Anibal Garcia-Hernandez

Instituto de Astrofísica de Canarias, San Cristóbal de La Laguna, Canary Islands, Spain

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Publications (7)2.51 Total impact

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    ABSTRACT: It is shown that graphite oxide (GO) and heavily ozonized C60 and C70 fullerenes known as “fullerene ozopolymers” are both paramagnetic materials with a very strong ESR signal at room temperature. When thermally annealed also the paramagnetic centres are gradually lost in large part. This occurs already at 350°C in the case of GO while for the fullerene ozopolymers a higher temperature is required, reaching at the end the same results. The half width of the ESR signal is linked to the distribution of the paramagnetic centres. Once again, striking analogies were found in the half width of the ESR signal measured on GO and on fullerene ozopolymers at least in the temperature range comprised between 25°C. and 450°C. Similarly, the same g-factor values, which are diagnostic for the understanding of the chemical nature of the paramagnetic centres, were found both on GO and on fullerene ozopolymers in all ranges of temperature considered.
    No preview · Article · Nov 2015 · Fullerenes Nanotubes and Carbon Nanostructures
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    ABSTRACT: Graphite oxide (GO) and heavily ozonized C60 and C70 fullerenes known as “fullerene ozopolymers” were studied by TGA-FTIR (Thermogravimetry coupled with FT-IR spectroscopy), DTG (Differential Thermogravimetry) and DTA (Differential Thermal Analysis) in air flow. It was found that GO burns at 70°C higher temperature than the fullerene ozopolymers. This different behavior toward the thermal oxidation of GO is due to the size of the oxidized and staked graphene layers which are expected to be significantly larger than those of the fullerene ozopolymers. Furthermore, the latter should necessarily have a buckybowl shaped structure which should favor their reactivity with oxygen.
    No preview · Article · Oct 2015 · Fullerenes Nanotubes and Carbon Nanostructures
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    ABSTRACT: It is shown that heavily ozonized C60 or C70 fullerenes (known also as “fullerene ozopolymers”) are suitable substrates for the preparation of graphene or nanographene in place of graphite oxide (GO) by thermal reduction in inert atmosphere. TGA-FTIR study shows that the release profile of CO2 and CO from fullerene ozopolymers in the temperature range between 25°C and 900°C is comparable to that shown by GO. Furthermore, the FT-IR spectral evolution of fullerene ozopolymers from room temperature to 630°C under inert atmosphere is once again strikingly comparable to that observed on GO under the same conditions.
    No preview · Article · Oct 2015 · Fullerenes Nanotubes and Carbon Nanostructures
  • Source
    Nick Cox · Pedro García-Lario · Anibal García-Hernández · Arturo Manchado
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    ABSTRACT: We present first preliminary results from AKARI/FIS pointed observations of post-AGB stars and planetary nebulae (PNe). A first analysis of the radial (azimuthally averaged) profile of the observed sources shows no evidence for excess emission due to the presence of circumstellar dust. No (detached) circumstellar faint dust-shells are seen in the image maps. Also, we present here first results of aperture flux photometry at wavelengths of 65, 90, 140 and 160 micron. Results are compared with IRAS flux densities as well as the beta release of the FIS Bright Source Catalog. Finally, spectral energy distributions are given, by way of an example, for two individual targets in our sample. Comment: 6 pages, 6 figures, Contributed Talk, to be published in the proceedings of the conference "AKARI, a light to illuminate the misty Universe" held at University of Tokyo, Japan, 16-19 February 2009
    Full-text · Article · May 2009
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    ABSTRACT: Planetary nebulae (PNe) are ideal probes of dust formation and evolution in low- and intermediate- mass stars. The analysis of dust emission in the 5 to 40 micron range, however, has concentrated mainly on a relatively small number of nearby Galactic PNe. The analysis has been recently extended to fainter sources located in the direction of the Galactic Bulge, and in the Galactic Halo. In addition, a significant number of Magellanic Cloud PNe was also studied by us with Spitzer/IRS. This has allowed us to study the dust properties and derive preliminary conclusions which suggest that indeed there are strong differences in the characteristics of the dust observed from source to source which seem to be mainly dependent on the mass of the progenitor star and the metallicity. To complete the picture, we propose to study with Spitzer/IRS the mid-IR spectra of all Galactic Disk PNe smaller than 4 arcsec (excluding duplications) to reveal the relations between nebular dust, gas abundance, and metallicity, at early evolutionary stages, as most compact PN are expected to be dynamically young. With the proposed observations we will solve some key questions on the formation and evolution of dust in PNe, such as the dependency of mass-loss with metallicity and progenitor mass. The statistical properties inferred from the proposed observations will be analyzed relatively to those of Magellanic Clouds, Galactic Halo and Galactic Bulge PNe, to asses the main characteristics of the particular solid state features detected in C-rich and O-rich PN as a function of the stellar population, with a large metallicity baseline. In addition, the proposed observations will yield alpha-element abundances with lower uncertainties than previosuly calculated. The Galactic distribution of our targets are such that metallicity gradients will also be derived from this program. The large and complete sample will fill a gap in the Spitzer program, with a strong legacy value.
    No preview · Article · Mar 2008
  • Dieter Engels · Anibal Garcia-Hernandez · Pedro Garcia-Lario
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    ABSTRACT: We propose to use the last Spitzer observing cycle to increase the sample of hidden post-AGB stars with mid-infrared spectroscopy. We selected predominantly GLIMPSE selected obscured objects tentatively identified as stars which have just abandoned the Asymptotic Giant Branch (AGB) and are now evolving to the planetary nebula (PN) stage. They are expected to represent the higher mass fraction of former oxygen-rich AGB stars and are suspected to be precursors of type I PNe. A high fraction of them shows a near infrared excess discovered in the GLIMPSE bands, indicating that as a consequence of strongly decreasing mass loss rates the surrounding circumstellar dust shell is becoming transparent again. Our sample is relatively faint at IRAS wavelengths so that they do not appear in IRAS color-selected samples. Based on our experience with Spitzer observations of an IRAS color selected sample, we will be able 1) to determine/confirm the chemical composition of the dust as O-rich, and possibly identify new mixed chemistry sources 2) to determine the dust grain structure (amorphous vs. crystalline) 3) to identify the solid state features superimposed on the dust continuum, in particular the relative amount of Al, Mg, Fe in the dust crystalline silicates 4) to study the correlation of the above observational properties with the evolutionary stage of the source, and 5) to identify new young infrared PNe, which may be associated to the high mass population of PNe The proposed Spitzer observing program is part of our efforts to understand the transition from AGB to PN while the stars are still in the earliest stages of the post-AGB phase. We found an astonishingly wide diversity of dust properties in their shells, probably because we observe them at different stages of their fast evolution. Due to their faintness at wavelengths shorter than 3 micron, Spitzer observations will be the last chance to study dust shells of hidden post-AGB stars for a long time.
    No preview · Article · Mar 2008
  • Pedro Garcia-Lario · Dieter Engels · Anibal Garcia-Hernandez · Jose V. Perea Calderon
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    ABSTRACT: We propose to take 5-37 micron IRS spectra of heavily obscured transition objects evolving from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) stage. The sample contains a complete flux-limited sample selected by IRAS colors. Their strong obscuration makes them optically invisible and some of them are not even detectable in the near infrared (< 3 micron), so that they are almost inaccesible from the ground, except at millimeter and radio wavelengths. These ``hidden post-AGB'' stars are expected to represent the higher mass fraction of former AGB stars, which were not covered by previous studies of (usually optical bright) post-AGB stars. The proposed Spitzer observations will allow: a) to determine the dominant chemistry of the circumstellar shells (O-rich vs. C-rich) for a major part of the sample b) to determine the composition of the dust in terms of their structure (amorphous vs. crystalline) c) to test the ISO result that disc-like geometries of the dust are connected with large fractions of crystalline dust, and d) to identify young infrared PN and peculiar sources with mixed chemistry. We expect that the properties of the solid-state features present in the mid-infrared spectral energy distributions will allow to order the sample into an evolutionary sequence. This will be a major step to improve our understanding of the fast developing processes occuring during the``hidden phase'' of post-AGB evolution.
    No preview · Article · May 2006