Facing problems in the determination of stellar temperatures and gravities: Galactic globular clusters

We analysed red giant branch stars in 16 Galactic globular clusters, computing their atmospheric parameters both from the photometry and from excitation and ionisation balances. The spectroscopic parameters are lower than the photometric ones and this discrepancy increases with decreasing metallicity, reaching differences of ~350 K in effective temperature and ~1 dex in surface gravity at [Fe/H] ~ –2.5 dex. We demonstrate that the spectroscopic parameters are inconsistent with the position of the stars in the colour-magnitude diagram, providing overly low temperatures and gravities, and predicting that the stars are up to about 2.5 magnitudes brighter than the observed magnitudes. The parameter discrepancy is likely due to inadequacies in the adopted physics; in particular the assumption of a one-dimensional geometry could be the origin of the observed slope between iron abundances and excitation potential that leads to low temperatures. However, the current modelling of 3D/NLTE radiative transfer for giant stars seems to be unable to totally erase this slope. We conclude that the spectroscopic parameters are incorrect for metallicity lower than –1.5 dex and that photometric temperatures and gravities should be adopted for these red giant stars. We provide a simple relation to correct the spectroscopic temperatures in order to put them onto a photometric scale.

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The GOTHAM survey: chemical evolution of Milky Way globular clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317007086 ◽

2017 ◽

Vol 13 (S334) ◽

pp. 25-28

Author(s):

Bruno Dias ◽

Beatriz Barbuy ◽

Ivo Saviane ◽

Enrico V. Held ◽

Gary Da Costa ◽

...

Keyword(s):

Chemical Evolution ◽

Globular Cluster ◽

Globular Clusters ◽

Stellar Population ◽

Milky Way ◽

Open Clusters ◽

Giant Stars ◽

Globular Cluster System ◽

Galactic Globular Clusters ◽

Red Giant

AbstractMilky Way globular clusters are excellent laboratories for stellar population detailed analysis that can be applied to extragalactic environments with the advent of the 40m-class telescopes like the ELT. The globular cluster population traces the early evolution of the Milky Way which is the field of Galactic archaeology. We present our GlObular clusTer Homogeneous Abundance Measurement (GOTHAM) survey. We derived radial velocities, Teff, log(g), [Fe/H], [Mg/Fe] for red giant stars in one third of all Galactic globular clusters that represent well the Milky Way globular cluster system in terms of metallicity, mass, reddening, and distance. Our method is based on low-resolution spectroscopy and is intrinsically reddening free and efficient even for faint stars. Our [Fe/H] determinations agree with high-resolution results to within 0.08 dex. The GOTHAM survey provides a new metallicity scale for Galactic globular clusters with a significant update of metallicities higher than [Fe/H] > -0.7. We show that the trend of [Mg/Fe] with metallicity is not constant as previously found, because now we have more metal-rich clusters. Moreover, peculiar clusters whose [Mg/Fe] does not match Galactic stars for a given metallicity are discussed. We also measured the CaII triplet index for all stars and we show that the different chemical evolution of Milky Way open clusters, field stars, and globular clusters implies different calibrations of calcium triplet to metallicity.

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Extending the Red Giant Branch versus Metallicity Calibration toward Metal-poor Systems: Near-Infrared Photometry of the Galactic Globular Clusters M56 and M15

The Astronomical Journal ◽

10.1086/301363 ◽

2000 ◽

Vol 119 (5) ◽

pp. 2274-2281 ◽

Cited By ~ 10

Author(s):

Valentin D. Ivanov ◽

Jordanka Borissova ◽

Almudena Alonso-Herrero ◽

Tatiana Russeva

Keyword(s):

Globular Clusters ◽

Near Infrared ◽

Infrared Photometry ◽

Galactic Globular Clusters ◽

Red Giant

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The multipopulation phenomenon in Galactic globular clusters: M4 and M22

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309991219 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 326-332

Author(s):

S. Villanova ◽

G. Piotto ◽

A. F. Marino ◽

A. P. Milone ◽

A. Bellini ◽

...

Keyword(s):

High Resolution ◽

Spectroscopic Data ◽

Globular Clusters ◽

Stellar Population ◽

The Other ◽

Wide Field ◽

Galactic Globular Clusters ◽

Red Giant ◽

Process Elements ◽

Different Characteristics

AbstractWe present an abundance analysis based on high-resolution spectra of red-giant-branch (RGB) stars in the Galactic globular clusters NGC 6121 (M4) and NGC 6656 (M22). Our aim was to study their stellar population in the context of the multipopulation phenomenon recently discovered to affect some globular clusters. Analysis was performed for the following elements: O, Na, Mg, Al, Ca, Fe, Y, and Ba. Spectroscopic data were completed by high-precision wide-field U BV IC ground-based photometry and HST/ACS observations. For M4, we find a well-defined Na–O anticorrelation composed of two distinct groups of stars with significantly different Na and O content. The two groups of Na-rich and Na-poor stars populate two different regions along the RGB. As regards M22, Na and O follow the well-known anticorrelation found in many other GCs. However, at odds with M4, it appears to be continuous without any hint of clumpiness. On the other hand, we identified two clearly separated groups of stars with significantly different abundances of the s-process elements Y, Zr and Ba. The relative numbers of the members of both groups are very similar to the ratio of the stars in the two subgiant branches of M22 recently found by Piotto (2009). The s-element-rich stars are also richer in iron and have higher Ca abundances. This makes M22 the second cluster after ω Centauri where an intrinsic spread in Fe was found. Both spectroscopic and photometric results imply the presence of two stellar populations in M4 and M22, even if both clusters have completely different characteristics.

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