2,204 research outputs found
NGC 6819: testing the asteroseismic mass scale, mass loss, and evidence for products of non-standard evolution
We present an extensive peakbagging effort on Kepler data of 50 red
giant stars in the open star cluster NGC 6819. By employing sophisticated
pre-processing of the time series and Markov Chain Monte Carlo techniques we
extracted individual frequencies, heights and linewidths for hundreds of
oscillation modes.
We show that the "average" asteroseismic parameter , derived
from these, can be used to distinguish the stellar evolutionary state between
the red giant branch (RGB) stars and red clump (RC) stars.
Masses and radii are estimated using asteroseismic scaling relations, both
empirically corrected to obtain self-consistency as well as agreement with
independent measures of distance, and using updated theoretical corrections.
Remarkable agreement is found, allowing the evolutionary state of the giants to
be determined exclusively from the empirical correction to the scaling
relations. We find a mean mass of the RGB stars and RC stars in NGC 6819 to be
and ,
respectively. The difference is
almost insensitive to systematics, suggesting very little RGB mass loss, if
any.
Stars that are outliers relative to the ensemble reveal overmassive members
that likely evolved via mass-transfer in a blue straggler phase. We suggest
that KIC 4937011, a low-mass Li-rich giant, is a cluster member in the RC phase
that experienced very high mass-loss during its evolution. Such over- and
undermassive stars need to be considered when studying field giants, since the
true age of such stars cannot be known and there is currently no way to
distinguish them from normal stars.Comment: 21 pages, 11 figure
Search for lithium-rich giants in 32 open clusters with high-resolution spectroscopy
Lithium-rich giant stars are rare and their existence challenges our
understanding of stellar structure and evolution. We profit from the
high-quality sample gathered with HARPS and UVES, in order to search for
Li-rich giants and to identify the Li enrichment mechanisms responsible. We
derive stellar parameters for 247 stars belonging to 32 open clusters, with
0.07 Ga < ages < 3.6 Ga. We employed the spectral synthesis technique code
FASMA for the abundance analysis of 228 stars from our sample. We also
determined ages, distances, and extinction using astrometry and photometry from
Gaia and PARSEC isochrones to constrain their evolutionary stage. Our sample
covers a wide range of stellar masses from 1 to more than 6 solar masses where
the majority of the masses are above 2 solar masses. We have found 14 canonical
Li-rich giant stars which have experienced the first dredge-up. This
corresponds to 6% of our total sample, which is higher than what is typically
found for field stars. Apart from the canonical limit, we use the maximum Li
abundance of the progenitor stars as a criterion for Li enrichment. We find Li
enhancement also among eight stars which have passed the first dredge up and
show strong Li lines based on the fact that stars at the same evolutionary
stage in the same cluster have significantly different Li abundances. We
confirm that giants with higher Li abundance correspond to a higher fraction of
fast-rotating giants, suggesting a connection between Li enhancement and
stellar rotation as predicted by stellar models. Our Li-rich giants are found
in various evolutionary stages implying that no unique Li production mechanism
is responsible for Li enrichment but rather different intrinsic or external
mechanisms can be simultaneously at play.Comment: accepted in A&A, online data will be available in CD
Disentangling the electronic and phononic glue in a high-Tc superconductor
Unveiling the nature of the bosonic excitations that mediate the formation of
Cooper pairs is a key issue for understanding unconventional superconductivity.
A fundamen- tal step toward this goal would be to identify the relative weight
of the electronic and phononic contributions to the overall frequency (\Omega)
dependent bosonic function, \Pi(\Omega). We perform optical spectroscopy on
Bi2212 crystals with simultaneous time- and frequency-resolution; this
technique allows us to disentangle the electronic and phononic contributions by
their different temporal evolution. The strength of the interaction
({\lambda}~1.1) with the electronic excitations and their spectral distribution
fully account for the high critical temperature of the superconducting phase
transition.Comment: 9 pages, 4 figure
Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
Asteroseismic constraints on K giants make it possible to infer radii, masses
and ages of tens of thousands of field stars. Tests against independent
estimates of these properties are however scarce, especially in the metal-poor
regime. Here, we report the detection of solar-like oscillations in 8 stars
belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2
Mission during its Campaign 2. Making use of independent constraints on the
distance, we estimate masses of the 8 stars by utilising different combinations
of seismic and non-seismic inputs. When introducing a correction to the Delta
nu scaling relation as suggested by stellar models, for RGB stars we find
excellent agreement with the expected masses from isochrone fitting, and with a
distance modulus derived using independent methods. The offset with respect to
independent masses is lower, or comparable with, the uncertainties on the
average RGB mass (4-10%, depending on the combination of constraints used). Our
results lend confidence to asteroseismic masses in the metal poor regime. We
note that a larger sample will be needed to allow more stringent tests to be
made of systematic uncertainties in all the observables (both seismic and
non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.Comment: 6 pages, 3 figures, accepted for publication in MNRA
Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons
The understanding of how the sub-nanoscale exchange interaction evolves in macroscale correlations and ordered phases of matter, such as magnetism and superconductivity, requires to bridging the quantum and classical worlds. This monumental challenge has so far only been achieved for systems close to their thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics of the macroscale magnetic order parameter in the Heisenberg antiferromagnet KNiF 3 triggered by the impulsive optical generation of spin excitations with the shortest possible nanometre wavelength and femtosecond period. Our magneto-optical pump-probe experiments also demonstrate the coherent manipulation of the phase and amplitude of these femtosecond nanomagnons, whose frequencies are defined by the exchange energy. These findings open up opportunities for fundamental research on the role of short-wavelength spin excitations in magnetism and strongly correlated materials; they also suggest that nanospintronics and nanomagnonics can employ coherently controllable spin waves with frequencies in the 20 THz domain
Femtosecond phononic coupling to both spins and charges in a room-temperature antiferromagnetic semiconductor
Spintronics is postulated on the possibility to employ the magnetic degree of freedom of electrons for computation and couple it to charges. In this view the combination of the high frequency of spin manipulations offered
by antiferromagnets, with the wide tunability of the electronic properties peculiar of semiconductors, provides
a promising and intriguing platform. Here we explore this scenario in α-MnTe, which is a semiconductor
antiferromagnetically ordered at room temperature. Relying on a Raman mechanism and femtosecond laser
pulses, we drive degenerate modes of coherent optical phonons, which modulate the chemical bonds involved
in the superexchange interaction. The spectrally resolved measurements of the transient reflectivity reveal a
coherent modulation of the band gap at the frequency of 5.3 THz. The detection of the rotation of the polarization,
typically associated with magneto-optical effects, shows coherent and incoherent contributions. Modeling how
the ionic motion induced by the phonons affects the exchange interaction in the material, we calculate the
photoinduced THz spin dynamics: the results predict both a coherent and incoherent response, the latter of which
is consistent with the experimental observation. Our work demonstrates that the same phonon modes modulate
both the charge and magnetic degree of freedom, suggesting the resonant pumping of phonons as a viable way
to link spin and charge dynamics even in nonlinear regimes
Kepler red-clump stars in the field and in open clusters: Constraints on core mixing
Convective mixing in helium-core-burning (HeCB) stars is one of the outstanding issues in stellar modelling. The precise asteroseismic measurements of gravity-mode period spacing (&dela;σ1) have opened the door to detailed studies of the near-core structure of such stars, which had not been possible before. Here, we provide stringent tests of various core-mixing scenarios against the largely unbiased population of red-clump stars belonging to the old-open clusters monitored by Kepler, and by coupling the updated precise inference on &dela;σ1 in thousands of field stars with spectroscopic constraints. We find that models with moderate overshooting successfully reproduce the range observed of &dela;σ1 in clusters. In particular, we show that there is no evidence for the need to extend the size of the adiabatically stratified core, at least at the beginning of the HeCB phase. This conclusion is based primarily on ensemble studies of &dela;σ1 as a function of mass and metallicity. While &dela;σ1 shows no appreciable dependence on the mass, we have found a clear dependence of &dela;σ1 on metallicity, which is also supported by predictions from models
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