Doctor of Philosophy

dissertationThe hippocampus (HPP) plays an important role in episodic memory, or memory for an event that occurs in a specific place and time, and there is evidence to suggest that the HPP is involved in processing spatiotemporal information in order to form context;ual representations of memory events. The HPP is not a homogeneous structure, but instead is comprised of anatomically distinct subregions, including the dentate gyrus (DG), CA3, and CA1, associated with separate mnemonic processing functions that contribute to episodic memory formation. Specifically, the DG is thought to support spatial processing functions, whereas the CA1 subregion has been implicated in temporal processing. Despite considerable advances in our understanding of the unique contributions of HPP subregions to learning and memory processes, the role of the dorsal DG (dDG) in spatial processing as it relates to spatial representations is not entirely understood or agreed upon. Given the importance of spatial representations to spatial navigation and episodic memory function, the current investigation sought to further define the role of the dDG in spatial processing through a series of studies that explored the nature of spatial memory representations. The results suggest that the dDG plays a critical role in (1) the integration of multimodal information into unique representations of the spatial environment via conjunctive encoding, (2) the reduction of interference among similar spatial locations via spatial pattern separation, and (3) the formation of temporal associations among distinct spatial events via temporal integration. Taken together, the present findings provide evidence for a dynamic role for the dDG in spatial processing by demonstrating the importance of an intact dDG across a variety of spatial tasks and under a variety of learning and memory demands

The orbital motion of the Arches cluster — clues on cluster formation near the galactic center

The Arches cluster is one of the most massive, young clusters in the Milky Way. Located inside the central molecular zone in the inner 200 pc of the Galactic center, it formed in one of the most extreme star-forming environments in the present-day Galaxy. Its young age of only 2.5 Myr allows us to observe the cluster despite the strong tidal shear forces in the inner Galaxy. The orbit of the cluster determines its dynamical evolution, tidal stripping, and hence its fate. We have measured the proper motion of the Arches cluster relative to the ambient field from Keck/NIRC2 LGS-AO and VLT/NAOS-CONICA NGS-AO observations taken 4.3 years earlier. When combined with the radial velocity, we derive a 3D space motion of 232 ± 30 km/s for the Arches. This motion is exceptionally large when compared to molecular cloud orbits in the GC, and places stringent constraints on the formation scenarios for starburst clusters in dense, nuclear environments

The orbital motion of the Quintuplet cluster - a common origin for the Arches and Quintuplet clusters?

We investigate the orbital motion of the Quintuplet cluster near the Galactic center with the aim of constraining formation scenarios of young, massive star clusters in nuclear environments. Three epochs of adaptive optics high-angular resolution imaging with Keck/NIRC2 and VLT/NACO were obtained over a time baseline of 5.8 years, delivering an astrometric accuracy of 0.5-1 mas/yr. Proper motions were derived in the cluster reference frame and were used to distinguish cluster members from the majority of field stars. Fitting the cluster and field proper motion distributions with 2D gaussian models, we derive the orbital motion of the cluster for the first time. The Quintuplet is moving with a 2D velocity of 132 +/- 15 km/s with respect to the field along the Galactic plane, which yields a 3D orbital velocity of 167 +/- 15 km/s when combined with the previously known radial velocity. From a sample of 119 stars measured in three epochs, we derive an upper limit to the velocity dispersion in the core of the Quintuplet cluster of sigma_1D < 10 km/s. Knowledge of the three velocity components of the Quintuplet allows us to model the cluster orbit in the potential of the inner Galaxy. Comparing the Quintuplet's orbit with the Arches orbit, we discuss the possibility that both clusters originated in the same area of the central molecular zone. [abridged]Comment: 40 pages, 12 figures, accepted for publication in Ap

Discovery of low-metallicity stars in the central parsec of the Milky Way

We present a metallicity analysis of 83 late-type giants within the central 1 pc of the Milky Way. K-band spectroscopy of these stars were obtained with the medium-spectral resolution integral-field spectrograph NIFS on Gemini North using laser-guide star adaptive optics. Using spectral template fitting with the MARCS synthetic spectral grid, we find that there is large variation in metallicity, with stars ranging from [M/H] $<$ -1.0 to above solar metallicity. About 6\% of the stars have [M/H] $<$ -0.5. This result is in contrast to previous observations, with smaller samples, that show stars at the Galactic center have approximately solar metallicity with only small variations. Our current measurement uncertainties are dominated by systematics in the model, especially at [M/H] $>$ 0, where there are stellar lines not represented in the model. However, the conclusion that there are low metallicity stars, as well as large variations in metallicity is robust. The metallicity may be an indicator of the origin of these stars. The low-metallicity population is consistent with that of globular clusters in the Milky Way, but their small fraction likely means that globular cluster infall is not the dominant mechanism for forming the Milky Way nuclear star cluster. The majority of stars are at or above solar metallicity, which suggests they were formed closer to the Galactic center or from the disk. In addition, our results indicate that it will be important for star formation history analyses using red giants at the Galactic center to consider the effect of varying metallicity.Comment: 11 pages, 10 figures, ApJ Accepte

Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center

We present new kinematic measurements and modeling of a sample of 116 young stars in the central parsec of the Galaxy in order to investigate the properties of the young stellar disk. The measurements were derived from a combination of speckle and laser guide star adaptive optics imaging and integral field spectroscopy from the Keck telescopes. Compared to earlier disk studies, the most important kinematic measurement improvement is in the precision of the accelerations in the plane of the sky, which have a factor of six smaller uncertainties (~10 uas/yr/yr). We have also added the first radial velocity measurements for 8 young stars, increasing the sample at the largest radii (6"-12") by 25%. We derive the ensemble properties of the observed stars using Monte-Carlo simulations of mock data. There is one highly significant kinematic feature (~20 sigma), corresponding to the well-known clockwise disk, and no significant feature is detected at the location of the previously claimed counterclockwise disk. The true disk fraction is estimated to be ~20%, a factor of ~2.5 lower than previous claims, suggesting that we may be observing the remnant of what used to be a more densely populated stellar disk. The similarity in the kinematic properties of the B stars and the O/WR stars suggests a common star formation event. The intrinsic eccentricity distribution of the disk stars is unimodal, with an average value of = 0.27 +/- 0.07, which we show can be achieved through dynamical relaxation in an initially circular disk with a moderately top-heavy mass function.Comment: 65 pages, 22 figures, 8 tables, submitted to Ap

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

We investigate the circumstellar disc fraction as determined from L-band excess observations of the young, massive Arches and Quintuplet clusters residing in the central molecular zone of the Milky Way. The Quintuplet cluster was searched for L-band excess sources for the first time. We find a total of 26 excess sources in the Quintuplet cluster and 21 in the Arches cluster, of which 13 are new detections. With the aid of proper motion membership samples, the disc fraction of the Quintuplet cluster was derived for the first time to be 4.0 +/- 0.7%. There is no evidence for a radially varying disc fraction in this cluster. In the case of the Arches cluster, a disc fraction of 9.2 +/- 1.2% approximately out to the cluster's predicted tidal radius, r < 1.5 pc, is observed. This excess fraction is consistent with our previously found disc fraction in the cluster in the radial range 0.3 < r < 0.8 pc. In both clusters, the host star mass range covers late A- to early B-type stars, 2 < M < 15 Msun, as derived from J-band photospheric magnitudes. We discuss the unexpected finding of dusty circumstellar discs in these UV intense environments in the context of primordial disc survival and formation scenarios of secondary discs. We consider the possibility that the L-band excess sources in the Arches and Quintuplet clusters could be the high-mass counterparts to T Tauri pre-transitional discs. As such a scenario requires a long pre-transitional disc lifetime in a UV intense environment, we suggest that mass transfer discs in binary systems are a likely formation mechanism for the B-star discs observed in these starburst clusters.Comment: 47 pages, 22 figures, accepted by A&

Recent Results and Perspectives for Precision Astrometry and Photometry with Adaptive Optics

Large ground-based telescopes equipped with adaptive optics (AO) systems have ushered in a new era of high-resolution infrared photometry and astrometry. Relative astrometric accuracies of <0.2 mas have already been demonstrated from infrared images with spatial resolutions of 55-95 mas resolution over 10-20'' fields of view. Relative photometric accuracies of 3% and absolute photometric accuracies of 5%-20% are also possible. I will review improvements and current limitations in astrometry and photometry with adaptive optics of crowded stellar fields. These capabilities enable experiments such as measuring orbits for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole and its environment, and identifying individual stars in young star clusters, which can be used test the universality of the initial mass function.Comment: SPIE Conference Proceedin

The Quintuplet Cluster: Extended Structure and Tidal Radius

The Quintuplet star cluster is one of only three known young ($<10$ Myr) massive (M $>10^4$ M$_\odot$) clusters within $\sim100$ pc of the Galactic Center. In order to explore star cluster formation and evolution in this extreme environment, we analyze the Quintuplet's dynamical structure. Using the HST WFC3-IR instrument, we take astrometric and photometric observations of the Quintuplet covering a $120''\times120''$ field-of-view, which is $19$ times larger than those of previous proper motion studies of the Quintuplet. We generate a catalog of the Quintuplet region with multi-band, near-infrared photometry, proper motions, and cluster membership probabilities for $10,543$ stars. We present the radial density profile of $715$ candidate Quintuplet cluster members with $M\gtrsim4.7$ M$_\odot$ out to $3.2$ pc from the cluster center. A $3\sigma$ lower limit of $3$ pc is placed on the tidal radius, indicating the lack of a tidal truncation within this radius range. Only weak evidence for mass segregation is found, in contrast to the strong mass segregation found in the Arches cluster, a second and slightly younger massive cluster near the Galactic Center. It is possible that tidal stripping hampers a mass segregation signature, though we find no evidence of spatial asymmetry. Assuming that the Arches and Quintuplet formed with comparable extent, our measurement of the Quintuplet's comparatively large core radius of $0.62^{+0.10}_{-0.10}$ pc provides strong empirical evidence that young massive clusters in the Galactic Center dissolve on a several Myr timescale.Comment: 25 pages (21-page main text, 4-page appendix), 18 figures, submitted to Ap

The Fate of Binaries in the Galactic Center: The Mundane and the Exotic

The Galactic Center (GC) is dominated by the gravity of a super-massive black hole (SMBH), Sagittarius A$^*$, and is suspected to contain a sizable population of binary stars. Such binaries form hierarchical triples with the SMBH, undergoing Eccentric Kozai-Lidov (EKL) evolution, which can lead to high eccentricity excitations for the binary companions' mutual orbit. This effect can lead to stellar collisions or Roche-lobe crossings, as well as orbital shrinking due to tidal dissipation. In this work we investigate the dynamical and stellar evolution of such binary systems, especially with regards to the binaries' post-main-sequence evolution. We find that the majority of binaries (~75%) is eventually separated into single stars, while the remaining binaries (~25%) undergo phases of common-envelope evolution and/or stellar mergers. These objects can produce a number of different exotic outcomes, including rejuvenated stars, G2-like infrared-excess objects, stripped giant stars, Type Ia supernovae (SNe), cataclysmic variables (CVs), symbiotic binaries (SBs), or compact object binaries. We estimate that, within a sphere of 250 Mpc radius, about 7.5 to 15 Type Ia SNe per year should occur in galactic nuclei due to this mechanism, potentially detectable by ZTF and ASAS-SN. Likewise we estimate that, within a sphere of 1 Gpc$^3$ volume, about 10 to 20 compact object binaries form per year that could become gravitational wave sources. Based on results of EKL-driven compact object binary mergers in galactic nuclei by Hoang at al. (2018), this compact object binary formation rate translates to about 15 to 30 events per year detectable by Advanced LIGO.Comment: 8 pages, 3 figures, accepted by Ap