Cygnus OB2 - a young globular cluster in the Milky Way

The morphology and stellar content of the Cygnus OB2 association has been determined using 2MASS infrared observations in the J, H, and K bands. The analysis reveals a spherically symmetric association of 2 deg in diameter with a half light radius of 13', corresponding to Rh = 6.4 pc at an assumed distance of 1.7 kpc. The interstellar extinction for member stars ranges from Av = 5m to 20m, which led to a considerable underestimation of the association size and richness in former optical studies. From the infrared colour-magnitude diagram, the number of OB member stars is estimated to 2600 +/- 400, while the number of O stars amounts to 120 +/- 20. This is the largest number of O stars ever found in a galactic massive star association. The slope of the initial mass function has been determined from the colour-magnitude diagram to Gamma=-1.6 +/- 0.1. The total mass of Cyg OB2 is estimated to (4-10)*10^4 Msol, where the primary uncertainty comes from the unknown lower mass cut-off. Using the radial density profile of the association, the central mass density is determined to rho0=40-150 Msol pc^-3. Considering the mass, density, and size of Cyg OB2 it seems untenable to classify this object still as OB association. Cygnus OB2 more closely resembles a young globular cluster like those observed in the Large Magellanic Cloud or in extragalactic star forming regions. It is therefore suggested to re-classify Cygnus OB2 as young globular cluster - an idea which goes back to Reddish et al. (1966). Cygnus OB2 would then be the first object of this class in the Milky Way.Comment: 10 pages, 8 figures, accepted for publication in A&

The Galactic 26Al Problem and The Close Binary SNIb/c Solution?

The origin of the long-lived radioactive 26Al, which has been observed in the Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission, has been a persistent problem for over twenty years. Wolf-Rayet (WR) winds were thought to be the most promising source, but their calculated 26Al yields are not consistent with recent analyses of the 1.809 MeV emission from the nearest WR star and nearby OB associations. The expected 26Al yield from the WR star exceeds by as much as a factor of 3, that set by the 2-sigma upper limit on the 1.809 MeV emission, while the WR yields in the OB associations are only about 1/3 of that required by the 1.809 MeV emission. We suggest that a solution to these problems may lie in 26Al from a previously ignored source: explosive nucleosynthesis in the core collapse SNIb/c supernovae of WR stars that have lost most of their mass to close binary companions. Recent nucleosynthetic calculations of SNIb/c suggest that their 26Al yields depend very strongly on the final, pre-supernova mass of the WR star, and that those with final masses around 6 to 8 solar masses are expected to produce as much as 0.01 solar masses of 26Al per supernova. Such binary SNIb/c make up only a small fraction of the current SNIb/c and only about 1% of all Galactic core collapse supernovae. They appear to be such prolific sources that the bulk of the present 26Al in the Galaxy may come from just a few hundred close binary SNIb/c and the intense 1.809 MeV emission from nearby OB associations may come from just one or two such supernova.Comment: Accepted for publication in Astrophysical Journal Letters, 611,10 August 200

Globular Cluster Formation in M82

We present high resolution mid-infrared (mid-IR; 11.7 and 17.65 micron) maps of the central 400 pc region of the starburst galaxy M82. Seven star forming clusters are identified which together provide ~ 15% of the total mid-IR luminosity of the galaxy. Combining the mid-IR data with thermal radio measurements and near- and mid-IR line emission, we find that these young stellar clusters have inferred masses and sizes comparable to globular clusters. At least 20% of the star formation in M82 is found to occur in super-star clusters.Comment: 12 pages including three color figures; accepted for publication in Ap

New results on 26Al production in classical novae

The production of 26Al by explosions of classical novae has been computed by means of a hydrodynamic code that follows both the accretion and the explosion stages. A special emphasis has been put on the analysis of the influence of the initial abundances of the accreted envelope, as well as on the nuclear reaction rates involved. With the most recent values of chemical composition and reaction rates available, 26Al production is lowered with respect to previous computations. According to our results, the final contribution of novae to the galactic 26Al is at most 0.4 M_sun, which is a small part of the estimated 26Al in the Galaxy derived from COMPTEL observations of the 1809 keV emission.Comment: Accepted for publication in ApJ Letters (16 pages, 1 figure, aasms4

Galactic Point Sources of TeV Antineutrinos

High energy cosmic ray experiments have identified an excess from the region of the Galactic Plane in a limited energy range around $10^{18}$ eV (EeV). This is very suggestive of neutrons as candidate primaries, because the directional signal requires relatively-stable neutral primaries, and time-dilated neutrons can reach Earth from typical Galactic distances when the neutron energy exceeds an EeV. We here point out that if the Galactic messengers are neutrons, then those with energies below an EeV will decay in flight, providing a flux of cosmic antineutrinos above a TeV which is {\it observable} at a kilometer-scale neutrino observatory. The expected event rate per year above 1 TeV in a detector such as IceCube, for example, is 20 antineutrino showers (all flavors) and a $1^\circ$ directional signal of 4 $\bar \nu_\mu$ events. A measurement of this flux can serve to identify the first extraterrestrial point source of TeV antineutrinos.Comment: matches published versio