66 research outputs found
Birth and early evolution of a planetary nebula
The final expulsion of gas by a star as it forms a planetary nebula --- the
ionized shell of gas often observed surrounding a young white dwarf --- is one
of the most poorly understood stages of stellar evolution. Such nebulae form
extremely rapidly (about 100 years for the ionization) and so the formation
process is inherently difficult to observe. Particularly puzzling is how a
spherical star can produce a highly asymmetric nebula with collimated outflows.
Here we report optical observations of the Stingray Nebula which has become an
ionized planetary nebula within the past few decades. We find that the
collimated outflows are already evident, and we have identified the nebular
structure that focuses the outflows. We have also found a companion star,
reinforcing previous suspicions that binary companions play an important role
in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199
An Interpretation of Late Quaternary Glacial Flow Indicators in the Baie des Chaleurs Region, Northern New Brunswick
A sequence of late Quaternary geologic events in northern New Brunswick is determined from striation analysis derived from published data, open file reports, and field research conducted by the authors since 1985. These data are integrated with clast provenance and clast fabric trend analysis, as well as information from other studies in the surrounding area. South of the Baie des Chaleurs, a complicated Late Wisconsinan glacial history is preserved in the form of erosive features including nailhead striae, miniature crag-and-tails, and various scars, striations, and fractures. The rarity of sedimentary deposits and datable materials precludes simple stratigraphie interpretation. Based on over 1,000 striation sites, we conclude four major phases of glacial flow affected the area during the Late Wisconsinan: 1) an early flow to the southeast which reflects local Appalachian ice; 2) a second phase of glacial flow to the east indicating a Laurentide ice influence in western New Brunswick; 3) a third phase of glacial flow to the north-northeast, which may represent ice response to drawdown in the Baie des Chaleurs; and 4) a final multidirectional flow indicating localized ice response during the last stages of Late Wisconsinan glaciation. The absence of Canadian Shield erratics in northern New Brunswick is explained in terms of ice streaming along the St. Lawrence channel beneath a southward-flowing Laurentide Ice Sheet. Basal ice debris (including Shield erratics) was apparently truncated and removed by the obliquely flowing ice stream, leaving relatively clean ice in the Ice Sheet as it entered Gaspésie and ultimately New Brunswick.On a déterminé la séquence des événements géologiques du Quaternaire supérieur survenus dans le nord du Nouveau-Brunswick à partir de l'analyse des stries tirée de diverses sources. Au sud de la baie des Chaleurs, l'évolution glaciaire complexe du Wisconsinien supérieur peut être retracée à partir des formes d'érosion glaciaire dont les clouures, les crag-and-tails miniatures, les cicatrices de toutes sortes, les stries et les fractures. La rareté des sédiments et des matériaux pouvant être datés excluent toute interprétation fondée sur la stratigraphie. En s'appuyant sur plus de 1000 sites de stries, on croit que la région a connu quatre épisodes glaciaires pendant le Wisconsinien supérieur: 1 ) le premier mouvement, de faible importance, s'est fait vers le SE en provenance des Appalaches; 2) le deuxième écoulement glaciaire vers PE démontre la force des glaces laurentidiennes au nord du Nouveau-Brunswick; 3) le troisième mouvement glaciaire vers le NNE pourrait être une réaction à l'affaissement qui s'est produit dans la baie des Chaleurs; 4) le dernier mouvement multidirectionnel s'est fait à partir de calottes localisées pendant les derniers stades de la glaciation wisconsinienne. On explique l'absence des blocs erratiques dans le nord du Nouveau-Brunswick par un écoulement des glaces le long du chenal laurentien interrompant l'Inlandsis laurentidien s'écoulant vers le sud. Les débris glaciaires de fond (y compris les blocs erratiques du Bouclier) ont apparemment été tronqués et enlevés par un courant glaciaire oblique, épurant ainsi la glace qui pénétrait en Gaspésie, puis au Nouveau-Brunswick.Een Interpretatie van Laat-glaciale Ijsbewegingsindicaties in de Chaleur Bay Regio, Noord New Brunswick. De Kwartair geologische kartering van noord New Brunswick begon met de studies van Robert Chalmers aan het einde van de 19e eeuw. Sindsdien zijn verschillende pogingen gedaan om de glaciale geschiedenis van dit gebied te ontravelen en uiteindelijk zijn Chalmers ideeen herontdekt en aangepast aan moderne theorien en modellen van glaciale processen en isostasie. De afwezigheid van stratigrafische secties en de aanwezigheid van vêle glaciale erosieve en morphologische vormen heeft geleid tot de ontwikkeling van een "erosie-stratigrafie ". Deze informatie wordt gesteund door till lithologische en ges-teente orientatie studies. Het resultaat van dit gedetailleerde karterings project is de her-kenning van 4 verschillende ijs-bewegingen. Datering van deze gebeurtenissen is een probleem vanwege een gebrek aan dateer-bare materialen. Een eerste zuidoostelijke ijsbeweging is geregistreerd in het oostelijke deel van het gebied en in de Chaleur Coastal Plain. De tweede en sterkste gebeurtenis geeft een oostwaartse ijsbeweging te zien, die waarschijnlijk gedurende het gehele Wisconsinan (Weichselien) en mogelijk het gehele Pleistoceen van belang kan zijn geweest. Een derde ijsbeweging is in noord-oostelijke richt-ing in de Baie des Chaleurs. In een latere fase, lokale ijsbewegingen vonden plaats in verschillende richtingen vanaf de hooglanden en mogelijk vanuit Baie des Chaleurs. Er is een poging gedaan deze gebeurtenissen in een régionale context te plaatsen, waarin de interactie van de Laurentide Ice Sheet met het Appalachian Ice Complex en het concept van een Laurentian ijsstroomkanaal centraal staan
The high-velocity outflow in the proto-planetary nebula Hen 3-1475
The proto-planetary nebula Hen 3-1475 shows a remarkable highly collimated
optical jet with an S-shaped string of three pairs of knots and extremely high
velocities. We present here a detailed analysis of the overall morphology,
kinematic structure and the excitation conditions of these knots based on deep
ground-based high dispersion spectroscopy complemented with high spatial
resolution spectroscopy obtained with STIS onboard HST, and WFPC2 [N II]
images. The spectra obtained show double-peaked, extremely wide emission line
profiles, and a decrease of the radial velocities with distance to the source
in a step-like fashion. We find that the emission line ratios observed in the
intermediate knots are consistent with a spectrum arising from the
recombination region of a shock wave with shock velocities ranging from 100 to
150 km/s. We propose that the ejection velocity is varying as a function of
time with a quasi-periodic variability (with timescale of the order of 100
years) and the direction of ejection is also varying with a precession period
of the order of 1500 years.Comment: 19 pages, 8 figures, accepted for publication in A&
Nebular emission lines in IRAS 17347-3139
We report the detection of nebular emission lines in the optical and
mid-infrared spectra of IRAS 17347-3139, a heavily obscured OH/IR star which
may be rapidly evolving from the AGB to the PN stage. The presence of emission
lines is interpreted as a clear indication that the ionization of its
circumstellar envelope has already started. This source belongs to the rare
class of objects known as `OHPNe' displaying both OH maser and radio continuum
emission. However, unlike the rest of stars in this class, prominent C-rich
dust features are detected in its mid-infrared spectrum, which makes the
analysis of this star particularly interesting.Comment: 2 pages, 2 figures, Proc. IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge
Univ. Press
Spitzer/IRS spectroscopy of high mass precursors to planetary nebulae
We present Spitzer/IRS observations of a small sample of heavily obscured
IRAS sources displaying both the infrared and OH maser emission characteristic
of OH/IR stars on the asymptotic giant branch (AGB), but also radio continuum
emission typical of ionized planetary nebulae (PNe), the so-called OHPNe. Our
observations show that their mid-infrared spectra are dominated by the
simultaneous presence of strong and broad amorphous silicate absorption
features together with crystalline silicate features, originated in their
O-rich circumstellar shells. Out of the five sources observed, three of them
are clearly non-variable at infrared wavelengths, confirming their post-AGB
status, while the remaining two still show strong photometric fluctuations, and
may still have not yet departed from the AGB. One of the non-variable sources
in the sample, IRAS 17393-2727, displays a strong [Ne II] nebular emission at
12.8 microns, indicating that the ionization of its central region has already
started. This suggests a rapid evolution from the AGB to the PN stage. We
propose that these heavily obscured OHPNe represent the population of high mass
precursors to PNe in our Galaxy.Comment: To appear in The Astrophysical Journal Letters (scheduled in the 2007
September 1 issue
Photometry and low resolution spectroscopy of hot post-AGB candidates
We have obtained Johnson U, B, V and Cousins R, I photometry and low
resolution spectra of a small sample of hot post-AGB candidates. Using the
present data in combination with JHK data from 2MASS, infrared data from the
MSX catalog and the IRAS fluxes, we have studied the spectral energy
distribution (SED) of these stars. Using the DUSTY code we have estimated the
dust temperatures, the distances to the stars, the mass-loss rates, angular
radii of the inner boundary of the dust envelopes and dynamical ages from the
tip of the AGB. These candidates have also been imaged through a narrow band
H-alpha filter, to search for nebulosity around the central stars. Our H-alpha
images revealed the bipolar morphology of the low excitation PN IRAS 17395-0841
with an angular extent of 2.8arcsec. The bipolar lobes of IRAS 17423-1755 in
H-alpha were found to have an angular extent of 3.5arcsec (south-east lobe) and
2.2arcsec (north-west lobe). The dust envelope characteristics, low resolution
spectrum and IRAS colors suggest that IRAS 18313-1738 is similar to the
proto-planetary nebula (PPN) HD 51585. The SED of IRAS 17423-1755, IRAS
18313-1738 and IRAS 19127+1717 show a warm dust component (in addition to the
cold dust) which may be due to recent and ongoing mass-loss.Comment: 20 pages, 6 figures, h-alpha figure compressed with XV, paper
accepted for publication in Astronomy & Astrophysic
A survey for water maser emission towards planetary nebulae. New detection in IRAS 17347-3139
We report on a water maser survey towards a sample of 27 planetary nebulae
(PNe) using the Robledo de Chavela and Medicina single-dish antennas, as well
as the Very Large Array (VLA). Two detections have been obtained: the already
known water maser emission in K 3-35, and a new cluster of masers in IRAS
17347-3139. This low rate of detections is compatible with the short life-time
of water molecules in PNe (~100 yr). The water maser cluster at IRAS 17347-3139
are distributed on a ellipse of size ~ 0.2" x 0.1", spatially associated with
compact 1.3 cm continuum emission (simultaneously observed with the VLA). From
archive VLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral index alpha =
0.76 +- 0.03 is derived for this radio source, which is consistent with either
a partially optically thick ionized region or with an ionized wind. However,
the latter scenario can be ruled out on mass-loss considerations, thus
indicating that this source is probably a young PN. The spatial distribution
and the radial velocities of the water masers are suggestive of a rotating and
expanding maser ring, tracing the innermost regions of a torus formed at the
end of the AGB phase. Given that the 1.3 cm continuum emission peak is located
near one of the tips of the major axis of the ellipse of masers, we speculate
on a possible binary nature of IRAS 17347-3139, where the radio continuum
emission could belong to one of the components and the water masers would be
associated with a companion.Comment: Accepted by The Astrophysical Journal. 25 pages, 6 figure
HE 3-1475 AND ITS JETS
We present spectra and high-resolution images taken with HST, the NTT, the
VLA, and the MPIA/ESO 2.2m of the emission-line star He 3-1475 which we suggest
is a post-AGB star. The star is at the origin of a 15-arcsec-long structure
containing symmetrically opposing bright knots. The knots have radial
velocities of about 500 km/s from the center of He 3-1475 to the ends of the
jets. HST snapshots show that the core of He 3-1475 is unipolar with a star at
the SE end and the nebula fanning out toward the NW. VLA observations show the
presence of OH masers, which are positioned parallel to the optical jets. A
model is proposed that accounts for all of the observational data. This unusual
object may link the OH/IR stars having extreme outflow velocities with highly
bipolar planetary nebulae.Comment: 14 pages, uu-encoded postcript. 6 figures available on request from
Matt Bobrowski ([email protected]
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