First Stellar Abundances in NGC 6822 from VLT‐UVES and Keck‐HIRES Spectroscopy

Venn, Kim A.; Lennon, D. J.; Kaufer, A.; McCarthy, James K.; Przybilla, N.; Kudritzki, Rolf‐Peter; Lemke, Michael; Skillman, Evan D.; Smartt, S. J.

doi:10.1086/318424

Cited by 125 publications

(177 citation statements)

References 52 publications

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“…The galaxy appears to have experienced a relatively steady rate of star formation over the last ∼11 Gyr, although there are signs of a recent increase (Clementini et al 2003;Komiyama et al 2003;Gallart et al 1996), and it has a large and well studied AGB population. NGC 6822 has been the subject of multiple studies to estimate its iron abundance (Venn et al 2001;Tolstoy et al 2001;Davidge 2003b;Clementini et al 2003;Kang et al 2006). Sibbons et al (2012; hereafter Paper I) estimated the global iron abundance of the AGB population of the galaxy from the calculated C/M ratio; using JHK photometry of a ∼3 deg 2 area centred on NGC 6822 (α(2000.0) = 19 h 44 m 56 s , δ(2000.0) = −14 • 48 06 ).…”

Section: Aims Of This Papermentioning

confidence: 99%

Spectral classification of photometrically selected AGB candidates in NGC 6822

Sibbons

Ryan

Napiwotzki

et al. 2015

A&A

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Context. 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. Aims. 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 4 kpc radial limit imposed in our previous work. Methods. Spectra of 323 sources, distributed across an area of 2 deg 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.Results. 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%. Conclusions. Based on the new JHK classification criteria, revised estimates of the global C/M ratio, 0.95 ± 0.04, and iron abundance, −1.38 ± 0.06 dex, are presented for NGC 6822.

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Section: Aims Of This Papermentioning

confidence: 99%

Spectral classification of photometrically selected AGB candidates in NGC 6822

Sibbons

Ryan

Napiwotzki

et al. 2015

A&A

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“…From this we obtain 12 + log (O/H) = 8.16 ± 0.05. On the other hand, from A-type supergiant stars, Venn et al (2001) derived the chemical abundance of iron to be 12 + log (Fe/H) = 7.01 ± 0.20. These data will be used as observational constraints for the present component of the ISM.…”

Section: Observational Constraintsmentioning

confidence: 99%

“…The present age of the galaxy is assumed to be 13.5 Gyr. In this table we also present the Fe/H value by Venn et al (2001) from A-type supergiants. Because most of the Fe in H ii regions and PNe is trapped in dust grains (Peimbert & Peimbert 2010;Shields 1978;Delgado-Inglada et al 2009), we do not present the gaseous Fe/H abundance.…”

Section: Observational Constraintsmentioning

confidence: 99%

Planetary nebulae as observational constraints in chemical evolution models for NGC 6822

Hernández-Martínez

Carigi

Peña

et al. 2011

A&A

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Aims. Chemical evolution models are useful for understanding the formation and evolution of stars and galaxies. Model predictions will be more robust when more observational constraints are used. We present chemical evolution models for the dwarf irregular galaxy NGC 6822 using chemical abundances of old and young planetary nebulae (PNe) and H ii regions as observational constraints. We use two sets of chemical abundances, one derived from collisionally excited lines (CELs) and one from recombination lines (RLs). We use our models as a tool to distinguish between both procedures for abundance determinations. Methods. In our chemical evolution code the chemical contribution of low and intermediate mass stars is time-delayed, while for the massive stars the chemical contribution follows the instantaneous recycling approximation. Our models have two main free parameters: the mass-loss rate of a well-mixed outflow and the upper mass limit, M up , of the initial mass function (IMF). To reproduce the gaseous mass and the present-day O/H value we need to vary the outflow rate and the M up value. Results. We calculate two models with different M up values that reproduce the constraints adequately. The abundances of old PNe agree with our models and support the star-formation history derived independently from photometric data. Both require an early wellmixed wind, lasting 5.3 Gyr, to reproduce the observed gaseous mass in the galaxy. In addition, by assuming a fraction of binaries producing SNIa of 1%, the models fit the Fe/H abundance ratio as derived from A supergiants. The first model (M4C), which assumes M up = 40 M , fits within errors smaller than 2σ the O/H, Ne/H, S/H, Ar/H and Cl/H abundances obtained from CELs for old and young PNe and H ii regions. The second model (M1R), which adopts M up = 80 M , reproduces within 2σ errors the O/H, C/H, Ne/H and S/H abundances adopted from RLs. Both models reproduce the increase of the O, Ne, S, and Ar elements during the last 6 Gyr. We are not able to match the observed N/O ratios in either case, which suggests that the N yields of LIMS need to be improved. Model M1R does not provide a good fit to the Cl/H and Ar/H ratios, because the SN yields of those elements for m > 40 M are not adequate and need to be improved (two sets of yields were tried). From these results we are unable to conclude which set of abundances (the one from CELs or the one from RLS) represents the real abundances in the ISM better. We discuss the predicted ΔY/ΔO values, finding that the value from model M1R agrees better with data for other galaxies from the literature than the value from model M4C.

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“…The galaxy is embedded in a large HI envelope (Weldrake et al 2003). Previous spectroscopic studies, although incomplete, of supergiants (Venn et al 2001), HII regions (Chandar et al 2000), and red giant branch (RGB) stars (Tolstoy et al 2001) found evidence of a gradient in [O/H] and measured an average [Fe/H] = −0.9 dex with a spread of 1.5 dex. Most importantly, this galaxy has a large and widely distributed intermediate-age population (Battinelli et al 2006) with a clear spread in metallicity (Cioni & Habing 2005).…”

Section: Introductionmentioning

confidence: 99%

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

Kacharov

Rejkuba²,

Cioni

2012

A&A

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Aims. We calibrate spectroscopically the C-versus (vs.) M-type asymptotic giant branch (AGB) star selection made using near-IR photometry, and investigate the spatial distribution of the C/M ratio in NGC 6822, based on low resolution spectroscopy and near-IR photometry. Methods. We obtained low resolution multi-object spectroscopy with the VIMOS instrument at the ESO VLT of ∼800 stars in seven fields centred on NGC 6822. The spectroscopic classification of giant stars in NGC 6822 and foreground dwarf contaminants was made by comparing more than 500 good quality spectra with the spectroscopic atlas of Turnshek et al. (1985). The sample of spectroscopically confirmed AGB stars in NGC 6822 is divided into C-and M-rich giants to constrain the C vs. M AGB star selection criteria based on photometry. The larger near-IR photometric sample is then used to investigate the C/M ratio gradients across the galaxy.Results. We present the largest catalogue of near-IR photometry and spectra of AGB stars in NGC 6822 with 150 C-stars and 122 M-stars. Seventy-nine percent of the C-stars in our catalogue are redder than (J − K) 0 = 1.2 mag, and 12% are brighter than K 0 = 16.45 mag and bluer than (J − K) 0 = 1.2 mag. The remaining 9% are mixed with the M-type AGB stars, 88% of which have colours (J − H) 0 > 0.73 mag and (J − K) 0 between 0.9 mag and 1.2 mag. The remainder are mixed with dwarfs and C-type stars. The foreground dwarfs have preferably colours (J − H) 0 < 0.73 mag (95%). Using the proposed criteria, we estimate that the overall C/M ratio of the galaxy is around 0.8 with a spread between 0.2 < C/M < 1.8. These results suggest that the metallicity index [Fe/H] is between −1.2 dex and −1.3 dex according to the different calibrations and that there is a significant spread of about 0.4 ÷ 0.6 dex. We also discuss age rather than metallicity variations that could explain the C/M ratio trends.

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First Stellar Abundances in NGC 6822 from VLT‐UVES and Keck‐HIRES Spectroscopy

Abstract: and was made possible by the generous financial support of the W. M. Keck Foundation.

Cited by 125 publications

References 52 publications

Spectral classification of photometrically selected AGB candidates in NGC 6822

Spectral classification of photometrically selected AGB candidates in NGC 6822

Planetary nebulae as observational constraints in chemical evolution models for NGC 6822

Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

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