15,496 research outputs found
An updated catalog of OH-maser-emitting planetary nebulae
Aims. We studied the characteristics of planetary nebulae (PNe) that show
both OH maser and radio continuum emission (hereafter OHPNe). These have been
proposed to be very young PNe, and therefore, they could be key objects for
understanding the formation and evolution of PNe. Methods. We consulted the
literature searching for interferometric observations of radio continuum and OH
masers toward evolved stars, including the information from several surveys. We
also processed radio continuum and OH maser observations toward PNe in the Very
Large Array data archive. The high positional accuracy provided by
interferometric observations allow us to confirm or reject the association
between OH maser and radio continuum emission. Results. We found a total of six
PNe that present both OH maser and radio continuum emissions, as confirmed with
radio interferometric observations. These are bona fide OHPNe. The confirmed
OHPNe present a bipolar morphology in resolved images of their ionized emission
at different wavelengths, suggesting that the OH maser emission in PNe is
related to nonspherical mass-loss phenomena. The OH maser spectra in PNe
present a clear asymmetry, tending to show blueshifted emission with respect to
the systemic velocity. Their infrared colors suggest that most of these objects
are very young PNe. OHPNe do not form a homogeneous group, and seem to
represent a variety of different evolutionary stages. We suggest that OH masers
pumped in the AGB phase may disappear during the post-AGB phase, but reappear
once the source becomes a PN and its radio continuum emission is amplified by
the OH molecules. Therefore, OH maser emission could last significantly longer
than the previously assumed 1000 yr after the end of the AGB phase. This maser
lifetime may be longer in PNe with more massive central stars, which ionize a
larger amount of gas in the envelope.Comment: 16 pages, 5 figures, 4 tables. Accepted for publication by Astronomy
& Astrophysic
Sub-arcsecond Morphology of Planetary Nebulae
Planetary nebulae (PNe) can be roughly categorized into several broad
morphological classes. The high quality images of PNe acquired in recent years,
however, have revealed a wealth of fine structures that preclude simplistic
models for their formation. Here we present narrow-band, sub-arcsecond images
of a sample of relatively large PNe that illustrate the complexity and variety
of small-scale structures. This is especially true for bipolar PNe, for which
the images reveal multi-polar ejections and, in some cases, suggest turbulent
gas motions. Our images also reveal the presence or signs of jet-like outflows
in several objects in which this kind of component has not been previously
reported.Comment: 7 pages, 7 figures, Accepted for publication in PAS
Invisible decays of ultra-high energy neutrinos
Gamma-ray bursts (GRBs) are expected to provide a source of ultra high energy
cosmic rays, accompanied with potentially detectable neutrinos at neutrino
telescopes. Recently, IceCube has set an upper bound on this neutrino flux well
below theoretical expectation. We investigate whether this mismatch between
expectation and observation can be due to neutrino decay. We demosntrate the
phenomenological consistency and theoretical plausibility of the neutrino decay
hypothesis. A potential implication is the observability of majoron-emitting
neutrinoless double beta decay.Comment: 11 pages, 3 figures. To appear in Frontiers High Energy Physic
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
- …