91,361 research outputs found
Understanding of the Retarded Oxidation Effects in Silicon Nanostructures
In-depth understanding of the retarded oxidation phenomenon observed during
the oxidation of silicon nanostructures is proposed. The wet thermal oxidation
of various silicon nanostructures such as nanobeams, concave/convex nanorings
and nanowires exhibits an extremely different and complex behavior. Such
effects have been investigated by the modeling of the mechanical stress
generated during the oxidation process explaining the retarded regime. The
model describes the oxidation kinetics of silicon nanowires down to a few
nanometers while predicting reasonable and physical stress levels at the
Si/SiO interface by correctly taking into account the relaxation effects
in silicon oxide through plastic flow
Planetary companions orbiting M giants HD 208527 and HD 220074
Aims. The purpose of the present study is to research the origin of planetary
companions by using a precise radial velocity (RV) survey.
Methods. The high-resolution spectroscopy of the fiber-fed Bohyunsan
Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy
Observatory (BOAO) is used from September 2008 to June 2012.
Results. We report the detection of two exoplanets in orbit around HD 208527
and HD 220074 exhibiting periodic variations in RV of 875.5 +/- 5.8 and 672.1
+/- 3.7 days. The RV variations are not apparently related to the surface
inhomogeneities and a Keplerian motion of the planetary companion is the most
likely explanation. Assuming possible stellar masses of 1.6 +/- 0.4 and 1.2 +/-
0.3 M_Sun, we obtain the minimum masses for the exoplanets of 9.9 +/- 1.7 and
11.1 +/- 1.8 M_Jup around HD 208527 and HD 220074 with an orbital semi-major
axis of 2.1 +/- 0.2 and 1.6 +/- 0.1 AU and an eccentricity of 0.08 and 0.14,
respectively. We also find that the previously known spectral classification of
HD 208527 and HD 220074 was in error: Our new estimation of stellar parameters
suggest that both HD 208527 and HD 220074 are M giants. Therefore, HD 208527
and HD 220074 are so far the first candidate M giants to harbor a planetary
companion.Comment: 7 pages, 9 figures, 4 tables, accepted for publisation in Astronomy &
Astrophysic
Constraints on SN Ia progenitor time delays from high-z SNe and the star formation history
We re-assess the question of a systematic time delay between the formation of
the progenitor and its explosion in a type Ia supernova (SN Ia) using the
Hubble Higher-z Supernova Search sample (Strolger et al. 2004). While the
previous analysis indicated a significant time delay, with a most likely value
of 3.4 Gyr, effectively ruling out all previously proposed progenitor models,
our analysis shows that the time-delay estimate is dominated by systematic
errors, in particular due to uncertainties in the star-formation history. We
find that none of the popular progenitor models under consideration can be
ruled out with any significant degree of confidence. The inferred time delay is
mainly determined by the peak in the assumed star-formation history. We show
that, even with a much larger Supernova sample, the time delay distribution
cannot be reliably reconstructed without better constraints on the
star-formation history.Comment: accepted for publication in MNRA
Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae
Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and
galactic chemical evolution. They are thought to be thermonuclear explosions of
carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar
mass limit in binaries. Previous studies have suggested that He novae may be
progenitor candidates of SNe Ia. However, the mass retention efficiencies
during He nova outbursts are still uncertain. In this article, we aim to study
the mass retention efficiencies of He nova outbursts and to investigate whether
SNe Ia can be produced through He nova outbursts. Using the stellar evolution
code Modules for Experiments in Stellar Astrophysics, we simulated a series of
multicycle He-layer flashes, in which the initial WD masses range from 0.7 to
1.35 Msun with various accretion rates. We obtained the mass retention
efficiencies of He nova outbursts for various initial WD masses, which can be
used in the binary population synthesis studies. In our simulations, He nova
outbursts can increase the mass of the WD to the Chandrasekar mass limit and
the explosive carbon burning can be triggered in the center of the WD; this
suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass
retention efficiencies in the present work are lower than those of previous
studies, which leads to a lower birthrates of SNe Ia through the WD + He star
channel. Furthermore, we obtained the elemental abundances distribution at the
moment of explosive carbon burning, which can be used as the initial input
parameters in studying explosion models of SNe Ia.Comment: 8 pages, 12 figures, 2 tables, published in Astronomy & Astrophysics
(A&A 604, A31, 2017
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