1,614 research outputs found
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey
Context
A large number of magnetic white dwarfs discovered in the SDSS have so far only been analyzed by visual comparison of the observations with relatively simple models of the radiation transport in a magnetised stellar atmosphere.
Aims We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS.
Methods
We calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs (WDs) with magnetic field strengths of between 1 MG and 1200 MG for different angles between the magnetic field vector and the line of sight,and for effective temperatures between 7000 K and 50 000 K. We used a least squares minimization scheme with an evolutionary algorithm to find the best-fit magnetic field geometry of the observed data. We used centered dipoles or dipoles that had been shifted
along the dipole axis to model the coadded SDSS fiber spectrum of each object.
Result
We analyzed the spectra of all known magnetic hydrogen-rich (DA) WDs from the SDSS (97 previously published, plus 44 newly discovered) and also investigated the statistical properties of the magnetic field geometries of this sample.
Conclusions
The total number of known magnetic white dwarfs has already been more than tripled by the SDSS and more objects are expected after more systematic searches. The magnetic fields have strengths of between ≈1 and 900 MG. Our results further support the claims that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and that of either another source of progenitor types or binary evolution is needed. Clear indications of non-centered dipoles exist in about ∼50%, of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars
Zeeman tomography of magnetic white dwarfs. III, The 70–80 Megagauss magnetic field of PG 1015+014
Aims. We analyse the magnetic field geometry of the magnetic DA white dwarf PG 1015+014 with our Zeeman tomography method.
Methods. This study is based on rotation-phase resolved optical flux and circular polarization spectra of PG 1015+014 obtained with FORS1
at the ESO VLT. Our tomographic code makes use of an extensive database of pre-computed Zeeman spectra. The general approach has been described in Papers I and II of this series.
Results. The surface field strength distributions for all rotational phases of PG 1015+014 are characterised by a strong peak at 70 MG. A separate peak at 80 MG is seen for about one third of the rotation cycle. Significant contributions to the Zeeman features arise from regions
with field strengths between 50 and 90 MG. We obtain equally good simultaneous fits to the observations, collected in five phase bins, for two different field parametrizations: (i) a superposition of individually tilted and off-centred zonal multipole components; and (ii) a truncated multipole expansion up to degree l = 4 including all zonal and tesseral components. The magnetic fields generated by both parametrizations exhibit a similar global structure of the absolute surface field values, but differ considerably in the topology of the field lines. An effective photospheric temperature of T eff = 10 000 ± 1000 K was found.
Conclusions. Remaining discrepancies between the observations and our best-fit models suggest that additional small-scale structure of the magnetic field exists which our field models are unable to cover due to the restricted number of free parameters
Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae
Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects.
Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography.
Methods. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an
extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series.
Results. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree lmax = 3 or 5 and include all tesseral and sectoral components with
0 ≤ m ≤ l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, and CP Tuc, and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with
strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr.
Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and
direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure
Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820
Around 10% of white dwarfs exhibit global magnetic structures with fields
ranging from 1 kG to hundreds of MG. Recently, the first radiation
magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that
convection should be suppressed in their photospheres for magnetic fields with
strengths B 50 kG. These predictions are in agreement with our
knowledge of stellar physics (e.g. energy transfer in strong magnetic field
regions of the solar photosphere), but have yet to be directly confirmed from
white dwarf observations. We obtained COS far-UV spectroscopy of the weakly
magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional
non-magnetic, convective remnants (all in the range
9000-11,000 K). We fitted both the COS and the already available optical
spectra with convective and radiative atmospheric models. As expected, we find
that for two of the non-magnetic comparison stars only convective model fits
predict consistent values from both the optical and the FUV
spectra. In contrast, for WD2105-820 only the best fitting radiative model
produced consistent results.Comment: 8 pages, 7 figures, 1 table, accepted for publication in MNRA
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with
magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000
K and 50000 K. Through a least-squares minimization scheme with an evolutionary
algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95
previously published plus 14 newly discovered within SDSS, and five discovered
by SEGUE). Since we were limited to a single spectrum for each object we used
only centered magnetic dipoles or dipoles which were shifted along the magnetic
dipole axis. We also statistically investigated the distribution of
magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the high-field polar AR UMa
Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR
UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a
magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+
components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes
``enabled'' in the presence of both strong magnetic and electric fields. Our
attempt in fitting the overall optical+UV low state spectrum with single
temperature magnetic WD models remains rather unsatisfactory, indicating either
a shortcoming in the present models or a new physical process acting in AR UMa.
As a result, our estimate of the WD temperature remains somewhat uncertain,
Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and
present throughout the entire binary orbit, and a second bump near ~1650A,
which appears only near the inferior conjunction of the secondary star. These
are suggestive of low harmonic cyclotron emission produced by low-level
(M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no
evidence in the power spectrum of light variations for accretion in gas blobs.
The observed Ly alpha emission line shows a strong phase dependence with
maximum flux and redshift near orbital phase phi~0.3, strongly indicating an
origin on the trailing hemisphere of the secondary star. An additional Ly alpha
absorption feature with similar phasing as the Ly alpha emission, but a
\~700km/s blueshift could tentatively be ascribed to absorption of WD emission
in a moderately fast wind. We derive a column density of neutral hydrogen of
NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.Comment: 26 pages, 10 figures, AAS TeX 5.0, accepted for publication in the
Astrophysical Journa
Examining the relationship between rheumatoid arthritis, multimorbidity and adverse health-related outcomes: a systematic review protocol
Background:
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterised by articular inflammation and systemic complications. Multimorbidity (the presence of two or more long-term health conditions) is highly prevalent in people with RA but the effect of multimorbidity on mortality and other health-related outcomes is poorly understood.
Objective:
To determine what is known about the effect, if any, of multimorbidity on mortality and health-related outcomes in individuals with RA.
Design:
Systematic review of the literature. The following electronic medical databases will be searched: MEDLINE, Embase, Cumulative Index to Nursing and Allied Health Literature, PsycINFO, The Cochrane Library and Scopus. Included studies will be quality appraised using the Quality in Prognostic Studies tool developed by the Cochrane Prognosis Methods Group. A narrative synthesis of findings will be undertaken and meta-analyses considered, if appropriate. This protocol adheres to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols 2015 guidelines, ensuring the quality of the review.
Conclusions:
Understanding the influence of multimorbidity on mortality and other health-related outcomes in RA will provide an important basis of knowledge with the potential to improve future clinical management of RA. PROSPERO registration number: CRD42019137756
Seismology Of A Massive Pulsating Hydrogen Atmosphere White Dwarf
We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M-star = 0.88 +/- 0.02 M-circle dot and T-eff = 12,100 +/- 140 K. These values are consistent with those derived from the the optical spectra and photometric colors.CNPqFAPERGS/PronexUS National Science Foundation AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009MICINN grant AYA08-1839/ESPESF EUROCORES Program EuroGENESIS (MICINN grant) EUI2009-04170Generalitat de Catalunya 2009SGR315EU-FEDER fundsAGENCIA through the Programa de Modernizacion Tecnologica BID 1728/OC-ARCONICET PIP 112-200801-00940Astronom
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