4,042 research outputs found
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars
Probing Elemental Abundances in SNR 1987A Using XMM-Newton
We report on the latest (2007 Jan) observations of supernova remnant (SNR)
1987A from the XMM-Newton mission. Since the 2003 May observations of Haberl et
al. (2006), 11 emission lines have experienced increases in flux by factors ~ 3
to 10, with the 775 eV line of O VIII showing the greatest increase; we have
observed 6 lines of Fe XVII and Fe XVIII previously unreported by XMM-Newton. A
two-shock model representing plasmas in non-equilibrium ionization is fitted to
the EPIC-pn spectra, yielding temperatures of ~ 0.4 and ~ 3 keV, as well as
elemental abundances for N, O, Ne, Mg, Si, S and Fe. We demonstrate that the
abundance ratio of N and O can be constrained to less than ~20% accuracy.
Within the same confidence interval, the same analysis suggests that the C+N+O
abundance varies from ~ 1.1 to 1.4 X 10^-4. Normalizing our obtained abundances
by the Large Magellanic Cloud (LMC) values of Hughes, Hayashi & Koyama (1998),
we find that O, Ne, Mg and Fe are under-abundant, while Si and S are
over-abundant, consistent with the findings of Aschenbach (2007). Such a result
has implications for both the single-star and binary accretion/merger models
for the progenitor of SNR 1987A. In the context of the binary merger scenario
proposed by Morris & Podsiadlowski (2006, 2007), material forming the inner,
equatorial ring was expelled after the merger, implying that either our derived
Fe abundance is inconsistent with typical LMC values or that iron is
under-abundant at the site of the progenitor star of SNR 1987A.Comment: 14 pages, 10 diagrams (2 omitted). Accepted by Ap
X-ray to NIR emission from AA Tauri during the dim state - Occultation of the inner disk and gas-to-dust ratio of the absorber
AA Tau is a well-studied, nearby classical T Tauri star, which is viewed
almost edge-on. A warp in its inner disk periodically eclipses the central
star, causing a clear modulation of its optical light curve. The system
underwent a major dimming event beginning in 2011 caused by an extra absorber,
which is most likely associated with additional disk material in the line of
sight toward the central source. We present new XMM-Newton X-ray, Hubble Space
Telescope FUV, and ground based optical and near-infrared data of the system
obtained in 2013 during the long-lasting dim phase. The line width decrease of
the fluorescent H disk emission shows that the extra absorber is located at
au. Comparison of X-ray absorption () with dust extinction (),
as derived from measurements obtained one inner disk orbit (eight days) after
the X-ray measurement, indicates that the gas-to-dust ratio as probed by the
to ratio of the extra absorber is compatible with the ISM ratio.
Combining both results suggests that the extra absorber, i.e., material at
au, has no significant gas excess in contrast to the elevated
gas-to-dust ratio previously derived for material in the inner region
(au).Comment: 16 pages, 12 figures, accepted by A&
Huygens' Principle for the Klein-Gordon equation in the de Sitter spacetime
In this article we prove that the Klein-Gordon equation in the de Sitter
spacetime obeys the Huygens' principle only if the physical mass of the
scalar field and the dimension of the spatial variable are tied by
the equation . Moreover, we define the incomplete Huygens'
principle, which is the Huygens' principle restricted to the vanishing second
initial datum, and then reveal that the massless scalar field in the de Sitter
spacetime obeys the incomplete Huygens' principle and does not obey the
Huygens' principle, for the dimensions , only. Thus, in the de Sitter
spacetime the existence of two different scalar fields (in fact, with m=0 and
), which obey incomplete Huygens' principle, is equivalent to
the condition (in fact, the spatial dimension of the physical world). For
these two values of the mass are the endpoints of the so-called in
quantum field theory the Higuchi bound. The value of the
physical mass allows us also to obtain complete asymptotic expansion of the
solution for the large time. Keywords: Huygens' Principle; Klein-Gordon
Equation; de Sitter spacetime; Higuchi Boun
50GHz Ge waveguide electro-absorption modulator integrated in a 220nm SOI photonics platform
We report waveguide-integrated Ge electro-absorption modulators operating at 1615nm wavelength with 3dB bandwidth beyond 50GHz and a capacitance of 10fF, A 2V voltage swing enables 4.6dB DC extinction ratio for 4.1dB insertion loss
Nonlinear multidimensional cosmological models with form fields: stabilization of extra dimensions and the cosmological constant problem
We consider multidimensional gravitational models with a nonlinear scalar
curvature term and form fields in the action functional. In our scenario it is
assumed that the higher dimensional spacetime undergoes a spontaneous
compactification to a warped product manifold. Particular attention is paid to
models with quadratic scalar curvature terms and a Freund-Rubin-like ansatz for
solitonic form fields. It is shown that for certain parameter ranges the extra
dimensions are stabilized. In particular, stabilization is possible for any
sign of the internal space curvature, the bulk cosmological constant and of the
effective four-dimensional cosmological constant. Moreover, the effective
cosmological constant can satisfy the observable limit on the dark energy
density. Finally, we discuss the restrictions on the parameters of the
considered nonlinear models and how they follow from the connection between the
D-dimensional and the four-dimensional fundamental mass scales.Comment: 21 pages, LaTeX2e, minor changes, improved references, fonts include
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