Tip Of Asymptotic Giant Branch Research Articles

The porous envelope and circumstellar wind matter of the closest carbon star, CW Leonis

AbstractRecent abrupt changes of CW Leonis may indicate that we are witnessing the moment that the central carbon star is evolving off the Asymptotic Giant Branch (AGB) and entering into the pre-planetary nebula (PPN) phase. The recent appearance of a red compact peak at the predicted stellar position is possibly an unveiling event of the star, and the radial beams emerging from the stellar position resemble the feature of the PPN Egg Nebula. The increase of light curve over two decades is also extraordinary, and it is possibly related to the phase transition. Decadal-period variations are further found in the residuals of light curves, in the relative brightness of radial beams, and in the extended halo brightness distribution. Further monitoring of the recent dramatic and decadal-scale changes of this most well-known carbon star CW Leonis at the tip of AGB is still highly essential, and will help us gain a more concrete understanding on the conditions for transition between the late stellar evolutionary phases.

A Spitzer Space Telescope survey of extreme asymptotic giant branch stars in M32

We investigate the population of cool, evolved stars in the Local Group dwarf elliptical galaxy M32, using Infrared Array Camera observations from the Spitzer Space Telescope. We construct deep mid-infrared colour-magnitude diagrams for the resolved stellar populations within 3.5 arcmin of M32's centre, and identify those stars that exhibit infrared excess. Our data is dominated by a population of luminous, dust-producing stars on the asymptotic giant branch (AGB) and extend to approximately 3 mag below the AGB tip. We detect for the first time a sizeable population of `extreme' AGB stars, highly enshrouded by circumstellar dust and likely completely obscured at optical wavelengths. The total dust-injection rate from the extreme AGB candidates is measured to be $7.5 \times 10^{-7}$ ${\rm M}_{\odot} \, {\rm yr}^{-1}$, corresponding to a gas mass-loss rate of $1.5 \times 10^{-4}$ ${\rm M}_{\odot} \, {\rm yr}^{-1}$. These extreme stars may be indicative of an extended star-formation epoch between 0.2 and 5 Gyr ago.

Effects of nuclear cross sections on19F nucleosynthesis at low metallicities

The origin of fluorine is a longstanding problem in nuclear astrophysics. It is widely recognized that Asymptotic Giant Branch (AGB) stars are among the most important contributors to the Galactic fluorine production. In general, extant nucleosynthesis models overestimate the fluorine production by AGB stars with respect to observations. In this paper we review the relevant nuclear reaction rates involved in the fluorine production/destruction. We perform this analysis on a model with initial mass M=2 M$_\odot$ and Z=0.001. We found that the major uncertainties are due to the $^{13}$C($\alpha$,n)$^{16}$O, the $^{19}$F($\alpha$,p)$^{22}$Ne and the $^{14}$N(p,$\gamma$)$^{15}$O reactions. A change of the corresponding reaction rates within the present experimental uncertainties implies surface $^{19}$F variations at the AGB tip lower than 10\%. For some $\alpha$ capture reactions, however, larger variations in the rates of those processes cannot be excluded. Thus, we explore the effects of the variation of some $\alpha$ capture rates well beyond the current published uncertainties. The largest $^{19}$F variations are obtained by varying the $^{15}$N($\alpha$,$\gamma$)$^{19}$F and the $^{19}$F($\alpha$,p)$^{22}$Ne reactions. The analysis of some $\alpha$ capture processes assuming a wider uncertainty range determines $^{19}$F abundances in better agreement with recent spectroscopic fluorine measurements at low metallicity. In the framework of the latter scenario the $^{15}$N($\alpha$,$\gamma$)$^{19}$F and the $^{19}$F($\alpha$,p)$^{22}$Ne reactions show the largest effects on fluorine nucleosynthesis. The presence of poorly known low energy resonances make such a scenario, even if unlikely, possible. We plan to directly measure these resonances.

The pulsation of AGB stars in the Magellanic Cloud clusters NGC 1978 and 419

The intermediate-age Magellanic Cloud clusters NGC 1978 and NGC 419 are each found to contain substantial numbers of pulsating AGB stars, both oxygen-rich and carbon-rich. Each cluster also contains two pulsating asymptotic giant branch (AGB) stars which are infrared sources with a large mass loss rate. Pulsation masses have been derived for the AGB variables, from the lowest luminosity O-rich variables to the most evolved infrared sources. It is found that the stars in NGC 1978 have a mass of 1.55Msun early on the AGB while the NGC 419 stars have a mass of 1.87Msun early on the AGB. These masses are in good agreement with those expected from the cluster ages determined by main-sequence turnoff fitting. Nonlinear pulsation models fitted to the highly evolved AGB stars show that a substantial amount of mass loss has occurred during the AGB evolution of these stars. An examination of the observed mass loss on the AGB, and the AGB tip luminosities, shows that in both clusters the mass loss rates computed from the formula of Vassiliadis & Wood (1993) reproduce the observations reasonably well. The mass loss rates computed from the formula of Blocker (1995) terminate the AGB in both clusters at a luminosity which is much too low.

The Asymptotic Giant Branch of NGC 205: The Characteristics of Carbon Stars and M Giants Identified fromJHK′ Images

J, H, and K' images are used to investigate the asymptotic giant branch (AGB) content of the Local Group dwarf elliptical galaxy NGC 205. The AGB on the (K, H-K) and (K, J-K) color-magnitude diagrams consists of two sequences: a near-vertical plume of giants with spectral types K and M and a red arm containing C stars. There are 320 C stars with Mbol 1.5 within 2' of the nucleus. C stars account for 10% of the integrated luminosity of AGB stars brighter than Mbol = -3.75 near the center of NGC 205, and this is in excellent agreement with what is measured in intermediate-age clusters in the LMC. The most luminous AGB star has Mbol = -6.5, although variability introduces an uncertainty of a few tenths of a magnitude when using this as an estimate of the AGB tip brightness. Comparisons with models suggest that the brightest AGB stars formed within the past 0.1 Gyr and that the previous episode of star formation occurred a few tenths of a Gyr earlier. These results are consistent with star formation in NGC 205 being triggered by interactions with M31. These data also demonstrate that near-infrared imaging provides an efficient means of identifying C stars in nearby galaxies. The techniques used here to identify C stars and probe the AGB are well suited to studies of galaxies outside of the Local Group using data obtained with adaptive optics systems on large ground-based telescopes.

THE BRIGHTEST ASYMPTOTIC GIANT BRANCH STARS IN THE INNER BULGE OF M31

JHK images with angular resolutions approaching the diffraction limit of the 3.6 m Canada-France-Hawaii Telescope are used to investigate the bright asymptotic giant branch (AGB) content of the M31 bulge. The AGB tip in a field 26 from the galaxy center occurs at K = 15.6, which is significantly fainter than measured in previous ground-based studies that sampled similar projected distances from the center of M31 but were affected by crowding. Within 26 of the center of M31, the number density of bright AGB stars scales with r-band surface brightness and the K brightness of the AGB tip does not vary measurably with radius. It is concluded that the infrared bright AGB stars (1) belong to the bulge and not the disk and (2) are well mixed throughout the inner bulge, suggesting that they formed at a time when the overall structural properties of the M31 bulge were imprinted. The bolometric luminosity functions (LFs) of the M31 bulge and Baade's window are in excellent agreement, while the brightest AGB stars in the M31 bulge, the Galactic bulge, and M32 have similar MK. Barring a fortuitous tuning of age and metallicity to produce AGB tips with similar brightnesses, it is suggested that the brightest stars in M32 and the bulges of M31 and the Milky Way belong to an old, metal-rich population; these stars are bright not because they have a young or intermediate age, but because they have a high metallicity.

The Detection of Bright Asymptotic Giant Branch Stars in the Nearby Elliptical Galaxy Maffei 1

We have used the adaptive optics system on the Canada-France-Hawaii Telescope to study a field 6' from the center of the heavily obscured elliptical galaxy Maffei 1. Our near diffraction-limited H and K' images reveal an excess population of objects with respect to a background field, and we conclude that the asymptotic giant branch (AGB) tip in Maffei 1 occurs at K = 20 ± 0.25. Assuming that stars at the AGB tip in Maffei 1 have the same intrinsic luminosity as those in the bulge of M31, then the actual distance modulus of Maffei 1 is 28.2 ± 0.3 if AV = 5.1, which corresponds to a distance of 4.4 Mpc. This is in excellent agreement with the distance estimated from surface brightness fluctuations in the infrared and confirms that Maffei 1 is too distant to affect the dynamics of the Local Group significantly.

Multiple shell Planetary Nebulae: Evolutionary paths and observed configurations

Recent observations of multiple shell planetary nebulae confirm the existing subdivision into two morphological types: attached halo and detached halo. The multiple shell phenomenon appears in about twenty-five percent of round and elliptical planetary nebulae of the Instituto de Astrofisica de Canarias morphological survey. The analysis of these nebulae together with that of their central stars lead to the interpretation that detached halo nebulae are formed through discontinuous mass loss at the Asymptotic Giant Branch tip. The attached halo nebulae, instead, are probably produced via dynamical nebular shaping.

The stellar content of NGC 3109

view Abstract Citations (12) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Stellar Content of NGC 3109 Davidge, T. J. Abstract Deep B, V, and I CCD images are used to investigate the stellar content of a field along the major axis of the nearby dwarf galaxy NGC 3109. Stars spanning a wide range of ages are detected, and the youngest have ages between 10 and 30 million years. The width of the main sequence can be explained solely by observational uncertainties, indicating that star- to-star differences in reddening are small. The total line-of-sight reddening, derived from the main-sequence locus, is E( B - V) ~ 0.14. The distance modulus of NGC 3109 is rederived from published Cepheid photometry after assuming that the absorption measured in this field is representative of the galaxy in general, and the result is μ_0_ ~ 25.05. The luminosity function of upper main-sequence stars follows a well-defined power law with exponent 0.37+/-0.01. This is flatter than what is typically seen in other galaxies, and it is suggested that the shallow luminosity function may be the consequence of stochastic variations in the local star-forming history. Asymptotic giant branch (AGB) stars have also been detected. The AGB tip occurs near M_bol_~ -7.0, indicating that an intermediate-age population is present. Moreover, the bolometric luminosity function of AGB stars is identical to that seen in the Small Magellanic Cloud, a galaxy with similar integrated absolute brightness. Finally, the locus of the red giant branch is very blue, indicating that the old and intermediate-age populations are extremely metal-poor, with [Fe/H] <= - 1.6. Publication: The Astronomical Journal Pub Date: April 1993 DOI: 10.1086/116518 Bibcode: 1993AJ....105.1392D Keywords: Astronomical Photometry; Asymptotic Giant Branch Stars; Dwarf Galaxies; Interstellar Extinction; Star Distribution; Stellar Luminosity; Brightness Distribution; Cepheid Variables; Distance; Magellanic Clouds; Metallicity; Star Formation; Stellar Color; Astrophysics; GALAXIES: INDIVIDUAL: NGC 3109; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (13) NED (1)