3,454 research outputs found
Masses and Radii of the Nuclei with N>=Z in an Alpha-Cluster Model
In the framework of a recently developed alpha-cluster model a nucleus is
represented as a core (alpha-cluster liquid drop with dissolved excess neutron
pairs in it) and a nuclear molecule on its surface. From analysis of
experimental nuclear binding energies one can find the number of alpha-clusters
in the molecule and calculate the nuclear charge radii. It was shown that for
isotopes of one Z with growing A the number of alpha-clusters in the molecule
decreases to three, which corresponds to the nucleus 12C for even Z and 15N for
odd Z, and the specific density of the core binding energy \rho grows and
reaches its saturation value. In this paper it is shown that the value
\rho=2.55 MeV/fm^3 explains the particular number of excess neutrons in stable
nuclei.Comment: 7 pages, 3 eps figures, submitted as a contribution to the
Proceedings of the International Conference, Messina, Italy, October 5-9,
200
Study of crack-initiation phenomena associated with stress corrosion of aluminum alloys Annual report, 6 Jun. 1967 - 6 Jun. 1968
Electron microscopic, fractographic, and microprobe observations of crack initiation and propagation phenomena associated with stress corrosion of aluminum alloy
Wireless Telemetry System for Implantable Sensors
Advanced testing of medical treatments involves experimentation on small laboratory animals, such as genetically modified mice. These subjects are used to help researchers develop medication and cures for humans. To understand the effects of the treatments, innovative telemetry systems are developed, that enable remote real-time cardiac monitoring. The latest research in the field of cardiac monitoring has revealed two major limitations with wireless implantable systems: a) the current size of implantable electronics limits the physical size of the system to larger subjects; and b) the systems only interface with one sensor type (e.g., pressure sensor only). This research focuses on the design of a wireless telemetry system architecture, intended to retrieve blood pressure and volume data. A physical prototype is created that is 2.475 cm3 and weights 4.01 g. This thesis will enable a path towards miniaturization, leading to the incorporation of a wireless system into small laboratory animals
SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and
Astrophysic
On the universality of luminosity-metallicity and mass-metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3
We study relations between global characteristics of low-redshift (0 < z < 1)
compact star-forming galaxies, including absolute optical magnitudes, Hbeta
emission-line luminosities (or equivalently star-formation rates), stellar
masses, and oxygen abundances. The sample consists of 5182 galaxies with
high-excitation HII regions selected from the SDSS DR7 and SDSS/BOSS DR10
surveys adopting a criterion [OIII]4959/Hbeta > 1. These data were combined
with the corresponding data for high-redshift (2 < z < 3) star-forming
galaxies. We find that in all diagrams low-z and high-z star-forming galaxies
are closely related indicating a very weak dependence of metallicity on stellar
mass, redshift, and star-formation rate. This finding argues in favour of the
universal character of the global relations for compact star-forming galaxies
with high-excitation HII regions over redshifts 0 < z < 3.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Searching for metal-deficient emission-line galaxy candidates: the final sample of the SDSS DR12 galaxies
We present a spectroscopic study of metal-deficient dwarf galaxy candidates,
selected from the SDSS DR12. The oxygen abundances were derived using the
direct method in galaxies with the electron temperature-sensitive emission line
[OIII]4363A measured with an accuracy better than 30%. The oxygen abundances
for the remaining galaxies with larger uncertainties of the [OIII]4363A line
fluxes were calculated using a strong-line semi-empirical method by Izotov and
Thuan. The resulting sample consists of 287 low-metallicity candidates with
oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient
(XMD) candidates with 12+log O/H<7.35. Ten out of sixteen XMDs known so far (or
~60%) have been discovered by our team using the direct method. Three XMDs were
found in the present study. We study relations between global parameters of
low-metallicity galaxies, including absolute optical magnitudes, Hbeta
luminosities (or equivalently star formation rates), stellar masses,
mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies
strongly deviate to lower metallicities in L-Z, L(Hbeta)-Z and Mstar-Z diagrams
than in relations obtained for large samples of low-redshift, star-forming
galaxies with non-restricted metallicities. These less chemically evolved
galaxies with stellar masses ~10^6-10^8Msun, Hbeta luminosities ~10^38-10^41
erg/s, SFR~0.01-1.0Msun/yr, and sSFR~50 Gyr^-1 have physical conditions which
may be characteristic of high-redshift low-mass star-forming galaxies which are
still awaiting discovery.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
New candidates for extremely metal-poor emission-line galaxies in the SDSS/BOSS DR10
We present a spectroscopic study of eight extremely low-metallicity candidate
emission-line galaxies with oxygen abundances possibly below 12 +log O/H =
7.35. These galaxies were selected from Data Release 10 of the Sloan Digital
Sky Survey/Baryon Oscillation Spectroscopic Survey (SDSS/BOSS DR10). We will
call these extremely metal-deficient galaxies XMD galaxies. The electron
temperature-sensitive emission line [O~{\sc iii}] 4363 is detected in
three galaxies and marginally detected in two galaxies, allowing for abundance
determination by a "direct" method. Because of large uncertainties in the [O
{\sc iii}]4363\AA\ line fluxes, we also calculated oxygen abundance in
these galaxies together with the remaining three galaxies using a strong-line
semi-empirical method. This method gives oxygen abundances higher than 7.35 for
three galaxies with detected [O {\sc iii}]4363\AA\ line and lower than
7.35 for the remaining five objects of the sample. The newly-discovered
galaxies represent excellent targets for follow-up spectroscopic observations
with the largest telescopes to improve the oxygen abundance determination and
to increase the number of these very rare low-metallicity objects. The extreme
location of the most massive and luminous XMD galaxies and XMD candidates in
the stellar mass-metallicity diagram implies that these galaxies may be genuine
young objects.
With stellar masses of up to 10 - 10, the galaxies
are not chemically enriched and strongly deviate to lower metallicity as
compared to the relation obtained for a large sample of low-redshift,
star-forming galaxies.Comment: 10 pages, 5 figures, 4 tables, accepted in Astronomy and Astrophysic
The Mg II 2797, 2803 emission in low-metallicity star-forming galaxies from the SDSS
We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming
galaxies with oxygen abundances 12 + logO/H ~ 7.5-8.4. Redshifts of selected
galaxies are in the range z~0.36-0.70. This allows us to detect the redshifted
MgII 2797,2803 emission lines. Our aim is to use these lines for the magnesium
abundance determination. The MgII emission was detected in ~2/3 of the
galaxies. We find that the MgII 2797 emission-line intensity follows a trend
with the excitation parameter x= O^{2+}/O that is similar to that predicted by
CLOUDY photoionised HII region models, suggesting a nebular origin of MgII
emission. The Mg/O abundance ratio is lower by a factor ~2 than the solar
ratio. This is probably the combined effect of interstellar MgII absorption and
depletion of Mg onto dust. However, the effect of dust depletion in selected
galaxies, if present, is small, by a factor of ~2 lower than that of iron.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 8
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