Allyson A. Sheffield

Probing the Halo from the Solar Vicinity to the Outer Galaxy: Connecting Stars in Local Velocity Structures to Large-scale Clouds

(Abridged) This paper presents the first connections made between two local features in velocity-space found in a survey of M giant stars and stellar spatial inhomogeneities on global scales. Comparison to cosmological, chemodynamical stellar halo models confirm that the M giant population is particularly sensitive to rare, recent and massive accretion events. These events can give rise to local observed velocity sequences - a signature of a small fraction of debris from a common progenitor, passing at high velocity through the survey volume, near the pericenters of their eccentric orbits. The majority of the debris is found in much larger structures, whose morphologies are more cloud-like than stream-like and which lie at the orbital apocenters. Adopting this interpretation, the full-space motions represented by the observed velocity features are derived under the assumption that the members within each sequence share a common velocity. Orbit integrations are then used to trace the past and future trajectories of these stars across the sky revealing plausible associations with large, previously-discovered, cloud-like structures. The connections made between nearby velocity structures and these distant clouds represent preliminary steps towards developing coherent maps of such giant debris systems. These maps promise to provide new insights into the origin of debris clouds, new probes of Galactic history and structure, and new constraints on the high-velocity tails of the local dark matter distribution that are essential for interpreting direct detection experiments.

A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities

The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure) and A13 are stellar overdensities at estimated heliocentric distances of

Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk

d \sim 11

Spectroscopic follow-up of the Hercules–Aquila Cloud

kpc and 15 kpc observed at low Galactic latitudes towards the anticenter of our Galaxy. While these overdensities were initially thought to be remnants of a tidally-disrupted satellite galaxy, an alternate scenario is that they are composed of stars from the Milky Way (MW) disk kicked out to their current location due to interactions between a satellite galaxy and the disk. To test this scenario, we study the stellar populations of the Monoceros Ring and A13 by measuring the number of RR Lyrae and M giant stars associated with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae stars in the two structures and measure radial velocities to compare with previously measured velocities for M giant stars in the regions of the Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars (

A spectroscopic survey of late-type giants in the Milky Way disk and local halo substructure

f_{RR:MG}

Identifying Contributions to the Stellar Halo from Accreted, Kicked-out, and In Situ Populations

) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae stars (116 in the Monoceros Ring and 37 in A13) and find that both structures have very low

Exploring halo substructure with giant stars. XIV. The nature of the Triangulum-Andromeda stellar features

f_{RR:MG}

Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?

. The results support a scenario in which stars in A13 and Mon/GASS formed in the MW disk. We discuss a possible association between Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar velocity distributions and

Disk Heating, Galactoseismology, and the Formation of Stellar Halos

f_{RR:MG}

Chemical Abundances of Hydrostatic and Explosive Alpha-elements in Sagittarius Stream Stars

.