1,740 research outputs found
Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos
We establish new dynamical constraints on the mass and abundance of compact
objects in the halo of dwarf spheroidal galaxies. In order to preserve
kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi
dSph) against gravitational scattering, we place upper limits on the density of
compact objects as a function of their assumed mass. The mass of the dark
matter constituents cannot be larger than 1000 solar masses at a halo density
in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario
in which dark halo cores are formed by two-body relaxation processes. Our
bounds on the fraction of dark matter in compact objects with masses >3000
solar masses improve those based on dynamical arguments in the Galactic halo.
In particular, objects with masses solar masses can comprise no
more than a halo mass fraction . Better determinations of the
velocity dispersion of old overdense regions in dSphs may result in more
stringent constraints on the mass of halo objects. For illustration, if the
preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed,
compact objects with masses above solar masses could be excluded
from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Switching Exponent Scaling near Bifurcation Points for Non-Gaussian Noise
We study noise-induced switching of a system close to bifurcation parameter values where the number of stable states changes. For non-Gaussian noise, the switching exponent, which gives the logarithm of the switching rate, displays a non-power-law dependence on the distance to the bifurcation point. This dependence is found for Poisson noise. Even weak additional Gaussian noise dominates switching sufficiently close to the bifurcation point, leading to a crossover in the behavior of the switching exponent
Analytic solution of a magnetized tori with magnetic polarization around Kerr black holes
We present the first family of magnetically polarized equilibrium tori around
a Kerr black hole. The models were obtained in the test fluid approximation by
assuming that the tori is a linear media, making it is possible to characterize
the magnetic polarization of the fluid through the magnetic susceptibility
. The magnetohydrodynamic (MHD) structure of the models was solved by
following the Komissarov approach, but with the aim of including the magnetic
polarization of the fluid, the integrability condition for the magnetic
counterpart was modified. We build two kinds of magnetized tori depending on
whether the magnetic susceptibility is constant in space or not. In the models
with constant , we find that the paramagnetic tori () are
more dense and less magnetized than the diamagnetic ones () in the
region between the inner edge, , and the center of the disk, ;
however, we find the opposite behavior for . Now, in the models with
non-constant , the tori become more magnetized than the Komissarov
solution in the region where , and less
magnetized when . Nevertheless, it is worth
mentioning that in all solutions presented in this paper the magnetic pressure
is greater than the hydrodynamic pressure. These new equilibrium tori can be
useful for studying the accretion of a magnetic media onto a rotating black
hole.Comment: 7 pages, 15 .pdf figures, 1 table, Accepted for publication in
Astronomy & Astrophysics (A&A
A Spatiotemporal Synthesis of High-Resolution Salinity Data with Aquaculture Applications
Technological advancement and the desire to better monitor shallow habitats in the Chesapeake Bay, Maryland, United States led to the initiation of several high-resolution monitoring programs such as ConMon (short for “Continuous Monitoring”) measuring oxygen, salinity, and chlorophyll-a at a 15-minute frequency. These monitoring efforts have yielded an enormous volume of data and insight into the condition of the tidal water of the Bay. But this information is underutilized in documenting the fine-scale variability of water quality, which is critical in identifying the link between water quality and ecological responses, partly due to the challenges in integrating monitoring data collected at different frequencies and locations. In a project to understand the environmental suitability of aquaculture sites and the future potential overlap between aquaculture and submerged aquatic vegetation, we developed a spatiotemporal synthesis of ConMon data with data from long-term, fixed-station seasonal monitoring. Here, we present our generalized additive model-based approach to predict salinity at high frequency (15 minutes) and fine spatial resolution (~100 meters) in the Maryland portion of the Bay, its major tributaries, and the shallow tidal creeks that exchange with the tributaries. Predictive performance was validated to be 1 PSU (practical salinity unit) in root mean square error using de novo monitoring. The resulting data provide insights into the environmental suitability of aquaculture, specifically the sensitivity of the Easter oyster (Crassostrea virginica) to low salinity stress. The spatiotemporal synthesis approach has potential applications for integrated monitoring and potential linkage with high-resolution water quality models for shallow habitats
Formation and fate of the born-again planetary nebula HuBi 1
We present the first 3D radiation-hydrodynamic simulations on the formation
and evolution of born-again planetary nebulae (PNe), with particular emphasis
to the case of HuBi1, the inside-out PN. We use the extensively-tested GUACHO
code to simulate the formation of HuBi1 adopting mass-loss and stellar wind
terminal velocity estimates obtained from observations presented by our group.
We found that, if the inner shell of HuBi1 was formed by an explosive very late
thermal pulse (VLTP) ejecting material with velocities of 300 km
s, the age of this structure is consistent with that of 200 yr
derived from multi-epoch narrow-band imaging. Our simulations predict that, as
a consequence of the dramatic reduction of the stellar wind velocity and photon
ionizing flux during the VLTP, the velocity and pressure structure of the outer
H-rich nebula are affected creating turbulent ionized structures surrounding
the inner shell. These are indeed detected in Gran Telescopio Canarias MEGARA
optical observations. Furthermore, we demonstrate that the current relatively
low ionizing photon flux from the central star of HuBi1 is not able to
completely ionize the inner shell, which favors previous suggestions that its
excitation is dominated by shocks. Our simulations suggest that the kinetic
energy of the H-poor ejecta of HuBi1 is at least 30 times that of the clumps
and filaments in the evolved born-again PNe A30 and A78, making it a truly
unique VLTP event.Comment: 9 pages, 10 figures, no tables; accepted to MNRA
Affordable Rotating Fluid Demonstrations for Geoscience Education: The DIYnamics Project
Demonstrations using rotating tanks of fluid can help demystify otherwise counterintuitive behaviors of atmospheric, oceanic, and planetary interior fluid motions. But the expense and complicated assembly of existing rotating table platforms limit their appeal for many schools, especially those below the university level. Here, we introduce Do-It-Yourself Dynamics (DIYnamics), a project developing extremely low-cost rotating tank platforms and accompanying teaching materials. The devices can be assembled in a few minutes from household items, all available for purchase online. Ordering, assembly, and operation instructions are available on the DIYnamics website. Videos using these and other rotating tables to teach specific concepts such as baroclinic instability are available on the DIYnamics YouTube channel—including some in Spanish. The devices, lesson plans, and demonstrations have been successfully piloted at multiple middle schools, in a university course, and at public science outreach events. These uses to date convince us of the DIYnamics materials’ pedagogical value for instructors from well-versed university professors to K–12 science teachers with little background in fluid dynamics
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