1,110 research outputs found
Synthesis, characterization and investigation of tautomeric, potentiometric and antimicrobial properties of a novel unsymmetric Schiff base and its Fe(III) and Ni(II) complexes
Abstract A new unsymmetric Schiff base [(2OH)R-CH=N-(C6H4)-CH=N-R'(2OH) R=phenyl, R' = naphthyl] with its Fe(III) and Ni(II) complexes were synthesized by a two step method. Diimine Schiff base and its complexes were characterized by elemental analysis, mass spectra, IR, 1H/13C-NMR spectra, TGA analysis, electronic and magnetic measurements. The phenol-imine and keto-amine tautomerism of the unsymmetric Schiff base was investigated with NMR techniques and UV-visible spectra in different solvents. Also, the protonation constants of the ligand and the stability constants of its Ni(II) and Fe(III) complexes were determined potentiometrically in 1:1(v/v) ethanol-water mixture at an ionic strength of 0.5 mol·L-1 KCl and at 25.0 ± 0.1 oC. The antifungal, antimicrobial activities and the minimum inhibitory concentration (MIC) values of the compounds were evaluated against Escherichia coli (0157:H7), Micrococcus luteus (NRRLB 4975), Bacillus cereus (RSKK 863) and Candida albicans (ATCC 16231).
Cold gas outflows from the Small Magellanic Cloud traced with ASKAP
Feedback from massive stars plays a critical role in the evolution of the
Universe by driving powerful outflows from galaxies that enrich the
intergalactic medium and regulate star formation. An important source of
outflows may be the most numerous galaxies in the Universe: dwarf galaxies.
With small gravitational potential wells, these galaxies easily lose their
star-forming material in the presence of intense stellar feedback. Here, we
show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic
hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming
bar of the galaxy. The outflows are cold, , and may have formed
during a period of active star formation million years (Myr) ago. The
total mass of atomic gas in the outflow is solar masses, , or % of the total atomic gas of the galaxy. The inferred
mass flux in atomic gas alone, , is up to an order of magnitude greater than the star
formation rate. We suggest that most of the observed outflow will be stripped
from the SMC through its interaction with its companion, the Large Magellanic
Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen
encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018,
http://dx.doi.org/10.1038/s41550-018-0608-
A runaway collision in a young star cluster as the origin of the brightest supernova
Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded
\cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P,
2006CBET..695....1F}. Its progenitor might have been a very massive (
\msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen
in the spectrum of the supernova, because stars \msun are believed to
have shed their hydrogen envelopes several hundred thousand years before the
explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have
arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we
show that the collision frequency of massive stars in a dense and young cluster
(of the kind to be expected near the center of a galaxy) is sufficient to
provide a reasonable chance that SN 2006gy resulted from such a bombardment. If
this is the correct explanation, then we predict that when the supernova fades
(in a year or so) a dense cluster of massive stars becomes visible at the site
of the explosion
The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49
The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO
243-49 galaxy exhibits variability with a possible recurrence time of a few
hundred days. Finding the origin of this variability would constrain the still
largely unknown properties of this extraordinary object. Since it exhibits an
intensity-hardness behavior characteristic of black-hole X-ray transients, we
have analyzed the variability of HLX-1 in the framework of the disk instability
model that explains outbursts of such systems. We find that the long-term
variability of HLX-1 is unlikely to be explained by a model in which outbursts
are triggered by thermal-viscous instabilities in an accretion disc. Possible
alternatives include the instability in a radiation-pressure dominated disk but
we argue that a more likely explanation is a modulated mass-transfer due to
tidal stripping of a star on an eccentric orbit around the intermediate-mass
black hole. We consider an evolutionary scenario leading to the creation of
such a system and estimate the probability of its observation. We conclude,
using a simplified dynamical model of the post-collapse cluster, that no more
than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar
mergers in the dense collapsed cores of young clusters - could have a few times
1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for
mass transfer at periapse, while avoiding collisional destruction or being
scattered into the IMBH by 2-body encounters. The finite but low probability of
this configuration is consistent with the uniqueness of HLX-1. We note,
however, that the actual response of a standard accretion disk to bursts of
mass transfer may be too slow to explain the observations unless the orbit is
close to parabolic (and hence even rarer) and/or additional heating, presumably
linked to the highly time-dependent gravitational potential, are invoked.Comment: 8 pages, 2 figures. Additional figure, extended discussion. To be
published in ApJ, June 10, 2011, v734 -
Radio observations of NGC 6388: an upper limit on the mass of its central black hole
We present the results of deep radio observations with the Australia
Telescope Compact Array (ATCA) of the globular cluster NGC 6388. We show that
there is no radio source detected (with a r.m.s. noise level of 27 uJy) at the
cluster centre of gravity or at the locations of the any of the Chandra X-ray
sources in the cluster. Based on the fundamental plane of accreting black holes
which is a relationship between X-ray luminosity, radio luminosity and black
hole mass, we place an upper limit of 1500 M_sun on the mass of the putative
intermediate-mass black hole located at the centre of NGC 6388. We discuss the
uncertainties of this upper limit and the previously suggested black hole mass
of 5700 M_sun based on surface density profile analysis.Comment: 6 pages, 2 figures, accepted for publication in MNRA
Bovine oocytes in secondary follicles grow and acquire meiotic competence in severe combined immunodeficient mice
A rigorous methodology is developed
that addresses numerical and
statistical issues when developing group contribution (GC) based property
models such as regression methods, optimization algorithms, performance
statistics, outlier treatment, parameter identifiability, and uncertainty
of the prediction. The methodology is evaluated through development
of a GC method for the prediction of the heat of combustion (Δ<i>H</i><sub>c</sub><sup>o</sup>) for pure components. The results showed that robust regression
lead to best performance statistics for parameter estimation. The
bootstrap method is found to be a valid alternative to calculate parameter
estimation errors when underlying distribution of residuals is unknown.
Many parameters (first, second, third order group contributions) are
found unidentifiable from the typically available data, with large
estimation error bounds and significant correlation. Due to this poor
parameter identifiability issues, reporting of the 95% confidence
intervals of the predicted property values should be mandatory as
opposed to reporting only single value prediction, currently the norm
in literature. Moreover, inclusion of higher order groups (additional
parameters) does not always lead to improved prediction accuracy for
the GC-models; in some cases, it may even increase the prediction
error (hence worse prediction accuracy). However, additional parameters
do not affect calculated 95% confidence interval. Last but not least,
the newly developed GC model of the heat of combustion (Δ<i>H</i><sub>c</sub><sup>o</sup>) shows predictions of great accuracy and quality (the most data
falling within the 95% confidence intervals) and provides additional
information on the uncertainty of each prediction compared to other
Δ<i>H</i><sub>c</sub><sup>o</sup> models reported in literature
The LOFAR Two-metre Sky Survey: the radio view of the cosmic star formation history
© 2023 Oxford University Press. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stad1602We present a detailed study of the cosmic star formation history over per cent of cosmic time (), using deep, radio continuum observations that probe star formation activity independent of dust. The Low Frequency Array Two Metre Sky Survey has imaged three well-studied extragalactic fields, Elais-N1, Bo\"otes and the Lockman Hole, reaching rms sensitivity at . The availability of high-quality ancillary data from ultraviolet to far-infrared wavelengths has enabled accurate photometric redshifts and the robust separation of radio-bright AGN from their star-forming counterparts. We capitalise on this unique combination of deep, wide fields and robustly-selected star-forming galaxies to construct radio luminosity functions and derive the cosmic star formation rate density. We carefully constrain and correct for scatter in the relation, which we find to be . Our derived star formation rate density lies between previous measurements at all redshifts studied. We derive higher star formation rate densities between and than are typically inferred from short wavelength emission; at earlier times, this discrepancy is reduced. Our measurements are generally in good agreement with far-infrared and radio-based studies, with small offsets resulting from differing star formation rate calibrations.Peer reviewe
Classification of compact radio sources in the Galactic plane with supervised machine learning
Generation of science-ready data from processed data products is one of the
major challenges in next-generation radio continuum surveys with the Square
Kilometre Array (SKA) and its precursors, due to the expected data volume and
the need to achieve a high degree of automated processing. Source extraction,
characterization, and classification are the major stages involved in this
process. In this work we focus on the classification of compact radio sources
in the Galactic plane using both radio and infrared images as inputs. To this
aim, we produced a curated dataset of ~20,000 images of compact sources of
different astronomical classes, obtained from past radio and infrared surveys,
and novel radio data from pilot surveys carried out with the Australian SKA
Pathfinder (ASKAP). Radio spectral index information was also obtained for a
subset of the data. We then trained two different classifiers on the produced
dataset. The first model uses gradient-boosted decision trees and is trained on
a set of pre-computed features derived from the data, which include
radio-infrared colour indices and the radio spectral index. The second model is
trained directly on multi-channel images, employing convolutional neural
networks. Using a completely supervised procedure, we obtained a high
classification accuracy (F1-score>90%) for separating Galactic objects from the
extragalactic background. Individual class discrimination performances, ranging
from 60% to 75%, increased by 10% when adding far-infrared and spectral index
information, with extragalactic objects, PNe and HII regions identified with
higher accuracies. The implemented tools and trained models were publicly
released, and made available to the radioastronomical community for future
application on new radio data.Comment: 27 pages, 15 figures, 9 table
LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Accepted for publication in Astronomy and AstrophysicsAims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one α high, calculated between 1.3 GHz and 5 GHz, is -0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.Peer reviewedFinal Accepted Versio
- …