201 research outputs found
The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula
We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L?) 2.3, T* 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* 0.61-0.62 M? post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models
Tangential Motions and Spectroscopy within NGC 6720, the Ring Nebula
We have combined recent Hubble Space Telescope WFPC2 images in the [O III]
5007 and [N II] 6583 lines with similar images made 9.557 years earlier to
determine the motion of the Ring Nebula within the plane of the sky. Scaled
ratio images argue for homologous expansion, that is, larger velocities scale
with increasing distance from the central star. The rather noisy pattern of
motion of individual features argues for the same conclusion and that the
silhouetted knots move at the same rate as the surrounding gas. These
tangential velocities are combined with information from a recent high
resolution radial velocity study to determine a dynamic distance, which is in
basic agreement with the distance determined from the parallax of the central
star. We have also obtained very high signal to noise ratio moderate resolution
spectra (9.4 Angstrom) along the major and minor axes of the nebula and from
this determined the electron temperatures and density in the multiple
ionization zones present. These results confirm the status of the Ring Nebula
as one of the older planetary nebulae, with a central star transitioning to the
white dwarf cooling curve. (Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA Contract No. NAS 5-26555 and the San Pedro Martir Observatory
operated by the Universidad Nacional Autonoma de Mexico.)Comment: Astronomical Journal, in pres
Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)
We report on the discovery of recently ionized hydrogen-deficient gas in the
immediate circumstellar environment of the final helium shell flash star V4334
Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object
spectroscopy we have found spatially extended (about 2") emission from [N II],
[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines
we have identified two components located at velocities -350 +/-50 and +200
+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583
feature at zero intensity corresponds to a velocity spread of about 1500 km/s.
Based on the available data it is not possible to conclusively determine the
mechanism of ionization. Both photo-ionization, from a rapidly evolving central
star, and shock excitation, as the result of the collision of the fast ouflows
with slower circumstellar matter, could account for the observed lines. The
central star is still hidden behind strong dust absorption, since only a faint
highly reddened continuum is apparent in the spectra. Theory states that it
will become hotter and will retrace its post-asymptotic giant branch evolution
towards the planetary nebula domain. Our detection of the ionized ejecta from
the very late helium shell flash marks the beginning of a new phase in this
star's amazingly rapid evolution.Comment: 11 pages, 2 figures. Accepted by ApJ
Physical Structure of Planetary Nebulae. I. The Owl Nebula
The Owl Nebula is a triple-shell planetary nebula with the outermost shell
being a faint bow-shaped halo. We have obtained deep narrow-band images and
high-dispersion echelle spectra in the H-alpha, [O III], and [N II] emission
lines to determine the physical structure of each shell in the nebula. These
spatio-kinematic data allow us to rule out hydrodynamic models that can
reproduce only the nebular morphology. Our analysis shows that the inner shell
of the main nebula is slightly elongated with a bipolar cavity along its major
axis, the outer nebula is a filled envelope co-expanding with the inner shell
at 40 km/s, and the halo has been braked by the interstellar medium as the Owl
Nebula moves through it. To explain the morphology and kinematics of the Owl
Nebula, we suggest the following scenario for its formation and evolution. The
early mass loss at the TP-AGB phase forms the halo, and the superwind at the
end of the AGB phase forms the main nebula. The subsequent fast stellar wind
compressed the superwind to form the inner shell and excavated an elongated
cavity at the center, but has ceased in the past. At the current old age, the
inner shell is backfilling the central cavity.Comment: 10 pages, 6 figures, 1 table, to appear in the Astronomical Journa
A Young Stellar Cluster in the Nucleus of NGC 4449
We have obtained 1-2 A resolution optical Echellette spectra of the nuclear
star cluster in the nearby starburst galaxy NGC 4449. The light is clearly
dominated by a very young (6-10 Myr) population of stars. For our age dating,
we have used recent population synthesis models to interpret the observed
equivalent width of stellar absorption features such as the HI Balmer series
and the CaII triplet around 8500 A. We also compare the observed spectrum of
the nuclear cluster to synthesized spectra of simple stellar populations of
varying ages. All these approaches yield a consistent cluster age. Metallicity
estimates based on the relative intensities of various ionization lines yield
no evidence for significant enrichment in the center of this low mass galaxy:
the metallicity of the nuclear cluster is about one fourth of the solar value,
in agreement with independent estimates for the disk material of NGC 4449.Comment: 24 pages (incl. 7 figures), accepted by AJ, March 2001 issue revised
version with minor changes and additions, one additional figur
The HI shell G132.6-0.7-25.3: A Supernova Remnant or an Old Wind-Blown Bubble?
Data from the Canadian Galactic Plane Survey reveal an abundance of HI shells
and arcs in the disk of our galaxy. While their shape is suggestive of stellar
winds or supernovae influence, very few of these structures have been examined
in detail thus far. A fine example is an HI shell in the outer Galaxy with no
continuum counterpart discovered in the survey's pilot project. Its size and
kinematics suggest that it was created by the winds of a single late-type O
star which has since evolved off the main sequence or by a supernova explosion.
A B1 Ia star at the centre of the shell, in projection, is a possible candidate
for energy source if the shell is assumed to be wind-blown. The shell's shape
implies a surprisingly small scale height of less than about 30 pc for the
surrounding gas if the elongation is due to evolution in a density gradient.Comment: 25 pages, 6 figures. Accepted for publication in the Astronomical
Journa
Tele-medicine versus face-to-face consultation in Endocrine Outpatients Clinic during COVID-19 outbreak: a single-center experience during the lockdown period
The COVID-19 outbreak in Italy is the major concern of Public Health in 2020: measures of containment were progressively expanded, limiting Outpatients' visit
The Halo and Rings of the Planetary Nebula NGC 40 in the Mid-Infrared
We present imaging and spectroscopy of NGC 40 acquired using the Spitzer
Space Telescope (Spitzer), and the Infrared Space observatory (ISO). These are
used to investigate the nature of emission from the central nebular shell, from
the nebular halo, and from the associated circumnebular rings. It is pointed
out that a variety of mechanisms may contribute to the mid-infrared (MIR)
fluxes, and there is evidence for a cool dust continuum, strong ionic
transitions, and appreciable emission by polycyclic aromatic hydrocarbons
(PAHs). Prior observations at shorter wavelengths also indicate the presence of
warmer grains, and the possible contribution of H2 transitions. It is suggested
that an apparent jet-like structure to the NE of the halo represents one of the
many emission spokes that permeate the shell. The spokes are likely to be
caused by the percolation of UV photons through a clumpy interior shell, whilst
the jet-like feature is enhanced due to locally elevated electron densities; a
result of interaction between NGC 40 and the interstellar medium. It is finally
noted that the presence of the PAH, 21 microns and 30 microns spectral features
testifies to appreciable C/O ratios within the main nebular shell. Such a
result is consistent with abundance determinations using collisionally excited
lines, but not with those determined using optical recombination linesComment: 13 pages, 8 figures, Accepted for publication in MNRAS. 37 pages in
arXi
Detection of circumstellar material in a normal Type Ia Supernova
Type Ia supernovae are thought to be thermonuclear explosions of accreting
white dwarfs that reach a critical mass limit. Despite their importance as
cosmological distance indicators, the nature of their progenitors has remained
controversial. Here we report the detection of circumstellar material in a
normal Type Ia supernova. The expansion velocities, densities and dimensions of
the circumstellar envelope indicate that this material was ejected from the
progenitor system. The relatively low expansion velocities appear to favor a
progenitor system where a white dwarf accretes material from a companion star
which is in the red-giant phase at the time of explosion.Comment: 25 pages, 7 figures. Accepted for publication in Science. Full
resolution version at
http://www.hq.eso.org/~fpatat/science/sn06X/preprint.pdf . The original paper
can be found at http://www.sciencemag.org/cgi/content/abstract/114300
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