1,541 research outputs found
The Core Composition of a White Dwarf in a Close Double Degenerate System
We report the identification of the double degenerate system NLTT 16249 that
comprises a normal, hydrogen-rich (DA) white dwarf and a peculiar,
carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. We
disentangled the observed spectra and constrained the properties of both
stellar components. In the evolutionary scenario commonly applied to the
sequence of DQ white dwarfs, both carbon and nitrogen would be dredged up from
the core. The C/N abundance ratio (~ 50) in the atmosphere of this unique DQ
white dwarf suggests the presence of unprocessed material (14N) in the core or
in the envelope. Helium burning in the DQ progenitor may have terminated early
on the red-giant branch after a mass-ejection event leaving unprocessed
material in the core although current mass estimates do not favor the presence
of a low-mass helium core. Alternatively, some nitrogen in the envelope may
have survived an abridged helium-core burning phase prior to climbing the
asymptotic giant-branch. Based on available data, we estimate a relatively
short orbital period (P <~ 13 hrs) and on-going spectroscopic observations will
help determine precise orbital parameters.Comment: Accepted for publication in ApJ Letter
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope I: Is the Massive Star Cluster Double?
We present first results from a high resolution multi-band survey of the
Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged
Westerlund 2 with the Advanced Camera for Surveys through the , ,
and filters and with the Wide Field Camera 3 in the , ,
and filters. We derive the first high resolution pixel-to-pixel map of
the color excess of the gas associated with the cluster, combining
the H () and Pa () line observations. We
demonstrate that, as expected, the region is affected by significant
differential reddening with a median of ~mag. After separating
the populations of cluster members and foreground contaminants using a
vs. color-magnitude diagram, we identify a pronounced
pre-main-sequence population in Westerlund 2 showing a distinct turn-on. After
dereddening each star of Westerlund 2 individually in the color-magnitude
diagram we find via over-plotting PARSEC isochrones that the distance is in
good agreement with the literature value of ~kpc. With
zero-age-main-sequence fitting to two-color-diagrams, we derive a value of
total to selective extinction of . A spatial density map of
the stellar content reveals that the cluster might be composed of two clumps.
We estimate the same age of 0.5-2.0 Myr for both clumps. While the two clumps
appear to be coeval, the northern clump shows a lower stellar
surface density.Comment: 24 pages, 27 figures, 7 tables; Accepted for publication to The
Astronomical Journa
Multi-site breathers in Klein-Gordon lattices: stability, resonances, and bifurcations
We prove the most general theorem about spectral stability of multi-site
breathers in the discrete Klein-Gordon equation with a small coupling constant.
In the anti-continuum limit, multi-site breathers represent excited
oscillations at different sites of the lattice separated by a number of "holes"
(sites at rest). The theorem describes how the stability or instability of a
multi-site breather depends on the phase difference and distance between the
excited oscillators. Previously, only multi-site breathers with adjacent
excited sites were considered within the first-order perturbation theory. We
show that the stability of multi-site breathers with one-site holes change for
large-amplitude oscillations in soft nonlinear potentials. We also discover and
study a symmetry-breaking (pitchfork) bifurcation of one-site and multi-site
breathers in soft quartic potentials near the points of 1:3 resonance.Comment: 34 pages, 12 figure
Hubble Tarantula Treasury Project. IV. The extinction law
We report on the study of interstellar extinction across the Tarantula nebula
(30 Doradus), in the Large Magellanic Cloud, using observations from the Hubble
Tarantula Treasury Project in the 0.3 - 1.6 micron range. The considerable and
patchy extinction inside the nebula causes about 3500 red clump stars to be
scattered along the reddening vector in the colour-magnitude diagrams, thereby
allowing an accurate determination of the reddening slope in all bands. The
measured slope of the reddening vector is remarkably steeper in all bands than
in the the Galactic diffuse interstellar medium. At optical wavelengths, the
larger ratio of total-to-selective extinction, namely Rv = 4.5 +/- 0.2, implies
the presence of a grey component in the extinction law, due to a larger
fraction of large grains. The extra large grains are most likely ices from
supernova ejecta and will significantly alter the extinction properties of the
region until they sublimate in 50 - 100 Myr. We discuss the implications of
this extinction law for the Tarantula nebula and in general for regions of
massive star formation in galaxies. Our results suggest that fluxes of strongly
star forming regions are likely to be underestimated by a factor of about 2 in
the optical.Comment: 15 pages, 10 figures, accepted for publication in MNRAS; Updated
references, corrected typos in Table
A Kohn-Sham system at zero temperature
An one-dimensional Kohn-Sham system for spin particles is considered which
effectively describes semiconductor {nano}structures and which is investigated
at zero temperature. We prove the existence of solutions and derive a priori
estimates. For this purpose we find estimates for eigenvalues of the
Schr\"odinger operator with effective Kohn-Sham potential and obtain
-bounds of the associated particle density operator. Afterwards,
compactness and continuity results allow to apply Schauder's fixed point
theorem. In case of vanishing exchange-correlation potential uniqueness is
shown by monotonicity arguments. Finally, we investigate the behavior of the
system if the temperature approaches zero.Comment: 27 page
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