249 research outputs found
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 Galactic centre mini-spiral in the mm-regime
The mini-spiral is a feature of the interstellar medium in the central ~2 pc
of the Galactic center. It is composed of several streamers of dust and ionised
and atomic gas with temperatures between a few 100 K to 10^4 K. There is
evidence that these streamers are related to the so-called circumnuclear disk
of molecular gas and are ionized by photons from massive, hot stars in the
central parsec. We attempt to constrain the emission mechanisms and physical
properties of the ionized gas and dust of the mini-spiral region with the help
of our multiwavelength data sets. Our observations were carried out at 1.3 mm
and 3 mm with the mm interferometric array CARMA in California in March and
April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the
NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of
the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an
inverted synchrotron spectrum. We find electron densities within the range
0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with
a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and
extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe
a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm
observations of the extended emission of the mini-spiral, which we interpret as
evidence that there are a range of contributions to the thermal free-free
emission by the ionized gas emission and by dust at 1.3 mm.Comment: 9 pages, 11 figures, accepted to A&
A strongly magnetized pulsar within grasp of the Milky Way's supermassive black hole
The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr)
A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of
intense star formation near the black hole a few Myr ago, which might have left
behind a young neutron star traveling deep into SgrA*'s gravitational
potential. On 2013 April 25, a short X-ray burst was observed from the
direction of the Galactic center. Thanks to a series of observations with the
Chandra and the Swift satellites, we pinpoint the associated magnetar at an
angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin
period and its derivative (P=3.7635537(2) s and \dot{P} = 6.61(4)x10^{-12}
s/s), confirmed by quasi simultaneous radio observations performed with the
Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure
of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We
have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*.
Simulations of its possible motion around SgrA* show that it is likely (~90%
probability) in a bound orbit around the black hole. The radiation front
produced by the past activity from the magnetar passing through the molecular
clouds surrounding the Galactic center region, might be responsible for a large
fraction of the light echoes observed in the Fe fluorescence features.Comment: ApJ Letters in pres
GRMHD simulations of accretion onto Sgr A*: How important are radiative losses?
We present general relativistic magnetohydrodynamic (GRMHD) numerical
simulations of the accretion flow around the supermassive black hole in the
Galactic centre, Sagittarius A* (Sgr A*). The simulations include for the first
time radiative cooling processes (synchrotron, bremsstrahlung, and inverse
Compton) self-consistently in the dynamics, allowing us to test the common
simplification of ignoring all cooling losses in the modeling of Sgr A*. We
confirm that for Sgr A*, neglecting the cooling losses is a reasonable
approximation if the Galactic centre is accreting below ~10^{-8} Msun/yr i.e.
Mdot < 10^{-7} Mdot_Edd. But above this limit, we show that radiative losses
should be taken into account as significant differences appear in the dynamics
and the resulting spectra when comparing simulations with and without cooling.
This limit implies that most nearby low-luminosity active galactic nuclei are
in the regime where cooling should be taken into account.
We further make a parameter study of axisymmetric gas accretion around the
supermassive black hole at the Galactic centre. This approach allows us to
investigate the physics of gas accretion in general, while confronting our
results with the well studied and observed source, Sgr A*, as a test case. We
confirm that the nature of the accretion flow and outflow is strongly dependent
on the initial geometry of the magnetic field. For example, we find it
difficult, even with very high spins, to generate powerful outflows from discs
threaded with multiple, separate poloidal field loops.Comment: Resubmitted to MNRAS, including modifications in response to referee
report. 13 pages, 15 figure
The formation history of our Galaxy's nuclear stellar disc constrained from HST observations of the Quintuplet field
Until recently it was thought that the nuclear stellar disc at the centre of
our Galaxy was formed via quasi-continuous star formation over billions of
years. However, an analysis of GALACTICNUCLEUS survey data indicates that >80%
of the mass of the stellar disc formed at least 8 Gyr ago and about 5% roughly
1 Gyr ago. Our aim is to derive new constraints on the formation history of the
nuclear stellar disc. We analysed a catalogue of HST/WFC3-IR observations of
the Quintuplet cluster field. From this catalogue, we selected about 24000
field stars that probably belong to the nuclear stellar disc. We used red clump
giants to deredden the sample and fit the resulting F153M luminosity function
with a linear combination of theoretical luminosity functions created from
different stellar evolutionary models. We find that >70% of the stellar
population in the nuclear disc probably formed more than 10 Gyr ago, while ~15%
formed in an event (or series of events) ~1Gyr ago. Up to 10% of the stars
appear to have formed in the past tens to hundreds of Myr. These results do not
change significantly for reasonable variations in the assumed mean metallicity,
sample selection, reddening correction, or stellar evolutionary models. We
confirm previous work that changed the formation paradigm for stars in the
Galactic Centre. The nuclear stellar disc is indeed a very old structure. There
seems to have been little star formation activity between its formation and
about 1 Gyr ago.Comment: Accepted for publication in A&
Does functionalisation enhance CO2 uptake in interpenetrated MOFs? An examination of the IRMOF-9 series
The effect of pore functionalisation (-I, -OH, -OCH3) on a series of topologically equivalent, interpenetrated metal-organic frameworks (MOFs) was assessed by both simulation and experiment. Counter-intuitively, a decreased affinity for CO2 was observed in the functionalised materials, compared to the non-functionalised material. This result highlights the importance of considering the combined effects of network topology and chemical functionality in the design of MOFs for enhanced CO2 adsorptionRavichandar Babarao, Campbell J. Coghlan, Damien Rankine, Witold M. Bloch, Gemma K. Gransbury, Hiroshi Sato, Susumu Kitagawa, Christopher J. Sumby, Matthew R. Hill and Christian J. Doona
Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA
Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany
The 2014 interferometric imaging beauty contest
This is the final version of the article. Available from SPIE via the DOI in this record.From Conference Volume 9146: Optical and Infrared Interferometry IV. Jayadev K. Rajagopal; Michelle J. Creech-Eakman; Fabien Malbet. Montréal, Quebec, Canada, June 22, 2014Here we present the results of the 6th biennial optical interferometry imaging beauty contest. Taking advantage of a unique opportunity, the red supergiant VY CMa and the Mira variable R Car were observed in the astronomical H-band with three 4-telescope configurations of the VLTI-AT array using the PIONIER instrument. The community was invited to participate in the subsequent image reconstruction and interpretation phases of the project. Ten groups submitted entries to the beauty contest, and we found reasonable consistency between images obtained from independent workers using quite different algorithms. We also found that significant differences existed between the submitted images, much greater than in past beauty contests that were all based on simulated data. A novel crowd-sourcing" method allowed consensus median images to be constructed, filtering likely artifacts and retaining real features." We definitively detect strong spots on the surfaces of both stars as well as distinct circumstellar shells of emission (likely water/CO) around R Car. In a close contest, Joel Sanchez (IAA-CSIC/Spain) was named the winner of the 2014 interferometric imaging beauty contest. This process has shown that new comers" can use publicly-available imaging software to interpret VLTI/PIONIER imaging data, as long as sufficient observations are taken to have complete uv coverage { a luxury that is often missing. We urge proposers to request adequate observing nights to collect sufficient data for imaging and for time allocation committees to recognise the importance of uv coverage for reliable interpretation of interferometric data. We believe that the result of the proposed broad international project will contribute to inspiring trust in the image reconstruction processes in optical interferometry
The Galactic Center: A Laboratory for Fundamental Astrophysics and Galactic Nuclei
As the closest example of a galactic nucleus, the Galactic center presents an
exquisite laboratory for learning about supermassive black holes (SMBH) and
their environs. Detailed studies of stellar dynamics deep in the potential well
of a galaxy, with exisiting and future large ground-based telescopes, offer
several exciting directions in the coming decade. First, it will be possible to
obtain precision measurements of the Galaxy's central potential, providing both
a unique test of General Relativity (GR) and a detection of the extended dark
matter distribution that is predicted to exist around the SMBH. Tests of
gravity have not previously been possible on the mass scale of a SMBH.
Similarly, only upper limits on the extended matter distribution on small
scales currently exist; detection of dark matter on these scales is an
important test of Lambda-CDM and the detection of stellar remnants would reveal
a population that may dominate the stellar dynamics on the smallest scales.
Second, our detailed view of the SMBH and its local gas and stellar environment
provides insight into how SMBHs at the centers of galaxies form, grow and
interact with their environs as well as on the exotic processes at work in the
densest stellar clusters in the Universe. The key questions, still unanswered,
of when and how SMBHs formed in the early universe, and the myriad ways in
which feedback from SMBHs can affect structure formation, can be informed by
directly observing the physical processes operating at the SMBH.Comment: An ASTRO2010 White Paper. Animations and high resolution images can
be found at
http://www.astro.ucla.edu/~ghezgroup/gc/pictures/Future_GCorbits.shtm
Near-infrared spectra of Galactic stellar clusters detected on Spitzer/GLIMPSE images
We present near-infrared spectroscopic observations of massive stars in three
stellar clusters located in the direction of the inner Galaxy. One of them, the
Quartet, is a new discovery while the other two were previously reported as
candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and
GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we
firmly establish the nature of the brightest stars in these clusters, yielding
new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an
early WC star in GLIMPSE20. We combine this information with the available
photometric measurements from 2MASS, to estimate cluster masses, ages, and
distances. The presence of several massive stars places the Quartet and
GLIMPSE20 among the small sample of known Galactic stellar clusters with masses
of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about
3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant
stars identified in GLIMPSE13 indicates that it is another massive (~ 6500
Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of
about 3 kpc.Comment: aastex macro, 21 pages, 15 figures. ApJ, accepte
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