J/A+A/601/A31 Reduced spectra of NGC 6528 stars (Liu+, 2017)
Determination of robust metallicities for metal-rich red giant branch stars.
An application to the globular cluster NGC 6528.
Liu C., Ruchti G., Feltzing S., Primas F.
<Astron. Astrophys. 601, A31 (2017)>
=2017A&A...601A..31L 2017A&A...601A..31L (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, globular ; Stars, normal ; Spectroscopy
Keywords: Galaxy: bulge - globular clusters: individual: NGC 6528 -
stars: atmospheres - stars: fundamental parameters
Abstract:
Here we explore robust ways of determining the iron content of
metal-rich giant stars. We aim to understand what biases and
shortcomings the widely applied methods suffer from.
In this study we were mainly concerned with standard methods of
analysing stellar spectra. These include the analysis of individual
lines to determine stellar parameters, and analysis of the broad wings
of certain lines (e.g. Halpha and calcium lines) to determine
effective temperature and surface gravity for the stars.
For NGC 6528 we find that [Fe/H]=+0.04dex with a scatter of 0.07dex,
which gives an error in the derived mean abundance of 0.02dex.
Our work has two important conclusions for analysis of metal-rich red
giant branch stars. Firstly, for spectra with S/N of below about 35
per reduced pixel, [Fe/H] becomes too high. Secondly, determination of
Teff using the wings of the Hα line results in [Fe/H] values
about 0.1dex higher than if excitational equilibrium is used. The last
conclusion is perhaps unsurprising, as we expect the NLTE effect to
become more prominent in cooler stars and we can not use the the wings
of the Hα line to determine Teff for the cool stars in our
sample. We therefore recommend that in studies of metal-rich red giant
stars care should be taken to obtain sufficient calibration data to
enable use of the cooler stars.
Description:
The 1D spectra of globular cluster NGC 6528 stars could be directly
used to measure stellar parameters and elemental abundances, and/or as
benchmarks to calibrate metal-rich populations in bulge.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 121 75 Basic information for all stars analysed in
this study
list.dat 121 66 List of gfits spectra
fits/* . 66 Individual fits spectra
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See also:
J/AJ/84/1305 : UBV photometry in NGC 6528 (van den Bergh+, 1979)
J/A+AS/127/167 : V,I CCD photometry of NGC 6528 (Richtler+ 1998)
J/A+A/385/67 : NGC 6528 HST VI photometry & proper motions (Feltzing+, 2002)
J/A+A/565/A8 : Stroemgren photometry near NGC 6528 (Calamida+, 2014)
J/A+A/573/A13 : Metal abundances in 6 globular cluster (Dias+, 2015)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Seq [1/34] Sequential number of the source
4- 7 A4 --- ID Identification number of the source
9- 32 A24 --- Name Source name (2MASS or VVV)
34- 35 I2 h RAh Right ascension (J2000)
37- 38 I2 min RAm Right ascension (J2000)
40- 44 F5.2 s RAs Right ascension (J2000)
46 A1 --- DE- Declination sign (J2000)
47- 48 I2 deg DEd Declination (J2000)
50- 51 I2 arcmin DEm Declination (J2000)
53- 54 I2 arcsec DEs Declination (J2000)
56- 60 F5.2 mag Jmag0 ?=- De-reddened J magnitude (1)
62- 66 F5.2 mag Hmag0 ?=- De-reddened H magnitude (1)
68- 73 F6.3 mag Kmag0 ?=- De-reddened K magnitude (1)
75- 81 F7.2 km/s RV Radial velocity
83- 86 F4.2 km/s e_RV rms uncertainty on RV
87 A1 --- n_RV [c] Note on RV (2)
89- 98 A10 "date" Obs.Date Observation date
100-105 A6 s ExpTime Exposure time
107-117 A11 --- Inst Instrument
119-121 I3 --- S/N Signal-to-noise ratio measured from the
spectrum (3)
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Note (1): De-reddened magnitudes have been derived adopting the reddening law by
Cardelli et al. (1989ApJ...345..245C 1989ApJ...345..245C).
Note (2): c: The radial velocity is adopted from
Carretta et al. (2001AJ....122.1469C 2001AJ....122.1469C).
Note (3): The average S/N of the final co-added spectrum for each star was
estimated using the SPLOT task within IRAF at three short wavelength regions
(574.4-574.7nm, 604.7-606.3nm, and 606.8-607.6nm).
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Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Seq [1/34] Sequentail number of the source
4- 8 I5 --- Nx Number of pixels along X-axis
10- 32 A23 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.sss)
34- 40 F7.2 0.1nm blambda Lower value of wavelength interval
42- 48 F7.2 0.1nm Blambda Upper value of wavelength interval
50- 58 F9.7 0.1nm dlambda Resolution
60- 62 I3 Kibyte size Size of FITS file
64- 84 A21 --- FileName Name of FITS file, in subdirectory fits
86-121 A36 --- Title Title of the FITS file
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Acknowledgements:
Cheng Liu, cheng(at)astro.lu.se
References:
Zoccali et al., 2004A&A...423..507Z 2004A&A...423..507Z, Metal content of NGC 6528
(End) Cheng Liu [Lund Obs., Sweden], Patricia Vannier [CDS] 16-Feb-2017