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

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4. The velocity dispersion of the carbon stars

The broad velocity dispersion of [FORMULA] = 113 [FORMULA] 14 km s-1 (TR91) of the ALRW91 C-stars is apparently comparable with a Galactic Bulge dispersion. A Galactic Bulge membership leads to the mystery outlined in Sect. 1 and the question arises if there is an alternative explanation for the velocity dispersion, which does not imply a Bulge membership. In the following sub-sections various alternatives related to the Sagittarius dwarf galaxy are explored.

4.1. Are they member of the Sagittarius dwarf galaxy?

The average heliocentric radial velocity of the SDG stars is [FORMULA] = 140 [FORMULA] 2 km s-1, which corresponds in galacto-centric coordinates with 172 km s-1 and a dispersion of [FORMULA] = 11.4 [FORMULA] 0.7 km s-1 (Ibata et al. 1995, 1997). The average radial velocity of the ALRW91 C-stars is [FORMULA] = - 44 [FORMULA] 20 km s-1.

It is immediately evident from both the average velocities and their dispersion that the majority or even all of the ALRW91 C-stars cannot be member of the SDG.

Numerical calculation (see Edelsohn & Elmegreen 1997, Johnston et al. 1995 and Velázquez & White 1995) indicated that the velocity dispersion of the SDG stars does not change significantly on approaching and after crossing of the Galactic plane. Johnston et al. also demonstrated for moving groups with stars stripped from the dwarf galaxy, that their radial velocities change but that they do maintain a small velocity dispersion.

THE ALRW91 C-STARS CANNOT BE MEMBER OF THE SDG, NOR CAN THEY BE A TIDALLY STRIPPED MOVING GROUP.

4.2. Are they formed during crossing the disc?

4.2.1. In the galactic anti-centre direction?

If the SDG is moving towards the galactic mid-plane (Edelsohn & Elmegreen 1997 and references cited therein) then the most recent crossing occurred in the direction of the galactic anti-centre. At the present position this should have resulted in a small velocity dispersion similar to the one of the SDG stars. The broad velocity distribution of the C-stars does not support the possibility that these stars have been dragged along the SDG orbit from the anti-centre to its present position.

THE ALRW91 C-STARS WERE NOT FORMED IN THE GALACTIC ANTI-CENTRE DIRECTION .

4.2.2. On the recent approach towards the disc?

Hydrodynamical calculations of the interaction of the SDG with a gaseous H I disc by Ibata&Razoumov (1998) indicate that on approach of the SDG part of the H I layer is first pulled out of the disc due to the attractive influence of the dwarf galaxy. The temperature and density of the gas is at this stage not favourable to turn part of the gas into stars. This will change when the SDG gets nearer to the midplane and part of the material is compressed to densities high enough to sustain star formation. The SDG is located at about 6 kpc out of the Galactic midplane. This is too far away to invoke star formation.

THE ALRW91 C-STARS ARE NOT FORMED ON APPROACH OF THE GALACTIC MIDPLANE .

4.2.3. On a recent crossing?

Star formation could be invoked from the material residing in either the SDG and/or near the impact spot in the Galactic disc, when the SDG is nearby the galactic plane and crosses it. Stars formed from SDG material are SDG members and can be ignored in this discussion (see Sect. 4.1). Near the Galactic plane the SDG starts to compress the material that it pulled out from the disc earlier on its approach (see Sect. 4.2.2). Star formation is mainly triggered during and after the crossing of the Galactic plane 4. The SDG pushes material out of the Galactic disc. A small fraction of the newly formed stars is dragged along the orbit, while the majority of the material together with some young stars is moving away from the SDG. Part of these young stars are moving with different velocities towards us while another part of the stars are moving away from us. As a consequence, the distribution of radial velocities is considerably broader than expected from a SDG membership alone. The resulting distribution can even mimic a Bulge-like velocity dispersion. Fig. 2 gives a schematic view of this process. If the motion of the dwarf galaxy is perpendicular to the galactic plane then the resulting radial velocity distribution after crossing the galactic plane will be symmetric around zero. The situation sketched in Fig. 2 implies that more stars will be found moving towards us, thus giving a negative value for the average radial velocity.

[FIGURE] Fig. 2. Schematic view of the broad, bulge like, radial velocity distribution for the material from an induced star formation event, caused by the crossing from a dwarf galaxy through the Galactic mid-plane. Note that the majority of the new material will not be moving along the dwarf galaxy orbit. The dotted ellipse and the dotted line indicate respectively the current position and radial velocity (not on scale) for the dwarf galaxy

A RECENT STAR FORMATION EVENT, INDUCED BY THE CROSSING THROUGH THE GALACTIC PLANE OF THE SDG, CAN ACCOUNT FOR BOTH THE AVERAGE RADIAL VELOCITY AND THE VELOCITY DISPERSION OF THE ALRW91 C-STARS.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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