230 research outputs found
Oxygen and nitrogen abundances of HII regions in six spiral galaxies
Spectroscopic observations of 63 HII regions in six spiral galaxies (NGC 628,
NGC 783, NGC 2336, NGC 6217, NGC 7331, and NGC 7678) were carried out with the
6-meter telescope (BTA) of Russian Special Astrophysical Observatory with the
Spectral Camera attached to the focal reducer SCORPIO in the multislit mode
with a dispersion of 2.1A/pixel and a spectral resolution of 10A. These
observations were used to estimate the oxygen and nitrogen abundances and the
electron temperatures in HII regions through the recent variant of the strong
line method (NS calibration). The parameters of the radial distribution (the
extrapolated central intercept value and the gradient) of the oxygen and
nitrogen abundances in the disks of spiral galaxies NGC 628, NGC 783, NGC 2336,
NGC 7331, and NGC 7678 have been determined. The abundances in the NGC 783, NGC
2336, NGC 6217, and NGC 7678 are measured for the first time. Galaxies from our
sample follow well the general trend in the luminosity - central metallicity
diagram for spiral and irregular galaxies.Comment: 11 pages, 7 figures, 6 tables. Accepted for publication in MNRA
HST observations of the cometary blue compact dwarf galaxy UGC 4483: a relatively young galaxy?
We present V and I photometry of the resolved stars in the cometary blue
compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary
Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram
(CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric
errors less than 0.2 mag. It reveals not only a young stellar population of
blue main-sequence stars and blue and red supergiants, but also an older
evolved population of red giant and asymptotic giant branch stars. The measured
magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a
distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/-
0.2 Mpc. The youngest stars are associated with the bright H II region at the
northern tip of the galaxy. The population of older stars is found throughout
the low-surface-brightness body of the galaxy and is considerably more spread
out than the young stellar population, suggesting stellar diffusion. The most
striking characteristics of the CMD of UGC 4483 are the very blue colors of the
red giant stars and the high luminosity of the asymptotic giant branch stars.
Both of these characteristics are consistent with either: 1) a very low
metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and
an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from
the ionized gas emission lines) and a relatively young age of the oldest
stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not
exclude the possibility that UGC 4483 is a relatively young galaxy having
formed its first stars only ~ 2 Gyr ago.Comment: 37 pages, 15 PS figures, to appear in Ap
The metallicity extremes of the Sagittarius dSph using SALT spectroscopy of PNe
In this work we present the first spectroscopic results obtained with the
Southern African Large Telescope (SALT) telescope during its
perfomance-verification phase. We find that the Sagittarius dwarf spheroidal
galaxy (Sgr) Sgr contains a youngest stellar population with [O/H] -0.2 and age
t>1 Gyr, and an oldest population with [O/H]=-2.0. The values are based on
spectra of two planetary nebulae (PNe), using empirical abundance
determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He.
We confirm the high abundances of PN StWr2-21 with 12+log(O/H) = 8.57+/-0.02
dex. The other PN studied, BoBn1, is an extraordinary object in that the neon
abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield
the original stellar metallicity, corresponding to 12+log(O/H) = 6.72+/-0.16
dex which is 1/110 of the solar value. The actual [O/H] is much higher: third
dredge-up enriched the material by a factor of ~12 in oxygen, ~240 in nitrogen
and ~70 in neon. Neon as well as nitrogen and oxygen content may have been
produced in the intershell of low-mass AGB stars. Well defined broad WR lines
are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67% for dSph galaxies.Comment: 14 pages, 4 figures, accepted to MNRA
Interactions and star formation activity in Wolf-Rayet galaxies
We present the main results of the PhD Thesis carried out by
L\'opez-S\'anchez (2006), in which a detailed morphological, photometrical and
spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies was
realized. The main aims are the study of the star formation and O and WR
stellar populations in these galaxies and the role that interactions between
low surface companion objects have in the triggering of the bursts. We analyze
the morphology, stellar populations, physical conditions, chemical abundances
and kinematics of the ionized gas, as well as the star-formation activity of
each system.Comment: 16 pages, 15 figure
An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies
(abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions
in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations
of 38 HII regions in 28 emission-line galaxies. These emission-line galaxies
were selected mainly from the Data Release 6 (DR6) of the Sloan Digital Sky
Survey (SDSS) as metal-deficient galaxy candidates. We collect photometric and
high-quality spectroscopic data for a large uniform sample of star forming
galaxies including new observations. Our aim is to study the
luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its
low-metallicity end and compare it with that for higher-redshift galaxies. From
our new observations we find that the oxygen abundance in 61 out of the 66 HII
regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes
27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor
star-forming galaxies (XBCDs). Among them are 10 HII regions with
12+logO/H<7.3. The new sample is combined with a further 93 low-metallicity
galaxies with accurate oxygen abundance determinations from our previous
studies, yielding in total a high-quality spectroscopic data set of 154 HII
regions. 9000 more galaxies with oxygen abundances, based mainly on the
Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with
respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in
its oxygen abundance (7.0<12+logO/H<8.8), allowing us to probe the L-Z relation
in the nearby universe down to the lowest currently studied metallicity level.
The L-Z relation established on the basis of the present sample is consistent
with previous ones obtained for emission-line galaxies.Comment: 27 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
The DEEP2 Galaxy Redshift Survey: Discovery of Luminous, Metal-poor, Sta r-forming Galaxies at Redshifts z~0.7
We have discovered a sample of 17 metal-poor, yet luminous, star-forming
galaxies at redshifts z~0.7. They were selected from the initial phase of the
DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536
galaxies as those showing the temperature-sensitive [OIII]l4363 auroral line.
These rare galaxies have blue luminosities close to L*, high star formation
rates of 5 to 12 solar masses per year, and oxygen abundances of 1/3 to 1/10
solar. They thus lie significantly off the luminosity-metallicity relation
found previously for field galaxies with strong emission lines at redshifts
z~0.7. The prior surveys relied on indirect, empirical calibrations of the R23
diagnostic and the assumption that luminous galaxies are not metal-poor. Our
discovery suggests that this assumption is sometimes invalid. As a class, these
newly-discovered galaxies are: (1) more metal-poor than common classes of
bright emission-line galaxies at z~0.7 or at the present epoch; (2) comparable
in metallicity to z~3 Lyman Break Galaxies but less luminous; and (3)
comparable in metallicity to local metal-poor eXtreme Blue Compact Galaxies
(XBCGs), but more luminous. Together, the three samples suggest that the
most-luminous, metal-poor, compact galaxies become fainter over time.Comment: This is a .tgz file. It should create the following files: texto.tex,
tab1.tex, f1.eps and f2.eps. The LaTeX style used is emulateapj.cls, version
November 26, 2004. This submission is 5 pages long, one table and two
figures. To appear in ApJ
«Общие критерии» и безопасность программно-конфигурируемых сетей
«Common criteria» (ISO 15408) is a universally recognized and broadly applicable approach to information security solutions management and evaluation. «Common criteria» leans on developing a shared conceptual basis for key security solution modules including protection profiles and security targets. Conceptual basis development implies defining the following elements: security objectives and assumptions (for the environment and the object), threats and security policies, as well as functional and assurance requirements. The specifics of SDN (software defined network) security solutions is largely driven by fundamental architectural principles of SDN technology itself − primarily by the separation of control and data flows, − and by conditions imposed by Open Flow protocol application. However, proactive (threats and policies), passive (objectives and assumptions) and reactive (requirements) aspects of security management remain highly relevant for this type of security solutions. This paper discusses the Common Criteria application specifics for assessing the SDN security and practical MTUCI (Moscow Technical University of Communications and Informatics) experience in the development of the protection profile. A new class of network attacks on SDN switches and controllers can involve either data or control components. In addition to traditional vulnerabilities, centralization of management functions paves way for new security threats by isolating controller activity and administrative message exchange. Therefore, identifying and analyzing threats, policies and requirements specific to SDN control module security becomes an emerging priority.«Общие критерии» (ISO 15408) – общепризнанный и широко применимый подход к управлению и оценке решений в области информационной безопасности. «Общие критерии» опираются на разработку общей концептуальной основы для ключевых решений безопасности, включая профили защиты и целевые объекты безопасности. Концептуальная основа разработки подразумевает определение следующих элементов: цели и предположения безопасности (для среды и объекта), угрозы и политики безопасности, а также функциональные требования и требования к обеспечению безопасности. Специфика решений по обеспечению безопасности SDN во многом обусловлена фундаментальными архитектурными принципами самой технологии SDN – в первую очередь разделением потоков управления и данных, а также условиями применения протокола OpenFlow. Тем не менее, проактивные (угрозы и политики), пассивные (цели и предположения) и реактивные (требования) аспекты управления безопасностью остаются весьма актуальными для такого типа решений безопасности. В статье рассматриваются особенности применения единых критериев оценки безопасности SDN и практического опыта Московского технического университета связи и информатики при разработке профиля защиты. Новый класс сетевых атак на коммутаторы и контроллеры SDN может использовать как данные, так и компоненты управления. В дополнение к традиционным уязвимостям централизация функций управления открывает путь для новых угроз безопасности путем изоляции деятельности контроллера и обмена управляющими сообщениями. Поэтому выявление и анализ угроз, политик и требований, специфичных для безопасности модуля управления SDN, становится новым приоритетом
Mechanical Behavior of Polycrystalline Rhenium under 3-Points Bending at a Low Homological Temperature
Mechanical behaviour of polycrystalline rhenium under 3-points bending at such low homological temperature as room is discussed. Two metallurgical technologies (electron beam melting and powder metallurgy) were used for the samples preparation. Fine-grained samples (PM metal) exhibit some plasticity prior the failure, while coarse-grained ones (EBM metal) behave like a brittle solid. The intergranular fracture is the fracture mode of rhenium in both cases. Basal slip and prismatic slip of dislocations are not active in rhenium at low homological temperature, but the grain boundary sliding occurs under these conditions. Therefore, polycrystalline rhenium cannot be machined at room temperature despite the growth of grain boundary cracks are braked in the samples due to grain boundary sliding. © Published under licence by IOP Publishing Ltd.Russian Science Foundation, RSF: 18-19-00217This work is supported by the Russian Science Foundation (#18-19-00217)
PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release
We present a low-resolution spectroscopic survey of the Orion nebula which
data we release for public use. In this article, we intend to show the possible
applications of this dataset analyzing some of the main properties of the
nebula. We perform an integral field spectroscopy mosaic of an area of ~5' X 6'
centered on the Trapezium region of the nebula, including the ionization front
to the south-east. The analysis of the line fluxes and line ratios of both the
individual and integrated spectra allowed us to determine the main
characteristics of the ionization throughtout the nebula.The final dataset
comprises 8182 individual spectra, which sample each one a circular area of
\~2.7" diameter. The data can be downloaded as a single row-stacked spectra
fits file plus a position table or as an interpolated datacube with a final
sampling of 1.5"/pixel. The integrated spectrum across the field-of-view was
used to obtain the main integrated properties of the nebula, including the
electron density and temperature, the dust extinction, the Halpha integrated
flux (after correcting for dust reddening), and the main diagnostic line
ratios. The individual spectra were used to obtain line intensity maps of the
different detected lines. These maps were used to study the distribution of the
ionized hydrogen, the dust extinction, the electron density and temperature,
and the helium and oxygen abundance...Comment: 13 pages, 8 figures, accepted for publishing in Astronomy &
Astrophysic
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