257 research outputs found
Doppler imaging an X-ray flare on the ultrafast rotator BO Mic - A contemporaneous multiwavelength study using XMM-Newton and VLT
We present an analysis of contemporaneous photospheric, chromospheric and
coronal structures on the highly active K-dwarf star BO Mic (Speedy Mic). We
localize a moderate flare in the stellar atmosphere and study its energetics,
size and thermal behaviour. The analysis is based on strictly simultaneous
X-ray, UV- and optical observations carried out by XMM-Newton and the VLT. We
use Doppler imaging and related methods for the localization of features. Based
on X-ray spectroscopy we study the the coronal plasma in and outside the flare.
The flare emits in X-rays and UV, but is not detected in white light; it is
located at intermediate latitude between an extended spot group and the weakly
spotted pole. We estimate its height below 0.4 stellar radii making it clearly
distinct in longitude and height from the prominences found about two stellar
radii above the surface. While BO Mic's photospheric brightness is modulated
due to extended starspots, neither the chromospheric nor the X-ray emission
show a pronounced rotational modulation.Comment: Accepted by A&
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
According to our current understanding, a subclass of the upper main sequence
chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic.
Nevertheless, chemical inhomogeneities were recently discovered on their
surfaces. At the same time, no global magnetic fields stronger than 1-100 G are
detected by modern studies. The goals of our study are to search for magnetic
field in the HgMn binary system 66 Eri and to investigate chemical spots on the
stellar surfaces of both components. Our analysis is based on high quality
spectropolarimetric time-series observations obtained during 10 consecutive
nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study line profile variability. Chemical spot geometry was
reconstructed using multi-line Doppler imaging. We report a non-detection of
magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field.
Circular polarization profiles also do not indicate any signatures of complex
surface magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the HgMn
component we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these elements
reconstructed with the Doppler imaging technique, show relative underabundance
on the hemisphere facing the secondary component. The contrast of chemical
inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.Comment: 13 pages, 14 figure
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters
High-resolution spectropolarimetric observations provide simultaneous
information about stellar magnetic field topologies and three-dimensional
distributions of chemical elements. Here we present analysis of a unique full
Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap
star HD 24712. The goal of our work is to examine circular and linear
polarization signatures inside spectral lines and to study variation of the
stellar spectrum and magnetic observables as a function of rotational phase. HD
24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over
a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and
resolving power exceeding 100000. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the spectral lines
of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to
study the morphology of the Stokes profile shapes in individual spectral lines
and in LSD Stokes profiles corresponding to different line masks. From the LSD
Stokes V profiles we measured the longitudinal component of the magnetic field,
, with an accuracy of 5-10 G. We also determined the net linear
polarization from the LSD Stokes Q and U profiles. We determined an improved
rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured
from the cores of Halpha and Hbeta lines. The analysis of measurements
showed no evidence for a significant radial magnetic field gradient in the
atmosphere of HD 24712. We used our and net linear polarization
measurements to determine parameters of the dipolar magnetic field topology. We
found that magnetic observables can be reasonably well reproduced by the
dipolar model. We discovered rotational modulation of the Halpha core and
related it a non-uniform surface distribution of rare-earth elements.Comment: Accepted for publication in A&
X-rays from accretion shocks in T Tauri stars: The case of BP Tau
We present an XMM-Newton observation of the classical T Tauri star BP Tau. In
the XMM-Newton RGS spectrum the O {\sc vii} triplet is clearly detected with a
very weak forbidden line indicating high plasma densities and/or a high UV flux
environment. At the same time concurrent UV data point to a small hot spot
filling factor suggesting an accretion funnel shock as the site of the X-ray
and UV emission. Together with the X-ray data on TW Hya these new observations
suggest such funnels to be a general feature in classical T Tauri stars.Comment: 4 pages, 4 figures, accepted by A&
Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique
opportunity to study the topology of stellar magnetic fields in detail and to
investigate magnetically driven processes of spot formation. Here we aim to
derive the surface magnetic field geometry and chemical abundance distributions
for the extraordinary Ap star HD 75049. This object hosts a surface field of
~30 kG, one of the strongest known for any non-degenerate star. We used
time-series of high-resolution HARPS intensity and circular polarisation
observations. These data were interpreted with the help of magnetic Doppler
imaging and model atmospheres incorporating effects of a non-solar chemical
composition and a strong magnetic field. We refined the rotational period of HD
75049 to Prot=4.048267+/-0.000036 d. We also derived basic stellar parameters,
Teff=10250+/-250 K and logg=4.3+/-0.1. Magnetic Doppler imaging revealed that
the field topology of HD 75049 is poloidal and dominated by a dipolar
contribution with a peak surface field strength of 39 kG. At the same time,
deviations from the classical axisymmetric oblique dipolar configuration are
significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance
contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Nd is
found to be enhanced close to the magnetic pole, whereas Si and Cr are
concentrated predominantly at the magnetic equator. The iron distribution shows
low-contrast features both at the magnetic equator and the pole. The morphology
of the magnetic field and the properties of chemical spots in HD 75049 are
qualitatively similar to those of Ap stars with weaker fields. Consequently,
whatever mechanism forms and sustains global magnetic fields in
intermediate-mass main-sequence stars, it operates in the same way over the
entire observed range of magnetic field strengths.Comment: accepted for publication in A&
The search for magnetic fields in mercury-manganese stars
We performed a highly sensitive search for magnetic fields on a large set of
HgMn stars. With the aid of a new polarimeter attached to the HARPS
spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular
polarization spectra of 41 single and double HgMn stars. Using a multi-line
analysis technique on each star, we co-added information from hundreds of
spectral lines resulting in significantly greater sensitivity to the presence
of magnetic fields, including very weak fields. For the 47 individual objects
studied, including 6 components of SB2 systems, we do not detect any magnetic
fields at greater than the 3 sigma level. The lack of detection in the circular
polarization profiles indicates that if strong fields are present on these
stars, they must have complex surface topologies. For simple global fields, our
detection limits imply upper limits to the fields present of 2-10 Gauss in the
best cases. We conclude that HgMn stars lack large-scale magnetic fields,
typical for spotted magnetic Ap stars, sufficient to form and sustain the
chemical spots observed on HgMn stars. Our study confirms that in addition to
magnetically altered atomic diffusion, there exists another differentiation
mechanism operating in the atmospheres of late-B main sequence stars which can
compositional inhomogeneities on their surfaces.Comment: 12 pages, 8 figures, 2 table
Are there tangled magnetic fields on HgMn stars?
Several recent spectrophotometric studies failed to detect significant global
magnetic fields in late-B HgMn chemically peculiar stars, but some
investigations have suggested the presence of strong unstructured or tangled
fields in these objects. We used detailed spectrum synthesis analysis to search
for evidence of tangled magnetic fields in high-quality observed spectra of 8
slowly rotating HgMn stars and one normal late-B star. We also evaluated recent
sporadic detections of weak longitudinal magnetic fields in HgMn stars based on
the moment technique. Our analysis of the Zeeman broadening of magnetically
sensitive spectral lines reveals no evidence of tangled magnetic fields in any
of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the
mean magnetic field modulus -- much smaller than the field strengths implied by
studies based on differential magnetic line intensification and quadratic field
diagnostics. The new HARPSpol longitudinal field measurements for the extreme
HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero
at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted
HgMn star HD 11753 shows that the recent moment technique measurements
retrieved from the same data are incompatible with the lack of circular
polarization signatures in the spectrum of this star. We conclude that there is
no evidence for substantial tangled magnetic fields on the surfaces of studied
HgMn stars. We cannot independently confirm the presence of very strong
quadratic or marginal longitudinal fields for these stars, so results from the
moment technique are likely to be spurious.Comment: 12 pages, 11 figures; accepted for publication in A&
Neutralization of SARS-CoV-2 by highly potent, hyperthermostable, and mutation-tolerant nanobodies
Monoclonal anti-SARS-CoV-2 immunoglobulins represent a treatment option for COVID-19. However, their production in mammalian cells is not scalable to meet the global demand. Single-domain (VHH) antibodies (also called nanobodies) provide an alternative suitable for microbial production. Using alpaca immune libraries against the receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein, we isolated 45 infection-blocking VHH antibodies. These include nanobodies that can withstand 95°C. The most effective VHH antibody neutralizes SARS-CoV-2 at 17–50 pM concentration (0.2–0.7 µg per liter), binds the open and closed states of the Spike, and shows a tight RBD interaction in the X-ray and cryo-EM structures. The best VHH trimers neutralize even at 40 ng per liter. We constructed nanobody tandems and identified nanobody monomers that tolerate the K417N/T, E484K, N501Y, and L452R immune-escape mutations found in the Alpha, Beta, Gamma, Epsilon, Iota, and Delta/Kappa lineages. We also demonstrate neutralization of the Beta strain at low-picomolar VHH concentrations. We further discovered VHH antibodies that enforce native folding of the RBD in the E. coli cytosol, where its folding normally fails. Such “fold-promoting” nanobodies may allow for simplified production of vaccines and their adaptation to viral escape-mutations
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
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