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Register a new userAbsolute proper motion of the Galactic open cluster M67
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A. Bellini
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We derived the absolute proper motion (PM) of the old, solar-metallicity Galactic open cluster M67 using observations collected with CFHT (1997) and with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to determine the absolute PM of the cluster. We find (mu_alpha cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50 pc, we explore the influence of the Galactic potential, with and without the bar and/or spiral arms, on the galactic orbit of the cluster.
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We present the first measurement of the absolute proper motions of IRAS 00259+5625 (CB3, LBN594) associated with the HI loop called the NGC281 superbubble that extends from the Galactic plane over ~300 pc toward decreasing galactic latitude. The proper motion components measured with VERA are (mu_alpha cos(delta), mu_delta) = (-2.48 +/- 0.32, -2.85 +/- 0.65) mas yr^{-1}, converted into (mu_l cos(b), mu_b) = (-2.72 +/- 0.32, -2.62 +/- 0.65) mas yr^{-1} in the Galactic coordinates. The measured proper motion perpendicular to the Galactic plane (mu_b) shows vertical motion away from the Galactic plane with a significance of about ~4-sigma. As for the source distance, the distance measured with VERA is marginal, 2.4^{+1.0}_{-0.6} kpc. Using the distance, an absolute vertical motion (v_{b}) of -17.9 +/- 12.2 km s^{-1} is determined with ~1.5-sigma significance. The tendency of the large vertical motion is consistent with previous VLBI results for NGC 281 associated with the same superbubble. Thus, our VLBI results indicate the superbubble expansion motion whose origin is believed to be sequential supernova explosions.
In this paper we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 ($omega$ Cen) as part of the HST large program GO-14118+14662. We analyzed a field 17 arcmin South-West of the center of $omega$ Cen and computed PMs with an epoch span of $sim$15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is: $(mu_alpha cosdelta , mu_delta) = (-3.341 pm 0.028 , -6.557 pm 0.043)$ mas yr$^{-1}$. Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass $(mu_alpha cosdelta , mu_delta) = (-3.238 pm 0.028, -6.716 pm 0.043)$ mas yr$^{-1}$. This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for $omega$ Cen in the literature by more than 5$sigma$, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of $omega$ Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for $omega$ Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.
We have derived the absolute proper motion (PM) of the globular cluster M55 using a large set of CCD images collected with the du Pont telescope between 1997 and 2008. We find (PM_RA*cos(DEC), PM_DEC) = (-3.31 +/- 0.10, -9.14 +/- 0.15) mas/yr relative to background galaxies. Membership status was determined for 16 945 stars with 14<V<21 from the central part of the cluster. The PM catalogue includes 52 variables of which 43 are probable members of M55. This sample is dominated by pulsating blue straggler stars but also includes 5 eclipsing binaries, three of which are main sequence objects. The survey also identified several candidate blue, yellow and red straggler stars belonging to the cluster. We detected 15 likely members of the Sgr dSph galaxy located behind M55. The average PM for these stars was measured to be (PM_RA*cos(DEC), PM_DEC)=(-2.23 +/- 0.14, -1.83 +/- 0.24) mas/yr.
We present a proper motion survey of the Galactic plane, using IPHAS data and POSS-I Schmidt plate data as a first epoch, that probes down to proper motions below 50 milliarcseconds per year. The IPHAS survey covers the northern plane ($|b| < 5^{circ}$) with CCD photometry in the $r$, $i$ and H${alpha}$ passbands. We examine roughly 1400 sq. deg. of the IPHAS survey area and draw up a catalogue containing 103058 objects with significant proper motions below 150 millarcseconds per year in the magnitude range 13.5$< r <$19. Our survey sample contains large samples of white dwarfs and subdwarfs which can be identified using a reduced proper motion diagram. We also found several objects with IPHAS colours suggesting H${alpha}$ emission and significant proper motions. One is the known cataclysmic variable GD552; two are known DB white dwarfs and five others are found to be non-DA (DB and DC) white dwarfs, which were included in the H$alpha$ emission line catalogue due to their lack of absorption in the H$alpha$ narrow-band.
Stars in open clusters are expected to share an identical abundance pattern. Establishing the level of chemical homogeneity in a given open cluster deserves further study as it is the basis of the concept of chemical tagging to unravel the history of the Milky Way. M67 is particularly interesting given its solar metallicity and age as well as being a dense cluster environment. We conducted a strictly line-by-line differential chemical abundance analysis of two solar twins in M67: M67-1194 and M67-1315. Stellar atmospheric parameters and elemental abundances were obtained with high precision using Keck/HIRES spectra. M67-1194 is essentially identical to the Sun in terms of its stellar parameters. M67-1315 is warmer than M67-1194 by ~ 150 K as well as slightly more metal-poor than M67-1194 by ~ 0.05 dex. M67-1194 is also found to have identical chemical composition to the Sun, confirming its solar twin nature. The abundance ratios [X/Fe] of M67-1315 are similar to the solar abundances for elements with atomic number Z <= 30, while most neutron-capture elements are enriched by ~ 0.05 dex, which might be attributed to enrichment from a mixture of AGB ejecta and r-process material. The distinct chemical abundances for the neutron-capture elements in M67-1315 and the lower metallicity of this star compared to M67-1194, indicate that the stars in M67 are likely not chemically homogeneous. This poses a challenge for the concept of chemical tagging since it is based on the assumption of stars forming in the same star-forming aggregate.
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