720 research outputs found
Differential rotation decay in the radiative envelopes of CP stars
Stars of spectral classes A and late B are almost entirely radiative. CP
stars are a slowly rotating subgroup of these stars. It is possible that they
possessed long-lived accretion disks in their T Tauri phase. Magnetic coupling
of disk and star leads to rotational braking at the surface of the star.
Microscopic viscosities are extremely small and will not be able to reduce the
rotation rate of the core of the star. We investigate the question whether
magneto-rotational instability can provide turbulent angular momentum
transport. We illuminate the question whether or not differential rotation is
present in CP stars. Numerical MHD simulations of thick stellar shells are
performed. An initial differential rotation law is subject to the influence of
a magnetic field. The configuration gives indeed rise to magneto-rotational
instability. The emerging flows and magnetic fields transport efficiently
angular momentum outwards. Weak dependence on the magnetic Prandtl number
(~0.01 in stars) is found from the simulations. Since the estimated time-scale
of decay of differential rotation is 10^7-10^8 yr and comparable to the
life-time of A stars, we find the braking of the core to be an ongoing process
in many CP stars. The evolution of the surface rotation of CP stars with age
will be an observational challenge and of much value for verifying the
simulations.Comment: 8 pages, 11 figures; submitted to Astron. & Astrophy
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
Binary coalescence from case A evolution -- mergers and blue stragglers
We constructed some main-sequence mergers from case A binary evolution and
studied their characteristics via Eggleton's stellar evolution code. Both total
mass and orbital angular momentum are conservative in our binary evolutions.
Some mergers might be on the left of the ZAMS as defined by normal surface
composition on a CMD because of enhanced surface helium content. The study also
shows that central hydrogen content of the mergers is independent of mass. As a
consequence, we fit the formula of magnitude and B-V of the mergers when they
return back to thermal equilibrium with maximum error 0.29 and 0.037,
respectively. Employing the consequences above, we performed Monte Carlo
simulations to examine our models in NGC 2682 and NGC 2660. In NGC 2682, binary
mergers from our models cover the region with high luminosity, but its
importance is much less than that of AML. Our results are well-matched to the
observations of NGC2660 if there is about 0.5Mo of mass loss in the merger
process.Comment: 14 pages, 12 figures. accepted by MNRA
X-ray rotational modulation of a supersaturated star in IC 2391
We present evidence of X-ray rotational modulation on VXR45, a young fast
rotator star, member of IC 2391. It is a dG9 spectral type star whose
rotational period and X-ray luminosity make it a supersaturated star. Our X-ray
observation, made with EPIC/PN on XMM-Newton, covers about two photometric
rotational periods. The detection of rotational modulation implies the presence
of structural inhomogeneities. Possible interpretations are presented and
discussed.Comment: 4 pages with 4 PostScript figures. Accepted for publication in
Astronomy & Astrophysics Letter
In vitro cell cytotoxicity profile and morphological response to polyoxometalate-stabilised gold nanoparticles
The size and redox properties of molecular polyoxometalates (POMs) make them extremely relevant for bioapplications: from disrupting tumour growth and enzyme inhibition, to DNA-intercalating agents and antimicrobial applications. Their unique ability to reversibly dominate and receive electrons, coupled with their high anionic charge, also makes them suitable for the preparation of zero-valent state metal nanoparticles (NPs) from molecular precursors. Polyoxometalate-stabilised nanoparticles (NPs@POM) are therefore an ideal delivery vehicle for bioactive POMs. Here we show how POM-stabilised gold NPs (AuNPs@POM) are massively internalised into Vero (kidney epithelial) and B16 (skin melanoma) cell lines with variable cytotoxic effects. Cell viability assays and quantification of cytoplasmic membrane composition revealed that the Vero cell line was unaltered by the internalisation of these hybrid particles; while their internalisation in B16 tumour cells produced mild cytotoxic effects and an antiproliferative cell cycle arrest in the G0/G1 and G2/M phases. The observed perturbation of the tumour cell line combined with the high degree of internalisation means that these (or similar) NPs@POM could serve as candidates for a range of bioapplications in diagnostics or therapy
The role of estimated fetal weight discordance in dichorionic twin pregnancies
Evaluation of relative fetal growth in the form of estimated fetal weight discordance (EFWd) is a necessary element of any ultrasound examination in twin pregnancies. It is one of the criteria for the diagnosis of selective fetal growth restriction (sFGR) according to the most established worldwide guidelines. Apart from the effectiveness of this parameter for the diagnosis of sFGR, it may also be used as an independent factor for risk stratification of neonatal and maternal complications. Furthermore, numerous studies have proven the greater prognostic value of EFWd in dichorionic pregnancies, which may result from differences in the pathogenesis of fetal growth abnormalities in mono- and dichorionic pregnancies. Because of the variability of this parameter throughout pregnancy, there is an ongoing discussion regarding replacing or individualizing it with percentile charts. An additional element, complementary to EFWd in assessing the risk of complications in twin pregnancies is the use of this measurement in combination with Doppler assessment, which increases its predictive value. The use of EFWd as one of the factors influencing care and decision-making in dichorionic twin pregnancies seems to be a simple and effective method, however, further research assessing the use and possible applications of this indicator is necessary
Gold nanoprism-nanorod face off: comparing the heating efficiency, cellular internalization and thermoablation capacity
[Aim]: This work compares the synthesis, heating capability, cellular internalization and thermoablation capacity of two different types of anisotropic gold nanoparticles: gold nanorods (NRs) and nanoprisms (NPrs). [Methods]: Both particles possess surface plasmon resonance absorption bands in the near-IR, and their heating efficiency upon irradiation with a continuous near-IR laser (1064 nm) was evaluated. The cellular internalization, location and toxicity of these PEG-stabilized NPrs and NRs were then assessed in the Vero cell line by transmission electron microscopy and inductively coupled plasma mass spectrometry analysis, and their ability to induce cell death upon laser irradiation was then evaluated and compared. [Results & conclusion]: Although both nanoparticles are highly efficient photothermal converters, NRs possessed a more efficient heating capability, yet the in vitro thermoablation studies clearly demonstrated that NPrs were more effective at inducing cell death through photothermal ablation due to their greater cellular internalization.This work was supported by Shanghai Jiao Tong University, Fondo Social de la DGA (grupos DGA), Ministerio de la Economía y Competitividad del Gobierno de España for the public funding of Proyectos I+D+I – Programa Estatal de Investigación, Desarrollo e Innovación Orientada a los Retos de la Sociedad (project no. SAF2014–54763-C2–2-R) and the ERC-Starting grant 239931-NANOPUZZLE. For financial support SG Mitchell acknowledges the Fundación General CSIC (Programa ComFuturo); A Artiga acknowledges the Ministerio de Educación, Cultura y Deportes for an FPU grant
(FPU014/06249); G Alfranca acknowledges the Ministry of Education for a China Scholarship Council (CSC) grant; and M Moros acknowledges the European Commission for an MCSA Fellowship (grant agreement no. 660228). TPeer Reviewe
Four ultra-short period eclipsing M-dwarf binaries in the WFCAM Transit Survey
We report on the discovery of four ultra-short period (P<0.18 days) eclipsing
M-dwarf binaries in the WFCAM Transit Survey. Their orbital periods are
significantly shorter than of any other known main-sequence binary system, and
are all significantly below the sharp period cut-off at P~0.22 days as seen in
binaries of earlier type stars. The shortest-period binary consists of two M4
type stars in a P=0.112 day orbit. The binaries are discovered as part of an
extensive search for short-period eclipsing systems in over 260,000 stellar
lightcurves, including over 10,000 M-dwarfs down to J=18 mag, yielding 25
binaries with P<0.23 days. In a popular paradigm, the evolution of short period
binaries of cool main-sequence stars is driven by loss of angular momentum
through magnetised winds. In this scheme, the observed P~0.22 day period
cut-off is explained as being due to timescales that are too long for
lower-mass binaries to decay into tighter orbits. Our discovery of low-mass
binaries with significantly shorter orbits implies that either these timescales
have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic
activity, or that the mechanism for forming these tight M-dwarf binaries is
different from that of earlier type main-sequence stars.Comment: 22 pages, 17 figures, 3 tables Accepted for publication in MNRA
Characterization and comparison of enterococcus spp. Isolates from feces of healthy dogs and urine of dogs with utis
Enterococcus spp. are opportunistic pathogens of both humans and animals characterized by high resistance to antimicrobials. Dogs could be intestinal carriers or suffer from Enterococcus infections, mainly urinary tract infections (UTIs). This study aimed to analyze and compare En-terococcus spp. isolated from healthy dog stools and sick dog urine. Overall, 51 isolates (29 from stools and 22 from UTI) were characterized at species level and tested for antimicrobial resistance, biofilm production and presence of resistance and virulence genes. E. faecium and E. faecalis resulted as equally distributed in stools samples, while E. faecalis predominated among UTI isolates. HLAR phenotype was detected in 47.1% isolates; 64.7% isolates were resistant to ampicillin (47.1% with a MIC ≥ 64 µg/mL). High levels of resistance were recorded for fluoroquinolones (enrofloxacin 74.5%, ciprofloxacin 66.7%), clindamycin (84.3%), tetracycline (78.4%) and quinupristin–dalfopristin (78.4%). No vancomycin resistant strains were detected. All but one isolate were multidrug-resistant. Most detected resistance genes were tetM (70.5%), pbp4 (52.9%) and aph(3′ )-IIIa (39.2%). All isolates were able to produce biofilm, but isolates from UTIs and belonging to E. faecalis more frequently resulted in strong biofilm producers. Most detected virulence genes were asa1 (52.9%), gelE (41.2%), cylA (37.3%) and esp (35.3%); all of them resulted as more frequently associated to E. faecalis. No particular differences emerged between isolates from feces and UTI, considering all evaluated aspects. Our results confirm pet dogs as carriers of multidrug-resistant enterococci; stool microflora could be considered as the most probable source of enterococcal UTI and E. faecalis carried by dogs seems to be more virulent than E. faecium, justifying its more frequent involvement in urinary tract infections
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