Sgr A* (minor update for Astrolog 5.4x users) . .
Poseidia
First, note that Abramov's Astrolog 5.41A's default
B1950 value for Sgr A* was 17,42,29.30 ,-28,59,18.5,
..very close to the corrected position cited below.
- - From GCNEWS, Galactic Center Research:
http://www.mpifr-bonn.mpg.de/gcnews/cumulabs.shtml
See #153, 'The Position of Sgr A* at the Galactic Center'
F. Yusef-Zadeh, D. Choate, W. Cotton .. Mon May 17 1999):
http://www.mpifr-bonn.mpg.de/gcnews/gcnews/Vol.10/ZA...@OSSENU.ASTRO.NWU.EDU_sgrpos.txt
For Astrolog 5.4x users, you can update your "fixstars.ast"
with the correction for Sgr A*, using epoch B1950 or J2000.
The newer 2000 reference frame is definitely more accurate,
as Astrolog is using the (J2000) SWEPH/JPL-DE406 ephemeris,
and conversion from the older 1950 values shows some error.
You can see this error by activating both of these entries:
Gal.CenterB1950,SgrA*,1950,17,42,29.3076,-28,59,18.484,
Gal.CenterJ2000,SgrA*,2000,17,45,40.0383,-29,00,28.069,
*proper motion(?) 0.0007,0.014
*Note that Sgr A's proper motion is very near zero, and so
is probably better left at the zero default "0.000, 0.00",
since the margin of error is attributed to other factors,
as these pages excerpted below describe in great detail.
Remember that proper motion values in Astrolog 5.41 use
rectascension in *seconds of time* per century, whereas
all declination values use arc-seconds per century. See?
I quote from page #154 linked from this GCNEWS website:
http://www.mpifr-bonn.mpg.de/gcnews/gcnews/Vol.10/dba...@bkypsr2.berkeley.EdU_9906048.txt
[begin quote #154]
"Proper motions and radial velocities of luminous
infrared stars in the galactic center have provided
strong evidence for a dark mass of $2.5\times 10^6$ \Msun$~$
in the central 0.05 pc of the galaxy. The leading hypothesis
for this mass is a black hole. High angular resolution
measurements at radio wavelengths find a compact radio
source, Sagittarius (Sgr) A$^*$, that is either the
faint glow from a small amount of material accreting onto
the hole with low radiative efficiency or a miniature AGN
core-jet system.
We provide in this paper a full report on the
first program that has measured the apparent proper
motion of Sgr A$^*$ with respect to background extragalactic
reference frame. Our current result is:
$$\mu_{l,*}=[-6.18\pm 0.19]~~{\rm mas~y}^{-1}$$
$$\mu_{b,*}=[-0.65\pm 0.17]~~{\rm mas~y}^{-1}.$$
The observations were obtained with the NRAO Very Large
Array at 4.9 GHz over sixteen years. The proper motion of
Sgr A$^*$ provides an estimate of its mass based on
equipartition of kinetic energy between the hole and
the surrounding stars. The measured motion is largest
in galactic longitude. This component of the motion is
consistent with the secular parallax that results from
the rotation of the solar system about the center, which
is a global measure of Oort's constants (A-B), with no
additional peculiar motion of Sgr A$^*$. The current
uncertainty in Oort's galactic rotation constants limits
the use of this component of the proper motion for a mass
inference. In latitude we find a small, and weakly significant,
peculiar motion of Sgr A$^*$, $-19\pm 7$ km s$^{-1}$ after
correction for the motion of the solar system with respect
to the local standard of rest.
We consider sources of peculiar motion of Sgr A$^*$ ranging
from unstable radio wave propagation through intervening
turbulent plasma to the effects of asymmetric masses in the
center. These fail to account for a significant peculiar motion.
One can appeal to an $m=1$ dynamical instability that numerical
simulations have revealed. However, the measurement of a
latitude peculiar proper motion of comparable magnitude and
error but with opposite sign in the companion paper by
\citeauthor{Reid99} (\citeyear{Reid99}) leads us to conclude
at the present time that our errors may be underestimated, and
that the actual peculiar motion might therefore be closer to zero."
[end quote #154, PROPER MOTION OF THE COMPACT, NONTHERMAL
RADIO SOURCE IN THE GALACTIC CENTER, SAGITTARIUS A*]
Here's a quote from #156 (linked from the same website):
http://www.mpifr-bonn.mpg.de/gcnews/gcnews/Vol.10/re...@cfa.harvard.edu_reid.txt
[begin quote #156]
"The {\it apparent} motion of \sgrab can be used to
estimate the mass and elucidate the nature of this unusual
source. An apparent motion of \sgrab can be attributed to
at least three possible components:
1) a secular motion induced by the orbit of the Sun about
the Galactic Center,
2) a yearly oscillation owing to the Earth's orbital
motion around the Sun (trigonometric parallax), and
3) a possible motion of \sgrab with respect to the dynamical
center of the Galaxy.
Measurement of, or limits for, these components of \sgra's
apparent motion can provide unique information on the
circular rotation speed (\tnot) of the Local Standard of Rest
(LSR) and the peculiar motion of the Sun (\vsun), the
distance to the center of the Galaxy (\rnot), and the
nature of \sgrab itself.
Given the excellent agreement in the global and local
measures of the angular rotation rate of the Galaxy, and
the lack of a detected peculiar motion for \sgra, it is
likely that \sgrab is at the dynamical center of the Galaxy."
[end quote #156, The Proper Motion of Sgr A*: I. First VLBA.. ]
Daniel Joseph Min
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