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P1: MRM CB240-03 November 20, 1999 20 12:55 Char Count= 0 The Andromeda galaxy (M31) Fig. 5. 7. Possible spiral arm segments are indicated. The location of M32 is indicated by E. In the figure the major axis is vertical. The youngest associations are shown as filled circles. (plotted as open circles). Inspection of this figure also shows that the most active region of star formation occurs between 9 kpc and 15 kpc from the nucleus of M31. Magnier et al. (1997a) have used the positions of individual stars in V versus U − V diagrams, in conjunction with theoretical evolutionary tracks, to derive the age distributions of stars in different associations.

These authors also found the CN λ 4170 feature to be enhanced in the spectra of M31 globular clusters. Ponder et al. (1998) have used the Hubble Space Telescope to obtain spectra of the M31 globular clusters M II, M IV, K 58, and K 280. These authors find that the M31 clusters generally have stronger Mg2 lines than do Galactic clusters with similar λλ 2600–3000 colors. Furthermore, the NH feature at λ 3360 in M31 globulars is greatly enhanced relative to its strength in Galactic globulars. This shows that the M31 globulars must be overabundant in nitrogen.

From color diagrams obtained with the IRAS satellite Xu & Helou (1994) conclude that very small grains (but not polycyclic aromatic hydrocarbons) are only half as abundant in M31 as they are in high-latitude Galactic “cirrus” clouds. Very small grains are the ˚ bump in the interstellar absorption curve. leading candidate for the carrier of the 2,175 A These IRAS observations are consistent with recent Hubble Space Telescope observations ˚ bump is weaker (Hutchings et al. 1992, Bianchi et al. 1996), which show that the 2,175 A and narrower in M31 than it is in the Galaxy.

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