scholarly journals Silicon Carbide as the Carrier of the 21μm Feature

2003 ◽  
Vol 209 ◽  
pp. 315-315
Author(s):  
A. K. Speck ◽  
A. M. Hofmeister
Keyword(s):  
Silicon Carbide ◽  
Solid State ◽  
Ir Spectra ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Transient Species ◽  
Physical Conditions ◽  
Show Evidence ◽  
Central Stars

Some proto-planetary nebulae (PPNe) exhibit an enigmatic feature in their infrared (IR) spectra at ~21 μm. PPNe which display this feature are all C-rich and all show evidence for s-process enhancements in their photospheres, indicative of efficient dredge-up during the ascent of the asymptotic giant branch (AGB). Furthermore, this 21 μm feature is not seen in the spectra of either the precursors to PPNe, the AGB stars, or the successors of PPNe, planetary nebulae (PNe). However the 21 μm feature has been seen in the spectra of PNe with Wolf-Rayet central stars. Therefore the carrier of this feature is unlikely to be a transient species that only exists in the PPNe phase. It is more likely that the physical conditions in the AGB stars and PNe conspire against the observation of an IR feature at 21 μm. This feature has been attributed to various molecular and solid state species, none of which satisfy all constraints, although TiC and PAHs have seemed the most viable.

scholarly journals New models for the evolution of central stars of planetary nebulae: Faster and Brighter

2016 ◽  
Vol 12 (S323) ◽  
pp. 179-183
Author(s):  
Marcelo M. Miller Bertolami
Keyword(s):  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Short Article ◽  
Intermediate Mass Stars ◽  
New Models ◽  
Central Stars

AbstractThe post-asymptotic giant branch (AGB) phase is arguably one of the least understood phases of the evolution of low- and intermediate- mass stars. The recent post-AGB evolutionary sequences computed by Miller Bertolami (2016) are at least three to ten times faster than those previously published by Vassiliadis & Wood (1994) and Blöcker (1995) which have been used in a large number of studies. This is true for the whole mass and metallicity range. The new models are also ~0.1–0.3 dex brighter than the previous models with similar remnant masses. In this short article we comment on the main reasons behind these differences, and discuss possible implications for other studies of post-AGB stars or planetary nebulae.

Dust Structure Around Asymptotic Giant Branch Stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 525-526 ◽  
Author(s):  
Devendra Raj Upadhyay ◽  
Lochan Khanal ◽  
Priyanka Hamal ◽  
Binil Aryal
Keyword(s):  
Inclination Angle ◽  
Asymptotic Giant Branch ◽  
Color Temperature ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Physical Conditions ◽  
The Core ◽  
Cavity Structure ◽  
Temperature Ranges ◽  
Giant Branch Stars

AbstractThis paper presents mass, temperature profile, and the variation of Planck’s function in different regions around asymptotic giant branch (AGB) stars. The physics of the interstellar medium (ISM) is extremely complex because the medium is very inhomogeneous and is made of regions with fairly diverse physical conditions. We studied the dust environment such as flux, temperature, mass, and inclination angle of the cavity structure around C-rich asymptotic giant branch stars in 60 μm and 100 μm wavelengths band using Infrared Astronomical Survey. We observed the data of AGB stars named IRAS 01142+6306 and IRAS 04369+4501. Flexible image transport system image was downloaded from Sky View Observatory; we obtained the surrounding flux density using software Aladin v2.5. The average dust color temperature and mass are found to be 25.08 K, 23.20 K and 4.73 × ;1026 kg (0.00024 M⊙), 2.58 × 1028 kg (0.013 M⊙), respectively. The dust color temperature ranges from 18.76 K ± 3.16 K to 33.21K ± K and 22.84 K ± 0.18 K to 24.48 K ± 0.63 K. The isolated cavity like structure around the AGB stars has an extension of 45.67 pc × 17.02 pc and 42.25 pc × 17.76 pc, respectively. The core region is found to be edge-on having an inclination angle of 79.46° and 73.99°, respectively.

scholarly journals Evolutionary Tracks for Central Stars of Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 463-472 ◽  
Author(s):  
Detlef Schönberner
Keyword(s):  
Mass Loss ◽  
White Dwarf ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Helium Burning ◽  
Hydrogen Burning ◽  
Stellar Envelope ◽  
Thermal Pulse ◽  
Pulse Cycle ◽  
Central Stars

Our understanding of the evolution of Central Stars of Planetary Nebulae (CPN) has made considerable progress during the last years. This was possible since consistent computations through the asymptotic giant branch (AGB), with thermal pulses and (in some cases) mass loss taken into account, became available (Schönberner, 1979, 1983; Kovetz and Harpaz, 1981; Harpaz and Kovetz, 1981; Iben, 1982, 1984; Wood and Faulkner, 1986). It turned out that the evolution depends very sensitively on the inital conditions on the AGB. More precisely, the evolution of an AGB remnant is a function of the phase of the thermal-pulse cycle during which this remnant was created on the tip of the AGB by the planetary-nebula (PN) formation process (Iben, 1984, 1987). This was first shown by Schönberner (1979), and then fully explored by Iben (1984). In short, two major modes of PAGB evolution to the white dwarf stage are possible, according to the two main phases of a thermally pulsing AGB star: the hydrogen-burning or helium-burning mode. If, for instance, the PN formation, i.e. the removal of the stellar envelope by mass loss, happens during a luminosity peak that follows a thermal pulse of the helium-burning shell, the remnant leaves the AGB while still burning helium as the main energy supplier (Härm and Schwarzschild, 1975). On the other hand, PN formation may also occur during the quiescent hydrogen-burning phase on the AGB, and the remnant continues then to burn mainly hydrogen on its way to becoming a white dwarf.

scholarly journals New IR-Observations of Post AGB Stars and Proto-Planetary Nebulae

1989 ◽  
Vol 131 ◽  
pp. 445-445 ◽  
Author(s):  
W.E. van der Veen ◽  
H. J. Habing ◽  
T. R. Geballe
Keyword(s):  
Selection Criteria ◽  
Large Fraction ◽  
Broad Band ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Be Stars ◽  
Ir Sources ◽  
Visual Magnitude ◽  
Small Band

A sample was selected from the IRAS Point Source Catalogue based on the following selection criteria: very red (“cold”) IRAS-colours: roughly F25/F12 > 2.5 and F60/F25 < 1.2; and low IR-variability: VAR < 30. These non-variable IR-sources may be stars that have evolved beyond the AGB (Asymptotic Giant Branch); a large fraction (40%) is associated with known planetary nebulae (Van der Veen and Habing, 1987, Astron. Astrophys., in press). To determine the nature of the other 60% additional observations were made mainly in the infrared: 1–13 μm, during 4 observing runs: ESO (La Silla, Chile) in July 1986 and June 1987; UKIRT (Hawaii) in August 1986 and June 1987. A total number of 58 sources was observed. A summary of the observations: -IR broad band photometry at 1.2, 1.6, 2.2, 3.8 and 4,6 μm for all 58 sources. -IR broad band photometry at 8.4, 9.7 and 12.8 μm for 19 sources. -IR small band photometry for 4 sources in the ranges 2–2.5 μm and 3–3.5 μm. -IR spectroscopy for 10 sources in the ranges 2–2.5 μm and 3–3.5 μm, -V, R, I observations (0.55, 0.7 and 0.9 μm) for 5 sources associated with a star of visual magnitude 8–9. These observations were carried out by D. de Winter (Amsterdam) with the 0.5-m ESO telescope at La Silla (Chile). -Walraven photometry (0.32, 0.36, 0.38, 0.43 and 0.54 μm) for 21 stars brighter than V = 15 and within 10“ from the IRAS position. These observations were carried out by M. van Haarlem (Leiden) with the 0.9-m Dutch telescope at La Silla (Chile).

scholarly journals NLTE analysis of central stars of highly excited Planetary Nebulae

1997 ◽  
Vol 180 ◽  
pp. 132-132
Author(s):  
T. Rauch ◽  
J. Köppen ◽  
R. Napiwotzki ◽  
K. Werner
Keyword(s):  
White Dwarfs ◽  
State Of The Art ◽  
Planetary Nebulae ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Spectral Analyses ◽  
Central Stars

Very hot central stars (CSPN) of highly excited planetary nebulae (PN) display directly the formation of white dwarfs. Only a few of these CSPN have been analyzed so far due to their low brightness and thus, the interpretation of their evolutionary status is hampered by statistical incompleteness. In the last decade many spectral analyses of very hot post-AGB stars by means of state-of-the-art NLTE model atmospheres have been performed (e.g. Rauch et al. 1996; Werner & Rauch 1994; Rauch & Werner 1995) and our picture of post-AGB evolution has been improved.

scholarly journals Stellar wind models of central stars of planetary nebulae

2020 ◽  
Vol 635 ◽  
pp. A173 ◽  
Author(s):  
J. Krtička ◽  
J. Kubát ◽  
I. Krtičková
Keyword(s):  
Mass Loss ◽  
White Dwarf ◽  
Planetary Nebula ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Planetary Nebulae ◽  
Terminal Velocity ◽  
Asymptotic Giant Branch ◽  
Central Stars ◽  
Loss Rates

Context. Fast line-driven stellar winds play an important role in the evolution of planetary nebulae, even though they are relatively weak. Aims. We provide global (unified) hot star wind models of central stars of planetary nebulae. The models predict wind structure including the mass-loss rates, terminal velocities, and emergent fluxes from basic stellar parameters. Methods. We applied our wind code for parameters corresponding to evolutionary stages between the asymptotic giant branch and white dwarf phases for a star with a final mass of 0.569 M⊙. We study the influence of metallicity and wind inhomogeneities (clumping) on the wind properties. Results. Line-driven winds appear very early after the star leaves the asymptotic giant branch (at the latest for Teff ≈ 10 kK) and fade away at the white dwarf cooling track (below Teff = 105 kK). Their mass-loss rate mostly scales with the stellar luminosity and, consequently, the mass-loss rate only varies slightly during the transition from the red to the blue part of the Hertzsprung–Russell diagram. There are the following two exceptions to the monotonic behavior: a bistability jump at around 20 kK, where the mass-loss rate decreases by a factor of a few (during evolution) due to a change in iron ionization, and an additional maximum at about Teff = 40−50 kK. On the other hand, the terminal velocity increases from about a few hundreds of km s−1 to a few thousands of km s−1 during the transition as a result of stellar radius decrease. The wind terminal velocity also significantly increases at the bistability jump. Derived wind parameters reasonably agree with observations. The effect of clumping is stronger at the hot side of the bistability jump than at the cool side. Conclusions. Derived fits to wind parameters can be used in evolutionary models and in studies of planetary nebula formation. A predicted bistability jump in mass-loss rates can cause the appearance of an additional shell of planetary nebula.

scholarly journals Spectral analysis of the O(He)-type central stars of the planetary nebulae K 1-27 and LoTr 4

2011 ◽  
Vol 7 (S283) ◽  
pp. 482-483
Author(s):  
Nicole Reindl ◽  
Ellen Ringat ◽  
Thomas Rauch ◽  
Klaus Werner ◽  
Jeffrey. W. Kruk
Keyword(s):  
Spectral Analysis ◽  
Planetary Nebulae ◽  
Agb Stars ◽  
Evolutionary Sequence ◽  
Preliminary Results ◽  
Pure Helium ◽  
R Coronae Borealis Stars ◽  
Central Stars

AbstractThe four known O(He) stars are the only amongst the hottest post-AGB stars whose atmospheres are composed of almost pure helium. Thus, their evolution deviates from the hydrogen-deficient post-AGB evolutionary sequence of carbon-dominated stars like e.g. PG 1159 stars. The origin of the O(He) stars is still not explained. They might be either post-early AGB stars or the progeny of R Coronae Borealis stars. We present preliminary results of a non-LTE spectral analysis based on FUSE and HST/COS observations.

scholarly journals Heavy elements in planetary nebulae: A theorist's gold mine

2011 ◽  
Vol 7 (S283) ◽  
pp. 127-130
Author(s):  
Amanda I. Karakas ◽  
Maria Lugaro
Keyword(s):  
Neutron Capture ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Gold Mine ◽  
Agb Stars ◽  
Capture Process ◽  
Wide Range ◽  
Model Predictions

AbstractObservations of planetary nebulae have revealed a wealth of information about the composition of heavy elements synthesized by the slow neutron capture process (the s process). In some of these nebulae the abundances of neutron-capture elements are enriched by factors of 10 to 30 times the solar value, indicating that these elements were produced in the progenitor star while it was on the asymptotic giant branch (AGB). In this proceedings we summarize results of our recent full s-process network predictions covering a wide range of progenitor masses and metallicities. We compare our model predictions to observations and show how this can provide important insights into nucleosynthesis processes occurring deep within AGB stars.

scholarly journals Improved Neutron-Capture Element Abundances in Planetary Nebulae

2009 ◽  
Vol 26 (3) ◽  
pp. 339-344 ◽  
Author(s):  
N. C. Sterling ◽  
H. L. Dinerstein ◽  
S. Hwang ◽  
S. Redfield ◽  
A. Aguilar ◽  
...  
Keyword(s):  
Cross Sections ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Rate Coefficients ◽  
Atomic Data ◽  
Agb Stars ◽  
Iron Species ◽  
Element Abundances ◽  
Ionization Balance ◽  
Initial Results

AbstractSpectroscopy of planetary nebulae (PNe) provides the means to investigate s-process enrichments of neutron(n)-capture elements that cannot be detected in Asymptotic Giant Branch (AGB) stars. However, accurate abundance determinations of these elements present a challenge. Corrections for unobserved ions can be large and uncertain, since in many PNe only one ion of a given n-capture element has been detected. Furthermore, the atomic data governing the ionization balance of these species are not well-determined, inhibiting the derivation of accurate ionization corrections. We present initial results of a program that addresses these challenges. Deep high-resolution optical spectroscopy of ∼20 PNe has been performed to detect emission lines from trans-iron species including Se, Br, Kr, Rb and Xe. The optical spectral region provides access to multiple ions of these elements, which reduces the magnitude and importance of uncertainties in the ionization corrections. In addition, experimental and theoretical efforts are providing determinations of the photoionization cross sections and recombination rate coefficients of Se, Kr and Xe ions. These new atomic data will make it possible to derive robust ionization corrections for these elements. Together, our observational and atomic data results will enable n-capture element abundances to be determined with unprecedented accuracy in ionized nebulae.

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