643 research outputs found
Tracing early evolutionary stages of high-mass star formation with molecular lines
Despite its major role in the evolution of the interstellar medium, the
formation of high-mass stars (M > 10 Msol) is still poorly understood. Two
types of massive star cluster precursors, the so-called Massive Dense Cores
(MDCs), have been observed, which differ in their mid-infrared brightness. The
origin of this difference is not established and could be the result of
evolution, density, geometry differences, or a combination of these. We compare
several molecular tracers of physical conditions (hot cores, shocks) observed
in a sample of mid-IR weak emitting MDCs with previous results obtained in a
sample of exclusively mid-IR bright MDCs. The aim is to understand the
differences between these two types of object. We present single-dish
observations of HDO, H2O-18, SO2 and CH3OH lines at lambda = 1.3 - 3.5 mm. We
study line profiles and estimate abundances of these molecules, and use a
partial correlation method to search for trends in the results. The detection
rates of thermal emission lines are found to be very similar between mid-IR
quiet and bright objects. The abundances of H2O, HDO (1E-13 to 1E-9 in the cold
outer envelopes), SO2 and CH3OH differ from source to source but independently
of their mid-IR flux. In contrast, the methanol class I maser emission, a
tracer of outflow shocks, is found to be strongly anti-correlated with the 12
micron source brightnesses. The enhancement of the methanol maser emission in
mid-IR quiet MDCs may indicate a more embedded nature. Since total masses are
similar between the two samples, we suggest that the matter distribution is
spherical around mid-IR quiet sources but flattened around mid-IR bright ones.
In contrast, water emission is associated with objects containing a hot
molecular core, irrespective of their mid-IR brightness. These results indicate
that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.Comment: 15 pages, 6 figures, 11 tables, accepted for publication in A&A the
11/06/201
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
Ground-State SiO Maser Emission Toward Evolved Stars
We have made the first unambiguous detection of vibrational ground-state
maser emission from SiO toward six evolved stars. Using the Very Large Array,
we simultaneously observed the v=0, J=1-0, 43.4-GHz, ground-state and the v=1,
J=1-0, 43.1-GHz, first excited-state transitions of SiO toward the oxygen-rich
evolved stars IRC+10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the
S-type star chi Cyg. We detected at least one v=0 SiO maser feature from six of
the seven stars observed, with peak maser brightness temperatures ranging from
10,000 K to 108,800 K. In fact, four of the seven v=0 spectra show multiple
maser peaks, a phenomenon which has not been previously observed. Ground-state
thermal emission was detected for one of the stars, RX Boo, with a peak
brightness temperature of 200 K. Comparing the v=0 and the v=1 transitions, we
find that the ground-state masers are much weaker with spectral characteristics
different from those of the first excited-state masers. For four of the seven
stars the velocity dispersion is smaller for the v=0 emission than for the v=1
emission, for one star the dispersions are roughly equivalent, and for two
stars (one of which is RX Boo) the velocity spread of the v=0 emission is
larger. In most cases, the peak flux density in the v=0 emission spectrum does
not coincide with the v=1 maser peak. Although the angular resolution of these
VLA observations were insufficient to completely resolve the spatial structure
of the SiO emission, the SiO spot maps produced from the interferometric image
cubes suggest that the v=0 masers are more extended than their v=1
counterparts
Field-induced magnetic anisotropy in La0.7Sr0.3CoO3
Magnetic anisotropy has been measured for the ferromagnetic La0.7Sr0.3CoO3
perovskite from an analysis of the high-field part of the magnetization vs.
field curves, i.e., the magnetic saturation regime. These measurements give a
magnetic anistropy one order of magnitude higher than that of reference
manganites. Surprisingly, the values of the magnetic anisotropy calculated in
this way do not coincide with those estimated from measurements of coercive
fields which are one order of magnitude smaller. It is proposed that the reason
of this anomalous behaviour is a transition of the trivalent Co ions under the
external magnetic field from a low-spin to an intermediate-spin state. Such a
transition converts the Co3+ ions into Jahn-Teller ions having an only
partially quenched orbital angular momentum, which enhances the intra-atomic
spin-orbit coupling and magnetic anisotropy.Comment: Accepted of publication in Europhysics Letters, 11 pages, 5 figure
Comparison of high spatial resolution stereo-PIV measurements in a turbulent boundary layer with available DNS dataset
In the present contribution, the aptitude of Stereoscopic Particle Image Velocimetry (SPIV) and of Direct Numerical Simulations (DNS) to investigate coherent structures of near wall turbulence is evaluated. For this purpose, the general properties and constraints of the two techniques are first reviewed. Then, data obtained from stereo-PIV experiments in a boundary layer and DNS in a channel flow are considered. Some statistics of the velocity fields are computed, and the results obtained from the two approaches compared
A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3
ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K
The multiferroic phases of (Eu:Y)MnO3
We report on structural, magnetic, dielectric, and thermodynamic properties
of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the
multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the
interference of magnetic contributions of the 4f-ions. In addition, it offers
the possibility to continuously tune the influence of the A-site ionic radii.
For small concentrations x <= 0.1 we find a canted antiferromagnetic and
paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric
polarization coexists with the features of a long wavelength incommensurate
spiral magnetic phase analogous to the observations in TbMnO3. In the
intermediate concentration range around x = 0.2 a multiferroic scenario is
realized combining weak ferroelectricity and weak ferromagnetism, presumably
due to a canted spiral magnetic structure.Comment: 8 pages, 8 figure
Evolution of massive protostars: the IRAS 18151-1208 region
The study of physical and chemical properties of massive protostars is
critical to better understand the evolutionary sequence which leads to the
formation of high-mass stars. IRAS 18151-1208 is a nearby massive region (d =
3kpc, L ~ 20000 Lsun) which splits into three cores: MM1, MM2 and MM3
(separated by 1'-2'). We aim at (1) studying the physical and chemical
properties of the individual MM1, MM2 and MM3 cores; (2) deriving their
evolutionary stages; (3) using these results to improve our view of the
evolutionary sequence of massive cores. The region was observed in the CS,
C34S, H2CO, HCO+, H13CO+, and N2H+ lines at mm wavelengths with the IRAM 30m
and Mopra telescopes. We use 1D and 2D modeling of the dust continuum to derive
the density and temperature distributions, which are then used in the RATRAN
code to model the lines and constrain the abundances of the observed species.
All the lines were detected in MM1 and MM2. MM3 shows weaker emission, or even
is undetected in HCO+ and all isotopic species. MM2 is driving a newly
discovered CO outflow and hosts a mid-IR-quiet massive protostar. The abundance
of CS is significantly larger in MM1 than in MM2, but smaller than in a
reference massive protostar such as AFGL2591. In contrast the N2H+ abundance
decreases from MM2 to MM1, and is larger than in AFGL2591. Both MM1 and MM2
host an early phase massive protostar, but MM2 (and mid-IR-quiet sources in
general) is younger and more dominated by the host protostar than MM1
(mid-IR-bright). The MM3 core is probably in a pre-stellar phase. We find that
the N2H+/C34S ratio varies systematically with age in the massive protostars
for which the data are available. It can be used to identify young massive
protostars.Comment: 19 pages, 17 figures, accepted by A&A the 3 June 200
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