829 research outputs found
Tachoastrometry: astrometry with radial velocities
Spectra of composite systems (e.g., spectroscopic binaries) contain spatial
information that can be retrieved by measuring the radial velocities (i.e.,
Doppler shifts) of the components in four observations with the slit rotated by
90 degrees in the sky. By using basic concepts of slit spectroscopy we show
that the geometry of composite systems can be reliably retrieved by measuring
only radial velocity differences taken with different slit angles. The spatial
resolution is determined by the precision with which differential radial
velocities can be measured. We use the UVES spectrograph at the VLT to observe
the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum
expected separation of 23 milli-arcseconds. We measure an astrometric signal in
radial velocity of 276 \ms, which corresponds to a separation between the two
components at the time of the observations of 18 milli-arcseconds. The
stars were aligned east-west. We describe a simple optical device to
simultaneously record pairs of spectra rotated by 180 degrees, thus reducing
systematic effects. We compute and provide the function expressing the shift of
the centroid of a seeing-limited image in the presence of a narrow slit.The
proposed technique is simple to use and our test shows that it is amenable for
deriving astrometry with milli-arcsecond accuracy or better, beyond the
diffraction limit of the telescope. The technique can be further improved by
using simple devices to simultaneously record the spectra with 180 degrees
angles.With tachoastrometry, radial velocities and astrometric positions can be
measured simultaneously for many double line system binaries in an easy way.
The method is not limited to binary stars, but can be applied to any
astrophysical configuration in which spectral lines are generated by separate
(non-rotational symmetric) regions.Comment: Accepted for publication in A&
Wavelets: a powerful tool for studying rotation, activity, and pulsation in Kepler and CoRoT stellar light curves
Aims. The wavelet transform has been used as a powerful tool for treating
several problems in astrophysics. In this work, we show that the time-frequency
analysis of stellar light curves using the wavelet transform is a practical
tool for identifying rotation, magnetic activity, and pulsation signatures. We
present the wavelet spectral composition and multiscale variations of the time
series for four classes of stars: targets dominated by magnetic activity, stars
with transiting planets, those with binary transits, and pulsating stars.
Methods. We applied the Morlet wavelet (6th order), which offers high time and
frequency resolution. By applying the wavelet transform to the signal, we
obtain the wavelet local and global power spectra. The first is interpreted as
energy distribution of the signal in time-frequency space, and the second is
obtained by time integration of the local map. Results. Since the wavelet
transform is a useful mathematical tool for nonstationary signals, this
technique applied to Kepler and CoRoT light curves allows us to clearly
identify particular signatures for different phenomena. In particular, patterns
were identified for the temporal evolution of the rotation period and other
periodicity due to active regions affecting these light curves. In addition, a
beat-pattern signature in the local wavelet map of pulsating stars over the
entire time span was also detected.Comment: Accepted for publication on A&
The WFCAM Multi-wavelength Variable Star Catalog
Stellar variability in the near-infrared (NIR) remains largely unexplored.
The exploitation of public science archives with data-mining methods offers a
perspective for the time-domain exploration of the NIR sky. We perform a
comprehensive search for stellar variability using the optical-NIR multi-band
photometric data in the public Calibration Database of the WFCAM Science
Archive (WSA), with the aim of contributing to the general census of variable
stars, and to extend the current scarce inventory of accurate NIR light curves
for a number of variable star classes. We introduce new variability indices
designed for multi-band data with correlated sampling, and apply them for
pre-selecting variable star candidates, i.e., light curves that are dominated
by correlated variations, from noise-dominated ones. Pre-selection criteria are
established by robust numerical tests for evaluating the response of
variability indices to colored noise characteristic to the data. We find 275
periodic variable stars and an additional 44 objects with suspected variability
with uncertain periods or apparently aperiodic variation. Only 44 of these
objects had been previously known, including 11 RR~Lyrae stars in the outskirts
of the globular cluster M3 (NGC~5272). We provide a preliminary classification
of the new variable stars that have well-measured light curves, but the
variability types of a large number of objects remain ambiguous. We classify
most of the new variables as contact binary stars, but we also find several
pulsating stars, among which 34 are probably new field RR~Lyrae and 3 are
likely Cepheids. We also identify 32 highly reddened variable objects close to
previously known dark nebulae, suggesting that these are embedded young stellar
objects. We publish our results and all light-curve data as the WFCAM Variable
Star Catalog.Comment: 21 pages, 11 figure
Transient Charging and Discharging of Spin-polarized Electrons in a Quantum Dot
We study spin-polarized transient transport in a quantum dot coupled to two
ferromagnetic leads subjected to a rectangular bias voltage pulse.
Time-dependent spin-resolved currents, occupations, spin accumulation, and
tunneling magnetoresistance (TMR) are calculated using both nonequilibrium
Green function and master equation techniques. Both parallel and antiparallel
leads' magnetization alignments are analyzed. Our main findings are: a
dynamical spin accumulation that changes sign in time, a short-lived pulse of
spin polarized current in the emitter lead (but not in the collector lead), and
a dynamical TMR that develops negative values in the transient regime. We also
observe that the intra-dot Coulomb interaction can enhance even further the
negative values of the TMR.Comment: 7 pages, 6 figures. Typos corrections corresponding to the published
versio
Learn to Eat and Get a Move On
Healthy dieting and the practice of physical activities are deemed fundamental for the adequate growth, development and maintenance of health in young people. This project takes into account these priority areas and is directed towards the educational community, giving a voice to the students through projects such as "Ambassadors of Health" and "E- P@articipate". With "Learn to Eat and Get a Move On" a transversal involvement between community partners, projects and disciplinary areas were achieved, namely through the projects "Mediterranean Diet", "Geodesic Greenhouse" and physical education, with activities being developed in the areas of diet, health and environment. In the greenhouse, aromatic seeds were planted to highlight their importance in a diet, as well as a way to reduce or replace salt
The Galactic O-Star Spectroscopic Survey (GOSSS). III. 142 additional O-type systems
This is the third installment of GOSSS, a massive spectroscopic survey of
Galactic O stars, based on new homogeneous, high signal-to-noise ratio, R~2500
digital observations selected from the Galactic O-Star Catalog (GOSC). In this
paper we present 142 additional stellar systems with O stars from both
hemispheres, bringing the total of O-type systems published within the project
to 590. Among the new objects there are 20 new O stars. We also identify 11 new
double-lined spectroscopic binaries (SB2s), of which 6 are of O+O type and 5 of
O+B type, and an additional new tripled-lined spectroscopic binary (SB3) of
O+O+B type. We also revise some of the previous GOSSS classifications, present
some egregious examples of stars erroneously classified as O-type in the past,
introduce the use of luminosity class IV at spectral types O4-O5.5, and adapt
the classification scheme to the work of Arias et al. (2016).Comment: Accepted for publication in ApJS. This version has minor corrections
regarding typos and reference
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