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Here are some of the issues related to the formation and evolution of the Galactic thin disk:

Many of the basic observational constraints on the properties of the Galactic disk are still uncertain. At this time, we do not have reliable information about the star formation history of the disk. We do not know how the metallicity distribution and the stellar velocity dispersions in the disk have evolved with time. One might have expected that these observational questions were well understood by now, but this is not yet so. The basic observational problem is the difficulty of measuring ages for individual stars.

The younger stars of the Galactic disk show a clear abundance gradient of about 0.07 dex kpc-1, outlined nicely by the cepheids (Luck et al. 2006). In the outer disk, for the older stars, the abundance gradient appears to be even stronger: the abundance gradient (and the gradient in the ratio of alpha-elements to Fe) have flattened with time towards the solar values. A striking feature of the radial abundance gradient in the Galaxy is that it flattens for R > 12 kpc at an [Fe/H] value of about -0.5 (Carney et al. 2005). A similar flattening of the abundance gradient is seen in the outer regions of the disk of M31 (Worthey et al. 2005).

The relation between the stellar age and the mean metallicity and velocity dispersion are the fundamental observables that constrain the chemical and dynamical evolution of the Galactic thin disk. The age-metallicity relation (AMR) in the solar neighborhood is still uncertain. Different authors find different relations, ranging from a relatively steep decrease of metallicity with age from [Rocha-Pinto et al. 2004] to almost no change of mean metallicity with age from [Nordström et al. 2004]. Much of the earlier work indicated that a large scatter in metallicity was seen at all ages, which was part of the motivation to invoke large-scale radial mixing of stars within the disk. This mixing, predicted by Sellwood & Binney (2002), is generated by resonances with the spiral pattern, and is able to move stars from one near-circular orbit to another. It would bring stars from the inner and outer disks, with their different mean abundances, into the solar neighborhood. Radial mixing is potentially an important feature of the evolution of the disk. At this stage, it is a theoretical concept, and it is not known how important it is in the Galactic disk. We are not aware of any strong observational evidence at this stage for the existence of radial mixing. More recent results on the AMR (e.g. Wylie de Boer , unpublished) indicate that there is a weak decrease of mean metallicity with age in the Galactic thin disk, but that the spread in metallicity at any age is no more than about 0.10 dex. If this is correct, then radial mixing may not be so important for chemically mixing the Galactic disk.

The age-velocity dispersion relation (AVR) is also not well determined observationally. The velocity dispersion of stars appears to increase with age, and this is believed to be due to the interaction of stars with perturbers such as giant molecular clouds and transient spiral structure. But there is a difference of opinion about the duration of this heating. One view is that the stellar velocity dispersion σ increases steadily for all time, ∼ t0.2-0.5, based on Wielen 1977's work using chromospheric ages and kinematics for the McCormick dwarfs. Another view (e.g. Quillen & Garnett 2000), based on the data for subgiants from Edvardsson et al. (1993) is that the heating takes place for the first ∼ 2 Gyr, but then saturates when σ ≈ 20 km s-1 because the stars of higher velocity dispersion spend most of their orbital time away from the Galactic plane where the sources of heating lie. Data from Soubiran et al. (2008) support this view. Again, much of the difference in view goes back to the difficulty of measuring stellar ages. Accurate ages from asteroseismology would be very welcome. Accurate ages and distances for a significant sample of red giants would allow us to measure the AMR and AVR out to several kpc from the Sun. This would be a great step forward in understanding the chemical and dynamical evolution of the Galactic disk.

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