586 research outputs found
Irradiated brown dwarfs
We have observed the post common envelope binary WD0137-349 in the near
infrared , and bands and have determined that the photometry varies
on the system period (116 min). The amplitude of the variability increases with
increasing wavelength, indicating that the brown dwarf in the system is likely
being irradiated by its 16500 K white dwarf companion. The effect of the
(primarily) UV irradiation on the brown dwarf atmosphere is unknown, but it is
possible that stratospheric hazes are formed. It is also possible that the
brown dwarf (an L-T transition object) itself is variable due to patchy cloud
cover. Both these scenarios are discussed, and suggestions for further study
are made.Comment: 5 pages, 2 figures. Proceedings from "Brown dwarfs come of age"
meeting in Fuerteventura 201
Parameterization of sea-salt optical properties and physics of the associated radiative forcing
The optical properties of sea-salt aerosol have been parameterized at shortwave and longwave wavelengths. The optical properties were parameterized in a simple functional form in terms of the ambient relative humidity based on Mie optical property calculations. The proposed parameterization is tested relative to Mie calculations and is found to be accurate to within a few percent. In the parameterization, the effects of the size distribution on the optical properties are accounted for in terms of effective radius of the sea-salt size distribution. This parameterization differs from previous works by being formulated directly with the wet sea-salt size distribution and, to our knowledge, this is the first published sea-salt parameterization to provide a parameterization for both shortwave and longwave wavelengths. <br><br> We have used this parameterization in a set of idealized 1-D radiative transfer calculations to investigate the sensitivity of various attributes of sea-salt forcing, including the dependency on sea-salt column loading, effective variance, solar angle, and surface albedo. From these sensitivity tests, it is found that sea-salt forcings for both shortwave and longwave spectra are linearly related to the sea-salt loading for realistic values of loadings. The radiative forcing results illustrate that the shortwave forcing is an order of magnitude greater than the longwave forcing results and opposite in sign, for various loadings. Forcing sensitivity studies show that the influence of effective variance for sea-salt is minor; therefore, only one value of effective variance is used in the parameterization. The dependence of sea-salt forcing with solar zenith angle illustrates an interesting result that sea-salt can generate a positive top-of-the-atmosphere result (i.e. warming) when the solar zenith angle is relatively small (i.e. <30&deg;). Finally, it is found that the surface albedo significantly affects the shortwave radiative forcing, with the forcing diminishing to zero as the surface albedo tends to unity
Further investigation of white dwarfs in the open clusters NGC2287 and NGC3532
We report the results of a CCD imaging survey, complimented by astrometric
and spectroscopic follow-up studies, that aims to probe the fate of
heavy-weight intermediate mass stars by unearthing new, faint, white dwarf
members of the rich, nearby, intermediate age open clusters NGC3532 and
NGC2287. We identify a total of four white dwarfs with distances, proper
motions and cooling times which can be reconciled with membership of these
populations. We find that WDJ0643-203 in NGC2287, with an estimated mass of
M=1.02-1.16Msun, is potentially the most massive white dwarf so far identified
within an open cluster. Guided by the predictions of modern theoretical models
of the late-stage evolution of heavy-weight intermediate mass stars, we
conclude that there is a distinct possibility it has a core composed of O and
Ne. We also determine that despite the cooling times of the three new white
dwarfs in NGC3532 and the previously known degenerate member NGC3532-10
spanning ~90Myr, they all have remarkably similar masses (M~0.9-1Msun). This is
fully consistent with the results from our prior work on a heterogeneous sample
of ~50 white dwarfs from 12 stellar populations, on the basis of which we
argued that the stellar initial mass-final mass relation is less steep at
Minit>4Msun than in the adjacent lower initial mass regime. This change in the
gradient of the relation could account for the secondary peak observed in the
mass distribution of the field white dwarf population and mitigate the need to
invoke close binary evolution to explain its existence. Spectroscopic
investigation of numerous additional candidate white dwarf members of NGC3532
unearthed by a recent independent study would be useful to confirm (or
otherwise) these conclusions.Comment: 8 Figures, 8 tables. Accepted for publication in MNRA
WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe
There is a striking and unexplained dearth of brown dwarf companions in close
orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the
frequency of stellar and planetary companions. Although rare and relatively
short-lived, these systems leave detectable evolutionary end points in the form
of white dwarf - brown dwarf binaries and these remnants can offer unique
insights into the births and deaths of their parent systems. We present the
discovery of a close (orbital separation ~ 0.006 AU) substellar companion to a
massive white dwarf member of the Praesepe star cluster. Using the cluster age
and the mass of the white dwarf we constrain the mass of the white dwarf
progenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of the
white dwarf means the substellar companion must have been engulfed by the B
star's envelope while it was on the late asymptotic giant branch (AGB). Hence,
the initial separation of the system was ~2 AU, with common envelope evolution
reducing the separation to its current value. The initial and final orbital
separations allow us to constrain the combination of the common envelope
efficiency (alpha) and binding energy parameters (lambda) for the AGB star to
alpha lambda ~3. We examine the various formation scenarios and conclude that
the substellar object was most likely to have been captured by the white dwarf
progenitor early in the life of the cluster, rather than forming in situ.Comment: Accepted for publication in ApJ
Component masses of young, wide, non-magnetic white dwarf binaries in the SDSS DR7
We present a spectroscopic component analysis of 18 candidate young, wide,
non-magnetic, double-degenerate binaries identified from a search of the Sloan
Digital Sky Survey Data Release 7 (DR7). All but two pairings are likely to be
physical systems. We show SDSS J084952.47+471247.7 + SDSS J084952.87+471249.4
to be a wide DA+DB binary, only the second identified to date. Combining our
measurements for the components of 16 new binaries with results for three
similar, previously known systems within the DR7, we have constructed a mass
distribution for the largest sample to date (38) of white dwarfs in young,
wide, non-magnetic, double-degenerate pairings. This is broadly similar in form
to that of the isolated field population with a substantial peak around M~0.6
Msun. We identify an excess of ultra-massive white dwarfs and attribute this to
the primordial separation distribution of their progenitor systems peaking at
relatively larger values and the greater expansion of their binary orbits
during the final stages of stellar evolution. We exploit this mass distribution
to probe the origins of unusual types of degenerates, confirming a mild
preference for the progenitor systems of high-field-magnetic white dwarfs, at
least within these binaries, to be associated with early-type stars.
Additionally, we consider the 19 systems in the context of the stellar initial
mass-final mass relation. None appear to be strongly discordant with current
understanding of this relationship.Comment: 20 pages, 5 Tables, 7 figures. accepted for publication in MNRA
X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were
monitored for 35 days and 12 days with ASCA and RXTE (and EUVE for Ton S180).
The short time scale (hours-days) variability patterns were very similar across
energy bands, with no evidence of lags between any of the energy bands studied.
The fractional variability amplitude was almost independent of energy band. It
is difficult to simultaneously explain soft Seyferts stronger variability,
softer spectra, and weaker energy-dependence of the variability relative to
hard Seyferts. The soft and hard band light curves diverged on the longest time
scales probed, consistent with the fluctuation power density spectra that
showed relatively greater power on long time scales in the softest bands. The
simplest explanation is that a relatively hard, rapidly-variable component
dominates the total X-ray spectrum and a slowly-variable soft excess is present
in the lowest energy channels of ASCA. Although it would be natural to identify
the latter with an accretion disk and the former with a corona surrounding it,
a standard thin disk could not get hot enough to radiate significantly in the
ASCA band, and the observed variability time scales are much too short. The
hard component may have a more complex shape than a pure power-law. The most
rapid factor of 2 flares and dips occurred within ~1000 sec in Ark 564 and a
bit more slowly in Ton S180. The speed of the luminosity changes rules out
viscous or thermal processes and limits the size of the individual emission
regions to <~15 Schwarzschild radii (and probably much less), that is, to
either the inner disk or small regions in a corona
Aerosol indirect effects on glaciated clouds. Part I: Model description
Various improvements were made to a state-of-the-art aerosol–cloud model and comparison of the model results with observations from field campaigns was performed. The strength of this aerosol–cloud model is in its ability to explicitly resolve all the known modes of heterogeneous cloud droplet activation and ice crystal nucleation. The model links cloud particle activation with the aerosol loading and chemistry of seven different aerosol species. These improvements to the model resulted in more accurate prediction especially of droplet and ice crystal number concentrations in the upper troposphere and enabled the model to directly sift the aerosol indirect effects based on the chemistry and concentration of the aerosols. In addition, continental and maritime cases were simulated for the purpose of validating the aerosol–cloud model and for investigating the critical microphysical and dynamical mechanisms of aerosol indirect effects from anthropogenic solute and solid aerosols, focusing mainly on glaciated clouds. The simulations showed that increased solute aerosols reduced cloud particle sizes by about 5 μm and inhibited warm rain processes. Cloud fractions and their optical thicknesses were increased quite substantially in both cases. Although liquid mixing ratios were boosted, there was however a substantial reduction of ice mixing ratios in the upper troposphere owing to the increase in snow production aloft. These results are detailed in the subsequent parts of this study
Up-regulation of endothelial delta-like 4 expression correlates with vessel maturation in bladder cancer.
PURPOSE: Angiogenesis and vascular endothelial growth factor (VEGF) expression are associated with a poor outcome in bladder cancer. To understand more about the mechanisms, we studied the role of delta-like 4 (DLL4), an endothelial-specific ligand of the Notch signaling pathway, in bladder cancer angiogenesis. EXPERIMENTAL DESIGN: The expression of DLL4, CD34, and VEGF were studied in a cohort of 60 bladder tumors and 10 normal samples using quantitative PCR. In situ hybridization was used to study the pattern of DLL4 expression in 22 tumor and 9 normal samples. Serial sections were also stained for CD34 and alpha-smooth muscle actin (alpha-SMA) using conventional immunohistochemistry. RESULTS: The expression of DLL4 was significantly up-regulated in superficial (P < 0.01) and invasive (P < 0.05) bladder cancers. DLL4 expression significantly correlated with CD34 (P < 0.001) and VEGF (P < 0.001) expression. The in situ hybridization studies showed that DLL4 was highly expressed within bladder tumor vasculature. Additionally, DLL4 expression significantly correlated with vessel maturation as judged by periendothelial cell expression of alpha-SMA, 98.7% of DLL4-positive tumor vessels coexpressed alpha-SMA, compared with 64.5% of DLL4-negative tumor vessels (P < 0.001). High DLL4 expression may have prognostic value in superficial and invasive bladder. CONCLUSION: DLL4 expression is associated with vascular differentiation in bladder cancer; thus, targeting DLL4 may be a novel antiangiogenic therapy
A provisional database for the silicon content of foods in the United Kingdom
Si may play an important role in bone formation and connective tissue metabolism. Although biological interest in this element has recently increased, limited literature exists on the Si content of foods. To further our knowledge and understanding of the relationship between dietary Si and human health, a reliable food composition database, relevant for the UK population, is required. A total of 207 foods and beverages, commonly consumed in the UK, were analysed for Si content. Composite samples were analysed using inductively coupled plasma–optical emission spectrometry following microwave-assisted digestion with nitric acid and H2O2. The highest concentrations of Si were found in cereals and cereal products, especially less refined cereals and oat-based products. Fruit and vegetables were highly variable sources of Si with substantial amounts present in Kenyan beans, French beans, runner beans, spinach, dried fruit, bananas and red lentils, but undetectable amounts in tomatoes, oranges and onions. Of the beverages, beer, a macerated whole-grain cereal product, contained the greatest level of Si, whilst drinking water was a variable source with some mineral waters relatively high in Si. The present study provides a provisional database for the Si content of UK foods, which will allow the estimation of dietary intakes of Si in the UK population and investigation into the role of dietary Si in human health.<br /
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