J/A+A/574/A102 AGB candidates in NGC 6822 (Sibbons+, 2015)
Spectral classification of photometrically selected AGB candidates in NGC 6822.
Sibbons L.F., Ryan S.G., Napiwotzki R., Thompson G.P.
<Astron. Astrophys., 574, A102-102 (2015)>
=2015A&A...574A.102S 2015A&A...574A.102S
ADC_Keywords: Galaxies, nearby ; Stars, giant ; Photometry, infrared ;
MK spectral classification
Keywords: techniques: photometric - techniques: spectroscopic -
stars: AGB and post-AGB - stars: carbon - galaxies: dwarf -
galaxies: stellar content
Abstract:
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 2deg2, 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.
Description:
Target sources have been selected from potential AGB candidates that
were identified using JHK photometry in Paper I (Sibbons et al. 2012,
J/A+A/540/A135). The spectroscopic observations were arried out in
service mode on the nights of 2011-08-30 and 31 using the AAOmega
multi-fibre spectrograph at the 3.9m Anglo-Australian Telescope (AAT).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 101 15 Futher 11 inner region and 4 outer region sources
that show high excitation [OIII] emission lines
table10.dat 101 74 74 candidate AGB sources in the inner region (≤3kpc)
for which spectra with S/N≥10 were obtained
table11.dat 101 73 73 candidate AGB sources in the outer region
(≤5kpc) for which spectra have been obtained
table12.dat 101 43 43 candidate AGB sources in the inner and outer
regions for which we are able to offer a tentative
classification despite the low S/N of the spectra
table13.dat 28 26 Spectral source counterpart IDs
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See also:
J/A+A/540/A135 : AGB population of NGC 6822 (Sibbons+, 2012)
Byte-by-byte Description of file: table4.dat table10.dat table11.dat table12.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- Reg [io] inner or outer region of NGC 6822
2 A1 --- f_ID [*] Misclassified C-type sources (1)
4- 9 I6 --- ID Source identification number
11- 13 A3 --- n_ID [1-4, ] Source in other references (2)
15- 24 F10.6 deg RAdeg Right ascension (J2000.0)
26- 35 F10.6 deg DEdeg Declination (J2000.0)
37- 41 F5.2 mag Jmag ?=- Reddening corrected J magnitude
43- 47 F5.2 mag Hmag ?=- Reddening corrected H magnitude
49- 53 F5.2 mag Kmag ?=- Reddening corrected K magnitude
55- 58 A4 --- phT Photometric type (C, M or Unclear)
59 A1 --- n_phT [*] * for rejected from the photometric sample
in Paper I (3)
61- 64 F4.2 kpc Dist Radial distance from NGC 6822 center
66- 69 A4 --- spT Spectroscopic type (Uncl=Unclear) (4)
71- 75 A5 --- spTK Kang et al. (2006, J/A+A/454/717) type (4)
77- 83 A7 --- spTD Demers et al. (2006, J/A+A/546/905) type (4)
84- 86 I3 --- [LDB2002] [1/900]? Identification number in Letarte
et al. 2002 (J/AJ/123/832)
88- 92 F5.3 mag R-I ?=- R-I colour index
94- 98 F5.3 mag CN-TiO ?=- CN-TiO index
100-101 I2 --- S/N [6/70] Signal-to-noise ratio (<10 for table12)
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Note (1): * = C-type sources misclassified as dK or dK7 by the
matching program from the rest of the sample.
Note (2): Reference number for source in common (in table13) with the
catalogues:
1 = Kang et al. 2006, Cat. J/A+A/454/717),
2 = Demers et al. 2006, Cat. J/A+A/546/905),
3 = Whitelock et al. 2013, Cat. J/MNRAS/428/2216)
4 = Kacharov et al. 2012, Cat. J/A+A/537/A108)
Note (3): sources marked with a * were rejected from the photometric sample
in Paper I (Sibbons et al., 2012, Cat. J/A+A/540/A135) as they did not meet
their strict quality control criterion but nevertheless could be classified
by them.
Note (4): Source classifications preceded by a "d" denote dwarf stars,
"e" indicates the presence of Hα in emission - many also show [SII]
and [NII] (see Sect. 4.1.2) and a colon ":" indicates a tentative
classification.
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Byte-by-byte Description of file: table13.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 I6 --- ID Source identification number
8- 10 I3 --- [KSK2006] ?=- Kang et al. 2006, J/A+A/454/717 ,number,
<[KSK2006] C NNN> in Simbad
12- 15 I4 --- [DBA2006] ?=- Demers et al. 2006, J/A+A/546/905) number,
<[DBA2006] NNN> in Simbad
17- 22 I6 --- [KRC2012] ?=- Kacharov et al. 2012, J/A+A/537/A108)
number, <[KRC2012] NNNNNN> in Simbad
24- 28 I5 --- [WMF2013] ?=- Whitelock et al. 2013, J/MNRAS/428/2216
number, <[WMF2013] NNNNN> in Simbad
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History:
From electronic version of the journal
References:
Sibbons et al., Paper I 2012A&A...540A.135S 2012A&A...540A.135S, Cat. J/A+A/540/A135
(End) Patricia Vannier [CDS] 09-Mar-2015