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

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

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

Status and preliminary results of the ANAIS experiment at Canfranc

ANAIS (Annual Modulation with NaI's) is an experiment planned to investigate seasonal modulation effects in the signal of galactic WIMPs using up to 107 kg of NaI(Tl) in the Canfranc Underground Laboratory (Spain). A prototype using one single crystal (10.7 kg) is being developed before the installation of the complete experiment; the first results presented here show an average background level of 1.2 counts/(keV kg day) from threshold (Ethr~4 keV) up to 10 keV.Comment: 3 pages, 2 figures, talk delivered at the 7th International Workshop on Topics in Astroparticle and Underground Physics (TAUP 2001), September 2001, Laboratori Nazionali del Gran Sasso, Italy (to appear in the Conference Proceedings, Nucl. Phys. B (Proc. Suppl.)

A Measurement of Photon Production in Electron Avalanches in CF4

This paper presents a measurement of the ratio of photon to electron production and the scintillation spectrum in a popular gas for time pro jection chambers, carbon tetrafluoride (CF4), over the range of 200 to 800 nm; the ratio is measured to be 0.34+/-0.04. This result is of particular importance for a new generation of dark matter time projection chambers with directional sensitivity which use CF4 as a fill gas.Comment: 19 pages, including appendix. 8 figure

The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks

We systematically examine how the presence in a binary affects the final core structure of a massive star and its consequences for the subsequent supernova explosion. Interactions with a companion star may change the final rate of rotation, the size of the helium core, the strength of carbon burning and the final iron core mass. Stars with initial masses larger than \sim 11\Ms that experiece core collapse will generally have smaller iron cores at the time of the explosion if they lost their envelopes due to a previous binary interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger helium and metal cores if they have a close companion, since the second dredge-up phase which reduces the helium core mass dramatically in single stars does not occur once the hydrogen envelope is lost. We find that the initially more massive stars in binary systems with masses in the range 8 - 11\Ms are likely to undergo an electron-capture supernova, while single stars in the same mass range would end as ONeMg white dwarfs. We suggest that the core collapse in an electron-capture supernova (and possibly in the case of relatively small iron cores) leads to a prompt explosion rather than a delayed neutrino-driven explosion and that this naturally produces neutron stars with low-velocity kicks. This leads to a dichotomous distribution of neutron star kicks, as inferred previously, where neutron stars in relatively close binaries attain low kick velocities. We illustrate the consequences of such a dichotomous kick scenario using binary population synthesis simulations and discuss its implications. This scenario has also important consequences for the minimum initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio