212 research outputs found
Adaptive Optics Observations of the Galactic Center Young Stars
Adaptive Optics observations have dramatically improved the quality and
versatility of high angular resolution measurements of the center of our
Galaxy. In this paper, we quantify the quality of our Adaptive Optics
observations and report on the astrometric precision for the young stellar
population that appears to reside in a stellar disk structure in the central
parsec. We show that with our improved astrometry and a 16 year baseline,
including 10 years of speckle and 6 years of laser guide star AO imaging, we
reliably detect accelerations in the plane of the sky as small as 70
microarcsec/yr/yr (~2.5 km/s/yr) and out to a projected radius from the
supermassive black hole of 1.5" (~0.06 pc). With an increase in sensitivity to
accelerations by a factor of ~6 over our previous efforts, we are able to
directly probe the kinematic structure of the young stellar disk, which appears
to have an inner radius of 0.8". We find that candidate disk members are on
eccentric orbits, with a mean eccentricity of = 0.30 +/- 0.07. Such
eccentricities cannot be explained by the relaxation of a circular disk with a
normal initial mass function, which suggests the existence of a top-heavy IMF
or formation in an initially eccentric disk.Comment: 7 pages, 4 figures, SPIE Astronomical Telescopes and Instrumentation
201
Testing for periodicities in near-IR light curves of Sgr A
We present the results of near-infrared (2 μm) monitoring of Sgr A*-IR with 1 minute time sampling using laser guide star adaptive optics (LGS AO) system at the Keck II
telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2006 May and 2007 August. Sgr A*-IR is detected at all times and is continuously variable. These observations allow us to investigate Nyquist sampled periods ranging from
about 2 minutes to an hour. Of particular interest are periods of ~20 min, which corresponds to a quasi-periodic (QPO) signal claimed based upon previous near-infrared observations and interpreted as the orbit of a ’hot spot’ at or near the last stable orbit of a spinning black hole.
We investigate these claims by comparing periodograms of the light curves with models for red noise and find no significant deviations that would indicate QPO activity at any time scale probed in the study. We find that the variability of Sgr A* is consistent with a model based on
correlated noise with a power spectrum having a frequency dependence of ~ f^(2.5), consistent with that observed in AGNs. Furthermore, the periodograms show power down to the minimum sampling time of 2 min, well below the period of the last stable orbit of a maximally spinning black hole, indicating that the Sgr A*-IR light curves observed in this study is unlikely to be from the Keplerian motion of a single ’hot spot’ of orbiting plasma
Penggugusan Provinsi di Indonesia Berdasarkan Kondisi Kesehatan
Untuk melaksanakan penelitian di Indonesia yang dapat mewakili 33 provinsi, sampling bertahap banyak dilakukan, dan tahap awalnya adalah memilih provinsi. Pada penelitian bidang kesehatan, agar provinsi terpilih mewakili kondisi kesehatan penduduk Indonesia, seyogyanya provinsi dikelompokkan berdasarkan variabel terkait kesehatan. Untuk itu, secara statistik dapat dilakukan analisis gugus (cluster analysis) memakai data dari berbagai sumber, dengan 27 variabel mencakup prevalensi beberapa penyakit infeksi dan status gizi, akses ke pelayanan kesehatan, status demografi, indeks pembangunan manusia, dan aspek keuangan. Hasil akhir menunjukkan bahwa ada 4 gugus provinsi di Indonesia, pada masing-masing gugus terdapat sebanyak 4, 8, 7, dan 14 provinsi. Proses penggugusan dengan analisis gugus semacam ini dapat diterapkan dengan memakai data yang diperbaharui dan hasilnya dapat dimanfaatkan sebagai salah satu pertimbangan untuk sampling provinsi di Indonesia.Amultistage sampling procedure is often used in conducting a research that represents all 33 provinces in Indonesia, and the first step for the procedure is the sample selection of provinces. In the area of health research, it is recommended that the province selection is based on the stratification of provinces using health related variables. Cluster analysis is a statistical technique possibly employed utilizing data from many sources. In this particular application, it involves 27 important health variables which reflect important communicable diseases and nutritional status, access to health services, demographic situation, human development index, and financial factor. This cluster analysis produces four clusters of province, with each of them comprising of 4, 8, 7, and 14 provinces. This statistical clustering technique of provinces can be implemented and considered in the sampling process of provinces in Indonesia using the updated data
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The presence of organophosphorus compounds (OPs) in the environmental counterparts has become an important problem because of their toxicity. In this study, the photocatalytic degradation reactions of the three OPs with hydroxyl radical were investigated by both experimental and quantum chemical methods. Photocatalytic degradation kinetics of the examined organophosphorus compounds were investigated under UV-A irradiation using TiO2 as the photocatalyst. The effects of the initial concentrations on the degradation rate have been examined. There was an observable loss of OPs in the presence of TiO2 photocatalyst under UV-A at 0.2 g TiO2 per 100 mL. The quantum chemical calculations have been carried out by the density functional theory (DFT) at B3LYP/6-31g(d) level. The reaction pathways were modelled to find the most probable mechanism for OPs with the OH radical and to determine the primary intermediates. The rate constants of the eight reaction paths were calculated by the transition state theory. Conductor-like polarizable continuum model (CPCM) was used as the solvation model with the intention of understanding the water effect. The theoretical results were in agreement with experimental ones. © 2021 Serbian Chemical Society. All rights reserved.18,164Acknowledgement. The authors of this research has greatfully acknowledgemented to financially support of Tekirdag Namık Kemal University Research Project with the project number of NKUBAP.01.GA.18.164
3D stellar kinematics at the Galactic center: measuring the nuclear star cluster spatial density profile, black hole mass, and distance
We present 3D kinematic observations of stars within the central 0.5 pc of
the Milky Way nuclear star cluster using adaptive optics imaging and
spectroscopy from the Keck telescopes. Recent observations have shown that the
cluster has a shallower surface density profile than expected for a dynamically
relaxed cusp, leading to important implications for its formation and
evolution. However, the true three dimensional profile of the cluster is
unknown due to the difficulty in de-projecting the stellar number counts. Here,
we use spherical Jeans modeling of individual proper motions and radial
velocities to constrain for the first time, the de-projected spatial density
profile, cluster velocity anisotropy, black hole mass (), and
distance to the Galactic center () simultaneously. We find that the inner
stellar density profile of the late-type stars, to
have a power law slope , much more shallow than
the frequently assumed Bahcall Wolf slope of . The measured
slope will significantly affect dynamical predictions involving the cluster,
such as the dynamical friction time scale. The cluster core must be larger than
0.5 pc, which disfavors some scenarios for its origin. Our measurement of
and
kpc is consistent with that derived from stellar
orbits within 1 of Sgr A*. When combined with the orbit of
S0-2, the uncertainty on is reduced by 30% ( kpc).
We suggest that the MW NSC can be used in the future in combination with
stellar orbits to significantly improve constraints on .Comment: 7 pages, 3 figures, 2 tables, ApJL accepte
The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole
Stars with short orbital periods at the center of our galaxy offer a powerful
and unique probe of a supermassive black hole. Over the past 17 years, the W.
M. Keck Observatory has been used to image the Galactic center at the highest
angular resolution possible today. By adding to this data set and advancing
methodologies, we have detected S0-102, a star orbiting our galaxy's
supermassive black hole with a period of just 11.5 years. S0-102 doubles the
number of stars with full phase coverage and periods less than 20 years. It
thereby provides the opportunity with future measurements to resolve
degeneracies in the parameters describing the central gravitational potential
and to test Einstein's theory of General Relativity in an unexplored regime.Comment: Science, in press (published Oct 5, 2012). See Science Online for the
Supplementary Material, or here:
http://www.astro.ucla.edu/~ghezgroup/gc/research/S02_S0102_orbits.htm
The Keplerian orbit of G2
We give an update of the observations and analysis of G2 - the gaseous red
emission-line object that is on a very eccentric orbit around the Galaxy's
central black hole and predicted to come within 2400 Rs in early 2014. During
2013, the laser guide star adaptive optics systems on the W. M. Keck I and II
telescopes were used to obtain three epochs of spectroscopy and imaging at the
highest spatial resolution currently possible in the near-IR. The updated
orbital solution derived from radial velocities in addition to Br-Gamma line
astrometry is consistent with our earlier estimates. Strikingly, even ~6 months
before pericenter passage there is no perceptible deviation from a Keplerian
orbit. We furthermore show that a proposed "tail" of G2 is likely not
associated with it but is rather an independent gas structure. We also show
that G2 does not seem to be unique, since several red emission-line objects can
be found in the central arcsecond. Taken together, it seems more likely that G2
is ultimately stellar in nature, although there is clearly gas associated with
it.Comment: Proceedings of IAU Symposium #303, "The Galactic Center: Feeding and
Feedback in a Normal Galactic Nucleus"; 2013 September 30 - October 4, Santa
Fe New Mexico (USA
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