Structure and shaping processes within the extended atmospheres of AGB stars

We present recent studies using the near-infrared instrument AMBER of the VLT Interferometer (VLTI) to investigate the structure and shaping processes within the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER observations of the Mira variable S Ori have revealed wavelength-dependent apparent angular sizes. These data were successfully compared to dynamic model atmospheres, which predict wavelength-dependent radii because of geometrically extended molecular layers. Most recently, AMBER closure phase measurements of several AGB stars have also revealed wavelength-dependent deviations from 0/180 deg., indicating deviations from point symmetry. The variation of closure phase with wavelength indicates a complex non-spherical stratification of the extended atmosphere, and may reveal whether observed asymmetries are located near the photosphere or in the outer molecular layers. Concurrent observations of SiO masers located within the extended molecular layers provide us with additional information on the morphology, conditions, and kinematics of this shell. These observations promise to provide us with new important insights into the shaping processes at work during the AGB phase. With improved imaging capabilities at the VLTI, we expect to extend the successful story of imaging studies of planetary nebulae to the photosphere and extended outer atmosphere of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F. Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for Astrophysics, Manchester, UK, 201

Modelling the alumina abundance of oxygen-rich evolved stars in the Large Magellanic Cloud

In order to determine the composition of the dust in the circumstellar envelopes of oxygen-rich asymptotic giant branch (AGB) stars we have computed a grid of modust radiative-transfer models for a range of dust compositions, mass-loss rates, dust shell inner radii and stellar parameters. We compare the resulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec Large Magellanic Cloud (LMC) sample, finding good overall agreement for stars with a mid-infrared excess. We use these models to fit a sample of 37 O-rich AGB stars in the LMC with optically thin circumstellar envelopes, for which 5$-$35-$\mu$m Spitzer infrared spectrograph (IRS) spectra and broadband photometry from the optical to the mid-infrared are available. From the modelling, we find mass-loss rates in the range $\sim 8\times10^{-8}$ to $5\times10^{-6}$ M$_{\odot}\ \mathrm{yr}^{-1}$, and we show that a grain mixture consisting primarily of amorphous silicates, with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra. Furthermore, we show from dust models that the AKARI [11]$-$[15] versus [3.2]$-$[7] colour-colour diagram, is able to determine the fractional abundance of alumina in O-rich AGB stars.Comment: 22 pages, 17 figures, accepted MNRA

Detailed Abundances for a Large Sample of Giant Stars in the Globular Cluster 47 Tucanae (NGC 104)

47 Tuc is an ideal target to study chemical evolution and GC formation in massive more metal-rich GCs since is the closest, massive GC. We present chemical abundances for O, Na, Al, Si, Ca, Ti, Fe, Ni, La, and Eu in 164 red giant branch (RGB) stars in the massive globular cluster 47 Tuc using spectra obtained with both the Hydra multi-fiber spectrograph at the Blanco 4-m telescope and the FLAMES multi-object spectrograph at the Very Large Telescope. We find an average [Fe/H]=--0.79$\pm$0.09 dex, consistent with literature values, as well as over-abundances of alpha-elements ([\alpha/\mbox{Fe}]\sim0.3 dex). The n-capture process elements indicate that 47 Tuc is r-process dominated ([Eu/La]=+0.24), and the light elements O, Na, and Al exhibit star-to-star variations. The Na-O anti-correlation, a signature typically seen in Galactic globular clusters, is present in 47 Tuc, and extends to include a small number of stars with [O/Fe] $\sim$\,--0.5. Additionally, the [O/Na] ratios of our sample reveal that the cluster stars can be separated into three distinct populations. A KS-test demonstrates that the O-poor/Na-rich stars are more centrally concentrated than the O-rich/Na-poor stars. The observed number and radial distribution of 47 Tuc's stellar populations, as distinguished by their light element composition, agrees closely with the results obtained from photometric data. We do not find evidence supporting a strong Na-Al correlation in 47 Tuc, which is consistent with current models of AGB nucleosynthesis yields.Comment: Accepted for publication in The Astrophysical Journa

A multi-wavelength view of the central kiloparsec region in the Luminous Infrared Galaxy NGC1614

The Luminous Infrared Galaxy NGC1614 hosts a prominent circumnuclear ring of star formation. However, the nature of the dominant emitting mechanism in its central ~100 pc is still under debate. We present sub-arcsecond angular resolution radio, mid-infrared, Pa-alpha, optical, and X-ray observations of NGC1614, aimed at studying in detail both the circumnuclear ring and the nuclear region. The 8.4 GHz continuum emission traced by the Very Large Array (VLA) and the Gemini/T-ReCS 8.7 micron emission, as well as the Pa-alpha line emission, show remarkable morphological similarities within the star-forming ring, suggesting that the underlying emission mechanisms are tightly related. We used an HST/NICMOS Pa-alpha map of similar resolution to our radio maps to disentangle the thermal free-free and non-thermal synchrotron radio emission, from which we obtained the intrinsic synchrotron power-law for each individual region within the central kpc of NGC1614. The radio ring surrounds a relatively faint, steep-spectrum source at the very center of the galaxy, suggesting that the central source is not powered by an AGN, but rather by a compact (r < 90 pc) starburst. Chandra X-ray data also show that the central kpc region is dominated by starburst activity, without requiring the existence of an AGN. We also used publicly available infrared data to model-fit the spectral energy distribution of both the starburst ring and a putative AGN in NGC1614. In summary, we conclude that there is no need to invoke an AGN to explain the observed bolometric properties of the galaxy.Comment: 13 pages, 7 figures, 5 tables. Accepted for publication in Ap

A study of rotating globular clusters - the case of the old, metal-poor globular cluster NGC 4372

Aims: We present the first in-depth study of the kinematic properties and derive the structural parameters of NGC 4372 based on the fit of a Plummer profile and a rotating, physical model. We explore the link between internal rotation to different cluster properties and together with similar studies of more GCs, we put these in the context of globular cluster formation and evolution. Methods: We present radial velocities for 131 cluster member stars measured from high-resolution FLAMES/GIRAFFE observations. Their membership to the GC is additionally confirmed from precise metallicity estimates. Using this kinematic data set we build a velocity dispersion profile and a systemic rotation curve. Additionally, we obtain an elliptical number density profile of NGC 4372 based on optical images using a MCMC fitting algorithm. From this we derive the cluster's half-light radius and ellipticity as r_h=3.4'+/-0.04' and e=0.08+/-0.01. Finally, we give a physical interpretation of the observed morphological and kinematic properties of this GC by fitting an axisymmetric, differentially rotating, dynamical model. Results: Our results show that NGC 4372 has an unusually high ratio of rotation amplitude to velocity dispersion (1.2 vs. 4.5 km/s) for its metallicity. This, however, puts it in line with two other exceptional, very metal-poor GCs - M 15 and NGC 4590. We also find a mild flattening of NGC 4372 in the direction of its rotation. Given its old age, this suggests that the flattening is indeed caused by the systemic rotation rather than tidal interactions with the Galaxy. Additionally, we estimate the dynamical mass of the GC M_dyn=2.0+/-0.5 x 10^5 M_Sun based on the dynamical model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3 M_Sun/L_Sun, representative of an old, purely stellar population.Comment: Accepted for publication in A&A, 12 pages, 14 figures, 2 table

The Early Evolution of Massive Stars: Radio Recombination Line Spectra

Velocity shifts and differential broadening of radio recombination lines are used to estimate the densities and velocities of the ionized gas in several hypercompact and ultracompact HII regions. These small HII regions are thought to be at their earliest evolutionary phase and associated with the youngest massive stars. The observations suggest that these HII regions are characterized by high densities, supersonic flows and steep density gradients, consistent with accretion and outflows that would be associated with the formation of massive stars.Comment: ApJ in pres

AGB Sodium Abundances in the Globular Cluster 47 Tucanae (NGC 104)

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high resolution spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be =-18.56 km s^-1 (sigma=10.21 km s^-1) and =-0.68 (sigma=0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction <20% of Na-rich stars in 47 Tuc may fail to ascend the AGB. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. [abridged]Comment: Accepted for publication in the Astronomical Journal; 15 pages; 8 figures; 4 table

Revealing the mid-infrared emission structure of IRAS 16594-4656 and IRAS 07027-7934

TIMMI2 diffraction-limited mid-infrared images of a multipolar proto-planetary nebula IRAS 16594-4656 and a young [WC] elliptical planetary nebula IRAS 07027-7934 are presented. Their dust shells are for the first time resolved (only marginally in the case of IRAS 07027-7934) by applying the Lucy-Richardson deconvolution algorithm to the data, taken under exceptionally good seeing conditions (<0.5"). IRAS 16594-4656 exhibits a two-peaked morphology at 8.6, 11.5 and 11.7 microns which is mainly attributed to emission from PAHs. Our observations suggest that the central star is surrounded by a toroidal structure observed edge-on with a radius of 0.4" (~640 AU at an assumed distance of 1.6 kpc) with its polar axis at P.A.~80 degrees, coincident with the orientation defined by only one of the bipolar outflows identified in the HST optical images. We suggest that the material expelled from the central source is currently being collimated in this direction and that the multiple outflow formation has not been coeval. IRAS 07027-7934 shows a bright, marginally extended emission (FWHM=0.3") in the mid-infrared with a slightly elongated shape along the N-S direction, consistent with the morphology detected by HST in the near-infrared. The mid-infrared emission is interpreted as the result of the combined contribution of small, highly ionized PAHs and relatively hot dust continuum. We propose that IRAS 07027-7934 may have recently experienced a thermal pulse (likely at the end of the AGB) which has produced a radical change in the chemistry of its central star.Comment: 35 pages, 8 figures (figures 1, 2, 4 and 6 are in low resolution) accepted for publication in Ap