Retention Fractions for Globular Cluster Neutron Stars

Fokker-Planck models are used to give estimates for the retention fractions for newly-born neutron stars in globular clusters as a function of kick velocity. These can be used to calculate the present day numbers of neutron stars in globular clusters and in addressing questions such as the origin of millisecond pulsars. As an example, the Population I kick velocity distribution of Lyne & Lorimer (1994) is used to estimate the retained fractions of neutron stars originating as single stars and in binary systems. For plausible initial conditions fewer than 4% of single neutron stars are retained. The retention fractions from binary systems can be 2 to 5 times higher. The dominant source of retained neutron stars is found to be through binary systems which remain bound after the first supernova, ie. high-mass X-ray binaries. The retained fraction decreases with an increasing number of progenitors, but the retention fraction decreases more slowly than the number of progenitors increases. On balance, more progenitors give more neutron stars in the cluster.Comment: To appear in MNRAS, 15 pages, LaTeX, requires MN and epsf styles, includes 3 PS Figures, compressed, uuencoded forma

The outskirts of globular clusters as modified gravity probes

In the context of theories of gravity modified to account for the observed dynamics of galactic systems without the need to invoke the existence of dark matter, a prediction often appears regarding low acceleration systems: wherever $a$ falls below $a_{0}$ one should expect a transition from the classical to the modified gravity regime.This modified gravity regime will be characterised by equilibrium velocities which become independent of distance, and which scale with the fourth root of the total baryonic mass, $V^{4} \propto M$. The two above conditions are the well known flat rotation curves and Tully-Fisher relations of the galactic regime. Recently however, a similar phenomenology has been hinted at, at the outskirts of Galactic globular clusters, precisely in the region where $a<a_{0}$. Radial profiles of the projected velocity dispersion have been observed to stop decreasing along Keplerian expectations, and to level off at constant values beyond the radii where $a<a_{0}$. We have constructed gravitational equilibrium dynamical models for a number of globular clusters for which the above gravitational anomaly has been reported, using a modified Newtonian force law which yields equilibrium velocities equivalent to MOND. We find models can be easily constructed having an inner Newtonian region and an outer modified gravity regime, which reproduce all observational constraints, surface brightness profiles, total masses and line of sight velocity dispersion profiles. Through the use of detailed single stellar population models tuned individually to each of the globular clusters in question, we derive estimates of the total masses for these systems. Interestingly, we find that the asymptotic values of the velocity dispersion profiles are consistent with scaling with the fourth root of the total masses, as expected under modified gravity scenarios.Comment: Accepted in ApJ, 13 pages, 7 figure

Can high-velocity stars reveal black holes in globular clusters?

We estimate the number of individual, fast-moving stars observable in globular clusters under the assumption that the clusters contain massive central black holes which follow the galactic black-hole mass vs. sigma relationship. We find that radial velocity measurements are unlikely to detect such stars, but that proper motion studies could reveal such stars, if they exist, in the most likely clusters. Thus, HST proper motion studies can test this hypothesis in a few nearby clusters.Comment: 10 pages, 1 figure in AASTeX v5.0. Accepted for publication in ApJ Letter

The Initial Mass Functions in the Super-Star-Clusters NGC 1569A and NGC 1705-1

I use recent photometric and stellar velocity dispersion measurements of the super-star-clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their present-day luminosity/mass (L_V/M) ratios. I then use the inferred L_V/M ratios, together with population synthesis models of evolving star-clusters, to constrain the initial-mass-functions (IMFs) in these objects. I find that (L_V/M)_solar=28.9 in 1569A, and (L_V/M)_solar=126 in 1705-1. It follows that in 1569A the IMF is steep with alpha~2.5 for m**(-alpha)dm IMFs which extend to 0.1 M_sun. This implies that most of the stellar mass in 1569A is contained in low-mass (< 1 M_sun) stars. However, in 1705-1 the IMF is either flat, with alpha<2$, or it is truncated at a lower mass-limit between 1 and 3 M_sun. I compare the inferred IMFs with the mass functions (MFs) of Galactic globular clusters. It appears that 1569A has a sufficient reservoir of low-mass stars for it to plausibly evolve into an object similar to Galactic globular clusters. However, the apparent deficiency of low-mass stars in 1705-1 may make it difficult for this SSC to become a globular cluster. If low-mass stars do dominate the cluster mass in 1705-1, the large L_V/M ratio in this SSC may be evidence that the most massive stars have formed close to the cluster cores.Comment: ApJ, in press. 19 Pages, Latex; [email protected]

Central Proper-Motion Kinematics of NGC 6752

We present proper motions derived from WFPC2 imaging for stars in the core of the peculiar globular cluster NGC 6752. The central velocity dispersion in both components of the proper motion is 12 km/s. We discuss the implications of this result as well as the intrinsic difficulties in making such measurements. We also give an alternative correction for the 34-row problem in the WFPC2 CCDs.Comment: 25 pages, 7 figures, 1 table included. Accepted for publication in A

Heavy Sneutrinos as Dark Matter

We calculate the relic density of very heavy, stable scalar neutrinos in the minimal supersymmetric standard model. We include all two-body final states, as well as the effects of co-annihilation with scalar electrons. We find that the sneutrino relic density is in the cosmologically interesting region 0.1\ltwid\Omega_{\sn}h^2\ltwid 1.0 for 550\gev\ltwid\msn\ltwid 2300\gev. For nominal values of the parameters of our galactic halo, recent results from the Heidelberg--Moscow direct detection experiment set an upper limit on \Omega_{\sn} which is lower by a factor ranging from two to ten, depending on \msn.Comment: 7 pages in LaTeX, plus 1 uuencoded figure (uses epsf), UCSBTH--94--29, UMN--TH--1306/9

Star counts in NGC 6397

I-band CCD images of a large area of the nearby globular cluster NGC~6397 have been used to construct a surface density profile and two luminosity and mass functions. The surface density profile extends out to 14\arcm from the cluster center and shows no sign of a tidal cutoff. The inner profile is a power-law with slope -0.8 steepening to -1.7 outside of 1\arcm. The mass functions are for fields at 4\arcm\ and 11\arcm from the cluster center and confirm the upturn in the mass function for stars less massive than about 0.4 M\solar. There appears to be an excess of low-mass stars over higher-mass stars in the outer field with respect to the inner, in qualitative agreement with expectations for mass segregation.Comment: 16 pages + 7 pages of tables, LaTeX using AASTeX macros, 11 figures available by request, IoA preprin

Sensitivity plots for WIMP modulation searches

Prospects of WIMP searches using the annual modulation signature are discussed on statistical grounds, introducing sensitivity plots for the WIMP-nucleon scalar cross section.Comment: 3 pages, 2 figures, talk given at TAUP'99, september 199