145 research outputs found
The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion
We present a semi-analytic model for shaping the nebula around eta Carinae
that accounts for the simultaneous production of bipolar lobes and an
equatorial disk through a rotating surface explosion. Material is launched
normal to the surface of an oblate rotating star with an initial kick velocity
that scales approximately with the local escape speed. Thereafter, ejecta
follow ballistic orbital trajectories, feeling only a central force
corresponding to a radiatively reduced gravity. Our model is conceptually
similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we
modify it to an explosion instead of a steady line-driven wind, we include a
rotationally-distorted star, and we treat the dynamics somewhat differently.
Continuum-driving avoids the disk inhibition that normally operates in
line-driven winds. Our model provides a simple method by which rotating hot
stars can simultaneously produce intrinsically bipolar and equatorial mass
ejections, without an aspherical environment or magnetic fields. Although
motivated by eta Carinae, the model may have generic application to other LBVs,
B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less
influential, our model generalizes to produce bipolar morphologies without
disks, as seen in many PNe.Comment: ApJ accepted, 9 page
Near-infrared integral field spectroscopy of the Homunculus nebula around Eta Carinae using Gemini/CIRPASS
This work presents the first integral field spectroscopy of the Homunculus
nebula around Eta Carinae in the near-infrared spectral region (J band). We
confirmed the presence of a hole on the polar region of each lobe, as indicated
by previous near-IR long-slit spectra and mid-IR images. The holes can be
described as a cylinder of height (i.e. the thickness of the lobe) and diameter
of 6.5 and 6.0x10^{16} cm, respectively. We also mapped the blue-shifted
component of He I 10830 seen towards the NW lobe. Contrary to previous works,
we suggested that this blue-shifted component is not related to the Paddle but
it is indeed in the equatorial disc.
We confirmed the claim of Smith (2005) and showed that the spatial extent of
the Little Homunculus matches remarkably well the radio continuum emission at 3
cm, indicating that the Little Homunculus can be regarded as a small HII
region. Therefore, we used the optically-thin 1.3 mm radio flux to derive a
lower limit for the number of Lyman-continuum photons of the central source in
Eta Car. In the context of a binary system, and assuming that the ionising flux
comes entirely from the hot companion star, the lower limit for its spectral
type and luminosity class ranges from O5.5 III to O7 I. Moreover, we showed
that the radio peak at 1.7 arcsec NW from the central star is in the same
line-of-sight of the `Sr-filament' but they are obviously spatially separated,
while the blue-shifted component of He I 10830 may be related to the radio peak
and can be explained by the ultraviolet radiation from the companion star.Comment: Accepted for publication on MNRAS (15 pages, 12 EPS figures). A
version of this paper with high-resolution figures is available at
ftp://www.astro.iag.usp.br/pub/mairan/homunc.pd
The Structure of the Homunculus: I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of Eta Carinae
High resolution long-slit spectra obtained with the Phoenix spectrograph on
Gemini South provide our most accurate probe of the three dimensional structure
of the Homunculus around eta Car. The new near-infrared spectra dramatically
confirm the double-shell structure inferred previously from thermal dust
emission, resolving the nebula into a very thin outer shell seen in H2 21218,
and a warmer, thicker inner layer seen in [Fe II] 16435. The thin H2 skin hints
that the most important mass loss during the 19th century eruption had a very
short duration of less than 5 yr. H2 emission traces the majority of the mass
in the nebula, and has an average density of order 10^6.5 cm-3. This emission,
in turn, yields our first definitive picture of the exact shape of the nebula,
plus a distance of 2350pm50 pc and an inclination angle of 41deg (the polar
axis is tilted 49deg from the plane of the sky). The distribution of the H2
emission provides the first measure of the latitude dependence of the speed,
mass loss, and kinetic energy associated with eta Car's 19th century explosion.
Almost 75 percent of the total mass and more than 90 percent of the kinetic
energy in the ejecta were released at high latitudes. This rules out a model
for the bipolar shape wherein an otherwise spherical explosion was pinched at
the waist by a circumstellar torus. Also, the ejecta could not have been
deflected toward polar trajectories by a companion star, since the kinetic
energy of the polar ejecta is greater than the binding energy of the putative
binary system. Instead, most of the mass appears to have been directed poleward
by the explosion itself. [abridged]Comment: 25 pages, figs 2 and 3 in color. Accepted by Ap
Two new species of Odontostilbe historically hidden under O. microcephala (Characiformes: Cheirodontinae)
Specimens historically identified as Odontostilbe microcephala from the upper rio ParanĂĄ and Andean piedmont tributaries of the rĂo Paraguay are reviewed and split in three species. We found that the distribution of O. microcephala is restricted to the Andean slope of the rĂo Paraguay basin. The species is distinguished from congeners with subterminal mouth by the elongate body, usually 10-12 gill rakers on upper branch and smaller horizontal orbital diameter (24.6-32.8 % HL, mean 28.7%). Specimens from upper rio ParanĂĄ constitute two new species, diagnosed from other Cheirodontinae by the presence of mesopterygoid teeth, grouped on median portion and forming a continuous row. The new species are distinguished from each other by having premaxillary teeth with five cusps vs. nine cusps and by the number of lamellae in left and right sides of central median raphe of olfactory rosette with 20-21 vs. 11-12.EspĂ©cimes historicamente identificados com Odontostilbe microcephala do rio ParanĂĄ e tributĂĄrios do rĂo Paraguay, foram revisados e separados em trĂȘs espĂ©cies. A distribuição de O. microcephala Ă© restrita ao sopĂ© andino da bacia do rĂo Paraguay. A espĂ©cie Ă© distinta das congĂȘneres com boca subterminal pela forma alongada, geralmente 10-12 rastros branquiais no ramo superior e menor diĂąmetro horizontal da Ăłrbita (24,6-32,8 % CC, mĂ©dia 28,7%). EspĂ©cimes do alto rio ParanĂĄ constituem duas espĂ©cies novas diagnosticadas de outros Cheirodontinae pela presença de dentes no mesopterigoide, agrupados em sua porção mĂ©dia e formando uma fileira continua. As novas espĂ©cies distinguem-se por ter dentes premaxilares com cinco cĂșspides vs. nove cĂșspides e pelo nĂșmero de lamelas nos lados esquerdo e direito da rafe central da roseta olfativa com 20-21 vs. 11-12
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