Photodissociation Regions Radiative Transfer Dr. Thomas G. Bisbas tbisbas@ufl.edu
Interstellar Radiation Field In the solar neighbourhood, the ISRF is dominated by six components Schematic sketch of the energy density of the interstellar radiation field at different frequencies.
Structure of ionized regions Ionized gas Ionization Front Molecular Gas Massive Stars Molecular Gas Kallias Ioannidis
Photodissociation Regions The region where the atomic phase occurs is known as Photodissociation Region (PDR). Studying PDRs is a key aspect to understand the role played by the FUV radiation (6eV < hν < 13.6eV) in governing the physical and chemical structure and determining the thermal balance of the neutral ISM in galaxies.
Photodissociation Regions The source of FUV photons need not be restricted to stars. The accretion of material onto supermassive black holes in active galactic nuclei (AGN) produces strong ultraviolet and often X-ray emission that impacts the surrounding gas and dust, creating large PDRs. Supernova explosions also enrich the surrounding ISM with metals thus changing its metallicity and therefore the overall gas and dust temperature profiles. Examining such extreme environments can provide insight into the dynamics of non-local ISM observed at the centres of merger and starburst galaxies.
Molecular hydrogen in the Universe The bulk of baryonic non-stellar mass of the Universe consists of hydrogen with a total fractional abundance of ~70%. It is the gas that regulates the evolution of galaxies across the epoch times. Molecular hydrogen is responsible for star formation and in general plays a fundamental role in many astrophysical contexts. It is found in all cold (T~10K) and dark regions where UV photons emitted by stars do not penetrate.
Molecular hydrogen in the Universe Molecular hydrogen is not visible in optical wavelengths and does not emit radiation which can be captured by radio telescopes. In particular it lacks a permanent dipole moment and can only change ro-vibrational state through weak quadrupolar transition with high excitation temperatures >500 K that are sub-thermally populated and so difficult to detect in the cold bulk of the ISM. The pure rotational transitions of molecular hydrogen lie at mid-infrared wavelengths that are largely inaccessible from the ground, due to telluric absorption and strong background radiation. Because of this, we observe molecular hydrogen indirectly using carbon monoxide as a tracer. CO is also abundant in the Universe and it formed in places where molecular hydrogen is also formed. It also emits radiation detectable by radiotelescopes such as ALMA, JCMT, VLA etc.
Heating mechanisms in diffuse clouds The hydrogen that is found deep inside Diffuse Clouds is almost entirely neutral. Even if an OB star (or stars) were nearby, no UV photons at wavelengths less than 912A can penetrate deep into these diffuse clouds, since (by definition) they will have all been used up in ionising the hydrogen found in the outer edge of the cloud. Thus deep in these diffuse clouds we cannot invoke hydrogen photoionisation as the main heating mechanism. Only atoms with ionization potentials less than 13.6eV can be ionised, by the remaining lower energy photons
Heating mechanisms in diffuse clouds Hydrog en Helium Carbon Nitroge n Oxygen Hydrogen has an ionization potential of 13.6eV Helium (the next most abundant element) has ionization potential higher than for hydrogen (24eV for neutral HeI and 54eV for HeII), so photoionisation of this element is impossible in the interior of a diffuse cloud. OI has ionization potential of 13.6eV (very very close to the hydrogen one) and NI has 14.4eV, so again these elements are unsuitable to be photoionized in the innermost part of a diffuse cloud. However, neutral carbon (CI) has lower ionization potential (11.3eV, corresponding to a wavelength of 1110A), so potentially photoionisation of CI could provide kinetic energy of photoelectrons which could heat the gas.
Photoelectric effect The photoelectric effect is the effect at which metals emit electrons when light shines upon them. Electrons emitted in this manner may be called photoelectrons. It was first proposed by Albert Einstein in 1905 who was awarded the Nobel Prize in 1921 for that. The reason behind this award is that it is the first experiment which showed that the light apart from wave can also behave as a particle, or as a discrete wave packet (a photon) carrying energy E=hf (h is Planck's constant, f is the frequency).
Photoelectric heating The main heating process for the deep layers within the diffuse ISM is the ejection of photoelectrons from the small dust grains that exist with the gas. Such grains have a relatively low work function (analogous to atomic ionization potential energy), denoted W. The energy E of the ejected electron is thus: Far-ultraviolet photons absorbed by a grain will create energetic (several ev) electrons. E = hv W h is Planck's constant, v is photon's frequency While these electrons diffuse in the grain, they will lose energy through collisions. However, if during this diffusion process they reach the surface with enough energy to overcome the work function W of the grain, and the Coulomb potential φ (if the grain is positively charged), they can be injected into the gas phase with excess kinetic energy.
Photoelectric heating Polycyclic Aromatic Hydrocarbons (PAH) are large molecules, organic compounds, which contain only carbon and hydrogen. It has been suggested that they have been formed as early as the first couple of billions of years after the Big Bang, in association with the formation of new stars and exoplanets. It is also suggested that PAHs account for significant percentage of all carbon in the Universe, and that they are potential sites for abiologic syntheses of materials required by the earliest forms of life. The photoelectric heating due to PAHs is more efficient than grains, as the electrons do not suffer collisions once they absorb an FUV photon.
Photon heating by H 2 After photodissociation of a molecule, the fragments will carry away some of the photon energy as kinetic energy, heating the gas. After (re-)formation of a molecule, the newly formed species may be left in a vibrationally excited state. Infrared photons can also vibrationally excite molecules directly. In both cases, collisional de-excitation can then heat the gas. Dust-gas heating In the ISM, gas and dust are not in thermodynamical equilibrium (they have different temperatures). If the dust is warmer than the gas, gas atoms bouncing off a grain can be an important gas heating source. If the gas is warmer, this process is really a cooling process.
Cosmic rays Cosmic rays: they are NOT rays! They are high-energy relativistic particles with energies of order of several hundred MeV whereas their energy density in Milky Way is estimated to be 1 ev/cm3. They were discovered in 1912 by V. Hess. Cosmic rays are in principle consisting of ~90% hydrogen nuclei (protons), ~9% helium nuclei, while the remaining ~1% corresponds to heavier nuclei than helium. Since cosmic rays carry charge, they follow the magnetic field lines of the entire Galaxy and are therefore bound to it. In general, every particle moving at such high energies can be considered as a cosmic ray particle. Victor F Hess in the balloon s basket sometime between 1911 and 1912. 24 years later Hess won the Nobel Prize.
Cosmic-ray and X-ray heating Cosmic ray particles consist primarily of high-energy protons and electrons. Their origin is mainly due to supernova explosions, although other mechanisms also exist. Cosmic ray protons and X-rays can both ionize hydrogen atoms: (p,x) + HI (p', X') + HII + e Cosmic rays can penetrate a diffuse cloud and can contribute as a heating source well inside a dense area. A high-energy proton can ionize a gas atom. The substantial kinetic energy of the resulting primary electron can be lost through collisions with other electrons or through ionization or excitation of gas atoms or molecules. X-rays are so energetic that the primary electron created by X-ray absorption can be energetic enough to lead to secondary ionization.
Heating vs cooling The most dominant heating mechanism in diffuse clouds is the dust photoelectric heating. Cooling of the diffuse ISM gas is mainly due to Forbidden Line emission, particularly in low-lying states in neutral OI and especially in singly ionised carbon, CII. Very strong forbidden lines occur in the farinfrared: CII 158μm, OI 146μm. Putting in the dust grain heating rates and forbidden line cooling rates for typical diffuse ISM cloud densities of around n~100 cm -3, gives estimates of kinetic temperatures of ~ 70 100 Kelvin.
Example of PDRs Density: n=1000 cm-3 Radiation field: χ=10χ0 (Draine) Chemical network of 33 species and 330 reactions
CI dominated CO dominated CII dominated H2 dominated HI dominated Example of PDRs
Benchmarking...
Example of PDRs
Example of PDRs
Heating functions
Cooling functions
Local emissivities
Radiative transfer If gas velocity is zero
Radiative transfer DO i=1,itot Calculate Tex, Bnu(Tex) ENDDO DO velocity = min, max DO i=1,itot Calculate optical depth at velocity ENDDO DO i=1,itot-1 (integration) Calculate Radiative Transfer ENDDO ENDDO Convert to units of brightness temperature and/or antenna temperature
Brightness temperature of CO
Optically thin vs Optically thick
Radiative transfer in 3D simulations
Radiative transfer in 3D simulations