J. B. Rice

The magnetic field topology of the weak-lined T Tauri star V410 Tauri - New strategies for Zeeman-Doppler imaging

We present a detailed temperature and magnetic investigation of the T Tauri star V410 Tau by means of a simultaneous Doppler- and Zeeman-Doppler Imaging. Moreover we introduce a new line profile reconstruction method based on a singular value decomposition (SVD) to extract the weak polarized line profiles. One of the key features of the line profile reconstruction is that the SVD line profiles are amenable to radiative transfer modeling within our Zeeman-Doppler Imaging code iMap. The code also utilizes a new iterative regularization scheme which is independent of any additional surface constraints. To provide more stability a vital part of our inversion strategy is the inversion of both Stokes I and Stokes V profiles to simultaneously reconstruct the temperature and magnetic field surface distribution of V410 Tau. A new image-shear analysis is also implemented to allow the search for image and line profile distortions induced by a differential rotation of the star. The magnetic field structure we obtain for V410 Tau shows a good spatial correlation with the surface temperature and is dominated by a strong field within the cool polar spot. The Zeeman-Doppler maps exhibit a large-scale organization of both polarities around the polar cap in the form of a twisted bipolar structure. The magnetic field reaches a value of almost 2 kG within the polar region but smaller fields are also present down to lower latitudes. The pronounced non-axisymmetric field structure and the non-detection of a differential rotation for V410 Tau supports the idea of an underlying

Spatially resolving the accretion shocks on the rapidly-rotating M0 T-Tauri star MN Lupi

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Doppler imaging of stellar surface structure - XIX. The solar-type components of the close binary

-type dynamo, which is predicted for weak-lined T Tauri stars.

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We obtained high-resolution, high-quality VLT/UVES spectra to reconstruct the two-dimensional surface structure of the rapidly-rotating classical T-Tauri star MN Lupi on two separate nights. Both surface maps show a structured warm (5000 K) band centered around the pole at a latitude of ≈65 ◦ . Located within the band are two hot spots with temperatures of approximately or possibly even in excess of 5800 K, i.e. 2000 K above the effective photospheric temperature. Both maps appear with an adjacent equatorial band of temperature 3400 K, some 400−500 K below the effective photospheric temperature. While we interpret the two hot spots and the warm high-latitude band to be the heating points from two accretion impacts at the time of our observations and their redistributed energy trailed due to the fast stellar rotation, respectively, the cool equatorial band may not be cool after all but due to obscuration of the stellar surface by the innermost region of the disk. The fact that the hot spots appear at high stellar latitude is in agreement with the magnetospheric accretion model that proposes material funnelling onto the star along a predominantly dipolar magnetic field at roughly 50 ◦ latitude. The evidence of ongoing disk accretion, together with the very fast rotation of MN Lupi of just 3−4 times below its break-up velocity, suggests that the accretion mechanism is the cause of its rapid surface rotation. We present a model of magnetic star-disk coupling for MN Lupi that predicts a polar surface magnetic field of ≈ 3k G.

Coronae Borealis

We present the first Doppler image for both stellar components of the F9+G0 ZAMS binary σ 2 CrB and found evidence for the coexistence of cool and warm spots on both stars. Cool spots appear mainly at polar or high latitudes while a confined equatorial warm belt appears on the trailing hemisphere of each of the two stars with respect to the orbital motion. We also present an update of the TempMap imaging code that allows us to solve the stellar surface temperature distribution on both binary components simultaneously, including photometric input. Several test reconstructions are performed to demonstrate its reliability and robustness. Our new orbital solution results in very precise masses for both components - good to 0.4% - and confirms the spectral classifications of F9 and G0 for the primary and secondary, respectively. The visual component, σ 1 CrB, seems to be G4 rather than G0. All three components are on or very close to the ZAMS which is also confirmed by the relatively high lithium abundance of about twenty times the solar abundance. Photometric light variations are detected with a period of 1.157 ± 0.002 days that we interpret to be the rotation period of both binary components. A 0. m 04 dimming in y together with a reddening of 0. m 01 in b − y during the year 2000 suggests a long-term spot variability compatible with a period of at least 260 days.

Doppler imaging of stellar surface structure. XVI. A time-series analysis of the moderately-rotating K1-giant σ Geminorum

We present a simultaneous photometric and spectroscopic imaging analysis of the long-period RS CVn binary σ Gem, covering 3.6 consecutive rotation cycles with high time resolution. From six overlapping but consecutive Doppler maps we trace the evolution of individual spots throughout the time range covered. All spots group either along a band at approximately +45°latitude and a width of 30°, or appear centered at the equator. No polar spot is detected. We did not find a conclusive migration pattern from the cross-correlation maps from one rotation to the next and attribute this to a masking effect of short-term spot changes.

Binary-induced magnetic activity? ⋆ Time-series echelle spectroscopy and photometry of HD 123351 = CZ CVn

Context. Multi-wavelength time-series observations with high cadence and long duration are needed to resolve and understand the many variations of magnetically active late-type stars, which is an approach often used to observe the Sun. Aims. We present a first and detailed study of the bright and active K 0IV-III star HD 123351. Methods. We acquired a total of 955 high-resolution STELLA echelle spectra during the years 2006‐2010 and a total of 2,260 photometric V IC data points during 1998‐2010. These data are complemented by some spectra from CFHT and KPNO. Results. The star is found to be a single-lined spectroscopic binary with a period of 147.8919±0.0003 days and a large eccentricity of e=0.8086±0.0001. The rms of the orbital solution is just 47 m s −1 , making it the most precise orbit ever obtained for an active binary system. The rotation period is constrained from long-term photometry to be 58.32±0.01 days. It shows that HD 123351 is a very asynchronous rotator, rotating five times slower than the expected pseudo-synchronous value. Two spotted regions persisted

Doppler imaging of stellar surface structure XXIII. the ellipsoidal K giant binary ζ Andromedae

Aims. We present the first Doppler images of the bright RS CVn-type binary ζ And. The star is a magnetically active Kl giant with its rotation synchronized to the 17.8-day orbital period. Our revised lithium abundance of log n = 1.2 places ζ And in the vicinity of Li-rich RGB stars but it is nevertheless a Li-normal chromospherically active binary star. The star seems to undergo its first standard dredge-up dilution. Methods. Four consecutive Doppler images were obtained from a continuous 67-night observing run at NSO-McMath in 1996/97. An additional single image was obtained from a continuous 19-night run at KPNO in 1997/98. These unique data allow to compute a small time series of the evolution of the stars surface structure. All line-profile inversions are done with a modified TempMap version that takes into account the non-spherical shape of the star. Representative test reconstructions are performed and demonstrate the codes reliability and robustness. Results. High and low-latitude spot activity was recovered together with an asymmetric polar cap-like feature. The latter dominated the first half of the two-month time series in 1996/97. The second half showed mostly medium-to-high latitude activity and only a fainter polar spot. The coolest areas were restored with a temperature contrast of about 1000 ± 200 K. Some weaker features at equatorial latitudes were also recovered but these could be partially spurious and appear blurred due to imperfect phase coverage. We use our line profiles to reconstruct an average non-sphericity of R pole /R point = 0.96 which would, if not taken into account, mimic a temperature difference pole-to-equator of ≈ 220 K, especially at the phases of quadrature. Finally, we apply two different methods for restoring surface differential rotation and found a weak solar-type rotation law with a shear ΔΩ ≈ 0.95°/day (α = ΔΩ/Ω eq = +0.049 ± 0.003), i.e. roughly a factor of four weaker at a rotation rate roughly 1.5 times faster than the Suns.

The William-Wehlau Spectropolarimeter: Observing Hot Stars in All Four Stokes Parameters

We introduce a new polarimeter unit, which, mounted at the Cassegrain focus of any telescope and fiber-connected to a fixed CCD spectrograph, is able to measure all Stokes parameters I, Q, U, and V across spectral Lines of bright stellar targets and other

Doppler imaging of the helium-variable star a Centauri

Aims. The helium-peculiar star a Cen exhibits interesting line profile variations of elements such as iron, nitrogen and oxygen in addition to its well-known extreme helium variability. The objective of this paper is to use new high signal-to-noise, high-resolution spectra to perform a quantitative measurement of the helium, iron, nitrogen and oxygen abundances of the star and determine the relation of the concentrations of the heavier elements on the surface of the star to the helium concentration and perhaps to the magnetic field orientation. Methods. Doppler images have been created for the elements helium, iron, nitrogen and oxygen using the programs described in earlier papers by Rice and others. An alternative surface abundance mapping code has been used to model the helium line variations after our Doppler imaging of certain individual helium lines produced mediocre results. Results. Doppler imaging of the helium abundance of a Cen confirms the long-known existence of helium-rich and helium-poor hemispheres on the star and we measure a difference of more than two orders of magnitude in helium abundance from one side of the star to the other. Helium is overabundant by a factor of about 5 over much of the helium-rich hemisphere. Of particular note is our discovery that the helium-poor hemisphere has a very high abundance of 3 He, approximately equal to the 4 He abundance. a Cen is therefore a new member of the small group of helium-3 stars and the first well-established magnetic member of the class. For the three metals investigated here, there are two strong concentrations of abundance near the equator at longitude roughly 135 ◦ consistent with the positive magnetic maximum and two somewhat weaker concentrations of abundance near longitude 315 ◦ on the equator near where the helium concentration is centered and roughly where the negative peak of the magnetic field would be found. Another strong concentration is found near the equator at about longitude 45 ◦ and this is not explainable in terms of any simple symmetry with the