Star formation efficiency

Fig. 1. Predicted and observed [a/Fe] ratios in Sextan. The curves refer to different star formation efficiencies (v), indicated on the top left side of the Figure. The lowest curves refer to the lowest s. The central line is the best fit to the data and the other two curves represent the maximum and minimum v which can be acceptable. The wind parameter is assumed to be Wi = 9.

A simplified version of the argument has been given by Mould (1984).2 With the above assumption about star formation efficiency, in a continuum approximation,... 

Purely chemical models (no dynamics) have been computed by several authors by varying the number of bursts, the time of occurrence of bursts, tburst, the star formation efficiency, the type of galactic wind, the IMF and the nucleosynthesis prescriptions (Marconi et al. 1994 Kunth et al.1995 Bradamante et al.1998). The main conclusions of these papers can be summarized as follows ... 

The number of bursts should be Nburats < 10, the star formation efficiency should vary from 0.1 to 0.7 Gyr-1 for either Salpeter or Scalo (1986) IMF but Salpeter IMF is... 

Figure 9. The log(N/0) vs. 12 +log(0/H) for a sample of BCG. The data are from Recchi (2002). Overimposed are three models with a single burst of star formation and different star formation efficiency. In particular, the dotted line corresponds to an efficiency v = Gyr l, the continuous fine to v = 2.50yr 1 and the dashed fine to / = 5Gj/r 1. The burst duration is 100 Myr. As one can see, the saw-tooth behaviour typical of a bursting mode of star fotmation is evident.

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

If we take the total observed mass in the Universe as 2 x 109MSun and divide this by the dynamic timescale for a GMC, this suggests that star formation is occurring at a rate of 500 MSun yr-1, which is 100 times the currently observed formation rate. This calculation is riddled with assumptions and approximations, including the efficiency of star formation. 

G.P.C. analysis it was concluded the polymer fraction eluting at 27 ml. was a coupled dimer or two-arm star. The DVB/RLi ratio in this case was 3.0 (corrected for 44% EVB). From this observation it became of interest to study the influence of DVB/RLi ratio on the efficiency of star formation. The reaction time, temperature, and arm molecular weight were held constant while the DVB/RLi ratio varied. 

Figure 6 illustrates the effect of the DVB/RLi ratio. It was observed that as the DVB/RLi ratio increased, the percentage of unlinked linear material decreased. At the lower DVB/RLi ratios, linear coupled two arm stars were formed (see Ref. 15), while at the higher ratios, the linear material was not coupled. Thus, there is a strong influence on the efficiency of the star-branched polymer formation as the DVB/RLi ratio is varied for the case of poly(butadienyllithium). At the low molar ratios of DVB/RLi and two arm coupling reaction competes with the star-formation process. 

For polyisoprenyllithium anions, it was also observed that increased linking efficiency was achieved at higher DVB/RLi ratios, Figure 10. In comparison, the polyisoprenyllithium anions link more efficiently than polybutadienyllithium. Also, in contrast to polybutadienyllithium anions, polyisoprene star formation does not result in coupled dimer at the lower DVB/RLi ratios. This is perhaps reflected in the faster cross-over rate from the polydienyllithium anion to the vinylbenzylanion (reaction... 

Type lb supernovae appear in or near regions of star formation in spiral galaxies (Porter and Filippenko 1987). However, Huang (1987) has found that Type II supernovae, most of which probably have main sequence masses between 8 and 15 Mq, also occur in star-formation regions. Therefore this constraint is consistent with but does not require the hypothesis that Type lb have more massive progenitors than Type II. Type lb also could have somewhat less massive progenitors than Type II. In the white-dwarf model, the initial masses are not expected to be low. If the white dwarf accretes at 10 8 M0 yr-1 at low efficiency (a few percent) from the wind of a companion, and the white dwarf needs to accumulate a few tenths of a solar mass of helium before it explodes, then the companion must be able to lose several solar masses in its wind. It would need to be initially a star of at least intermediate mass (> 5 MG). 

Different timescales for star formation (Worthey et al. 1992) in the sense that star formation should be more efficient in more massive gaiaxies (v oc Mf). In this case, the situation couid be such that a gaiactic wind occurs eariier in more massive systems, the inverse wind scenario (Matteucci 1994). 

FIGURE 19. (Courtesy of Kurt Adelberger). An illustration of the principles behind the Lyman break technique. Hot stars have flat far-UV continua, but emit fewer photons below 912 A, the limit of the Lyman series of hydrogen (top panel). These photons are also efficiently absorbed by any H I associated with the sites of star formation (middle panel) and have a short mean free path—typically only 40 A—in the IGM at z = 3. Consequently, when observed from Earth (bottom panel), the spectrum of a star forming galaxy at z cs 3 exhibits a marked break near 4000 A. With appropriately chosen broad-band filters, this spectral discontinuity gives rise to characteristic colours objects at these redshifts appear blue in (G — 72.) and red in ([/ — G). For this reason, such galaxies are sometimes referred to as [/-dropouts. A more quantitative description of the Lyman break technique can be found in Steidel, Pettini, Hamilton (1995). 

Furthermore, grains are efficient infrared (IR) emitters and provide a cooling agent which, in the process of gravitational collapse of interstellar clouds into stars, very much facilitates star formation. 

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