Bibcode
Costa, Edgardo; Méndez, René A.; Pedreros, Mario H.; Moyano, Maximiliano; Gallart, Carme; Noël, Noelia; Baume, Gustavo; Carraro, Giovanni
Referencia bibliográfica
The Astronomical Journal, Volume 137, Issue 5, pp. 4339-4360 (2009).
Fecha de publicación:
5
2009
Número de citas
37
Número de citas referidas
34
Descripción
We present the first results of a ground-based program to determine the
proper motion of the Magellanic Clouds (MCs) relative to background
quasars (QSO), being carried out using the Iréneé du Pont
2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields
have been targeted in the Small Magellanic Cloud (SMC) over a time base
of six years, and with seven epochs of observation. One quasar field was
targeted in the Large Magellanic Cloud (LMC), over a time base of five
years, and with six epochs of observation. The shorter time base in the
case of the LMC is compensated by the much larger amount of high-quality
astrometry frames that could be secured for the LMC quasar field (124
frames), compared to the SMC fields (an average of roughly 45 frames).
In this paper, we present final results for field Q0557-6713 in the LMC
and field Q0036-7227 in the SMC. From field Q0557-6713, we have obtained
a measured proper motion of μαcos δ = +1.95
± 0.13 mas yr-1, μδ = +0.43
± 0.18 mas yr-1 for the LMC. From field Q0036-7227, we
have obtained a measured proper motion of μα
cosδ = +0.95 ± 0.29 mas yr-1,
μδ = -1.14 ± 0.18 mas yr-1 for the
SMC. Although we went through the full procedure for another SMC field
(QJ0036-7225), on account of unsolvable astrometric difficulties caused
by blending of the QSO image, it was impossible to derive a reliable
proper motion. Current model rotation curves for the plane of the LMC
indicate that the rotational velocity (V rot) at the position
of LMC field Q0557-6713 can be as low as 50 km s-1, or as
high as 120 km s-1. A correction for perspective and rotation
effects leads to a center of mass proper motion for the LMC of
μα cosδ = +1.82 ± 0.13 mas
yr-1, μδ = +0.39 ± 0.15 mas
yr-1 (V rot = 50 km s-1), and to
μα cosδ = +1.61 ± 0.13 mas
yr-1, μδ = +0.60 ± 0.15 mas
yr-1 (V rot = 120 km s-1). Assuming
that the SMC has a disk-like central structure, but that it does not
rotate, we obtain a center of mass proper motion for the SMC of
μα cosδ = +1.03 ± 0.29 mas
yr-1, μδ = -1.09 ± 0.18 mas
yr-1. Our results are in reasonable agreement with most
previous determinations of the proper motion of the MCs, including
recent Hubble Space Telescope measurements. Complemented with published
values of the radial velocity of the centers of the LMC and SMC, we have
used our proper motions to derive the galactocentric (gc) velocity
components of the MCs. For the LMC, we obtain V gc,t = +315
± 20 km s-1, V gc,r = +86 ± 17 km
s-1 (V rot = 50 km s-1), and V
gc,t = +280 ± 24 km s-1, V gc,r
= +94 ± 17 km s-1 (V rot = 120 km
s-1). For the SMC, we obtain V gc,t = +258
± 50 km s-1, V gc,r = +20 ± 44 km
s-1. These velocities imply a relative velocity between the
LMC and SMC of 84 ± 50 km s-1, for V
rot,LMC = 50 km s-1, and 62 ± 63 km
s-1 for V rot,LMC = 120 km s-1. Albeit
our large errors, these values are not inconsistent with the standard
assumption that the MCs are gravitationally bound to each other.
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