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Astron. Astrophys. 351, 920-924 (1999)

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3. Chemical evolution of the Galaxy

If HVCs are extragalactic, the chemical evolution of the Galactic disc is strongly affected by continual but episodic infall of metal-poor gas from HVCs which mixes slowly with the rest of the interstellar medium.

The infall rate of low metallicity gas should be increasing now, or at least remain constant on timescales compared to the Hubble time. The reasons which sustain this hypothesis are based on a interpretation of the chemical evolution in the local Galactic disc:

  • The G-dwarf distribution in the solar neighbourhood (Rocha-Pinto & Maciel 1996) with a sharp rise in numbers close to [FORMULA] requires either a sharp increase in the SFR (stellar formation rate) at that metallicity, or the accumulation of stars formed at different epochs but at the same metallicity and both can be explained using a rising or near constant infall rate of metal-poor gas.

  • Star formation in the past 5 Gyr has a long-term average slow-rise according to stellar activity data (Vaughan & Preston 1980, Barry 1988).

  • The 9Be abundance plotted versus iron abundance has a loop-back close to [FORMULA], implying a fall in Fe abundance while 9Be abundance increases, which can be well explained with a rising infall rate (Casuso & Beckman 1997).

An inference presented in BL99 is that the accretion of gas in the Milky Way is decreasing. The present value would be 7.5 [FORMULA]/yr. However, in their simple dynamical model, they do not take into account the increasing gravitational potential of the discs due to the accretion itself. The increasing mass of our Galaxy could counterbalance the declining density of the baryonic material in the intergalactic medium leading to an increasing accretion rate of material into the Galaxy. This will be discussed quantitatively in the next section.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999