UCLA

The proximity of the Milky Way Galactic center (GC) allows a unique perspective into how supermassive black holes (SMBHs) affect their surrounding stellar population. Several uncertainties still remain about the stellar population at the GC, notably the frequency of binary star systems at the GC and the distribution of dark stellar remnants surrounding the SMBH. In this thesis, we used over 13 years of Keck Observatory adaptive optics (AO) photometric flux measurements of the stellar population within approximately a half parsec of the GC SMBH in an effort to address the uncertainties about the GC stellar binary population and to probe the density of unseen stars or compact objects near the SMBH.

First, we present an ≈ 11.5 year adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central ∼ 0.4 pc (∼ 10′′) of the Milky Way nuclear star cluster. In the study, we measured the photometry of 563 stars using the Keck II NIRC2 imager (K′-band, λ0 = 2.124 μm). We achieved a photometric uncertainty floor of ∆mK′ ∼ 0.03 (≈ 3%), comparable to the highest precision achieved in other AO studies. Approximately half of our sample (50 � 2%) shows variability. 52 � 5% of known early-type young stars and 43 � 4% of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recovered the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrained the Galactic center eclipsing binary fraction of known early-type stars to be at least 2.4 � 1.7%. We found no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We additionally identified a new periodic variable at the GC, S2-36, with a 39.43 day photometric period.

Second, we present the discovery of the first known eclipsing, ellipsoidal binary star system (S2-36) at the GC, with a binary orbital period of ≈ 78.8 days at a projected distance of ≈ 2 arcsec from Sgr A*. As the first dynamically soft stellar binary discovered in the region, S2-36 offers powerful constraints on the dark cusp density within ∼ 0.1 pc of the SMBH. We used new and existing near-infrared adaptive optics Keck Observatory imaging data collected over 13 years to measure the photometric and astrometric properties of S2-36. S2-36’s proper motion suggests a distance < ≈ 0.2 pc from the SMBH. A trended multi-band periodicity search of the photometric measurements in the K′- (λ0 = 2.124 μm) and H-band −4 (λ0 = 1.633 μm) reveals a periodic signal with false-alarm probability of less than 10 . These results, combined with our flux and color measurements, suggest S2-36 is an ellipsoidal binary at the GC. Best-fit astrophysical binary models to the observed light curve consist of two red giant stars where the photometric variability originates from ellipsoidal variation and eclipses. However, with photometry measurements alone, our model fits have degeneracies in age and metallicity of the component stars (age ≈ 3 to 13.5 Gyr, [Fe/H] ≈ −1.5 to +0.5). The presence of an old, dynamically soft binary at the GC over its entire lifetime implies few encounters with sources that would disrupt the binary. Under the assumption that the binary has spent its lifetime in the GC environment, the binary’s survival places an upper limit of ∼ 104 stellar-mass black holes (M ∼ 10M⊙) within ≈ 0.1 pc of the SMBH, and a local two-body relaxation time < ≈ 5 � 10^8 yr. Our observations are consistent with X-ray estimates of compact objects in the GC and provide a powerful new constraint on the presence of the dark cusp at the GC.

Finally, we present new methodology and initial results for a photometric study to constrain the stellar binary fraction of the GC. With 873 stars, a larger stellar sample than previously ever used for a photometric binary search of GC stars, we detected a total of five likely periodic signals with our updated methods; two of the likely stars (S4-308 and S3-438) have not been previously identified as periodic variables in any previous study. We then provide an overview of the calculation of our experiment’s sensitivity to photometric binaries and detail a Bayesian framework to combine the sensitivity and our experiment’s detections to constrain the intrinsic GC stellar binary fraction. The methods outlined can be employed on future photometric studies of the GC stellar population to constrain its binary fraction, particularly for short-period binaries.