We study the populations of massive stars in the Carina region and their energetic feedback and ejection of $^{26}$Al. We did a census of the stellar populations in young stellar clusters within a few degrees of the Carina Nebula. For each star we es
timated the mass, based on the spectral type and the host cluster age. We used population synthesis to calculate the energetic feedback and ejection of $^{26}$Al from the winds of the massive stars and their supernova explosions. We used 7 years of INTEGRAL observations to measure the $^{26}$Al signal from the region. The INTEGRAL $^{26}$Al signal is not significant with a best-fit value of about 1.5e-5 ph/cm^2/s, approximately half of the published Compton Gamma Ray Observatory (CGRO) result, but in agreement with the latest CGRO estimates. Our analysis of the stellar populations in the young clusters leads to an expected signal of half the observed value, but the results are consistent within 2 sigma. We find that the fraction of $^{26}$Al ejected in Wolf-Rayet winds is high, and the observed signal is unlikely to be caused by $^{26}$Al ejected in supernovae alone, indicating a strong wind ejection of $^{26}$Al. Due to the lack of prominent O stars, regions with ages $gtrsim$10 Myr are often neglected in studies of OB associations. We find that in the Carina region such clusters contribute significantly to the stellar mass and the energetics of the region.
In the dense stellar environment of the globular clusters, compact binaries are produced dynamically. Therefore the fraction of type Ia supernovae that explode in globular clusters is expected to be higher than the fraction of mass residing in these.
We have searched for globular clusters at the positions of observed type Ia supernovae. We used archival HST images and literature data, covering the positions either before the supernovae exploded, or long enough after that the supernovae have faded below the luminosities of globular clusters. We did not find evidence for globular clusters at any of the supernova positions. For 18 type Ia supernovae, the observations are sensitive enough that any globular cluster would have been detected, and for further 17 type Ia supernovae, the brighter globular clusters would have been detected. Correcting for incompleteness, we derive a 90% upper limit of 0.09 on the fraction of type Ia supernovae that explode in globular clusters for the full sample and 0.22 for the sample of supernovae in late-type galaxies. This allows us to limit enhancements per unit stellar mass for a coeval population eta_{co}<50 (100) with 90% (99%) confidence. We find that by observing the positions of a sample of less than 100 type Ia supernovae in the outer parts of early-type galaxies, it will be possible to probe the currently favoured range of eta_{co}~1-10.
The mixing of ejecta from young stars into the interstellar medium is an important process in the interplay between star formation and galaxy evolution. A unique window into these processes is provided by the radioactive isotopes $^{26}$Al, traced by
its $gamma$-ray decay lines at 1.8 MeV. With a mean lifetime of $sim$1 Myr it is a long-term tracer of nucleosynthesis for massive stars. Our population synthesis code models the ejection of $^{26}$Al, together with the $^{60}$Fe, the kinetic energy and UV radiation for a population of massive stars. We have applied the code to study the nearby Orion region and the more massive Carina region and found good agreement with observational constraints.
We investigate the spatial coincidence of ultra-luminous X-ray sources (ULXs) with young massive stellar clusters. In particular we perform astrometry on Chandra and HST data of two ULXs that are possibly associated with such clusters. To date M82 X-
1 is the only ULX claimed to be coincident with a young massive stellar cluster. We remeasure the position of this source with a high accuracy and find that the position of the X-ray source is 0.65 arcsec away from the stellar cluster, corresponding to an offset significance of 3 sigma. We also report the discovery of a new candidate, based on observations of NGC 7479. One of the ULXs observed in three X-ray observations is found to be spatially coincident (within 1 sigma of the position error) with a young super-cluster observed in the HST images. In the brightest state, the absorbed luminosity of the ULX is a few times $10^{40}$ erg s$^{-1}$, and in the faintest state below the detection limit of $sim4$ times $10^{39}$ erg s$^{-1}$. The luminosity in the brightest state requires an accreting black hole mass of at least 100 M$_{odot}$ assuming isotropic emission. However it is possible that the source is contaminated by X-ray emission from the nearby supernova SN2009jf. In this case the luminosity of the ULX is in a range where it is strongly debated whether it is a super-Eddington stellar mass black hole or an intermediate mass black hole. The colours of the host cluster indicate a young stellar population, with an age between 10 and 100 Myr. The total stellar mass of the cluster is $sim5cdot10^{5}$M$_{odot}$.
The formation and evolution of low-mass X-ray binaries (LMXBs) is not well understood. The properties of a population of LMXBs depend on a number of uncertain aspects of binary evolution, and population studies offers a relatively new way of probing
binary interactions. We have studied the shape of the faint end of the X-ray luminosity function (LF) of LMXBs in nearby galaxies with Chandra and in the Milky Way using the Swift all-sky monitor. We find a clear difference between the LF of LMXBs in globular clusters (GCs) and those outside, with a relative lack of faint GC sources. This indicates a difference in the composition of the two populations.
In the single degenerate scenario for Type Ia supernova (SNeIa), a white dwarf (WD) must gain a significant amount of matter from a companion star. Because the accreted mass carries angular momentum, the WD is likely to achieve fast spin periods, whi
ch can increase the critical mass, $M_{crit}$, needed for explosion. When $M_{crit}$ is higher than the maximum mass achieved by the WD, the WD must spin down before it can explode. This introduces a delay between the time at which the WD has completed its epoch of mass gain and the time of the explosion. Matter ejected from the binary during mass transfer therefore has a chance to become diffuse, and the explosion occurs in a medium with a density similar to that of typical regions of the interstellar medium. Also, either by the end of the WDs mass increase or else by the time of explosion, the donor may exhaust its stellar envelope and become a WD. This alters, generally diminishing, explosion signatures related to the donor star. Nevertheless, the spin-up/spin-down model is highly predictive. Prior to explosion, progenitors can be super-$M_{Ch}$ WDs in either wide binaries with WD companions, or else in cataclysmic variables. These systems can be discovered and studied through wide-field surveys. Post explosion, the spin-up/spin-down model predicts a population of fast-moving WDs, low-mass stars, and even brown dwarfs. In addition, the spin-up/spin-down model provides a paradigm which may be able to explain both the similarities and the diversity observed among SNeIa.
We study the populations of X-ray sources in the Milky Way in the 15-55 keV band using a deep survey with the BAT instrument aboard the Swift observatory. We present the logN-logS distributions of the various source types and we analyze their variabi
lity and spectra. For the low-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries (HMXBs) we derive the luminosity functions to a limiting luminosity of L_X~7 times10^{34} erg s/s. Our results confirm the previously found flattening of the LMXB luminosity function below a luminosity of L_X~10^{37} erg s/s. The luminosity function of the HMXBs is found to be significantly flatter in the 15-55 keV band than in the 2-10 keV band. From the luminosity functions we estimate the ratios of the hard X-ray luminosity from HMXBs to the star-formation rate, and the LMXB luminosity to the stellar mass. We use these to estimate the X-ray emissivity in the local universe from X-ray binaries and show that it constitutes only a small fraction of the hard X-ray background.
We assemble a census of the most massive stars in Orion, then use stellar isochrones to estimate their masses and ages, and use these results to establish the stellar content of Orions individual OB associations. From this, our new population synthes
is code is utilized to derive the history of the emission of UV radiation and kinetic energy of the material ejected by the massive stars, and also follow the ejection of the long-lived radioactive isotopes 26Al and 60Fe. In order to estimate the precision of our method, we compare and contrast three distinct representations of the massive stars. We compare the expected outputs with observations of 26Al gamma-ray signal and the extent of the Eridanus cavity. We find an integrated kinetic energy emitted by the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent with the energy thought to be required to create the Eridanus superbubble. We also find good agreement between our model and the observed 26Al signal, estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion region. Our population synthesis approach is demonstrated for the Orion region to reproduce three different kinds of observable outputs from massive stars in a consistent manner: Kinetic energy as manifested in ISM excavation, ionization as manifested in free-free emission, and nucleosynthesis ejecta as manifested in radioactivity gamma-rays. The good match between our model and the observables does not argue for considerable modifications of mass loss. If clumping effects turn out to be strong, other processes would need to be identified to compensate for their impact on massive-star outputs. Our population synthesis analysis jointly treats kinematic output and the return of radioactive isotopes, which proves a powerful extension of the methodology that constrains feedback from massive stars.
We developed a new population synthesis code for groups of massive stars, where we model the emission of different forms of energy and matter from the stars of the association. In particular, the ejection of the two radioactive isotopes 26Al and 60Fe
is followed, as well as the emission of hydrogen ionizing photons, and the kinetic energy of the stellar winds and supernova explosions. We investigate various alternative astrophysical inputs and the resulting output sensitivities, especially effects due to the inclusion of rotation in stellar models. As the aim of the code is the application to relatively small populations of massive stars, special care is taken to address their statistical properties. Our code incorporates both analytical statistical methods applicable to small populations, as well as extensive Monte Carlo simulations. We find that the inclusion of rotation in the stellar models has a large impact on the interactions between OB associations and their surrounding interstellar medium. The emission of 26Al in the stellar winds is strongly enhanced, compared to non-rotating models with the same mass-loss prescription. This compensates the recent reductions in the estimates of mass-loss rates of massive stars due to the effects of clumping. Despite the lower mass-loss rates, the power of the winds is actually enhanced for rotating stellar models. The supernova power (kinetic energy of their ejecta) is decreased due to longer lifetimes of rotating stars, and therefore the wind power dominates over supernova power for the first 6 Myr after a burst of star-formation. For populations typical of nearby star-forming regions, the statistical uncertainties are large and clearly non-Gaussian.
We study the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low luminosity limit and comparison betw
een globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of 8e35 erg/s, about 2-3 times deeper than previous investigations. We confirm the presence of the low luminosity break in the overall LMXB XLF at log(L_X)=37.2-37.6 below which the luminosity distribution follows a constant dN/d(ln L). Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest that there is an enhanced fraction of helium accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the 4 times higher ionization temperature of He, such systems are subject to accretion disk instabilities at approximately 20 times higher mass accretion rate, and therefore are not observed as persistent sources at low luminosities.
