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News, highlights from the publications of the Stellar Activity group.
Spectangular - a software tool for spectral disentangling
The software Spectangular is for spectral disentangling via singular value decomposition with global optimization of the orbital parameters of the stellar system or radial velocities of the individual observations.
Transmission spectroscopy of HAT-P-19b
Hot Jupiter exoplanets are gas giants orbiting their host star in only a few days. If their orbit lie in the same plane as our line of sight, the planet transits its host star once per orbit. This event yields the opportunity to study the atmosphere of this far-away world. We investigated the inflated hot Jupiter HAT-P-19b for spectroscopic signatures of a cloud-free atmosphere.
Spot evolution on the red giant star XX Triangulum
Solar spots can be observed in great detail and their evolution can be studied in real-time. Their decay rate is of great interest as it is directly related to the magnetic diffusivity, which itself is a key quantity for the length of a magnetic-activity cycle. Is a linear spot decay also detectable on other stars, and is this in agreement with the large range of solar and stellar activity cycle lengths?
Do we observe the underlying dynamo?
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 have determined whether mean-field dynamo models of late-F type dwarf stars can reproduce the surface features - in particular the high-latitude spots - recovered in Doppler maps.
The Radius Discrepancy in Low-mass Stars
A long-standing issue in the theory of low mass stars is the discrepancy between predicted and observed radii and effective temperatures. In spite of the increasing availability of very precise radius determinations from Detached Eclipsing Binaries (DEBs) and direct interferometric measurements of radii of single stars, there is no unanimous consensus on the extent (or even the existence) of the discrepancy and on its connection with other stellar properties (e.g. metallicity, magnetic activity). The best data currently available (i.e. with relative error below 5%) are used to investigate the radius discrepancy phenomenon.
Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae
The single rapidly-rotating G0 giant 31 Comae has been a puzzle because of the absence of photometric variability despite its strong chromospheric and coronal emissions. As a Hertzsprung-gap giant, it is expected to be at the stage of rearranging its moment of inertia, hence likely also its dynamo action, which could possibly be linked with its missing photospheric activity. Our aim is to detect photospheric activity, obtain the rotation period, and use it for a first Doppler image of the star's surface. Its morphology could be related to the evolutionary status.
Tables of the list of Doppler images
A summary of the results of the Doppler imaging technique.
First time-series optical photometry from Antarctica
Eradicating the problems associated with the Earth's day-night cycle is mandatory for long and continuous time-series photometry and had been achieved with either large ground-based networks of observatories at different geographic longitudes or when conducted from space. A third possibility is offered by a polar location with astronomically-qualified site characteristics. We present the first scientific stellar time-series optical photometry from Dome C in Antarctica and analyze approximately 13,000 CCD frames acquired in July 2007. The optical pilot telescope of the "International Robotic Antarctic Infrared Telescope'', named "small IRAIT'' (sIRAIT), and its UBVRI CCD photometer were used in BVR for a continuous 243 hours (10.15 days) with a duty cycle of 98 % and a cadence of 155 s. The prime targets were the chromospherically active, spotted binary star V841 Cen and the non-radially pulsating delta Scuti star V1034 Cen.
Light-curve inversions with truncated least-squares principal components: Tests and application to HD 291095 = V1355 Orionis
We present a new inversion code that reconstructs the stellar surface spot configuration from the light curve of a rotating star. Our code employs a method that uses the truncated least-squares estimation of the inverse problem’s objects principal components. We use spot filling factors as the unknown objects. Various test cases that represent a rapidly rotating K subgiant are used for the forward problem.
Zeeman-Doppler imaging of II Peg
Late-type stars in general possess complicated magnetic surface fields which makes their detection and in particular their modeling and reconstruction challenging. In this work we present a new Zeeman-Doppler imaging code which is especially designed for the application to late-type stars. This code uses a new multi-line cross-correlation technique by means of a principal component analysis to extract and enhance the quality of individual polarized line profiles.
HD 6286 - active, triple, ellipsoidal and eclipsing binary
We present a detailed analysis of the star HD 6286 = BE Psc from 16 years of spectroscopic and 19 years of photometric observations. The star is an evolved, chromospherically active and eclipsing binary consisting of a K1 giant plus a F6 dwarf/subgiant in a circular orbit with a period of 35.671 days. A faint, close visual companion of spectral type ≈G0 makes the system triple.
The ellipsoidal K giant binary ζ Andromedae
We present the first Doppler images of the bright RS CVn-type binary ζ And. The star is a magnetically active K1 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.
HD 1405 - First Doppler images of the very young K2-dwarf PW Andromedae
We present the first Doppler images of the single pre-main-sequence star PW And. Its position in the HR-diagram suggests it to be in the rapid-braking phase just prior to arrival on the ZAMS. Images are obtained from seven individual spectral lines as well as from 5-nm wide chunks of spectrum that invert a total of 58 line profiles simultaneously.
Spatially resolving the accretion shocks on the rapidly-rotating M0 T-Tauri star MN Lupi
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.
LQ Hya
We reconstruct a time series of 28 surface temperature maps (Doppler-images) of the spotted single K2-dwarf LQ Hya from 35 consecutive stellar rotations in Nov.-Dec. 1996. Two more maps are obtained from data in late April and early May 2000, just two weeks apart. All maps show spot activity preferably at low latitudes between -20° and +50°, with a concentration in a band centered at around +30°, and with only occasional evidence for a higher-latitude spot extension.
HD 171488
We present the first Doppler image of the single G0-dwarf HD 171488. As a 30-Myr young field star with a lithium abundance of ~140-times that of the Sun and a rotation 20 times faster, it is hardly a "solar analog" but could be coined an "infant Sun". Its position in the H-R diagram suggests it to be in the rapid-braking phase just prior to arrival on the ZAMS.
HD 31993
We present two Doppler images from two consecutive stellar rotations of the single K2-giant HD 31993. Each Doppler image is reconstructed from spectra obtained within a single stellar rotation.
The σ² CrB binary system
Pseudo three-dimensional view of the binary system. The maps are those from the full-spectrum inversion. Note that the obtained ratio of the stellar radii of the two components is 0.955 but that they are shown at different size during the two conjunction phases (90° and 270° , respectively) for better 3-D viewing.
UZ Librae
We present eight Doppler images of UZ Librae from the years 1994, 1996, 1997, 1998 and 2000. These maps indicate a preferred temperature distribution from one year to the other and suggest preferred spot locations for at least the seven years of our observations.
The Pleiades G-dwarf HII314
We present the first Doppler image of a solar-type G dwarf in the Pleiades open cluster obtaine from high-resolution Gecko CFHT spectra.
σ Geminorum
We present a simultaneous photometric and spectroscopic imaging analysis of the long-period RS CVn binary sigma Gem, covering 3.6 consecutive rotation cycles with high time resolution.
Smallest KPNO telescope discovers biggest starspot on XX Tri (HD 12545)
Imagine a sunspot group with dimensions exceeding the size of the entire Sun! Precisely, a size of 12 x 20 solar radii, i.e. 60 times the extension of the largest sunspot group ever observed or 10,000 times its areal surface coverage. Hard to do? Not if this super starspot is located on the surface of the K0 giant star XX Triangulum (HD 12545), a cool star that is approximately ten times larger and two times more massive than the Sun. In any case, this spot is by far the largest ever observed and will create some miracles for theorists yet to be explained. This discovery was announced by a group of astronomers led by Klaus G. Strassmeier of the University of Vienna, Austria, and was based on observations made with one of the smallest telescopes on Kitt Peak, the 0.9-m coude feed telescope.
Rotation, activity, and lithium in cool stars
We have used the SES@STELLA-I as well as the Amadeus and/or the Wolfgang to obtain time-series high-resolution optical echelle spectroscopy and VI and/or by photometry for a sample of 60 active stars.