113 research outputs found
The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra
Spectroscopic observations obtained with the VLT of one planetary nebula (PN)
in Sextans A and of five PNe in Sextans B and of several HII regions (HII) in
these two dwarf irregular galaxies are presented. The extended spectral
coverage, from 320.0 to 1000.0nm, and the large telescope aperture allowed us
to detect a number of emission lines, covering more than one ionization stage
for several elements (He, O, S, Ar). The electron temperature (Te) diagnostic
[OIII] line at 436.3 nm was measured in all six PNe and in several HII allowing
for an accurate determination of the ionic and total chemical abundances by
means of the Ionization Correction Factors method. For the time being, these
PNe are the farthest ones where such a direct measurement of the Te is
obtained. In addition, all PNe and HII were also modelled using the
photoionization code CLOUDY. The physico-chemical properties of PNe and HII are
presented and discussed. A small dispersion in the oxygen abundance of HII was
found in both galaxies: 12 + (O/H)=7.60.2 in SextansA, and
7.80.2 in SextansB. For the five PNe of SextansA, we find that 12 +
(O/H)=8.00.3, with a mean abundance consistent with that of HII. The
only PN known in SextansA appears to have been produced by a quite massive
progenitor, and has a significant nitrogen overabundance. In addition, its
oxygen abundance is 0.4 dex larger than the mean abundance of HII, possibly
indicating an efficient third dredge-up for massive, low-metallicity PN
progenitors. The metal enrichment of both galaxies is analyzed using these new
data.Comment: 19 pages, 11 figures, Accepted by A&
FLIERs and Other Microstructures in Planetary Nebulae. IV. Images of Elliptical PNs from the Hubble Space Telescope
?????We report new results from high spatial resolution Wide Field Planetary Camera 2 imaging studies of FLIERs and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucleus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density ?104 cm-3. Can neutral knots account for the properties of FLIERs? We compare two broad classes of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevaporated gas swept by winds into head-tail shapes. Both classes of models fail basic consistency tests. Hence an entirely new conceptual paradigm is needed to account for the phenomenology of FLIERs
The formation of stars in groups
Observations of the dust and gas around embedded stellar clusters reveal some
of the processes involved in their formation and evolution. Large scale mass
infall with rates dM/dt=4e-4 solar masses/year is found to be disrupted on
small scales by protostellar outflows. Observations of the size and velocity
dispersion of clusters suggest that protostellar migration from their
birthplace begins at very early times and is a potentially useful evolutionary
indicator.Comment: 8 page conference proceedings for "The Earliest Phases of Massive
Star Birth" (Third Boulder Hot Star Workshop), ed. P. Crowthe
The physical structure of the point-symmetric and quadrupolar planetary nebula NGC 6309
We analyse the point-symmetric planetary nebula NGC 6309 in terms of its
three-dimensional structure and of internal variations of the physical
conditions to deduce the physical processes involved in its formation. We used
VLA-D 3.6-cm continuum, ground-based, and HST-archive imaging as well as long
slit high- and low-dispersion spectroscopy. The low-dispersion spectra indicate
a high excitation nebula, with low to medium variations of its internal
physical conditions. In the optical images, the point-symmetric knots show a
lack of [NII] emission as compared with similar features previously known in
other PNe. A rich internal structure of the central region is seen in the HST
images, resembling a deformed torus. Long slit high-dispersion spectra reveal a
complex kinematics in the central region. The spectral line profiles from the
external regions of NGC 6309 indicate expanding lobes (~40 km/s) as those
generally found in bipolar nebulae. Finally, we have found evidence for the
presence of a faint halo, possibly related to the envelope of the AGB-star
progenitor. Our data indicate that NGC 6309 is a quadrupolar nebula with two
pairs of bipolar lobes whose axes are oriented PA=40 and PA=76. Equatorial and
polar velocities for these two pairs of lobes are 29 and 86 km/s for the
bipolar system at PA=40 and 25 and 75 km/s for the bipolar system at PA=76.
There is also a central torus that is expanding at 25 km/s. Kinematical age for
all these structures is around 3700 to 4000 yr. We conclude that NGC 6309 was
formed by a set of well-collimated bipolar outflows (jets), which were ejected
in the initial stages of its formation as a planetary nebula. These jets carved
the bipolar lobes in the previous AGB wind and their remnants are now observed
as the point-symmetric knots tracing the edges of the lobes.Comment: To be published in Astronomy & Astrophysics; 11 pages, 8 figures, 4
table
Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulae IC
2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are
composed of a markedly elongated inner shell, and of a less aspherical outer
shell expanding at a considerably higher velocity than the inner one. Embedded
in the outer shells of both nebulae are found several low-ionization knots. In
IC 2553, the knots show a point-symmetric distribution with respect to the
central star: one possible explanation for their formation is that they are the
survivors of pre-existing point-symmetric condensations in the AGB wind, a fact
which would imply a quite peculiar mass-loss geometry from the giant
progenitor. In the case of NGC 5882, the lack of symmetry in the distribution
of the observed low-ionization structures makes it possible that they are the
result of in situ instabilities.Comment: 20 pages including 1 table and 6 figures. ApJ accepted. Also
available at http://andromeda.roque.ing.iac.es/~sanchez/ingpub/index2000.htm
The Local Group Census: planetary nebulae in IC 10, Leo A and Sextans A
In the framework of our narrow-band survey of the Local Group galaxies, we
present the results of the search for planetary nebulae (PNe) in the dwarf
irregular galaxies IC10, LeoA and SextansA. Using the standard on-band/off-band
technique, sixteen new candidate PNe have been discovered in the closest
starburst galaxy, IC10. The optical size of this galaxy is estimated to be much
larger than previously thought, considering the location of the new PNe in an
area of 3.6 kpc X 2.7 kpc. We also confirm the results of previous studies for
the other two dwarf irregular galaxies, with the detection of one candidate PN
in LeoA and another one in SextansA. We review the number of planetary nebulae
discovered in the Local Group to date and their behaviour with metallicity. We
suggest a possible fall in the observed number of PNe when [Fe/H]-1.0,
which might indicate that below this point the formation rate of PNe is much
lower than for stellar populations of near Solar abundances. We also find
non-negligible metallicity effects on the [OIII] luminosity of the brightest PN
of a galaxy.Comment: 10 pages, 4 figures, A&A accepte
High-velocity collimated outflows in planetary nebulae: NGC 6337, He 2-186, and K 4-47
We have obtained narrow-band images and high-resolution spectra of the
planetary nebulae NGC 6337, He 2-186, and K 4-47, with the aim of investigating
the relation between their main morphological components and several
low-ionization features present in these nebulae. The data suggest that NGC
6337 is a bipolar PN seen almost pole on, with polar velocities higher than 200
km/s. The bright inner ring of the nebula is interpreted to be the "equatorial"
density enhancement. It contains a number of low-ionization knots and outward
tails that we ascribe to dynamical instabilities leading to fragmentation of
the ring or transient density enhancements due to the interaction of the
ionization front with previous density fluctuations in the ISM. The lobes show
a pronounced point-symmetric morphology and two peculiar low-ionization
filaments whose nature remains unclear. The most notable characteristic of He
2-186 is the presence of two high-velocity (higher than 135 km/s) knots from
which an S-shaped lane of emission departs toward the central star. K 4-47 is
composed of a compact core and two high-velocity, low-ionization blobs. We
interpret the substantial broadening of line emission from the blobs as a
signature of bow shocks, and using the modeling of Hartigan, Raymond, & Hartman
(1987), we derive a shock velocity of 150 km/s and a mild inclination of the
outflow on the plane of the sky. We discuss possible scenarios for the
formation of these nebulae and their low-ionization features. In particular,
the morphology of K 4-47 hardly fits into any of the usually adopted mass-loss
geometries for single AGB stars. Finally, we discuss the possibility that
point-symmetric morphologies in the lobes of NGC 6337 and the knots of He 2-186
are the result of precessing outflows from the central stars.Comment: 16 pages plus 7 figures, ApJ accepted. Also available at
http://www.iac.es/publicaciones/preprints.htm
New young planetary nebulae in IPHAS
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory. DOI: 10.1051/0004-6361/200811575Aims. We search for very small-diameter galactic planetary nebulae (PNe) representing the earliest phases of PN evolution. The IPHAS catalogue of H-emitting stars provides a useful basis for this study since all sources present in this catalogue must be of small angular diameter. Methods. The PN candidates are selected based on their location in two colour-colour diagrams: IPHAS (r' - H) vs. (r' - i'), and 2MASS (J - H) vs. (H - ). Spectroscopic follow-up was carried out on a sample of candidates to confirm their nature. Results. We present a total of 83 PN candidates. We were able to obtain spectra or find the classification from the literature for 35 candidates. Five of these objects are likely to be new PNe, including one large bipolar PN discovered serendipitously close to an emission-line star. PN distances deduced from extinction-distance relations based on IPHAS field-star photometry are presented for the first time. These yield distance estimates for our objects in the range 2 kpc and 6 kpc. From the data in hand, we conclude that four of the discovered objects are probably young PNe.Peer reviewe
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