The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures >3000 to 10000K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6+/-1.4km/s. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere.We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.
We herein determine the average integrated mass-loss from stars belonging to the dominant metal-poor population ([Fe/H]~-1.7) of the Galactic globular cluster {omega} Centauri (NGC 5139) during their red giant and horizontal branch (HB) evolution.
We present the iron abundance of 24 asymptotic giant branch (AGB) stars, members of the globular cluster 47 Tucanae, obtained with high-resolution spectra collected with the FEROS spectrograph at the MPG/ESO 2.2m Telescope. To test the reliability of the chemical analysis, 11 red giant branch (RGB) stars were also studied.
The ratio of C- and M-type asymptotic giant branch (AGB) stars is commonly used to estimate the metallicity of extragalactic populations. Sources in the AGB population must therefore be accurately classified as either C- or M-type. Spectroscopic data are presented for candidate C- and M-type AGB stars, previously classified using JHK photometry, in the Local Group dwarf galaxy NGC 6822. This paper aims to evaluate the success of the JHK classification criteria used in order to determine the level of error associated with this method, and to refine the criteria for future studies. The success rate of a second independent method of source classification, the CN-TiO method, is also examined. We also review the validity of the 4kpc radial limit imposed in our previous work. Spectra of 323 sources, distributed across an area of 2deg^2^, were taken using the AAOmega multi-fibre spectrograph on the Anglo-Australian Telescope and have been classified using an automated classification system and spectral standards from the literature. Nearly half (135) of these sources were selected in common with a photometric catalogue that relied on the CN-TiO method. Within this sample we were able to classify 158 sources, including 82 C-type giants and one anomalous M-type giant, all members of NGC 6822, and 75 foreground K- and M-type dwarf sources. All but three of the giant sources are located within 3 kpc of the galactic centre. Using this spectroscopic sample, new JHK photometric criteria for the isolation and classification of C- and M-type AGB stars have been derived. The error rate in the CN-TiO method, arising from stars incorrectly classified as C-type, has been estimated to be ~7%. Based on the new JHK classification criteria, revised estimates of the global C/M ratio, 0.95+/-0.04, and iron abundance, -1.38+/-0.06dex, are presented for NGC 6822.
NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close proximity and apparent isolation provide a unique opportunity to study galactic evolution without any obvious strong external influences. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population and metallicity in NGC 6822. Using deep, high quality JHK photometry, taken with WFCAM on UKIRT, carbon- and oxygen-rich AGB stars have been isolated. The ratio between their number, the C/M ratio, has then been used to derive the [Fe/H] abundance across the galaxy.
We investigate the resolved stellar content of early-type dwarf galaxies in the Centaurus A group, to estimate their intermediate-age population fractions. We use near-infrared photometric data taken with the VLT/ISAAC instrument, together with previously analyzed archival HST/ACS data. The combination of the optical and infrared wavelength range permits us to firmly identify luminous asymptotic giant branch stars, which are indicative of an intermediate-age population in these galaxies. We consider one dwarf spheroidal (CenA-dE1) and two dwarf elliptical (SGC1319.1-4216 and ESO269-066) galaxies that are dominated by an old population. The most recent periods of star formation are estimated to have taken place between 2 and 5Gyr ago for SGC1319.1-4216 and ESO269-066, and approximately 9Gyr ago for CenA-dE1. For ESO269-066, we find that the intermediate-age populations are significantly more centrally concentrated than the predominantly old underlying stars. The intermediate-age population fraction is found to be small in the target galaxies, consistent with fractions of up to 15% of the total population. These values might be larger by a factor of two or three, if we considered the observational limitations and the recent discussion about the uncertainties in theoretical models. We suggest that there is a correlation between intermediate-age population fraction and proximity to the dominant group galaxy, with closer dwarfs having slightly smaller fractions, although our sample is too small to draw firm conclusions. Even when considering our results as lower limits, the intermediate-age population fractions for the studied dwarfs are clearly much smaller than those found in similar dwarfs around the Milky Way, but comparable to what is seen for the low-mass M31 companions. Our results confirm our previous work about early-type dwarfs in the Centaurus A group.
We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made with the Infrared Survey Facility, over an area of about 42x42arcmin^2^. The colour-magnitude diagram shows a broad well-populated giant branch with a tip that slopes downwards from red to blue, as might be expected given Fornax's known range of age and metallicity. The extensive asymptotic giant branch (AGB) includes seven Mira variables and 10 periodic semiregular variables. Five of the seven Miras are known to be carbon rich. Their pulsation periods range from 215 to 470d, indicating a range of initial masses. Three of the Fornax Miras are redder than typical Large Magellanic Cloud (LMC) Miras of similar period, probably indicating particularly heavy mass-loss rates. Many, but not all, of the characteristics of the AGB are reproduced by isochrones from Marigo et al. (2008A&A...482..883M) for a 2Gyr population with a metallicity of Z=0.0025.
We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the near-UV (NUV) bandpass ({lambda}_eff_~2310{AA}), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the far-UV (FUV) bandpass ({lambda}_eff_~1528{AA}). We find that NUV emission is correlated with optical to near-infrared emission, leading to higher detection fractions among the brightest, and hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission and possibly the FUV emission are anticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.
It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anticorrelations of light-element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here, we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 red giant branch (RGB) and eight AGB stars, deriving Fe, Na, O, Mg, and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M4 (with contrasting results), since the same tools and methods were used.
As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination of {delta}>-45{deg}, which corresponds to 266{deg}>l>-20{deg} at b=0{deg}. All sources were selected from the Wide-field Infrared Survey Explorer Catalog of Galactic HII Regions, and have infrared angular diameters >=260". The Galactic distribution of these "large" HII regions is similar to that of the previously known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values, compared with that of previous HRDS surveys. We discover seven sources with extremely narrow RRLs <10km/s. If half the line width is due to turbulence, these seven sources have thermal plasma temperatures <1100K. These temperatures are lower than any measured for Galactic HII regions, and the narrow-line components may arise instead from partially ionized zones in the HII region photodissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the size of the previously known census of Galactic HII regions.
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