It has been claimed in the recent literature that a non-trivial relation between the mass of the most-massive star, mmax, in a star cluster and its embedded star cluster mass (the mmax-Mecl relation) is falsified by observations of the most-massive s
tars and the Halpha luminosity of young star clusters in the starburst dwarf galaxy NGC 4214. Here it is shown by comparing the NGC 4214 results with observations from the Milky Way that NGC 4214 agrees very well with the predictions of the the mmax-Mecl relation and the integrated galactic stellar initial mass function (IGIMF) theory and that this difference in conclusions is based on a high degree of degeneracy between expectations from random sampling and those from the mmax-Mecl relation, but are also due to interpreting mmax as a truncation mass in a randomly sampled IMF. Additional analysis of galaxies with lower SFRs than those currently presented in the literature will be required to break this degeneracy.
Observational studies are showing that the galaxy-wide stellar initial mass function are top-heavy in galaxies with high star-formation rates (SFRs). Calculating the integrated galactic stellar initial mass function (IGIMF) as a function of the SFR o
f a galaxy, it follows that galaxies which have or which formed with SFRs > 10 Msol yr^-1 would have a top-heavy IGIMF in excellent consistency with the observations. Consequently and in agreement with observations, elliptical galaxies would have higher M/L ratios as a result of the overabundance of stellar remnants compared to a stellar population that formed with an invariant canonical stellar initial mass function (IMF). For the Milky Way, the IGIMF yields very good agreement with the disk- and the bulge-IMF determinations. Our conclusions are that purely stochastic descriptions of star formation on the scales of a pc and above are falsified. Instead, star formation follows the laws, stated here as axioms, which define the IGIMF theory. We also find evidence that the power-law index beta of the embedded cluster mass function decreases with increasing SFR. We propose further tests of the IGIMF theory through counting massive stars in dwarf galaxies.
Recent evidence based independently on spectral line strengths and dynamical modelling point towards a non-universal stellar Initial Mass Function (IMF), probably implying an excess of low-mass stars in elliptical galaxies with a high velocity disper
sion. Here we show that a time-independent bottom-heavy IMF is compatible neither with the observed metal-rich populations found in giant ellipticals nor with the number of stellar remnants observed within these systems. We suggest a two-stage formation scenario involving a time-dependent IMF to reconcile these observational constraints. In this model, an early strong star-bursting stage with a top-heavy IMF is followed by a more prolonged stage with a bottom-heavy IMF. Such model is physically motivated by the fact that a sustained high star formation will bring the interstellar medium to a state of pressure, temperature and turbulence that can drastically alter the fragmentation of the gaseous component into small clumps, promoting the formation of low-mass stars. This toy model is in good agreement with the different observational constrains on massive elliptical galaxies, such as age, metallicity, alpha-enhancement, M/L, or the mass fraction of the stellar component in low-mass stars.
We introduce a new method to measure the dispersion of mmax values of star clusters and show that the observed sample of mmax is inconsistent with random sampling from an universal stellar initial mass function (IMF) at a 99.9% confidence level. The
scatter seen in the mmax-Mecl data can be mainly (76%) understood as being the result of observational uncertainties only. The scatter of mmax values at a given Mecl are consistent with mostly measurement uncertainties such that the true (physical) scatter may be very small. Additionally, new data on the local star-formation regions Taurus-Auriga and L1641 in Orion make stochastically formed stellar populations rather unlikely. The data are however consistent with the local IGIMF (integrated galactic stellar initial mass function) theory according to which a stellar population is a sum of individual star-forming events each of which is described by well defined physical laws. Randomly sampled IMFs and henceforth scale-free star formation seems to be in contradiction to observed reality.
The question whether or not massive stars can form in isolation or only in star clusters is of great importance for the theory of (massive) star-formation as well as for the stellar initial mass function of whole galaxies (IGIMF-theory). While a seem
ingly easy question it is rather difficult to answer. Several physical processes (e.g. star-loss due to stellar dynamics or gas expulsion) and observational limitations (e.g. dust obscuration of young clusters, resolution) pose severe challenges to answer this question. In this contribution we will present the current arguments in favour and against the idea that all O stars form in clusters.
Over the past years observations of young and populous star clusters have shown that the stellar initial mass function (IMF) can be conveniently described by a two-part power-law with an exponent alpha_2 = 2.3 for stars more massive than about 0.5 Ms
ol and an exponent of alpha_1 = 1.3 for less massive stars. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters, and that the mass function of these can be described as a power-law (the embedded cluster mass function, ECMF) with an exponent beta ~2. These two results imply that the integrated galactic IMF (IGIMF) for early-type stars cannot be a Salpeter power-law, but that they must have a steeper exponent. An application to star-burst galaxies shows that the IGIMF can become top-heavy. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies. In this contribution the IGIMF theory is described, and the accompanying contribution by Pflamm-Altenburg, Weidner & Kroupa (this volume) documents the applications of the IGIMF theory to galactic astrophysics.
Context. The mass discrepancy in massive O stars represents a long-standing problem in stellar astrophysics with far-reaching implications for the chemical and dynamical feedback in galaxies. Aims. Our goal is to investigate this mass discrepancy by
comparing state-of-the-art model masses with model-independent masses determined from eclipsing binaries. Methods. Using stellar evolution models and a recent calibration of stellar parameters for O-star spectral sub-classes, we present a convenient way to convert observed solar metallicity O star spectral types into model masses, which we subsequently compare to our dynamical mass compilation. We also derive similar
Super Star Clusters (Mecl > 10^5 Msol) are the largest stellar nurseries in our local Universe, containing hundreds of thousands to millions of young stars within a few light years. Many of these systems are found in external galaxies, especially in
pairs of interacting galaxies, and in some dwarf galaxies, but relatively few in disk galaxies like our own Milky Way. We show that a possible explanation for this difference is the presence of shear in normal spiral galaxies which impedes the formation of the very large and dense super star clusters but prefers the formation of loose OB associations possibly with a less massive cluster at the center. In contrast, in interacting galaxies and in dwarf galaxies, regions can collapse without having a large-scale sense of rotation. This lack of rotational support allows the giant clouds of gas and stars to concentrate into a single, dense and gravitationally bound system.
With the use of N-body calculations the amount and properties of escaping stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas expulsion are studied over the first 5 Myr of their existence. Besides the number of stars also di
fferent initial radii and binary populations are examined as well as virialised and collapsing clusters. It is found that these clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with considerable velocities up to more than 100 km/s. Even with their mean velocities between 2 and 8 km/s these stars will still be travelling between 2 and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in star-forming regions not necessarily be counted as evidence for the isolated formation of stars.
Aims. Current and upcoming space missions may be able to detect moons of transiting extra-solar planets. In this context it is important to understand if exomoons are expected to exist and what their possible properties are. Methods. Using estimates
for the stability of exomoon orbits from numerical studies, a list of 87 known transiting exoplanets is tested for the potential to host large exomoons. Results. For 92% of the sample, moons larger than Luna can be excluded on prograde orbits, unless the parent exoplanets internal structure is very different from the gas-giants of the solar system. Only WASP-24b, OGLE2-TR-L9, CoRoT-3b and CoRoT-9b could have moons above 0.4 moplus, which is within the likely detection capabilities of current observational facilities. Additionally, the range of possible orbital radii of exomoons of the known transiting exoplanets, with two exceptions, is below 8 Jupiter-radii and therefore rather small.
