198 research outputs found
Diversity of chemistry and excitation conditions in the high-mass star forming complex W33
The object W33 is a giant molecular cloud that contains star forming regions
at various evolutionary stages from quiescent clumps to developed H II regions.
Since its star forming regions are located at the same distance and the primary
material of the birth clouds is probably similar, we conducted a comparative
chemical study to trace the chemical footprint of the different phases of
evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment
(APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We
detected 27 transitions of 10 different molecules in the APEX data and 52
transitions of 16 different molecules in the SMA data. The chemistry on scales
larger than 0.2 pc, which are traced by the APEX data, becomes more
complex and diverse the more evolved the star forming region is. On smaller
scales traced by the SMA data, the chemical complexity and diversity increase
up to the hot core stage. In the H II region phase, the SMA spectra resemble
the spectra of the protostellar phase. Either these more complex molecules are
destroyed or their emission is not compact enough to be detected with the SMA.
Synthetic spectra modelling of the HCO transitions, as detected with the
APEX telescope, shows that both a warm and a cold component are needed to
obtain a good fit to the emission for all sources except for W33 Main1. The
temperatures and column densities of the two components increase during the
evolution of the star forming regions. The integrated intensity ratios
NH(32)/CS(65) and
NH(32)/HCO(43) show clear trends as a
function of evolutionary stage, luminosity, luminosity-to-mass ratio, and
H peak column density of the clumps and might be usable as chemical
clocks.Comment: 66 pages, 28 figures, 8 tables, accepted for publication at A&
Resolving the distance controversy for Sharpless 269: A possible kink in the outer arm
Sharpless 269 (S269) is one of a few HII regions in the outer spiral arm of
the Milky Way with strong water maser emission. Based on data from the Very
Long Baseline Interferometry (VLBI) Exploration of Radio Astrometry (VERA)
array, two parallax measurements have been published, which differ by nearly
. Each distance estimate supports a different structure for the outer
arm. Moreover, given its large Galactocentric radii, S269 has special relevance
as its proper motion and parallax have been used to constrain the Galactic
rotation curve at large radii. Using recent Very Long Baseline Array (VLBA)
observations, we accurately measure the parallax and proper motion of the water
masers in S269. We interpret the position and motion of S269 in the context of
Galactic structure, and possible optical counterparts. S269's 22 GHz water
masers and two close-by quasars were observed at 16 epochs between 2015 and
2016 using the VLBA. We measure an annual parallax for S269 of 0.241
0.012 mas corresponding to a distance from the Sun of
kpc by fitting four maser spots. The mean proper motion for S269 was estimated
as mas and mas for
and respectively, which corresponds
to the motion expected for a flat Galactic rotation curve at large radius. This
distance estimate, Galactic kinematic simulations and observations of other
massive young stars in the outer region support the existence of a kink in the
outer arm at 140 degrees. Additionally, we find more than 2,000
optical sources in the Gaia DR2 catalog within 125 pc radius around the 3D
position of the water maser emission; from those only three sources are likely
members of the same stellar association that contains the young massive star
responsible for the maser emission (S269 IRS 2w).Comment: Accepted for publication in A&
Anomalous peculiar motions of high-mass young stars in the Scutum spiral arm
We present trigonometric parallax and proper motion measurements toward 22
GHz water and 6.7 GHz methanol masers in 16 high-mass star-forming regions.
These sources are all located in the Scutum spiral arm of the Milky Way. The
observations were conducted as part of the Bar and Spiral Structure Legacy
(BeSSeL) survey. A combination of 14 sources from a forthcoming study and 14
sources from the literature, we now have a sample of 44 sources in the Scutum
spiral arm, covering a Galactic longitude range from 0 to 33. A
group of 16 sources shows large peculiar motions of which 13 are oriented
toward the inner Galaxy. A likely explanation for these high peculiar motions
is the combined gravitational potential of the spiral arm and the Galactic bar.Comment: 27 pages, 52 figures, 4 tables, accepted for publication in A&
High field x-ray diffraction study on a magnetic-field-induced valence transition in YbInCu4
We report the first high-field x-ray diffraction experiment using synchrotron
x-rays and pulsed magnetic fields exceeding 30 T. Lattice deformation due to a
magnetic-field-induced valence transition in YbInCu4 is studied. It has been
found that the Bragg reflection profile at 32 K changes significantly at around
27 T due to the structural transition. In the vicinity of the transition field
the low-field and the high-field phases are observed simultaneously as the two
distinct Bragg reflection peaks: This is a direct evidence of the fact that the
field-induced valence state transition is the first order phase transition. The
field-dependence of the low-field-phase Bragg peak intensity is found to be
scaled with the magnetization.Comment: 5 pages, 6 figures, submitted to J. Phys. Soc. Jp
Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}
For describing the first-order isostructural valence phase transition in
mixed valence compounds we develop a new approach based on the lattice Anderson
model. We take into account the Coulomb interaction between localized f and
conduction band electrons and two mechanisms of electron-lattice coupling. One
is related to the volume dependence of the hybridization. The other is related
to local deformations produced by f- shell size fluctuations accompanying
valence fluctuations. The large f -state degeneracy allows us to use the 1/N
expansion method. Within the model we develop a mean-field theory for the
first-order valence phase transition in YbInCu_{4}. It is shown that the
Coulomb interaction enhances the exchange interaction between f and conduction
band electron spins and is the driving force of the phase transition. A
comparison between the theoretical calculations and experimental measurements
of the valence change, susceptibility, specific heat, entropy, elastic
constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a
good quantitative agreement is found. On the basis of the model we describe the
evolution from the first-order valence phase transition to the continuous
transition into the heavy-fermion ground state in the series of compounds
YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of
YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.
Optical study of the electronic phase transition of strongly correlated YbInCu_4
Infrared, visible and near-UV reflectivity measurements are used to obtain
conductivity as a function of temperature and frequency in YbInCu_4, which
exhibits an isostructural phase-transition into a mixed-valent phase below
T_v=42 K. In addition to a gradual loss of spectral weight with decreasing
temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the
mixed-valent phase. This feature can be described in terms of excitations into
the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of
resistivity, provides a direct reflection of the onset of coherence in this
strongly correlated electron system.Comment: 4 pages, 3 figures (to appear in Phys. Rev. B
Valence Fluctuations Revealed by Magnetic Field Scan: Comparison with Experiments in YbXCu_4 (X=In, Ag, Cd) and CeYIn_5 (Y=Ir, Rh)
The mechanism of how critical end points of the first-order valence
transitions (FOVT) are controlled by a magnetic field is discussed. We
demonstrate that the critical temperature is suppressed to be a quantum
critical point (QCP) by a magnetic field. This results explain the field
dependence of the isostructural FOVT observed in Ce metal and YbInCu_4.
Magnetic field scan can lead to reenter in a critical valence fluctuation
region. Even in the intermediate-valence materials, the QCP is induced by
applying a magnetic field, at which the magnetic susceptibility also diverges.
The driving force of the field-induced QCP is shown to be a cooperative
phenomenon of the Zeeman effect and the Kondo effect, which creates a distinct
energy scale from the Kondo temperature. The key concept is that the closeness
to the QCP of the FOVT is capital in understanding Ce- and Yb-based heavy
fermions. It explains the peculiar magnetic and transport responses in CeYIn_5
(Y=Ir, Rh) and metamagnetic transition in YbXCu_4 for X=In as well as the sharp
contrast between X=Ag and Cd.Comment: 14 pages, 9 figures, OPEN SELECT in J. Phys. Soc. Jp
Suppression of the structural phase transition in by large magnetic fields
The transition in CeLaTh is measured
as a function of applied magnetic field using both resistivity and
magnetization. The transition temperature decreases with
increasing magnetic field, reaching zero temperature at around 56 T. The
magnetic-field dependence of the transition temperature may be fitted using a
model that invokes the field and temperature dependence of the entropy of the
-electron moments of the phase, suggesting that the volume
collapse in Ce and its alloys is primarily driven by entropic considerations
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