C-rich Proto-planetary Nebulae Research Articles

On the carriers of the 21 μm emission feature in post-asymptotic giant branch stars

The mysterious 21micron emission feature seen in sixteen C-rich proto-planetary nebulae (PPNe) remains unidentified since its discovery in 1989. Over a dozen of materials are suggested as the carrier candidates. In this work we quantitatively investigate eight inorganic and one organic carrier candidates in terms of elemental abundance constraints, while previous studies mostly focus on their spectral profiles (which could be largely affected by grain size, shape and clustering effects). It is found that: (1) five candidates (TiC nanoclusters, fullerenes coordinated with Ti atoms, SiS$_2$, doped-SiC, and SiO$_2$-coated SiC dust) violate the abundance constraints (i.e. they require too much Ti, S or Si to account for the emission power of the 21micron band, (2) three candidates (carbon and silicon mixtures, Fe$_2$O$_3$, and Fe$_3$O$_4$),while satisfying the abundance constraints, exhibit secondary features which are not detected in the 21micron sources, and (3) nano FeO, neither exceeding the abundance budget nor producing undetected secondary features, seems to be a viable candidate, supporting the suggestions of Posch et al. 2004.

On the inorganic carriers of the 21 micron emission feature in post-AGB stars

AbstractThe mysterious 21 μm emission feature seen in only 12 C-rich proto-planetary nebulae (PPNe) remains unidentified since its discovery in 1989. Over a dozen materials have been suggested as the carrier candidates while none of them has received general acceptance. We investigate the inorganic carrier candidates by applying the observational constraints of the feature strength and associated features. It is found that: (1) three candidates, TiC clusters, fullerenes with Ti impurity atoms, and SiS2, are not abundant enough to account for the emission power of the 21 μm band, (2) five candidates, doped-SiC, SiO2-mantled SiC dust, carbon and silicon mixtures, Fe2O3, and Fe3O4, all show associated features which are either not detected in the 21 μm sources or detected but with a much lower strength, and (3) FeO, which satisfies the abundance constraints, does not display any associated features which are not seen in the 21 μm sources. Moreover, FeO is more likely to survive in the C-rich environment than Fe2O3and Fe3O4. Thus FeO seems to be the most plausible one among the inorganic carrier candidates.

ITAL]Infrared Space Observatory's[/ITAL] Discovery of C[TINF]4[/TINF]H[TINF]2[/TINF], C[TINF]6[/TINF]H[TINF]2[/TINF], and Benzene in CRL 618

We report on the detection with the Infrared Space Observatory (ISO), for the first time in the circumstellar medium, of the polyacetylenic chains C4H2 and C6H2 and of benzene (C6H6) in the direction of the proto-planetary nebula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C2H2. Benzene is 40 times less abundant than acetylene. We suggest that the chemistry in CRL 618 has been strongly modified by the UV photons coming from the hot central star and by the shocks associated with its high-velocity winds. All the infrared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense torus that surrounds the central star. C4H2 and C6H2 have also been detected in CRL 2688, so it seems that C-rich proto-planetary nebulae are the best organic chemistry factories in space.

On the Transience of Circumstellar Shells about |b| ≥ 10o OH/IR Stars. I. Basic Statistics

There are 62 OH/IR stars in the Arecibo sky with |b| ≥ 10°, |l| < 90°, and a 25 μm flux S(25) ≥ 2 Jy. These were identified as a result of a complete OH survey of color-selected IRAS sources, which also identified four O-rich proto-planetary nebulae (PPNs). Since OH/IR stars evolve into PPNs, the ~103 yr expansion age of one, 18095+2704, calibrates an ~1670 yr average net duration for 1612 MHz emission from high-latitude stars. This is comparable with the 770 yr wind travel time from the photosphere out to the 1612 MHz emission zone of WX Psc and the ~500 yr duration of the luminosity spike following after a thermal pulse. There is, however, only one PPN candidate without an OH maser to match to the sample's 118 OH/IR-star color mimics (circumstellar shells with the IR colors of OH/IR stars that do not exhibit 1612 MHz masers), where eight are implied if mimics independently evolve through a PPN phase. Most mimics are therefore likely to be the immediate precursors of OH/IR stars. The average net duration of heavy mass loss in a superwind is then estimated from the ratio between the number of IR sources and PPNs at ~3700 yr. However, the absence of any O-rich PPNs to match to OH/IR stars and mimics in the Galactic anticenter region (AGC), even though there are two carbon stars and four C-rich PPN with appropriate IR colors there, offers a strong hint that most OH/IR stars in the AGC exit their asymptotic giant branch phase as C-rich PPNs. This carries the direct implication that the superwind phase in high-latitude stars is usually a cyclical phase.

Infrared Space ObservatoryObservations of IRAS 16594−4656: A New Proto–Planetary Nebula with a Strong 21 Micron Dust Feature

Based on ISO short-wavelength spectrograph (SWS) + long-wavelength spectrograph (LWS) observations, IRAS 16594-4656 is identified as a new member of the rare class of C-rich proto-planetary nebulae with the 21 ?m dust feature in emission. In addition, the major polycyclic aromatic hydrocarbon (PAH) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 ?m are detected, together with unusually strong features at 12.6 and 13.4 ?m, very rarely observed, attributed to PAH species with a high degree of hydrogenation. The shapes and relative intensities of the various features found are consistent with a mixed population of transient decomposition products of large hydrogenated amorphous carbonaceous (HAC) grains, consisting of fullerenes with different degrees of hydrogenation, which we suggest to be the main carriers of the strong 21 ?m feature, and a combination of small and large partially dehydrogenated cationic PAH molecules excited by the visible radiation coming from the central star. Additional features corresponding to crystalline silicates (mostly pyroxenes) are also tentatively detected, together with a possible very broad unidentified emission feature centered around 30 ?m, previously reported as appearing in combination with the 21 ?m feature in other proto-planetary nebulae candidates. The combination of features corresponding to O-rich and C-rich dust grains, if real, would suggest a recent change to a C-rich chemistry in the outer envelope of IRAS 16594-4656. With the help of optical ground-based spectroscopy and recently obtained Hubble Space Telescope (HST) images, we identify IRAS 16594-4656 as a new transition object between the asymptotic giant branch (post-AGB) and the planetary nebula (PN) stage. The ground-based optical spectrum shows only the Balmer lines in emission and a faint, probably shock-excited, [O I] emission over a very red continuum where strong, probably circumstellar, diffuse interstellar absorption bands (DIBs) at 5780 and 6281-6284 ? are clearly seen. HST WFPC2 continuum images reveal the presence of a bright central star surrounded by a bipolar reflection nebula with a multiple-axis morphology and a maximum size of ~5''?11''.