Star formation burst

The resulting [Ca/Fe] versus [Fe/H] plot is shown in Fig. 1, where except for a few outliers that will have to be manually inspected, a clear trend appears [Ca/Fe] slowly rises with [Fe/H] until it reaches a maximum and then declines again for the most metal-rich stars (RGB-a according to [4]). This nicely confirms a previous finding by [8] and [9]. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their a-enhancement would be SNe type la intervention. No simple explanation is provided for the rise of [Ca/Fe] at low [Fe/H], although a series of star formation bursts should be the likely cause. 

From the point of view of GCE, one is interested primarily in effects averaged over long periods of time of the order of Gyr but in dwarf galaxies which may have experienced only a few star formation bursts over a Hubble time the sporadic character may have appreciable effects, especially when one bears in mind that much of the abundance data for such objects comes from H II regions which are intrinsically the result of a current burst, and there is indeed evidence for a cosmic dispersion in certain element abundance ratios such as N/O in such objects (see Chapter 11). 

Interesting variants on the simplest star formation laws include stochastic self-propagating star formation (Gerola Seiden 1978 Dopita 1985), self-regulating star formation (Arimoto 1989 Hensler Burkert 1990), stochastic star-formation bursts (Matteucci Tosi 1985), separate laws for the halo and disk, the latter including terms that account for cloud collisions and induced star formation from interactions between massive stars and clouds (Ferrini et al. 1992, 1994), and the existence of a threshold surface gas density for star formation (Kennicutt 1989 Chamcham, Pitts Tayler 1993). 

Regardless of the details concerning self-enrichment and winds, the existence of isolated star formation bursts will also affect the iron-oxygen and iron-a relations, introducing scatter in Fe/O and possibly gaps in the iron abundance distribution function. When the interval between successive bursts exceeds the evolution time for SN la (maybe about 1 Gyr), iron will build up in the ISM resulting in an enhanced Fe/O ratio in the second burst so that one can end up with [Fe/O] > 0 (Gilmore Wyse 1991) see Fig. 8.7. 

Alternative wind models assume a selective outflow of enriched material, either taking place at the time of star formation bursts in dwarf galaxies (Matteucci Tosi 1985) or taking place continuously from the hot component of the ISM which is also assumed to be metal-enriched (Vader 1987 Ferreras, Scannapieco Silk 2002). An analytical model by Lynden-Bell (1992) supposes that, of the return fraction 1 — a from a generation of stars, a fraction 1 — / escapes, leading via Eq. (7.34) to... 

Our results suggest that dSphs contain multiple stellar components with different spatial, kinematic and metallicity distributions, and rule out formation scenarios involving single bursts of star formation. 

The strengths of the model are its natural connection to supemovae and star formation and that the supernova remnant would have enough time to form iron via the decay of nickel and cobalt to possibly produce the claimed iron lines. Moreover, it is expected to be a baryon-clean environment. The model is, however, very sensitive to the fine tuning of parameters. Moreover, GRB030329 places a rather strict limit of a few hours on the delay between the SN and the GRB and thus rules out the supranova model for at least this particular burst. 

Bursts of star formation in merging subsystems made of gas (Tinsley Larson 1979). In this picture star formation stops after the last burst and gas is lost via stripping or wind. 

Merging of early formed stellar systems in a wide redshift range and preferentially at late epochs (Kauffmann et al. 1993). A burst of star formation can occur during the major merging where 30% of the stars can be formed (Kauffmann 1996). 

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 principal sources of primary N are thought to be intermediate mass stars, with masses 4 M/Mq 7, during the asymptotic giant branch (AGB) phase. A corollary of this hypothesis is that the release of N into the ISM should lag behind that of O which, as we have seen, is widely believed to be produced by massive stars which explode as Type II supernovae soon after an episode of star formation. Henry et al. (2000) calculated this time delay to be approximately 250 Myr at low metallicities the (N/O) ratio could then perhaps be used as a clock with which to measure the past rate of star formation, as proposed by Edmunds Pagel (1978). Specifically, in metal-poor galaxies which have only recently experienced a burst of star formation one may expect to find values of (N/O) below the primary plateau at (N/O) —1.5, provided the fresh Oxygen has been mixed with the ISM (Larsen, Sommer-Larsen, Pagel 2001). 

These stars have been of central concern in a myriad of observational and theoretical works. No wonder They indeed play a key role in many chapters of astrophysics. In particular, they influence the physical and chemical states of their circumstellar environments or of the interstellar medium through their intense radiation and mass losses during their non-explosive phases of evolution, and even more so, as a result of their final supernova explosions. They may act as triggers of star formation, are essential agents of the evolution of the nuclidic content of the galaxies, accelerate particles to cosmic ray energies, and leave neutron stars or black holes at the end of their evolution. They are also the progenitors of certain 7-ray bursts. 

Virtually all models for the ultimate energy source of GRBs involve an endpoint of stellar evolution, particularly of the most massive stars. Thus it has been proposed that the burst rate must be proportional to the overall cosmic star formation rate. This view is supported by the fact that the typical redshifts (z 1) associated with GRB host galaxies correspond to an epoch of early active star formation in the Universe. Burst counterparts also tend to be... 

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