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Register a new userIs the distant globular cluster Pal 14 in a deep-freeze?
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We investigate the velocity dispersion of Pal 14, an outer Milky-Way globular cluster at Galactocentric distance of 71 kpc with a very low stellar density (central density 0.1-0.2 Msun/pc^3). Due to this low stellar density the binary population of Pal 14 is likely to be close to the primordial binary population. Artificial clusters are generated with the observed properties of Pal 14 and the velocity dispersion within these clusters is measured as Jordi et al. (2009) have done with 17 observed stars of Pal 14. We discuss the effect of the binary population on these measurements and find that the small velocity dispersion of 0.38 km/s which has been found by Jordi et al. (2009) would imply a binary fraction of less than 0.1, even though from the stellar density of Pal 14 we would expect a binary fraction of more than 0.5. We also discuss the effect of mass segregation on the velocity dispersion as possible explanation for this discrepancy, but find that it would increase the velocity dispersion further. Thus, either Pal 14 has a very unusual stellar population and its birth process was significantly different than we see in todays star forming regions, or the binary population is regular and we would have to correct the observed 0.38 km/s for binarity. In this case the true velocity dispersion of Pal 14 would be much smaller than this value and the cluster would have to be considered as kinematically frigid, thereby possibly posing a challenge for Newtonian dynamics but in the opposite sense to MOND.
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We present 21-cm observations of four Galactic globular clusters, as part of the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties and likelihood of association with the cluster. We conclude that an association of the HI cloud and Pal 4 is possible, but that a chance coincidence between Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether. New, more stringent upper limits were derived for the other three clusters: M 3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas and the interaction of compact clouds with the Galactic Halo gas.
We report the detection of a pair of degree-long tidal tails associated with the globular cluster Palomar 14, using images obtained at the CFHT. We reveal a power-law departure from a King profile at large distances to the cluster center. The density map constructed with the optimal matched filter technique shows a nearly symmetrical and elongated distribution of stars on both sides of the cluster, forming a S-shape characteristic of mass loss. This evidence may be the telltale signature of tidal stripping in action. This, together with its large Galactocentric distance, imposes strong constraints on its orbit and/or origin: i) it must follow an external orbit confined to the peripheral region of the Galactic halo and/or ii) it formed in a satellite galaxy later accreted by the Milky Way.
We present evidence for mass segregation in the outer-halo globular cluster Palomar 14, which is intuitively unexpected since its present-day two-body relaxation time significantly exceeds the Hubble time. Based on archival Hubble Space Telescope imaging, we analyze the radial dependence of the stellar mass function in the clusters inner 39.2 pc in the mass range of 0.53-0.80 M_sun, ranging from the main-sequence turn-off down to a V-band magnitude of 27.1 mag. The mass function at different radii is well approximated by a power law and rises from a shallow slope of 0.6+/-0.2 in the clusters core to a slope of 1.6+/-0.3 beyond 18.6 pc. This is seemingly in conflict with the finding by Beccari et al. (2011), who interpret the clusters non-segregated population of (more massive) blue straggler stars, compared to (less massive) red giants and horizontal branch stars, as evidence that the cluster has not experienced dynamical segregation yet. We discuss how both results can be reconciled. Our findings indicate that the cluster was either primordially mass-segregated and/or used to be significantly more compact in the past. For the latter case, we propose tidal shocks as the mechanism driving the clusters expansion, which would imply that Palomar 14 is on a highly eccentric orbit. Conversely, if the cluster formed already extended and with primordial mass segregation, this could support an accretion origin of the cluster.
We present an analysis of the radial dependence of the stellar mass function in the diffuse outer-halo globular cluster Palomar 14. Using archival HST/WFPC2 data of the clusters central 39 pc (corresponding to ~0.85*r_h) we find that the mass function in the mass range of 0.55 to 0.85 solar masses is well approximated by a power-law at all radii. The mass function steepens with increasing radius, from a shallow power-law slope of 0.66+/-0.32 in the clusters centre to a slope of 1.61+/-0.33 beyond the core radius, showing that the cluster is mass-segregated. This is seemingly in conflict with its long present-day half-mass relaxation time of ~20 Gyr, and with the recent finding by Beccari et al. (2011), who interpret the clusters non-concentrated population of blue straggler stars as evidence that dynamical segregation has not affected the cluster yet. We discuss this apparent conflict and argue that the cluster must have either formed with primordial mass segregation, or that its relaxation time scale must have been much smaller in the past, i.e. that the cluster must have undergone a significant expansion.
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.
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