emphasis on the possible changes of X-ray activity manifestation with stellar evolution.

On Stellar X-Ray Emission

Stellar X-ray astronomy represents an entirely new astronomical discipline which has emerged during the past five years. It lies at the crossroads of solar physics, stellar physics, and general astrophysics. The present review is concerned with the main physical problems which arise in connection with a study of the stellar X-ray data. A central issue is the extent to which the extrapolation from solar physics is justified and the definition (if possible) of the limits to such extrapolation. The observational properties of X-ray emission from stars are considered along with the solar analogy and the modeling of X-ray emission from late-type stars, the modeling of X-ray emission from early-type stars, the physics of stellar X-ray emission, stellar X-ray emission in the more general astrophysical context, and future prospects.

Empirical Models of Stellar Flares: Constraints on Flare Theories

ABSTRACTA review is presented on empirical flare models of UV Cet type stars based upon optical,UV, X-ray, and radio observations. The observational constraints on the flare energetics, nature of radiation sources, and flare structures are discussed, with special attention to the geometrical dimension and the magnetic field of the flaring region. The hot-plasma model in the solar analogy is critically examined by comparing it with observations. Possible future observations are suggested to tighten the constraints on the stellar flare theories.