Spectral classification of photometrically selected AGB candidates in NGC 6822

L. F. Sibbons, S. G. Ryan, R. Napiwotzki, G. P. Thompson
2015 Astronomy and Astrophysics  
Context. Spectroscopic data are presented for 162 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 − T iO method, is also examined. We also
more » ... ined. 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 an automated classification system and spectral standards from the literature. 135 of these sources were selected in common with the photometric catalogue of Letarte et al. (2002) which relied on the CN − T iO method. Results. Within this sample we were able to classify 162 sources, including 82 C-type giants and 1 anomalous M-type giant, all members of NGC 6822, and a further 75 foreground K-and M-type dwarf sources. All but 3 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 − T iO 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. 2 Sibbons et al: AGB Population in NGC 6822 Applying the following colour and magnitude criteria to our photometric data nearly 3800 potential C-and Mtype AGB sources were identified in NGC 6822; K 0 ≤ 17.41 mag, (J−K) 0 ≥ 0.74 mag and (J−H) 0 ≥ 0.72 mag. The colour boundary between the C-and M-type sources was set at (J − K) 0 = 1.20 mag. An examination of the stellar density profiles of the AGB and red giant branch (RGB) candidate sources led us to conclude that the strongest AGB candidates lay within 4 kpc of the galactic centre. Beyond that limit it was inferred that the density of the NGC 6822 sources had dropped to a level that made it impossible to reliably disentangle them from the remaining MW foreground contamination. Therefore the candidate AGB sources were split into two groups: (1) those within 4 kpc of the galactic center (2368 sources), and (2) those beyond 4 kpc (1387 sources). We will discuss the sources selected for follow up spectroscopy in the context of these two groups. As the spectral data were collected using a multiobject fibre-fed spectrograph (see Sect. 2.2), we were able to target ∼ 400 sources. The 2, 368 candidate AGB stars in the inner region were cross-matched with the catalogue of Letarte et al. (2002) who had previously classified 904 C-type AGB stars in a small field lying fully within the 4 kpc radial limit, using the CN − T iO method; 635 sources were found to be common to both catalogues. From this sample of 635 stars, 68 sources within 3 kpc (all classified as M-type in Paper I) were selected for spectroscopic follow up. A further 67 sources also within 3 kpc (46 C-type, 15 M-type and six unclassified according to Paper I) were selected from the JHK photometric counterparts for another 235 sources belonging to the catalogue of Letarte et al. (2002) . These sources were included in our target list as although they were astrometrically matched with sources in the catalogue of Letarte et al. (2002) they were excluded from our analysis in Paper I, by the stringent quality control criterion applied, which resulted in the removal of some genuine AGB stars from the sample. The six sources that were unclassified in Paper I were without a magnitude measurement in at least one of the three photometric bands. A total of 135 sources were selected for spectroscopic follow up in the inner region, all of which had been classified as C-type stars by Letarte et al. (2002) . The majority of sources in the outer region were expected to be foreground interlopers; 188 sources all at a radial distance of ≥ 5 kpc were selected. The vast majority of candidate AGB sources in the outer region were classified as M-type; this is reflected in the photometric classifications of the target outer 188 sources (178 M-type and 10 C-type). Our reasoning for only selecting target sources at ≤ 3 kpc or ≥ 5 kpc is explained below. The RA and Dec positions quoted for our spectroscopic targets are taken directly from the catalogues of Paper I. AAOmega data
doi:10.1051/0004-6361/201423981 fatcat:37kpxkt5wjfe7nmmr5wuye4afm