Hierarchical Star-Formation in M33: Fundamental properties of the star-forming regions

Star-formation within galaxies appears on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often sub-structure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the local group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (~20pc) to kpc scales. We find evidence for a continuum of star-forming group sizes, from pc to kpc scales. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power-law with an index, -2, similar to that found for clusters and GMCs. Additionally, the groups follow a similar mass-radius relation as GMCs. The size distribution of the groups is best described by a log-normal distribution and we show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a log-normal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ~4kpc, suggesting a change in the structure of the star-forming ISM, possibly reflected in the lack of GMCs beyond this radius. (abridged)Comment: 12 pages, 16 figures, accepted MNRA

Age Dating Stellar Populations in the Near Infrared: An absolute age indicator from the presence/absence of red supergiants

The determination of age is a critical component in the study of a population of stellar clusters. In this letter we present a new absolute age indicator for young massive star clusters based on J-H colour. This novel method identifies clusters as older or younger than 5.7 +/- 0.8 Myr based on the appearance of the first population of red supergiant stars. We test the technique on the stellar cluster population of the nearby spiral galaxy, M83, finding good agreement with the theoretical predictions. The localisation of this technique to the near-IR promises that it may be used well into the future with space-- and ground--based missions optimised for near-IR observations.Comment: 5 pages, 5 figures. Accepted to MNRAS, November 201

Electronic transport coefficients from ab initio simulations and application to dense liquid hydrogen

Using Kubo's linear response theory, we derive expressions for the frequency-dependent electrical conductivity (Kubo-Greenwood formula), thermopower, and thermal conductivity in a strongly correlated electron system. These are evaluated within ab initio molecular dynamics simulations in order to study the thermoelectric transport coefficients in dense liquid hydrogen, especially near the nonmetal-to-metal transition region. We also observe significant deviations from the widely used Wiedemann-Franz law which is strictly valid only for degenerate systems and give an estimate for its valid scope of application towards lower densities

Gas expulsion in massive star clusters?. Constraints from observations of young and gas-free objects

The final, definitive version of this paper has been published in A&A, Vol 587, A53, February 2016, doi: 10.1051/0004-6361/201526685. Reproduced with permission from Astronomy & Astrophysics, © ESO.Context. Gas expulsion is a central concept in some of the models for multiple populations and the light-element anti-correlations in globular clusters. If the star formation efficiency was around 30 per cent and the gas expulsion happened on the crossing timescale, this process could preferentially expel stars born with the chemical composition of the proto-cluster gas, while stars with special composition born in the centre would remain bound. Recently, a sample of extragalactic, gas-free, young massive clusters has been identified that has the potential to test the conditions for gas expulsion. Aims: We investigate the conditions required for residual gas expulsion on the crossing timescale. We consider a standard initial mass function and different models for the energy production in the cluster: metallicity-dependent stellar winds, radiation, supernovae and more energetic events, such as hypernovae, which are related to gamma ray bursts. The latter may be more energetic than supernovae by up to two orders of magnitude. Methods: We computed a large number of thin-shell models for the gas dynamics, and calculated whether the Rayleigh-Taylor instability is able to disrupt the shell before it reaches the escape speed. Results: We show that the success of gas expulsion depends on the compactness index of a star cluster C5 ≡ (M∗/ 105 M⊙)/(rh/ pc), with initial stellar mass M∗ and half-mass radius rh. For given C5, a certain critical, local star formation efficiency is required to remove the rest of the gas. Common stellar feedback processes may not lead to gas expulsion with significant loss of stars above C5 ≈ 1. Considering pulsar winds and hypernovae, the limit increases to C5 ≈ 30. If successful, gas expulsion generally takes place on the crossing timescale. Some observed young massive clusters have 1 <C5< 10 and are gas-free at ≈10 Myr. This suggests that gas expulsion does not affect their stellar mass significantly, unless powerful pulsar winds and hypernovae are common in such objects. By comparison to observations, we show that C5 is a better predictor for the expression of multiple populations than stellar mass. The best separation between star clusters with and without multiple populations is achieved by a stellar winds-based gas expulsion model, where gas expulsion would occur exclusively in star clusters without multiple populations. Single and multiple population clusters also have little overlap in metallicity and age. Conclusions: Globular clusters should initially have C5 ≲ 100, if the gas expulsion paradigm was correct. Early gas expulsion, which is suggested by the young massive cluster observations, hence would require special circumstances, and is excluded for several objects. Most likely, the stellar masses did not change significantly at the removal of the primordial gas. Instead, the predictive power of the C5 index for the expression of multiple populations is consistent with the idea that gas expulsion may prevent the expression of multiple populations. On this basis, compact young massive clusters should also have multiple populations.Peer reviewe

Investigation of a0-f0 mixing

We investigate the isospin-violating mixing of the light scalar mesons a0(980) and f0(980) within the unitarized chiral approach. Isospin-violating effects are considered to leading order in the quark mass differences and electromagnetism. In this approach both mesons are generated through meson-meson dynamics. Our results provide a description of the mixing phenomenon within a framework consistent with chiral symmetry and unitarity, where these resonances are not predominantly q q-bar states. Amongst the possible experimental signals, we discuss observable consequences for the reaction J/Psi -> phi pi0 eta in detail. In particular we demonstrate that the effect of a0-f0 mixing is by far the most important isospin-breaking effect in the resonance region and can indeed be extracted from experiment.Comment: 15 pages, 9 figures; discussion extended, title changed, version published in Phys. Rev.

The Young Cluster Population of M82 Region B

We present observations obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope of the "fossil" starburst region B in the nearby starburst galaxy M82. By comparing UBVI photometry with models, we derive ages and extinctions for 35 U-band selected star clusters. We find that the peak epoch of cluster formation occurred ~ 150 Myr ago, in contrast to earlier work that found a peak formation age of 1.1 Gyr. The difference is most likely due to our inclusion of U-band data, which are essential for accurate age determinations of young cluster populations. We further show that the previously reported turnover in the cluster luminosity function is probably due to the neglect of the effect of extended sources on the detection limit. The much younger cluster ages we derive clarifies the evolution of the M82 starburst. The M82-B age distribution now overlaps with the ages of: the nuclear starburst; clusters formed on the opposite side of the disk; and the last encounter with M81, some 220 Myr ago.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter

On the circularly polarized optical emission from AE Aquarii

The reported nightly mean value of the circular polarization of optical emission observed from the close binary system AE Aqr is 0.06% (+-) 0.01%. We discuss a possibility that the observed polarized radiation is emitted mainly by the white dwarf or its vicinity. We demonstrate that this hypothesis is rather unlikely since the contribution of the white dwarf to the optical radiation of the system is too small. This indicates that the polarimetric data on AE Aqr cannot be used for the evaluation of the surface magnetic field strength of the white dwarf in this system.Comment: 4 pages, 3 figures, accepted for publication in Astron. & Astrophy

Silicon isotopes in Antarctic sponges : an interlaboratory comparison

Cycling of deepwater silicon (Si) within the Southern Ocean, and its transport into other ocean basins, may be an important player in the uptake of atmospheric carbon, and global climate. Recent work has shown that the Si isotope (denoted by δ29Si or δ30Si) composition of deep sea sponges reflects the availability of dissolved Si during growth, and is a potential proxy for past deep and intermediate water silicic acid concentrations. As with any geochemical tool, it is essential to ensure analytical precision and accuracy, and consistency between methodologies and laboratories. Analytical bias may exist between laboratories, and sponge material may have matrix effects leading to offsets between samples and standards. Here, we report an interlaboratory evaluation of Si isotopes in Antarctic and sub-Antarctic sponges. We review independent methods for measuring Si isotopes in sponge spicules. Our results show that separate subsamples of non-homogenized sponges measured by three methods yield isotopic values within analytical error for over 80% of specimens. The relationship between δ29Si and δ30Si in sponges is consistent with kinetic fractionation during biomineralization. Sponge Si isotope analyses show potential as palaeoceaongraphic archives, and we suggest Southern Ocean sponge material would form a useful additional reference standard for future spicule analyses