Convection zone

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Fig. 1. Evolutionary tracks (labelled in Mq) and isochrones (in Myr) for low-mass stars taken from two models [8,31]. The epochs of photospheric Li depletion (and hence Li-burning in the core of a fully convective star or at the convection zone base otherwise) and the development of a radiative core are indicated. The numbers to the right of the tracks indicate the fraction of photospheric Li remaining at the point where the radiative core develops and at the end of Li burning.

Accretion Li abundances can be altered in two ways by accretion. During PMS Li depletion the additional mass will lead to increased Li depletion at a given Teff when the star reaches the ZAMS [26], If accretion occurs after Li-burning has ceased then the convective zone is enriched with Li. Too much accretion is required to be compatible with observations of disks around PMS stars unless the accreted material is H/He-deficient. But then accretion of sufficient H/He depleted material to explain the Li abundance scatter would also lead to (for instance) Fe abundance anomalies of order 0.2-0.3dex - much higher than allowed by current observational constraints [38]. 

We have also measured the lithium abundances in the samples of unmixed and mixed stars [6]. When low mass stars, such as those in our sample, evolve through the red giant branch, the degree of dilution of the lithium increases as the convective zone penetrates deeper and thus we expect a decline of the lithium abundance. In the mixed stars the lithium has never been detected, the upper limit of the lithium abundance is log N(Li) < 0.0, on the contrary in all the unmixed stars but one, the lithium line is visible and log N(Li) is > 0.20. In these stars as expected, the lithium abundance decreases when the gravity decreases (Fig. 3-b). 

What happens for cooler (i.e. less massive) stars on the red side of the Li dip As we shall see now, the stellar mass or the effective temperature of the dip is a transition point for stellar structure and evolution. First of all it is a transition as far as the rotation history of the stars is concerned. Indeed the physical processes responsible for surface velocity are different, or at least operate with different timescales on each side of the dip. At the age of the Hyades, the stars hotter than the dip still have their initial velocity while cooler stars have been efficiently spun down (Fig. 1). This behavior is linked to the variation of the thickness of the superficial H-He convection zone which gets rapidly deeper as Teff decreases from 7500 to 6000K (e.g. TC98). Below 6600 K, the stars have a sufficiently deep... 

These complementary observational constraints indicate that another process participates to the transport of AM in solar-type stars, while MC and turbulence are successful in more massive stars. The two most likely candidates are the large-scale magnetic field which could be present in the radiative zone and the internal gravity waves (hereafter 1GW) which are generated by the external convective zone. As we just explained, the observations suggest that the efficiency of this process is finked to the growth of the convective enveiope. This is a characteristics of 1GW. 

Abstract. We discuss the evidence gathered from helioseismology about considerable mixing below the solar convection zone. The evidence is obtained directly through inversions and also through more subtle, somewhat indirect signatures if mixing. 

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

As stars become older, lithium at their surface becomes gradually depleted by mixing with deeper layers at temperatures above 2.5 x 106 K where it is destroyed by the (p, a) reaction, Eq. (4.49). This destruction takes place more rapidly in cooler stars with deeper outer convection zones, so that there is a trend for lithium abundance to decrease with both stellar age and diminishing surface temperature in cooler stars some depletion takes place already in the pre-main-sequence stage. Thus, in the young Pleiades cluster ( 108 yr), lithium has its standard abundance down to Teff = 5500 K (type G5), whereas in the older Hyades cluster ( 6 x 108yr) it is noticeably depleted below Tc t = 6300 K (F7) and also in... 

Fig. 5.7. Evolutionary tracks for Z = 0.02 (near solar metallicity) stars with different masses in the HR diagram. (Luminosities are in solar units.) Points labelled 1 define the ZAMS and points labelled 2 the terminal main sequence (TAMS), the point where central hydrogen is exhausted. The Schonberg-Chandrasekhar limit may be reached either before or after this (for M > 1.4 Af0). Points marked 3 show the onset of shell hydrogen-burning. Few stars are found in the Hertzsprung gap between point 4 and point 5 , where the surface convection zone has grown deep enough to bring nuclear processed material to the surface in the first dredge-up. Adapted from Iben (1967).

Figure 6.4 also illustrates a mechanism whereby 13C may be introduced into the inter-shell convection zone of a low- or intermediate-mass star. When that zone reaches its maximum extension, it covers almost the whole inter-shell region apart from a thin interval marked A. The mass of this region decreases from about... 

Figure 6.5 shows a computation of the development of physical conditions during a pulse, according to the old picture. The 13 C source is activated at a relatively low temperature at the moment when the inter-shell convection zone reaches up to the level of the pocket. Later, after exhaustion of 13C, the temperature rises... 

Given that seed nuclei are exposed to a flux of neutrons at T = 1.5 x 108 K, nn = 109 cm-3 for 20 years during a pulse, and r0 = 0.3 mb-1, estimate the overlap fraction r between successive inter-shell convective zones. 

The luminosity then decreased rapidly from 20 to 0.5 Lq, where Lq is the Sun s present luminosity, whilst the surface temperature stabilised at around 4460 K. The Sun looked like an orange. The convective zone was resorbed and covered the star like a blanket. Although just 1% of the mass, it occupies today 30% of the radius ... 

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