3,585 research outputs found
Fundamental Parameters of Cepheids: Masses and Multiplicity
Masses determined from classical Cepheids in binary systems are a primary
test of both pulsation and evolutionary calculations. The first step is to
determine the orbit from ground-based radial velocities. Complementary
satellite data from Hubble, FUSE, IUE, and Chandra provide full information
about the system. A summary of recent results on masses is given. Cepheids have
also provided copious information about the multiplicity of massive stars, as
well as the distribution of mass ratios and separations. This provides some
important constraints for star formation scenarios including differences
between high and low mass results and differences between close and wide
binaries
Challenges in Cepheid Evolution and Pulsation Modeling
Cepheids have long been used as standard candles to determine distances
around the Milky Way and to nearby galaxies. A discrepancy still remains for
Hubble Constant determinations using Cepheids vs. the cosmic microwave
background or calibrations to the tip of the red-giant branch. Therefore,
refinement of Cepheid period-luminosity relations continues to be an active
topic of research.
Cepheids are also important laboratories for testing stellar physics. This
paper explores outstanding questions in Cepheid evolution and pulsation
modeling. We examine the discrepancy between Cepheid masses determined from
pulsation properties and binary orbital dynamics and those determined using
stellar evolution models. We review attempts to resolve the discrepancy by
including rotation, convective overshooting, and mass loss. We review the
impact of uncertainties in nuclear reaction rates on Cepheid evolution and the
extent of blue loops in the Hertzsprung-Russell diagram. We consider
implications for Cepheids of stellar opacity revisions suggested in light of
findings for the Sun and other types of variable stars.
We apply the 1-D open-source MESA stellar evolution code and the MESA radial
stellar pulsation (RSP) nonlinear hydrodynamics code to investigate changes in
input physics for Cepheid models. We touch on progress in 2-D and 3-D stellar
modeling applied to Cepheids. Additional areas in which Cepheid models are
being tested against observations include: predicting the edges of the Cepheid
pulsation instability strip; predicting period-change rates and implications
for instability strip crossings; explaining period and amplitude modulations
and periodicities that may be non-radial pulsation modes; discovering what can
be learned from Cepheid observations in X-ray, ultraviolet, and radio
wavelengths. We also show a few examples of Cepheid light curves from NASA TESS
photometry.Comment: 10 pages, 17 figures. Revised and updated version of conference
proceedings submitted for 2023 Society for Astronomical Sciences Symposium on
Telescope Science, June 22-24, 2023, eds. J.C. Martin, R.K. Buchheim, R.M.
Gill, W. Green, and J. Menk
Domain Walls in MQCD and Monge-Ampere Equation
We study Witten's proposal that a domain wall exists in M-theory fivebrane
version of QCD (MQCD) and that it can be represented as a supersymmetric
three-cycle in G_2 holonomy manifold. It is shown that equations defining the
U(1) invariant domain wall for SU(2) group can be reduced to the Monge-Ampere
equation. A proof of an algebraic formula of Kaplunovsky, Sonnenschein and
Yankielowicz is presented. The formal solution of equations for domain wall is
constructed.Comment: Latex, 18 pages, section 4.2 modified, typos correcte
Packing and Hausdorff measures of stable trees
In this paper we discuss Hausdorff and packing measures of random continuous
trees called stable trees. Stable trees form a specific class of L\'evy trees
(introduced by Le Gall and Le Jan in 1998) that contains Aldous's continuum
random tree (1991) which corresponds to the Brownian case. We provide results
for the whole stable trees and for their level sets that are the sets of points
situated at a given distance from the root. We first show that there is no
exact packing measure for levels sets. We also prove that non-Brownian stable
trees and their level sets have no exact Hausdorff measure with regularly
varying gauge function, which continues previous results from a joint work with
J-F Le Gall (2006).Comment: 40 page
On the Evolution Equation for Magnetic Geodesics
In this paper we prove the existence of long time solutions for the parabolic
equation for closed magnetic geodesics.Comment: In this paper we prove the existence of long time solutions for the
parabolic equation for closed magnetic geodesic
The 3′–5′ proofreading exonuclease of archaeal family-B DNA polymerase hinders the copying of template strand deaminated bases
Archaeal family B polymerases bind tightly to the deaminated bases uracil and hypoxanthine in single-stranded DNA, stalling replication on encountering these pro-mutagenic deoxynucleosides four steps ahead of the primer–template junction. When uracil is specifically bound, the polymerase–DNA complex exists in the editing rather than the polymerization conformation, despite the duplex region of the primer-template being perfectly base-paired. In this article, the interplay between the 3′–5′ proofreading exonuclease activity and binding of uracil/hypoxanthine is addressed, using the family-B DNA polymerase from Pyrococcus furiosus. When uracil/hypoxanthine is bound four bases ahead of the primer–template junction (+4 position), both the polymerase and the exonuclease are inhibited, profoundly for the polymerase activity. However, if the polymerase approaches closer to the deaminated bases, locating it at +3, +2, +1 or even 0 (paired with the extreme 3′ base in the primer), the exonuclease activity is strongly stimulated. In these situations, the exonuclease activity is actually stronger than that seen with mismatched primer-templates, even though the deaminated base-containing primer-templates are correctly base-paired. The resulting exonucleolytic degradation of the primer serves to move the uracil/hypoxanthine away from the primer–template junction, restoring the stalling position to +4. Thus the 3′–5′ proofreading exonuclease contributes to the inability of the polymerase to replicate beyond deaminated bases
Benzyne in V4334 Sqr: A Quest for the Ring with SOFIA/EXES
Large aromatic molecules are ubiquitous in both circumstellar and interstellar environments. Detection of small aromatic molecules, such as benzene (C6H6) and benzyne (C6H4), are rare in astrophysical environments. Detection of such species will have major implications for our understanding of the astrochemistry involved in the formation of the molecules necessary for life, including modeling the chemical pathways to the formation of larger hydrocarbon molecules. We conducted a search for the infrared 18 μm spectral signature of benzyne in V4334 Sgr with the Stratospheric Observatory for Infrared Astronomy (SOFIA)/Echelon-Cross-Echelle Spectrograph (EXES) finding no evidence for a feature at the sensitivity of our observations
Effect of Exercise Training and +Gz Acceleration Training on Men
Countermeasures for reduction in work capacity (maximal oxygen uptake and strength) during spaceflight and enhanced orthostatic intolerance during re-entry, landing and egress from the return vehicle are continuing problems. The purpose for this study was to test the hypothesis that passive-acceleration training; supine, interval, exercise plus acceleration training and exercise combined with acceleration training would improve orthostatic tolerance in ambulatory men; and that addition of the aerobic exercise conditioning would not alter this improved tolerance from that of passive-acceleration training. Seven men (24-38 yr) underwent "Passive" training on the Ames human-powered centrifuge (HPC) for 30 min, "Exercise" training on the cycle ergometer with constant +Gz acceleration; and "Combined" exercise training at 40% to 90% of the HPC +Gz(max) exercise level. Maximal supine exercise loads increased significant (P<0.05) by 8.3% (Passive), 12.6% (Exercise), and by 15.4% (Combined) after training, but their post-training maximal oxygen uptakes and maximal heart rates were unchanged. Maximal time to fatigue (endurance) was unchanged with Passive was increased (P<0.05) with Exercise and Combined training. Thus, the exercise in the Exercise and Combined training Phases resulted in greater maximal loads and endurance without effect on maximal oxygen uptake or heart rate. There was a 4% to 6% increase (P<0.05) in all four quadriceps muscle volumes (right and left) after post-Combined training. Resting pre-tilt heart rate was elevated by 12.9% (P<0.05) only after Passive training suggesting that the exercise training attenuated the HR response. Plasma volume (% Delta) was uniformly decreased by 8% to 14% (P<0.05) at tilt-tolerance pre- vs. post-training indicating essentially no effect of training on the level of hypovolemia. Post-training tilt-tolerance time and heart rate were increased (P<0.05) only with Passive training by 37.8% and by 29.1%, respectively. Thus, addition of exercise training appeared to attenuate the increased Passive tilt-tolerance
Elephants classify human ethnic groups by odor and garment color
Animals can benefit from classifying predators or other dangers into categories, tailoring their escape strategies to the type and nature of the risk. Studies of alarm vocalizations have revealed various levels of sophistication in classification [1-5]. In many taxa, reactions to danger are inflexible, but some species can learn the level of threat presented by the local population of a predator [6-8] or by specific, recognizable individuals [9-10]. Some species distinguish several species of predator, giving differentiated warning calls and escape reactions; here we explore an animal’s classification of sub-groups within a species. We show that elephants distinguish at least two Kenyan ethnic groups, and can identify them by olfactory and color cues independently. In the Amboseli ecosystem, Kenya, Maasai warriors demonstrate virility by spearing elephants (Loxodonta africana), but Kamba agriculturalists pose little threat. Elephants showed greater fear when they detected the scent of garments previously worn by Maasai than by Kamba men, and reacted aggressively to the color associated with Maasai warriors. Elephants are therefore able to classify members of a single species into sub-groups that pose different degrees of danger
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