1,660 research outputs found
Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star
We have derived the mean broadening profile of the star V102 in the region of
the open cluster IC4665 from high resolution spectroscopy. At a projected
equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong
deviation from classical rotation. We discuss several scenarios, the most
plausible being strong differential rotation in latitudinal direction. For this
scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8
rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a
spectral type of A9 and support photometric measurements classifying IC4665
V102 as a non-member of IC4665. At such early spectral type this is the
strongest case of differential rotation observed so far. Together with three
similar stars, IC4665 V102 seems to form a new class of objects that exhibit
extreme latitudinal shear in a very shallow convective envelope.Comment: accepted for A&A Letter
Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F and later
and derived ``overall'' broadening functions in selected wavelength regions
utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of
the projected rotational velocity were derived from the first zero
of the Fourier transformed profiles and the shapes of the profiles were
analyzed for effects of differential rotation. The broadening profiles of 70
stars rotating faster than km s show no indications of multiplicity nor of spottedness. In
those profiles we used the ratio of the first two zeros of the Fourier
transform to search for deviations from rigid rotation. In the vast
majority the profiles were found to be consistent with rigid rotation. Five
stars were found to have flat profiles probably due to cool polar caps, in
three stars cuspy profiles were found. Two out of those three cases may be due
to extremely rapid rotation seen pole on, only in one case ( km
s) solar-like differential rotation is the most plausible explanation
for the observed profile. These results indicate that the strength of
differential rotation diminishes in stars rotating as rapidly as v \sin{i} \ga
50 km s.Comment: 10 pages, accepted for publication in A&
Can stellar activity make a planet seem misaligned?
Several studies have shown that the occultation of stellar active regions by
the transiting planet can generate anomalies in the high-precision transit
light curves, and these anomalies may lead to an inaccurate estimate of the
planetary parameters (e.g., the planet radius). Since the physics and geometry
behind the transit light curve and the Rossiter- McLaughlin effect
(spectroscopic transit) are the same, the Rossiter-McLaughlin observations are
expected to be affected by the occultation of stellar active regions in a
similar way. In this paper we perform a fundamental test on the spin-orbit
angles as derived by Rossiter-McLaughlin measurements, and we examine the
impact of the occultation of stellar active regions by the transiting planet on
the spin-orbit angle estimations. Our results show that the inaccurate
estimation on the spin-orbit angle due to stellar activity can be quite
significant (up to 30 degrees), particularly for the edge-on, aligned, and
small transiting planets. Therefore, our results suggest that the aligned
transiting planets are the ones that can be easily misinterpreted as misaligned
owing to the stellar activity. In other words, the biases introduced by
ignoring stellar activity are unlikely to be the culprit for the highly
misaligned systems.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Estimates of Active Region Area Coverage through Simultaneous Measurements of He I 5876 and 10830 Lines
Simultaneous, high-quality measurements of the neutral helium triplet
features at 5876~\AA\ and 10830~\AA, respectively, in a sample of solar-type
stars are presented. The observations were made with ESO telescopes at the La
Silla Paranal Observatory under program ID 088.D-0028(A) and MPG Utility Run
for FEROS 088.A-9029(A). The equivalent widths of these features combined with
chromospheric models are utilized to infer the fractional area coverage, or
filling factor, of magnetic regions outside of spots. We find that the majority
of the sample is characterized by filling factors less than unity. However,
discrepancies occur among the coolest K-type and warmest and most rapidly
rotating F-type dwarf stars. We discuss these apparently anomalous results and
find that in the case of K-type stars they are an artifact of the application
of chromospheric models best suited to the Sun than to stars with significantly
lower . The case of the F-type rapid rotators can be explained
with the measurement uncertainties of the equivalent widths, but they may also
be due to a non-magnetic heating component in their atmospheres. With the
exceptions noted above, preliminary results suggest that the average heating
rates in the active regions are the same from one star to the other, differing
in the spatially integrated, observed level of activity due to the area
coverage. Hence, differences in activity in this sample are mainly due to the
filling factor of active regions.Comment: Accepted for publication in The Astrophysical Journa
Rapid magnetic flux variability on the flare star CN Leonis
We present UVES/VLT observations of the nearby flare star CN Leo covering the
Wing-Ford FeH band near 1mu with high spectral resolution. Some of the FeH
absorption lines in this band are magnetically sensitive and allow a
measurement of the mean magnetic flux on CN Leo. Our observations, covering
three nights separated by 48 hours each, allow a clear detection of a mean
magnetic field of Bf~2.2kG. The differential flux measurements show a
night-to-night variability with extremely high significance. Finally, our data
strongly suggest magnetic flux variability on time scales as low as 6 hours in
line with chromospheric variability.Comment: 4 pages, accepted for publication as A&A Lette
The Origin of Enhanced Activity in the Suns of M67
We report the results of the analysis of high resolution photospheric line
spectra obtained with the UVES instrument on the VLT for a sample of 15
solar-type stars selected from a recent survey of the distribution of H and K
chromospheric line strengths in the solar-age open cluster M67. We find upper
limits to the projected rotation velocities that are consistent with solar-like
rotation (i.e., v sini ~< 2-3 km/s) for objects with Ca II chromospheric
activity within the range of the contemporary solar cycle. Two solar-type stars
in our sample exhibit chromospheric emission well in excess of even solar
maximum values. In one case, Sanders 1452, we measure a minimum rotational
velocity of vsini = 4 +/- 0.5 km/s, or over twice the solar equatorial
rotational velocity. The other star with enhanced activity, Sanders 747, is a
spectroscopic binary. We conclude that high activity in solar-type stars in M67
that exceeds solar levels is likely due to more rapid rotation rather than an
excursion in solar-like activity cycles to unusually high levels. We estimate
an upper limit of 0.2% for the range of brightness changes occurring as a
result of chromospheric activity in solar-type stars and, by inference, in the
Sun itself. We discuss possible implications for our understanding of angular
momentum evolution in solar-type stars, and we tentatively attribute the rapid
rotation in Sanders 1452 to a reduced braking efficiency.Comment: accepted by Ap
Multi-wavelength observations of Proxima Centauri
We report simultaneous observations of the nearby flare star Proxima Centauri
with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and
X-ray observations cover the star's quiescent state, as well as its flaring
activity and allow us to probe the stellar atmospheric conditions from the
photosphere into the chromosphere, and then the corona during its different
activity stages. Using the X-ray data, we investigate variations in coronal
densities and abundances and infer loop properties for an intermediate-sized
flare. The optical data are used to investigate the magnetic field and its
possible variability, to construct an emission line list for the chromosphere,
and use certain emission lines to construct physical models of Proxima
Centauri's chromosphere.
We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA
during the more active states of Proxima Centauri. For the intermediate flare,
we find two secondary flare events that may originate in neighbouring loops,
and discuss the line asymmetries observed during this flare in H i, He i, and
Ca ii lines. The high time-resolution in the H alpha line highlights strong
temporal variations in the observed line asymmetries, which re-appear during a
secondary flare event. We also present theoretical modelling with the stellar
atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&
Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal
In this paper, we present new spectropolarimetric observations of the
planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters
at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL),
respectively. We detected the magnetic field of the star at three epochs in
2008. It is a weak magnetic field of only a few Gauss, oscillating between a
predominant toroidal component in January and a dominant poloidal component in
June and July. A magnetic polarity reversal was observed relative to the
magnetic topology in June 2007. This is the second such reversal observed in
two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2
years. This is the first detection of a magnetic cycle for a star other than
the Sun. The role of the close-in massive planet in the short activity cycle of
the star is questioned.
Tau Boo has strong differential rotation, a common trend for stars with
shallow convective envelope. At latitude 40 deg., the surface layer of the star
rotates in 3.31 d, equal to the orbital period. Synchronization suggests that
the tidal effects induced by the planet may be strong enough to force at least
the thin convective envelope into corotation. Tau Boo shows variability in the
Ca H & K and Halpha throughout the night and on a night to night time scale. We
do not detect enhancement in the activity of the star that may be related to
the conjunction of the planet. Further data is needed to conclude about the
activity enhancement due to the planet.Comment: 9 pages, 5 figures, 3 tables Accepted to MNRA
The lived user experience of virtual environments: Initial steps of a phenomenological analysis in a safety training setting
Virtual environments (VEs) are making their way into various sectors of life to enhance and support human activity, including learning. VEs have been used in various contexts for training, and in many cases they are designed to model or simulate - as accurately and authentically as possible - a specific work context. In striving for authenticity, visual and representative realism tends to receive most of the development input, despite of several studies that challenge its importance. New training avenues have raised the importance of rigorous phenomenological descriptions for a deeper understanding of user experience in the actual context of use. This paper reports the preliminary steps in a phenomenological analysis of how employees working in actual hazardous settings experience virtual safety training environments. Such open-ended research project can reveal new aspects of user experience that can advice the development and evaluation of human-computer interaction in digital technology-enhanced training contexts
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