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We present some preliminary results of simultaneous monitoring of the two flare stars, UV Ceti and EQ Peg with EXOSAT and ground based optical spectro-scopy. A strong correlation, found between the Hγ emission and the soft X-ray flux, over several hours of observation of UV Ceti, suggests that much of the low level X-ray flux which has previously been considered ‘quiescent’, in fact derives from small flare events. The implication is that coronal heating in M type dwarfs may result directly from the frequent occurrence of micro-flares.

During the era of the Solar Maximum Mission (SMM) observations of the Sun, significant developments were taking place in the parallel field of stellar astrophysics which led to the concept of the so-called “solar-stellar connection.” In the same way that the developments in the earlier part of this century had used solar observations, such as limb darkening, together with stellar observations, such as flux distributions and line profiles, to create the field of stellar atmospheres, the fields of stellar outer atmospheres and stellar activity developed as connections were made between the highly dynamic, highly structured phenomena of the Sun revealed by Skylab and SMM, and apparently analogous phenomena on stars. Broadly speaking, the most important concepts were: 1. The detection of X-ray-emitting coronae on many types of stars, together with the fact that such coronal emissions could vary widely for stars of identical spectral type. This played an important role in the rejection of acoustic heating mechanisms for the corona. 2. The discovery of a dividing line in the H-R diagram at which stars evolving off the main sequence seemed to abruptly lose their coronae and appeared to acquire massive stellar winds instead. 3. The establishment of relations between coronal emission and rotation of stars. 4. The establishment of age-versus-activity relations. 5. The discovery of stellar activity cycles analogous to the 11-year solar cycle. 6. The development of flux-flux relationships to see whether plages and active regions are the same on all active late-type stars and of techniques to begin to map spots and active regions on other stars. 7. The observations of the high-energy components of flares on other stars in the ultraviolet and X-ray regions. 8. The exploration of the limits of the fraction of the stellar thermonuclear power that is or can be channeled into chromospheric, coronal, or flare phenomena. This ranges from non-magnetic basal levels of chromospheric emission to extreme X-ray emission in coronae and flares.

We report on the first successful coordinated observations of stellar flares carried out on March 28, 1984 simultaneously over a wide range of wavelengths, from UV to microwaves, using the IUE satellite, three ESO telescopes at La Silla (Chile) and the VLA at Socorro (NM, USA).

In the solar corona the opacities of some of the prominent X-ray emission lines are on the order of τ ≈ 1 over typical coronal path lengths. We present and discuss a particular solution of the radiative transfer problem involving an extended, spherically symmetric coronal shell radiating isotropic, homogeneous emission in which single-scattering also takes place. Within the context of this simplified model we find that scattered radiation is an important contribution to the total emergent resonance line flux and that for the He-like family of resonance (r), intercombination (i), and forbidden (f) lines, the ratio G=(f + i)/r would decrease as a function of optical depth for disk-center emission in an extended spherically symmetric corona.

The flight of a multilayer-coated mirror on a sounding rocket experiment on March 17, 1988 which was used to obtain photometric images of the sun just prior to solar eclipse is discussed. The rocket payload and the choice of passband are described. The design of the multilayer, the fabrication of the mirror, and the reflectance measurements are addressed.

A tensor formulation of the equation of radiative transfer is derived in a seven-dimensional Riemannian space such that the resulting equation constitutes a divergence in any coordinate system. After being transformed to a spherically symmetric comoving coordinate system, the transfer equation contains partial derivatives in angle and frequency, as well as optical depth due to the effects of aberration and the Doppler shift. However, by virtue of the divergence form of this equation, the divergence theorem may be applied to yield a numerical differencing scheme which is expected to be stable and to conserve luminosity. It is shown that the equation of transfer derived by this method in a Lagrangian coordinate system may be reduced to that given by Castor (1972), although it is, of course, desirable to leave the equation in divergence form.

The first detection of both quiescent and flaring soft X-ray emission from a dMe flare star, Proxima Centauri (dM5e) is reported. The data are analyzed for temporal variability and spectral characteristics. The quiescent state is characterized by a mean X-ray luminosity of 1.5 x 10 to the 27th erg s/s, corresponding to a mean surface flux of 700,000 erg s/sq cm-s, and an inferred temperature of 4-million K. The flare that is detected has a peak flux of 7.4 x 10 to the 27th erg s/s and a peak temperature of 17-million K. The implications of these data for models of the quiescent and flare coronae of dMe stars are discussed.

The Einstein Imaging Particle Counter observed a major X-ray flare in its entirety during a 5-hr period of simultaneous observations, with the IUE, of the dM5e flare star Proxima Centauri in August, 1980. The detailed X-ray light curve, temperature determinations during various intervals, and UV line fluxes obtained before, during, and after the flare indirectly indicate a 'two-ribbon flare' prominence eruption. The calculated ratio of coronal to bolometric luminosity for the event is about 100 times the solar ratio. The Proxima Cen corona is analyzed in the context of static loop models, in light of which it is concluded that less than 6% of the stellar surface seems to be covered by X-ray emitting active regions.

Results are reported for a three-day coordinated observing program to monitor the flare star Proxima Centauri in the X-ray, optical, and radio spectrum. During this interval 30 optical flares and 12 possible radio bursts were observed. The SAS 3 X-ray satellite made no X-ray detections. An upper limit of 0.08 on the X-ray/optical luminosity ratio is derived for the brightest optical flare. The most sensitive of the radio telescopes failed to detect 6-cm emission during one major and three minor optical flares, and on this basis an upper limit on the flare radio emission (1 hundred-thousandth of the optimal luminosity) is derived.

The paper reports the detection of two X-ray flares from each of the nearby dMe stars, AT Mic and AD Leo, with the A-2 experiment on board HEAO 1. A spectrum obtained during the brighter AT Mic flare, the first X-ray spectrum of a stellar flare, is well matched by a thermal model with a temperature 3 x 10 to the 7 K and an iron K-alpha emission line. The X-ray luminosities derived are in the range 1.3-16 x 10 to the 30th ergs/s, while emission measures are in the range 1.1-14 x 10 to the 53rd/cu cm. The estimated Lx/Lopt ratios exceed unity and are inconsistent with Mullan's flare model. Several scenarios to explain this discrepancy are proposed.

Analysis of recent ultraviolet observations of the Capella binary system (Alpha Aur) indicates a dense geometrically narrow chromosphere-corona transition region in the Capella system primary (G5 III) similar in many respects to a solar active region. An examination of the coronal energy balance, together with the coronal base pressure derived from the line fluxes, predicts a corona with a mean temperature of 1.2 million K and a large stellar wind consistent with observations.

A possible explanation is proposed for the observation that late-type stars falling in a certain region of the H-R diagram exhibit no X-ray emission and hence appear not to have coronae. It is argued that, due to the low surface gravity of these stars, the hot X-ray emitting loops in their atmospheres simply become cool and emit at longer wavelengths. This argument is demonstrated quantitatively by considering the effect on a typical solar loop of scaling it up to parameters appropriate to a supergiant.

The ultraviolet and near-infrared spectra of O I in Arcturus are analyzed by a 15-level 14-transition model for O I and the Ayres-Linsky (1975) model chromosphere. It is found that the anomalously bright O I resonance lines at 1302, 1305, and 1306 A can be readily explained by a Ly-beta-pumped fluorescence mechanism as originally proposed by Bowen (1974). Observed equivalent widths of the near-infrared triplet and singlet lines are also consistent with the model predictions, but the intercombination lines at 1355 and 1359 A and near-infrared quintet lines may pose a problem.

The preliminary results of the Einstein Observatory stellar X-ray survey are presented. To date, 143 soft X-ray sources have been identified with stellar counterparts, leaving no doubt that stars in general constitute a pervasive class of low-luminosity galactic X-ray sources. Stars along the entire main sequence, of all luminosity classes, pre-main sequence stars as well as very evolved stars have been detected. Early type OB stars have X-ray luminosities in the range 10 to the 31st to 10 to the 34th ergs/s; late type stars show a somewhat lower range of X-ray emission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late type main-sequence stars show little dependence of X-ray emission levels upon stellar effective temperature; similarly, the observations suggest weak, if any, dependence of X-ray luminosity upon effective gravity. Instead, the data show a broad range of emission levels (about three orders of magnitude) throughout the main sequence later than F0.

Soft X-ray emission was detected from Gamma Hya and 37 Com, at levels of 6-8 x 10 to the -13th ergs/sq cm per sec, during 0.2-4.0 keV observations of eight G-K giant stars obtained with the Einstein Observatory's Imaging Proportional Counter. Upper limits of about 3 x 10 to the 13th ergs/sq cm per sec were determined for the remaining stars. These results are judged to strengthen the Ayres et al (1981) findings that stellar coronae whose temperature is greater than one million K are either very weak or entirely absent among red giants and supergiants, while a large range in coronal emission exists among the yellow giant and supergiant stars.

Possible wind models for late-type stars which appear not to have hot coronae and transition regions are investigated; taking Arcturus and the prototypical star, models with T less than approximately 20,000 K are considered, and solutions with mass loss rates of 10 to the -9th solar masses per year are sought. The radiation pressure of L-alpha resonance scattering can exceed the force of gravity in the chromosphere and initiate a net outflow, but is insufficient to sustain a wind, except perhaps in the presence of an additional momentum input term such as Alfven wave pressure. It is concluded that L-alpha radiation-pressure-initiated winds can occur in stars to the right of the Linsky-Haisch dividing line in the H-R diagram between stars with and without transition regions and presumably hot coronae, and that the existence of these winds may explain energetically the absence of hot coronae in these stars.

We report on a coordinated program involving X-ray, ultraviolet, optical, and radio observations of the dM5e flare star Proxima Centauri. We detected one major X-ray flare event with L/sub x/(0.2--4.0 keV)roughly-equal6.0 x 10/sup 27/ ergs s/sup -1/, T = 1.7 x 10/sup 7/ K, and EM = 7.5 x 10/sup 50/ cm/sup -3/ during the rise phase and L/sub x/roughly-equal7.4 x 10/sup 27/ ergs s/sup -1/, T = 1.2 x 10/sup 7/ K, and EM = 12.0 x 10/sup 50/ cm/sup -3/ during the decay phase. This is the first detection of a time-resolved stellar X-ray flare that shows changes in its spectral flux distribution. We detected no ultraviolet, optical or radio emission corresponding to this flare, but we did detect a total of five optical and 12 possible radio flares, including one event with simultaneous radio and optical emission. We interpret the absence of optical and ultraviolet emission at the time of the X-ray flare in terms of an arch model in which the flare cools predominently by X-ray radiation. The observed 20 min expotential cooling time is consistent with an electron density of 1.0 x 10/sup 11/ cm/sup -3/ during the decay phase and a flare of totalmore » arch length of ..pi.. x 10/sup 10/ cm, comparable to the size of the star itself. We conclude that we have observed an X-ray flare more like a typical strong solar flare than heretofore seen on a flare star.« less

During a series of spectral scans in search of flares on the dMe star YZ CMi the authors observed prominent chromospheric emission lines of Hγ λ4340, Hδ λ4100, Ca II H λ3968 blended with H λ3970, Ca II K λ3934, and Hζ λ3889. During one five-minute scan the authors recorded the following peculiar transient phenomena: (1) the appearance of an exceptionally strong line at ≡λ4007; (2) the simultaneous appearance of another prominent feature at ≡λ4276; and (3) a change in the ratio of (Ca II H+H)/Ca II K. The authors discuss the possible origin of this unusual transient spectrum.

An atlas of IUE short-wavelength spectra for a set of 14 bright G0 V-G5 V stars are presented, and it is shown that these manifest a range of qualitatively different chromospheric and transition region spectra and significant differences in radiative flux originating at the temperature minimum level. A comprehensive survey of observational data and physical parameters of these stars has been performed, and tabular summaries of the data are presented. It is concluded that the UV continuum longward of about 1600 A is a diagnostic of the temperature minimum. A considerable range of minimum temperature for stars of similar effective temperature and spectral type is found. The temperature minimum of the sun is highly structured on spatial scales of about one arcsec.