3,260 research outputs found
Reconstruction of Stellar Orbits Close to Sagittarius A*: Possibilities for Testing General Relativity
We have reconstructed possible orbits for a collection of stars located
within 0.5 arcsec of Sgr A*. These orbits are constrained by observed stellar
positions and angular proper motions. The construction of such orbits serves as
a baseline from which to search for possible deviations due to the unseen mass
distribution in the central 1000 AU of the Galaxy. We also discuss the
likelihood that some of these stars may eventually exhibit detectable
relativistic effects, allowing for interesting tests of general relativity
around the 2.6 x 10^6 solar mass central object.Comment: 20 pages, 5 figures submitted to Astrophysical Journal, substantial
changes and additions based on referee's comment
Dense gas in the dust lane of Centaurus A
The interstellar medium of Centaurus A (NGC 5128) has been studied
extensively in recent years, using mostly molecular lines tracing low to medium
density gas (500 to several cm). The amount and distribution of
the dense molecular gas was largely unknown. Here we present new millimeter
data of the HCN(1-0), CS(2-1), and CS(3-2) rotational transitions towards the
nearby radio galaxy Centaurus A observed with the SEST on La Silla, Chile. We
obtained spectra of the HCN(1-0) emission which traces dense 10 cm
molecular gas at the center and along the prominent dust lane at offset
positions and . We also
obtained a few spectra of CS(2-1) and (3-2) tracing densities of
cm. The emission in these lines is weak and reaches a few mK at the
available angular resolutions of 54 - 36.
At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at
0.064, and decreases to 0.02 to 0.04 in the dust lane.Comment: 7 pages, 5 figures, accepted by A.&A. 04. May 200
Near infrared imaging of the broad absorption line quasar BAL QSO 0134+3253
In this paper we present near infrared (NIR) imaging data of the host galaxy
of the broad absorption line quasar (BALQ) at z=2.169, serendipitously found
close to 3C48. The data were obtained with the ESO-VLT camera ISAAC during
period 67. We find extended, rest-frame optical emission around the BALQ after
subtracting a scaled stellar point spread function from the quasar nucleus in
J, H, and Ks. The extended rest-frame optical emission can be interpreted as an
approximately 2 Gyr old stellar population composing the host galaxy of the
BALQ or a stellar population of similar age associated with an intermediate
(z=1.667) absorption system spectroscopically identified by Canalizo & Stockton
(1998) simultaneously. The rest-frame-UV emission on the other hand is
dominated by a young, 500 Myr old stellar population. The UV/optical colors
resemble a mixture of the two populations, of which the young one accounts for
about 80%. Assuming that the residual emission is located at the BALQ redshift,
we find that the host galaxy has a resolved flux of about 10% of the BALQ flux.
The physical scale is quite compact, typical for radio quiet QSOs or Lyman
break galaxies at these redshifts, indicating that the systems are still in the
process of forming.Comment: 14 pages, referee style, 6 figures, 4 tables, accepted for
publication in A&
A Black Hole in the Galactic Center Complex IRS 13E?
The IRS 13E complex is an unusual concentration of massive, early-type stars
at a projected distance of ~0.13 pc from the Milky Way's central supermassive
black hole Sagittarius A* (Sgr A*). Because of their similar proper motion and
their common nature as massive, young stars it has recently been suggested that
IRS 13E may be the remnant of a massive stellar cluster containing an
intermediate-mass black hole (IMBH) that binds its members gravitationally in
the tidal field of Sgr A*. Here, we present an analysis of the proper motions
in the IRS~13E environment that combines the currently best available data with
a time line of 10 years. We find that an IMBH in IRS 13E must have a minimum
mass of ~10^4 solar masses in order to bind the source complex gravitationally.
This high mass limit in combination with the absence so far of compelling
evidence for a non-thermal radio and X-ray source in IRS 13E make it appear
unlikely that an IMBH exists in IRS 13E that is sufficiently massive to bind
the system gravitationally.Comment: accepted by AP
The instrumental polarization of the Nasmyth focus polarimetric differential imager NAOS/CONICA (NACO) at the VLT - Implications for time-resolved polarimetric measurements of Sgr A*
We report on the results of calibrating and simulating the instrumental
polarization properties of the ESO VLT adaptive optics camera system
NAOS/CONICA (NACO) in the Ks-band. We use the Stokes/Mueller formalism for
metallic reflections to describe the instrumental polarization. The model is
compared to standard-star observations and time-resolved observations of bright
sources in the Galactic center. We find the instrumental polarization to be
highly dependent on the pointing position of the telescope and about 4% at
maximum. We report a polarization angle offset of 13.28{\deg} due to a position
angle offset of the half-wave plate that affects the calibration of NACO data
taken before autumn 2009. With the new model of the instrumental polarization
of NACO it is possible to measure the polarization with an accuracy of 1% in
polarization degree. The uncertainty of the polarization angle is < 5{\deg} for
polarization degrees > 4%. For highly sampled polarimetric time series we find
that the improved understanding of the polarization properties gives results
that are fully consistent with the previously used method to derive the
polarization. The small difference between the derived and the previously
employed polarization calibration is well within the statistical uncertainties
of the measurements, and for Sgr A* they do not affect the results from our
relativistic modeling of the accretion process.Comment: 16 pages, 15 figures, 5 tables, accepted by A&A on 2010 October 1
Stellar interactions in dense and sparse star clusters
Stellar encounters potentially affect the evolution of the protoplanetary
discs in the Orion Nebula Cluster (ONC). However, the role of encounters in
other cluster environments is less known. We investigate the effect of the
encounter-induced disc-mass loss in different cluster environments. Starting
from an ONC-like cluster we vary the cluster size and density to determine the
correlation of collision time scale and disc-mass loss. We use the NBODY6++
code to model the dynamics of these clusters and analyze the effect of
star-disc encounters. We find that the disc-mass loss depends strongly on the
cluster density but remains rather unaffected by the size of the stellar
population. The essential outcome of the simulations are: i) Even in clusters
four times sparser than the ONC the effect of encounters is still apparent. ii)
The density of the ONC itself marks a threshold: in less dense and less massive
clusters it is the massive stars that dominate the encounter-induced disc-mass
loss whereas in denser and more massive clusters the low-mass stars play the
major role for the disc mass removal. It seems that in the central regions of
young dense star clusters -- the common sites of star formation -- stellar
encounters do affect the evolution of the protoplanetary discs. With higher
cluster density low-mass stars become more heavily involved in this process.
This finding allows for the extrapolation towards extreme stellar systems: in
case of the Arches cluster one would expect stellar encounters to destroy the
discs of most of the low- and high-mass stars in several hundred thousand
years, whereas intermediate mass stars are able to retain to some extant their
discs even under these harsh environmental conditions.Comment: accepted by Astronomy and Astrophysic
The stars of the galactic center
We consider the origin of the so-called S stars orbiting the supermassive
black hole at the very center of the Galaxy. These are usually assumed to be
massive main-sequence stars. We argue instead that they are the remnants of
low-to-intermediate mass red giants which have been scattered on to near-radial
orbits and tidally stripped as they approach the central black hole. Such stars
retain only low-mass envelopes and thus have high effective temperatures. Our
picture simultaneously explains why S stars have tightly-bound orbits, and the
observed depletion of red giants in the very center of the Galaxy.Comment: 9 pages, 1 figure, ApJ Letters, in pres
Research on Computational and Display Requirements for Human Control of Space Vehicle Boosters, Phase III. Part II - Computer Programs and Data Package Final Report, 1 Mar. - 31 Aug. 1967
Hybrid computer program for real time trajectory determination of human controlled space vehicle booste
Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars
A new statistical method for pinpointing the massive black hole (BH) in the
Galactic Center on the IR grid is presented and applied to astrometric IR
observations of stars close to the BH. This is of interest for measuring the IR
emission from the BH, in order to constrain accretion models; for solving the
orbits of stars near the BH, in order to measure the BH mass and to search for
general relativistic effects; and for detecting the fluctuations of the BH away
from the dynamical center of the stellar cluster, in order to study the stellar
potential. The BH lies on the line connecting the two images of any background
source it gravitationally lenses, and so the intersection of these lines fixes
its position. A combined search for a lensing signal and for the BH shows that
the most likely point of intersection coincides with the center of acceleration
of stars orbiting the BH. This statistical detection of lensing by the BH has a
random probability of ~0.01. It can be verified by deep IR stellar
spectroscopy, which will determine whether the most likely lensed image pair
candidates (listed here) have identical spectra.Comment: 4 pages, 2 figures, submitted to ApJ
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