1,194,850 research outputs found
The size--density relation of extragalactic HII regions
We investigate the size--density relation in extragalactic HII regions, with
the aim of understanding the role of dust and different physical conditions in
the ionized medium. First, we compiled several observational data sets for
Galactic and extragalactic HII regions and confirm that extragalactic HII
regions follow the same size (D)--density (n) relation as Galactic ones.
Motivated by the inability of static models to explain this, we then modelled
the evolution of the size--density relation of HII regions by considering their
star formation history, the effects of dust, and pressure-driven expansion. The
results are compared with our sample data whose size and density span roughly
six orders of magnitude. We find that the extragalactic size--density relation
does not result from an evolutionary sequence of HII regions but rather
reflects a sequence with different initial gas densities (``density
hierarchy''). Moreover, the size of many HII regions is limited by dust
absorption of ionizing photons, rather than consumption by ionizing neutral
hydrogen. Dust extinction of ionizing photons is particularly severe over the
entire lifetime of compact HII regions with typical gas densities of greater
than 10^3 cm^{-3}. Hence, as long as the number of ionizing photons is used to
trace massive star formation, much star-formation activity could be missed.
Such compact dense environments, the ones most profoundly obscured by dust,
have properties similar to ``maximum--intensity starbursts''. This implies that
submillimeter and infrared wavelengths may be necessary to accurately assess
star formation in these extreme conditions both locally and at high redshift.Comment: 18 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
The role of dust in "active" and "passive" low-metallicity star formation
We investigate the role of dust in star formation activity of extremely
metal-poor blue compact dwarf galaxies (BCDs). Observations suggest that star
formation in BCDs occurs in two different regimes: "active" and "passive". The
"active" BCDs host super star clusters (SSCs), and are characterised by compact
size, rich H2 content, large dust optical depth, and high dust temperature; the
"passive" BCDs are more diffuse with cooler dust, and lack SSCs and large
amounts of H2. By treating physical processes concerning formation of stars and
dust, we are able to simultaneously reproduce all the above properties of both
modes of star formation (active and passive). We find that the difference
between the two regimes can be understood through the variation of the
"compactness" of the star-forming region: an "active" mode emerges if the
region is compact (with radius \la 50 pc) and dense (with gas number density
\ga 500 cm). The dust, supplied from Type II supernovae in a compact
star-forming region, effectively reprocesses the heating photons into the
infrared and induces a rapid H2 formation over a period of several Myr. This
explains the high infrared luminosity, high dust temperature, and large H2
content of active BCDs. Moreover, the gas in "active" galaxies cools (\la 300
K) on a few dynamical timescales, producing a "run-away" star formation episode
because of the favourable (cool) conditions. The mild extinction and relatively
low molecular content of passive BCDs can also be explained by the same model
if we assume a diffuse region (with radius \ga 100 pc and gas number density
\la 100 cm). We finally discuss primordial star formation in
high-redshift galaxies in the context of the "active" and "passive" star
formation scenario.Comment: Astronomy and Astrophysics, in press, 16 pages, 8 figure
Rhomboid prism pair for rotating the plane of parallel light beams
An optical system is described for rotating the plane defined by a pair of parallel light beams. In one embodiment a single pair of rhomboid prisms have their respective input faces disposed to receive the respective input beams. Each prism is rotated about an axis of revolution coaxial with each of the respective input beams by means of a suitable motor and gear arrangement to cause the plane of the parallel output beams to be rotated relative to the plane of the input beams. In a second embodiment, two pairs of rhomboid prisms are provided. In a first angular orientation of the output beams, the prisms merely decrease the lateral displacement of the output beams in order to keep in the same plane as the input beams. In a second angular orientation of the prisms, the input faces of the second pair of prisms are brought into coincidence with the input beams for rotating the plane of the output beams by a substantial angle such as 90 deg
Coulomb-type interaction under Lorentz symmetry breaking effects
Based on models of confinement of quarks, we analyse a relativistic scalar
particle subject to a scalar potential proportional to the inverse of the
radial distance and under the effects of the violation of the Lorentz symmetry.
We show that the effects of the Lorentz symmetry breaking can induced a
harmonic-type potential. Then, we solve the Klein-Gordon equation analytically
and discuss the influence of the background of the violation of the Lorentz
symmetry on the relativistic energy levels.Comment: 10 pages, no figur
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