7,780 research outputs found
Red-giant stars in eccentric binaries
The unparalleled photometric data obtained by NASAâs Kepler Space Telescope has led to improved understanding of red-giant stars and binary stars. We discuss the characterization of known eccentric system, containing a solar-like oscillating red-giant primary component. We also report several new binary systems that are candidates for hosting an oscillating companion. A powerful approach to study binary stars is to combine asteroseimic techniques with light curve fitting. Seismology allows us to deduce the properties of red giants. In addition, by modeling the ellipsoidal modulations we can constrain the parameters of the binary system. An valuable independent source are ground-bases, high-resolution spectrographs
Performance of Conformable Ablators in Aerothermal Environments
Conformable Phenolic Impregnated Carbon Ablator, a cousin of Phenolic Impregnated Carbon Ablator (PICA), was developed at NASA Ames Research Center as a lightweight thermal protection system under the Fundamental Aeronautics Program. PICA is made using a brittle carbon substrate, which has a very low strain to failure. Conformable PICA is made using a flexible carbon substrate, a felt in this case. The flexible felt significantly increases the strain to failure of the ablator. PICA is limited by its thermal mechanical properties. Future NASA missions will require heatshields that are more fracture resistant than PICA and, as a result, NASA Ames is working to improve PICAs performance by developing conformable PICA to meet these needs. Research efforts include tailoring the chemistry of conformable PICA with varying amounts of additives to enhance mechanical properties and testing them in aerothermal environments. This poster shows the performance of conformable PICA variants in arc jets tests. Some mechanical and thermal properties will also be presented
New Stagnation Arc Jet Model Design for Testing ADEPT 3-D Carbon Cloth
The ADEPT concept has been considered as an entry, descent and landing system to enable Human Mars class missions. Ground rules for the Mars studies required aerocapture, orbit, and then entry. The design utilizes a 3-D woven carbon cloth fabric as both heatshield and primary structure and design guidelines required 6 layers remaining after all entry events. The peak heating predicted for the ADEPT carbon cloth was 35 Wcm2 and resulting temperatures were predicted to be 1400K. Predictions for carbon mass loss were performed using equilibrium thermochemistry, which is only accurate for T2000K. Carbon oxidation is kinetically controlled at T2000K, and mass loss drops off considerably from equilibrium values. Design of the cloth thickness and mass would be significantly reduced if kinetics were considered. This effort was to design a stagnation test article design that could be used in the AHF with varying levels of oxygen where the results could be used to develop an engineering model to describe the recession rate of the carbon as a function of the partial pressure of monotomic oxygen
Internal rotation of the red-giant star KIC 4448777 by means of asteroseismic inversion
In this paper we study the dynamics of the stellar interior of the early
red-giant star KIC 4448777 by asteroseismic inversion of 14 splittings of the
dipole mixed modes obtained from {\it Kepler} observations. In order to
overcome the complexity of the oscillation pattern typical of red-giant stars,
we present a procedure which involves a combination of different methods to
extract the rotational splittings from the power spectrum. We find not only
that the core rotates faster than the surface, confirming previous inversion
results generated for other red giants (Deheuvels et al. 2012,2014), but we
also estimate the variation of the angular velocity within the helium core with
a spatial resolution of and verify the hypothesis of a sharp
discontinuity in the inner stellar rotation (Deheuvels et al. 2014). The
results show that the entire core rotates rigidly with an angular velocity of
about ~nHz and provide evidence for an
angular velocity decrease through a region between the helium core and part of
the hydrogen burning shell; however we do not succeed to characterize the
rotational slope, due to the intrinsic limits of the applied techniques. The
angular velocity, from the edge of the core and through the hydrogen burning
shell, appears to decrease with increasing distance from the center, reaching
an average value in the convective envelope of
~nHz. Hence, the core in KIC~4448777 is
rotating from a minimum of 8 to a maximum of 17 times faster than the envelope.
We conclude that a set of data which includes only dipolar modes is sufficient
to infer quite accurately the rotation of a red giant not only in the dense
core but also, with a lower level of confidence, in part of the radiative
region and in the convective envelope.Comment: accepted for publication on Ap
Tracing kinematical and physical asymmetries in the jet from DG Tau B
Stellar jets can be highly asymmetric and have multiple velocity components.
To clarify the origin of jet asymmetries and constrain their launch mechanism
we study the physical and kinematical structure of the flow emitted by DG Tau
B. The analysis of deep spectra taken at the KECK telescope allows us to infer
the physical properties (the electron and total density, ne and nh, the
ionisation fraction, xe, and the temperature, te) and the spatial distribution
of the velocity components in the two jet lobes. The presence of dust grains in
the jet is investigated by estimating the gas-phase abundance of calcium with
respect to its solar value. At the base of the jet the lines are broad (~100
km/s) and up to three velocity components are detected. At 5" from the source,
however, only the denser and more excited high velocity components survive and
the lines are narrower (~10-30 km/s). The jet is strongly asymmetric both in
velocity and in its physical structure. The red lobe, slower (~140 km/s) and
more collimated, presents low ionisation fractions (xe~0.1-0.4) and
temperatures (te<5e3 K), while the total density is up to ~2.5e4 ccm. The blue
lobe, faster (~-320 km/s) and less collimated, is also less dense (nh~1e4 ccm)
but highly excited (te up to ~5e4 K and xe up to 0.9). The estimated mass loss
rate is similar in the two lobes (~6-8e-9 Msol/yr), suggesting that the
ejection power is comparable on the two sides of the system, as expected from a
magneto-centrifugal ejection mechanism, and that the observed asymmetries are
due to different mass load and propagation properties in an inhomogeneous
environment. Calcium is strongly depleted, indicating that the jet contains
dust grains and, therefore, should originate from a region of the disk
extending beyond the dust sublimation radius. The depletion is lower for higher
velocities, consistent with dust destruction by shocks.Comment: 14 pages, 9 figures, accepted by A&
Spin down of the core rotation in red giants
The space mission Kepler provides us with long and uninterrupted photometric
time series of red giants. We are now able to probe the rotational behaviour in
their deep interiors using the observations of mixed modes. We aim to measure
the rotational splittings in red giants and to derive scaling relations for
rotation related to seismic and fundamental stellar parameters. We have
developed a dedicated method for automated measurements of the rotational
splittings in a large number of red giants. Ensemble asteroseismology, namely
the examination of a large number of red giants at different stages of their
evolution, allows us to derive global information on stellar evolution. We have
measured rotational splittings in a sample of about 300 red giants. We have
also shown that these splittings are dominated by the core rotation. Under the
assumption that a linear analysis can provide the rotational splitting, we
observe a small increase of the core rotation of stars ascending the red giant
branch. Alternatively, an important slow down is observed for red-clump stars
compared to the red giant branch. We also show that, at fixed stellar radius,
the specific angular momentum increases with increasing stellar mass. Ensemble
asteroseismology indicates what has been indirectly suspected for a while: our
interpretation of the observed rotational splittings leads to the conclusion
that the mean core rotation significantly slows down during the red giant
phase. The slow-down occurs in the last stages of the red giant branch. This
spinning down explains, for instance, the long rotation periods measured in
white dwarfsComment: Accepted in A&
Photo- and Electron-Production of Mesons on Nucleons and Nuclei
In these lectures I will show some results obtained with the chiral unitary
approach applied to the photo and electroproduction of mesons. The results for
photoproduction of and , together with
related reactions will be shown, having with common denominator the excitation
of the resonance which is one of those dynamically generated in
the chiral unitary approach. Then I will show results obtained for the reaction which reproduce the bulk of the data except for a
pronounced peak, giving support to a new mesonic resonance, X(2175). Results
will also be shown for the electromagnetic form factors of the
resonance, also dynamically generated in this approach. Finally, I will show
some results on the photoproduction of the in nuclei, showing that
present experimental results claiming a shift of the mass in the
medium are tied to a particular choice of background and are not conclusive.
One the other hand, the same experimental results show unambiguously a huge
increase of the width in the nuclear medium.Comment: Lecture at the "International School of Nuclear Physics", 29th Course
Quarks in Hadrons and Nuclei, Erice, Italy, September 2007. Note added in
Proofs concerning the mixed events technique and other comments on omega
productio
Lipid biomarkers reveal trophic relationships and energetic tradeâoffs in contrasting phenotypes of the coldâwater coral Desmophyllum dianthus in Comau Fjord, Chile
Benthic suspension feeders like corals and sponges are important bioengineers in many marine habitats, from the shallow tropics to the depth of polar oceans. While they are generally considered opportunistic, little is known about their actual in situ diet. To tackle this limitation, fatty acid trophic markers (FATMs) have been employed to gain insights into the composition of their diet. Yet, these in situ studies have not been combined with physiological investigations to understand how physiological limitations may modulate the biochemistry of these organisms. Here, we used the cold-water coral (CWC) Desmophyllum dianthus in its natural habitat in Comau Fjord (Northern Patagonia, Chile) as our model species to assess the trophic ecology in response to contrasting physico-chemical conditions (variable vs. stable) and ecological drivers (food availability) at three shallow sites and one deep site. We took advantage of the expression of two distinct phenotypes with contrasting performance (growth, biomass, respiration) coinciding with the differences in sampling depth. We analysed the corals' fatty acid composition to evaluate the utility of FATM profiles to gain dietary insights and assess how performance trade-offs potentially modulate an organism's FATM composition. We found that 20:1(n-9) zooplankton markers dominated the deep high-performance phenotype, while 20:5(n-3) and 22:6(n-3) diatom and flagellate markers, respectively, are more prominent in shallow low-performance phenotype. Surprisingly, both energy stores and performance were higher in the deep phenotype, in spite of measured lower zooplankton availability. Essential FA concentrations were conserved across sites, likely reflecting required levels for coral functioning and survival. While the deep high-performance phenotype met with these requirements, the low-performance phenotype appeared to need more energy to maintain functionality in its highly variable environment, potentially causing intrinsic re-allocations of energy and enrichment in certain essential markers (20:5(n-3), 22:6(n-3)). Our analysis highlights the biological and ecological insights that can be gained from FATM profiles in CWCs, but also cautions the reliability of FATM as diet tracers under limiting environmental conditions that may also be applicable to other marine organisms. Read the free Plain Language Summary for this article on the Journal blog
Nonradial and nonpolytropic astrophysical outflows IX. Modeling T Tauri jets with a low mass-accretion rate
Context: A large sample of T Tauri stars exhibits optical jets, approximately
half of which rotate slowly, only at ten per cent of their breakup velocity.
The disk-locking mechanism has been shown to be inefficient to explain this
observational fact.
Aims: We show that low mass accreting T Tauri stars may have a strong stellar
jet component that can effectively brake the star to the observed rotation
speed.
Methods: By means of a nonlinear separation of the variables in the full set
of the MHD equations we construct semi- analytical solutions describing the
dynamics and topology of the stellar component of the jet that emerges from the
corona of the star.
Results: We analyze two typical solutions with the same mass loss rate but
different magnetic lever arms and jet radii. The first solution with a long
lever arm and a wide jet radius effectively brakes the star and can be applied
to the visible jets of T Tauri stars, such as RY Tau. The second solution with
a shorter lever arm and a very narrow jet radius may explain why similar stars,
either Weak line T Tauri Stars (WTTS) or Classical T Tauri Stars (CTTS) do not
all have visible jets. For instance, RY Tau itself seems to have different
phases that probably depend on the activity of the star.
Conclusions: First, stellar jets seem to be able to brake pre-main sequence
stars with a low mass accreting rate. Second, jets may be visible only part
time owing to changes in their boundary conditions. We also suggest a possible
scenario for explaining the dichotomy between CTTS and WTTS, which rotate
faster and do not have visible jets
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