260 research outputs found
Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster
The aims of this work are to provide accurate photometry in multiple
near-infrared broadband filters, to determine the power-law index of the
extinction-law toward the central parsec of the Galaxy, to provide measurements
of the absolute extinction toward the Galactic center, and finally to measure
the spatial variability of extinction on arcsecond scales.We use adaptive
optics observations of the central parsec of the Milky Way. Absolute values for
the extinction in the H, Ks, and L'-bands as well as of the power-law indices
of the H to Ks and Ks to L' extinction-laws are measured based on the
well-known properties of red clump stars. Extinction maps are derived based on
H-Ks and Ks-L' colors. We present Ks-band photometry for ~7700 stars (H and L'
photometry for a subset). From a number of recently published values we compute
a mean distance of the Galactic center of R_0=8.03+-0.15 kpc, which has an
uncertainty of just 2%. Based on this R_0 and on the RC method, we derive
absolute mean extinction values toward the central parsec of the Galaxy of
A_H=4.48+-0.13 mag, A_Ks=2.54+-0.12$ mag, and A_L'=1.27+-0.18 mag. We estimate
values of the power-law indices of the extinction-law of
alpha_{H-Ks}=2.21+-0.24 and alpha_{Ks-L'}=1.34+-0.29. A Ks-band extinction map
for the Galactic center is computed based on this extinction law and on stellar
H-Ks colors. Mean extinction values in a circular region with 0.5" radius
centered on Sagittarius A* are A_{H, SgrA*}=4.35+-0.12, A_{Ks,
SgrA*}=2.46+-0.03, and A_{L', SgrA*}=1.23+-0.08.Comment: accepted for publication by Astronomy & Astrophysics; please contact
RS for higher quality figure
Stellar Orbits Near Sagittarius A*
We present new SHARP/NTT stellar proper motion and accelaration data covering
an interval from 1992 to 2000: 1) We combine the high precision but shorter
time scale NIRC/Keck data with the lower precision but longer time scale
SHARP/NTT data set; 2) We statistically correct the observed accelerations for
geometrical projection effects; 3) We exclude star S8 from the analysis of the
amount and position of the central mass. We show that the stars S2, and most
likely S1 and S8 as well, are on bound, fairly inclined (), and
eccentric () orbits around a central dark mass. The combination of
both data sets results in a position of this central mass of 48
E and 18 S of the nominal radio position of Sgr A*.
The mean statistically corrected enclosed mass derived from accelerations is
M_acc=(5 +/- 3) x 10^6 solar masses with current radial separations of S1 and
S2 from SgrA* of about . This enclosed mass estimate is derived from
individual stellar orbits as close to the massive black hole at the center of
the Milky Way as currently possible. Although the uncertainties are large this
estimate is fully consistent with the enclosed mass range of (2.6-3.3)x10^6
solar masses derived by Genzel et al. (2000) from radial and/or proper motion
velocities of a homogenized sample of sources.Comment: 32 pages, 15 figures, accepted by MNRA
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
Stellar dynamics in the central arcsecond of our galaxy
We present proper motions for 40 stars at projected distances
from Sagittarius A* (Sgr A*). We find evidence on a level for
radial anisotropy of the cluster of stars within of Sgr A*. We find no
evidence for a stationary source or variable source at the position of Sgr A*.
We confirm/find accelerated motion for 6 stars, with 4 stars having passed the
pericenter of their orbits during the observed time span. We
calculated/constrained the orbital parameters of these stars. All orbits have
moderate to high eccentricities. We discuss the possible bias in detecting
preferentially orbits with high eccentricities. We find that the center of
acceleration for all the orbits coincides with the radio position of Sgr A*.
From the orbit of the star S2, the currently most tightly constrained one, we
determine the mass of Sgr A* to be M and its
position to mas East and mas South of the nominal radio
position. The mass estimate for the central dark mass from the orbit of S2 is
fully consistent with the mass estimate of M
obtained from stellar proper motions within of Sgr A* using a
Leonard-Merritt mass estimator. We find that radio astronomical observations of
the proper motion of Sgr A* in combination with its intrinsic source size place
at the moment the tightest constraints on the mass density of Sgr A*, which
must exceed .Comment: 51 pages, 16 Figures, reviewed ms submitted to ap
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
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&
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