17,309 research outputs found
Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes - diffuse interstellar bands connection
We present high-resolution (R~15000) VLT/UVES optical spectra of two
planetary nebulae (PNe; Tc 1 and M 1-20) where C60 (and C70) fullerenes have
already been found. These spectra are of high-quality (S/N > 300) for PN Tc 1,
which permits us to search for the expected electronic transitions of neutral
C60 and diffuse interstellar bands (DIBs). Surprisingly, we report the
non-detection of the most intense optical bands of C60 in Tc 1, although this
could be explained by the low C60 column density estimated from the C60
infrared bands if the C60 emission peaks far away from the central star. The
strongest and most common DIBs in both fullerene PNe are normal for their
reddening. Interestingly, the very broad 4428 A DIB and the weaker 6309 A DIB
are found to be unusually intense in Tc 1. We also report the detection of a
new broad (FWHM~5 A) unidentified band at ~6525 A. We propose that the 4428 A
DIB (probably also the 6309 A DIB and the new 6525 A band) may be related to
the presence of larger fullerenes (e.g., C80, C240, C320, and C540) and
buckyonions (multishell fullerenes such as C60@C240 and C60@C240@C540) in the
circumstellar envelope of Tc 1.Comment: Accepted for publication in Astronomy & Astrophysics Letters (6
pages, 4 figures, and 1 Table
Radiative corrections to from three generations of Majorana neutrinos and sneutrinos
In this work we study the radiative corrections to the mass of the lightest
Higgs boson of the MSSM from three generations of Majorana neutrinos and
sneutrinos. The spectrum of the MSSM is augmented by three right handed
neutrinos and their supersymmetric partners. A seesaw mechanism of type I is
used to generate the physical neutrino masses and oscillations that we require
to be in agreement with present neutrino data. We present a full one-loop
computation of these Higgs mass corrections, and analyze in full detail their
numerical size in terms of both the MSSM and the new (s)neutrino parameters. A
critical discussion on the different possible renormalization schemes and their
implications is included.Comment: 42 pages, 39 figures, 1 appendix, version published in AHE
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
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