2,614 research outputs found
Spin transition in GdN@C, detected by low-temperature on-chip SQUID technique
We present a magnetic study of the GdN@C molecule, consisting of a
Gd-trimer via a Nitrogen atom, encapsulated in a C cage. This molecular
system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI)
applications. We used a low-temperature technique able to detect small magnetic
signals by placing the sample in the vicinity of an on-chip SQUID. The
technique implemented at NHMFL has the particularity to operate in high
magnetic fields of up to 7 T. The GdN@C shows a paramagnetic
behavior and we find a spin transition of the GdN structure at 1.2 K. We
perform quantum mechanical simulations, which indicate that one of the Gd ions
changes from a state () to a state (), likely due to a charge transfer between the C cage and the ion
Cation occupancy determination in manganese zinc ferrites using Fourier transform infrared spectroscopy
The magnetic and electric properties of ferrites are influenced by the cation distribution within the crystalline spinel lattice. Methods such as extended x-ray-absorption fine structure(EXAFS) have been used to determine cation occupancies within the crystalline structure of materials such as manganesezincferrite (MZFO); however, it is not practical to be used for daily analysis. Fourier transform infrared (FTIR)spectroscopy is another technique which has the potential to determine cation occupancy while offering speed and convenience. In the literature it has been demonstrated that in ferrite systems FTIR data can be correlated to cation percentages when comparing tetrahedral (Td) and octahedral (Oh) sites. FTIRspectra were collected on a series of MZFO nanoparticles in the range from 200 to 600cm−1 and two absorbance peaks were observed. The first absorption region shifted with changing sample composition as calculated from transmission EXAFS experiments and elemental analysis. The data was normalized to the maximum of the peak of interest and the shifts were correlated to cation occupancy
Retrospective Predictors of Successful Career Change Among Older Workers: Perceived Objective Success Versus Perceived Subjective Success
Investigating the Magnetospheres of Rapidly Rotating B-type Stars
Recent spectropolarimetric surveys of bright, hot stars have found that ~10%
of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG).
The prominent paradigm describing the interaction between the stellar winds and
the surface magnetic field is the magnetically confined wind shock (MCWS)
model. In this model, the stellar wind plasma is forced to move along the
closed field loops of the magnetic field, colliding at the magnetic equator,
and creating a shock. As the shocked material cools radiatively it will emit
X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and
characterizing the hot wind material confined by the magnetic fields of these
stars. Some B-type stars are found to have very short rotational periods. The
effects of the rapid rotation on the X-ray production within the magnetosphere
have yet to be explored in detail. The added centrifugal force due to rapid
rotation is predicted to cause faster wind outflows along the field lines,
leading to higher shock temperatures and harder X-rays. However, this is not
observed in all rapidly rotating magnetic B-type stars. In order to address
this from a theoretical point of view, we use the X-ray Analytical Dynamical
Magnetosphere (XADM) model, originally developed for slow rotators, with an
implementation of new rapid rotational physics. Using X-ray spectroscopy from
ESA's XMM-Newton space telescope, we observed 5 rapidly rotating B-type stars
to add to the previous list of observations. Comparing the observed X-ray
luminosity and hardness ratio to that predicted by the XADM allows us to
determine the role the added centrifugal force plays in the magnetospheric
X-ray emission of these stars.Comment: IAUS Conference Proceeding
Photometric variability of the LAMOST sample of magnetic chemically peculiar stars as seen by TESS
High-quality light curves from space missions have opened up a new window on
the rotational and pulsational properties of magnetic chemically peculiar (mCP)
stars and have fuelled asteroseismic studies. They allow the internal effects
of surface magnetic fields to be probed and numerous astrophysical parameters
to be derived with great precision. We present an investigation of the
photometric variability of a sample of 1002 mCP stars discovered in the LAMOST
archival spectra with the aims of measuring their rotational periods and
identifying interesting objects for follow-up studies. TESS photometry was
available for 782 mCP stars and was analysed using a Fourier two-term frequency
fit to determine the stars' rotational periods. The rotational signal was then
subtracted from the light curve to identify non-rotational variability. A
pixel-level blending analysis was performed to check whether the variability
originates in the target star or a nearby blended neighbour. We investigated
correlations between the rotational periods, fractional age on the main
sequence, mass, and several other observables. We present rotational periods
and period estimates for 720 mCP stars. In addition, we identified four
eclipsing binary systems that likely host an mCP star, as well as 25 stars with
additional signals consistent with pulsation (12 stars with frequencies above
10 d and 13 stars with frequencies below 10 ). We find that more
evolved stars have longer rotation periods, in agreement with the assumption of
the conservation of angular momentum during main-sequence evolution. With our
work, we increase the sample size of mCP stars with known rotation periods and
identify prime candidates for detailed follow-up studies. This enables two
paths towards future investigations: population studies of even larger samples
of mCP stars and the detailed characterisation of high-value targets.Comment: 30 pages, 9 figures, 1 table. Accepted for publication in the Journal
of Astronomy and Astrophysics (A&A
MOBSTER -- VII. Using light curves to infer magnetic and rotational properties of stars with centrifugal magnetospheres
Early-type B stars with strong magnetic fields and rapid rotation form
centrifugal magnetospheres (CMs), as the relatively weak stellar wind becomes
magnetically confined and centrifugally supported above the Kepler co-rotation
radius. CM plasma is concentrated at and above the Kepler co-rotation radius at
the intersection between the rotation and magnetic field axis. Stellar rotation
can cause these clouds of material to intersect the viewer's line-of-sight,
leading to photometric eclipses. However, for stars with strong () magnetic fields and rapid rotation, CMs can become optically thick enough
for emission to occur via electron scattering. Using high-precision space
photometry from a sample of stars with strong H emission, we apply
simulated light curves from the Rigidly Rotating Magnetosphere model to
directly infer magnetic and rotational properties of these stars. By comparing
the values inferred from photometric modelling to those independently
determined by spectropolarimetry, we find that magnetic obliquity angle
, viewer inclination and critical rotation fraction can be
approximately recovered for 3 of the 4 stars studied here. However, there are
large discrepancies between the optical depth at the Kepler radius expected from magnetometry, and the values required to match the
observations. We show that of order unity is needed to
reasonably match the light curve morphology of our sample stars.Comment: 15 pages, 8 figures. Accepted for publication in MNRA
Discovery Of A Magnetic Field In The Rapidly Rotating O-Type Secondary Of The Colliding-Wind Binary HD 47129 (Plaskett\u27s Star)
We report the detection of a strong, organized magnetic field in the secondary component of the massive O8III/I+O7.5V/III double-lined spectroscopic binary system HD 47129 (Plaskett\u27s star) in the context of the Magnetism in Massive Stars survey. Eight independent Stokes V observations were acquired using the Echelle SpectroPolarimetric Device for the Observations of Stars (ESPaDOnS) spectropolarimeter at the Canada-France-Hawaii Telescope and the Narval spectropolarimeter at the Telescope Bernard Lyot. Using least-squares deconvolution we obtain definite detections of signal in Stokes V in three observations. No significant signal is detected in the diagnostic null (N) spectra. The Zeeman signatures are broad and track the radial velocity of the secondary component; we therefore conclude that the rapidly rotating secondary component is the magnetized star. Correcting the polarized spectra for the line and continuum of the (sharp-lined) primary, we measured the longitudinal magnetic field from each observation. The longitudinal field of the secondary is variable and exhibits extreme values of -810 +/- 150 and +680 +/- 190 G, implying a minimum surface dipole polar strength of 2850 +/- 500 G. In contrast, we derive an upper limit (3 sigma) to the primary\u27s surface magnetic field of 230 G. The combination of a strong magnetic field and rapid rotation leads us to conclude that the secondary hosts a centrifugal magnetosphere fed through a magnetically confined wind. We revisit the properties of the optical line profiles and X-ray emission - previously interpreted as a consequence of colliding stellar winds - in this context. We conclude that HD 47129 represents a heretofore unique stellar system - a close, massive binary with a rapidly rotating, magnetized component - that will be a rich target for further study
Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations
Recent spectropolarimetric studies of 7 SPB and Cep stars have
suggested that photospheric magnetic fields are more common in B-type pulsators
than in the general population of B stars, suggesting a significant connection
between magnetic and pulsational phenomena. We present an analysis of new and
previously published spectropolarimetric observations of these stars. New
Stokes observations obtained with the high-resolution ESPaDOnS and Narval
instruments confirm the presence of a magnetic field in one of the stars
( Lup), but find no evidence of magnetism in 5 others. A re-analysis
of the published longitudinal field measurements obtained with the
low-resolution FORS1/2 spectropolarimeters finds that the measurements of all
stars show more scatter from zero than can be attributed to Gaussian noise,
suggesting the presence of a signal and/or systematic under-estimation of error
bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO
archive demonstrates that small changes in reduction procedure lead to
substantial changes in the inferred longitudinal field, and substantially
reduces the number of field detections at the 3 level. Furthermore, we
find that the published periods are not unique solutions to the time series of
either the original or the revised FORS1/2 data. We conclude that the reported
field detections, proposed periods and field geometry models for Pyx,
15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction
procedures, and that magnetic fields at the reported strength are no more
common in SPB/ Cep stars than in the general population of B stars.Comment: 10 pages, 5 figures, accepted for publication in ApJ, 2012, typo
correcte
The MiMeS Project: Overview and Current Status
The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration
among many of the foremost international researchers of the physics of hot,
massive stars, with the basic aim of understanding the origin, evolution and
impact of magnetic fields in these objects. At the time of writing, MiMeS Large
Programs have acquired over 950 high-resolution polarised spectra of about 150
individual stars with spectral types from B5-O4, discovering new magnetic
fields in a dozen hot, massive stars. The quality of this spectral and magnetic
mat\'eriel is very high, and the Collaboration is keen to connect with
colleagues capable of exploiting the data in new or unforeseen ways. In this
paper we review the structure of the MiMeS observing programs and report the
status of observations, data modeling and development of related theory.Comment: Proceedings of IAUS272: Active OB star
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