Metallicity and photospheric abundances in field K and M dwarfs

Elemental abundances in late-type stars are of interest in several ways: they determine the location of the stars in the HR diagram and therefore their ages, as well as the atmospheric structure in their middle and upper photospheres. Especially in the case of chromospherically active late-type stars the question arises to what degree the upper photosphere is influenced by the nearby chromosphere. Analysing S/N ∼ 200 and Δλ/λ ∼ 20 000 data, we found a mean metallicity index [M/H] = −0.2 for programme K and M field stars based on an analysis of spectra in the region 5500–9000 Å. We also found that the Ca  I 6162-Å transition is a potential surface gravity indicator for K-type stars. For the chromospheric activity interval 4.4 < log  F _{Mg II}  < 6.6 we did not find any chromospheric activity impact on photospheric and upper photospheric transitions. With the derived metallicity, we confirmed the Li abundance from our previous paper and thus its dependence on the Mg  II chromospheric activity index. The nature of the spectrum for the active M-type star Gl 896A is explained by pure rotation of 14 km s⁻¹. As far as the lithium–rotation relation is concerned, the spectrum of Gl 517 is rotationally broadened as well, by 12 km s⁻¹, and the Li abundance is the second highest in our sample of stars. However, there is no link between very high Li abundance, 2.2 dex, in the K dwarf star Gl 5 and stellar rotation.     

The solar surface differential rotation from disk-integrated chromospheric fluxes

Disk-integrated solar chromospheric Caii K-line (3933.68 ) fluxes have been measured almost daily at Sacramento Peak Observatory since 1977. Using observing windows selected to mimic seasonal windows for chromospheric measurements of lower Main-Sequence stars such as those observed by Mount Wilson Observatory's HK Project, we have measured the solar rotation from the modulation of the Caii K-line flux. We track the change of rotation period from the decline of cycle 21 through the maximum of cycle 22. This variation in rotation period is shown to behave as expected from the migration of active regions in latitude according to Maunder's butterfly diagram, including an abrupt change in rotation period at the transition from cycle 21 to cycle 22. These results indicate the successful detection of solar surface differential rotation from disk-integrated observations. We argue that the success of our study compared to previous investigations of the solar surface differential rotation from disk-integrated fluxes lies primarily with the choice of the length of the time-series window. Our selection of 200 days is shorter than in previous studies whose windows are typically on the order of one year. The 200-day window is long enough to permit an accurate determination of the rotation period, yet short enough to avoid complications arising from active region evolution. Thus, measurements of the variation of rotation period in lower Main-Sequence stars, especially those that appear to be correlated with long-term changes in chromospheric activity (i.e., cycles), are probably evidence for stellar surface differential rotation.     

Further images of α Persei G dwarfs

We present two images of intermediate and low axial inclination G dwarfs (AP 149 and AP 193) in the young open cluster α Persei, and compare these with previous images of intermediate and high axial inclination objects in this cluster. All stars show starspots at high latitudes, with one star exhibiting a strong polar spot. Although low-latitude features are found on all stars to some degree, the detection of spots on AP 193 is marginal. The apparent difference in starspot morphology from one object to the next is probably the result of a stellar magnetic cycle, although the exact effect on the starspot distribution throughout a cycle is unknown.
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius.     

Stellar Tomography

I review recent progress in the field of stellar surface imaging, with particular reference to advanced methods for mapping surface-brightness inhomogeneities and the surface vector magnetic field on magnetically active late-type stars. New signal enhancement techniques, utilising profile information from hundreds or thousands of photospheric lines simultaneously, allow images to be derived for stars several magnitudes fainter than was previously possible. For brighter stars, the same techniques make it possible to map features as small as two or three degrees in extent on the stellar surface. This opens up whole new areas of research, such as the ability to use starspot tracking to study surface differential rotation patterns on single and binary stars, and to follow the secular evolution of the magnetic field itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Late-type members of young stellar kinematic groups – I. Single stars

This is the first paper of a series aimed at studying the properties of late-type members of young stellar kinematic groups. We concentrate our study on classical young moving groups such as the Local Association (Pleiades moving group,     , IC 2391 supercluster (35 Myr), Ursa Major group (Sirius supercluster, 300 Myr), and Hyades supercluster (600 Myr), as well as on recently identified groups such as the Castor moving group (200 Myr). In this paper we compile a preliminary list of single late-type possible members of some of these young stellar kinematic groups. Stars are selected from previously established members of stellar kinematic groups based on photometric and kinematic properties as well as from candidates based on other criteria such as their level of chromospheric activity, rotation rate and lithium abundance. Precise measurements of proper motions and parallaxes taken from the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, and published radial velocity measurements are used to calculate the Galactic space motions ( U , V , W ) and to apply Eggen's kinematic criteria in order to determine the membership of the selected stars to the different groups. Additional criteria using age-dating methods for late-type stars will be applied in forthcoming papers of this series. A further study of the list of stars compiled here could lead to a better understanding of the chromospheric activity and their age evolution, as well as of the star formation history in the solar neighbourhood. In addition, these stars are also potential search targets for direct imaging detection of substellar companions.     

Differential rotation of giant stars

From a set of high-resolution spectral observations of late type giant stars we used Doppler imaging to derive time-series temperature maps of the stellar surfaces. Using these temperature maps, it is possible to track the temporal changes of the spot features and derive estimates of the strength and sign of the differential surface rotation of these stars. Looking into the latitudinal changes of the surface maps, it is also possible to derive meridional flows on these stars. But due to the lower accuracy of the latitudes of the reconstructed spot features, the data requirements are higher than for the detection of differential rotation. Nevertheless, a correlation between the differential rotation and meridional flow estimates is suggested. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling the time variation of the surface differential rotation in AB Doradus and LQ Hydrae

Sequences of Doppler images of the young, rapidly rotating late-type stars AB Dor and LQ Hya show that their equatorial angular velocity and the amplitude of their surface differential rotation vary versus time. Such variations can be modelled to obtain information on the intensity of the azimuthal magnetic stresses within stellar convection zones. We introduce a simple model in the framework of the mean-field theory and discuss briefly the results of its application to those solar-like stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold

From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.
We reconstruct the large-scale magnetic geometry of the targets as a low-order  (ℓ < 10)  spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component.     

Stellar analogs of solar magnetic activity

The techniques and principal results of observational studies of stellar activity are summarized. Both chromospheric and coronal emission clearly track surface magnetic field properties, but it is not well known how the detailed relation between the emission and surface magnetic fields varies with spectral type. For lower Main-Sequence stars of the same spectral type, there is clear evidence of a close relationship between mean activity level and rotation period P _rot. There is also less definitive evidence for a similar dependence on convective overturn time _c , such that activity depends on the single parameter Ro = P _rot/ _c . For single stars, stellar rotation, and magnetic activity both decline smoothly with age. This implies a feedback between angular momentum loss rate and activity level. Temporal variations in mean stellar activity level mimic the solar cycle only for old stars like the Sun, being much more irregular for younger stars. The characteristic timescale of the variations (the cycle period) appears to depend on Ro for old stars, but shows no clear dependence on either rotation rate or spectral type for younger stars. Further data on mean activity and its variation for a large number of lower Main-Sequence stars should contribute significantly to our understanding of the causes of stellar magnetic activity.     

A phenomenological interpretation of stellar chromospheres

An attempt is made to develop a phenomenological interpretation of stellar chromospheres. The following problems are examined: observed emission powers of magnesium chromospheres on stars based on the ultraviolet doublet, 2800 Mgii, observations; dependence of chromosphere emission on spectral and luminosity classes; stellar chromospheres as an accidental event; chromospheres of stars-components of binary systems; stars with the chromospheres of solar type (S) and nonsolar (NS) type; distribution of stars by means of the type of their chromosphere on luminosity class; stars with superpower magnesium emission; emission measures for both the magnesium and calcium chromospheres; interrelation between chromosphere, transition zone and corona; chromospheric activity and rotation of stars; possibility of the existence of chromospheres on hot stars; phenomenological picture of stellar chromospheres; stars without the line 2800 Mgii, in emission or in absorption; syndrome of red giant HD 4174. At the end, the problem of heating of stellar chromospheres is discussed.     

Recent observations of AB Dor and interpretation

We use minimal empirical modelling techniques to interpret recent (2006–2007) photometry and spectroscopy of AB Dor. We compare, in particular, broadband (B and V) maculation effects with emission features in high‐resolution Ca II K‐line spectroscopy. We also compare emission effects in the Ca II Kand Hα lines observed at different rotational phases. We refer to a broader multiwavelength campaign, of which these optical data were a part, involving X‐ray and microwave observations to be published later. The broadband light curves are characterized by one outstanding macula, whereas the emission lines suggest 4 possible main chromospheric activity sites (‘faculae’). These appear at a similar latitude and with comparable size to the main umbra, but there are significant displacements in longitude. However, one strong facular concentration near phase zero may have a physical relationship to the main macula. The derived longitudes of these features would have been affected by differential rotation operating over the several months between the spectroscopic and photometric observations, but the difference of at least ∼30° between facula and umbra appears too great to allow their coincidence. The possibility of a large bipolar surface structure is considered, keeping in mind the bipolar character of solar activity centres: the activity of rapidly rotating cool stars being generally compared with that of the Sun, scaled up by a few orders of magnitude. Observed microwave activity may link to this same main photospheric and chromospheric centre picked up by the optical analysis. Characterization of macular and facular contributions in stellar activity sites would be improved with a closer timing of observations and higher signal to noise ratios in emission line data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Basic stellar parameter determination for active cool eclipsing binaries

We examine procedures for the determination of reliable parameter sets characterizing stars in short period eclipsing binary systems of later dwarf, generally sunlike, spectral types. These stars have magnetodynamically active components and form a useful extension to the solar laboratory. Their much more rapid rotation than that of the Sun implies correspondingly enhanced indications of activity-particularly with relatively large maculation effects in broadband data.We concentrate on five such systems, for which we discuss new data and analyses-CG Cyg, ER Vul, YY Gem, BH Vir and UV Psc. Our main aim is to enable more reliable absolute comparisons between the basic properties of such stars and the Sun. Some indications of the ages of these binaries can also provide further pointers to the relative importance of different parameters (e.g. rotation, age, etc.), in determining stellar activity.This article originally appeared as Poster JD12 G42 at the 22nd General Assembly of the IAU, The Hague (van Woerden, 1994).     

Metallicity effects on the chromospheric activity–age relation for late-type dwarfs

We show that there is a relationship between the age excess, defined as the difference between the stellar isochrone and chromospheric ages, and the metallicity as measured by the index [Fe/H] for late-type dwarfs. The chromospheric age tends to be lower than the isochrone age for metal-poor stars, and the opposite occurs for metal-rich objects. We suggest that this could be an effect of neglecting the metallicity dependence of the calibrated chromospheric emission–age relation. We propose a correction to account for this dependence. We also investigate the metallicity distributions of these stars, and show that there are distinct trends according to the chromospheric activity level. Inactive stars have a metallicity distribution which resembles the metallicity distribution of solar neighbourhood stars, while active stars appear to be concentrated in an activity strip on the log  R '_HK × [Fe/H] diagram. We provide some explanations for these trends, and show that the chromospheric emission–age relation probably has different slopes on the two sides of the Vaughan–Preston gap.     

The Emergence of Magnetic Field into Stellar Atmospheres

The magnetic activity in the atmosphere of the Sun, possibly of most other magnetically active late-type stars, is a consequence of the emergence of magnetic flux from the stellar interior. Research in this field has to consider the rise of magnetized plasma from the depths of the convective envelope through the stellar surface and out across the corona. In the present review, a number of key questions, difficulties and results are discussed.     

Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity——comparison with the Sun

The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from the HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory.We show that for stars of F, G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycle, is different: it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of chromospheric and coronal activity confirms that the Sun belongs to stars with a low level of chromospheric activity and stands apart among these stars by its minimum level of coronal radiation and minimum level of variations in photospheric flux.     

A facular model based on the wings of the Ca ii lines

We develop a relatively simple procedure for deriving models of upper photospheric regions based on the damping wings of the Ca ii resonance and infrared triplet lines. The procedure is used to derive a facular model but can also be applied to late-type stars. We compare our model to that of Chapman.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff Member, Laboratory Astrophysics Division, National Bureau of Standards.     

Photospheric Magnetic Activity in a Proxy for the Young Sun: HD 134319

We present the results of a long-term photometric monitoring of the young single main-sequence star HD134319. It shows short-term variability of the optical-band continuum flux with a period of 4.448 days. The variability is attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere, whose visibility is modulated by the star's rotation. Maps of the photospheric spot pattern have been obtained with light curve inversion techniques based on the Maximum Entropy and the Tikhonov regularization criteria. The overall spot pattern shows evidence for two long-lasting active longitudes located about 180° apart, with a total area of at least 16% of the stellar surface (assuming an inclination of the stellar rotation axis of 90° on the line of sight). The longitude distribution of the spot pattern and its total area do not show any clear evidence for a long-term variation along the five years of observations. A comparison with recent mean field dynamo models is also addressed, suggesting a possible interpretation of such a behaviour. Singularity, spectral type, youth and a high level of photospheric and chromospheric activity make HD134319 a suitable proxy for studying the magnetic activity of the young Sun not far after its arrival on the zero age main sequence.     

X-ray stars observed in LAMOST spectral survey

X-ray stars have been studied since the beginning of X-ray astronomy. Investigating and studying the chromospheric activity from X-ray stellar optical spectra is highly significant in providing insights into stellar magnetic activity. The big data of LAMOST survey provides an opportunity for researching stellar optical spectroscopic properties of X-ray stars. We inferred the physical properties of X-ray stellar sources from the analysis of LAMOST spectra. First, we cross-matched the X-ray stellar catalogue (12254 X-ray stars) from ARXA with LAMOST data release 3 (DR3), and obtained 984 good spectra from 713 X-ray sources. We then visually inspected and assigned spectral type to each spectrum and calculated the equivalent width (EW) of H\(\alpha\) line using the Hammer spectral typing facility. Based on the EW of H\(\alpha\) line, we found 203 spectra of 145 X-ray sources with H\(\alpha\) emission above the continuum. For these spectra we also measured the EWs of H\(\beta\), H\(\gamma\), H\(\delta\) and Ca ii IRT lines of these spectra. After removing novae, planetary nebulae and OB-type stars, we found there are 127 X-ray late-type stars with H\(\alpha\) line emission. By using our spectra and results from the literature, we found 53 X-ray stars showing H\(\alpha\) variability; these objects are Classical T Tauri stars (CTTs), cataclysmic variables (CVs) or chromospheric activity stars. We also found 18 X-ray stars showing obvious emissions in the Ca ii IRT lines. Of the 18 X-ray stars, 16 are CTTs and 2 are CVs. Finally, we discussed the relationships between the EW of H\(\alpha\) line and X-ray flux.     

CCD-Doppler measurements of solar differential rotation

Using the AT1 CCD camera at the Echelle spectrograph of the GCT at Tenerife, solar Doppler rotation measurements in the photospheric lines Fe I 6301.5 Å and 6302.5 Å and in the chromospheric line Na-D₂ 5890.0 Å have been made. The line shifts measured at different heliographic latitudes around the limb were corrected for observer motion and converted into sidereal rotation rates. At the equator the observed chromospheric rotation rate is about 8 % larger than the photospheric rate, and the average observed Doppler rotation rate is not very much different from the mean rotation rates deduced from all published tracer works and all published Doppler works. Near the poles (where tracer methods rely on extrapolation) both the chromospheric and the photospheric rotation rate are slightly smaller than the all Doppler rate and are considerably smaller than the extrapolated all tracer rate. If all previous measurements of solar rotation are taken into account, a surface rotation law with lower error bounds than previously possible can be derived.     

Basal heating in the atmospheres of cool stars

Summary This paper reviews observational evidence concerning the existence of so-calledbasal heating that occurs in the outer atmospheres of all stars with convective envelopes. Effects of basal heating depend primarily on the effective temperature, with little sensitivity to surface gravity or elemental abundances. Basal heating occurs predominantly in the chromosphere, possibly in the (lower) transition region, but not at an observable level in coronae (except perhaps in early F-type and in M-type dwarf stars). Basal fluxes are observed in the slowest rotators where it shows no significant modulation. The basal flux level is observed directly on the Sun only over regions void of intrinsically strong photospheric fields. There is substantial quantitative observational and theoretical evidence that the basal emission from stellar outer atmospheres is caused by the dissipation of acoustic waves generated by turbulent convection. The magnetic canopy turns out to be of little consequence, but effects of intrinsically weak fields on the basal mechanism cannot be entirely ruled out. Solar observations constrain the spatio-temporal character of the basal atmosphere and the acoustic flux levels as a function of height, resulting in a model in which intermittent wave dissipation causes emission characteristic of both cool and warm atmospheric areas, in which — at least in the solar case — a time-averaged chromospheric temperature rise may not even exist.