231 research outputs found
Third-dredge-up oxygen in planetary nebulae
The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf
galaxy appear to result from nearly twin stars, except that third-dredge-up
carbon is more abundant in He 2-436. A thorough photoionization-model analysis
implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436,
indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with
respect to carbon is (7 +/- 4)%. Excess nitrogen in Wray 16-423 suggests third
dredge-up of late CN-cycle products even in these low-mass,
intermediate-metallicity stars.Comment: To appear in Astron. Astrophys. Lett. (Latex, 5 pages, 1 postscript
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Detection of deuterium Balmer lines in the Orion Nebula
The detection and first identification of the deuterium Balmer emission
lines, D-alpha and D-beta, in the core of the Orion Nebula is reported.
Observations were conducted at the 3.6m Canada-France-Hawaii Telescope, using
the Echelle spectrograph Gecko. These lines are very narrow and have identical
11 km/s velocity shifts with respect to H-alpha and H-beta. They are probably
excited by UV continuum fluorescence from the Lyman (DI) lines and arise from
the interface between the HII region and the molecular cloud.Comment: 4 pages, latex, 1 figure, 1 table, accepted for publication in
Astronomy & Astrophysics, Letter
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
A radio-continuum and photoionization-model study of the two planetary nebulae in the Sagittarius dwarf galaxy
Radio continuum observations at 1.4, 4.8 and 8.6 GHz of the two Planetary
Nebulae (PNe) in the Sagittarius dwarf galaxy reveal the elongated shape of
Wray 16-423 and the extreme compactness of He 2-436. He 2-436 is confirmed as
subject to local dust extinction.
Photoionization models for both PNe are obtained from two different codes,
allowing theoretical uncertainties to be assessed. Wray 16-423, excited by a
star of Teff 1.07x10^5K, is an ellipsoidal, matter-bounded nebula, except for a
denser sector. He 2-436, excited by a 7x10^4K star, includes two
radiation-bounded shells, with the inner one possibly corresponding to a
transitory event. Both stars are on the same (H-burning) evolutionary track of
initial mass (1.2+/-0.1) Msun and may be twins, with the PN ejection of Wray
16-423 having occured ~1500 years before He 2-436.
The PN abundances re-inforce the common origin of the parent stars,
indicating almost identical depletions with respect to solar for O, Ne, Mg, S,
Cl, Ar, and K (-0.55+/-0.07 dex), large identical overabundances for He and
strong overabundances for carbon, particularly in He2-436. Excess nitrogen
makes Wray 16-423 nearly a Type I PN. These PNe provide a means to calibrate
both metallicity and age of the Sagittarius stellar population, and they
confirm that the youngest, most metal-rich population has an age of 5Gyr and a
metallicity of [Fe/H]=-0.55, in agreement with the slope of the red giant
branch. (Abridged abstract)Comment: To appear in Astron. Astrophys. (Latex, 17 pages, 1 postscript
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The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A
The isotope ratio, 85Rb/87Rb, places constraints on models of the
nucleosynthesis of heavy elements, but there is no precise determination of the
ratio for material beyond the Solar System. We report the first measurement of
the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A
for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)
that differs significantly from the meteoritic value of 2.59. The Rb/K
elemental abundance ratio for the cloud also is lower than that seen in
meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values
indicates that the interstellar 85Rb abundance in this direction is lower than
the Solar System abundance. We attribute the lower abundance to a reduced
contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are
consistent with much less r-process synthesis for the solar neighborhood
compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter
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