* Astronomy

Title: Why is the Milky Way X-factor Constant?
Authors: Desika Narayanan (Arizona), Philip Hopkins (Berkeley)

The CO-H2 conversion factor (Xco; otherwise known as the X-factor) is observed to be remarkably constant in the Milky Way and in the Local Group (aside from the SMC). To date, our understanding of why Xco should be so constant remains poor. Using a combination of extremely high resolution (~ 1 pc) galaxy evolution simulations and molecular line radiative transfer calculations, we suggest that Xco displays a narrow range of values in the Galaxy due to the fact that molecular clouds share very similar physical properties. In our models, this is itself a consequence of stellar feedback competing against gravitational collapse. GMCs whose lifetimes are regulated by radiative feedback show a narrow range of surface densities, temperatures and velocity dispersions with values comparable to those seen in the Milky Way. As a result, the X-factors from these clouds show reasonable correspondence with observed data from the Local Group, and a relatively narrow range. On the other hand, feedback-free clouds collapse to surface densities that are larger than those seen in the Galaxy, and hence result in X-factors that are systematically too large compared to the Milky Way's. We conclude that radiative feedback within GMCs can generate cloud properties similar to those observed in the Galaxy, and hence a roughly constant Milky Way X-factor in normal, quiescent clouds.

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Title: An Optical Survey for mm-Sized Interstellar Meteoroids
Authors: R. Musci, R. J. Weryk, P. Brown, M. D. Campbell-Brown, P. A. Wiegert

We report high resolution multi-station observations of meteors by the Canadian Automated Meteor Observatory (CAMO) recorded from June 2009 to August 2010. Our survey has a limiting detection magnitude of +5 mag in R-band, equivalent to a limiting meteoroid mass of ~2*E-7 kg. The high metric trajectory accuracy (of the order of 30 m perpendicular to the solution and 200 m along-track) allows us to determine velocities with average uncertainty of < 1.5% in speed and ~0.4 degr in radiant direction. A total of 1739 meteors had measured orbits. The data has been searched for meteors in hyperbolic orbits, which are potentially of interstellar origin. We found 22 potential hyperbolic meteors among our sample, with only two of them having a speed at least three sigma above the hyperbolic limit. For our one year survey we find no clear evidence of interstellar meteoroids at mm-sizes in a weighted time-area product of ~1*E4 km²*h. Backward integrations performed for these 22 potentially hyperbolic meteors to check for close encounters with planets show no considerable changes in their orbits. Detailed examination leads us to conclude that our few identified events are most likely the result of measurement error. We find an upper limit of f_ISP < 2*E-4/(km²*h) for the flux of interstellar meteoroids at Earth with a limiting mass of m > 2*E-7 kg.

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Clocking the mosh pit of interstellar space

The space between the stars in the Milky Way and all other galaxies is full of dust and gas, the raw materials from which stars and planets are made.
But the dynamics of these galactic mosh pits, which are perhaps best known through the spectacular images from the Hubble Space Telescope of towering nebulas caught in the act of churning out stars, are still mysterious. Now, an international team of scientists, including two from the University of Wisconsin-Madison, has clocked the turbulence of the warm ionised gas in interstellar space, a measurement that promises a better handle on a process responsible for regulating the appearance and composition of the Milky Way.
The work is reported today (Oct. 5) in the journal Nature.
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