Eccentric double white dwarfs as LISA sources in globular clusters

We consider the formation of double white dwarfs (DWDs) through dynamical interactions in globular clusters. Such interactions can give rise to eccentric DWDs, in contrast to the exclusively circular population expected to form in the Galactic disk. We show that for a 5-year Laser Interferometer Space Antenna (LISA) mission and distances as far as the Large Magellanic Cloud, multiple harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher than 8 for at least a handful of eccentric DWDs, given their formation rate and typical lifetimes estimated from current cluster simulations. Consequently the association of eccentricity with stellar-mass LISA sources does not uniquely involve neutron stars, as is usually assumed. Due to the difficulty of detecting (eccentric) DWDs with present and planned electromagnetic observatories, LISA could provide unique dynamical identifications of these systems in globular clusters.Comment: Published in ApJ 665, L5

On the origin of planets at very wide orbits from the re-capture of free floating planets

In recent years several planets have been discovered at wide orbits (>100 AU) around their host stars. Theoretical studies encounter difficulties in explaining their formation and origin. Here we propose a novel scenario for the production of planetary systems at such orbits, through the dynamical recapture of free floating planets (FFPs) in dispersing stellar clusters. This process is a natural extension of the recently suggested scenario for the formation of wide stellar binaries. We use N-body simulations of dispersing clusters with 10-1000 stars and comparable numbers of FFPs to study this process. We find that planets are captured into wide orbits in the typical range ~100-10^6 AU, and have a wide range of eccentricities (thermal distribution). Typically, 3-6 x (f_FFP/1) % of all stars capture a planetary companion with such properties (where f_FFP is the number of FFP per star). The planetary capture efficiency is comparable to that of capture-formed stellar-binaries, and shows a similar dependence on the cluster size and structure. It is almost independent of the specific planetary mass; planets as well as sub-stellar companions of any mass can be captured. The capture efficiency decreases with increasing cluster size, and for a given cluster size the it increases with the host/primary mass. More than one planet can be captured around the same host and planets can be captured into binary systems. Planets can also be captured into pre-existing planetary and into orbits around black holes and massive white dwarfs, if these formed early enough before the cluster dispersal. In particular, stellar black holes have a high capture efficiency (>50 % and 5-10 x (f_FFP/1) % for capture of stars and planetary companions, respectively) due to their large mass. Finally, although rare, two FFPs or brown dwarfs can become bound and form a FFP-binary system with no stellar host.Comment: ApJ, in press. Added two figure

Mass Segregation in Globular Clusters

We present the results of a new study of mass segregation in two-component star clusters, based on a large number of numerical N-body simulations using our recently developed dynamical Monte Carlo code. Specifically, we follow the dynamical evolution of clusters containing stars with individual masses m_1 as well as a tracer population of objects with individual masses m_2=\mu m_1, using N=10^5 total stars. For heavy tracers, which could represent stellar remnants such as neutron stars or black holes in a globular cluster, we characterize in a variety of ways the tendency for these objects to concentrate in or near the cluster core. In agreement with simple theoretical arguments, we find that the characteristic time for this mass segregation process varies as 1/\mu. For models with very light tracers (\mu <~ 10^-2), which could represent free-floating planets or brown dwarfs, we find the expected depletion of light objects in the cluster core, but also sometimes a significant enhancement in the halo. Using these results we estimate the optical depth to gravitational microlensing by planetary mass objects or brown dwarfs in typical globular clusters. For some initial conditions, the optical depth in the halo due to very low-mass objects could be much greater than that of luminous stars. If we apply our results to M22, using the recent null detection of Sahu, Anderson, & King (2001), we find an upper limit of ~25% at the 63% confidence level for the current mass fraction of M22 in the form of very low-mass objects.Comment: Accepted for publication in ApJ. Minor revisions reflecting the new results of Sahu et al. on M22. 13 pages in emulateapj style, including 9 figures and 3 table

The optical companion to the binary millisecond pulsar J1824-2452H in the globular cluster M28

We report on the optical identification of the companion star to the eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster M28 (NGC 6626). This star is at only 0.2" from the nominal position of the pulsar and it shows optical variability (~ 0.25 mag) that nicely correlates with the pulsar orbital period. It is located on the blue side of the cluster main sequence, ~1.5 mag fainter than the turn-off point. The observed light curve shows two distinct and asymmetric minima, suggesting that the companion star is suffering tidal distortion from the pulsar. This discovery increases the number of non-degenerate MSP companions optically identified so far in globular clusters (4 out of 7), suggesting that these systems could be a common outcome of the pulsar recycling process, at least in dense environments where they can be originated by exchange interactions.Comment: accepted for publication on ApJ, 17 pages, 5 figure

The binary fraction in the globular cluster M10 (NGC 6254): comparing core and outer regions

We study the binary fraction of the globular cluster M10 (NGC 6254) as a function of radius from the cluster core to the outskirts, by means of a quan- titative analysis of the color distribution of stars relative to the fiducial main sequence. By taking advantage of two data-sets, acquired with the Advanced Camera for Survey and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have studied both the core and the external regions of the cluster. The binary fraction is found to decrease from 14% within the core, to 1.5% in a region between 1 and 2 half-mass radii from the cluster centre. Such a trend and the derived values are in agreement with previous results ob- tained in clusters of comparable total magnitude. The estimated binary fraction is sufficient to account for the suppression of mass segregation observed in M10, without any need to invoke the presence of an intermediate-mass black hole in its centre.Comment: Accepted for publication in ApJ (22 pages, 7 figures, 3 tables

Intermediate mass black holes in AGN disks: I. Production & Growth

Here we propose a mechanism for efficiently growing intermediate mass black holes (IMBH) in disks around supermassive black holes. Stellar mass objects can efficiently agglomerate when facilitated by the gas disk. Stars, compact objects and binaries can migrate, accrete and merge within disks around supermassive black holes. While dynamical heating by cusp stars excites the velocity dispersion of nuclear cluster objects (NCOs) in the disk, gas in the disk damps NCO orbits. If gas damping dominates, NCOs remain in the disk with circularized orbits and large collision cross-sections. IMBH seeds can grow extremely rapidly by collisions with disk NCOs at low relative velocities, allowing for super-Eddington growth rates. Once an IMBH seed has cleared out its feeding zone of disk NCOs, growth of IMBH seeds can become dominated by gas accretion from the AGN disk. However, the IMBH can migrate in the disk and expand its feeding zone, permitting a super-Eddington accretion rate to continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of migrators. We highlight the remarkable parallel between the growth of IMBH in AGN disks with models of giant planet growth in protoplanetary disks. If an IMBH becomes massive enough it can open a gap in the AGN disk. IMBH migration in AGN disks may stall, allowing them to survive the end of the AGN phase and remain in galactic nuclei. Our proposed mechanisms should be more efficient at growing IMBH in AGN disks than the standard model of IMBH growth in stellar clusters. Dynamical heating of disk NCOs by cusp stars is transferred to the gas in a AGN disk helping to maintain the outer disk against gravitational instability. Model predictions, observational constraints and implications are discussed in a companion paper (Paper II).Comment: 11 pages, 4 figures, MNRAS (accepted

The effects of fly-bys on planetary systems

Most of the observed extrasolar planets are found on tight and often eccentric orbits. The high eccentricities are not easily explained by planet-formation models, which predict that planets should be on rather circular orbits. Here we explore whether fly-bys involving planetary systems with properties similar to those of the gas giants in the solar system, can produce planets with properties similar to the observed planets. Using numerical simulations, we show that fly-bys can cause the immediate ejection of planets, and sometimes also lead to the capture of one or more planets by the intruder. More common, however, is that fly-bys only perturb the orbits of planets, sometimes leaving the system in an unstable state. Over time-scales of a few million to several hundred million years after the fly-by, this perturbation can trigger planet-planet scatterings, leading to the ejection of one or more planets. For example, in the case of the four gas giants of the solar system, the fraction of systems from which at least one planet is ejected more than doubles in 10^8 years after the fly-by. The remaining planets are often left on more eccentric orbits, similar to the eccentricities of the observed extrasolar planets. We combine our results of how fly-bys effect solar-system-like planetary systems, with the rate at which encounters in young stellar clusters occur. For example, we measure the effects of fly-bys on the four gas giants in the solar system. We find, that for such systems, between 5 and 15 per cent suffer ejections of planets in 10^8 years after fly-bys in typical open clusters. Thus, encounters in young stellar clusters can significantly alter the properties of any planets orbiting stars in clusters. As a large fraction of stars which populate the solar neighbourhood form in stellar clusters, encounters can significantly affect the properties of the observed extrasolar planets.Comment: 22 pages, 15 figures, 5 tables. Accepted for publication in MNRA

The blue stragglers formed via mass transfer in old open clusters

In this paper, we present the simulations for the primordial blue stragglers in the old open cluster M67 based on detailed modelling of the evolutionary processes. The principal aim is to discuss the contribution of mass transfer between the components of close binaries to the blue straggler population in M67. First, we followed the evolution of a binary of 1.4M$_\odot$+0.9M$_\odot$. The synthetic evolutionary track of the binary system revealed that a primordial blue straggler had a long lifetime in the observed blue straggler region of color-magnitude diagram. Second, a grid of models for close binary systems experiencing mass exchange were computed from 1Gyr to 6Gyr in order to account for primordial blue-straggler formation in a time sequence. Based on such a grid, Monte-Carlo simulations were applied for the old open cluster M67. Adopting appropriate orbital parameters, 4 primordial blue stragglers were predicted by our simulations. This was consistent with the observational fact that only a few blue stragglers in M67 were binaries with short orbital periods. An upper boundary of the primordial blue stragglers in the color-magnitude diagram (CMD) was defined and could be used to distinguish blue stragglers that were not formed via mass exchange. Using the grid of binary models, the orbital periods of the primordial BSs could be predicted. Compared with the observations, it is clear that the mechanism discussed in this work alone cannot fully predict the blue straggler population in M67. There must be several other processes also involved in the formation of the observed blue stragglers in M67.Comment: 11 pages, 6 figures, A&A accepte

The ACS Survey of Galactic Globular Clusters. XII. Photometric Binaries along the Main-Sequence

The fraction of binary stars is an important ingredient to interpret globular cluster dynamical evolution and their stellar population. We investigate the properties of main-sequence binaries measured in a uniform photometric sample of 59 Galactic globular clusters that were observed by HST WFC/ACS as a part of the Globular Cluster Treasury project. We measured the fraction of binaries and the distribution of mass-ratio as a function of radial location within the cluster, from the central core to beyond the half-mass radius. We studied the radial distribution of binary stars, and the distribution of stellar mass ratios. We investigated monovariate relations between the fraction of binaries and the main parameters of their host clusters. We found that in nearly all the clusters, the total fraction of binaries is significantly smaller than the fraction of binaries in the field, with a few exceptions only. Binary stars are significantly more centrally concentrated than single MS stars in most of the clusters studied in this paper. The distribution of the mass ratio is generally flat (for mass-ratio parameter q>0.5). We found a significant anti-correlation between the binary fraction in a cluster and its absolute luminosity (mass). Some, less significant correlation with the collisional parameter, the central stellar density, and the central velocity dispersion are present. There is no statistically significant relation between the binary fraction and other cluster parameters. We confirm the correlation between the binary fraction and the fraction of blue stragglers in the cluster.Comment: 43 Pages, 52 figures, accepted for publication in A&

The Multiple Origin of Blue Straggler Stars: Theory vs. Observations

In this chapter we review the various suggested channels for the formation and evolution of blue straggler stars (BSSs) in different environments and their observational predictions. These include mass transfer during binary stellar evolution - case A/B/C and D (wind Roche-lobe overflow) mass transfer, stellar collisions during single and binary encounters in dense stellar cluster, and coupled dynamical and stellar evolution of triple systems. We also explore the importance of the BSS and binary dynamics in stellar clusters. We review the various observed properties of BSSs in different environments (halo and bulge BSSs, BSSs in globular clusters and BSSs in old open clusters), and compare the current observations with the theoretical predictions for BSS formation. We try to constrain the likely progenitors and processes that play a role in the formation of BSSs and their evolution. We find that multiple channels of BSS formation are likely to take part in producing the observed BSSs, and we point out the strengths and weaknesses of each the formation channel in respect to the observational constraints. Finally we point out directions to further explore the origin of BSS, and highlight eclipsing binary BSSs as important observational tool.Comment: Chapter 11, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe