J/ApJ/725/331 Astrometry in the Galactic Center (Yelda+, 2010)
Improving Galactic center astrometry by reducing the effects of geometric
distortion.
Yelda S., Lu J.R., Ghez A.M., Clarkson W., Anderson J., Do T., Matthews K.
<Astrophys. J., 725, 331-352 (2010)>
=2010ApJ...725..331Y 2010ApJ...725..331Y
ADC_Keywords: Galactic plane ; Milky Way ; Positional data ; Models ;
Proper motions ; Photometry, infrared
Keywords: astrometry - Galaxy: kinematics and dynamics -
instrumentation: adaptive optics
Abstract:
We present significantly improved proper motion measurements of the
Milky Way's central stellar cluster. These improvements are made
possible by refining our astrometric reference frame with a new
geometric optical distortion model for the W. M. Keck II 10m
telescope's adaptive optics camera (NIRC2) in its narrow field mode.
For the first time, this distortion model is constructed from on-sky
measurements and is made available to the public in the form of FITS
files. When applied to widely dithered images, it produces residuals
in the separations of stars that are a factor of ∼3 smaller compared
with the outcome using previous models. By applying this new model,
along with corrections for differential atmospheric refraction, to
widely dithered images of SiO masers at the Galactic center (GC), we
improve our ability to tie into the precisely measured radio Sgr
A*-rest frame. The resulting infrared reference frame is ∼2-3 times
more accurate and stable than earlier published efforts. In this
reference frame, Sgr A* is localized to within a position of 0.6mas
and a velocity of 0.09mas/yr, or ∼3.4km/s at 8kpc (1σ). Also,
proper motions for members of the central stellar cluster are more
accurate, although less precise, due to the limited number of these
wide field measurements. We define a reference frame with SiO masers
and this reference frame's stability should improve steadily with
future measurements of the SiO masers in this region
(∝t-3/2). This is essential for achieving the necessary
reference frame stability required to detect the effects of general
relativity and extended mass on short-period stars at the GC.
Description:
Two types of GC observations were obtained with the NIRC2 narrow
camera and the LGSAO system (laser guide star (LGS) adaptive optics)
at Keck.
First, two sets of deep GC observations centered roughly on Sgr A*
offer a test of the new distortion model because something is changed
in each. In a data set from 2007 May, previously reported in Ghez et
al. (2008ApJ...689.1044G 2008ApJ...689.1044G), 103 frames were obtained with a camera
orientation of PA=0° (May 17) and another 20 frames were collected
at PA=200° (May 20). In a new data set collected on 2008 May 15,
the first 22 images were obtained with the LGSAO system and the
remaining 112 images were taken with the NGSAO (natural guide star AO)
system.
Second, three new epochs of observations, designed to measure the
relative positions of seven IR-bright SiO masers, were carried out in
2008 May, 2009 June, and 2010 May, bringing the total number of such
observations to six.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table7.dat 70 1279 Galactic center secondary IR astrometric standards
(with corrections from erratum: ApJ, 726, 114)
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See also:
II/270 : TCS-CAIN: NIR Survey of the Galactic plane (Cabrera-Lavers+, 2006)
J/MNRAS/417/114 : HST/NICMOS Galactic Center survey catalogue (Dong+, 2011)
J/A+A/534/A117 : NIR polarimetry of sources near SgrA* (Buchholz+, 2011)
J/A+A/511/A18 : NIR observations of stars near SgrA* (Schoedel+, 2010)
J/ApJ/707/L114 : The orbit of S2 star around Sgr A* (Gillessen+, 2009)
J/A+A/502/91 : Proper motions of stars near SgrA* (Schoedel+, 2009)
J/A+A/499/483 : NIR observations of sources in galactic center (Buchholz+,
2009)
J/ApJ/670/1115 : IRS spectra at 38 positions in Galactic center (Simpson+,
2007)
J/ApJ/659/1241 : Stellar variability in Galactic Center (Rafelski+, 2007)
J/ApJ/643/1011 : Early-type stars in the center of the Galaxy (Paumard+, 2006)
J/ApJ/642/861 : N-band imaging of the Galactic Center (Viehmann+, 2006)
J/ApJ/470/864 : JHKL photometry of the Galactic Center (Blum+, 1996)
http://www.astro.ucla.edu/~ghezgroup/distortion/ : Ghez Research Group
NIRC2 additional data
Byte-by-byte Description of file: table7.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Source name
11- 14 F4.1 mag Kpmag K' band magnitude
16- 22 F7.2 yr EpIR Average time of the IR positional measurements
24- 28 F5.2 arcsec Rad Radius from Sgr A*
30- 36 F7.4 arcsec oRA Offset in Right Ascension from Sgr A*
38- 40 F3.1 mas e_oRA Uncertainty in oRA (1)
42- 48 F7.4 arcsec oDE Offset in Declination from Sgr A*
50- 52 F3.1 mas e_oDE Uncertainty in oDE (1)
54- 57 F4.1 mas/yr pmRA Right Ascension proper motion
59- 61 F3.1 mas/yr e_pmRA Uncertainty in pmRA (2)
63- 66 F4.1 mas/yr pmDE Declination proper motion
68- 70 F3.1 mas/yr e_pmDE Uncertainty in pmDE (2)
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Note (1): Includes centroiding, alignment and residual distortion (∼1mas)
errors, but do not include error in position of Sgr A* (0.55mas,
0.59mas in RA and Dec, respectively).
Note (2): Velocity errors do not include error in velocity of Sgr A*
(0.09mas/yr, 0.14mas/yr in RA and Dec, respectively).
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 07-Aug-2012