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Astron. Astrophys. 338, 435-441 (1998)

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6. Challenges

6.1. More carbon stars wanted

It will become very important to increase the sample size of the C-stars related to the SDG. Ng (1997) argues that still a considerable number of C-stars could be found in the databases with long period variable stars from the various micro-lensing projects (see the contributions described in Ferlet et al. 1997).

The distances of these stars can be obtained from their periods and K-band luminosities. The velocity dispersion of a significant number of carbon Miras & semiregular variables located at a distance comparable to the SDG will provide an independent verification of the hypothesis that the ALRW91 C-stars are the result of an induced star formation event (see Sect 4.2.3).

Note that special care should be taken to avoid mixing the older, metal-richer SDG carbon stars with the essentially younger ALRW91-like C-stars 9

6.2. Comparison with theoretical models

The ALRW91 & the SDG C-stars offer a great opportunity to study along the same line of sight two distinct populations of carbon stars (Sect. 5.5). The distribution of both the ALRW91 and SDG C-stars ought to be compared with the theoretical models from Marigo (1998) using for each group respectively the SMC and LMC metallicity. Moreover, the difference in metallicity and the similarity in age between the ALRW91 C-stars and those from the Fornax dwarf galaxy (Stetson et al. 1998) provides furthermore the possibility to tune the theoretical models to even lower metallicities.

Such a comparison would provide an independent verification of the age of each group of C-stars and support the assertion that the ALRW91 C-stars did form from material originating from our Galactic disc.

It is not clear what kind of event lead to the formation of the SDG C-stars [FORMULA] 4 Gyr ago. It is possibly a time-stamp of the encounter with the LMC which deflected the SDG in a closer orbit around our Galaxy (Zhao 1998). This encounter should have left in the LMC traces of a population of carbon stars with a comparable age.

6.3. Radial velocities of young stars

In Sect. 4.2.3 it is mentioned that the hydrodynamical calculations of the interaction between the SDG and the Galactic H I disc (Ibata & Razoumov 1998) provide an indication that, after a collision of the SDG with the Galactic plane, gaseous and stellar material will be scattered around in all directions. The scattering results in a small number of young stars in the CMDs from Marconi et al. (1998, see also Fig. 3 Ng 1997). A comparison of the velocity dispersion between the ALRW91 C-stars and the young stars found at a SDG related distance should provide an independent verification of the induced star formation scenario suggested in this paper.

6.4. Chemo-dynamical formation models

The models thus far consider only the gravitational interaction between the Galaxy and the SDG (Ibata & Lewis 1998, Ibata & Razoumov 1998, and Nair & Miralda-Escudé 1998), but do not take into account an encounter, induced, star formation event. Improved models should therefore be employed to trace the stars from such an event.

The SDG offers the opportunity to study in great detail encounter induced star formation events. Chemo-dynamical models (Carraro et al. 1998a, Gerritsen 1997) should be explored, which take into account the induced star formation when a SDG like object crosses our Galactic disc, to determine when star formation occurs, when carbon stars will emerge, and to determine to which extent the newly formed stars can be dragged along the orbit.

As an aside, induced star formation due to the crossing of a dwarf spheroidal galaxy through the galactic mid-plane might be one of the processes responsible for the elusive nature of the so-called halo carbon stars (see Groenewegen et al. 1997 and references cited therein). Hydrodynamical calculations of the interaction between the SDG and the Galactic H I disc (Ibata & Razoumov 1998) indicate that after a collision material from the H I disc is pulled/pushed out of the plane to [FORMULA] 10 kpc heights. i.e. comparable to the heights of the halo carbon stars. This material is partly made up out of stars (see Sect. 4.2.3) results in a prominent signature and remains clearly visible for at least half an orbit.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998