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Giovanni Covone » 3.A census of systems in the Interstellar Medium, part 2

A census of systems in the ISM

We continue our survey of objects in the interstellar medium.

We will review the most important observational properties of the following systems.

Reflection Nebulae.
Dark Nebulae and Bok globules.
Supernovae remnants.

Reflection Nebulae

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: $T < 2.5 \times 10^4$ Kelvin

Can you explain the blue color of the reflection nebulae?

Comparison with HII region: different source, different radiation.

The Witch Nebula, associated with the bright star Rigel, in Orion. Credit: NASA.

Spectra of HII regions

Five spectra of Galactic HII regions, from Magrini et al. (2005).

Photodissociation regions (PRD)

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.

Density value: $\sim 10^3 - 10^7 \, {\rm cm}^{-3}$

FUV photons (energy: 6–13.6 eV) strongly influence the gas chemistry and are the most important source of heat.

Where they occur: region of interstellar gas that is dense and cold enough to remain neutral.
Characterized by low a column density: penetration of FUV photons from distant, massive stars.
Example: the gas at the boundary of a giant molecular cloud. In other words, the outer edge of a bubble of gas that is being heated by intense ultraviolet radiation.

Scheme of a generic PRD

A schematic illustration of a PDR (from Hollenbach & Tielens, 1997).

PDR in the Orion Complex

A PDR region in the Orion Bar. The so-called BNKL region contains a massive protostar.Credit: NASA, ESO.

Dark nebulae

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.

The Horse Head in Orion constellation. Image obtained with WFPC2, HST. Credit: NASA.

Bok globules

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 $10^{-2}$ 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.

Figure: Bok globules

Bok globules in the HII region IC2944. Image taken with the WFPC2, HST. Credit: NASA.

Supernova remnants

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 $10^6$ 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 supernova remnants

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?

The most famous young SNR, the Crab Nebula, a six-light-year-wide expanding remnant of a supernova exploded nearly 1,000 years ago.

Supernova remnant E0102-72

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.

Supernova remnant E0102-72, as seen at radio, optical and X-ray wavelength. Credit: NASA.

Old supernova remnants

Photoionized by X-rays emitted from dense cooling regions collisionally heated up to $10^6$ 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

Summary

A spectacular HST image of NGC 3603, a giant nebula hosting a massive young clusters. How many structures can you recognize in this field?Credit: NASA.