New directions

The bulge provides both a background screen for microlensing events and a rich population of stars for investigating phenomena like planetary transits. The recent Sagittarius Window Exoplanet Probe Survey (SWEEPS) (Sahu et al. 2006) has

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Figure 1.6. Detection of vertex deviation in the bulge/bar population as a function of abundance (Soto et al. 2006). The radial velocity (y-axis) is plotted against the transverse proper motion (x-axis); [Fe/H] is from Sadler et al. (1996) but using the scale of Fulbright et al. (2006a). The rotation of the axes for the metal-rich subset reflects the streaming of stars along the bar. This larger sample confirms the effect first noted by Zhao et al. (1994)

identified 16 transit candidates, some of which have periods <1 day. These ultra-short-period planets are the analogs of hot Jupiters, but they orbit M dwarfs. There is a hint that this class of object is preferentially associated with metal-rich stars, but considerably more work is required before we know how the incidence of bulge-transiting planets depends on metallicity. Although it is beyond the scope of this review, note that there have been several microlensing planet detections toward the bulge, yielding terrestrial-mass planets. A proposed spacecraft, the Microlensing Planet Finder (Bennett et al. 2004) would image continuously a several-square-degree area of the bulge from space, potentially discovering tens of terrestrial planet lenses and 50,000 transit events. Such a mission would settle the question of planetary incidence as a function of galactocentric distance and metallicity.

The advent of the nirspec high-dispersion infrared echelle spectrograph (McLean et al. 1998) at Keck II made possible the determination of detailed abundances for cool and obscured stars. This enables a push toward the Galactic Center. Rich & Origlia (2005) find that bulge M giants have [Fe/H] - Solar with [a/Fe] = +0.3, while Rich & Origlia (2007) find no gradient in both [Fe/H] and [a/Fe] within 4° of the Galactic center (Figure 1.5). Using these powerful infrared techniques but at lower spectral resolution, we plan to extend our work to the nucleus of our nearest spiral galaxy, M31. The OSIRIS spectrograph at Keck is capable of integral field spectroscopy and, in several-hour exposures, should be able to measure abundances for red giants in the bulge of M31, at R = 2,000-3,000. These observations might unveil for the first time the stellar populations that resemble those in the centers of giant elliptical galaxies.

Combining proper-motion studies with abundances and kinematics yields the surprising result (Figure 1.6) (Soto et al. 2006) that the more metal-rich stars in the bulge exhibit a strong vertex deviation (principal-axis rotation) of the velocity ellipsoid. If the metal-rich subset of stars in the bulge have a different formation history from the bulge as a whole, this would seem problematic for any "simple model" of chemical evolution, in which the chemical enrichment has taken place rapidly, without infall or outflow of material. The issue now becomes that of understanding the chemical/dynamical evolution of populations as they enrich past Solar metallicity.

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