The Extent of NGC 6822 Revealed by Its C Star Population

Letarte, B.; Demers, S.; Battinelli, P.; Kunkel, W. E.

doi:10.1086/338319

Cited by 87 publications

(209 citation statements)

References 38 publications

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“…Furthermore, a recent M 31 outer disk survey, using Sloan r , g and i filters, also suggests that bright C stars are lacking in the outskirt of the disk (Demers & Battinelli, in preparation). Such magnitude trend is also seen in NGC 6822 (Letarte et al 2002), albeit less pronounced because of the much shorter distance range. The bright limit of the apparent magnitudes of the LMC C stars, compiled by Kontizas et al (2001) shows, likewise, an obvious radial trend, over some 8 degrees.…”

Section: Discussionmentioning

confidence: 58%

“…The zero-point of the narrow band observations was obtained setting the average (CN-TiO) of blue stars equal to zero following a procedure similar to what adopted by Brewer et al (1995). This procedure is explained by Letarte et al (2002) and in the present paper we follow their definition of blue stars as stars in the colour range 0.0 < (R − I) 0 < 0.45. To convert observed (R − I) colours into intrinsic colours we adopt E(B − V) = 0.10 corresponding to the value for the SW2 field of Battinelli et al (2003).…”

Section: Discussionmentioning

confidence: 99%

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Exploratory C star search in the disk of M 31 beyond 30 kpc

Battinelli

Demers

2005

A&A

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Abstract. In order to investigate how far the intermediate-age population of the disk of M 31 extends we survey 450 arcmin 2 along its southern major axis, from 35 kpc to 43 kpc. We identified a handful of C stars. We conclude that we have reached the edge of the M 31 disk, at least as defined by intermediate age stars. There is no evidence of density enhancement AGB stars related to the Ferguson clump possibly associated with cluster G1. Radial velocity measures could provide confirmation that the most remote C star, at 40 kpc, is indeed a member of the disk population of M 31.

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Section: Discussionmentioning

confidence: 58%

Section: Discussionmentioning

confidence: 99%

Exploratory C star search in the disk of M 31 beyond 30 kpc

Battinelli

Demers

2005

A&A

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“…Candidate C-and M-type AGB stars were classified on the basis of their magnitudes and NIR colours to obtain the C/M ratio. However, a comparison of the classifications in Paper I with those in the work of Letarte et al (2002), who relied on the CN-TiO method to classify sources, suggested that up to ∼20% of the C-type star population may have been misclassified as M-type stars by the derived JHK criteria. A review of the estimated error in source classification in Paper I and the level of misclassification in the catalogue of Letarte et al (2002), which was previously assumed to be 100% accurate, are therefore the secondary and tertiary aims of this work.…”

Section: Aims Of This Papermentioning

confidence: 94%

“…However, a comparison of the classifications in Paper I with those in the work of Letarte et al (2002), who relied on the CN-TiO method to classify sources, suggested that up to ∼20% of the C-type star population may have been misclassified as M-type stars by the derived JHK criteria. A review of the estimated error in source classification in Paper I and the level of misclassification in the catalogue of Letarte et al (2002), which was previously assumed to be 100% accurate, are therefore the secondary and tertiary aims of this work. Using spectra of a sample of candidate C-and M-type AGB stars common to the catalogues of Paper I and Letarte et al (2002), the present work is the first to use spectra to examine sources that have been classified using both JHK photometry and CN-TiO data, with the object of quantifying the level of error associated with each technique.…”

Section: Aims Of This Papermentioning

confidence: 94%

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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“…Its optical structure is dominated by a bar that is 8 long (about 1.1 kpc), oriented almost N-S, and it contains a huge disk of H  of about 6 × 13 kpc at a position angle (PA) of 130 • centred on the optical centre (de Blok & Walter 2000;Weldrake et al 2003). In addition, a huge spheroidal stellar distribution with its long axis at PA of 64.5 • (almost perpendicular to the H  disk), composed mainly of intermediate-age stars, has been found (Letarte et al 2002;Battinelli et al 2006). Interestingly, the H  disk and the stellar spheroid do not share the same kinematics.…”

Section: Introductionmentioning

confidence: 99%

The planetary nebulae and H II regions in NGC 6822 revisited. Clues to AGB nucleosynthesis

García‐Rojas

Peña

Flores-Durán

et al. 2016

A&A

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Aims. The chemical behaviour of an ample sample of planetary nebulae (PNe) in NGC 6822 is analysed.Methods. Spectrophotometric data of 11 PNe and two H  regions were obtained with the OSIRIS spectrograph attached to the Gran Telescopio Canarias. Data for other 13 PNe and three H  regions were retrieved from the literature. Physical conditions and chemical abundances of O, N, Ne, Ar, and S were derived in a consistent way for 19 PNe and 4 H  regions.Results. Abundances in the PNe sample are widely distributed showing 12 + log (O/H) from 7.4 to 8.2 and 12 + log (Ar/H) from 4.97 to 5.80. Two groups of PNe can be differentiated: one old with low metallicity (12 + log (O/H) < 8.0 and 12 + log (Ar/H) < 5.7) and another younger one with metallicities similar to the values for H  regions. The old objects are distributed in a larger volume than the young ones. An important fraction of PNe (over 30%) was found to be highly N-rich (Peimbert Type I PNe). Such PNe occur at any metallicity. In addition, about 60% of the sample presents high ionization (He ++ /He ≥ 0.1), possessing a central star with effective temperature higher than 100 000 K. Possible biases in the sample are discussed. From comparison with stellar evolution models by Karakas (2010) and Fishlock et al. (2014) of the observed N/O abundance ratios, our PNe should have had initial masses that are lower than 4 M , although if the comparison is made with Ne vs. O abundances, the initial masses should have been lower than 2 M . It appears that these models of stars of 2−3 M are producing too much 22 Ne in the stellar surface at the end of the AGB. On the other hand, the comparison with another set of stellar evolution models with a different treatment of convection and on the assumptions about the overshoot of the convective core during the core H-burning phase, provided there is reasonable agreement between the observed and predicted N/O and Ne/H ratios if initial masses of more massive stars are about 4 M .

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The Extent of NGC 6822 Revealed by Its C Star Population

Cited by 87 publications

References 38 publications

Exploratory C star search in the disk of M 31 beyond 30 kpc

Exploratory C star search in the disk of M 31 beyond 30 kpc

Spectral classification of photometrically selected AGB candidates in NGC 6822

The planetary nebulae and H II regions in NGC 6822 revisited. Clues to AGB nucleosynthesis

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