arXiv subjects

D. A. Garcia-Hernandez

Publications and source records attributed to D. A. Garcia-Hernandez.

At least 91 records · Page 5Linked to original sources

arXiv2013

Nucleosynthesis in massive AGB stars with delayed superwinds: implications for the abundance anomalies in Globular Clusters

We present nucleosynthesis predictions for massive (5-7 solar masses) asymptotic giant branch (AGB) stars of solar metallicity where we delay the onset of the superwind to pulsation periods of P =700-800 days. We found that delaying the superwind in solar metallicity massive AGB stars results in a larger production of s-process elements, something that would be also expected at lower metallicities. These new models and the available observations show that massive C-O core AGB stars in our Galaxy and in the Magellanic Clouds experience considerable third dredge-up (TDU). Thus, if massive AGB stars at the metallicities of the Globular Clusters (GCs) also experience deep TDU, then these stars would not be good candidates to explain the abundance anomalies observed in most GCs. However, more massive AGB stars (e.g., near the limit of C-O core production) or super-AGB stars with O-Ne cores may not experience very efficient TDU, producing the high He abundances needed to explain the multiple populations observed in some GCs.

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arXiv2013

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.

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arXiv2012

Far- and mid-infrared spectroscopy of complex organic matter of astrochemical interest: coal, heavy petroleum fractions, and asphaltenes

The coexistence of a large variety of molecular species (i.e., aromatic, cycloaliphatic and aliphatic) in several astrophysical environments suggests that unidentified IR emission (UIE) occurs from small solid particles containing a mix of aromatic and aliphatic structures (e.g., coal, petroleum, etc.), renewing the astronomical interest on this type of materials. A series of heavy petroleum fractions namely DAE, RAE, BQ-1, and asphaltenes derived from BQ-1 were used together with anthracite coal and bitumen as model compounds in matching the band pattern of the emission features of proto-planetary nebulae (PPNe). All the model materials were examined in the mid-infrared (2.5-16.7 um) and for the first time in the far-infrared (16.7-200 um), and the IR bands were compared with the UIE from PPNe. The best match of the PPNe band pattern is offered by the BQ-1 heavy aromatic oil fraction and by its asphaltenes fraction. Particularly interesting is the ability of BQ-1 to match the band pattern of the aromatic-aliphatic C-H stretching bands of certain PPNe, a result which is not achieved neither by the coal model nor by the other petroleum fractions considered here. This study shows that a new interesting molecular model of the emission features of PPNe are asphaltene molecules which are composed by an aromatic core containing 3-4 condensed aromatic rings surrounded by cycloaliphatic (naphtenic) and aliphatic alkyl chains. It is instead shown the weakness of the model involving a mixture of PAHs for modeling the aromatic IR emission bands. The laboratory spectra of these complex organic compounds represent a unique data set of high value for the astronomical community; e.g., they may be compared with the Herschel Space Observatory spectra (~51-220 um) of several astrophysical environments such as (proto-) PNe, H II regions, reflection nebulae, star forming galaxies, and young stellar objects.

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arXiv2012

Infrared Study of Fullerene Planetary Nebulae

We present a study of 16 PNe where fullerenes have been detected in their Spitzer spectra. This large sample of objects offers an unique opportunity to test conditions of fullerene formation and survival under different metallicity environments as we are analyzing five sources in our own Galaxy, four in the LMC, and seven in the SMC. Among the 16 PNe under study, we present the first detection of C60 (possibly also C70) fullerenes in the PN M 1-60 as well as of the unusual 6.6, 9.8, and 20 um features (possible planar C24) in the PN K 3-54. Although selection effects in the original samples of PNe observed with Spitzer may play a potentially significant role in the statistics, we find that the detection rate of fullerenes in C-rich PNe increases with decreasing metallicity (5% in the Galaxy, 20% in the LMC, and 44% in the SMC). CLOUDY photoionization modeling matches the observed IR fluxes with central stars that display a rather narrow range in effective temperature (30,000-45,000 K), suggesting a common evolutionary status of the objects and similar fullerene formation conditions. The observed C60 intensity ratios in the Galactic sources confirm our previous finding in the MCs that the fullerene emission is not excited by the UV radiation from the central star. CLOUDY models also show that line- and wind-blanketed model atmospheres can explain many of the observed [NeIII]/[NeII] ratios by photoionization suggesting that possibly the UV radiation from the central star, and not shocks, are triggering the decomposition of the circumstellar dust grains. With the data at hand, we suggest that the most likely explanation for the formation of fullerenes and graphene precursors in PNe is that these molecular species are built from the photo-chemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures similar to that of HAC dust.

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arXiv2012

The hot R Coronae Borealis star DY Centauri is a binary

The remarkable hot R Coronae Borealis star DY Cen is revealed to be the first and only binary system to be found among the R Coronae Borealis (RCB) stars and their likely relatives, including the Extreme Helium stars and the hydrogen-deficient carbon stars. Radial velocity determinations from 1982-2010 have shown DY Cen is a single-lined spectroscopic binary in an eccentric orbit with a period of 39.67 days. It is also one of the hottest and most H-rich member of the class of RCB stars. The system may have evolved from a common-envelope to its current form.

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arXiv2012

A review on carbon-rich molecules in space

We present and discuss carbon-rich compounds of astrochemical interest such as polyynes, acetylenic carbon chains and the related derivative known as monocyanopolyynes and dicyanopolyynes. Fullerenes are now known to be abundant in space, while fulleranes - the hydrogenated fullerenes - and other carbon-rich compounds such as very large polycyclic aromatic hydrocarbons (VLPAHs) and heavy petroleum fractions are suspected to be present in space. We review the synthesis, the infrared spectra as well as the electronic absorption spectra of these four classes of carbon-rich molecules. The existence or possible existence in space of the latter molecules is reported and discussed.

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arXiv2012

High-resolution optical spectroscopy of DY Cen: diffuse interstellar bands in a proto-fullerene circumstellar environment?

We search high-resolution and high-quality VLT/UVES optical spectra of the hot R Coronae Borealis (RCB) star DY Cen for electronic transitions of the C60 molecule and diffuse interstellar bands (DIBs). We report the non-detection of the strongest C60 electronic transitions (e.g., those at ~3760, 3980, and 4024 A). Absence of C60 absorption bands may support recent laboratory results, which show that the ~7.0, 8.5, 17.4, and 18.8 um emission features seen in DY Cen - and other similar objects with PAH-like dominated IR spectra - are attributable to proto-fullerenes or fullerene precursors rather than to C60. DIBs towards DY Cen are normal for its reddening; the only exception is the DIB at 6284 A (possibly also the 7223A DIB) that is found to be unusually strong. We also report the detection of a new broad (FWHM~2 A) and unidentified feature centered at ~4000 A. We suggest that this new band may be related to the circumstellar proto-fullerenes seen at infrared wavelengths.

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arXiv2012

Modeling the dust emission from PN IC418

We construct a detailed model for the IR dust emission from the PN IC 418. We succeed to reproduce the emission from 2 to 200$μ$m. We can determine the amount of emitting dust as well as its composition, and compare to the depletion of elements determined for the photoionized region.

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arXiv2012

The Chemical composition of the post-AGB F-supergiant CRL 2688

We present an analysis of a high resolution (R~50,000) optical spectrum of the central region of the proto-planetary nebula CRL 2688. This object is thought to have recently moved off the AGB, and display abundance patterns of CNO and heavy elements that can provide us with important clues to understand the nucleosynthesis, dredge-up and mixing experienced by the envelope of the central star during its AGB stage of evolution. The analysis of the molecular features, presumably originated from the circumstellar matter provides further constraints on the chemistry and velocity of the expanding shell, expelled as a consequence of the strong mass loss experienced by the central star. We confirm that the central star shows a spectrum typical of an F-type supergiant with Teff=7250 K, log g=0.5 and [Fe/H]=-0.3 dex. We find that the abundance pattern of this object is characterized by enhancements of Carbon ([C/Fe]=0.6), Nitrogen ([N/Fe]=1.0) and Na ([Na/Fe]=0.7), similar to other previously known carbon-rich post-AGB stars. Yttrium is also enhanced while the [Ba/Y] ratio is very low (-1.0), indicating that only the light s-process elements are enhanced. The Zinc abundance is found to be normal, [Zn/Fe]=0.0, suggesting that there is no depletion of refractory elements. The Halpha, Na I and K I resonance lines show prominent emission components, whose helio-centric radial velocities are offsetted by -41 km/s relative to the photospheric metal-absorption lines. The molecular features of C_2 and CN also show emission components, whose velocities are consistent with the emission components of the Halpha, Na I, and K I lines. On the other hand, their absorption components are more highly blue shifted than the corresponding emission components, which suggests that the regions where the emission and absorption components arise are expanding at different velocities.

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arXiv2012

The nature of dust in compact Galactic planetary nebulae from Spitzer spectra

We present the Spitzer/IRS spectra of 157 compact Galactic PNe. These young PNe provide insight on the effects of dust in early post-AGB evolution, before much of the dust is altered or destroyed by the hardening stellar radiation field. Most of the selected targets have PN-type IRS spectra, while a few turned out to be misclassified stars. We inspected the group properties of the PN spectra and classified them based on the different dust classes (featureless, carbon-rich dust; oxygen-rich dust; mixed-chemistry dust) and subclasses (aromatic and aliphatic; crystalline and amorphous). All PNe are characterized by dust continuum and more than 80% of the sample shows solid state features above the continuum, in contrast with the Magellanic Cloud sample where only ~40% of the entire sample displays solid state features; this is an indication of the strong link between dust properties and metallicity. The Galactic PNe that show solid state features are almost equally divided among the CRD, ORD, and MCD. We analyzed dust properties together with other PN properties and found that (i) there is an enhancement of MCD PNe toward the Galactic center; (ii) CRD PNe could be seen as defining an evolutionary sequence, contrary to the ORD and MCD PNe; (iii) C- and O-rich grains retain different equilibrium temperatures, as expected from models; (iv) ORD PNe are highly asymmetric and CRD PNe highly symmetric; point-symmetry is statistically more common in MCD. We find that the Galactic Disk sample does not include MCD PNe, and the other dust classes are differently populated from high to low metallicity environments. The MCPNe seem to attain higher dust temperatures at similar evolutionary stages, in agreement with the observational findings of smaller dust grains in low metallicity interstellar environments.

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arXiv2011

High resolution spectroscopy of the high velocity hot post-AGB star LS III +52 24 (IRAS 22023+5249)

The first high-resolution (R~50,000) optical spectrum of the B-type star, LS III +52 24, identified as the optical counterpart of the hot post-AGB candidate IRAS 22023+5249 (I22023) is presented. We report the detailed identifications of the observed absorption and emission features in the full wavelength range (4290-9015 A) as well as the atmospheric parameters and photospheric abundances (under the Local Thermodinamic Equilibrium approximation) for the first time. The nebular parameters (Te, Ne) are also derived. We estimate Teff=24,000 K, log g=3.0, xi=7 kms-1 and the derived abundances indicate a slightly metal-deficient evolved star with C/O<1. The observed P-Cygni profiles of hydrogen and helium clearly indicate on-going post-AGB mass loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionisation has just started. The observed spectral features, large heliocentric radial velocity, atmospheric parameters, and chemical composition indicate that I22023 is an evolved post-AGB star belonging to the old disk population. The derived nebular parameters (Te=7000 K, Ne=1.2x104 cm-3) also suggest that I22023 may be evolving into a compact, young low-excitation Planetary Nebula. Our optical spectroscopic analysis together with the recent Spitzer detection of double-dust chemistry (the simultaneous presence of carbonaceous molecules and amorphous silicates) in I22023 and other B-type post-AGB candidates may point to a binary system with a dusty disk as the stellar origin common to the hot post-AGB stars with O-rich central stars.

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arXiv2011

Molecular processes from the AGB to the PN stage

Many complex organic molecules and inorganic solid-state compounds have been observed in the circumstellar shell of stars (both C-rich and O-rich) in the transition phase between Asymptotic Giant Branch (AGB) stars and Planetary Nebulae (PNe). This short (~100-10.000 years) phase of stellar evolution represents a wonderful laboratory for astrochemistry and provides severe constraints on any model of gas-phase and solid-state chemistry. One of the major challenges of present day astrophysics and astrochemistry is to understand the formation pathways of these complex organic molecules and inorganic solid-state compounds (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene in the case of a C-rich chemistry and oxides and crystalline silicates in O-rich environments) in space. In this review, I present an observational review of the molecular processes in the late stages of stellar evolution with a special emphasis on the first detections of fullerenes and graphene in PNe.

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arXiv2011

The formation of fullerenes: clues from new C60, C70, and (possible) planar C24 detections in Magellanic Cloud Planetary Nebulae

We present ten new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C60 and C70 abundances in space. Also, we report evidence for the possible detection of planar C24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ~6.6, 9.8, and 20 um. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), PAH clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C24). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young PNe. This is supported by the fact that many of our fullerene-detected PNe show altered [NeIII]/[NeII] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.

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arXiv2011

Dust around R Coronae Borealis stars: I. Spitzer/IRS observations

Spitzer/IRS spectra from 5 to 37 um for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the IRAS fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 um and 25 um are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 um luminosity and [8.0]-[24.0] color index.

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arXiv2011

Morphological classification of post-AGB stars

We present a complete study of the morphology of post-Asymptotic Giant Branch (AGB) stars. Post-AGB is a very short evolutionary phase between the end of the AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and 10,000 yrs). We have defined the end of the post-AGB phase and the beginning of the PN phase when the star is hot enough to fully ionize the hydrogen envelope. Post-AGB stars have a circumstellar shell that is illuminated by the central stars or partially ionized. However, this circumstellar shell is too small to be resolved from ground-based observations. Thus, we have used data from the Hubble Space Telescope (HST) database to resolve these shells. About 150 post-AGB were found in this database. Here we present the preliminary results on their morphological classification and the correlation with several parameters such as galactic latitude and IRAS fluxes. Our preliminary results show that 40% of the sample are stellar-like (S), 33 % bipolar (B), 12 % multi-polar (M) and 15 % elliptical (E).

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arXiv2011

Rubidium-rich Asymptotic Giant Branch stars in the Magellanic Clouds

The Magellanic Clouds (MCs) offer a unique opportunity to study the stellar evolution and nucleosynthesis of massive Asymptotic Giant Branch (AGB) stars in low metallicity environments where distances are known. Rubidium is a key element to distinguish between high mass AGB stars and low mass AGBs or other type of astronomical objects such as massive red supergiant stars. Theoretically, high mass AGBs are predicted to produce a lot of Rb. We present the discovery of massive Rb-rich AGB stars in the MCs, confirming for the first time that these stars also exist in other galaxies. Our findings show that these stars are generally brighter than the standard adopted luminosity limit (Mbol ~ -7.1) for AGB stars. The observations of massive MC AGBs are qualitatively predicted by the present theoretical models. However, these theoretical models are far from matching the extremely high Rb overabundances observed. This might be related with an incomplete present understanding of the atmospheres of these stars.

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arXiv2011

Far-infrared imaging of post-AGB stars and (proto)-planetary nebulae with the AKARI Far-Infrared Surveyor

By tracing the distribution of cool dust in the extended envelopes of post-AGB stars and (proto)-planetary nebulae ((P)PNe) we aim to recover, or constrain, the mass loss history experienced by these stars in their recent past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite was used to obtain far-infrared maps for a selected sample of post-AGB stars and (P)PNe. We derived flux densities (aperture photometry) for 13 post-AGB stars and (P)PNe at four far-infrared wavelengths (60, 90, 140, and 160 um). Radial (azimuthally averaged) profiles are used to investigate the presence of extended emission from cool dust. No (detached) extended emission is detected for any target in our sample at levels significant with respect to background and cirrus emission. Only IRAS 21046+4739 reveals tentative excess emission between 30 and 130". Estimates of the total dust and gas mass from the obtained maps indicate that the envelope masses of these stars should be large in order to be detected with the AKARI FIS. Imaging with higher sensitivity and higher spatial resolution is needed to detect and resolve, if present, any cool compact or extended emission associated with these evolved stars.

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arXiv2011

Are C60 molecules detectable in circumstellar shells of R Coronae Borealis stars?

The hydrogen-poor, helium-rich and carbon-rich character of the gas around R Coronae Borealis (RCB) stars has been suggested to be a site for formation of C60 molecules. This suggestion is not supported by observations reported here showing that infrared transitions of C60 are not seen in a large sample of RCB stars observed with the Infrared Spectrograph on the Spitzer Space Telescope. The infrared C60 transitions are seen, however, in emission and blended with PAH-features in spectra of DY Cen and possibly also of V854 Cen, the two least hydrogen-deficient (hydrogen deficiency of only ~10-100) RCB stars. The speculation is offered that C60 (and the PAHs) in the moderately H-deficient circumstellar envelopes may be formed by the decomposition of hydrogenated amorphous carbon but fullerene formation is inefficient in the highly H-deficient environments of most RCBs.

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