J/A+A/635/A161 Measured atmospheric parameters of NGC6397 stars (Jain+, 2020)
NGC 6397: The metallicity trend along the isochrone revisited.
Jain R., Prugniel P., Martins L., Lancon A.
<Astron. Astrophys. 635, A161 (2020)>
=2020A&A...635A.161J 2020A&A...635A.161J (SIMBAD/NED BibCode)
ADC_Keywords: Models, atmosphere; Clusters, globular; Spectroscopy ;
Effective temperatures ; Abundances, [Fe/H]
Keywords: methods: data analysis - techniques: spectroscopic -
stars: fundamental parameters
Abstract:
Recent work has used spectra of ∼5000 stars in NGC 6397 that were
extracted from a MUSE mosaic to determine the atmospheric parameters
for these stars by fitting the spectra against the Goettingen Spectral
Library. A significant change in metallicity between the turn off and
the red giant branch was found and was discussed as a possible
manifestation of predicted effects of atomic diffusion. However, the
small amplitude of the effect and inconsistency with earlier
measurements call for more attention before this result is
interpreted. Systematic effects due to the interpolation or to the
synthetic spectra cannot be ruled out at this level of precision.
We reanalyze the data with : the ELODIE and MILES reference libraries
in order to assess the robustness of the result. These empirical
libraries have a finer metallicity coverage down to approximately the
cluster metallicity turn-off. Methods. We used the ULySS full-spectrum
fitting package, together with the library interpolators to remeasure
the three atmospheric parameters effective temperature, surface
gravity, and [Fe/H] metallicity.
We find a very low [Fe/H] dispersion along the isochrone (0.07dex),
consistent with our error estimate (0.05dex). However, the [Fe/H]
trend is not reproducible. This shows that the data have the potential
to reveal patterns of the magnitude of the expected physical effects,
but the analysis methods need to be refined to cull systematic effects
that currently dominate the patterns.
Description:
The table contains the atmospheric parameters determined for stars of
NGC 6397. The spectra of these stars were observed using MUSE
spectrograph by Husser et al. 2016. We use the spectra of these stars
to estimate atmospheric parameters using ELODIE and MILES libraries.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table.dat 113 1587 Measured atmospheric parameters of NGC6397 stars
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See also:
J/AJ/133/1658 : ACS survey of galactic globular clusters (Sarajedini+, 2007)
J/A+A/588/A148 : NGC 6397 stars MUSE spectra (Husser+, 2016)
https://archive.stsci.edu/prepds/acsggct : The ACS Globular Cluster Survey
(astrometric and photometric catalogue)
http://muse-vlt.eu/science : MUSE science-verification data (extracted spectra)
Byte-by-byte Description of file: table.dat
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Bytes Format Units Label Explanations
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1- 8 I8 --- ID Designation of the star from Husser et al.
(2016, Cat. J/A+A/588/A148) (1)
10- 22 F13.9 deg RAdeg Star right ascension (J2000)
24- 35 F12.8 deg DEdeg Star declination (J2000)
37- 41 F5.3 [cm/s2] loggphot Surface gravity of the star
(determined photometrically)
43- 46 I4 K TeffE Effective temperature of the star predicted by
ELODIE library
48- 49 I2 K e_TeffE Error in TeffE
51- 55 F5.2 [-] [Fe/H]E Metallicity of the star predicted by
ELODIE fit
57- 60 F4.2 [-] e_[Fe/H]E Error in [Fe/H]E
62- 65 I4 K TeffM Effective temperature of the star predicted by
MILES library
67- 69 I3 K e_TeffM Error in TeffM
71- 75 F5.2 [-] [Fe/H]M Metallicity of the star predicted by MILES fit
77- 80 F4.2 [-] e_[Fe/H]M Error in [Fe/H]M
82- 87 F6.1 K Teffmean Mean of TeffE and TeffM
89- 93 F5.1 K e_Teffmean Error on the Teffmean
95-113 A19 --- GaiaDR2 GAIA star ID (2)
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Note (1): The ID of the star is taken from the header of the spectra and it is
defined as the ID from the reference photometric catalog. The reference
photometric catalog is from the ACS Globular Cluster Survey (Anderson et al.,
2008AJ....135.2055A 2008AJ....135.2055A).
The field of MUSE is larger than the ACS field so not all the stars have an
ACS ID.
For the stars that are a part of the ACS catalog the IDs are taken from there,
for other stars, it is a 9 digit number, the first two indicating the
pointing identifier (see http://muse-vlt.eu/science/globular-cluster-ngc-6397)
Note (2): We cross-identified the list of objects with Gaia, comparing the ACS
position given in the columns RAJ2000 and DEJ2000 with GAIA DR2 using the CDS
cross-match service.
The cross-identification in the crowded central area of the cluster is
challenging, in particular at the spatial resolution of MUSE. We tuned the
details of the cross-identification to ensure that less than 3/1000 of the
stars with repeated observations (those in the overlap between the pointings)
are identified with different GAIA stars.
The service queries the epoch-2000 coordinates calculated from the barycentric
ICRS coordinates at epoch 2015.5 and the proper motions when available.
The cross-match was carried out using a search radius 0.5 arcsec. We found
matches for 1540 stars. The angular separation of the stars that found the
matches varies from 0.01 to 0.4 arcsecond. There is a bimodal distribution in
the angular separation of the stars. The majority of the matches are within
0.2 arcsecond and we select stars that lie within 2sigma of the mean of this
main distribution (that is separation is confined to 0.05-0.15 arcseconds).
There is a secondary peak with smaller number of stars which have a systematic
shift of 0.3-0.4 arcseconds.
These stars also belong to the cluster but GAIA does not have the proper
motion information of these stars and thus they are transformed to J2000
coordinates without proper motion corrections. In the secondary peak, we
also select stars within 2sigma of the mean position offset (189 stars).
The corresponding GAIA ID of all these stars is in the last column of the
table.
The ID in the second column is as described in Husser et al.
(2016A&A...588A.148H 2016A&A...588A.148H), i.e. based on an initial catalog by Anderson et al.
(2008AJ....135.2055A 2008AJ....135.2055A). In the original data set there are multiple
observations for some stars and these observations in some instances are not
coincident on RA-Dec plane. Almost all of these repeated observations are
tied to the same GAIA stars. In table.dat these stars have been assigned the
same IDs by Husser and thus can be identified as being duplicates.
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Acknowledgements:
Rashi Jain, r.jain(at)unistra.fr
(End) Rashi Jain [Strasbourg Observatory], Patricia Vannier [CDS] 10-Feb-2020