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Post Info TOPIC: Interstellar medium
Blobrana


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RE: Interstellar medium
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Title: Nanodust in the Interstellar Medium in Comparison to the Solar System
Authors: Aigen Li, Ingrid Mann

Nanodust, which undergoes stochastic heating by single starlight photons in the interstellar medium, ranges from angstrom-sized large molecules containing tens to thousands of atoms (e.g. polycyclic aromatic hydrocarbon molecules) to grains of a couple tens of nanometers. The presence of nanograins in astrophysical environments has been revealed by a variety of interstellar phenomena: the optical luminescence, the near- and mid-infrared emission, the Galactic foreground microwave emission, and the ultraviolet extinction which are ubiquitously seen in the interstellar medium of the Milky Way and beyond. Nanograins (e.g. nanodiamonds) have also been identified as presolar in primitive meteorites based on their isotopically anomalous composition. Considering the very processes that lead to the detection of nanodust in the ISM for the nanodust in the solar system shows that the observation of solar system nanodust by these processes is less likely.

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Blobrana


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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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Blobrana


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Interstellar dust
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Title: Why don't clumps of cirrus dust gravitationally collapse?
Authors: Rudolph E. Schild, Carl H. Gibson, Theo M. Nieuwenhuizen, N. Chandra Wickramasinghe

We consider the Herschel-Planck infrared observations of presumed condensations of interstellar material at a measured temperature of approximately 14 K (Juvela et al., 2012), the triple point temperature of hydrogen. The standard picture is challenged that the material is cirrus-like clouds of ceramic dust responsible for Halo extinction of cosmological sources (Finkbeiner, Davis, and Schlegel 1999). Why would such dust clouds not collapse gravitationally to a point on a gravitational free-fall time scale of 10^8 years? Why do the particles not collide and stick together, as is fundamental to the theory of planet formation (Blum 2004; Blum and Wurm, 2008) in pre-solar accretion discs? Evidence from 3.3 µm and UIB emissions as well as ERE (extended red emission) data point to the dominance of PAH-type macromolecules for cirrus dust, but such fractal dust will not spin in the manner of rigid grains (Draine & Lazarian, 1998). IRAS dust clouds examined by Herschel-Planck are easily understood as dark matter Proto-Globular-star-Cluster (PGC) clumps of primordial gas planets, as predicted by Gibson (1996) and observed by Schild (1996).

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Blobrana


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Interstellar Meteoroids
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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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Blobrana


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Posts: 131433
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Interstellar medium
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Space 'snakes' revealed in first images

Astronomers have produced the first images of gas "snakes" spiralling in the Milky Way.
The pictures are of the swirling and churning gas that makes up the interstellar space of our galaxy.
A group of universities has taken 10 years to map the images from radio waves recorded 12 years ago.
Scientists have been trying to capture the image for more than 30 years.
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interstellagasb.jpg
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Credit CSIRO



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Blobrana


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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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Blobrana


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Title: Implications of the SPEAR FUV Maps on Our Understanding of the ISM
Authors: Eric J. Korpela (1), Martin Sirk (1), Jerry Edelstein (1), Kwangil Seon (2), Kyoung-Wook Min (3), Wonyong Han (2) ((1) Space Sciences Laboratory, University of California (2) Korea Astronomy and Space Science Institute (3) Korea Advance Institute of Science and Technology)

The distribution of a low-density transition temperature (10^4.5 - 10^5.5 K) gas in the interstellar medium conveys the character and evolution of diffuse matter in the Galaxy. This difficult to observe component of the ISM emits mainly in the far-ultraviolet (FUV) (912-1800 {\AA}) band. We describe spectral maps of FUV emission lines from the highly ionised species CIV and OVI likely to be the dominant cooling mechanisms of transition temperature gas in the ISM. The maps were obtained using an orbital spectrometer, SPEAR, that was launched in 2003 and has observed the FUV sky with a spectral resolution of ~ 550 and an angular resolution of 10'. We compare distribution of flux in these maps with three basic models of the distribution of transition temperature gas. We find that the median distribution of CIV and OVI emission is consistent with the spatial distribution and line ratios expected from a McKee-Ostriker (MO) type model of evaporative interfaces. However, the intensities are a factor of three higher than would be expected at the MO preferred parameters. Some high intensity regions are clearly associated with supernova remnants and superbubble structures. Others may indicate regions where gas is cooling through the transition temperature.

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