186 research outputs found
Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202
Some core-collapse supernovae show clear signs of interaction with dense
circumstellar material that often appears to be non-spherical. Circumstellar
nebulae around supernova progenitors provide clues to the origin of that
asymmetry in immediate pre-supernova evolution. Here I discuss outstanding
questions about the formation of the ring nebula around SN1987A and some
implications of similar ring nebulae around Galactic B supergiants. Several
clues hint that SN1987A's nebula may have been ejected in an LBV-like event,
rather than through interacting winds in a transition from a red supergiant to
a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental
parameters and circumstellar interactions", conference in honor of Virpi
Niemela's 70th birthda
NGC 602 Environment, Kinematics and Origins
The young star cluster NGC 602 and its associated HII region, N90, formed in
a relatively isolated and diffuse environment in the Wing of the Small
Magellanic Cloud. Its isolation from other regions of massive star formation
and the relatively simple surrounding HI shell structure allows us to constrain
the processes that may have led to its formation and to study conditions
leading to massive star formation. We use images from Hubble Space Telescope
and high resolution echelle spectrographic data from the Anglo-Australian
Telescope along with 21-cm neutral hydrogen (HI) spectrum survey data and the
shell catalogue derived from it to establish a likely evolutionary scenario
leading to the formation of NGC 602. We identify a distinct HI cloud component
that is likely the progenitor cloud of the cluster and HII region which
probably formed in blister fashion from the cloud's periphery. We also find
that the past interaction of HI shells can explain the current location and
radial velocity of the nebula. The surrounding Interstellar Medium is diffuse
and dust-poor as demonstrated by a low visual optical depth throughout the
nebula and an average HI density of the progenitor cloud estimated at 1 cm^-3.
These conditions suggest that the NGC 602 star formation event was produced by
compression and turbulence associated with HI shell interactions. It therefore
represents a single star forming event in a low gas density region.Comment: Accepted for publication in PASP. 25 pages, 10 figure
Jets and the shaping of the giant bipolar envelope of the planetary nebula KjPn 8
A hydrodynamic model involving cooling gas in the stagnation region of a
collimated outflow is proposed for the formation of the giant parsec-scale
bipolar envelope that surrounds the planetary nebula KjPn 8. Analytical
calculations and numerical simulations are presented to evaluate the model. The
envelope is considered to consist mainly of environmental gas swept-up by
shocks driven by an episodic, collimated, bipolar outflow. In this model, which
we call the ``free stagnation knot'' mechanism, the swept-up ambient gas
located in the stagnation region of the bow-shock cools to produce a high
density knot. This knot moves along with the bow-shock. When the central
outflow ceases, pressurization of the interior of the envelope stops and its
expansion slows down. The stagnation knot, however, has sufficient momentum to
propagate freely further along the axis, producing a distinct nose at the end
of the lobe. The model is found to successfully reproduce the peculiar shape
and global kinematics of the giant bipolar envelope of KjPn 8.Comment: 20 pages + 8 figures (in 1 tar-file 0.67 Mb
Supernova Remnants in the Magellanic Clouds III: An X-ray Atlas of LMC Supernova Remnants
We have used archival ROSAT data to present X-ray images of thirty-one
supernova remnants (SNRs) in the Large Magellanic Cloud (LMC). We have
classified these remnants according to their X-ray morphologies, into the
categories of Shell-Type, Diffuse Face, Centrally Brightened, Point-Source
Dominated, and Irregular. We suggest possible causes of the X-ray emission for
each category, and for individual features of some of the SNRs.Comment: 27 pages, 6 figures (9 figure files). To appear in the Supplement
Series of the Astrophysical Journal, August 1999 Vol. 123 #
Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N11
We present maps and a catalogue containing the J=1-0 12CO parameters of 29
individual molecular clouds in the second-brightest LMC star formation complex,
N11. In the southwestern part of N11, molecular clouds occur in a ring or shell
surrounding the major OB star association LH9. In the northeastern part, a
chain of molecular clouds delineates the rim of one of the so-called supergiant
shells in the LMC. The well-defined clouds have dimensions close to those of
the survey beam (diameters of 25 pc or less). Some of the clouds were also
observed in J=2-1 12CO, and in the lower two transitions of 13CO. Clouds mapped
with a twice higher angular resolution in J=2-1 12CO show substructure with
dimensions once again comparable to those of the mapping beam. The few clouds
for which we could model physical parameters have fairly warm (T(kin) = 60 -
150 K) and moderately dense (n(H2) = 3000 cm-3) gas. The northeastern chain of
CO clouds, although lacking in diffuse intercloud emission, is characteristic
of the more quiescent regions of the LMC and appears to have been subject to
relatively little photo-processing. The clouds forming part of the southwestern
shell or ring, however, are almost devoid of diffuse intercloud emission and
also exhibit other characteristics of an extreme photon-dominated region (PDR).Comment: 14 pages; accepted for publication in A&
SN 1987A's Circumstellar Envelope, II: Kinematics of the Three Rings and the Diffuse Nebula
We present several different measurements of the velocities of structures
within the circumstellar envelope of SN 1987A, including the inner, equatorial
ring (ER), outer rings (ORs), and the diffuse nebulosity at radii < 5 pc, based
on CTIO 4m and HST data. A comparison of STIS and WFPC2 [N II]6583 loci for the
rings show that the ER is expanding in radius at 10.5+-0.3 km/s, with the
northern OR expanding along the line of sight at about 26 km/s, and for the
southern OR, about 23 km/s. Similar results are found with CTIO 4m data.
Accounting for inclination, the best fit to all data show both ORs with an
expansion from the SN of 26 km/s. The ratio of the ER to OR velocities is
nearly equal to the ratio of ER to OR radii, so the rings are roughly
homologous, all having kinematic ages corresponding to about 20,000 yr before
the SN explosion. This makes previously reported, large compositional
differences between the ER and ORs difficult to understand. Additionally, a
grid of longslit 4m/echelle spectra centered on the SN shows two velocity
components over a region roughly coextensive with the outer circumstellar
envelope extending about 5 pc (20 arcsec) from the SN. One component is
blueshifted and the other redshifted from the SN centroid by about 10 km/s
each. These features may represent a bipolar flow expanding from the SN, in
which the ORs are propelled 10-15 km/s faster than that of the surrounding
envelope into which they propogate. The kinematic timescale for the entire
nebula is at least about 350,000 yr. The kinematics of these various structures
constrain possible models for the evolution of the progenitor and its formation
of a mass loss nebula.Comment: 25 pages AASTeX text plus 12 figures. ApJ, in pres
A Reanalysis of theUltraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)
extinction using data from the IUE final archive. Our new analysis takes
advantage of the improved signal--to--noise of the IUE NEWSIPS reduction, the
exclusion of stars with very low reddening, the careful selection of well
matched comparison stars, and an analysis of the effects of Galactic foreground
dust. Differences between the average extinction curves of the 30 Dor region
and the rest of the LMC are reduced compared to previous studies. We find that
there is a group of stars with very weak 2175 Ang. bumps that lie in or near
the region occupied by the supergiant shell, LMC 2, on the southeast side of 30
Dor. The average extinction curves inside and outside LMC 2 show a very
significant difference in 2175 Ang. bump strength, but their far--UV
extinctions are similar. While it is unclear whether or not the extinction
outside the LMC 2 region can be fit with the relation of Cardelli, Clayton and
Mathis (CCM), sightlines near LMC 2 cannot be fit with CCM due to their weak
2175 Ang. bumps. While the extinction properties seen in the LMC lie within the
range of properties seen in the Galaxy, the correlations of UV extinction
properties with environment seen in the Galaxy do not appear to hold in the
LMC.Comment: 29 pages, 10 figures, to be published in Ap
Supernova 1987A: Rotation and a Binary Companion
In this paper we provide a possible link between the structure of the bipolar
nebula surrounding SN1987A and the properties of its progenitor star. A Wind
Blwon Bubble (WBB) scenario is emplyed, in which a fast, tenuous wind from a
Blue Supergiant expands into a slow, dense wind, expelled during an earlier Red
Supergiant phase. The bipolar shapre develops due to a pole-to-equator density
contrast in the slow wind (ie, the slow wind forms a slow torus). We use the
Wind Compressed Disk (WCD) model of Bjorkman & Cassinelli (1992) to determine
the shape of the slow torus. In the WCD scenario, the shape of the torus is
determined by the rotation of the progenitor star. We then use a self-similar
semi-analytical method for wind blown bubble evolution to determine the shape
of the resulting bipolar nebula.
We find that the union of the wind-compressed-disk and bipolar-wind-blown-
bubble models allows us to recover the salient properties of SN1987A's
circumstellar nebula. In particular, the size, speed and density of SN1987A's
inner ring are easily reproduced in our calculations. An exploration of
parameter space shows the the red supergiant progenitor must be been rotating
at > 0.3 of its breakup speed. We conclude that the progenitor was most likely
spun up by a merger with a binary companion. Using a simple model for the
binary merger we find that the companion is likely to have had a mass > 0.5
M_sun.Comment: 30 pages, 4 figure
Photoevaporating flows from the cometary knots in the Helix nebula (NGC 7293)
We explain the Ha emission of the cometary knots in the Helix Nebula (NGC
7293) with an analytical model that describes the emission of the head of the
globules as a photoevaporated flow produced by the incident ionizing radiation
of the central star.We compare these models with the Ha emission obtained from
the HST archival images of the Helix Nebula. From a comparison of the Ha
emission with the predictions of the analytical model we obtain a rate of
ionizing photons from the central star of about 5e45 s^-1, which is consistent
with estimates based on the total Hb flux of the nebula. We also model the
tails of the cometary knots as a photoevaporated wind from a neutral shadow
region produced by the diffuse ionizing photon field of the nebula. A
comparison with the HST images allows us to obtain a direct determination of
the value of the diffuse ionizing flux. We compare the ratio of diffuse to
direct stellar flux as a function of radius inside an HII region with those
obtained from the observational data through the analytical tail and head wind
model. The agreement of this model with the values determined from the
observations of the knots is excellent.Comment: 9 pages, 5 figures, accepted for publication in Ap
Observational Study of the Multistructured Planetary Nebula NGC 7354
We present an observational study of the planetary nebula (PN) NGC 7354
consisting of narrowband Halpha and [NII]6584 imaging as well as low- and
high-dispersion long-slit spectroscopy and VLA-D radio continuum. According to
our imaging and spectroscopic data, NGC 7354 has four main structures: a quite
round outer shell and an elliptical inner shell, a collection of low-excitation
bright knots roughly concentrated on the equatorial region of the nebula, and
two symmetrical jet-like features, not aligned either with the shells' axes, or
with each other. We have obtained physical parameters like electron temperature
and electron density as well as ionic and elemental abundances for these
different structures. Electron temperature and electron density slightly vary
throughout the nebula. The local extinction coefficient c_Hbeta shows an
increasing gradient from south to north and a decreasing gradient from east to
west consistent with the number of equatorial bright knots present in each
direction. Abundance values show slight internal variations but most of them
are within the estimated uncertainties. In general, abundance values are in
good agreement with the ones expected for PNe. Radio continuum data are
consistent with optically thin thermal emission. We have used the interactive
three-dimensional modeling tool SHAPE to reproduce the observed morphokinematic
structures in NGC 7354 with different geometrical components. Our SHAPE model
is in very good agreement with our imaging and spectroscopic observations.
Finally, after modeling NGC 7354 with SHAPE, we suggest a possible scenario for
the formation of the nebula.Comment: Accepted for publication in AJ, 12 pages, 8 figure
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