Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988a%26a...205..105b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 205, no. 1-2, Oct. 1988, p. 105-124.
Astronomy and Astrophysics
Astrophysics
68
Asymptotic Giant Branch Stars, Hydroxyl Emission, Interstellar Masers, Stellar Evolution, Stellar Mass Ejection, Stellar Oscillations, Main Sequence Stars, Star Formation Rate, Stellar Envelopes, Variable Stars
Scientific paper
It now seems observationally well established that mass loss plays an important role in the evolution of stars on the upper asymptotic giant branch (AGB), a phase which all stars with main sequence masses up to 8 Msun go through. Observations of outflowing circumstellar material around Miras and long period OH/IR variables infer mass loss rates of a few times 10-7 Msunyr-1 for the Miras up to more than 10-4 Msunyr-1for the OH/IR variables. If the Miras and the OH/IR variables represent subsequent phases in the evolution of a single star then the relative scarcity of OH/IR variables tells us that the phase during which rapid mass loss occurs (the OH/IR star phase) must be shorter than the phase of more modest mass loss (the Mira phase). At present two scenarios exist for the evolution of the mass loss rate from being modest during the Mira phase to being very large during the OH/IR star phase. In one scenario the large increase in the mass loss rate is thought to occur suddenly due to a switch in the pulsation mode; whereas Miras are thought to pulsate in the first overtone the OH/IR variables with periods roughly two times longer should pulsate in the fundamental mode. The number of stars showing 1612 MHz OH emission as a function of the OH luminosity, however, is observed to be a continuous distribution over quite a large range in OH luminosity. In the second scenario this is interpreted to mean a continuous accelerated increase of the OH luminosity and of the mass loss rate from small to large values for a single star. We (re)propose that stellar pulsation in a single mode in combination with radiation pressure on dust causes the mass loss in all stars on the upper AGB, whether Miras or OH/IR variables. We derive an expression for the mass loss rate in terms of the basic stellar parameters mass and radius. We show that this expression leads to a time evolution of the mass loss rate in accord with that found by the authors of the second scenario. We present a model of the evolution on the upper AGB for stars with various main sequence masses. We incorporate mass loss according to the expression derived beginning at the moment a star starts to pulsate in the fundamental mode; this we assume is the onset of AGB evolution as a Mira variable. The moment when fundamental mode pulsation sets in is obtained from recent stellar pulsation models. We use this model of upper AGB evolution together with an essentially constant past star formation rate in the solar neighbourhood to obtain a model fit to the period distribution of classical Miras as well as to the 1612 MHz OH luminosity function of OH emitting Miras and OH/IR stars in the solar neighbourhood. The average past star formation rate as obtained from fitting the observed period distribution of classical Miras is in good agreement with that obtained by analysing the present day mass function of solar neighbourhood stars as outlined in Scalo (1986). We also calculate a model mass distribution for white dwarfs and a local white dwarf formation rate in good agreement with observations.
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