87 research outputs found
A 3D Photoionization Model of the Extreme Planetary Nebula NGC 6302
We present a 3D photoionization model of the PN NGC 6302, one of the most
complex objects of its kind. Our Mocassin model is composed of an extremely
dense circumstellar disk and a large pair of diffuse bipolar lobes, a
combination necessary to reproduce the observed spectrum. The masses of these
components gives a total nebular mass of 4.7Mo. Discrepancies between our model
fit and the observations are attributed to complex density inhomogeneities in
the nebula. The potential to resolve such discrepancies with more complex
models is confirmed by a range of models introducing small-scale structures.
Compared to solar abundances He is enhanced by 50%, C is slightly subsolar, O
is solar, and N is enhanced by a factor of 6. These imply a significant 3rd
dredge-up coupled with hot-bottom burning CN-cycle conversion of dredged-up C
to N.
The central star is partly obscured by the edge-on circumstellar disk and its
properties are not well constrained. Emission from a number of high-ionization
`coronal' lines provides constraints on the form of the high-energy ionizing
flux. Using a solar abundance stellar atmosphere we are unable to fit all of
the observed line fluxes, but a substantially better fit was obtained using a
220,000K H-deficient stellar atmosphere with L*=14,300 Lo. The H-deficient
nature of the central star suggests it has undergone a late thermal pulse, and
fits to evolutionary tracks imply a central star mass of 0.73-0.82Mo.
Timescales for these tracks suggest the object left the top of the AGB ~2100
years ago, in agreement with studies of the recent mass-loss event that formed
the bipolar lobes. Based on the modelled nebular and central star masses we
estimate the initial mass of the central star to be 5.5Mo, in agreement with
that derived from evolutionary tracks.
(Abstract truncated)Comment: 23 pages, 8 figures, 10 tables. Accepted for publication in MNRA
Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks
In our Galaxy, star formation occurs in a variety of environments, with a
large fraction of stars formed in clusters hosting massive stars. OB stars have
an important feedback on the evolution of protoplanetary disks around nearby
young stars and likely on the process of planet formation occurring in them.
The nearby massive association Cygnus OB2 is an outstanding laboratory to study
this feedback. It is the closest massive association to our Sun, and hosts
hundreds of massive stars and thousands of low mass members. In this paper, we
analyze the spatial variation of the disk fraction in Cygnus OB2 and we study
its correlation with the local values of Far and Extreme ultraviolet radiation
fields and the local stellar surface density. We present definitive evidence
that disks are more rapidly dissipated in the regions of the association
characterized by intense local UV field and large stellar density. In
particular, the FUV radiation dominates disks dissipation timescales in the
proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association,
EUV photons potentially induce a significant mass loss from the irradiated
disks across the entire association, but the efficiency of this process is
reduced at increasing distances from the massive stars due to absorption by the
intervening intracluster material. We find that disk dissipation due to close
stellar encounters is negligible in Cygnus OB2, and likely to have affected 1%
or fewer of the stellar population. Disk dissipation is instead dominated by
photoevaporation. We also compare our results to what has been found in other
young clusters with different massive populations, concluding that massive
associations like Cygnus OB2 are potentially hostile to protoplanetary disks,
but that the environments where disks can safely evolve in planetary systems
are likely quite common in our Galaxy.Comment: Accepted for publication in ApJS as part of the special issue on the
Chandra Cygnus OB2 Legacy Projec
CI observations in the CQ Tau proto-planetary disk: evidence for a very low gas-to-dust ratio ?
Gas and dust dissipation processes of proto-planetary disks are hardly known.
Transition disks between Class II (proto-planetary disks) and Class III (debris
disks) remain difficult to detect. We investigate the carbon chemistry of the
peculiar CQ Tau gas disk. It is likely a transition disk because it exhibits
weak CO emission with a relatively strong millimeter continuum, indicating that
the disk might be currently dissipating its gas content. We used APEX to
observe the two CI lines at 492GHz and 809 GHz in the disk orbiting CQ Tau. We
compare the observations to several chemical model predictions. We focus our
study on the influence of the stellar UV radiation shape and gas-to-dust ratio.
We did not detect the CI lines. However, our upper limits are deep enough to
exclude high-CI models. The only available models compatible with our limits
imply very low gas-to-dust ratio, of the order of a few, only. These
observations strengthen the hypothesis that CQ Tau is likely a transition disk
and suggest that gas disappears before dust.Comment: 5 pages, 5 figures, accepted for publication in A&
X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres
X-ray spectra from stellar coronae are reprocessed by the underlying
photosphere through scattering and photoionization events. While reprocessed
X-ray spectra reaching a distant observer are at a flux level of only a few
percent of that of the corona itself, characteristic lines formed by inner
shell photoionization of some abundant elements can be significantly stronger.
The emergent photospheric spectra are sensitive to the distance and location of
the fluorescing radiation and can provide diagnostics of coronal geometry and
abundance. Here we present Monte Carlo simulations of the photospheric Kalpha
doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source.
Fluorescent line strengths have been computed as a function of the height of
the radiation source, the temperature of the ionising X-ray spectrum, and the
viewing angle. We also illustrate how the fluorescence efficiencies scale with
the photospheric metallicity and the Fe abundance. Based on the results we make
three comments: (1) fluorescent Fe lines seen from pre-main sequence stars
mostly suggest flared disk geometries and/or super-solar disk Fe abundances;
(2) the extreme ~1400 mA line observed from a flare on V1486 Ori can be
explained entirely by X-ray fluorescence if the flare itself were partially
eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by
Swift during a large flare on II Peg is consistent with X-ray excitation and
does not require a collisional ionisation contribution. There is no convincing
evidence supporting the energetically challenging explanation of electron
impact excitation for observed stellar Fe Kalpha lines.Comment: 30 pages; accepted for publication in the Astrophysical Journa
Circumstellar discs: What will be next?
This prospective chapter gives our view on the evolution of the study of
circumstellar discs within the next 20 years from both observational and
theoretical sides. We first present the expected improvements in our knowledge
of protoplanetary discs as for their masses, sizes, chemistry, the presence of
planets as well as the evolutionary processes shaping these discs. We then
explore the older debris disc stage and explain what will be learnt concerning
their birth, the intrinsic links between these discs and planets, the hot dust
and the gas detected around main sequence stars as well as discs around white
dwarfs.Comment: invited review; comments welcome (32 pages
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
Anti-cancer effects and mechanism of actions of aspirin analogues in the treatment of glioma cancer
INTRODUCTION: In the past 25 years only modest advancements in glioma treatment have been made, with patient prognosis and median survival time following diagnosis only increasing from 3 to 7 months. A substantial body of clinical and preclinical evidence has suggested a role for aspirin in the treatment of cancer with multiple mechanisms of action proposed including COX 2 inhibition, down regulation of EGFR expression, and NF-κB signaling affecting Bcl-2 expression. However, with serious side effects such as stroke and gastrointestinal bleeding, aspirin analogues with improved potency and side effect profiles are being developed. METHOD: Effects on cell viability following 24 hr incubation of four aspirin derivatives (PN508, 517, 526 and 529) were compared to cisplatin, aspirin and di-aspirin in four glioma cell lines (U87 MG, SVG P12, GOS – 3, and 1321N1), using the PrestoBlue assay, establishing IC50 and examining the time course of drug effects. RESULTS: All compounds were found to decrease cell viability in a concentration and time dependant manner. Significantly, the analogue PN517 (IC50 2mM) showed approximately a twofold increase in potency when compared to aspirin (3.7mM) and cisplatin (4.3mM) in U87 cells, with similar increased potency in SVG P12 cells. Other analogues demonstrated similar potency to aspirin and cisplatin. CONCLUSION: These results support the further development and characterization of novel NSAID derivatives for the treatment of glioma
X-ray Absorption and Reflection in Active Galactic Nuclei
X-ray spectroscopy offers an opportunity to study the complex mixture of
emitting and absorbing components in the circumnuclear regions of active
galactic nuclei, and to learn about the accretion process that fuels AGN and
the feedback of material to their host galaxies. We describe the spectral
signatures that may be studied and review the X-ray spectra and spectral
variability of active galaxies, concentrating on progress from recent Chandra,
XMM-Newton and Suzaku data for local type 1 AGN. We describe the evidence for
absorption covering a wide range of column densities, ionization and dynamics,
and discuss the growing evidence for partial-covering absorption from data at
energies > 10 keV. Such absorption can also explain the observed X-ray spectral
curvature and variability in AGN at lower energies and is likely an important
factor in shaping the observed properties of this class of source.
Consideration of self-consistent models for local AGN indicates that X-ray
spectra likely comprise a combination of absorption and reflection effects from
material originating within a few light days of the black hole as well as on
larger scales. It is likely that AGN X-ray spectra may be strongly affected by
the presence of disk-wind outflows that are expected in systems with high
accretion rates, and we describe models that attempt to predict the effects of
radiative transfer through such winds, and discuss the prospects for new data
to test and address these ideas.Comment: Accepted for publication in the Astronomy and Astrophysics Review. 58
pages, 9 figures. V2 has fixed an error in footnote
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