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


What is the ISM

What is the ISM made of: a census of physical systems in the interstellar space.

We will study the physical composition of the ISM and their interaction with stars.

The interstellar medium plays a central role in the evolution of the galaxies: formation of new stars occur in dense molecular clouds, enriched by previous generation of stars; stars (mostly, the most massive and the evolved ones) diffuse intense stellar winds in the surrounding environment; massive stars end their life as supernovae, injecting large amount of matter and energy in the galaxy.

Hence, the abundance of heavy elements in the interstellar medium steadily increases (chemical “enrichment” or “pollution”).

A census of objects in the ISM

In this lecture we will make a detailed census of the structures observed in the ISM and a list of its physical components.

Aim of this lecture: an introduction to the observational properties of the most notable structures in the ISM.

The census of interstellar objects include:

  • Molecular clouds,
  • HII regions,
  • Planetary Nebulae,
  • Reflection nebulae,
  • Photodissociation regions,
  • Dark nebulae,
  • Supernova remnants

We will then classify these structures according to the physical parameters.

Typical parameters in the ISM

Density

Typical density of the Earth’s atmosphere n \sim 3 \times 10^{19}  molecules per cm3.

Vacuum in our laboratories, \sim 10^9 molecules per cm3.

Average value in the ISM within the Milky Way, n \sim 1 atoms per per cm3.

In hot and less dense regions, n \sim 10^{-2} atoms per per cm3.

Chemical composition

As a result of primordial nucleosynthesis, by number of nuclei the gas is roughly 89% H, 9% He and 2% elements heavier than H or He, with additional heavier elements (“metals” in astronomical parlance) present in trace amounts

Pressure on Earth and in Space

Typical values for the pressure in different physical situations, on Earh and in Space.

Typical values for the pressure in different physical situations, on Earh and in Space.


Chemical elements in the ISM

The table shows the chemical composition in mass fraction of the ISM.

The table shows the chemical composition in mass fraction of the ISM.


ISM role in stellar evolution

Schematic view of the link between evolution of stars and processes in the interstellar medium.

Schematic view of the link between evolution of stars and processes in the interstellar medium.


The Carina Nebula

Let’s start from a remarkable example of complex system in the ISM: the Carina Nebula

The Carina Nebula is a real maelstrom of star birth and death. Next image shows the Carina Nebula assembled from 48 frames taken with Hubble’s Advanced Camera for Surveys.
Images were taken in the light of neutral hydrogen.

Color information was added with data taken at the Cerro Tololo Inter-American Observatory in Chile. In the image, red corresponds to sulfur, green to hydrogen, and blue to oxygen emission.

Question: what are the wavelengths at which the mentioned elements are observed?

NGC 3372, the Carina Nebula

HST mosaic of the complex structure of the Carina Nebula. Also known as the Eta Carina Nebula, or NGC 3372, it is a large bright nebula that surrounds several open clusters of stars.

HST mosaic of the complex structure of the Carina Nebula. Also known as the Eta Carina Nebula, or NGC 3372, it is a large bright nebula that surrounds several open clusters of stars.


Clouds of molecular gas

Molecular clouds are among the largest and densest structures in the ISM.

Components: molecular hydrogen and other molecules, dust.

Detection via emission of CO molecules.

Ratio between the CO and H_2 numerical densities

\frac{n_{\rm CO}} {n_{H_2}} \sim 10^{-4}

  • Link with formation of stars.
  • Large range of mass.
  • Typical values of the temperature.
A molecular cloud enriched with dust, in the Carina Nebula. Credit: Hubble Heritage Team.

A molecular cloud enriched with dust, in the Carina Nebula. Credit: Hubble Heritage Team.


The Molecular Cloud Complex in Orion

The spatial distribution of the Orion Molecular Cloud Complex, as observed via its CO emission.

The spatial distribution of the Orion Molecular Cloud Complex, as observed via its CO emission.


HII regions

Definition: a region where hydrogen is mostly ionized.

  • Source of iniozation: UV radiation, with wavelength \lambda > 912 \, \AA.
  • UV radiation field produce mostly by hot and massive stars (spectral type: O, B).
  • Temeprature: T \sim 10^4 Kelvin.
  • Large range in mass.
  • Large range in density.
  • Tracer of on-going star-formation.
The Tarantula Nebula, a large HII region locate in the Large Magellanic Cloud. Credit: ESO.

The Tarantula Nebula, a large HII region locate in the Large Magellanic Cloud. Credit: ESO.


Spectrum of HII regions

Optical spectra provide tools to study of the HII region.

Spectrum is dominated by are recombination lines of hydrogen, helium and the so-called “forbidden” emission lines.

Source: [OII], [OIII], [NII] and other ions.

Why “forbidden”?
These transitions happen in physical conditions usually not met on Earth, therefore those lines are not observed in our laboratories.

In the picture: an emission line spectrum of a compact HII region in the Virgo intracluster field. Emission lines from oxygen ([OIII]) and hydrogen atoms, and ionized sulphur ([SII], [SIII]) are identified.

Typical spectrum of an HII region, where most of the flux in emitted in emission lines.
Credit: ESO.

Typical spectrum of an HII region, where most of the flux in emitted in emission lines. Credit: ESO.


HII regions in distant galaxies

HII regions are visible also at cosmological distances.

They dominate the optical spectrum and the colors of starburts and moderately star-forming galaxies.

Most distant objects known in the Universe are starburst galaxies, with spectra typical of HII regions.

In the picture, a portion of the galaxy Messier 83, in a color image. HII regions emit strong H-alpha lines, and appear red in this image.

Galaxy M83 observed with. A narrow filter centered on the H-alpha emission allows to see the bright HII regions. Credit: ESO.

Galaxy M83 observed with. A narrow filter centered on the H-alpha emission allows to see the bright HII regions. Credit: ESO.


Planetary Nebulae

Definition: Planetary Nebulae (PN) are photo-ionized region found around white dwarfs.

  • White dwarfs, final stage of the evolution of “ordinary” stars.
  • Temperature and spectra of white dwarfs.
  • Spectrum of a Planetary Nebula: formation and main lines.
  • Use of Planetary Nebula as kinematical tracers.
The planetary nebula IC 418, observed by HST. Credit: Hubble Heritage Team.

The planetary nebula IC 418, observed by HST. Credit: Hubble Heritage Team.


The Helix Nebula

The Helix Nebula, a bright planetary nebula. Credit: NASA, ESA.

The Helix Nebula, a bright planetary nebula. Credit: NASA, ESA.


Typical spectrum of PNe

Spectrum of Planetary Nebula, from Magrini et al. (2005). Note the very low continuum, and the impressive emission lines.

Spectrum of Planetary Nebula, from Magrini et al. (2005). Note the very low continuum, and the impressive emission lines.


I materiali di supporto della lezione

Chapter 1 of the book “The Physics and Chemistry of the Interstellar Medium“ by A.G.G.M. Tielens

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