Chemical abundance patterns in the inner Galaxy: the Scutum Red Supergiant Clusters

Ben Davies {1,2}, Livia Origlia {3}, Rolf-Peter Kudritzki {4}, Don F. Figer {1}, R. Michael Rich {5}, Francisco Najarro {6}, Ignacio Negueruela {7}, J. Simon Clark {8}

1. Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY, 14623, USA

2. School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

3. INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

4. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA

5. Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA

6. Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, Calle Serrano 121, 28006 Madrid, Spain

7. Departamento. de Fisica, Ingenieria de Sistemas y Teoria de la Senal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain

8. Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

The location of the Scutum Red-Supergiant (RSG) clusters at the end of the Galactic Bar makes them an excellent probe of the Galaxy's secular evolution; while the clusters themselves are ideal testbeds in which to study the predictions of stellar evolutionary theory. To this end, we present a study of the RSGs' surface abundances using a combination of high-resolution Keck/NIRSPEC H-band spectroscopy and spectral synthesis analysis. We provide abundance measurements for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of the stars studied are consistent with CNO burning and deep, rotationally enhanced mixing. The average $alpha$/Fe ratios of the clusters are solar, consistent with a thin-disk population. However, we find significantly sub-solar Fe/H ratios for each cluster, a result which strongly contradicts a simple extrapolation of the Galactic metallicity gradient to lower Galacto-centric distances. We suggest that a simple one-dimensional parameterization of the Galaxy's abundance patterns is insufficient at low Galactocentric distances, as large azimuthal variations may be present. Indeed, we show that the abundances of O, Si and Mg are consistent with independent measurements of objects in similar locations in the Galaxy. In combining our results with other data in the literature, we present evidence for large-scale ($sim$kpc) azimuthal variations in abundances at Galacto-centric distances of 3-5,kpc. While we cannot rule-out that this observed behaviour is due to systematic offsets between different measurement techniques, we do find evidence for similar behaviour in a study of the barred-spiral galaxy NGC~4736 which uses homogeneous methodology. We suggest that these azimuthal abundance variations could result from the intense but patchy star formation driven by the potential of the central bar.

Reference: To appear in ApJ
Status: Manuscript has been accepted