430 research outputs found
Recommended from our members
N 2 O emissions and NO 3 − leaching from two contrasting regions in Austria and influence of soil, crops and climate: a modelling approach
National emission inventories for UN FCCC reporting estimate regional soil nitrous oxide (N 2 O) fluxes by considering the amount of N input as the only influencing factor for N 2 O emissions. Our aim was to deepen the understanding of N 2 O fluxes from agricultural soils, including region specific soil and climate properties into the estimation of emission to find targeted mitigation measures for the reduction of nitrogen losses and GHG emissions. Within this project, N 2 O emissions and nitrate (NO 3 − ) leaching were modelled under spatially distinct environmental conditions in two agricultural regions in Austria taking into account region specific soil and climatic properties, management practices and crop rotations. The LandscapeDNDC ecosystem model was used to calculate N 2 O emissions and NO 3 − leaching reflecting different types of vegetation, management operations and crop rotations. In addition, N input and N fluxes were assessed and N 2 O emissions were calculated. This approach allowed identifying hot spots of N 2 O emissions. Results show that certain combinations of soil type, weather conditions, crop and management can lead to high emissions. Mean values ranged from 0.15 to 1.29 kg N 2 O–N ha −1 year −1 (Marchfeld) and 0.26 to 0.52 kg N 2 O–N ha −1 year −1 (Grieskirchen). Nitrate leaching, which strongly dominated N-losses, often reacted opposite to N 2 O emissions. Larger quantities of NO 3 − were lost during years of higher precipitation, especially if winter barley was cultivated on sandy soils. Taking into account the detected hot spots of N 2 O emissions and NO 3 − leaching most efficient measures can be addressed to mitigate environmental impacts while maximising crop production. © 2018, The Author(s)
Difference image photometry with bright variable backgrounds
Over the last two decades the Andromeda Galaxy (M31) has been something of a
test-bed for methods aimed at obtaining accurate time-domain relative
photometry within highly crowded fields. Difference imaging methods, originally
pioneered towards M31, have evolved into sophisticated methods, such as the
Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are
most widely used to survey variable stars, transients and microlensing events
in our own Galaxy. We show that modern difference image (DIA) algorithms such
as OIS, whilst spectacularly successful towards the Milky Way bulge, may
perform badly towards high surface brightness targets such as the M31 bulge.
Poor results can occur in the presence of common systematics which add spurious
flux contributions to images, such as internal reflections, scattered light or
fringing. Using data from the Angstrom Project microlensing survey of the M31
bulge, we show that very good results are usually obtainable by first
performing careful photometric alignment prior to using OIS to perform
point-spread function (PSF) matching. This separation of background matching
and PSF matching, a common feature of earlier M31 photometry techniques, allows
us to take full advantage of the powerful PSF matching flexibility offered by
OIS towards high surface brightness targets. We find that difference images
produced this way have noise distributions close to Gaussian, showing
significant improvement upon results achieved using OIS alone. We show that
with this correction light-curves of variable stars and transients can be
recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to
implement and is quick enough to be incorporated within real-time DIA
pipelines. (Abridged)Comment: 12 pages. Accepted for publication in MNRAS. Includes an expanded
discussion of DIA testing and results, including additional lightcurve
example
Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity
We investigate the nature of the long-period radial velocity variations in
Alpha Tau first reported over 20 years ago. We analyzed precise stellar radial
velocity measurements for Alpha Tau spanning over 30 years. An examination of
the Halpha and Ca II 8662 spectral lines, and Hipparcos photometry was also
done to help discern the nature of the long-period radial velocity variations.
Our radial velocity data show that the long-period, low amplitude radial
velocity variations are long-lived and coherent. Furthermore, Halpha equivalent
width measurements and Hipparcos photometry show no significant variations with
this period. Another investigation of this star established that there was no
variability in the spectral line shapes with the radial velocity period. An
orbital solution results in a period of P = 628.96 +/- 0.90 d, eccentricity, e
= 0.10 +/- 0.05, and a radial velocity amplitude, K = 142.1 +/- 7.2 m/s.
Evolutionary tracks yield a stellar mass of 1.13 +/- 0.11 M_sun, which
corresponds to a minimum companion mass of 6.47 +/- 0.53 M_Jup with an orbital
semi-major axis of a = 1.46 +/- 0.27 AU. After removing the orbital motion of
the companion, an additional period of ~ 520 d is found in the radial velocity
data, but only in some time spans. A similar period is found in the variations
in the equivalent width of Halpha and Ca II. Variations at one-third of this
period are also found in the spectral line bisector measurements. The 520 d
period is interpreted as the rotation modulation by stellar surface structure.
Its presence, however, may not be long-lived, and it only appears in epochs of
the radial velocity data separated by 10 years. This might be due to an
activity cycle. The data presented here provide further evidence of a planetary
companion to Alpha Tau, as well as activity-related radial velocity variations.Comment: 18 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
The CARMENES search for exoplanets around M dwarfs: Radial-velocity variations of active stars in visual-channel spectra
Previous simulations predicted the activity-induced radial-velocity (RV)
variations of M dwarfs to range from cm/s to km/s, depending on
various stellar and activity parameters. We investigate the observed relations
between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing
CARMENES high-resolution visual-channel spectra (m), which were
taken within the CARMENES RV planet survey during its first months of
operation. During this time, of the CARMENES-sample stars were observed
at least five times. From each spectrum we derived a relative RV and a measure
of chromospheric H emission. In addition, we estimated the chromatic
index (CRX) of each spectrum, which is a measure of the RV wavelength
dependence. Despite having a median number of only measurements per star,
we show that the RV variations of the stars with RV scatter of m/s and a
projected rotation velocity km/s are caused mainly by activity.
We name these stars `active RV-loud stars' and find their occurrence to
increase with spectral type: from for early-type M dwarfs
(MV) through for mid-type M dwarfs (MV) to
for late-type M dwarfs (MV). Their RV-scatter amplitude is
found to be correlated mainly with . For about half of the stars, we
also find a linear RVCRX anticorrelation, which indicates that their
activity-induced RV scatter is lower at longer wavelengths. For most of them we
can exclude a linear correlation between RV and H emission. Our results
are in agreement with simulated activity-induced RV variations in M dwarfs. The
RV variations of most active RV-loud M dwarfs are likely to be caused by dark
spots on their surfaces, which move in and out of view as the stars rotate.Comment: A&A accepte
On detection of the stochastic gravitational-wave background using the Parkes pulsar timing array
We search for the signature of an isotropic stochastic gravitational-wave
background in pulsar timing observations using a frequency-domain correlation
technique. These observations, which span roughly 12 yr, were obtained with the
64-m Parkes radio telescope augmented by public domain observations from the
Arecibo Observatory. A wide range of signal processing issues unique to pulsar
timing and not previously presented in the literature are discussed. These
include the effects of quadratic removal, irregular sampling, and variable
errors which exacerbate the spectral leakage inherent in estimating the steep
red spectrum of the gravitational-wave background. These observations are found
to be consistent with the null hypothesis, that no gravitational-wave
background is present, with 76 percent confidence. We show that the detection
statistic is dominated by the contributions of only a few pulsars because of
the inhomogeneity of this data set. The issues of detecting the signature of a
gravitational-wave background with future observations are discussed.Comment: 12 pages, 8 figures, 7 tables, accepted for publication in MNRA
The CARMENES search for exoplanets around M dwarfs: Wing asymmetries of H, Na I D, and He I lines
Stellar activity is ubiquitously encountered in M dwarfs and often
characterised by the H line. In the most active M dwarfs, H is
found in emission, sometimes with a complex line profile. Previous studies have
reported extended wings and asymmetries in the H line during flares. We
used a total of 473 high-resolution spectra of 28 active M dwarfs obtained by
the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths
with Near-infrared and optical Echelle Spectrographs) spectrograph to study the
occurrence of broadened and asymmetric H line profiles and their
association with flares, and examine possible physical explanations. We
detected a total of 41 flares and 67 broad, potentially asymmetric, wings in
H. The broadened H lines display a variety of profiles with
symmetric cases and both red and blue asymmetries. Although some of these line
profiles are found during flares, the majority are at least not obviously
associated with flaring. We propose a mechanism similar to coronal rain or
chromospheric downward condensations as a cause for the observed red
asymmetries; the symmetric cases may also be caused by Stark broadening. We
suggest that blue asymmetries are associated with rising material, and our
results are consistent with a prevalence of blue asymmetries during the flare
onset. Besides the H asymmetries, we find some cases of additional line
asymmetries in \ion{He}{i} D, \ion{Na}{i}~D lines, and the \ion{He}{i}
line at 10830\,\AA\, taken all simultaneously thanks to the large wavelength
coverage of CARMENES. Our study shows that asymmetric H lines are a
rather common phenomenon in M~dwarfs and need to be studied in more detail to
obtain a better understanding of the atmospheric dynamics in these objects.Comment: 28 pages, 67 figures, accepted to A&
Red Dots: A temperate 1.5 Earth-mass planet candidate in a compact multi-terrestrial planet system around GJ 1061
Abstract Small low-mass stars are favourable targets for the detection of rocky habitable planets. In particular, planetary systems in the solar neighbourhood are interesting and suitable for precise characterisation. The Red Dots campaigns seek to discover rocky planets orbiting nearby low-mass stars. The 2018 campaign targeted GJ 1061, which is the 20th nearest star to the Sun. For three consecutive months we obtained nightly, high-precision radial velocity measurements with the HARPS spectrograph. We analysed these data together with archival HARPS data. We report the detection of three planet candidates with periods of 3.204 ± 0.001, 6.689 ± 0.005 and 13.03 ± 0.03 days, which are close to 1:2:4 period commensurability. After several considerations related to the properties of the noise and sampling, we conclude that a 4th signal is most likely explained by stellar rotation, although it may be due to a planet. The proposed three-planet system (and the potential four-planet solution) is long-term dynamically stable. Planet-planet gravitational interactions are below our current detection threshold. The minimum masses of the three planets range from 1.4±0.2 to 1.8±0.3 M⊕. Planet d, with msin i = 1.64 ± 0.24M⊕, receives a similar amount of energy as Earth receives from the Sun. Consequently it lies within the liquid-water habitable zone of the star and has a similar equilibrium temperature to Earth. GJ 1061 has very similar properties to Proxima Centauri but activity indices point to lower levels of stellar activity
The CARMENES search for exoplanets around M dwarfs. Period search in H, Na I D, and Ca II IRT lines
We use spectra from CARMENES, the Calar Alto high-Resolution search for M
dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs, to
search for periods in chromospheric indices in 16 M0 to M2 dwarfs. We measure
spectral indices in the H, the Ca II infrared triplet (IRT), and the Na
I D lines to study which of these indices are best-suited to find rotation
periods in these stars. Moreover, we test a number of different period-search
algorithms, namely the string length method, the phase dispersion minimisation,
the generalized Lomb-Scargle periodogram, and the Gaussian process regression
with quasi-periodic kernel. We find periods in four stars using H and
in five stars using the Ca II IRT, two of which have not been found before. Our
results show that both H and the Ca II IRT lines are well suited for
period searches, with the Ca II IRT index performing slightly better than
H. Unfortunately, the Na I D lines are strongly affected by telluric
airglow, and we could not find any rotation period using this index. Further,
different definitions of the line indices have no major impact on the results.
Comparing the different search methods, the string length method and the phase
dispersion minimisation perform worst, while Gaussian process models produce
the smallest numbers of false positives and non-detections.Comment: 14 pages + 17 pages appendix, 9+16 figures, accepted to A&
- …