2,521 research outputs found
A model of the local region of the galaxy
A two-component model of the local interstellar medium was developed, based on the Radio Astronomy Explorer (RAE-1) satellite observations of the low frequency (0.2 - 10 MHz) galactic radio background spectrum. The model shows evidence for a thick spiral arm and a thick electron disk of 600 and 1200 pc slab thicknesses, respectively. The temperature difference between the north galactic pole and anticenter directions supports the concept of a local positive thermal gradient away from the galactic plane. A preliminary discussion of the influence of the Razin effect on the model is also presented
Spacelab 3 vapor crystal growth experiment
The Space Shuttle Challenger, with Spacelab 3 as its payload, was launched into orbit April 29, 1985. The mission, number 51-B, emphasized materials processing in space, although a wide variety of experiments in other disciplines were also carried onboard. One of the materials processing experiments on this flight is described, specifically the growth of single crystals of mercuric iodide by physical vapor transport
Turing pattern or system heterogeneity? A numerical continuation approach to assessing the role of Turing instabilities in heterogeneous reaction-diffusion systems
Turing patterns in reaction-diffusion (RD) systems have classically been
studied only in RD systems which do not explicitly depend on independent
variables such as space. In practise, many systems for which Turing patterning
is important are not homogeneous with ideal boundary conditions. In
heterogeneous systems with stable steady states, the steady states are also
necessarily heterogeneous which is problematic for applying the classical
analysis. Whilst there has been some work done to extend Turing analysis to
some heterogeneous systems, for many systems it is still difficult to determine
if a stable patterned state is driven purely by system heterogeneity or if a
Turing instability is playing a role. In this work, we try to define a
framework which uses numerical continuation to map heterogeneous RD systems
onto a sensible nearby homogeneous system. This framework may be used for
discussing the role of Turing instabilities in establishing patterns in
heterogeneous RD systems. We study the Schnakenberg and Gierer-Meinhardt models
with spatially heterogeneous production as test problems. It is shown that for
sufficiently large system heterogeneity (large amplitude spatial variations in
morphogen production) it is possible that Turing-patterned and base states
become coincident and therefore impossible to distinguish. Other exotic
behaviour is also shown to be possible. We also study a novel scenario in which
morphogen is produced locally at levels that could support Turing patterning
but on intervals/patches which are on the scale of classical critical domain
lengths. Without classical domain boundaries, Turing patterns are allowed to
bleed through; an effect noted by other authors. In this case, this phenomena
effectively changes the critical domain length. Indeed, we even note that this
phenomena may also effectively couple local patches together and drive
instability in this way.Comment: 10 figure
Progress on the hybridization of cultivated lentil Lens culinaris Medik. and wild species Lens tomentosus Ladizinsky
Non-Peer ReviewedThe domestication of lentil has produced bottleneck effects resulting in a narrow genetic basis which has resulted in reduced levels of resistance to biotic and abiotic stresses relative to its wild relatives. Phenotypic variability studies have identified wild lentil germplasm with resistance to anthracnose (Colletotrichum spp.), ascochyta blight (Ascochyta lentis), stemphylium blight (Stemphylium botryosum) and Orobanche spp. root-holoparasitic infection. To increase genetic diversity and resistance to biotic and abiotic stresses in new cultivars, introgression of desirable genes from crop wild relatives is necessary. Current evidences suggest seven taxa in the genus Lens Mill.: L. culinaris (ssp. culinaris and ssp. orientalis), L. odemensis, L. ervoides, L. nigricans, L. tomentosus and L. lamottei. Morphologically, the wild lentil species L. tomentosus most resembles L. orientalis although it can be distinguished as having a hairy pod. Previous attempts of crossing cultivated lentil L. culinaris with the wild species L. tomentosus, have failed to produce viable seeds, as with time growing embryos are gradually degraded and shrivelled, and non-viable seeds are formed; consequently an embryo rescue technique has been employed with limited success. However, this technique is time consuming, and also requires controlled growing environments and highly skilled technical personal. Using very comprehensive phenotyping technique, we have crossed cultivated lentil cultivar ‘Indian head’ with L. tomentosus and successfully produced viable F1 generation and it has currently being evaluated for phenological and morphological characteristics. Our results suggest successful results can be achieved by selecting phenotypically resembling lentil species as parents as an initial step
Age Constraints for an M31 Globular Cluster from Main Sequence Photometry
We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312
in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on
the Hubble Space Telescope. The cluster was included in deep observations taken
to measure the star formation history of the M31 halo. Overcoming a very
crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag
below the main sequence turnoff. These are the first observations to allow a
direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD
and luminosity function using a finely-spaced grid of isochrones that have been
calibrated using observations of Galactic clusters taken with the same camera
and filters. The luminosity difference between the subgiant and horizontal
branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc
and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison
to isochrones yields an age of 10 +2.5/-1 Gyr
Chemical Abundances Of Open Clusters From High-Resolution Infrared Spectra. I. NGC 6940
We present near-infrared spectroscopic analysis of 12 red giant members of
the Galactic open cluster NGC 6940. High-resolution (R45000) and high
signal-to-noise ratio (S/N > 100) near-infrared H and K band spectra were
gathered with the Immersion Grating Infrared Spectrograph (IGRINS) on the 2.7m
Smith Telescope at McDonald Observatory. We obtained abundances of H-burning
(C, N, O), (Mg, Si, S, Ca), light odd-Z (Na, Al, P, K), Fe-group
(Sc, Ti, Cr, Fe, Co, Ni) and neutron-capture (Ce, Nd, Yb) elements. We report
the abundances of S, P, K, Ce, and Yb in NGC 6940 for the first time. Many OH
and CN features in the H band were used to obtain O and N abundances. C
abundances were measured from four different features: CO molecular lines in
the K band, high excitation C I lines present in both near-infrared and
optical, CH and bands in the optical region. We have also determined
ratios from the R-branch band heads of first overtone (2-0) and
(3-1) (2-0) lines near 23440
\overset{\lower.5em\circ}{\mathrm{A}} and (3-1) lines at about
23730 \overset{\lower.5em\circ}{\mathrm{A}}. We have also investigated the HF
feature at 23358.3 \overset{\lower.5em\circ}{\mathrm{A}}, finding solar
fluorine abundances without ruling out a slight enhancement. For some elements
(such as the group), IGRINS data yield more internally
self-consistent abundances. We also revisited the CMD of NGC 6940 by
determining the most probable cluster members using Gaia DR2. Finally, we
applied Victoria isochrones and MESA models in order to refine our estimates of
the evolutionary stages of our targets.Comment: 16 pages, 10 figure
The impact of alpha/Fe enhanced stellar evolutionary tracks on the ages of elliptical galaxies
We complement our study of alpha/Fe enhanced stellar population models of
Lick absorption indices (Thomas et al. 2003) by comparing two sets of alpha/Fe
enhanced models. In both models the impact on Lick indices due to alpha/Fe
enhancement is accounted for through a modification of the stellar absorption
line-strengths using the response functions of Tripicco & Bell (1995). One set
of models, however, uses solar-scaled, the other alpha/Fe enhanced stellar
evolutionary tracks. Since the alpha/Fe enhanced tracks are hotter than the
solar-scaled ones (Salasnich et al. 2000), the correspondent stellar population
models have slightly weaker metallic indices (i.e. Mgb, etc.) and stronger
Balmer line indices (Hbeta) (Maraston et al 2003). Here we explore
quantitatively the impact of this effect on the alpha/Fe ratios, metallicities
and ages that are derived for elliptical galaxies. We find that the modest
decrease of the metallic indices Mgb and balance each other, such that
fully consistent alpha/Fe ratios are derived for stellar systems using alpha/Fe
enhanced models with either solar-scaled or alpha/Fe enhanced stellar tracks.
The decrease of the metallic indices and the increase of Hbeta conspire in a
way that also consistent metallicities are obtained. The derived ages, instead,
are significantly different. The inclusion of alpha/Fe enhanced stellar tracks
leads to the derivation of ages as high as 30 Gyr for elliptical galaxies. For
the same objects, ages not older than 15 Gyr are obtained, if alpha/Fe enhanced
models using solar-scaled tracks are adopted. This may indicate that current
stellar evolutionary models overestimate the bluing of stellar evolutionary
tracks due to alpha/Fe enhanced chemical mixtures at super-solar metallicities.Comment: 5 pages, 2 figures. Accepted by A&
Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.15383.xWe investigate a class of rapidly growing emission line galaxies, known as 'Green Peas', first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in Sloan Digital Sky Survey imaging. Their appearance is due to very strong optical emission lines, namely [O iii]λ5007 Å, with an unusually large equivalent width of up to ∼1000 Å. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some active galactic nuclei interlopers including eight newly discovered narrow-line Seyfert 1 galaxies. The star-forming Peas are low-mass galaxies (M∼ 108.5–1010 M⊙) with high star formation rates (∼10 M⊙ yr−1) , low metallicities (log[O/H]+ 12 ∼ 8.7) and low reddening [ E(B−V) ≤ 0.25 ] and they reside in low-density environments. They have some of the highest specific star formation rates (up to ∼10−8 yr−1 ) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myr. The few star-forming Peas with Hubble Space Telescope imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to luminous blue compact galaxies. They are also similar to high-redshift ultraviolet-luminous galaxies, e.g. Lyman-break galaxies and Lyα emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe.Peer reviewe
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