The PI and his team work on the scientific outcomes from the large and systematic survey for precision stellar elemental abundance analyses that is set out to map the spatial variations in the chemistry across the Milky Way Galactic disk. They employ the Apache Point Observatory Galactic Evolution Experiment (APOGEE) in the Sloan Digital Sky Survey (SDSS)-III survey, which is a large-scale, near-infrared (H-band), high-resolution (R ~ 30,000), high signal-to-noise (>100) spectroscopic survey of Milky Way stellar populations. The survey covers wavelengths from 1.51-1.68 microns which is a region that includes useful absorption lines from about 15 chemical elements, including alpha, odd-numbered, and iron peak elements. A three-year bright-time observing campaign from 2011-2014 will enable APOGEE to observe about 100,000 giants stars across the Galactic bulge, disk and halo, with the vast majority of these stars in the disk. This large catalog of chemical abundance data for stars all across the Milky Way from APOGEE is used to address a number of key questions related to the chemistry of the Galactic disk. The analysis of the data from the APOGEE pipeline gives the stellar parameters (e.g., effective temperatures, gravity, metallicity) and abundances for about 15 elements which are used to deduce additional stellar properties (star-by-star extinctions, distances, ages) that are needed to map the abundances in the disk and to constrain models of Galactic chemical evolution. These maps provide the three-dimensional distributions represent an order of magnitude improvement in what has been achieved before in terms of numbers of stars included, numbers of chemical elements probed, homogeneity in the collection, reduction, and calibration of the data, and spatial coverage, including vast regions of the disk never before probed because of the limitations from dust. As part of this work, the team plans to release a value-added APOGEE catalog of extinctions, distances and age estimates, along with the APOGEE data on abundances of about 15 chemical elements for about 100,000 stars from the thin and thick disks, and the reduction of these data into multidimensional maps of elemental abundances. These data should have significant value to the wider community of Galactic chemical evolution modelers.

Project Report

Ascertaining the formation and evolution of our home galaxy, the Milky Way, is fundamentally important to understanding our origins. The creation of stars like our Sun and the creation of all of the familiar chemical elements other than hydrogen and helium are intricately linked processes that drive the evolution of galaxies. During their lives stars synthesize hydrogen and helium into heavier elements, and, upon their deaths, stars can return some of those heavier elements to the surrounding gas, to be taken up in the formation of new stars. Through successive generations of stars, the abundance of heavier elements steadily increases. At a certain abundance threshold it is possible for stars to create planets, including rocky planets like Earth. This process can proceed at varying rates and with varying patterns of abundances in different galaxies, and even in different parts of the same galaxy, because it is a function of the numbers of stars of different masses that are formed and at what rates. Stars of different masses synthesize heavier elements in alternate ways, create different relative amounts of these elements, and have varying efficiencies in returning those elements back to the reservoir of star forming gas depending on the means by which they die. And while stars are continually forming in the disks of spiral galaxies, smaller "dwarf" galaxies independently form stars that can later be subsumed into larger galaxies that cannibalize them. It is currently thought that galaxy disks grow, inside out, by the continual accretion of originally independent systems. This complex interplay of in situ star formation and accretion of stars and gas from previously independent star systems, as well as a variety of dynamical processes that affect the vertical extent of disks and mix gas with varying degrees of efficiency, implies that the stars in galactic disks should have a varying pattern in the relative abundances of chemical elements as a function of radius and height. The main goal of this proposal was to make the first comprehensive mapping of the relative abundances of chemical elements across the Milky Way disk. We can read these varying patterns by looking at the spectra of stars, where the wavelengths of light are modulated by the relative abundances of chemical elements in the atmospheres of the stars, relative abundances that reflect the chemical abundances in the gas from which they formed. Among the hundreds of millions of known galaxies, the Milky Way is the one we can study in greatest detail, because of our position within it, which permits us to measure the properties of its individual stars precisely. However, one difficulty that has traditionally challenged such studies is that our view of our own Galactic disk is marred by clouds of dust intermixed with the stars that block visible light. This problem is alleviated by looking at the infrared light of stars, which is much less affected by the presence of dust. Our project takes advantage of data collected by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which uses a unique spectrograph that confers three advantages: (1) it operates in the infrared, which allows Milky Way view much less obscured by intervening dust. (2) It has high spectral resolving power, which allows it to accurately measure the abundances of fifteen key chemical elements. (3) It can observe up to 300 objects simultaneously over a large sky area, which allows rapid mapping of the sky. The P.I. of this proposal is the P.I. of the APOGEE project, while the other co-PIs play major roles in the processing and calibration of APOGEE data. Over the active period of this grant, APOGEE collected spectral data of over 146,000 stars, the majority distributed across all radii of the bulge and disk of the Milky Way. We are making these spectra, and derived measurements from them -- including the atmospheric properties, chemical abundances and Doppler velocities of the stars -- publicly available, and they can be used not only to understand the structure, dynamics and evolution of the Milky Way, but be applied to numerous other astrophysical problems. We have used these data to explore variations in the overall enrichment level and in the relative abundances of chemical elements as a function of Galactic radius and distance from the Galactic midplane. Among our many discoveries, we have found stark evidence for two chemically distinct populations of stars in the disk, and these populations show different spatial distributions and chemical trends within those spatial distributions. One of the populations, associated with older stars, shows almost no variation across the Galaxy, and this suggests that they shared a similar star formation history within a well-mixed, turbulent reservoir of gas. The other population shows strong chemical variations with position in the Galaxy, and is more prominent in the outer disk.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
Standard Grant (Standard)
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James Neff
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University of Virginia
United States
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