J/ApJ/741/100 Proper motions of Leo II member stars (Lepine+, 2011)
A first measurement of the proper motion of the Leo II dwarf spheroidal galaxy.
Lepine S., Koch A., Rich R.M., Kuijken K.
<Astrophys. J., 741, 100 (2011)>
=2011ApJ...741..100L 2011ApJ...741..100L
ADC_Keywords: Galaxies, nearby ; Proper motions ; Photometry, HST ;
Stars, fundamental
Keywords: galaxies: dwarf - galaxies: individual (Leo II) - Galaxy: halo -
Local Group - proper motions
Abstract:
We use 14 year baseline images obtained with the Wide Field and
Planetary Camera 2 on board the Hubble Space Telescope (HST) to
derive a proper motion for one of the Milky Way's most distant dwarf
spheroidal companions, Leo II, relative to an extragalactic background
reference frame. Astrometric measurements are performed in the
effective point-spread function formalism using our own developed
code. An astrometric reference grid is defined using 3224 stars that
are members of Leo II and brighter than a magnitude of 25 in the F814W
band. We identify 17 compact extragalactic sources, for which we
measure a systemic proper motion relative to this stellar reference
grid. We derive a proper motion
[µα,µδ]=[+104±113,-33±151]uas/yr for Leo II
in the heliocentric reference frame. Though marginally detected, the
proper motion yields constraints on the orbit of Leo II. Given a
distance of d∼230kpc and a heliocentric radial velocity vr=+79km/s,
and after subtraction of the solar motion, our measurement indicates a
total orbital motion vG=266.1±128.7km/s in the Galactocentric
reference frame, with a radial component vrG=21.5±4.3km/s and
tangential component vtG=265.2±129.4km/s. The small radial
component indicates that Leo II either has a low-eccentricity orbit or
is currently close to perigalacticon or apogalacticon distance. We see
evidence for systematic errors in the astrometry of the extragalactic
sources which, while close to being point sources, are slightly
resolved in the HST images. We provide a detailed catalog of the
stellar and extragalactic sources identified in the HST data which
should provide a solid early-epoch reference for future astrometric
measurements.
Description:
The dSph Leo II was first observed with the WFPC2 on 1994 May 15. In
addition to a pair of shallow 80s exposures in the F555W band, a
total of eight deep exposures (600s each) were obtained in each of
the F555W and F814W bands.
Leo II was reobserved on 2004 March 19. This time, only eight
exposures in the F814W band were obtained, 500-700s each.
Leo II was observed again on 2008 March 25, yielding eight exposures
of 1100s in each of the F555W and F814W bands.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 92 3227 Stellar members of Leo II defining the astrometric
grid
table2.dat 99 17 Extragalactic Reference sources in the Leo II field
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See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
J/ApJ/726/108 : Dispersion-supported stellar systems (Tollerud+, 2011)
J/ApJS/191/352 : Abundances in stars of Milky Way satellites (Kirby+, 2010)
J/AJ/134/566 : Leo II stellar kinematics (Koch+, 2007)
J/AJ/133/270 : Metallicities and ages in Leo II dSph (Koch+, 2007)
J/MNRAS/378/318 : Chemical abundances in Leo I and II dSph (Bosler+, 2007)
J/MNRAS/360/185 : VI photometry of the Leo II galaxy (Bellazzini+, 2005)
J/AJ/120/284 : Variables in Leo II dwarf spheroidal (Siegel+, 2000)
J/MNRAS/261/657 : Deep CCD photometry of Leo II (Demers+ 1993)
Byte-by-byte Description of file: table[12].dat
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Bytes Format Units Label Explanations
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1- 11 F11.7 deg RAdeg [168.355/168.395] Right Ascension in decimal
degrees (J2000)
13- 21 F9.7 deg e_RAdeg Uncertainty in RAdeg
23- 33 F11.7 deg DEdeg [22.144/22.185] Declination in decimal
degrees (J2000)
35- 43 F9.7 deg e_DEdeg Uncertainty in DEdeg
45- 49 F5.2 mas/yr pmRA [-4.1/3.6] Relative proper motion along RA (1)
51- 54 F4.2 mas/yr e_pmRA [0.02/3.6] Uncertainty in pmRA
56- 60 F5.2 mas/yr pmDE [-4.6/6.4] Relative proper motion along DE (1)
62- 65 F4.2 mas/yr e_pmDE [0.03/2.02] Uncertainty in pmDE
67- 71 F5.2 mag Vmag [19.62/27.56] HST/WFPC2 F555W band magnitude
73- 77 F5.2 mag Imag [18.25/27.82] HST/WFPC2 F814W band magnitude
79- 83 F5.2 mag V-I [-1.02/3.1] The HST (F555W-F814W) color index
85- 90 F6.2 --- Lambda [-21/1.1] Second moment of the source profile (2)
92 I1 --- WF [2/4] Wide Field camera frame in which the source
was detected
94- 99 A6 --- ID Extragalactic Reference source number (EGR-NN);
only for table 2
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Note (1): In the defined Leo II stellar grid.
Note (2): The second moment Λ is similar to a sharpness parameter,
sources having the same spread as the local PSF have Λ≃0.
Stars have a moment Λ between -0.4 and 0.4.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 25-Feb-2013