The magnetic Ap stars are characterized by the presence of large magnetic fields which undergo periodic variations. These magnetic field variations are accompanied by spectral variations caused by the inhomogeneous distribution of elements on the stellar surface. It is believed that the magnetic field plays an important role in determining this distribution. Accurate maps of the surface distribution of elements would provide valuable probes as to the field geometry as well as provide clues to the role of the magnetic fields in the atmospheres of these stars. We have developed a new technique for mapping the local equivalent width on a stellar surface from the observed spectral line variations.

An absorption line–splitting phenomenon, first reported by Cottrell and Lambert (1982a), has been shown to occur at about maximum light in the semi-regular pulsations of the R Coronae Borealis (RCB) star, RY Sgr (Lawson 1986). This has been interpreted as a shock wave propagating through the photospheric layers (Lawson and Cottrell 1986). We present spectroscopic observations of this star, taken to coincide with this line–splitting event. A sequence obtained during 1986 October revealed that this event extended over about 6 days (out of a period of about 40 days) and began at about the bluest B-V. This colour maximum, which corresponds to maximum photospheric temperatures and minimum radius, leads the V maximum by about 6 days.

The tremendous development of helioseismology in the last ten years has included extremely important results obtained through full disk measurements, both in Doppler shifts and broad band photometry. It was then looking very attractive to try to detect the same kind of small amplitude p-modes on other stars.

The two firmly established cases of many-mode observations, the Sun and Kurtz's Ap stars, suggest that in stars extremely small pulsation amplitudes are to be expected. If spectroscopic Doppler techniques are used to measure the velocity pattern, then it is obvious that with better resolution and higher S/N ratio of the spectral lines the sensitivity to detect velocity amplitudes increases. Observed pulsation frequencies of any star will put new constraints on the theory of stellar evolution. Besides addressing the two most important issues, namely determination of age and chemical composition, observed periods will also help resolve open questions in the physics of stellar interiors.

Following a summary of the observations which suggest that the outbursts of classical Be stars are caused by nonradial pulsations, properties, implications and requirements of a model based on this notion are evaluated. A preliminary analysis of new observations of μ Cen is presented which for the first time in a Be star reveals two relatively closely spaced non–commensurate periods. Such a result would render implausible speculations that the variability of Be stars is due to corotating surface features.

I discuss the general problem of determining the spatial structure in the outer atmospheres of active, late-type stars. There are several major differences between the problems of imaging photospheres and of imaging chromospheres. Because of these differences, chromospheric spectral imaging must be based on a direct deconvolution of the observed emission line profiles. I present results based on IUE spectra of AR Lac (=HD 210334).

An unconstrained four gaussian fit to the Mg II profile near the flare peak indicates that the flare occurred near the central meridian of the K1 IV star, perhaps above a spot. A more likely fit to the same data places the flare at +90 ±30 km s–1 relative to the K1 IV star, indicating significant downflowing plasma.

The abundances are usually determined assuming homogeneous concentrations for all elements in the line forming region. If diffusion driven by radiation occurs, this condition is not verified and the element stratification affects the equivalent widths as well as the line profiles. We shall illustrate these problems by a study of the gallium case. A synthetic spectrum of the Ga II resonance line λ 1414.401 Å has been computed assuming that gallium is stratified according a recent theoretical study of the diffusion of this element.

The fine structure of stellar photospheric convection (the stellar equivalent of solar granulation) can be analyzed with the aid of high-resolution spectroscopy. Photospheric absorption lines are slightly asymmetric and wavelength–shifted due to unequal photon contributions from bright and systematically Doppler–shifted granules and from darker intergranular areas. Numerical simulations of stellar surface convection in different stars have now been carried out, and such three–dimensional and time–dependent models predict the detailed stellar line profiles (including asymmetries and wavelength shifts), thus enabling a direct confrontation between observations and theory.

We have obtained FTS spectra of 11 stars of spectral types from MO to M4 III in the infrared K band. The shifts at line bottom of the lines of CO and Fe I show a dependence on excitation energy and line depth similar to that caused by convective motions in the photosphere of the Sun but with a larger amplitude. Time variability is small but important differences appear between individual stars of similar types. From these data it should be possible to obtain quantitative estimates of the convective velocities and horizontal temperature inhomogeneities.

Doppler imaging is a technique for deriving resolved images of rapidly rotating stars from a detailed analysis of very high signal-to-noise high resolution spectral line profiles. An improved version of this technique is presented, which now uses principles of maximum entropy image reconstruction to invert the line profile information. The effects that noise, finite resolution, and uncertainties in the assumed stellar physics have on the resultant images were explored through various test simulations. The technique is found to be efficient, accurate, and robust at deriving images of certain classes of stars from realistic quality data. Doppler images are presented of two spotted late-type stars, the RS CVn-type star HR 1099, and the FK Com-type star HD 199178. Both stars show surprisingly similar spot distributions. In each case, there is a single large cool spot straddling the pole, and a number of small cool spots at low latitudes. We expect that the small low latitude spots on each star will migrate poleward to join the polar spot, and suspect that the observed long-lived polar spots are the result of the poleward migration and merging of many active region complexes. If true, the poleward migration of starspots suggests that magnetic activity on very rapidly rotating stars is qualitatively different than that seen on our Sun. We suggest that the observed rotational trigger velocity for the appearance of large spots on late-type stars marks the transition from solar-type boundary layer dynamos to distributed dynamos, which occur only in more rapidly rotating stars. The sizes, locations, and migrations of the spots, however, may be more a result of the convective flow patterns than of any dynamo action, since the spots are quite long-lived.

We have determined Li abundances from high resolution, high signal-to-noise spectra near 6700 å of the F dwarfs in several galactic clusters and in the field. The Li-temperature profiles show large Li depletions in the mid–F stars indicating that matter has been circulated (or diffused) downward in the stars. This brings new observational evidence about the details of stellar envelope structure. Li depletions occur in stars with T = 6400–6900 K, with depletions of over an order of magnitude for those with T = 6500–6800 K. The relationship to rotation, galactic Li, and the halo star Li content is discussed.

For stars cooler than 6400°K, the lithium is observed to be depleted with respect to the universal value for Population I stars. The depletion depends on mass and age and occurs, both during the main-sequence (Hyades) and the pre-main-sequence (Pleiades).

We discuss the present limits to the velocity field diagnostics in stellar chromospheres achievable with ESO CAT+CES and IUE high resolution spectra.

Monitoring of active late-type dwarfs, spectroscopically with high resolution (λ/Δλ ∼ 105) and high S:N (>300) round their activity cycles, principally in the Call H resonance line, offers techniques to explore (a) plage filling factors (b) 3-dimensional chromospheric velocity fields (c) maps of surface activity via “Doppler Imaging”. In this paper we deal with the use of spectral signatures to derive plage cover.

A total of 48 nights of time series data have been obtained for the K giants: Arcturus, Pollux, and Aldebaran. A careful analysis of both single and multi–night sets using the earth's motion as a velocity calibrator has yielded stellar velocity time series accurate to ± 3 m/s per observation. Periodogram analyses of these sets have revealed the existence of oscillations with periods near 2.5 hrs and amplitudes of ± 5 m/s for both Pollux and Aldebaran, but not for Arcturus. Preliminary analysis of a 5-night set for Pollux using the CLEAN algorithm suggests at least three modes separated by about 35 microHertz.

I discuss the some of techniques used (and problems involved) in measuring stellar magnetic fields on cool stars, and detail how these measurements are broadening our understanding of stellar magnetic activity.

A new technique for detecting and measuring magnetic fields on cool stars is presented, incorporating both an improved observational approach and a rigorous LTE theoretical treatment. We have identified two lines in the near infrared, one of which is very sensitive to Zeeman broadening and the other relatively insensitive, that are easily accessible to CCD and Reticon detectors. The lines are modelled by solving numerically the equations of transfer for the Stokes parameters in a full model atmosphere, permitting computation of all relevant depth-dependent physics such as polarized line opacities and broadening sources. We have obtained high resolution, high S/N spectroscopic observations of several G and K dwarfs and have synthesized the two lines using two free parameters, namely, the average field strength, B, and the fraction, f, of the surface covered by fields. For the chromospherically active stars ε Eridani and Xi Boo A, we find B=1000, f=35% and B=1200, f=40%, respectively. A careful study has been made of various sources of broadening to determine their ability to mimic Zeeman broadening, and none is capable of doing so.

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

The first results of magnetic field measurements are presented here for HD 187474, a slowly rotating Ap star. From resolved Zeeman pattern the strength of the field and its mean inclination were obtained. From differential magnetic broadening a second value of the field strength has been deduced, which is compatible with the previous one. The “Robinson” method has been tested and a good agreement is found between observed and calculated Zeeman broadening of FeII lines. This method can therefore certainly be used to measure the surface field in slow rotating chemically peculiar stars.