2,682 research outputs found
Chasing the observational signatures of seed black holes at z > 7: candidate observability
Observing the light emitted by the first accreting black holes (BHs) would
dramatically improve our understanding of the formation of quasars at z > 6,
possibly unveiling the nature of their supermassive black hole (SMBH) seeds. In
previous works we explored the relative role of the two main competing BH seed
formation channels, Population III remnants (low-mass seeds) and direct
collapse BHs (high-mass seeds), investigating the properties of their host
galaxies in a cosmological context. Building on this analysis, we predict here
the spectral energy distribution and observational features of low- and
high-mass BH seeds selected among the progenitors of a z~6 SMBH. We derive the
processed emission from both accreting BHs and stars by using the
photo-ionization code Cloudy, accounting for the evolution of metallicity and
dust-to-gas mass ratio in the interstellar medium of the host galaxies, as
predicted by the cosmological data- constrained model GAMETE/QSOdust. We show
how future missions like JWST and ATHENA will be able to detect the light
coming from SMBH progenitors already at z~16. We build upon previous
complementary studies and propose a method based on the combined analysis of
near infrared (NIR) colors, IR excess (IRX) and UV continuum slopes (i.e.
color-color and IRX-Beta diagrams) to distinguish growing seed BH host galaxies
from starburst-dominated systems in JWST surveys. Sources selected through this
criterion would be the best target for follow-up X-ray observations.Comment: accepted for publicaztion in MNRA
Comparison of Theoretical Starburst Photoionisation Models for Optical Diagnostics
We study and compare different examples of stellar evolutionary synthesis
input parameters used to produce photoionisation model grids using the MAPPINGS
V modelling code. The aim of this study is to (a) explore the systematic
effects of various stellar evolutionary synthesis model parameters on the
interpretation of emission lines in optical strong-line diagnostic diagrams,
(b) characterise the combination of parameters able to reproduce the spread of
local galaxies located in the star-forming region in the Sloan Digital Sky
Survey, and (c) investigate the emission from extremely metal-poor galaxies
using photoionisation models. We explore and compare the stellar input ionising
spectrum (stellar population synthesis code [Starburst99, SLUG, BPASS], stellar
evolutionary tracks, stellar atmospheres, star-formation history, sampling of
the initial mass function) as well as parameters intrinsic to the H II region
(metallicity, ionisation parameter, pressure, H II region boundedness). We also
perform a comparison of the photoionisation codes MAPPINGS and CLOUDY. On the
variations in the ionising spectrum model parameters, we find that the
differences in strong emission-line ratios between varying models for a given
input model parameter are small, on average ~0.1 dex. An average difference of
~0.1 dex in emission-line ratio is also found between models produced with
MAPPINGS and CLOUDY. Large differences between the emission-line ratios are
found when comparing intrinsic H II region parameters. We find that
low-metallicity galaxies are better explained by a density-bounded H II region
and higher pressures better encompass the spread of galaxies at high redshift.Comment: 33 pages, 26 figures, accepted for publication in Ap
HELIOS-Retrieval: An Open-source, Nested Sampling Atmospheric Retrieval Code, Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation
We present an open-source retrieval code named HELIOS-Retrieval (hereafter
HELIOS-R), designed to obtain chemical abundances and temperature-pressure
profiles from inverting the measured spectra of exoplanetary atmospheres. In
the current implementation, we use an exact solution of the radiative transfer
equation, in the pure absorption limit, in our forward model, which allows us
to analytically integrate over all of the outgoing rays (instead of performing
Gaussian quadrature). Two chemistry models are considered: unconstrained
chemistry (where the mixing ratios are treated as free parameters) and
equilibrium chemistry (enforced via analytical formulae, where only the
elemental abundances are free parameters). The nested sampling algorithm allows
us to formally implement Occam's Razor based on a comparison of the Bayesian
evidence between models. We perform a retrieval analysis on the measured
spectra of the HR 8799b, c, d and e directly imaged exoplanets. Chemical
equilibrium is disfavored by the Bayesian evidence for HR 8799b, c and d. We
find supersolar C/O, C/H and O/H values for the outer HR 8799b and c
exoplanets, while the inner HR 8799d and e exoplanets have substellar C/O,
substellar C/H and superstellar O/H values. If these retrieved properties are
representative of the bulk compositions of the exoplanets, then they are
inconsistent with formation via gravitational instability (without late-time
accretion) and consistent with a core accretion scenario in which late-time
accretion of ices occurred differently for the inner and outer exoplanets. For
HR 8799e, we find that spectroscopy in the K band is crucial for constraining
C/O and C/H. HELIOS-R is publicly available as part of the Exoclimes Simulation
Platform (ESP; www.exoclime.org).Comment: 27 pages, 21 figures, 3 tables, published in A
The Semantic Web: Apotheosis of annotation, but what are its semantics?
This article discusses what kind of entity the proposed Semantic Web (SW) is, principally by reference to the relationship of natural language structure to knowledge representation (KR). There are three distinct views on this issue. The first is that the SW is basically a renaming of the traditional AI KR task, with all its problems and challenges. The second view is that the SW will be, at a minimum, the World Wide Web with its constituent documents annotated so as to yield their content, or meaning structure, more directly. This view makes natural language processing central as the procedural bridge from texts to KR, usually via some form of automated information extraction. The third view is that the SW is about trusted databases as the foundation of a system of Web processes and services. There's also a fourth view, which is much more difficult to define and discuss: If the SW just keeps moving as an engineering development and is lucky, then real problems won't arise. This article is part of a special issue called Semantic Web Update
Near-infrared photometry of WISE J085510.74-071442.5
Indexación: Web of ScienceAims. We aim at obtaining near-infrared photometry and deriving the mass, age, temperature, and surface gravity of WISE J085510.74 071442.5 (J0855 0714), which is the coolest object beyond the solar system currently known.
Methods. We used publicly available data from the archives of the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) to determine the emission of this source at 1.153 mu m (F110W) and 1.575 mu m (CH4-o ff). J0855 0714 was detected at both wavelengths with a signal-to-noise ratio of approximate to 10 (F110W) and approximate to 4 (CH4-off) at the peak of the corresponding point-spread-functions.
Results. This is the first detection of J0855 0714 in the H-band wavelengths. We measured 26.31 +/- 0.10 and 23.22 +/- 0.35 mag in F110W and CH4-o ff (Vega system). J0855 0714 remains unresolved in the HST images that have a spatial resolution of 0.22 0 0. Companions at separations of 0.5 AU (similar mass and brightness) and at similar to 1 AU approximate to 1 mag fainter in the F110W filter) are discarded. By combining the new data with published photometry, including non-detections, we build the spectral energy distribution of J0855 0714 from 0.89 through 22.09 mu m, and contrast it against current solar-metallicity models of planetary atmospheres. We determine that the best spectral fit yields a temperature of 225 250 K, a bolometric luminosity of log L/L-circle dot = 8 : 57, and a high surface gravity of log g = 5 : 0 (cm s(2)), which suggests an old age although a gravity this high is not fully compatible with evolutionary models. After comparing our data with the cooling theory for brown dwarfs and planets, we infer a mass in the interval 2 10 MJup for ages of 1 12 Gyr and high atmospheric gravities of log g greater than or similar to 3.5 (cm s(2)). If it had the age of the Sun, J0855 0714 would be a approximate to 5-M-Jup free-floating planetary-mass object.
Conclusions. J0855 0714 meets the mass values previously determined for free-floating planetary-mass objects discovered in starforming regions and young stellar clusters. Based on extrapolations of the substellar mass functions of young clusters to the field, as many J0855 0714-like objects as M5-L2 stars may be expected to populate the solar neighborhood.http://www.aanda.org/articles/aa/pdf/2016/08/aa28662-16.pd
Nebular Abundance Errors
The errors inherent to the use of the standard "ionization correction factor"
("i_CF") method of calculating nebular conditions and relative abundances of H,
He, N, O, Ne, S, and Ar in emission line nebulae have been investigated under
conditions typical for planetary nebulae. The photoionization code CLOUDY was
used to construct a series of model nebulae with properties spanning the range
typical of PNe. Its radial "profiles" of bright, frequently observed optical
emission lines were then summed over a variety of "apertures" to generate sets
of emission line measurements. These resulting line ratios were processed using
the i_CF method to "derive" nebular conditions and abundances. We find that for
lines which are summed over the entire nebula the i_CF-derived abundances
differ from the input abundances by less than 5% for He and O up to 25% or more
for Ne, S, and Ar. For resolved observations, however, the discrepancies are
often much larger and are systematically variable with radius. This effect is
especially pronounced in low-ionization zones where nitrogen and oxygen are
neutral or once-ionized such as in FLIERs, ansae and ionization fronts. We
argue that the reports of stellar-enriched N in the FLIERs of several PNe are
probably specious.Comment: 22 pages, 4 tables, and 1 figure. Accepted for publication in the
Astronomical Journal. Replaced to correct a referenc
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