527 research outputs found
Rings and arcs around evolved stars. II. The Carbon Star AFGL 3068 and the Planetary Nebulae NGC 6543, NGC 7009 and NGC 7027
We present a detailed comparative study of the arcs and fragmented ring-like
features in the haloes of the planetary nebulae (PNe) NGC 6543, NGC 7009, and
NGC 7027 and the spiral pattern around the carbon star AFGL 3068 using
high-quality multi-epoch HST images. This comparison allows us to investigate
the connection and possible evolution between the regular patterns surrounding
AGB stars and the irregular concentric patterns around PNe. The radial proper
motion of these features, ~15 km/s, are found to be consistent with the AGB
wind and their linear sizes and inter-lapse times (500-1900 yr) also agree with
those found around AGB stars, suggesting a common origin. We find evidence
using radiative-hydrodynamic simulations that regular patterns produced at the
end of the AGB phase become highly distorted by their interactions with the
expanding PN and the anisotropic illumination and ionization patterns caused by
shadow instabilities. These processes will disrupt the regular (mostly spiral)
patterns around AGB stars, plausibly becoming the arcs and fragmented rings
observed in the haloes of PNe.Comment: 13 pages, 9 figures, accepted for publication in MNRA
The Unusual Distributions of Ionized Material and Molecular Hydrogen in NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary Nebula
The planetary nebula NGC 6881 displays in the optical a quadrupolar
morphology consisting of two pairs of highly collimated bipolar lobes aligned
along different directions. An additional bipolar ejection is revealed by the
hydrogen molecular emission, but its wide hourglass morphology is very
different from that of the ionized material. To investigate in detail the
spatial distribution of molecular hydrogen and ionized material within NGC
6881, and to determine the prevalent excitation mechanism of the H2 emission,
we have obtained new near-IR Br-gamma and H2 and optical H-alpha and [N II]
images, as well as intermediate resolution JHK spectra. These observations
confirm the association of the H2 bipolar lobes to NGC 6881 and find that the
prevalent excitation mechanism is collisional. The detailed morphology and very
different collimation degree of the H2 and ionized bipolar lobes of NGC 6881
not only imply that multiple bipolar ejections have occurred in this nebula,
but also that the dominant shaping agent is different for each bipolar
ejection: a bipolar stellar wind most likely produced the H2 lobes, while
highly collimated outflows are carving out the ionized lobes into the thick
circumstellar envelope. The asymmetry between the southeast and northwest H2
bipolar lobes suggests the interaction of the nebula with an inhomogeneous
interstellar medium. We find evidence that places NGC 6881 in the H II region
Sh 2-109 along the Orion local spiral arm.Comment: 9 pages, 7 figures, 4 table
Spectroscopic confirmation of the planetary nebula nature of PM1-242, PM1-318 and PM1-333 and morphological analysis of the nebulae
We present intermediate resolution long-slit spectra and narrow-band Halpha,
[NII] and [OIII] images of PM1-242, PM318 and PM1-333, three IRAS sources
classified as possible planetary nebulae. The spectra show that the three
objects are true planetary nebulae and allow us to study their physical
properties; the images provide a detailed view of their morphology. PM1-242 is
a medium-to-high-excitation (e.g., HeII4686/Hbeta ~0.4; [NII]6584/Halpha ~0.3)
planetary nebula with an elliptical shape containing [NII] enhanced
point-symmetric arcs. An electron temperature [Te([SIII])] of ~10250 K and an
electron density [Ne([SII])] of ~2300 cm-3 are derived for PM1-242. Abundance
calculations suggest a large helium abundance (He/H ~0.29) in PM1-242. PM1-318
is a high-excitation (HeII4686/Hbeta ~1) planetary nebula with a ring-like
inner shell containing two enhanced opposite regions, surrounded by a fainter
round attached shell brighter in the light of [OIII]. PM1-333 is an extended
planetary nebula with a high-excitation (HeII4686/Hbeta up to ~0.9) patchy
circular main body containing two low-excitation knotty arcs. A low Ne([SII])
of ~450 cm-3 and Te([OIII]) of ~15000 K are derived for this nebula. Abundance
calculations suggest that PM1-333 is a type I planetary nebula. The lack of a
sharp shell morphology, low electron density, and high-excitation strongly
suggest that PM1-333 is an evolved planetary nebula. PM1-333 also shows two
low-ionization polar structures whose morphology and emission properties are
reminiscent of collimated outflows. We compare PM1-333 with other evolved
planetary nebulae with collimated outflows and find that outflows among evolved
planetary nebulae exhibit a large variety of properties, in accordance with
these observed in younger planetary nebula.Comment: Accepted in The Astronomical Journal, 23 pages, 6 figure
A disk inside the bipolar planetary nebula M2-9
Bipolarity in proto-planetary and planetary nebulae is associated with events
occurring in or around their cores. Past infrared observations have revealed
the presence of dusty structures around the cores, many in the form of disks.
Characterising those dusty disks provides invaluable constraints on the
physical processes that govern the final mass expulsion of intermediate-mass
stars. We focus this study on the famous M2-9 bipolar nebula, where the moving
lighthouse beam pattern indicates the presence of a wide binary. The compact
and dense dusty core in the center of the nebula can be studied by means of
optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines
with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are
interpreted using a dust radiative transfer Monte Carlo code. A disk-like
structure is detected perpendicular to the lobes and a good fit is found with a
stratified disk model composed of amorphous silicates. The disk is compact,
2535 mas at 8, and 3746 mas at 13. For
the adopted distance of 1.2 kpc, the inner rim of the disk is 15 AU. The
mass represents a few percent of the mass found in the lobes. The compactness
of the disk puts strong constraints on the binary content of the system, given
an estimated orbital period 90-120yr. We derive masses of the binary components
between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an
evolved star. We present different scenarios on the geometric structure of the
disk accounting for the interactions of the binary system, which includes an
accretion disk as well.Comment: 9 figures, A&A accepte
The Decline and Fall of the Youngest Planetary Nebula
The Stingray Nebula, aka Hen3-1357, appeared for the first time in 1990 when
bright nebular lines and radio emission that had not been observed before were
unexpectedly discovered (Parthasarathy et al. 1993). In the ensuing years the
nebula faded precipitously. We report changes in shape and large decreases in
its nebular emission-line fluxes based on well-calibrated images obtained by
the Hubble Space Telescope in 1996 and 2016. Hen3-1357 is now a "recombination
nebula".Comment: 5 rages, 2 figures, 1 tabl
Disk formation by asymptotic giant branch winds in dipole magnetic fields
This is the final version. Available from American Astronomical Society via the DOI in this recordWe present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s-1) outflow (10-5 M⊙ yr-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.NS
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