2,149 research outputs found
The polarization mode of the auroral radio emission from the early-type star HD142301
We report the detection of the auroral radio emission from the early-type
magnetic star HD142301. New VLA observations of HD142301 detected highly
polarized amplified emission occurring at fixed stellar orientations. The
coherent emission mechanism responsible for the stellar auroral radio emission
amplifies the radiation within a narrow beam, making the star where this
phenomenon occurs similar to a radio lighthouse. The elementary emission
process responsible for the auroral radiation mainly amplifies one of the two
magneto-ionic modes of the electromagnetic wave. This explains why the auroral
pulses are highly circularly polarized. The auroral radio emission of HD142301
is characterized by a reversal of the sense of polarization as the star
rotates. The effective magnetic field curve of HD142301 is also available
making it possible to correlate the transition from the left to the right-hand
circular polarization sense (and vice-versa) of the auroral pulses with the
known orientation of the stellar magnetic field. The results presented in this
letter have implications for the estimation of the dominant magneto-ionic mode
amplified within the HD142301 magnetosphere.Comment: 5 pages, 4 figures; accepted to MNRAS Letter
ENSO hindcast skill in the DWD - MPI-M - UHH seasonal prediction system
KlimawandelWe present an assessment of the El Niño Southern Oscillation (ENSO) hindcast skill in the DWD - MPI-M - UHH seasonal prediction system based on the earth system model MPI-ESM. The system is initialised from re-analysis in the atmospheric, oceanic and sea-ice component of the model. We use a hindcast ensemble with semi-annual start dates between 1981 and 2014 (10 member ensembles started every May and November for 6 months each). We find hindcast skill for Niño 3.4 sea surface temperatures up to 6 months ahead. Hindcast skill is higher for November start dates than for May start dates. In addition to the Niño 3.4 Index, we also assess hindcast skill for Niño3, the West Pacific Warm Water Volume and the zonal wind variability. In particular we focus on the difference in the hindcast skill in the May start dates for the 1997/98 and the 2014 November conditions - though for these two periods overall similar conditions were observed, the subsequent development with a strong El Niño in 1997/98 and a very weak El Niño in 2014 differed considerably
The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud: spectroscopy, orbital analysis, formation, and evolution
Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy
budget of galaxies and probe a critical phase in the evolution of massive stars
prior to core-collapse. It is not known whether core He-burning WR stars
(classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary
stripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due
to its complexity, our study focuses on the 44 WR binaries / binary candidates
of the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the
basis of radial velocity variations, composite spectra, or high X-ray
luminosities. Relying on a diverse spectroscopic database, we aim to derive the
physical and orbital parameters of our targets, confronting evolution models of
evolved massive stars at sub-solar metallicity, and constraining the impact of
binary interaction in forming them. Spectroscopy is performed using the Potsdam
Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is
performed using the code Spectangular or the shift-and-add algorithm.
Evolutionary status is interpreted using the Binary Population and Spectral
Synthesis (BPASS) code, exploring binary interaction and chemically-homogeneous
evolution.
No obvious dichotomy in the locations of apparently-single and binary WN
stars on the Hertzsprung-Russell diagram is apparent. According to commonly
used stellar evolution models (BPASS, Geneva), most apparently-single WN stars
could not have formed as single stars, implying that they were stripped by an
undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing
(e.g., during the red supergiant phase) are strongly underestimated in standard
stellar evolution models.Comment: accepted to A&A on 10.05.2019; 69 pages (25 main paper + 44
appendix); Corrigendum: Shenar et al. 2020, A&A, 641, 2: An unfortunate typo
in the implementation of the "transformed radius" caused errors of up to
~0.5dex in the derived mass-loss rates. This has now been correcte
The Polarization Mode of the Auroral Radio Emission from the Early-Type Star HD 142301
We report the detection of the auroral radio emission from the early-type magnetic star HD 142301. New VLA observations of HD 142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD 142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD 142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense (and vice versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD 142301 magnetosphere
40Ar–39Ar ages and isotope geochemistry of Cretaceous basalts in northern Madagascar: Refining eruption ages, extent of crustal contamination and parental magmas in a flood basalt province
The Madagascar Cretaceous igneous province exposed in the Mahajanga basin is represented by basalt and basaltic andesite lavas. New 40Ar–39Ar plateau ages (92.3 ± 2.0 Ma and 91.5 ± 1.3 Ma) indicate that the magmatism in the Mahajanga basin started about 92 Ma ago. Four geochemically distinct magma types (Groups A–D) are present. Group A and C rocks have low to moderate TiO2 (1.2–2.6 wt%), Nb (3–9 μg g−1) and Zr (82–200 μg g−1), and show large variations in ɛNdi (+0.1 to −10.8), 206Pb/204Pb (15.28 to 16.33) and γOs (+11.4 to +7378). The large isotopic variations, particularly in Os, Nd and Pb isotopic compositions, are likely due to crustal contamination. The low Pb isotope ratios observed in the Group A and C rocks suggest involvement of continental crust with low μ (238U/204Pb). Group B and D rocks have moderate to high TiO2 (2.2–4.9 wt%), Nb (8–24 μg g−1) and Zr (120–327 μg g−1). Age-corrected isotopes of Group B and D lavas show a small range in ɛNdi (+1.0 to +4.0) and a wide range in γOs (+128 to +1182). Values of 207Pb/204Pb are within the range for Groups A and C, but the Group D 206Pb/204Pb (16.52–17.08) and 208Pb/204Pb (37.51–38.01) values are higher, indicating a different crustal contaminant. Pb isotopic values of the Group B rocks seem to reflect the isotopic features of their mantle source. The magma groups of Mahajanga display a wide range of trace element and isotopic compositions that cannot be explained only by open-system crystallization processes but, rather, by distinct mantle sources
Identification of structurally re-engineered rocaglates as inhibitors against hepatitis E virus replication
Hepatitis E virus (HEV) infections are a leading cause of acute viral hepatitis in humans and pose a considerable threat to public health. Current standard of care treatment is limited to the off-label use of nucleoside-analog ribavirin (RBV) and PEGylated interferon-α, both of which are associated with significant side effects and provide limited efficacy. In the past few years, a promising natural product compound class of eukaryotic initiation factor 4A (eIF4A) inhibitors (translation initiation inhibitors), called rocaglates, were identified as antiviral agents against RNA virus infections. In the present study, we evaluated a total of 205 synthetic rocaglate derivatives from the BU-CMD compound library for their antiviral properties against HEV. At least eleven compounds showed inhibitory activities against the HEV genotype 3 (HEV-3) subgenomic replicon below 30 nM (EC50 value) as determined by Gaussia luciferase assay. Three amidino-rocaglates (ADRs) (CMLD012073, CMLD012118, and CMLD012612) possessed antiviral activity against HEV with EC50 values between 1 and 9 nM. In addition, these three selected compounds inhibited subgenomic replicons of different genotypes (HEV-1 [Sar55], wild boar HEV-3 [83-2] and human HEV-3 [p6]) in a dose-dependent manner and at low nanomolar concentrations. Furthermore, tested ADRs tend to be better tolerated in primary hepatocytes than hepatoma cancer cell lines and combination treatment of CMLD012118 with RBV and interferon-α (IFN-α) showed that CMLD012118 acts additive to RBV and IFN-α treatment. In conclusion, our results indicate that ADRs, especially CMLD012073, CMLD012118, and CMLD012612 may prove to be potential therapeutic candidates for the treatment of HEV infections and may contribute to the discovery of pan-genotypic inhibitors in the future. © 2022 The Author(s
A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
We present new radio/millimeter measurements of the hot magnetic star HR5907
obtained with the VLA and ALMA interferometers. We find that HR5907 is the most
radio luminous early type star in the cm-mm band among those presently known.
Its multi-wavelength radio light curves are strongly variable with an amplitude
that increases with radio frequency. The radio emission can be explained by the
populations of the non-thermal electrons accelerated in the current sheets on
the outer border of the magnetosphere of this fast rotating magnetic star. We
classify HR5907 as another member of the growing class of strongly magnetic
fast rotating hot stars where the gyro-synchrotron emission mechanism
efficiently operates in their magnetospheres. The new radio observations of
HR5907 are combined with archival X-ray data to study the physical condition of
its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the
presence of non-thermal spectral component. We suggest that non-thermal X-rays
originate a stellar X-ray aurora due to streams of non-thermal electrons
impacting on the stellar surface. Taking advantage of the relation between the
spectral indices of the X-ray power-law spectrum and the non-thermal electron
energy distributions, we perform 3-D modeling of the radio emission for HR5907.
The wavelength-dependent radio light-curves probe magnetospheric layers at
different heights above the stellar surface. A detailed comparison between
simulated and observed radio light-curves leads us to conclude that the stellar
magnetic field of HR5907 is likely non-dipolar, providing further indirect
evidence of the complex magnetic field topology of HR5907.Comment: 18 pages, 13 figures; accepted to MNRA
The Shortest-period Wolf-Rayet binary in the Small Magellanic Cloud: Part of a high-order multiple system
SMC AB 6 is the shortest-period (6.5d) Wolf-Rayet (WR) binary in the Small
Magellanic Cloud, and is therefore crucial for the study of binary interaction
and formation of WR stars at low metallicity. The WR component in AB 6 was
previously found to be very luminous (logL=6.3[Lsun]) compared to its reported
orbital mass (8Msun), placing it significantly above the Eddington limit.
Through spectroscopy and orbital analysis of newly acquired optical data taken
with UVES, we aim to understand the peculiar results reported for this system
and explore its evolutionary history.
Results: We find that AB 6 contains at least four stars. The 6.5d period WR
binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating
early O-type companion (O5.5 V, star B). Static N and He lines suggest the
presence of an emission line star (O5.5 I(f), star C). Finally, narrow
absorption lines portraying a long-term radial velocity variation show the
existence of a fourth star (O7.5 V, star D). Star D appears to form a second
140d period binary together with a fifth stellar member, which is a B-type
dwarf or a black hole. It is not clear that these additional components are
bound to the WR binary. The WR star is found to be less luminous than
previously thought (logL = 5.9[Lsun]) and, adopting 41Msun for star B, more
massive (18Msun). Correspondingly, the WR star does not exceed the Eddington
limit. We derive the initial masses of 60 and 40Msun for stars A and B and an
age of 3.9 Myr for the system. The WR binary likely experienced nonconservative
mass transfer in the past supported by the relatively rapid rotation of star B.
Conclusion: Our study shows that AB 6 is a multiple -- probably quintuple --
system. This finding resolves the previously reported puzzle of the WR primary
exceeding the Eddington limit and suggests that the WR star exchanged mass with
its companion in the past.Comment: 15 pages, 16 figures, accepted to A&A on the 30.4.2018. Comments
welcome
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