The absolute magnitude of field metal-poor horizontal branch stars

Gratton, R.

doi:10.1046/j.1365-8711.1998.01434.x

Cited by 58 publications

(58 citation statements)

References 27 publications

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“…Our zero point in Eq. (3) (see last row of Table 7) agrees with the results from the trigonometric parallax of RR Lyrae itself M V (RR) = 0.61 ± 0.11 (Benedict et al 2002), at [Fe/H] = −1.4 (Clementini et al 1995), and with Koen & Laney (1998) re-evaluation of Gratton (1998) Table 7 and discussion in Cacciari & Clementini 2003). Concerning the theoretical models, our M V (HB) is in excellent agreement with the family of models that predict a faint value of M V (RR) = 0.56 ± 0.12, namely Ferraro et al (1999), Straniero et al (1997), Demarque et al (2000), and VandenBerg et al (2000) models.…”

Section: Comparison With Other Determinations and Implications: The Dsupporting

confidence: 85%

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

Gratton

Bragaglia

Carretta

et al. 2003

A&A

297

387

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Abstract. New improved distances and absolute ages for the Galactic globular clusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the Main Sequence Fitting Method. We derived accurate estimates of reddening and metal abundance for these three clusters using a strictly differential procedure, where the Johnson B − V and Strömgren b − y colours and UVES high resolution spectra of turn-off stars and early subgiants belonging to the clusters were compared to similar data for field subdwarfs with accurate parallaxes measured by Hipparcos. The use of a reddening free temperature indicator (the profile of Hα) allowed us to reduce the error bars in reddening determinations to about 0.005 mag, and in metal abundances to 0.04 dex, in the scales defined by the local subdwarfs. Error bars in distances are then reduced to about 0.07 mag for each cluster, yielding ages with typical random errors of about 1 Gyr. We find that NGC 6397 and NGC 6752 have ages of 13.9 ± 1.1 and 13.8 ± 1.1 Gyr respectively, when standard isochrones without microscopic diffusion are used, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by other techniques sensitive to relative ages. If we use models that include the effects of sedimentation due to microscopic diffusion in agreement with our observations of NGC 6397, and take into account various sources of possible systematic errors with a statistical approach, we conclude that the age of the oldest globular clusters in the Galaxy is 13.4 ± 0.8 ± 0.6 Gyr, where the first error bar accounts for random effects, and the second one for systematic errors. This age estimate is fully compatible with the very recent results from WMAP, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z ≥ 2.5, in a standard ΛCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z ≥ 1.3. On the other hand, our new age estimate once combined with values of H 0 given by WMAP and by the HST Key Project, provides a robust upper limit at 95% level of confidence of Ω M < 0.57, independently of type Ia SNe, and strongly supports the need for a dark energy. The new cluster distances lead to new estimates of the horizontal branch luminosity, that may be used to derive the zero point of the relation between the horizontal branch absolute magnitude and metallicity: we obtain M V (HB) = (0.22 ± 0.05)([Fe/H] + 1.5) + (0.56 ± 0.07). This zero point is 0.03 mag shorter than obtained by Carretta et al. (2000) and within the error bar it agrees with, but it is more precise than most of the previous individual determinations of the RR Lyrae absolute magnitude. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m − M) 0 = 18.50 ± 0.09.

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Section: Comparison With Other Determinations and Implications: The Dsupporting

confidence: 85%

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

Gratton

Bragaglia

Carretta

et al. 2003

A&A

297

387

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“…For such stars, the temperature had to be estimated by an alternative method, the relationship between color and effective temperature (Gray 1992). Table 4 summarizes the best-fit models obtained for 13 stars of the sample, including T eff , log g, [Fe/H], and the distances to the objects, assuming M J = 0.70 for the FHB (Gratton 1998) and the magnitudes and colors of main-sequence stars by Gray (1992) and Koornneef (1983). Some of these results can be better visualized in Figure 5, which shows the diagnostic diagram proposed by Pier (1983) and Wilhelm et al (1999a).…”

Section: Discussionmentioning

confidence: 99%

“…They have been often used to study various aspects of the Galaxy, such as its kinematics, chemical evolution, and structure, due to several desirable characteristics. For example, their absolute magnitudes are confined to a narrow range (Gratton 1998), and they are numerous and luminous, making them suitable for studying the nature of both the inner and outer halo populations (Carollo et al 2007). Unfortunately, samples of the blue horizontal-branch (BHB) stars can be confounded by the presence of high-gravity main-sequence A-type stars or blue stragglers (hereafter A v stars) in the halo, due to similarities in the atmospheres of these stars.…”

Section: Introductionmentioning

confidence: 99%

A Search for Field Horizontal-Branch Stars Near the South Galactic Pole

Ortiz

Wilhelm

Costa³

et al. 2010

The Astronomical Journal

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Searches for field horizontal-branch (FHB) stars in the halo of the Galaxy in the past have been carried out by several techniques, such as objective-prism surveys and visual or infrared photometric surveys. By choosing adequate color criteria, it is possible to improve the efficiency of identifying bona fide FHB stars among the other objects that exhibit similar characteristics, such as main-sequence A-stars, blue stragglers, subdwarfs, etc. In this work, we report the results of a spectroscopic survey carried out near the south Galactic pole intended to validate FHB stars originally selected from the HK objective-prism survey of Beers and colleagues, based on near-infrared color indices. A comparison between the stellar spectra obtained in this survey with theoretical stellar atmosphere models allows us to determine T eff , log g, and [Fe/H] for 13 stars in the sample. Stellar temperatures were calculated from measured (B − V) o , when this measurement was available (16 stars). The color index criteria adopted in this work are shown to correctly classify 30% of the sample as FHB, 25% as non-FHB (main-sequence stars and subdwarfes), whereas 40% could not be distinguished between FHB and main-sequence stars. We compare the efficacy of different color criteria in the literature intended to select FHB stars, and discuss the use of the Mg ii 4481 line to estimate the metallicity.

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“…E(B − V ) taken from Huenemoerder et al (1984) (HBC), Hayes & Philip (1988) (HP), Gray et al (1996) (GCP), Gratton (1998) 1 Note that Gratton (1998) (G) adopted the reddening from Gray et al (1996) (GCP) for the stars in common, and Altman & de Boer (2000) (AB) adopted the reddening from Gratton (1998) for the stars in common.…”

Section: Parameters From the Visible Energy Distribution For Some Stamentioning

confidence: 99%

Stellar parameters for Pop II A-type stars from IUE spectra and new-ODF ATLAS9 model atmospheres

Castelli

Cacciari²

2001

A&A

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Abstract. Stellar parameters for twenty-seven field horizontal branch A-type stars, a post-AGB star (BD +32 2188), and a possible cool sdB star (BD +00 0145) were obtained by fitting the whole IUE energy distributions taken from the IUE-INES archive to the ultraviolet energy distributions predicted by new-ODF ATLAS9 model atmospheres, which include the Lyman-α H-H + and H-H quasi-molecular absorptions near 1400Å and 1600Å. The sample of stars was extensively studied by Kinman et al. (2000), who derived stellar parameters for them by using visual observations and also an ultraviolet color index. The effective temperatures obtained by fitting the IUE spectra to the new-ODF models agree with T eff derived by Kinman et al. (2000) for most of the stars in the sample. The gravities from UV agree with those from Kinman et al. (2000) for stars hotter than about 8700 K, while they are lower, on average, by 0.3 dex for the cooler stars. The same discrepancy is present when log g from the ultraviolet energy distribution is compared with log g from the visible energy distribution. The difference is insensitive to reddening, microturbulent velocity, metallicity, or mixing-length parameter for the treatment of the convection.

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The absolute magnitude of field metal-poor horizontal branch stars

Cited by 58 publications

References 27 publications

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

A Search for Field Horizontal-Branch Stars Near the South Galactic Pole

Stellar parameters for Pop II A-type stars from IUE spectra and new-ODF ATLAS9 model atmospheres

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