15,646 research outputs found
Persistence of invariant manifolds for nonlinear PDEs
We prove that under certain stability and smoothing properties of the
semi-groups generated by the partial differential equations that we consider,
manifolds left invariant by these flows persist under perturbation. In
particular, we extend well known finite-dimensional results to the setting of
an infinite-dimensional Hilbert manifold with a semi-group that leaves a
submanifold invariant. We then study the persistence of global unstable
manifolds of hyperbolic fixed-points, and as an application consider the
two-dimensional Navier-Stokes equation under a fully discrete approximation.
Finally, we apply our theory to the persistence of inertial manifolds for those
PDEs which possess them. teComment: LaTeX2E, 32 pages, to appear in Studies in Applied Mathematic
Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems
After a pedagogical introduction to the main concepts of synthetic
photometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS,
and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes
for various [Fe/H] and [alpha/Fe] combinations, and virtually any value of
reddening E(B-V) < 0.7. The successes and failures of model fluxes in
reproducing the observed magnitudes are highlighted. Overall, extant synthetic
fluxes predict quite realistic broad-band colours and bolometric corrections,
especially at optical and longer wavelengths: further improvements of the
predictions for the blue and ultraviolet spectral regions await the use of
hydrodynamic models where the microturbulent velocity is not treated as a free
parameter. We show how the morphology of the colour-magnitude diagram (CMD)
changes for different values of [Fe/H] and [alpha/Fe]; in particular, how
suitable colour combinations can easily discriminate between red giant branch
and lower main sequence populations with different [alpha/Fe], due to the
concomitant loops and swings in the CMD. We also provide computer programs to
produce tables of synthetic bolometric corrections as well as routines to
interpolate in them. These colour-Teff-metallicity relations may be used to
convert isochrones for different chemical compositions to various bandpasses
assuming observed reddening values, thus bypassing the standard assumption of a
constant colour excess for stars of different spectral type. We also show how
such an assumption can lead to significant systematic errors. The MARCS
transformations presented in this study promise to provide important
constraints on our understanding of the multiple stellar populations found in
globular clusters (e.g., the colours of lower main sequence stars are predicted
to depend strongly on [alpha/Fe]) and of those located towards/in the Galactic
bulge.Comment: MNRAS, accepted. Tables and programs to generate synthetic colours
and bolometric corrections in various photometric systems and for different
combination of E(B-V), [Fe/H], [alpha/Fe], Teff and logg available via CDS at
http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/MNRAS/444/39
On the use of Gaia magnitudes and new tables of bolometric corrections
The availability of reliable bolometric corrections and reddening estimates,
rather than the quality of parallaxes will be one of the main limiting factors
in determining the luminosities of a large fraction of Gaia stars. With this
goal in mind, we provide Gaia G, BP and RP synthetic photometry for the entire
MARCS grid, and test the performance of our synthetic colours and bolometric
corrections against space-borne absolute spectrophotometry. We find indication
of a magnitude-dependent offset in Gaia DR2 G magnitudes, which must be taken
into account in high accuracy investigations. Our interpolation routines are
easily used to derive bolometric corrections at desired stellar parameters, and
to explore the dependence of Gaia photometry on Teff, log(g), [Fe/H],
alpha-enhancement and E(B-V). Gaia colours for the Sun and Vega, and
Teff-dependent extinction coefficients, are also provided.Comment: MNRAS Letter. Solar colours: BP-G = 0.33, G-RP = 0.49, BP-RP = 0.82.
Mean extinction coefficients at turn-off: R_G = 2.740 , R_BP = 3.374, R_RP =
2.035. Interpolation routines available at
https://github.com/casaluca/bolometric-correction
The Kinetics of the Exchange of Tritium between Hypophosphorous Acid and Water
We have measured the rates of exchange of radioactive hydrogen (tritium) between tritiated water, HTO, and the thwo "undissociable" hydrogens of monobasic hydrophosphorous acid, H3PO2
- …