40360242
Disks in the Arches Cluster?Survival in a Starburst Environment
Deep Keck/NIRC2 HK'L' observations of the Arches cluster near the Galactic center reveal a significant population of near-infrared excess sources. We combine the L'-band excess observations with K'-band proper motions, which allow us to confirm cluster membership of excess sources in a starburst cluster for the first time. The robust removal of field contamination provides a reliable disk fraction down to our completeness limit of H = 19 mag, or ~5 M_⊙ at the distance of the Arches. Of the 24 identified sources with K' – L' > 2.0 mag, 21 have reliable proper motion measurements, all of which are proper motion members of the Arches cluster. VLT/SINFONI K'-band spectroscopy of 3 excess sources reveals strong CO bandhead emission, which we interpret as the signature of dense circumstellar disks. The detection of strong disk emission from the Arches stars is surprising in view of the high mass of the B-type main sequence host stars of the disks and the intense starburst environment. We find a disk fraction of 6% ± 2% among B-type stars in the Arches cluster. A radial increase in the disk fraction from 3% to 10% suggests rapid disk destruction in the immediate vicinity of numerous O-type stars in the cluster core. A comparison between the Arches and other high- and low-mass star-forming regions provides strong indication that disk depletion is significantly more rapid in compact starburst clusters than in moderate star-forming environments.
119298933
Increasing the Scientific Return of Stellar Orbits at the Galactic Center
We report a factor of $\sim$3 improvement in Keck Laser Guide Star Adaptive Optics (LGSAO) astrometric measurements of stars near the Galaxy's supermassive black hole (SMBH). By carrying out a systematic study of M92, we have produced a more accurate model for the camera's optical distortion. Updating our measurements with this model, and accounting for differential atmospheric refraction, we obtain estimates of the SMBH properties that are a factor of $\sim$2 more precise, and most notably, increase the likelihood that the black hole is at rest with respect to the nuclear star cluster. These improvements have also allowed us to extend the radius to which stellar orbital parameter estimates are possible by a factor of 2.
437892
A Spitzer Study of the Mass-Loss Histories of Three Bipolar Preplanetary Nebulae
We present the results of far-infrared imaging of extended regions around three bipolar preplanetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS 16342-3814, at 70 and 160 μm with the MIPS instrument on the Spitzer Space Telescope. After a careful subtraction of the point-spread function of the central star from these images, we place constraints on the existence of extended shells and thus on the mass outflow rates as a function of radial distance from these stars. We find no apparent extended emission in AFGL 2688 and OH 231.8+4.2 beyond 100'' from the central source. In the case of AFGL 2688, this result is inconsistent with a previous report of two extended dust shells made on the basis of ISO observations. We derive upper limits of 2.1 × 10-7 and 1.0 × 10-7 M⊙ yr-1 for the dust mass-loss rates of AFGL 2688 and OH 231.8, respectively, at 200'' from each source. In contrast to these two sources, IRAS 16342-3814 does show extended emission at both wavelengths, which can be interpreted as a very large dust shell with a radius of ~400'' and a thickness of ~100'', corresponding to 4 and 1 pc, respectively, at a distance of 2 kpc. However, this enhanced emission may also be Galactic cirrus; better azimuthal coverage is necessary for confirmation of a shell. If the extended emission is a shell, it can be modeled, with some assumptions about its dust properties, as enhanced mass outflow at a dust mass outflow rate of 1.5 × 10-6 M⊙ yr-1 superimposed on a steady outflow with a dust mass outflow rate of 1.5 × 10-7 M⊙ yr-1. Because of the size of the possible shell, it is likely that this shell has swept up a substantial mass of interstellar gas during its expansion, so these estimates are upper limits to the stellar mass-loss rate. We find a constant color temperature of 32 K throughout the circumstellar envelope of IRAS 16342-3814, which is consistent with heating by the interstellar radiation field.
16076130
A Power-Law Break in the Near-Infrared Power Spectrum of the Galactic Center Black Hole
Proposed scaling relations of a characteristic timescale in the X-ray power spectral density of galactic and supermassive black holes (BHs) have been used to argue that the accretion process is the same for small and large BHs. Here, we report on the discovery of this timescale in the near-infrared (NIR) radiation of Sgr A*, the 4 × 106 M ☉ BH at the center of our Galaxy, which is the most extreme sub-Eddington source accessible to observations. Previous simultaneous monitoring campaigns established a correspondence between the X-ray and NIR regime and thus the variability timescales are likely identical for the two wavelengths. We combined Keck and Very Large Telescope data sets to achieve the necessary dense temporal coverage, and a time baseline of four years allows for a broad temporal frequency range. Comparison with Monte Carlo simulations is used to account for the irregular sampling. We find a characteristic timescale of 154+124 –87 minutes (errors mark the 90% confidence limits) which is inconsistent with a recently proposed scaling relation that uses bolometric luminosity and BH mass as parameters. However, our result fits the expected value if only linear scaling with BH mass is assumed. We suggest that the luminosity-mass-timescale relation applies only to BH systems in the soft state. In the hard state, which is characterized by lower luminosities and accretion rates, there is just linear mass scaling, linking Sgr A* to hard state stellar mass BHs.
119174192
IMPROVING GALACTIC CENTER ASTROMETRY BY REDUCING THE EFFECTS OF GEOMETRIC DISTORTION
We present significantly improved proper motion measurements of the Milky Way’s central stellar cluster. These 
improvements are made possible by refining our astrometric reference frame with a new geometric optical distortion 
model for the W. M. Keck II 10 m telescope’s adaptive optics camera (NIRC2) in its narrow field mode. For the 
first time, this distortion model is constructed from on-sky measurements and is made available to the public in the 
form of FITS files.When applied to widely dithered images, it produces residuals in the separations of stars that are 
a factor of ~3 smaller compared with the outcome using previous models. By applying this new model, along with 
corrections for differential atmospheric refraction, to widely dithered images of SiO masers at the Galactic center 
(GC), we improve our ability to tie into the precisely measured radio Sgr A*-rest frame. The resulting infrared 
reference frame is ~2–3 times more accurate and stable than earlier published efforts. In this reference frame, 
Sgr A* is localized to within a position of 0.6 mas and a velocity of 0.09 mas yr^(−1), or ~3.4 km s^(−1) at 8 kpc (1σ). 
Also, proper motions for members of the central stellar cluster are more accurate, although less precise, due to the 
limited number of these wide field measurements. These proper motion measurements show that, with respect to 
Sgr A*, the central stellar cluster has no rotation in the plane of the sky to within 0.3 mas yr^(−1) arcsec^(−1), has no 
net translational motion with respect to Sgr A* to within 0.1 mas yr^(−1), and has net rotation perpendicular to the 
plane of the sky along the Galactic plane, as has previously been observed. While earlier proper motion studies 
defined a reference frame by assuming no net motion of the stellar cluster, this approach is fundamentally limited 
by the cluster’s intrinsic dispersion and therefore will not improve with time.We define a reference frame with SiO 
masers and this reference frame’s stability should improve steadily with future measurements of the SiO masers in 
this region (∝t^(−3/2)). This is essential for achieving the necessary reference frame stability required to detect the 
effects of general relativity and extended mass on short-period stars at the GC.
15084801
HIGH ANGULAR RESOLUTION INTEGRAL-FIELD SPECTROSCOPY OF THE GALAXY'S NUCLEAR CLUSTER: A MISSING STELLAR CUSP?
We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type (~1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young (~6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. In the survey region, we classified 60 stars as early-type (22 newly identified) and 74 stars as late-type (61 newly identified). We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K' < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, Σ(R)∝ R^(–Γ), is flat, with Γ = -0.27 ± 0.19. Monte Carlo simulations of the possible de-projected volume density profile, n(r)∝ r^(–γ), show that γ is less than 1.0 at the 99.7% confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than that predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey—stellar collisions, mass segregation from stellar remnants, or a recent merger event—mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions.
408151
Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?
We present new observations and analysis of G2—the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes (2006-2012) and include spectroscopy (R ~ 3600) centered on the hydrogen Brγ line as well as K' (2.1 μm) and L' (3.8 μm) imaging. Analysis of these observations shows the Brγ line emission has a positional offset from the L' continuum. This offset is likely due to background source confusion at L'. We therefore present the first orbital solution derived from Brγ line astrometry, which, when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1600 R_s), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L' astrometry. It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Brγ line emission appears unresolved in almost all epochs, which implies that the bulk of the emission resides in a compact region. The observations altogether suggest that while G2 has a gaseous component that is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.
122790046
Anisotropic power-law inflation for the Dirac-Born-Infeld theory
We find a new set of the Bianchi type I power-law expanding solutions in a string-motivated DiracBorn-Infeld theory. Stability analysis shows that these power-law inflationary solutions remain stable with or without the contribution of the Dirac-Born-Infeld effect. We also find a new set of Bianchi type I expanding power-law solutions in a two scalar Dirac-Born-Infeld model with an additional phantom field. It is shown that the inclusion of the phantom field turns the Bianchi type I power-law solutions unstable during the inflationary phase.
123036141
Anisotropically expanding universe in massive gravity
We study the cosmological implications of a ghost-free nonlinear massive gravity theory with a cosmological constant. We find that the massive terms serve as an effective cosmological constant for a large class of metric spaces with a compatible fiducial metric associated with the massive terms. A specific solution is solved as an example for this model under the Bianchi type I space and a compatible Bianchi type I fiducial metric. A stability analysis indicates that this set of solutions tends to be stable. Nonetheless, the anisotropically expanding solution will, in general, turn unstable when an additional scalar field with negative kinetic energy is introduced.
122039287
Anisotropic power-law inflation for a two scalar fields model
A special class of Bianchi type I expanding solutions in a string motivated theory with a single scalar field has been speculated to break the cosmic no-hair theorem that will not approach the late time isotropic expanding solution. We will show by a new perturbation approach that an unstable mode for the inflationary solutions exists when an additional phantom field is introduced. The result indicates that the existence of an unstable mode is closely related to the extra fields that could be present during the very early Universe.