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Interstellar and Circumstellar Medium

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Velocity channel images of an HC3N molecular line emission of CIT 6 observed with the VLA (red, Claussen et al. 2011), and modeled by an hydrodynamic radiative transfer simulation using the FLASH and SPARX codes (green). The position angles of the knots found in the model are indicated by arrows. A simple geometric model with an inclined Archimedes spiral-shell is drawn by a gray solid line in each channel.

Evidence of a Binary-Induced Spiral from an Incomplete Ring Pattern of CIT 6

Evolved-star dust production rates for the Large Magellanic Cloud from Riebel, Srinivasan, Sargent et al. (2012)

The Dust Input from Evolved Stars to The Magellanic Clouds

Map of the abundance of the CS molecule

Artist impression of fullerenes in front of a planetary nebulae.

The location of the Sun is given by a green circle, and the dotted line shows the Solar Circle, e.g. the circle that connects all points with the same distance to the Galactic Centre as the Sun. The dashed lines indicate the approximate location of the spiral arms in the Milky Way (Picture Credit: NAOJ; Masaaki Otsuka)

The location of the C60 containing Planetary Nebulae (blue symbols) in the Milky Way

Three infrared spectra taken by the Spitzer Space Telescope, showing the characteristic features of amorphous (at 10 and 18 micron) and crystalline (at 23, 28 and 33 micron) silicates seen in mass-losing Asymptotic Giant Branch stars. The spectra are ordered by increasing mass loss. The crystalline silicate feature at 33 micron becomes progressively stronger as the mass-loss rate goes up, while the 23 micron feature starts to go into self-absorption due to optical depth effects.

Crystallization of circumstellar silicates

lattice structure of crystalline silicate

lattice structure of amorphous silicate

crystalline silicates (1)

crystalline silicates (2)

The evolution of dust abundance

Spectral energy distribution of pulsed emission from a middle-aged pulsar B1951+32

CO(6-5) line at 690 GHz of the proto-planetary nebula CRL 618

Spectrum centered around 143 GHz of carbon star CW Leo

CO(2-1) emission from carbon star R Scl

Evolution of dust abundance

$The top panel indicates the total dust production in a starburst galaxy in terms of time after the start of the starburst; the star formation rate is constant and the starburst lasts 100 million years (indicated by the dashed line). The amorphization time scale is indicated by the dotted line, and the destruction time scale by the dash-dotted line. The resulting total silicate mass is shown in panel b) and the crystalline silicate mass is shown in panel c). Dividing these two quantities yields the crystalline fraction, or crystallinity, plotted in panel d), and it is apparent that under certain circumstances a high crystalline fraction may indeed occur in the interstellar medium of starburst galaxies$

The crystalline fraction of interstellar silicates in starburst galaxies

H2 1–0 O(3), PAH, and Brα line maps of N49 by using the AKARI/IRC spectra. For comparison, contours (black) are taken from an Hα image of the
HST/WFPC2 (Bilikova et al. 2007); these contours are 10%, 30%, 50%, and 70% of the peak. The black crossmarks the peak of the PAH emission at (α2000, δ2000)= (05h26m05.s14,−66d05'03"). The white crosses denote two clumps seen in the PAH line map. The scale bar represents 10" or 2.4 pc at 50 kpc. The color bars are given in units of W m−2 arcsec−2. North is up and east is to the left.

Molecular hydrogen, PAH, and Bralpha line maps of SNR N49 in the Large Magellanic Cloud

The space between the stars is not empty, but instead it is filled with gas and dust, and is referred to as the interstellar medium (ISM). Most of the ISM is occupied by low density gas, which makes up about 90% of the volume, but only 1% of the mass in the ISM, but intermediate and high density regions exist too.

Schematic chemical structure of the circumstellar shell of an AGB star
The life cycle of dust in the interstellar medium of a galaxy

Stars form the molecular cloud (center) and develop either as a high-mass (right) or a low-mass star (left). Upon a supernova explosion in case of high-mass stars, or a process of more gradual mass loss (AGB/Planetary Nebula phase), material enriched by the products of nucleosynthesis is ejected from the stars, and added to the ISM. Dust formation takes place in the stellar ejecta.

The densest regions, molecular clouds, are the areas where star formation occurs, and where the interstellar gas gets compacted into stars. These young stars are often surrounded by the remnants of the molecular clouds from which they formed, or, at a later stage, by a circumstellar disk from which planets are forming. Post-main-sequence stars, but also massive main-sequence stars, lose stellar mass, often enriched with the products of nucleosynthesis, which forms a chemically interesting circumstellar environment. The stellar ejecta eventually get deposited into to the ISM, and drive the chemical evolution of galaxies.

The interstellar and circumstellar (ICSM) group studies the physical and chemical properties of the ISM and circumstellar environments. The group is very diverse, consisting of observers (mostly infrared/submm) and modellers, studying various (non-stellar) aspects of the life cycle of matter in galaxies. Research topics include: the gas and dust mass loss history of Asymptotic Giant Branch (AGB) stars; the chemical composition of AGB envelopes; the effect of binarity on the physical structure of the circumstellar envelope; chemical composition of molecular and translucent clouds; astrochemistry; the circumstellar environment of young stars; proto-brown dwarfs; planetary nebulae; dust production by supernovae; the life cycle of gas and dust in galaxies, particularly the Magellanic Clouds; astromineralogy; dust in the early universe, and in active galaxies.

Schematic chemical structure of the circumstellar shell of an AGB star

The top half of the plot represents the chemistry in an oxygen-rich environment (C/O < 1), and the bottom half of the figure shows the carbon-rich counterpart. The logarithmic scale at the bottom shows the distance and temperature to the center of the star. Several distinct zones can be identified, namely the molecular formation region and the dust formation region, where conditions are governed by the density and the stellar radiation field, and the outermost region where the chemistry is dominated by the impinging interstellar radiation.