Active Late-type Stars Research Articles

XMM- Newton spectroscopy of stars in open clusters and star forming regions

We discuss observations of open clusters and star forming regions obtained with the RGS and EPIC instruments on board XMM- Newton. These observations provide a powerful tool to investigate the temperature structure, emission measure distribution and elemental abundances of stars from the pre-main sequence phase to post-main sequence evolution. We report in particular on EPIC and RGS spectroscopy of hot and cool stars in the very young cluster around σ Orionis, of two stars in the Hyades (the main-sequence star VB 50 and the clump giant VB 71) and of two pre-main sequence stars in Taurus–Auriga (the classical T Tauri star SU Aur and the weak-lined T Tauri star HD 283572). The implications of these observations for models of magnetic activity in late-type stars, of wind emission in early-type stars, and of X-ray production in the early stages of stellar evolution are briefly discussed.

Edge-on T Tauri stars

Using the U echelle spectrograph at the ESO VLT we obtained two-dimensional high-resolution (R = 50 000) spectra of the edge-on disk objects HH30 ∗ , HK Tau B, and HV Tau C. For comparison purposes we also observed with the same equipment both the classical T Tauri star HL Tau and the active late-type star LDN 1551-9. The spectra of all three observed edge-on disks consist of a T Tauri emission and absorption line spectrum with superimposed jet emission lines. Analysis of the spectra confirmed that the disks are completely opaque at visible wavelengths and that light from the central objects reaches us only via scattering layers above and below the disk planes. The central objects of our targets were found to be normal T Tauri stars showing moderate but different amounts of veiling of their photospheric spectra, indicating different accretion rates or evolutionary stages. We suggest that all classical T Tauri stars (CTTSs) show this observed morphology when viewed edge-on. Part of the jet emission from edge-on systems is directly visible to us in the forbidden lines as well as in Hα and He , a finding which contradicts the present paradigm of a pure magnetospheric accretion origin for the formation of hydrogen and helium emission lines in moderately active CTTSs. From a comparison with those Taurus-Auriga CTTSs for which the inclination is reliably known, we conclude that the view angle of CTTS systems is one of the key parameters governing apparent Hα emission strength in the T Tauri class. We discuss the various possible formation regions for the Na  D lines and show that profiles similar to observed ones can be formed at the base of the disk wind.

Mg II chromospheric radiative loss rates in cool active and quiet stars

The Mg  k emission line is a good indicator of the level of chromospheric activity in late-type stars. We investigate the dependence of this activity indicator on fundamental stellar parameters. To this purpose we use IUE observations of the Mg  k line in 225 late-type stars of luminosity classes I-V, with different levels of chromospheric activity. We first re-analyse the relation between Mg  k line luminosity and stellar absolute magnitude, performing linear fits to the points. The ratio of Mg  surface flux to total surface flux is found to be independent of stellar luminosity for evolved stars and to increase with decreasing luminosity for dwarfs. We also analyse the Mg  k line surface flux-metallicity connection. The Mg  k emission level turns out to be not dependent on metallicity. Finally, the Mg  k line surface flux-temperature relation is investigated by treating separately, for the first time, a large sample of very active and normal stars. The stellar surface fluxes in the k line of normal stars are found to be strongly dependent on the temperature and slightly dependent on the gravity, thus confirming the validity of recently proposed models. In contrast, data relative to RS CVn binaries and BY Dra stars, which show very strong chromospheric activity, are not justified in the framework of a description based only on acoustic waves and uniformly distributed magnetic flux tubes so that they require more detailed models.

Differential photometry of EK DRA and 29 DRA

We report on BVR precise photometry of active late-type stars EK DRA and 29 DRA in the spring of 2004. The analysis suggests a reasonable fit with single spot model for both cases in the season as far as the V-curve is concerned. The spots are generally cooler than surrounding photosphere and are at high latitudes. The maximal brightness changes for both stars implying a non-rotating component of activity signatures, the spots. We expect that the spot configuration is stable over the period of approximately three months. .

Evidences for butterfly diagrams for spots on active late-type stars

From long-term multicolour photometric observations of strongly spotted stars, one concludes usually on a presence of active longitudes. We argue the latitudinal effects in distribution spots over the surface and noticeable equatorward drift of the low-latitude boundary of the spotted region during the rising phase of activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Magnetic Cycles and Rotation in Active Late–Type Stars

Observational evidence, based both on spectroscopic Doppler imaging and long-term photometry, of strongly nonaxisymmetric spot distributions in magnetically very active late-type stars, with a special cyclic behaviour (the “flip-flop” effect), is presented. Theoretical implications of these results are discussed from the point of view of nonlinear mean-field dynamo theory.

Stars at γ-Ray Energies: the INTEGRAL Look at Stars

The European γ-ray satellite INTEGRAL, launched on October 17, 2002, is the successor to the highly successful American satellite Compton-GRO. Even though its main observational program focuses on “classical” high-energy sources like compact X-ray binaries or AGNs, some time is being devoted to γ-ray studies of massive stars and magnetically active late-type stars. We briefly describe here the four instruments of INTEGRAL, and summarize the ongoing stellar programs.

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.

Effect of chromospheric activity on the mean colours of late-type stars

We confirm the existence of an excess in the mean dereddened ( U B)0 colour of active single-lined spectroscopic giants over quiescent stars of the same spectral type and find evidence for this excess in ( B V)0, although at a lower scale, when plotted against (R I)0. This result suggests that the use of ( B V)0 calibrations to determine fundamental parameters or spectral types for late-type active stars should be taken with caution. Since magnetic activity is definitely involved, these and similar calibrations using (U B)0 and/or (B V)0 should not be used for the study of late-type young clusters' members or late-type pre-main sequence stars without some consideration. Instead, near-infrared colours like ( R I)0 should be better temperature indicators for these stars.

Stellar dynamos - perspectives and challenges

In this paper the present status of theoretical understanding - mostly relying on mean-field dynamo models - of global magnetic activity of late-type stars with convective envelopes is reviewed. The principles of the mean-field dynamo theory are introduced, and a discussion on some of the present-day problems and challenges is presented. The remaining sections are devoted to discuss the current status of solar and stellar dynamo models.

The Third [ITAL]Extreme Ultraviolet Explorer[/ITAL] Right Angle Program Catalog: The Last Years

We present 63 new extreme-ultraviolet sources from observations taken during the Right Angle Program (RAP) of the Extreme Ultraviolet Explorer (EUVE). This paper concentrates on data taken in the last years of the RAP, 1999 and 2000, with a sky coverage of 7%. We present sample light curves for several sources, including flare detections from the previously uncataloged source EUVE J0613-23.9B, the active late-type star EUVE J0728-20.9 (V372 Pup), and the cataclysmic variable EUVE J0332-25.9 (VY For). This final RAP catalog, combined with the EUVE all-sky survey, Lampton et al. faint list, and previous RAP catalogs, brings the total number of EUVE sources to nearly 1200.

A highly sensitive search for magnetic fields in B, A and F stars

Circular spectropolarimetric observations of 74 stars were obtained in an attempt to detect magnetic fields via the longitudinal Zeeman eect in their spectral lines. The sample observed includes 22 normal B, A and F stars, four emission-line Ba nd As tars, 25 Am stars, 10 HgMn stars, two Boo stars and 11 magnetic Ap stars. Using the Least-Squares Deconvolution multi-line analysis approach (Donati et al. 1997), high precision Stokes I and V mean signatures were extracted from each spectrum. We find absolutely no evidence for magnetic fields in the normal, Am and HgMn stars, with upper limits on longitu- dinal field measurements usually considerably smaller than any previously obtained for these objects. We conclude that if any magnetic fields exist in the photospheres of these stars, these fields are not ordered as in the magnetic Ap stars, nor do they resemble the fields of active late-type stars. We also detect for the first time a field in the A2pSr star HD 108945 and make new precise measurements of longitudinal fields in five previously known magnetic Ap stars, but do not detect fields in five other stars classified as Ap SrCrEu. We also report new results for several binary systems, including a newv sini for the rapidly rotating secondary of the Am- Del SB2 HD 110951.

Spectropolarimetry of starspots

We use the simulations of Stokes profiles for active late-type stars in order to assess the requirements to the observations and select the best observing strategy for these objects. The synthetic spectropolarimetry was computed using models of magnetic fields distribution for several rotational phases throughout the activity cycle. The simulations show that cyclic variations of the polarization profiles are more systematic and easier to observe and that averaging over single rotation periods may be used to increase the signal-to-noise ratio.

Origin of prolonged X-ray flares on active late-type stars

Soft X-ray data for prolonged flares in subgiants in RS CVn binary systems and some other active late-type stars (AB Dor, Algol) are analyzed. During these nonstationary events, a large amount of hot plasma with temperatures exceeding 108 K exists for many hours. Numerical simulations of gas-dynamical processes in the X-ray source—giant loops—can yield reliable estimates of the plasma parameters and flare-source size. This confirms that such phenomena exist while considerable energy is supplied to the top part of a giant loop or system of loops. Refined estimates of the flare energy (up to 1037 erg) and scales contradict the widely accepted idea that prolonged X-ray flares are associated with the evolution of local magnetic fields. The energy of the current component of the large-scale magnetic field arising during the ejection of magnetic field by plasma jets or stellar wind is estimated. Two cases are considered: a global stellar field and fields connecting regions with oppositely directed unipolar magnetic fields. The inferred energy of the current component of the magnetic field associated with distortion of the initial MHD configuration is close to the total flare energy, suggesting that large-scale magnetic fields play an important role in prolonged flares. The flare process encompasses some portion of a streamer belt and may propagate along the entire magnetic equator of the star during the most powerful prolonged events.

Optical properties of X-ray selected stars in NGC 2244 in the Rosette Nebula

We present high spatial resolution optical photometry of NGC 2244 in the Rosette Nebula using the NOAO MOSAIC Imager at Kitt Peak. We obtained BVIR and Hα images of the central cluster and surrounding nebulae, and present results for 138 sources selected from X-ray observations with the ROSAT PSPC and HRI. Color-magnitude diagrams of the cluster show a large number of the stars are still undergoing contraction onto the main sequence. The faintest X-ray selected cluster members have the highest X-ray-to-optical luminosities ( erg s-1; to 10-3) and indicate they are a population of young active late-type stars. Hα emission from the X-ray emitters is also remarkable. We have extended the detections of PMS stars in NGC 2244 well into the range of K spectral types. While most of the cluster stars are located in the color-magnitude diagrams in between the ZAMS and the 3 Myrs isochrone, significantly younger low mass stars exist and confirm earlier reports that star formation is still going on in the Rosette Nebula/NGC 2244 region.

The X-ray emission and the magnetic fields of late-type stars

We present the results of X-ray luminosities of some active late-type stars, based on data primarily from the ROSAT position sensitive proportional counter (PSPC). According to the observations, we divide the stars into four groups: single, wide binary, binary and RS CVn. We investigated the correlation between the X-ray emission and the hardness ratio, coronal temperature, magnetic field strength, magnetic flux density. Our results suggest that the magnetic field plays a very important role in stellar X-ray emission.

Can late-type active stars be explained by a dipole magnetic trap?

In this paper we review four different types of X-ray and/or radio observations of active late-type stars. We then consider if a single magnetic source configuration – a toroidal dipole magnetic trap – can possibly explain these various different observations. We conclude that, indeed, dipole magnetic confinement (similar to the magnetic configurations of the Earth's radiation belts and the case of Jupiter and the Io torus) can explain all the diverse observational data. We take this to be very strong observational support for this type of magnetic confinement scheme. We also consider that this magnetic configuration is only likely to be established and maintained in the most active stars.

Levels of coronal and chromospheric activity in late-type stars and various types of dynamo waves

We analyze the X-ray emission and chromospheric activity of late-type F, G, and K stars studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity L X/L bol. This indicates that the nature of processes associated with magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α-Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations found between various activity parameters and the Rossby number are consistent with our conclusions. Our analysis makes it possible to draw the first firm conclusions about the place of solar activity among analogous processes developing in active late-type stars.

The coronal FeXXI λ1354.094 line in AB Doradus

The active late-type star AB Doradus was observed in February 1996 with the Goddard High Resolution Spectrograph of the Hubble Space Telescope using the low resolution G140L grating. The observations covered one half of the star's rotation cycle (P =0 :514 d) with 11.5 min time resolution. The strong coronal FeXXI 1354.094 line formed at 10 7 K was analysed and its emission measure (EM) derived. This EM is much higher than that derived from recent XMM-Newton observations (Gudel et al. 2001), and earlier EXOSAT (Collier Cameron et al. 1988) and ASCA/EUVE (Mewe et al. 1996) data, as well, requiring a variability by a factor of 5. The physical reason for the variability remains unknown, since (outside flares) the observed broad band variability of AB Dor is much smaller.

Simultaneous Observations of Variability at All Atmospheric Levels of V824 Arae (HD 155555)

We conducted a multiwavelength campaign observing V824 Ara (HD 155555, G5 IV + K0 IV–V) continuously throughout one complete orbital cycle (~1.7 days) in early May of 1996. At the core of this campaign were observations using the GHRS on the Hubble Space Telescope (HST). In all, about 48,000 spectra, many in rapid readout mode, were obtained with the GHRS covering the C IV, Mg II, and Fe XXI wavelength regions at 11–15 separate phases. Simultaneous observations were made with the Extreme Ultraviolet Explorer (EUVE). Radio observations (3.5 and 6 cm) were conducted at the Australian Telescope, while ground-based visual spectroscopic and photometric observations were made at European Southern Observatory, Cerro Tololo Inter-American Observatory, the Anglo-Australian Telescope, and South African Astronomical Observatory. Additional ground-based observations were obtained before, during, and after the campaign. Our primary intent was to obtain a three-dimensional model of the atmosphere extending from the photosphere to the corona. Variability was clearly detected, including several flares observed in the HST, EUVE, and radio data. We present results from modeling the ultraviolet transition region lines using an anisotropic macroturbulence model. Previous studies of transition region lines in late-type active stars have used multiple Gaussians to fit the observed line profiles, adding broad components to account for the extended wings observed in several active systems, including V711 Tau (HR 1099). This broad component has been interpreted as arising from the continuous presence of microflaring. We demonstrate that anisotropic macroturbulence models can also explain the observed Mg II profiles.