The Relative Age of the Thin and Thick Galactic Disks

Liu, Wilson; Chaboyer, Brian

doi:10.1086/317231

Cited by 60 publications

(59 citation statements)

References 36 publications

(54 reference statements)

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“…The age of 7.9 Gyr for the local disk Hipparcos subgiants agrees very well with the age of the local disk white dwarfs based on the white dwarf luminosity function and modern cooling theory. The age of 13.5 Gyr for the halo globular cluster M92 and the field subdwarfs of the halo population suggests an appreciable age difference between the local disk subgiants of up to 5 Gyr or more, a conclustion reached earlier by Liu & Chaboyer (2000). This is supported by the discovery (Hansen et al 2002) that the faintest white dwarfs in the globular cluster M4 are 2.5 mag fainter than the faintest white dwarfs that define the local WD luminosity function.…”

Section: Discussionmentioning

confidence: 85%

The Age of the Oldest Stars in the Local Galactic Disk fromHipparcosParallaxes of G and K Subgiants

Sandage

Lubin

VandenBerg

2003

PUBL ASTRON SOC PAC

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ABSTRACT. We review the history of the discovery of field subgiant stars, the role that they played in the development of the early understanding of stellar evolution, and their importance in the age dating of the Galactic disk. We use the cataloged data from the Hipparcos satellite in this latter capacity.Based on Hipparcos parallaxes with relative accuracies of , the absolute magnitude of the lower , as well as to the envelope of the reddest main-sequence stars at . The rededge of the densest part of the distribution of field giants is, however, most readily matched by isochrones having [Fe/H . Such high metal abundances are evidently confirmed by the spectroscopically observed high ] ≈ ϩ0.23 metallicity (between [Fe/H and ϩ0.4) of the old thick-disk Galactic cluster NGC 6791, whose color-] p ϩ0.2 magnitude diagram can be made to fit either of these red boundaries by adopting suitable values for the reddening and distance modulus (from within the observed ranges of uncertainties of these quantities).The age of the field stars in the solar neighborhood is found to be Gyr (or Gyr if the stellar 7.9 ‫ע‬ 0.7 7.4 ‫ע‬ 0.7 models allow for the effects of diffusive processes) by fitting the theoretical isochrones for [Fe/H to the ] p ϩ0.37 lower envelope of the Hipparcos subgiants. However, this age is a function of metallicity. The models show a dependence of at for metallicities between 0.00 and ϩ0.37. The same grid dt p Ϫ3.99([Fe/H] Ϫ 0.37) M ∼ ϩ4 V of isochrones yields ages, in turn, of , , and 7.5 to 10 Gyr (depending on the assumed reddening) 4.0 ‫ע‬ 0.2 6.2 ‫ע‬ 0.5 for the old Galactic clusters M67, NGC 188, and NGC 6791 using metal abundances, distance moduli, and reddenings adopted in the text. Although the distance (and hence age) of NGC 6791 is somewhat uncertain, the ages of both the Galactic disk in the solar neighborhood and of NGC 6791 are, nevertheless, likely between 3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of ∼13.5 Gyr. The conclusion is the same as that reached earlier by Liu & Chaboyer, who used Hipparcos parallax stars that were mainly near the main-sequence turnoff rather than at the lower bound of the subgiant luminosity that we determine here. The most significant results are (1) the supermetallicity of the oldest local disk stars and (2) the large age difference between the most metal poor component of the halo and the thick and thin disk in the solar neighborhood, confirming Liu & Chaboyer. These facts are undoubtedly related and pose again the problem of the proper scenario for the timing of events in the formation of the halo and the Galactic disk in the solar neighborhood.1 Part review of the history of the discovery of subgiants and part new results from Hipparcos.

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Section: Discussionmentioning

confidence: 85%

The Age of the Oldest Stars in the Local Galactic Disk fromHipparcosParallaxes of G and K Subgiants

Sandage

Lubin

VandenBerg

2003

PUBL ASTRON SOC PAC

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“…In fact, several authors since Peebles & Dicke (1968) related the large overdensities required to the formation of globular clusters to violent phases connected to accretion episodes, that would have disrupted or at least considerably heated the thin disk. It is then interesting that a phase of low star formation followed the formation of the thick disk (see Gratton et al 1996Gratton et al , 2000Fuhrmann 1998;Liu & Chaboyer 2000), and of its population of globular clusters, like 47 Tuc (Zinn 1985), before the formation of the thin disk started. After this phase, only a few globular clusters actually formed; most of these objects (like e.g.…”

Section: The Epoch Of Galaxy Formationmentioning

confidence: 99%

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

Gratton

Bragaglia

Carretta

et al. 2003

A&A

295

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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“…Previous work on 47 Tuc that employed various stellar tracers, such as red giants (AlvesBrito et al 2005), subgiants and dwarfs (Carretta et al 2004) and asymptotic giant branch (AGB) stars (Wylie et al 2006), all agree on a mean metallicity near −0.7 dex; however, no consensus about an accurate absolute [Fe/H] has been reached, yet. In particular the age of 47 Tuc has been the subject of debate: while Gratton et al (2003) estimate an age of ∼11 Gyr, thus 2.6 Gyr younger than the old Galactic halo, McNamara (2001) and Liu & Chaboyer (2000) suggest an age in excess of 12.5 Gyr and thus comparable to the halo component. Several studies argue that 47 Tuc's color-magnitude diagram (CMD) bears a striking resemblance with the solar metallicity bulge clusters NGC 6553 and NGC 6528 (Ortolani et al 1995;Zoccali et al 2003).…”

Section: Introductionmentioning

confidence: 99%

A NEW ABUNDANCE SCALE FOR THE GLOBULAR CLUSTER 47 Tuc

Koch

McWilliam

2008

The Astronomical Journal

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We present chemical abundances for O, Na, Mg, Al, Si, Ca, Ti, and Fe in eight red giants and one turnoff star in the metal-rich globular cluster 47 Tuc, based on spectroscopy with the Magellan Inamori Kyocera Echelle highresolution spectrograph on the Magellan 6.5 m Clay telescope. A robust line by a line differential abundance analysis technique, relative to the K-giant Arcturus, was used to reduce systematic errors from atmospheric and atomic parameters. Our derived mean LTE [Fe/H] of −0.76 ± 0.01 ± 0.04 dex (random and systematic error, respectively) is more metal poor by about 0.1 dex than recent literature results. The chemical element ratios in this nearby globular cluster most closely resemble those of the Galactic bulge, although there is a non-negligible overlap with the composition of thick-disk stars. We find that the [Al/Fe] and [Na/Fe] ratios coincide with the upper boundary of the trends seen in the bulge and thick disk. There is only a small intrinsic scatter in the majority of the abundance ratios, indicating that 47 Tuc is mostly a rather chemically homogeneous system.

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The Relative Age of the Thin and Thick Galactic Disks

Cited by 60 publications

References 36 publications

The Age of the Oldest Stars in the Local Galactic Disk fromHipparcosParallaxes of G and K Subgiants

The Age of the Oldest Stars in the Local Galactic Disk fromHipparcosParallaxes of G and K Subgiants

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

A NEW ABUNDANCE SCALE FOR THE GLOBULAR CLUSTER 47 Tuc

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