Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.     

Einstein observations of cool stars

Observations of cool stars with the $\text{Einstein}$ Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant $\text{Einstein}$ results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.     

Evolution of activity signatures during the main sequence phase

We review recent work on the decay of magnetic activity signatures, such as chromospheric/transition region/coronal emission as well as mean flare emission, with age for solar and later type stars. Both implicit and explicit evolutionary representations are discussed. In terms of magnetic flux, as measured by excess chromospheric CaII luminosity, we show that a simple dynamo-rotation relation that incorporates both a saturated state with its characteristic critical rotation (or scale factor) as well as an asymptotic linear power law, i.e. a scale free relation, fits the extant data that includes the dMe stars. Introducing the saturated dynamo state, as exemplified by the dMe stars, into activity power-power diagrams, allows us to not only specify the saturated state, but to define evolutionary tracks that represent the decay from the saturated state. Using the quiescent coronal X-ray power (luminosity) as a basic measure of magnetic activity, we find simple monomial relations for both the saturated state (linear) and for the evolutionary tracks governing both quiescent activity and mean flare activity. In particular, the coronal power loss is found to vary quadratically with the chromospheric power loss, hence with magnetic flux.     

Radio wavelength observations of magnetic fields on active dwarf M,RS CVn and magnetic stars

The dwarf M stars YZ Canis Minoris and AD Leonis exhibit narrow-band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars AD Leonis and Wolf 424 emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process. We attribute them to coherent mechanisms such as an electron-cyclotron maser or coherent plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic of the gyrofrequency, the coronal magnetic field strength H = 250 G or 167 G and constraints on the plasma frequency imply an electron density of N_e = 6 × 10⁹cm⁻³. Coherent plasma radiation requires similar values of electron density but much weaker magnetic fields. Radio spikes from AD Leonis and Wolf 424 have rise times τ_R ⩽ 5 ms, indicating a linear size of L ⩽ 1.5 × 10⁸cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour. The shortest variation implies a linear size much less than that of the halo observed by VLBI techniques, and most probably sizes smaller than those of the component stars. The observed variations might be due to absorption by a thermal plasma located between the stars.     

Magnetic flux expulsion as an acceleration mechanism for stellar winds

Observations of the Sun show that magnetic flux is emerging through the surface in small scales in rather copious amounts. In order to maintain a steady state field strength, this flux must either be locally dissipated or explelled or both. We believe that magnetic reconnection and subsequent flux explusion is the most effective manner in which to achieve this. If new flux emerges into an already preexisting coronal magnetic field, the ambient field must be pushed aside to allow room for the new flux. If the ambient field strength decreases outward with radial distance as is expected for all stars, it may pinch off the emerging flux through magnetic reconnection and expell it outward. The net force on an isolated diamagnetic plasmoid produced by this process is shown to assume a particularly simple form, depending only on the plasmoid's mass, its temperature, and the radial gradient of the logarithm of the undisturbed magnetic pressure. If a sufficient number of these magnetic elements are produced per unit time, this process translates to a net outward magnetic force on the coronal plasma which can be greater that the gas pressure force. Thus, a stellar wind can be produced by magnetic forces alone without the need for a high coronal gas pressure — a mechanism which could be effective in explaining why stars, such as the late-type giants, which possess cool coronae nevertheless exhibit vigorous coronal expansions.     

A uvbyβ survey of northern-hemisphere active binaries

A recent detailed calibration of the uvby photometric system for late-type stars has opened the possibility to study photometric effects of stellar activity in the RS CVn grouup of binaries. In this communication, we present preliminary results for uvby Stromgren and Hβ photometry measurements of binary stars showing enhanced chromospheric activity and currently included among the RS CVn-like stars. It is shown that luminosity classes can be separated in terms of the uvby colours and that there is a general tendency for active stars to have smaller m₁ values than expected from standard relations. This δm₁ effect can be related to the degree of activity rather than to the actual metal content of their atmospheres.     

Late stages of stellar evolution – Herschel’s contributions

Cool objects glow in the infrared. The gas and solid-state species that escape the stellar gravitational attraction of evolved late-type stars in the form of a stellar wind are cool, with temperatures typically ?1500 K, and can be ideally studied in the infrared. These stellar winds create huge extended circumstellar envelopes with extents approaching 10¹⁹${10}^{19}$ cm. In these envelopes, a complex kinematical, thermodynamical and chemical interplay determines the global and local structural parameters. Unraveling the wind acceleration mechanisms and deriving the complicated structure of the envelopes is important to understand the late stages of evolution of ∼97% of stars in galaxies as our own Milky Way. That way, we can also assess the significant chemical enrichment of the interstellar medium by the mass loss of these evolved stars. The Herschel Space Observatory is uniquely placed to study evolved stars thanks to the excellent capabilities of the three infrared and sub-millimeter instruments on board: PACS, SPIRE and HIFI. In this review, I give an overview of a few important results obtained during the first two years of Herschel observations in the field of evolved low and intermediate mass stars, and I will show how the Herschel observations can solve some historical questions on these late stages of stellar evolution, but also add some new ones.     

Probing solar and stellar interior dynamics and dynamo

Solar and stellar activity is a result of complex interaction between magnetic field, turbulent convection and differential rotation in a star’s interior. Magnetic field is believed to be generated by a dynamo process in the convection zone. It emerges on the surface forming sunspots and starspots. Localization of the magnetic spots and their evolution with the activity cycle is determined by large-scale interior flows. Thus, the internal dynamics of the Sun and other stars hold the key to understanding the dynamo mechanism and activity cycles. Recently, significant progress has been made for modeling magnetohydrodynamics of the stellar interiors and probing the internal rotation and large-scale dynamics of the Sun by helioseismology. Also, asteroseismology is beginning to probe interiors of distant stars. I review key achievements and challenges in our quest to understand the basic mechanisms of solar and stellar activity.     

Formation of relativistic jets by collapsing stars to black holes

Formation of relativistic jets in the magnetosphere of collapsing stars is considered. These jets will be formed in the polar caps of magnetosphere of collapsing star, where the stellar magnetic field increases during the collapse and the charged particles are accelerated. The jets will generate non-thermal radiation. The analysis of dynamics and emission of particles in the stellar magnetosphere under collapse shows that collapsing stars can be powerful sources of relativistic jets.     

Thermal emission from isolated neutron stars and their surface magnetic field: Going quadrupolar?

In the last few years considerable observational resources have been devoted to study the thermal emission from isolated neutron stars. Detailed XMM and Chandra observations revealed a number of features in the X-ray pulse profile, like asymmetry, energy dependence, and possible evolution of the pulse profile over a time scale of months or years. Here we show that these characteristics may be explained by a patchy surface temperature distribution, which is expected if the magnetic field has a complex structure in which higher order multipoles contribute together with the dipole. We reconsider these effects from a theoretical point of view, and discuss their implications to the observational properties of thermally emitting neutron stars.     

Synoptic magnetic field in cycle 23 and in the beginning of the cycle 24

The SOHO/MDI data provide the uniform time series of the synoptic magnetic maps which cover the period of the cycle 23 and the beginning of the cycle 24. It is very interesting period because of the long and deep solar minimum between the cycles 23 and 24. Synoptic structure of the solar magnetic field shows variability during solar cycles. It is known that the magnetic activity contributes to the solar irradiance. The axisymmetrical distribution of the magnetic flux (Fig. 3c) is closely associated with the ‘butterfly’ diagram in the EUV emission (Benevolenskaya et al., 2001). And, also, the magnetic field (B_∥) shows the non-uniform distributions of the solar activity with longitude, so-called ‘active zones’, and ‘coronal holes’ in the mid-latitude. Polar coronal holes are forming after the solar maxima and they persist during the solar minima. SOHO/EIT data in the emission of Fe XII (195 Å) could be a proxy for the coronal holes tracking. The active longitudinal zones or active longitude exist due to the reappearance of the activity and it is clearly seen in the synoptic structure of the solar cycle. On the descending branch of the solar cycle 23 active zones are less pronounced comparing with previous cycles 20, 21 and 22. Moreover, the weak polar magnetic field precedes the long and deep solar minimum. In this paper we have discussed the development of solar cycles 23 and 24 in details.     

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4–6 M₀) stars and the evolution of their structure are presented. A distance-limited (d ≤ 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only α TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ∼10⁶K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.     

Magnetic activity in evolved stars

A rotational boundary, a coronal boundary, and a possible granulation boundary are discussed. Evidence for the magnetic nature of some of the observed activity in evolved stars is convincing for subgiants and giants, but much less so for luminosity classes II and Ib.     

Upper limits to ultraviolet line emission in fully convective M-dwarfs

Theoretical stellar structure models find that stars later than ∼dM3 are fully convective. It is widely believed that this should result in reduced non-radiative heating and activity levels, however, the observational evidence is sparse and somewhat contradictory. In order to expand the number of atmospheric diagnostics for these stars, we have obtained deep short wavelength (SWP) IUE spectra of 5 M dwarfs later than spectral type dM5. Exposure times ranged from 4 to 7 hours. Only upper limits were measured for chromospheric and transition region lines, thus appearing to rule out enhanced line emission activity. The fractional luminosities of C IV (λ1550), C II (λ1335), and C I (λ1657) relative to L_bol indicate that emission lines in the temperature range 6000 K to ∼1 × 10⁵K are up to an order of magnitude weaker than in earlier M dwarfs having radiative cores. The lower limits on line fluxes for Barnard's star are significantly below those of the other stars in the program.     

Emission heights of coronal bright points on Fe XII radiance map

The study of coronal bright points (BPs) is important for understanding coronal heating and the origin of the solar wind. Previous studies indicated that coronal BPs have a highly significant tendency to coincide with magnetic neutral lines in the photosphere. Here we further studied the emission heights of the BPs above the photosphere in the bipolar magnetic loops that are apparently associated with them. As BPs are seen in projection against the disk their true emission heights are unknown. The correlation of the BP locations on the Fe XII radiance map from EIT with the magnetic field features (in particular neutral lines) was investigated in detail. The coronal magnetic field was determined by an extrapolation of the photospheric field (derived from 2-D magnetograms obtained from the Kitt Peak observatory) to different altitudes above the disk. It was found that most BPs sit on or near a photospheric neutral line, but that the emission occurs at a height of about 5 Mm. Some BPs, while being seen in projection, still seem to coincide with neutral lines, although their emission takes place at heights of more than 10 Mm. Such coincidences almost disappear for emissions above 20 Mm. We also projected the upper segments of the 3-D magnetic field lines above different heights, respectively, on to the tangent x–y plane, where x is in the east–west and y in the south–north direction. The shape of each BP was compared with the respective field-line segment nearby. This comparison suggests that most coronal BPs are actually located on the top of their associated magnetic loops. Finally, we calculated for each selected BP region the correlation coefficient between the Fe XII intensity enhancement and the horizontal component of the extrapolated magnetic field vector at the same x–y position in planes of different heights, respectively. We found that for almost all the BP regions we studied the correlation coefficient, with increasing height, increases to a maximal value and then decreases again. The height corresponding to this maximum was defined as the correlation height, which for most bright points was found to range below 20 Mm.     

Coronal activity in F-, G-, and K-type stars; relations between parameters characterizing stellar structures and X-ray emission

A sample of 52 stars containing dwarfs and giants is subjected to a multidimensional factor analysis. The parameters used are the soft X-ray flux at the stellar surface F_x, the Ca II H+K line-core flux F_H+K, the stellar radius and mass. We find a high correlation between F_x and the Ca II H+K excess flux ΔF_H+K obtained by subtracting an observational lower-limit flux from F_H+K. We conclude that the lower-limit Ca II flux is uncorrelated with the stellar X-ray emission. The common-factor analysis shows that, for the present sample, F_x depends only on ΔF_H+K, and not on the stellar radius or mass. All stars included in our analysis follow the relation F_x ∝ Δ^1.4_H+K over almost four decades in F_x.     

V.L.A. observations of flare build-up in coronal loops

Very Large Array (V.L.A.) measurements at 20 cm wavelength map emission from coronal loops with second-of-arc angular resolution at time intervals as short as 3.3 seconds. The total intensity of the 20 cm emission describes the evolution and structure of the hot plasma that is detected by satellite X-ray observations of coronal loops. The circular polarization of the 20 cm emission describes the evolution, strength and structure of the coronal magnetic field. Preburst heating and magnetic changes that precede burst emission on time scales of between 1 and 30 minutes are discussed. Simultaneous 20 cm and soft X-ray observations indicate an electron temperature $\text{T}{}_{\mathrm{e}}\textcolor{red}{}\text{2.5 × 10}{}^7\text{K}$ and electron density $\text{N}{}_{\mathrm{e}}\textcolor{red}{}\text{10}{}^{10}\text{cm}{}^{\mathrm{?3}}$ during preburst heating in a coronal loop that was also associated with twisting of the entire loop in space. We also discuss the successive triggering of bursts from adjacent coronal loops; highly polarized emission from the legs of loops with large intensity changes over a 32 MHz change in observing frequency; and apparent motions of hot plasma within coronal loops at velocities V > 2,000 kilometerspersecond.     

IUE observations of cool stars

A summary of IUE results concerning late-type stars is presented. Observations show that high-temperature outer atmospheres, as indicated by N V, C IV emission at T ≈ 10⁵K, are generally present only in high-gravity (log g ? 2) stars. Objects with high-temperature emission tend not to exhibit cool circumstellar shells, and vice versa, although there are several transition objects, the hybrid atmosphere stars, which combine C IV emission with cool winds. Ultraviolet emission from stellar transition regions correlates well with chromospheric and X-ray emission. Transition-region line ratios indicate that many stars have differential emission measure distributions similar to the Sun's. Ultraviolet observations also give indications of important dynamical effects in low-gravity stars. Density diagnostics indicate extended chromospheres for some red giants and supergiants. In addition, the large widths of lines of high temperature ions in several luminous stars indicate supersonic motions.     

On the nature of coronal loops above the quiet sun network

The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network.     

Spectral and temporal studies of various late-type stars

The RS CVn stars Capella and σ² CrB have been measured with EXOSAT in soft and medium X-rays for about 24 hours each and the less active late-type star Procyon for about 6.5 hours. In addition, the RS CVn star γ. And was twice observed about one month apart for a total of about 7 hours, with the ME and the LE in the photometer mode only. All three RS CVn stars were detected with the ME-detector. The star σ² CrB showed a flare both in LE and ME with a rise time of about twelve minutes and a decay time of three hours. The active late-type stars σ² CrB and Capella show in the spectral region between 90 and 140 A lines from Fe XVIII to Fe XXIII, which can be resolved with the moderate resolution (Δγ ≈ 5 A) of the spectrometer. These lines are indicative of the presence of hot (≈ 10 MK) plasma like that in a Solar flare. In contrast, the spectrum of the cooler corona of the star Procyon does not show the hot Fe XXII and Fe XXIII lines but instead a blend at 175 A of Fe IX, X and XI lines that are formed in a typically quiet corona of a temperature around 1.5 MK. From the spectral intensities and the additional results of the simultaneous multi-color photometry coronal temperatures and emission measures are derived. There are indications in the spectra that the emission should be interpreted in terms of differential emission measure distribution models.