The Monoceros Stream of stars was first discovered as an over-density of blue stars in the Sloan ... (2004) subsequently uncovered evidence for the Canis Major.
Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis in honor of David L. Lambert ASP Conference Series, Vol. 336, 2005 Thomas G. Barnes III and Frank N. Bash
Metallicity of the Monoceros Stream from A/F-type Stars R. Wilhelm Physics Department, Texas Tech University, Lubbock, TX 79409-1051 T. C. Beers Department of Physics and Astronomy and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824 C. Allende Prieto McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712 H. J. Newberg Physics Department, Rensselaer Polytechnic Institute, Troy, NY 12180 B. Yanny Experimental Astrophysics Group, Fermi National Accelerator Lab, Batavia,IL 60510 Abstract. We have obtained metallicity estimates for a sample of A/F-type stars that appear to be members of the recently discovered Monoceros Stream. This sample of candidate main-sequence turn-off stars was chosen from the Sloan Digitized Sky Survey southern program spectra. The average metallicity, < [F e/H] >= −1.37 ± 0.04, is consistent with the abundance of six suspected Monoceros Stream globular clusters and with that of the metal-weak thick disk. There is some suggestion that the Monoceros turn-off stars are younger than the oldest population of stream globular clusters.
1.
Introduction
The Monoceros Stream of stars was first discovered as an over-density of blue stars in the Sloan Digital Sky Survey (Gunn et al. 1998; York et al. 2000; Abazajian et al. 2004, hereafter SDSS) by Newberg et al. (2002). The detection appeared as a ring-like structure located at low galactic latitudes (|b| < 30◦ ). Additional studies by Ibata et al. (2003), using photometry from the INT Wide Field Survey, Yanny et al. (2003) from SDSS spectra and photometry, and Rocha-Pinto et al. (2003) using photometrically identified 2MASS M-giants, confirmed the existence of the stream and placed it 15-20 kpc from the Galactic center with a prograde circular velocity of 215±25 km s−1 (Yanny et al. Erratum 2004). Martin et al. (2004) subsequently uncovered evidence for the Canis Major dwarf galaxy, apparently associated with the stream (l = 240◦ , b = −8◦ ). 371
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Martin et al. and Forbes, Strader, & Brodie (2004) have also identified globular clusters that appear to share the spatial and kinematic properties of Canis Major Dwarf and the Monoceros Stream. 2.
Analysis of the SDSS Data
Our present sample comprises stars from three separate spectroscopic plates in the SDSS southern program spectra, all chosen to be in the direction of the Monoceros Stream, and with photometry that is consistent with the stream’s main-sequence turn-off. We employ a combination of SDSS photometry and spectroscopy to determine metallicities, [Fe/H], for the sample stars. Our calibrations compare the input SDSS data to synthetic colors and spectroscopy in order to compute stellar effective temperature, surface gravity, and [Fe/H] (Allende Prieto et al. 2004; Wilhelm et al. 2003; Beers et al. 2004) To improve our abundance determination for the stream, we have removed stars with extremely low signal-to-noise; our expected uncertainty for a single star’s abundance estimate should be no worse than ±0.3 dex. 3.
Individual Spectroscopic Plate Results
The southern plates 1149 and 1154 are adjacent to one another, and in the direction of l ∼ 221◦ , b ∼ 21◦ . We find a low-dispersion population with a mean heliocentric velocity of < v >= 88 km s−1 and a dispersion of σv = 26 km s−1 , both consistent with the model predictions of Yanny et al. Erratum (2004). The average abundance for this sample is < [F e/H] >= −1.36, with a spread of σF e = ±0.45 dex. There is close agreement between the two plates. Plate 1150 is located at l ∼ 185◦ , b ∼ 28◦ , about 40◦ from the other two plates. This plate also exhibits a low-dispersion population (σ = 36 km s−1 ) and a mean velocity of < v >= 2 km s−1 , consistent with the model predictions. Despite being 40◦ from the other plates, the mean abundance is identical, < [F e/H] >= −1.39, with a spread of σF e = ±0.5 dex. This is strong evidence that a significant fraction of the stars on all three plates are sampling the same population (see Figure 1).
4.
Comparison to Globular Clusters
Forbes et al. (2004) suggested that age-metallicity relations may be used to locate sub-populations in the halo of the Galaxy. Table 1 lists the abundance for the Monoceros Stream, along with the old (age > 10 Gyrs) and the entire sample of suspected Monoceros/Canis Major globular clusters. It appears that the old clusters are more metal poor than the Monoceros Stream, suggesting that the A/F turn-off stars may be younger than the oldest globular clusters. This is also suggested by the position of the main-sequence turn-off, which is slightly blueward of the general halo turn-off. Although the difference in abundance between the stream stars and the old globular clusters is suggestive, a 0.3 dex shift could also arise from systematic differences between the abundance determinations.
Monoceros Stream
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Figure 1. The southern sample stars for Plates 1149/1154 and Plate 1150. The average metallicity is virtually identical for all plates. The solid line shows the predicted stream velocity; the dashed line is the halo mean velocity for the given directions.
We have also included the values for blue horizontal-branch (BHB) stars in the Sagittarius Stream analyzed by Wilhelm et al (2003). This sample exhibits much better agreement between the old globular cluster population and that of the BHB population, suggesting the two populations are coeval. Table 1.
Stream-Cluster Connection Object N Monoceros Stream Old Mon Globular Clusters All Mon Globular Clusters Sagittarius Stream Old Sgr Globular Clusters All Sgr Globular Clusters
5.
156 4 6 144 4 6
< [F e/H] >
σ[F e/H]
-1.37 -1.64 -1.39 -1.9 -1.86 -1.55
±0.5 ±0.2 ±0.5 ±0.6 ±0.1 ±0.5
Conclusion
The main sequence turn-off stars affiliated with the Monoceros Stream have an average metal abundance of < [F e/H] >=-1.37. This is significantly more metal rich than the BHB stars from the Sagittarius Stream. The abundance of the stream is more metal poor than the canonical thick disk, but is consistent with the metal-weak thick disk (see, e.g., Chiba & Beers 2000; Beers et al. 2002), suggesting that the stream and metal-weak thick disk may be related. If the Forbes et al. age-metallicity relation for the Canis Major Dwarf is correct, it possible that the A/F-type stars in the Monoceros Stream are younger than the oldest stream globular clusters. In contrast, the Sagittarius Stream appears to contain BHB stars that are as old as the oldest Sagittarius clusters, suggesting a slightly different star formation history for the two dwarf galaxies.
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Clearly, a homogeneous analysis of the clusters and the streams is needed to better explore these possibilities. 6.
Acknowledgments
Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington. T.C.B. acknowledges partial funding for this work from grants AST 0098508, AST 00-98549, and AST 04-06784, as well as from grant PHY 02-16783: Physics Frontiers Center/Joint Institute for Nuclear Astrophysics (JINA), awarded by the U.S. National Science Foundation. References Abazajian, K., et al. 2004, AJ, 128, 502 Allende Prieto, C., Beers, T.C., Li, Y., Newberg, H.J., Wilhelm, R., & Yanny, B. 2004, in Carnegie Observatories Astrophysics Series, Origin and Evolution of the Elements, ed. A. McWilliam & M. Rauch (Pasadena: Carnegie Observatories), 1 Beers, T.C., Allende Prieto, C., Wilhelm, R., Yanny, B., & Newberg, H.J., 2004, PASA, 21, 207 Beers, T.C., Drilling, J.S., Rossi, S., Chiba, M., Rhee, J., Fuhrmeister, B., Norris, J.E., & von Hippel, T. 2002, AJ, 124, 931 Chiba, M., & Beers, T.C. 2000, AJ, 119, 2843 Forbes, D., Strader, J. & Brodie, J. 2004, AJ, 127, 3394 Gunn, J. E., Carr, M., Rockosi, C., Sekiguchi, M., et al., 1998, AJ, 116, 3040 Ibata, R., Irwin, M., Lewis, G., Ferguson, A., & Tanvir, N. 2003, MNRAS, 340, 21 Martin, N., Ibata, R. A., Bellazzini, M., Irwin, M. J., Lewis, G. F., Dehnen, W, 2004, MNRAS, 348, 12 Newberg, H.J., Yanny, B., Rockosi, C., et al., 2002, ApJ, 569, 245 Rocha-Pinto, H., Majewski, S., Skrutskie, M., & Crane, J., 2003 ApJ, 594, 115 Wilhelm, R., Beers, T.C., Allende Prieto, C., Newberg, H.J., & Yanny, B., 2003, BAAS, 112.01 Yanny, B., Newberg, H. J., Grebel, Eva K., Kent, S., et al., 2003, ApJ, 588, 824 Yanny, B., Newberg, H. J., Grebel, Eva K., Kent, S., et al., 2004 Erratum, ApJ, 605, 575 York, D. G., Adelman, J., Anderson, J. E., et al., 2000, AJ, 120, 1579
