We continue our survey of objects in the interstellar medium.
We will review the most important observational properties of the following systems.
Definition: Reflection Nebulae are gas system scattering the light of a nearby star, which is not hot enough to fully ionized the hydrogen.
UV radiation from B-stars.
Temperature of the nearby star: Kelvin
Can you explain the blue color of the reflection nebulae?
Comparison with HII region: different source, different radiation.
Definition: PRD are dense where the hydrogen gas is neutral and where externally incident far-ultraviolet (FUV) radiation controls the chemical structure and dominates the thermal properties of the gas.
FUV photons (energy: 6–13.6 eV) strongly influence the gas chemistry and are the most important source of heat.
A PDR region in the Orion Bar. The so-called BNKL region contains a massive protostar.Credit: NASA, ESO.
First discovered by Herschel, which proposed a wrong interpretation (“holes in the heavens”).
Physical nature revealed by Barnard (photographic observations in early XX century).
Dark nebulae are mainly characterized by heavy attenuation caused by dust.
In the picture, a detail of the best known dark nebula, the Horse Head, also known as Barnard 33.
Bok globules are dense dark clouds of interstellar dust and gas in which star formation sometimes takes place, first observed by astronomer Bart Bok in the 1940s.
They are locate in HII regions.
They are small clouds, with size of the order of pc.
Typical mass: a few tens of solar masses.
They contain molecular hydrogen, carbon oxides and helium, and around one per cent (by mass) of silicate dust.
Bok made the hypothesis that thy were “similar to insect’s cocoons” that were undergoing gravitational collapse to form new stars from which stars and star clusters were born.
Supernova remnants (SNRs) are regions where the ISM is ionized by the expanding shock wave from a supernova explosion.
Energy injection of energy into the ISM by a supernova: effects on the surrounding medium.
The environment around the explosion is swept into the fast shell of dense material from the SN. The expanding shock wave consists of ejected material from the SN explosion and the interstellar material swept up and ionized along the way.
Small dense clouds are destroyed, and shocks are driven into larger clouds, initiating chemical reactions, unique to this environment. Low density gas bubbles of Kelvin are left behind.
Formation of super-bubbles (diameter: hundreds of parsecs in diameter).
Detection of SNR via their interaction with the ISM (high-velocity HI)
Main differences with respect to HII regions and planetary nebulae: the source of the ionizing photons and the role of the hydrodynamical shocks, providing an additional source of heating.
Two basic types of SNRs: young and old SNRs.
Young SNRs are crated by the wind coming from the central pulsar.
Relativistic electrons accelerated by the pulsar wind emit a strong UV synchrotron radiation, photo-inizing the surrounding ISM.
Pulsar winds are sometimes called “plerions” (from ancient Greek).
Problem: lack of young supernova remnants in our Galaxy. Selection effect or low star-formation rate?
In the picture we show a SNR located in the Small Magellanic Cloud, with a diameter of about 40 light years: the young SNR E0102-72
Radio waves (red colour) trace high-energy electrons spiraling around magnetic field lines in the shock wave expanding out from the detonated star.
Optical light (green color) traces clumps of relatively cool gas that includes oxygen.
X-rays (blue color) show relatively hot gas that has been heated to millions of degrees by an inward moving shock wave that has rebounded from a collision with existing or slower moving gas.
Photoionized by X-rays emitted from dense cooling regions collisionally heated up to K by the passage of the supernova blast wave through the ISM.
There are only a few hundreds known old supernova remnants in the Galaxy, most of which have been discovered in radio continuum and/or X-rays.
This number is much less than what we would expect from the Galactic supernova rate and their life time.
Probably, most old SNRs are difficult to detect
because of their faintness and the background contamination.
The filaments of faint supernova remnant Simeis 147. Credit: Sky Factoring
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