175 research outputs found
A photoionization model of the compact HII region G29.96-0.02
We present a detailed photoionization model of G29.96-0.02 (hereafter
G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the
Galaxy. This source has been observed extensively at radio and infrared
wavelengths. The most recent data include a complete ISO (SWS and LWS)
spectrum, which displays a remarkable richness in atomic fine-structure lines.
The number of observables is twice as much as the number available in previous
studies. In addition, most atomic species are now observed in two ionization
stages. The radio and infrared data on G29.96 are best reproduced using a
nebular model with two density components: a diffuse (n_e~680cm-3) extended (~1
pc) component surrounding a compact (~0.1 pc) dense (n_e~57000cm-3) core. The
properties of the ionizing star were derived using state-of-the-art stellar
atmosphere models. CoStar models yield an effective temperature of
\~30^{+2}_{-1} kK whereas more recent non-LTE line blanketed atmospheres with
stellar winds indicate somewhat higher values, Teff~32--38 kK. This range in
Teff is compatible with all observational constraints, including near-infrared
photometry and bolometric luminosity. The range 33-36 kK is also compatible
with the spectral type O5-O8 determined by Watson and Hanson (97) when recent
downward revisions of the effective temperature scale of O stars are taken into
account. The age of the ionizing star of G29.96 is found to be a few 10^6 yr,
much older than the expected lifetime of UCHII regions. Accurate gas phase
abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43
(1.3 and 3.5 times the solar values, respectively).Comment: Accepted in Astronomy and Astrophysic
A pan-Arctic assessment of the status of marine social-ecological systems
Marine social-ecological conditions in the Arctic are rapidly changing. With many transboundary issues, such as shifting ranges of fisheries, biodiversity loss, sea ice retreat, economic development and pollution, greater pan-Arctic assessment and co-management are necessary. We adapted the Ocean Health Index (OHI) to compile pan-Arctic data and evaluate ocean health for nine regions above the Arctic Circle to assess the extent to which pan-Arctic assessment is possible and identify broad social-ecological trends. While the quality and availability of data varied, we assessed and scored nine OHI goals, including the pressures and resilience measures acting upon them. Our results show the Arctic is sustainably delivering a range of benefits to people, but with room for improvement in all goals, particularly tourism, fisheries, and protected places. Successful management of biological resources and short-term positive impacts on biodiversity in response to climate change underlie these high goal scores. The OHI assesses the past and near-term future but does not account for medium- and long-term future risks associated with climate change, highlighting the need for ongoing monitoring, dynamic management, and strong action to mitigate its anticipated effects. A general increase in and standardisation of monitoring is urgently needed in the Arctic. Unified assessments, such as this one, can support national comparisons, data quality assessments, and discussions on the targeting of limited monitoring capabilities at the most pressing and urgent transboundary management challenges, which is a priority for achieving successful Arctic stewardship
Chemical abundance patterns in the inner Galaxy: the Scutum Red Supergiant Clusters
The location of the Scutum Red-Supergiant (RSG) clusters at the end of the
Galactic Bar makes them an excellent probe of the Galaxy's secular evolution;
while the clusters themselves are ideal testbeds in which to study the
predictions of stellar evolutionary theory. To this end, we present a study of
the RSGs' surface abundances using a combination of high-resolution H-band
spectroscopy and spectral synthesis analysis. We provide abundance measurements
for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of
the stars studied are consistent with CNO burning and deep, rotationally
enhanced mixing. The average a/Fe ratios of the clusters are solar, consistent
with a thin-disk population. However, we find significantly sub-solar Fe/H
ratios for each cluster, a result which strongly contradicts a simple
extrapolation of the Galactic metallicity gradient to lower Galacto-centric
distances. We suggest that a simple one-dimensional parameterization of the
Galaxy's abundance patterns is insufficient at low Galactocentric distances, as
large azimuthal variations may be present. Indeed, we show that the abundances
of O, Si and Mg are consistent with independent measurements of objects in
similar locations in the Galaxy. In combining our results with other data in
the literature, we present evidence for large-scale (~kpc) azimuthal variations
in abundances at Galacto-centric distances of 3-5kpc. While we cannot rule-out
that this observed behaviour is due to systematic offsets between different
measurement techniques, we do find evidence for similar behaviour in a study of
the barred-spiral galaxy NGC4736 which uses homogeneous methodology. We suggest
that these azimuthal abundance variations could result from the intense but
patchy star formation driven by the potential of the central bar.Comment: 13 pages, 9 figures, accepted to Ap
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISM
We have supplemented existing spectra of Barnard's Loop with high accuracy
spectrophotometry of one new position. Cloudy photoionization models were
calculated for a variety of ionization parameters and stellar temperatures and
compared with the observations. After testing the procedure with recent
observations of M43, we establish that Barnard's Loop is photoionized by four
candidate ionizing stars, but agreement between the models and observations is
only possible if Barnard's Loop is enhanced in heavy elements by about a factor
of 1.4. Barnard's Loop is very similar in properties to the brightest
components of the Orion-Eridanus Bubble and the Warm Ionized Medium (WIM). We
are able to establish models that bound the range populated in low-ionization
color-color diagrams (I([SII])/I(H{\alpha}) versus I([NII])/I(H{\alpha})) using
only a limited range of ionization parameters and stellar temperatures.
Previously established variations in the relative abundance of heavy elements
render uncertain the most common method of determining electron temperatures
for components of the Orion-Eridanus Bubble and the WIM based on only the
I([NII])/I(H{\alpha}) ratio, although we confirm that the lowest surface
brightness components of the WIM are on average of higher electron temperature.
The electron temperatures for a few high surface brightness WIM components
determined by direct methods are comparable to those of classical bright H II
regions. In contrast, the low surface brightness HII regions studied by the
Wisconsin H{\alpha} Mapper are of lower temperatures than the classical bright
HII regions
The stellar content, metallicity and ionization structure of HII regions
Observations of infrared fine-structure lines provide direct information on
the metallicity and ionization structure of HII regions and indirectly on the
hardness of the radiation field ionizing these nebulae. We have analyzed a
sample of Galactic and Magellanic Cloud HII regions observed by the Infrared
Space Observatory (ISO) to examine the interplay between stellar content,
metallicity and the ionization structure of HII regions. The observed
[SIV]10.5/[SIII]18.7 mum and [NeIII]15.5/[NeII]12.8 mum line ratios are shown
to be highly correlated over more than two orders of magnitude. We have
compared the observed line ratios to the results of photoionization models
using different stellar energy distributions. The derived characteristics of
the ionizing star depend critically on the adopted stellar model as well as the
(stellar) metallicity. We have compared the stellar effective temperatures
derived from these model studies for a few well-studied HII regions with
published direct spectroscopic determinations of the spectral type of the
ionizing stars. This comparison supports our interpretation that stellar and
nebular metallicity influences the observed infrared ionic line ratios. We can
explain the observed increase in degree of ionization, as traced by the
[SIV]\[SIII] and [NeIII]\[NeII] line ratios, by the hardening of the radiation
field due to the decrease of metallicity. The implications of our results for
the determination of the ages of starbursts in starburst galaxies are assessed.Comment: 9 pages; accepted for publication in A&A; figure 3 modifie
Understanding the Spectral Energy Distributions of the Galactic Star Forming Regions IRAS 18314-0720, 18355-0532 & 18316-0602
Embedded Young Stellar Objects (YSO) in dense interstellar clouds is treated
self-consistently to understand their spectral energy distributions (SED).
Radiative transfer calculations in spherical geometry involving the dust as
well as the gas component, have been carried out to explain observations
covering a wide spectral range encompassing near-infrared to radio continuum
wavelengths. Various geometric and physical details of the YSOs are determined
from this modelling scheme. In order to assess the effectiveness of this
self-consistent scheme, three young Galactic star forming regions associated
with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test
cases. They cover a large range of luminosity ( 40). The modelling of
their SEDs has led to information about various details of these sources, e.g.
embedded energy source, cloud structure & size, density distribution,
composition & abundance of dust grains etc. In all three cases, the best fit
model corresponds to the uniform density distribution.Comment: AAMS style manuscript with 3 tables (in a separate file) and 4
figures. To appear in Journal of Astronophysics & Astronom
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
New results on the time variation of the radial abundance gradients from planetary nebulae
New results on the time variation of the radial abundance gradients in the
galactic disk are presented on the basis of four different samples of planetary
nebulae. These comprise both smaller, homogeneous sets of data, and larger but
non-homogeneous samples. Four different chemical elements are considered,
namely, O, S, Ar, and Ne. Other objects such as open clusters, cepheids and HII
regions are also taken into account. Our analysis support our earlier
conclusions in the sense that, on the average, the radial abundance gradients
have flattened out during the last 6 to 8 Gyr, with important consequences for
models of the chemical evolution of the Galaxy.Comment: 2 pages, 1 figure, LaTeX, To be published in the Proceedings of the
IAU Symposium 234: Planetary Nebulae in Our Galaxy and Beyond, ed. M. J.
Barlow, R. H. Mende
Distant Voices: Learners' Stories About the Affective Side of Learning a Language at a Distance
Learning a language at a distance has its own special challenges. The remoteness of the learning context can mean isolation for the learner, communication difficulties for the teacher and problems of access for the researcher. Yet distance language learners are likely to be no more skilled in self-regulation than classroom learners, and to require high levels of support. Research tools are needed, therefore, which allow them to talk freely about their learning in order to help distance educators target support appropriately. This paper draws on data from two pilot ethnographic studies of distance language learners using think-aloud protocols to access their thought processes as they tackled two designated language tasks. They were carried out as part of a wider study in each case to investigate aspects of affect including beliefs, motivation and anxiety. The audio-taped voices provided rich insights into the advantages and disadvantages, pleasures and frustrations, comforts and anxieties of learning a language at a distance, and the strategies learners use to manage in a distance environment. The studies underlined the importance of listening to students and using their voices as a basis for discussion on improving aspects of the design and delivery of distance language courses
Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal an intense
signal of 1.809 MeV line emission, attributed to radioactive decay of 26Al,
that is closely correlated with 53 GHz free-free emission, originating from the
ionised interstellar medium. We modelled both emissions using a
multi-wavelength evolutionary synthesis code for massive star associations that
we applied to the known massive star populations in Cygnus. For all OB
associations and young open clusters in the field, we determined the population
age, distance, and richness as well as the uncertainties in all these
quantities from published photometric and spectroscopic data. We propagate the
population uncertainties in model uncertainties by means of a Bayesian method.
The young globular cluster Cyg OB2 turns out to be the dominant 26Al
nucleosynthesis and ionisation source in Cygnus. Our model reproduces the
ionising luminosity of the Cygnus region very well, yet it underestimates 26Al
production by about a factor of 2. We attribute this underestimation to
shortcomings of current nucleosynthesis models, and suggest the inclusion of
stellar rotation as possible mechanism to enhance 26Al production. We also
modelled 60Fe nucleosynthesis in the Cygnus region, yet the small number of
recent supernova events suggests only little 60Fe production. Consequently, a
detection of the 1.137 MeV and 1.332 MeV decay lines of 60Fe from Cygnus by the
upcoming INTEGRAL observatory is not expected.Comment: 24 pages, 7 figures, accepted for publication in A&
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