3,245 research outputs found
The Circumnuclear Ring of Ionized Gas in NGC3593
We present the results of narrow-band Halpha+NII imaging of the early-type
spiral NGC3593 in combination with a study of the flux radial profiles of the
NII (lambda: 654.80, 658.34 nm), Halpha, and SII (lambda: 671.65, 673.08 nm)
emission lines along its major axis. The galaxy is known to contain two
counterrotating stellar discs of different size and luminosity. We find that
the Halpha emission mainly derives from a small central region of 57 arcsec x
25 arcsec. It consists of a filamentary pattern with a central ring. This has a
diameter of about 17 arcsec (~ 0.6/h kpc) and it contributes about half of the
total Halpha flux. The ring is interpreted as the result of the interaction
between the acquired retrograde gas which later formed the smaller
counterrotating stellar disc and the pre-existing prograde gas of the galaxy.Comment: Accepted for pubblication in Astronomy and Astrophysics; one latex
file (corsini.tex), and 2 encapsulated postscript figures
(corsini_fig1.ps,corsini_fig2.ps). To be compiled with aa.cls latex2e macro
style (pslatex option): 6 pages after latex compilatio
Physical properties of high-mass clumps in different stages of evolution
(Abridged) Aims. To investigate the first stages of the process of high-mass
star formation, we selected a sample of massive clumps previously observed with
the SEST at 1.2 mm and with the ATNF ATCA at 1.3 cm. We want to characterize
the physical conditions in such sources, and test whether their properties
depend on the evolutionary stage of the clump.
Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2)
transitions and in the 22 GHz H2O maser line. Ammonia lines are a good
temperature probe that allow us to accurately determine the mass and the
column-, volume-, and surface densities of the clumps. We also collected all
data available to construct the spectral energy distribution of the individual
clumps and to determine if star formation is already occurring, through
observations of its most common signposts, thus putting constraints on the
evolutionary stage of the source. We fitted the spectral energy distribution
between 1.2 mm and 70 microns with a modified black body to derive the dust
temperature and independently determine the mass.
Results. The clumps are cold (T~10-30 K), massive (M~10^2-10^3 Mo), and dense
(n(H2)>~10^5 cm^-3) and they have high column densities (N(H2)~10^23 cm^-2).
All clumps appear to be potentially able to form high-mass stars. The most
massive clumps appear to be gravitationally unstable, if the only sources of
support against collapse are turbulence and thermal pressure, which possibly
indicates that the magnetic field is important in stabilizing them.
Conclusions. After investigating how the average properties depend on the
evolutionary phase of the source, we find that the temperature and central
density progressively increase with time. Sources likely hosting a ZAMS star
show a steeper radial dependence of the volume density and tend to be more
compact than starless clumps.Comment: Published in A&A, Vol. 556, A1
The B1 shock in the L1157 outflow as seen at high spatial resolution
We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in
the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K),
HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology
and spectral profiles has shown that the highest velocity gas is confined in a
few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall
of the gas cavity excavated by the shock expansion. A large velocity gradient
model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6
cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5
cm^-3 for the density of the high velocity bullets. The origin of the bullets
is still uncertain: they could be the result of local instabilities produced by
the interaction of the jet with the ambient medium or could be clump already
present in the ambient medium that are excited and accelerated by the expanding
outflow. The column densities of the observed species can be reproduced
qualitatively by the presence in B1 of a C-type shock and only models where the
gas reaches temperatures of at least 4000 K can reproduce the observed HC3N
column density.Comment: 13 pages, 12 figure
Mixed Early and Late-Type Properties in the Bar of NGC 6221: Evidence for Evolution along the Hubble Sequence?
Rotation curves and velocity dispersion profiles are presented for both the
stellar and gaseous components along five different position angles (P.A.=5,
50, 95, 125 and 155 degrees) of the nearby barred spiral NGC 6221. The observed
kinematics extends out to about 80" from the nucleus. Narrow and broad-band
imaging is also presented. The radial profiles of the fluxes ratio [NII]/Halpha
reveal the presence of a ring-like structure of ionized gas, with a radius of
about 9" and a deprojected circular velocity of about 280 km/s. The analysis of
the dynamics of the bar indicates this ring is related to the presence of an
inner Lindblad resonance (ILR) at 1.3 kpc. NGC6221 is found to exhibit
intermediate properties between those of the early-type barred galaxies: the
presence of a gaseous ring at an ILR, the bar edge located between the ILR's
and the corotation radius beyond the steep rising portion of the rotation
curve, the dust-lane pattern, and those of the late-type galaxies: an almost
exponential surface brightness profile, the presence of Halpha regions along
all the bar, the spiral-arm pattern. It is consistent with scenarios of
bar-induced evolution from later to earlier-type galaxies.Comment: 1 File ds7406.tar.gz which contains: one latex file (ds7406.tex), and
10 encsulated postscript figures (ds7406f**.eps). To be compiled with aa-l
latex2e macro style. To be published in A&A Sup. Serie
Near infrared and optical morphology of the dusty galaxy NGC972
Near infrared (NIR) and optical surface photometric analyses of the dusty
galaxy NGC972 are presented. The photometric profiles in the BVRJHK bands can
be fitted with a combination of gaussian and exponential profiles,
corresponding to a starburst nucleus and a stellar disk respectively. The
exponential scale length in the B-band is 2.8 times larger than in the K-band,
which implies a central B-band optical depth as high as 11. A bulge is absent
even in the NIR bands and hence the galaxy must be of a morphological type
later than the usually adopted Sb type. Relatively low rotational velocity and
high gas content also favor a later type, probably Sd, for the galaxy. Only one
arm can be traced in the distribution of old stars; the second arm, however,
can be traced in the distribution of dust and HII regions. Data suggest a short
NIR bar, which ends inside the nuclear ring. The slowly rising nature of the
rotation curve rules out a resonance origin of the the nuclear ring. The ring
is most likely not in the plane of the galaxy, given its circular appearance in
spite of the moderately high inclination of the galaxy. The off-planar nature
of the star forming ring, the unusually high fraction (30%) of the total mass
in molecular form, the presence of a nuclear starburst and the asymmetry of
spiral arms, are probably the result of a merger with a gas-rich companion
galaxy.Comment: Uses aas2pp4.sty and epsfig.sty, 12 pages To appear in Astronomical
Journal, October 199
Bird mortality related to collisions with ski–lift cables: do we estimate just the tip of the iceberg?
Collisions with ski–lift cables are an important cause of death for grouse species living close to alpine ski resorts. As several biases may reduce the detection probability of bird carcasses, the mortality rates related to these collisions are generally underestimated. The possibility that injured birds may continue flying for some distance after striking cables represents a major source of error, known as crippling bias. Estimating the crippling losses resulting from birds dying far from the ski–lift corridors is difficult and it is usually assessed by systematic searches of carcasses on both sides of the ski–lifts. Using molecular tracking, we were able to demonstrate that a rock ptarmigan hen flew up to 600 m after striking a ski–lift cable, a distance preventing its detection by traditional carcasses surveys. Given the difficulty in conducting systematic searches over large areas surrounding the ski–lifts, only an experiment using radio–tagged birds would allow us to estimate the real mortality rate associated with cable collision
Gause's exclusion principle revisited: artificial modified species and competition
Gause's principle of competition between two species is studied when one of
them is sterile. We study the condition for total extinction in the niche,
namely, when the sterile population exterminates the native one by an optimal
use of resources. A mathematical Lotka-Volterra non linear model of interaction
between a native and sterile species is proposed. The condition for total
extinction is related to the initial number of sterile individuals
released in the niche. In fact, the existence of a critical sterile-population
value is conjectured from numerical analysis and an analytical
estimation is found. When spatial diffusion (migration) is considered a
critical size territory is found and, for small territory, total extinction
exist in any case. This work is motived by the extermination agriculture
problem of fruit flies in our region.Comment: 11 pages. Published in Jour.Phys.A Math.Gen. 33, 4877 (2000
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