Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys. I. The low luminosity galaxy NGC 45

We present results from ACS and WFPC observations in the low luminosity galaxy NGC 45. We identified 28 young star cluster candidates. While the exact values of age, mass, and extinction depend somewhat on the choice of SSP models, we find no young clusters with masses higher than a few 1000 Msun for any model choice. We derive the luminosity function of young star clusters and find a slope of alpha=-1.94+-0.28. We also identified 19 old globular clusters and we estimate a specific frequency of globular clusters of S_N=1.4-1.9 which is significantly higher than observed for other late-type galaxies (e.g. SMC, LMC, M33). Most of these globular clusters appear to belong to a metal-poor population, although they coincide spatially with the location of the bulge of NGC 45.Comment: 16 pages,18 figures, accepted for publication in A&

Properties of star clusters - II. Scaleheight evolution of clusters

Until now, it has been impossible to observationally measure how star cluster scaleheight evolves beyond 1 Gyr as only small samples have been available. Here, we establish a novel method to determine the scaleheight of a cluster sample using modelled distributions and Kolmogorov-Smirnov tests. This allows us to determine the scaleheight with a 25 per cent accuracy for samples of 38 clusters or more. We apply our method to investigate the temporal evolution of cluster scaleheight, using homogeneously selected sub-samples of Kharchenko et al. (MWSC), Dias et al. (DAML02), WEBDA, and Froebrich et al. (FSR). We identify a linear relationship between scaleheight and log(age/yr) of clusters, considerably different from field stars. The scaleheight increases from about 40 pc at 1Myr to 75 pc at 1 Gyr,most likely due to internal evolution and external scattering events. After 1 Gyr, there is amarked change of the behaviour, with the scaleheight linearly increasing with log(age/yr) to about 550 pc at 3.5 Gyr. The most likely interpretation is that the surviving clusters are only observable because they have been scattered away from the mid-plane in their past. A detailed understanding of this observational evidence can only be achieved with numerical simulations of the evolution of cluster samples in the Galactic disc. Furthermore, we find a weak trend of an age-independent increase in scaleheight with Galactocentric distance. There are no significant temporal or spatial variations of the cluster distribution zero-point. We determine the Sun's vertical displacement from the Galactic plane as Z± = 18.5 ± 1.2 pc. ©2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

We present a simple analytical description of the disruption of star clusters in a tidal field, which agrees excellently with detailed N-body simulations. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to open clusters in the solar neighbourhood, based on the new cluster sample of Kharchenko et al. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr. This is a factor 5 shorter than derived from N-body simulations of clusters in the tidal field of the galaxy. (2) The present starformation rate in bound clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7 10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is half as small as that derived from the study of embedded clusters. The difference suggests that half of the clusters in the solar neighbourhood become unbound within 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of 3*10^4 M_sun. This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of -2, even if the physical upper mass limit is as high as 10^6 M_sun.Comment: 14 pages, 15 figures, to appear in Astronomy & Astrophysic

The star cluster - field star connection in nearby spiral galaxies I. Data analysis techniques and application to NGC 4395

It is generally assumed that a large fraction of stars are initially born in clusters. However, a large fraction of these disrupt on short timescales and the stars end up belonging to the field. Understanding this process is of paramount importance if we wish to constrain the star formation histories of external galaxies using star clusters. We attempt to understand the relation between field stars and star clusters by simultaneously studying both in a number of nearby galaxies. As a pilot study, we present results for the late-type spiral NGC 4395 using HST/ACS and HST/WFPC2 images. Different detection criteria were used to distinguish point sources (star candidates) and extended objects (star cluster candidates). Using a synthetic CMD method, we estimated the star formation history. Using simple stellar population model fitting, we calculated the mass and age of the cluster candidates. The field star formation rate appears to have been roughly constant, or to have possibly increased by up to about a factor of two, for ages younger than $\sim$300 Myr within the fields covered by our data. Our data do not allow us to constrain the star formation histories at older ages. We identify a small number of clusters in both fields. Neither massive ($>10^5$ M$_\odot$) clusters nor clusters with ages $\geq1$ Gyr were found in the galaxy and we found few clusters older than 100 Myr. Based on our direct comparison of field stars and clusters in NGC 4395, we estimate the ratio of star formation rate in clusters that survive for $10^7$ to $10^8$ years to the total star formation to be $\Gamma\sim0.03$. We suggest that this relatively low $\Gamma$ value is caused by the low star formation rate of NGC 4395.Comment: 16 pages, 20 figures, accepted for publication in A&

The emission positions of kHz QPOs and Kerr spacetime influence

Based the Alfven wave oscillation model (AWOM) and relativistic precession model (RPM) for twin kHz QPOs, we estimate the emission positions of most detected kHz QPOs to be at r=18+-3 km (R/15km) except Cir X-1 at r = 30\+-5 km (R/15km). For the proposed Keplerian frequency as an upper limit to kHz QPO, the spin effects in Kerr Spacetime are discussed, which have about a 5% (2%) modification for that of the Schwarzchild case for the spin frequency of 1000 (400) Hz.The application to the four typical QPO sources, Cir X-1, Sco X-1, SAX J1808.4-3658 and XTE 1807-294, is mentioned.Comment: Science China, Physics, Mechanics & Astronomy, 2010, 53, NO.

RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs

(Abridged) We report on ten weeks of RXTE observations of the X-ray transient XTE J1701-462. Comparisons with other sources suggest it had all the characteristics of the neutron star Z sources (the brightest persistent neutron star LMXBs). These include Z tracks in X-ray color diagrams and typical variability components detected in the power spectra. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate (Mdot) and luminosity dependencies in neutron star LMXBs. As its overall luminosity decreased, we observed a switch between two types of Z-source behavior, with most of the branches of the Z-track changing their shape and/or orientation. We interpret this switch as an extreme case of the longterm changes seen in the persistent Z sources and suggest that these result from changes in Mdot. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) might be more luminous (> 50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in Mdot, we propose that changes in Mdot can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will evolve into the branches observed in the X-ray color diagrams of the less luminous atoll sources, but not in a way that was previously suggested.Comment: Accepted for publication in ApJ, 11 page

Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models

The strong gravitational field of neutron stars in the brany universe could be described by spherically symmetric solutions with a metric in the exterior to the brany stars being of the Reissner-Nordstrom type containing a brany tidal charge representing the tidal effect of the bulk spacetime onto the star structure. We investigate the role of the tidal charge in orbital models of high-frequency quasiperiodic oscillations (QPOs) observed in neutron star binary systems. We focus on the relativistic precession model. We give the radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic oscillations. We show how the standard relativistic precession model modified by the tidal charge fits the observational data, giving estimates of the allowed values of the tidal charge and the brane tension based on the processes going in the vicinity of neutron stars. We compare the strong field regime restrictions with those given in the weak-field limit of solar system experiments.Comment: 26 pages, 6 figure