572 research outputs found
PEPSI deep spectra. I. The Sun-as-a-star
As part of the first Potsdam Echelle Polarimetric and Spectroscopic
Instrument (PEPSI) key-science project, we aim to provide well-exposed (viz.
deep) high-resolution spectra of representative stellar targets. These spectra
will be made available in form of (electronic) atlases. The first star in this
series of papers is our Sun. It also acts as a system-performance cornerstone.
The deep spectra in this paper are the results of combining up to 100
consecutive exposures per wavelength setting and are compared with other solar
flux atlases. Our software for the optimal data extraction and reduction of
PEPSI spectra is described and verified with the solar data. Three deep solar
flux spectra with a spectral resolution of up to 270,000, a continuous
wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio
(S/N) of between 2,000-8,000:1 depending on wavelength are presented.
Additionally, a time-series of 996 high-cadence spectra in one cross disperser
is used to search for intrinsic solar modulations. The wavelength calibration
based on Th-Ar exposures and simultaneous Fabry-Perot combs enables an absolute
wavelength solution within 10 m/s (rms) with respect to the HARPS laser-comb
solar atlas and a relative rms of 1.2 m/s for one day. For science
demonstration, we redetermined the disk-average solar Li abundance to
1.09+/-0.04 dex on the basis of 3D NLTE model atmospheres. We detected
disk-averaged p-mode RV oscillations with a full amplitude of 47 cm/s at 5.5
min. Comparisons with two solar FTS atlases, as well as with the HARPS solar
atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are
being taken with a sampling of one deep spectrum per day, and are supposed to
continue a full magnetic cycle of the Sun.Comment: in press, 13 pages, 8 figures, data available from pepsi.aip.d
PEPSI deep spectra. II. Gaia benchmark stars and other M-K standards
We provide a homogeneous library of high-resolution, high-S/N spectra for 48
bright AFGKM stars, some of them approaching the quality of solar-flux spectra.
Our sample includes the northern Gaia benchmark stars, some solar analogs, and
some other bright Morgan-Keenan (M-K) spectral standards. Well-exposed deep
spectra were created by average-combining individual exposures. The
data-reduction process relies on adaptive selection of parameters by using
statistical inference and robust estimators.We employed spectrum synthesis
techniques and statistics tools in order to characterize the spectra and give a
first quick look at some of the science cases possible. With an average
spectral resolution of R=220,000 (1.36 km/s), a continuous wavelength coverage
from 383 nm to 912 nm, and S/N of between 70:1 for the faintest star in the
extreme blue and 6,000:1 for the brightest star in the red, these spectra are
now made public for further data mining and analysis. Preliminary results
include new stellar parameters for 70 Vir and alpha Tau, the detection of the
rare-earth element dysprosium and the heavy elements uranium, thorium and
neodymium in several RGB stars, and the use of the 12C to 13C isotope ratio for
age-related determinations. We also found Arcturus to exhibit few-percent CaII
H&K and H-alpha residual profile changes with respect to the KPNO atlas taken
in 1999.Comment: in press, 15 pages, 7 figures, data available from pepsi.aip.d
Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae
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 Map code. The long-term photometric record is employed to compare our
map with previously published maps. Four cool spots were reconstructed, but no
polar spot was seen. The temperature difference to the photosphere of the spots
is between 990 and 280K. Two spots are reconstructed with a typical solar
morphology with an umbra and a penumbra. For the one isolated and relatively
round spot (A), we determine an umbral temperature of 990K and a penumbral
temperature of 180K below photospheric temperature. The umbra to photosphere
intensity ratio of EK Dra is approximately only half of that of a comparison
sunspot. A test inversion from degraded line profiles showed that the higher
spectral resolution of PEPSI reconstructs the surface with a temperature
difference that is on average 10% higher than before and with smaller surface
areas by 10-20%. PEPSI is therefore better suited to detecting and
characterising temperature inhomogeneities. With ten more years of photometry,
we also refine the spot cycle period of EK Dra to 8.90.2 years with a
continuing long-term fading trend. The temperature morphology of spot A so far
appears to show the best evidence for the existence of a solar-like penumbra
for a starspot. We emphasise that it is more the non-capture of the true umbral
contrast rather than the detection of the weak penumbra that is the limiting
factor. The relatively small line broadening of EK Dra, together with the only
moderately high spectral resolutions previously available, appear to be the
main contributors to the lower-than-expected spot contrasts when comparing to
the Sun.Comment: Accepted for A&
PEPSI deep spectra. III. A chemical analysis of the ancient planet-host star Kepler-444
We obtained an LBT/PEPSI spectrum with very high resolution and high
signal-to-noise ratio (S/N) of the K0V host Kepler-444, which is known to host
5 sub-Earth size rocky planets. The spectrum has a resolution of R=250,000, a
continuous wavelength coverage from 4230 to 9120A, and S/N between 150 and
550:1 (blue to red). We performed a detailed chemical analysis to determine the
photospheric abundances of 18 chemical elements, in order to use the abundances
to place constraints on the bulk composition of the five rocky planets. Our
spectral analysis employs the equivalent width method for most of our spectral
lines, but we used spectral synthesis to fit a small number of lines that
require special care. In both cases, we derived our abundances using the MOOG
spectral analysis package and Kurucz model atmospheres. We find no correlation
between elemental abundance and condensation temperature among the refractory
elements. In addition, using our spectroscopic stellar parameters and isochrone
fitting, we find an age of 10+/-1.5 Gyr, which is consistent with the
asteroseismic age of 11+/-1 Gyr. Finally, from the photospheric abundances of
Mg, Si, and Fe, we estimate that the typical Fe-core mass fraction for the
rocky planets in the Kepler-444 system is approximately 24 per cent. If our
estimate of the Fe-core mass fraction is confirmed by more detailed modeling of
the disk chemistry and simulations of planet formation and evolution in the
Kepler-444 system, then this would suggest that rocky planets in more
metal-poor and alpha-enhanced systems may tend to be less dense than their
counterparts of comparable size in more metal-rich systems.Comment: in press, 11 pages, 3 figures, data available from pepsi.aip.d
On a "New" Deformation of GL(2)
We refute a recent claim in the literature of a "new" quantum deformation of
GL(2).Comment: 4 pages, LATE
Warm and cool starspots with opposite polarities. A high-resolution Zeeman-Doppler-Imaging study of II Pegasi with PEPSI
We present a temperature and a magnetic-field surface map of the K2 subgiant
of the active binary II Peg. Employed are high resolution Stokes IV spectra
obtained with the new Potsdam Echelle Polarimetric and Spectroscopic Instrument
(PEPSI) at the Large Binocular Telescope (LBT). Our main result is that the
temperature features on II Peg closely correlate with its magnetic field
topology. We find a warm spot (350K warmer with respect to the effective
temperature) of positive polarity and radial field density of 1.1 kG coexisting
with a cool spot (780K cooler) of negative polarity of 2 kG. Several other cool
features are reconstructed containing both polarities and with (radial) field
densities of up to 2 kG. The largest cool spot is reconstructed with a
temperature contrast of 550 K, an area of almost 10% of the visible hemisphere,
and with a multipolar magnetic morphology. A meridional and an azimuthal
component of the field of up to +/-500G is detected in two surface regions
between spots with strong radial fields but different polarities. A force-free
magnetic-field extrapolation suggests that the different polarities of cool
spots and the positive polarity of warm spots are physically related through a
system of coronal loops of typical height of approx. 2 Rstar. While the H-alpha
line core and its red-side wing exhibit variations throughout all rotational
phases, a major increase of blue-shifted H-alpha emission was seen for the
phases when the warm spot is approaching the stellar central meridian
indicating high-velocity mass motion within its loop. We explain the warm spots
due to photospheric heating by a shock front from a siphon-type flow between
regions of different polarities while the majority of the cool spots is likely
formed due to the expected convective suppression like on the Sun.Comment: 12 pages, 8 figure
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