S. P. Järvinen

Spin up in RX J0806+15: the shortest period binary

RX J0806+15 has recently been identified as the binary system with the shortest known orbital period. We present a series of observations of RX J0806+15, including new optical observations taken one month apart. Using these observations and archival data, we find that the period of this system is decreasing over time. Our measurements imply that f = 6.11 x 10 - 1 6 Hz s - 1 , which is in agreement with a rate expected from the gravitational radiation for two white dwarfs orbiting at a given period. However, a smaller value off = 3.14 x 10 - 1 6 Hz s - 1 cannot be ruled out. Our result supports the idea that the 321.5-s period is the orbital period, that the system is the shortest period binary known so far and that it is one of the strongest sources of constant gravitational radiation in the sky. Furthermore, the decrease of the period strongly favours the unipolar inductor (or electric star) model rather than the accretion models.

EK Draconis - Magnetic activity in the photosphere and chromosphere

Context. As a young solar analogue, EKxa0Draconis provides an opportunity to study the magnetic activity of the infant Sun. Aims. We present three new surface temperature maps of EKxa0Draconis and compare them with previous results obtained from long-term photometry. Furthermore, we determined a set of stellar parameters and compared the determined values with the corresponding solar values. Methods. Atmospheric parameters were determined by comparing observed and synthetic spectra calculated with stellar atmosphere models. Surface temperature maps were obtained using the Occamian approach inversion technique. The differential rotation of EKxa0Dra was estimated using two different methods. Results. A detailed model atmosphere analysis of high resolution spectra of EKxa0Dra has yielded a self-consistent set of atmospheric parameters: T eff xa0= 5750xa0K, log gxa0= 4.5, [M/H]xa0= 0.0,

Doppler images and the underlying dynamo: the case of AF Leporis

xi_{rm t}

Want a PEPSI? Performance status of the recently commissioned high-resolution spectrograph and polarimeter for the 2x8.4m Large Binocular Telescope

xa0= 1.6xa0kmu2009s -1 . The evolutionary models imply that the star is slightly more massive than the Sun and has an age between 30-50xa0Myr, which agrees with the determined lithium abundance of

Mapping EK Draconis with PEPSI. Evidence for starspot penumbrae

log N

The nature of the X-ray flash of August 24 2005 - Photometric evidence for an on-axis z

(Li)xa0= 3.02. Moreover, the atmospheric parameters, as well as the wings of the Caxa0iixa08662xa0A, indicate that the photosphere of EKxa0Dra is very similar to the one of the present Sun, while their chromospheres differ. There also seems to be a correlation between magnetic features seen in the photosphere and chromosphere. The temperature images reveal spots of only 500xa0K cooler than the quiet photosphere. The mean spot latitude varies with time. The obtained differential rotation is very small, but the sign of it supports solar type differential rotation on EKxa0Dra.

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The (Zeeman-)Doppler imaging studies of solar-type stars very often reveal large high-latitude spots. This also includes F stars that possess relatively shallow convection zones, indicating that the dynamo operating in these stars differs from the solar dynamo. We aim to determine whether mean-field dynamo models of late-F type dwarf stars can reproduce the surface features recovered in Doppler maps. In particular, we wish to test whether the models can reproduce the high-latitude spots observed on some F dwarfs. The photometric inversions and the surface temperature maps of AF Lep were obtained using the Occamian-approach inversion technique. Low signal-to-noise spectroscopic data were improved by applying the least-squares deconvolution method. The locations of strong magnetic flux in the stellar tachocline as well as the surface fields obtained from mean-field dynamo solutions were compared with the observed surface temperature maps. The photometric record of AF Lep reveals both long- and short-term variability. However, the current data set is too short for cycle-length estimates. From the photometry, we have determined the rotation period of the star to be 0.9660+-0.0023 days. The surface temperature maps show a dominant, but evolving, high-latitude (around +65 degrees) spot. Detailed study of the photometry reveals that sometimes the spot coverage varies only marginally over a long time, and at other times it varies rapidly. Of a suite of dynamo models, the model with a radiative interior rotating as fast as the convection zone at the equator delivered the highest compatibility with the obtained Doppler images.

burst with continuous energy injection and an associated supernova?

PEPSI is the new fiber-fed and stabilized “Potsdam Echelle Polarimetric and Spectroscopic Instrument” for the Large Binocular Telescope (LBT). It covers the entire optical wavelength range from 384 to 913 nm in three exposures at resolutions of either R=λ/▵λ=50,000, 130,000 or 250,000. The R=130,000 mode can also be used with two dual-beam Stokes IQUV polarimeters. The 50,000-mode with its 12-pix sampling per resolution element is our “bad seeing” or “faint-object” mode. A robotic solar-disk-integration (SDI) telescope feeds solar light to PEPSI during day time and a 450-m fiber feed from the 1.8m VATT can be used when the LBT is busy otherwise. CCD characterization and a removal procedure for the spatial fixed-pattern noise were the main tasks left from the commissioning phase. Several SDI spectral time series with up to 300 individual spectra per day recovered the well-known solar 5-minute oscillation at a peak of 3 mHz (5.5min) with a disk-integrated radial-velocity amplitude of only 47 cm/s. Spectral atlases for 50 bright benchmark stars including the Sun were recently released to the scientific community, among them the ancient planet- system host Kepler-444. These data combine PEPSI’s high spectral resolution of R=250,000 with signal-to-noise ratio (S/N) of many hundreds to even thousands covering the entire optical to near-infrared wavelength range from 384 to 913 nm. Other early science cases were exoplanet transits including TRAPPIST-1, a spectrum of Boyajians star that revealed strong and structured but stable ISM Na D lines, a spectrum of Oph allowing a redetermination of the ISM Li line doublet, and a first Doppler image of the young solar analog EK Dra that revealed starspots with solar-like penumbrae.

Active Longitudes and Flip-Flops in Binary Stars

We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in spectral line profiles are inverted to a stellar surface temperature map using our

Magnetic activity in the photosphere and chromosphere

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