356 research outputs found
Orbital Evolution of Planets around Intermediate-Mass Giants
Around low- and intermediate-mass (1.5-3 M_sun) red giants, no planets have
been found inside 0.6 AU. Such a paucity is not seen in the case of 1 M_sun
main sequence stars. In this study, we examine the possibility that
short-period planets were engulfed by their host star evolving off the main
sequence. To do so, we have simulated the orbital evolution of planets,
including the effects of stellar tide and mass loss, to determine the critical
semimajor axis, a_crit, beyond which planets survive the RGB expansion of their
host star. We have found that a_crit changes drastically around 2 M_sun: In the
lower-mass range, a_crit is more than 1 AU, while a_crit is as small as about
0.2 AU in the higher-mass range. Comparison with measured semimajor axes of
known planets suggests that there is a lack of planets that only planet
engulfment never accounts for in the higher-mass range. Whether the lack is
real affects our understanding of planet formation. Therefore, increasing the
number of planet samples around evolved intermediate-mass stars is quite
meaningful to confirm robustness of the lack of planets.Comment: 4 pages, 3 figures, Part of PlanetsbeyondMS/2010 proceedings
http://arxiv.org/html/1011.660
The Stellar Abundances for Galactic Archeology (SAGA) Database - Compilation of the Characteristics of Known Extremely Metal-Poor Stars
We describe the construction of a database of extremely metal-poor (EMP)
stars in the Galactic halo whose elemental abundances have been determined. Our
database contains detailed elemental abundances, reported equivalent widths,
atmospheric parameters, photometry, and binarity status, compiled from papers
in the recent literature that report studies of EMP halo stars with [Fe/H] <
-2.5. The compilation procedures for this database have been designed to
assemble the data effectively from electronic tables available from online
journals. We have also developed a data retrieval system that enables data
searches by various criteria, and permits the user to explore relationships
between the stored variables graphically. Currently, our sample includes 1212
unique stars (many of which are studied by more than one group) with more than
15000 individual reported elemental abundances, covering all of the relevant
papers published by December 2007. We discuss the global characteristics of the
present database, as revealed by the EMP stars observed to date. For stars with
[Fe/H] < -2.5, the number of giants with reported abundances is larger than
that of dwarfs by a factor of two. The fraction of carbon-rich stars (among the
sample for which the carbon abundance is reported) amount to ~30 % for [Fe/H] <
-2.5. We find that known binaries exhibit different distributions of orbital
period, according to whether they are giants or dwarfs, and also as a function
of metallicity, although the total sample of such stars is still quite small.Comment: 24 pages, 10 figures, accepted by PASJ, final version. The SAGA
database is available at http://saga.sci.hokudai.ac.j
Transition of the Stellar Initial Mass Function Explored with Binary Population Synthesis
The stellar initial mass function (IMF) plays a crucial role in determining
the number of surviving stars in galaxies, the chemical composition of the
interstellar medium, and the distribution of light in galaxies. A key unsolved
question is whether the IMF is universal in time and space. Here we use
state-of-the-art results of stellar evolution to show that the IMF of our
Galaxy made a transition from an IMF dominated by massive stars to the
present-day IMF at an early phase of the Galaxy formation. Updated results from
stellar evolution in a wide range of metallicities have been implemented in a
binary population synthesis code, and compared with the observations of
carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying
the present-day IMF to Galactic halo stars causes serious contradictions with
four observable quantities connected with the evolution of AGB stars.
Furthermore, a comparison between our calculations and the observations of CEMP
stars may help us to constrain the transition metallicity for the IMF which we
tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion
of mass loss suppression in intermediate-mass AGB stars at low-metallicity.
This significantly reduces the overproduction of nitrogen-enhanced stars that
was a major problem in using the high-mass star dominated IMF in previous
studies. Our results also demonstrate that the use of the present day IMF for
all time in chemical evolution models results in the overproduction of Type I.5
supernovae. More data on stellar abundances will help to understand how the IMF
has changed and what caused such a transition.Comment: 8 pages, 2 figures, accepted by MNRAS Lette
Does trampoline or hard surface jumping influence lower extremity alignment?
[Purpose] To determine whether repetitive trampoline or hard surface jumping affects lower extremity alignment on jump landing. [Subjects and Methods] Twenty healthy females participated in this study. All subjects performed a drop vertical jump before and after repeated maximum effort trampoline or hard surface jumping. A three-dimensional motion analysis system and two force plates were used to record lower extremity angles, moments, and vertical ground reaction force during drop vertical jumps. [ Results] Knee extensor moment after trampoline jumping was greater than that after hard surface jumping. There were no significant differences between trials in vertical ground reaction force and lower extremity joint angles following each form of exercise. Repeated jumping on a trampoline increased peak vertical ground reaction force, hip extensor, knee extensor moments, and hip adduction angle, while decreasing hip flexion angle during drop vertical jumps. In contrast, repeated jumping on a hard surface increased peak vertical ground reaction force, ankle dorsiflexion angle, and hip extensor moment during drop vertical jumps. [Conclusion] Repeated jumping on the trampoline compared to jumping on a hard surface has different effects on lower limb kinetics and kinematics. Knowledge of these effects may be useful in designing exercise programs for different clinical presentations
Fermi-LAT and Suzaku Observations of the Radio Galaxy Centaurus B
Centaurus B is a nearby radio galaxy positioned in the Southern hemisphere
close to the Galactic plane. Here we present a detailed analysis of about 43
months of accumulated Fermi-LAT data of the gamma-ray counterpart of the source
initially reported in the 2nd Fermi-LAT catalog, and of newly acquired Suzaku
X-ray data. We confirm its detection at GeV photon energies, and analyze the
extension and variability of the gamma-ray source in the LAT dataset, in which
it appears as a steady gamma-ray emitter. The X-ray core of Centaurus B is
detected as a bright source of a continuum radiation. We do not detect however
any diffuse X-ray emission from the known radio lobes, with the provided upper
limit only marginally consistent with the previously claimed ASCA flux. Two
scenarios that connect the X-ray and gamma-ray properties are considered. In
the first one, we assume that the diffuse non-thermal X-ray emission component
is not significantly below the derived Suzaku upper limit. In this case,
modeling the inverse-Compton emission shows that the observed gamma-ray flux of
the source may in principle be produced within the lobes. This association
would imply that efficient in-situ acceleration of the radiating electrons is
occurring and that the lobes are dominated by the pressure from the
relativistic particles. In the second scenario, with the diffuse X-ray emission
well below the Suzaku upper limits, the lobes in the system are instead
dominated by the magnetic pressure. In this case, the observed gamma-ray flux
is not likely to be produced within the lobes, but instead within the nuclear
parts of the jet. By means of synchrotron self-Compton modeling we show that
this possibility could be consistent with the broad-band data collected for the
unresolved core of Centaurus B, including the newly derived Suzaku spectrum.Comment: Accepted for publication in A&A. 11 page
Transition of the stellar initial mass function explored using binary population synthesis
The stellar initial mass function (IMF) plays a crucial role in the determination of the number of surviving stars in galaxies, of the chemical composition of the interstellar medium and of the distribution of light in galaxies. A key unsolved question i
Fermi-LAT Observation of Supernova Remnant S147
We present an analysis of gamma-ray data obtained with the Large Area
Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the region
around SNR S147 (G180.0-1.7). A spatially extended gamma-ray source detected in
an energy range of 0.2--10 GeV is found to coincide with SNR S147. We confirm
its spatial extension at >5sigma confidence level. The gamma-ray flux is (3.8
\pm 0.6) x 10^{-8} photons cm^{-2} s^{-1}, corresponding to a luminosity of 1.3
x 10^{34} (d/1.3 kpc)^2 erg s^{-1} in this energy range. The gamma-ray emission
exhibits a possible spatial correlation with prominent Halpha filaments of
S147. There is no indication that the gamma-ray emission comes from the
associated pulsar PSR J0538+2817. The gamma-ray spectrum integrated over the
remnant is likely dominated by the decay of neutral pi mesons produced through
the proton--proton collisions in the filaments. Reacceleration of pre-existing
CRs and subsequent adiabatic compression in the filaments is sufficient to
provide the required energy density of high-energy protons.Comment: 12 pages, accepted for publication in Ap
Planet Engulfment by ~1.5-3 Solar-Mass Red Giants
Recent radial-velocity surveys for GK clump giants have revealed that planets
also exist around ~1.5-3 Msun stars. However, no planets have been found inside
0.6 AU around clump giants, in contrast to solar-type main-sequence stars, many
of which harbor short-period planets such as hot Jupiters. In this study we
examine the possibility that planets were engulfed by host stars evolving on
the red-giant branch (RGB). We integrate the orbital evolution of planets in
the RGB and helium burning (HeB) phases of host stars, including the effects of
stellar tide and stellar mass loss. Then we derive the critical semimajor axis
(or the survival limit) inside which planets are eventually engulfed by their
host stars after tidal decay of their orbits. Especially, we investigate the
impact of stellar mass and other stellar parameters on the survival limit in
more detail than previous studies. In addition, we make detailed comparison
with measured semimajor axes of planets detected so far, which no previous
study did. We find that the critical semimajor axis is quite sensitive to
stellar mass in the range between 1.7 and 2.1 Msun, which suggests a need for
careful comparison between theoretical and observational limits of existence of
planets. Our comparison demonstrates that all those planets are beyond the
survival limit, which is consistent with the planet-engulfment hypothesis.
However, on the high-mass side (> 2.1 Msun), the detected planets are orbiting
significantly far from the survival limit, which suggests that engulfment by
host stars may not be the main reason for the observed lack of short-period
giant planets. To confirm our conclusion, the detection of more planets around
clump giants, especially with masses > 2.5 Msun, is required.Comment: 12 pages, 6 figures, 1 table, accepted for publication in Ap
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