247 research outputs found
Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs
We have measured the profiles of the Ca II H and K chromospheric emission
lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar
masses) using multiple high resolution spectra obtained during six years with
the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,
equivalent widths, and line luminosities of Ca II H and K increase by a factor
of 3 with increasing stellar mass over this small range of stellar masses. We
fit the H and K lines with a double Gaussian model to represent both the
chromospheric emission and the non-LTE central absorption. Most of the sample
stars display a central absorption that is typically redshifted by ~0.1 km/s
relative to the emission, but the nature of this velocity gradient remains
unknown. The FWHM of the H and K lines increase with stellar luminosity,
reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent
widths and FWHM exhibit modest temporal variability in individual stars. At a
given value of M_v, stars exhibit a spread in both the equivalent width and
FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters
including rotation rate, age, and possibly metallicity, and to the spread in
stellar mass at a given M_v. The K line is consistently wider than the H line,
as expected, and its central absorption is more redshifted, indicating that the
H and K lines form at slightly different heights in the chromosphere where the
velocities are slightly different. The equivalent width of H-alpha correlates
with Ca II H and K only for stars having Ca II equivalent widths above ~2
angstroms, suggesting the existence of a magnetic threshold above which the
lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for
publication in PAS
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
The Influence of Magnetic Field on Oscillations in the Solar Chromosphere
Two sequences of solar images obtained by the Transition Region and Coronal
Explorer in three UV passbands are studied using wavelet and Fourier analysis
and compared to the photospheric magnetic flux measured by the Michelson
Doppler Interferometer on the Solar Heliospheric Observatory to study wave
behaviour in differing magnetic environments. Wavelet periods show deviations
from the theoretical cutoff value and are interpreted in terms of inclined
fields. The variation of wave speeds indicates that a transition from dominant
fast-magnetoacoustic waves to slow modes is observed when moving from network
into plage and umbrae. This implies preferential transmission of slow modes
into the upper atmosphere, where they may lead to heating or be detected in
coronal loops and plumes.Comment: 8 pages, 6 figures (4 colour online only), accepted for publication
in The Astrophysical Journa
Performance analysis of twin-spool water injected gas turbines using adaptive modeling.
ABSTRACT The development of an adaptive performance model for multi-spool gas turbines, equipped with the possibility of water injection is described. The model covers water injection at engine inlet, between the compressors and at the compressor exit. The selection of modification factors and the procedure for adapting component characteristics to overall performance data is discussed. A case of adaptation to overall performances is presented. The use of the model for studying overall engine and components performance is demonstrated. It is shown how operation with water injection modifies component operation, a fact that allows the identification of a wider range of the performance characteristics, in comparison to dry operation. This fact may also increase the diagnostic ability of techniques employing adaptive models. The sensitivity of diagnostic procedures to the different modes of operation of a gas turbine of the type described in the paper is also discussed
Quasi-periodic modulation of solar and stellar flaring emission by magnetohydrodynamic oscillations in a nearby loop
We propose a new model for quasi-periodic modulation of solar and stellar flaring emission. Fast magnetoacoustic oscillations of a non-flaring loop can interact with a nearby flaring active region. This interaction occurs when part of the oscillation situated outside the loop reaches the regions of steep gradients in magnetic field within an active region and produces periodic variations of electric
current density. The modulation depth of these variations is a few orders of magnitude greater than the amplitude of the driving oscillation. The variations of the current can induce current-driven plasma micro-instabilities and thus anomalous resistivity. This can periodically trigger magnetic reconnection, and hence acceleration of charged particles, producing quasi-periodic pulsations of X-ray,
optical and radio emission at the arcade footpoints
Fostering collective intelligence education
New educational models are necessary to update learning environments to the digitally shared communication and information. Collective intelligence is an emerging field that already has a significant impact in many areas and will have great implications in education, not only from the side of new methodologies but also as a challenge for education. This paper proposes an approach to a collective intelligence model of teaching using Internet to combine two strategies: idea management and real time assessment in the class. A digital tool named Fabricius has been created supporting these two elements to foster the collaboration and engagement of students in the learning process. As a result of the research we propose a list of KPI trying to measure individual and collective performance. We are conscious that this is just a first approach to define which aspects of a class following a course can be qualified and quantified.Postprint (published version
Spatially resolved signatures of bidirectional flows observed in inverted-Y shaped jets
Numerous apparent signatures of magnetic reconnection have been reported in the solar photosphere, including inverted-Y shaped jets. The reconnection at these sites is expected to cause localized bidirectional flows and extended shock waves; however, these signatures are rarely observed as extremely high spatial-resolution data are required. Here, we use Hα imaging data sampled by the Swedish Solar Telescope's CRisp Imaging SpectroPolarimeter to investigate whether bidirectional flows can be detected within inverted-Y shaped jets near the solar limb. These jets are apparent in the Hα line wings, while no signature of either jet is observed in the Hα line core, implying reconnection took place below the chromospheric canopy. Asymmetries in the Hα line profiles along the legs of the jets indicate the presence of bidirectional flows, consistent with cartoon models of reconnection in chromospheric anemone jets. These asymmetries are present for over two minutes, longer than the lifetimes of Rapid Blue Excursions, and beyond ±1 Å into the wings of the line indicating that flows within the inverted-Y shaped jets are responsible for the imbalance in the profiles, rather than motions in the foreground. Additionally, surges form following the occurrence of the inverted-Y shaped jets. This surge formation is consistent with models, which suggests such events could be caused by the propagation of shock waves from reconnection sites in the photosphere to the upper atmosphere. Overall, our results provide evidence that magnetic reconnection in the photosphere can cause bidirectional flows within inverted-Y shaped jets and could be the driver of surges
Observations of enhanced extreme ultraviolet continua during an X-class solar flare using <i>SDO</i>/EVE.
Observations of extreme-ultraviolet (EUV) emission from an X-class solar
flare that occurred on 2011 February 15 at 01:44 UT are presented, obtained
using the EUV Variability Experiment (EVE) onboard the Solar Dynamics
Observatory. The complete EVE spectral range covers the free-bound continua of
H I (Lyman continuum), He I, and He II, with recombination edges at 91.2, 50.4,
and 22.8 nm, respectively. By fitting the wavelength ranges blue-ward of each
recombination edge with an exponential function, lightcurves of each of the
integrated continua were generated over the course of the flare, as well as
emission from the free-free continuum (6.5-37 nm). The He II 30.4 nm and
Lyman-alpha 121.6 nm lines, and soft X-ray (0.1-0.8 nm) emission from GOES are
also included for comparison. Each free-bound continuum was found to have a
rapid rise phase at the flare onset similar to that seen in the 25-50 keV
lightcurves from RHESSI, suggesting that they were formed by recombination with
free electrons in the chromosphere. However, the free-free emission exhibited a
slower rise phase seen also in the soft X-ray emission from GOES, implying a
predominantly coronal origin. By integrating over the entire flare the total
energy emitted via each process was determined. We find that the flare energy
in the EVE spectral range amounts to at most a few per cent of the total flare
energy, but EVE gives us a first comprehensive look at these diagnostically
important continuum components.Comment: 6 pages, 3 figures, 1 table. Accepted to ApJ Letter
New M dwarf debris disk candidates in NGC 2547
With only six known examples, M-dwarf debris disks are rare, even though M
dwarfs constitute the majority of stars in the Galaxy. After finding a new M
dwarf debris disk in a shallow mid-infrared observation of NGC 2547, we present
a considerably deeper Spitzer-MIPS image of the region, with a maximum exposure
time of 15 minutes per pixel. Among sources selected from a previously
published membership list, we identify nine new M dwarfs with excess emission
at 24 micron tracing warm material close to the snow line of these stars, at
orbital radii of less than 1 AU. We argue that these are likely debris disks,
suggesting that planet formation is under way in these systems. Interestingly,
the estimated excess fraction of M stars appears to be higher than that of G
and K stars in our sample.Comment: 16 pages, 8 figures; accepted for publication in Ap
- …