Astronomy

The physical processes of emission lines in cosmic nebula

The physical processes of emission lines in cosmic nebula


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I think I understand how absorption lines in cosmic bodies occur. But after reading about the emission lines in quasars I am wondering more and more about the physical processes causing the emission lines in cosmic nebula. I hope you can explain these processes to me, more precisely from where to where energy needs to transition and if there are temperature requirements on the involved objects. Below are some of the statements that I encountered and I don't understand:

  1. Emission clouds/nebula need to be ionized - But they need to be only partially ionized, right? If they are fully ionized energy transitions of the electrons wouldn't be possible, so emission lines wouldn't be possible either and there would be only scattering.

  2. Only hot gases create emission lines - Is this really true? Wouldn't it be possible, that a cosmic cloud much cooler than the object illuminating it would also cause emission lines (see drawing below)?

  3. Regarding emission lines from stars I read that they occur due to recombination - is this true? But why only discrete initial energy of the electrons would allow them to recombine with a (partially) ionized atom?

So can you explain the physical processes of nebula emission lines in a bit more detail than the links do? Tnx


Just a short answer, and likely others will fill in more details.

  1. If there is ionization of some atoms, then generally there is recombination as well - you will have both processes going on, roughly in balance with each other. Typically when an electron recombines with an atom, it does so into some excited state. Then as it drops from that excited state into lower states, it emits one or more photons on the way down. This is how ionization gives you emission lines - it's the inverse process that matters for the emission.

  2. To have emission lines, the cloud only needs to be hotter than the background behind it. So in your sketch, an observer at the bottom would see an emission spectrum (as you've drawn), but an observer to the right would see an absorption spectrum (with the same lines) - from that perspective, more light is lost relative to the (hot) background source than is re-emitted in that direction by the cloud.

This link may help with some additional explanation of these ideas.


Physical Processes Commons &trade

Based on downloads in June 2021

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1) Understand how radiation propagates in space and interact with matter. Familiarize with the basic concepts of intensity, flux, and opacity. Solve the equation of radiative transfer.
2) Understand the physical properties and the generation of the main emission processes that make our Universe "shine" in different ways at different wavelengths, including black-body emission, bremsstrahlung, synchrotron, Compton and Inverse Compton scattering processes.
3) Understand how radiation produced in astronomical systems is modified by propagation through an intervening plasma or matter, through absorption and re-emission processes.
4) Understand the emission and absorption of radiation by atoms and molecules, and line formation mechanisms, that are of fundamental importance to interpret Astronomical data sets.
5) Become proficient at interpreting and modeling astronomical observations of electromagnetic radiation (e.g. recognizing the physical processes giving rise to different spectral features, derive physical properties of the emitting regions based on real Astronomical data sets from radio to the X-ray band).

Midterm and Final Project


FLIERs: Cosmic bullets or evaporating gas?

When a low-mass stars reaches the end of its life, it sheds its outer layers and forms a planetary nebula, a shell of hot, ionized gas. As seen from far away, a planetary nebula appears looks like a piece of cosmic art, with brilliant colors standing in stark contrast to the inky reaches of space. These objects have long been targets of study for astronomers because of their complexity and beauty.

If you flip through any catalog of planetary nebulae, you’ll notice that they vary wildly in shape, color and size. Some patterns and features, however, crop up again and again, variations on a theme. An example of this is a group of tiny structures with the mundane moniker of fast, low-ionization emission regions — FLIERs, for short. These dense clumps of relatively neutral gas appear to be moving away from the center of the nebula at high speeds — usually several times the speed of sound in the nebula!

FLIERs were first studied as part of a larger group of kinematic structures in planetary nebulae. After most low-mass stars leave the main sequence, they spend several million years as red giants, gradually shaking off their outer layers. When these stars enter the planetary nebula phase of their lives, their stellar winds may speed up dramatically, driving matter into the remains of the red giant envelope. This forms a shock wave, creating a colorful ring or shell around the star.

A group of astronomers (Balick et al. 1987) were interested in testing out this “interacting winds” model, and decided to perform a series of spectroscopic observations of a set of elliptical planetary nebulae. They chose to map the Hα and [N II] lines Hα is usually one of the strongest spectral lines in a planetary nebula, and [N II], singly-ionized nitrogen, lies right next to it. Using a method known as long-slit spectroscopy, the team observed eight elliptical planetary nebulae on the Mayall 4-meter telescope at Kitt Peak National Observatory, with the intent of mapping gaseous outflows and studying their motion.

All eight nebulae contained interesting structures, and three of them — NGC 3242, NGC 6826, and NGC 7662 — were found to include the knots we now refer to as FLIERs a fourth nebula, in the study, NGC 7009 would later be confirmed to have them, too. The knots appeared in pairs, on opposite sides of each nebula, and appeared to be quickly moving away from the central star.

The authors suggested that the knots, as well as the other microfeatures they observed, were the result of inhomogeneities in the red giant envelope. If the envelope was denser along its equatorial plane, the geometry of the shock would take on an elliptical shape. A reverse shock could also form, traveling towards the star and collimating the outflow into a more focused shape, leading to features like these knots. The knots would be photoionized by ultraviolet light from the central star on one side, and heated by collisions with the ambient gas on the other. Still, this explanation remained vague, and astronomers needed a deeper understanding of the physical processes leading to the formation of these structures.

FLIERs were heavily studied throughout the next decade, and many groups attempted to explain them. Any theory behind their formation would have to account for several key properties:

  • Their tendency to appear in pairs, lying opposite one another along a nebula’s symmetry axis
  • Their high rate of incidence (FLIERs showed up in roughly half of all planetary nebulae studied)
  • Their extreme velocities, often up to five times the speed of sound in the surrounding gas

Broadly speaking, as another group of astronomers (also led by Bruce Balick) put it in a 1998 paper, FLIER models fall into two distinct classes. The first are the “bullet” models, involving small, low-ionized fast-moving clumps of gas traveling through the surrounding gas of the nebula. They could be formed by interacting winds in a binary system or some other sort of stellar behavior. Unfortunately, observations show that both the morphology and ionization structure of FLIERs cannot be reproduced by bullet models they decrease in ionization with increased radius from the star, rather than having heavily ionized heads.

The other set of models involve photoevaporation, where ambient gas in the nebula is ionized by high-energy radiation from the central star. Astronomers had suggested that dense knots could then be accelerated to high speeds by either stellar winds or so-called exhaust gas released by the aforementioned photoevaporation. Balick et al. argued that these models, too, should be ruled out, because the acceleration mechanisms are too weak to ensure that the knots reach those high speeds.

Normally, I’d end a blog post of this sort by talking about the theory or model that ended up accepted by the astronomical community. Unfortunately, I can’t do that in here, because despite the ever-increasing sample size of planetary nebulae and the rise of hydrodynamic modeling codes, we don’t have a firm explanation of how FLIERs are formed. Therefore, I’m simply going to wrap things up with a gallery of some of the most striking examples of planetary nebulae that contain FLIERs. I hope you enjoy this cosmic art gallery.


The Carina Nebula: Star Birth in the Extreme

In celebration of the 17th anniversary of the launch and deployment of NASA's Hubble Space Telescope, a team of astronomers is releasing one of the largest panoramic images ever taken with Hubble's cameras. It is a 50-light-year-wide view of the central region of the Carina Nebula where a maelstrom of star birth - and death - is taking place.

Hubble's view of the nebula shows star birth in a new level of detail. The fantasy-like landscape of the nebula is sculpted by the action of outflowing winds and scorching ultraviolet radiation from the monster stars that inhabit this inferno. In the process, these stars are shredding the surrounding material that is the last vestige of the giant cloud from which the stars were born.

The immense nebula contains at least a dozen brilliant stars that are roughly estimated to be at least 50 to 100 times the mass of our Sun. The most unique and opulent inhabitant is the star Eta Carinae, at far left. Eta Carinae is in the final stages of its brief and eruptive lifespan, as evidenced by two billowing lobes of gas and dust that presage its upcoming explosion as a titanic supernova.

The fireworks in the Carina region started three million years ago when the nebula's first generation of newborn stars condensed and ignited in the middle of a huge cloud of cold molecular hydrogen. Radiation from these stars carved out an expanding bubble of hot gas. The island-like clumps of dark clouds scattered across the nebula are nodules of dust and gas that are resisting being eaten away by photoionization.

The hurricane blast of stellar winds and blistering ultraviolet radiation within the cavity is now compressing the surrounding walls of cold hydrogen. This is triggering a second stage of new star formation.

Our Sun and our solar system may have been born inside such a cosmic crucible 4.6 billion years ago. In looking at the Carina Nebula we are seeing the genesis of star making as it commonly occurs along the dense spiral arms of a galaxy.

The immense nebula is an estimated 7,500 light-years away in the southern constellation Carina the Keel (of the old southern constellation Argo Navis, the ship of Jason and the Argonauts, from Greek mythology).

This image is a mosaic of the Carina Nebula assembled from 48 frames taken with Hubble Space Telescope's Advanced Camera for Surveys. The Hubble 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. Red corresponds to sulfur, green to hydrogen, and blue to oxygen emission.

Credits:Hubble Image: NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA)
CTIO Image: N. Smith (University of California, Berkeley) and NOAO/AURA/NSF


The patchy environment of a rare cosmic explosion revealed

An Artists conception of FBOT. Credit: Bill Saxton, NRAO/AUI/NSF

Scientists from the National Centre for radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR) Pune used the upgraded Giant Metrewave Radio Telescope (uGMRT) to determine that AT 2018 cow, the first of a newly discovered class of cosmic explosions, has an extremely patchy environment. Sources like AT 2018cow release an enormous amount of energy, nonetheless fade extremely rapidly. This along with their extremely blue color has led to them being called FBOTs for Fast Blue Optical Transient. This is the first observational evidence of inhomogeneous emission from an FBOT. The origins of FBOTs are still under debate, but proposed models include explosion of a massive star, collision of an accreting neutron star and a star, merger of two white dwarfs, etc.

The FBOTs are difficult to find since they appear and vanish in the sky very quickly. However, several of them have been discovered in the past few years via the recent advent of surveys that scan the sky almost on daily basis. FBOTs that also emit in the radio are doubly rare, but are particularly interesting because radio observations help one to determine the properties of the environments of these explosions and their progenitors.

The FBOT AT2018cow was discovered on 16 June 2018. At a distance of about 215 million light-years, the cow showed luminosities much greater than that of normal supernovae. Prof. Poonam Chandra (NCRA-TIFR) and Dr. A. J. Nayana (a former Ph.D. student of Prof. Poonam Chandra) carried out radio observations of AT 2018cow with the uGMRT to determine the properties of its extended environment and emission region. "Our study has tremendously benefited by the unique low-frequency capabilities of the uGMRT. The uGMRT observations of the "cow" played an unique role in finding the non-uniform density around this explosion", says Nayana. She added, "Our work provides the first observational evidence of inhomogeneous emission from an FBOT. The density of the material around this explosion falls drastically around 0.1 light-year from the transient. This indicates that the progenitor star of AT2018cow was shedding mass much faster towards its end of life."

The green and red solid/dotted lines denote different theoretical models. The turn over point of this light curve enabled the determination of material velocity from the explosion, magnetic field strength, and environmental density at different distances from the explosion centre. Credit: A. J. Nayana and Poonam Chandra

AT 2018cow is also unusual in that it has been observable in the radio for a very long time. The longer one can observe the post explosion emission, the more distance the material that was ejected during the explosion has traveled. This allows one to study the large scale environment of the source. Dr. A. J. Nayana and Prof. Poonam Chandra have been observing the cow for

2 years with the uGMRT to understand its properties. "This is the first FBOT seen for this long at low radio frequencies and the uGMRT data gave crucial information about the environment of this transient.", Nayana said. Poonam Chandra explains, "This is the beauty of low-frequency radio observations. One gets to trace the footprints of the progenitor system much before it exploded. It is interesting that the material from the explosion is moving with speed greater than 20% speed of light even after

257 days post-explosion, without any deceleration".

The image inside the box is the AT2018cow. Credit: A. J. Nayana and Poonam Chandra

While the origin of FBOTs is still under debate, detailed radio observations can give hints about various physical parameters of these events like the speed of the material that came out of this explosion, the magnetic field strength, the rate by which the progenitor system sheds its mass before the explosion, etc. The uGMRT observations of the "cow" suggest that the progenitor erupted its material

100 times faster during the years close to its end-of-life compared to

23 years before the explosion. Also, AT2018cow showed inhomogeneities in the radio-emitting region whereas the other two radio bright FBOTs did not show these properties, making the "cow" unique in the group. "Observations of more FBOTs with the uGMRT will give information about their environments and progenitors to develop a comprehensive picture of the properties of these intriguing transients.", says Nayana.

The GMRT is an array of thirty 45-m antennas spread over 25 sq-km area in Khodad village, Narayangaon, India, built and operated by NCRA-TIFR, Pune. Currently it is one of the most sensitive low frequency radio telescope in the world.

The paper was published in the April 30, 2021 issue of The Astrophysical Journal Letters.


Nebula

With emission nebulae, ultraviolet radiation, generally coming from nearby or embedded hot stars, ionizes the interstellar gas atoms and light is emitted by the atoms as they interact with the free electrons in the nebula. Emission nebulae can be in the form of H II (ionized hydrogen) regions, planetary nebulae, or supernova remnants. With reflection nebulae light from a nearby star or stellar group is scattered (irregularly reflected) by the dust grains in the cloud. Reflection and emission nebulae are bright nebulae. In contrast dark nebulae are detected by what they obscure: the light from stars and other objects lying behind the cloud along our line of sight is significantly decreased or totally obscured by interstellar extinction. Dark nebulae contain approximately the same mixture of gas and dust as bright nebulae but there are no nearby stars to illuminate them. If the column density is sufficiently high, the majority of the hydrogen is likely to be present in molecular form (see molecular clouds).

The term ‘nebula’ was originally applied to any object that appeared fuzzy and extended in a telescope: over 100 were listed in the 18th-century Messier Catalog. The majority of these objects were later identified as galaxies and star clusters.


Physics of the Interstellar and Intergalactic Medium

This is a comprehensive and richly illustrated textbook on the astrophysics of the interstellar and intergalactic medium--the gas and dust, as well as the electromagnetic radiation, cosmic rays, and magnetic and gravitational fields, present between the stars in a galaxy and also between galaxies themselves.


Topics include radiative processes across the electromagnetic spectrum radiative transfer ionization heating and cooling astrochemistry interstellar dust fluid dynamics, including ionization fronts and shock waves cosmic rays distribution and evolution of the interstellar medium and star formation. While it is assumed that the reader has a background in undergraduate-level physics, including some prior exposure to atomic and molecular physics, statistical mechanics, and electromagnetism, the first six chapters of the book include a review of the basic physics that is used in later chapters. This graduate-level textbook includes references for further reading, and serves as an invaluable resource for working astrophysicists.


The term "Nebula" has varied in the history of astronomy. In pre-telescopic times it was used to distinguish objects which look non-stellar from the pointlike stars. Most "nebulae" known at that time have been shown to be open star clusters. The term "Nebula" was thus used for what we now call "Deepsky Object".

In early telescopic times, the nature of these objects was still widely unknown. With open clusters resolved, still all other deepsky objects were summarized as "Nebulae". Only the use of large telescopes, the discovery of spectroscopy and the invention of photography in the second half of the 19th century made it possible to distinguish "real" nebulae - i.e., gas and dust clouds - with certainty from objects made up of stars (globular clusters and galaxies).

  • Emission Nebulae: Emit light because the atoms in their gases are excited by high energy radiation of stars involved. They show emission line spectra.
  • Reflection Nebulae: Reflect light of nearby stars by their dust particles. Therefore, their spectra are the same as those of the stars, typically continuous spectra.
  • Absorption Nebulae or Dark Nebulae: Absorb light: Their gas component can be seen as absorption spectra in the light of background stars, their dust component by absorbing and reddening background light.

A more modern scheme distinguishes star-forming or pre-stellar nebulae (basically diffuse and dark nebulae) from post-stellar nebulae (basically planetary nebulae and supernova remnants). The first of these classes typically includes clouds of interstellar matter of a mass of several 100 or several 1,000 stars, while the latter is related to one specific star in advanced state of evolution, at or just beyond the end of its nuclear life.

There are a number of variations and special classes of nebulae such as the Herbig-Haro Nebulae (related to stars in the process of formation, and emit jets of gaseous material, thus often found near large diffuse nebulae with star formation) and Wolf-Rayet Nebulae (related to hot Wolf-Rayet stars, stars of some age that have ejected matter they now cause to shine).

On cosmic timescales, all these types of nebulae, in particular the bright nebulae, undergo rapid changes and have only comparatively short lifetimes, so that those we observe are all rather young objects. Planetary nebulae and supernova remnants usually have only a few thousands of years before they fade and spread their matter into the this interstellar matter of their environment, while star forming H II regions shine bright for the few 100,000 or million years they are brightened by the very hot massive O and/or B stars that formed within them. The giant molecular clouds have a somewhat longer life of some 10s of millions of years, while they form new stars and star clusters.

One should keep in mind that all Messier nebulae are members of our Milky Way Galaxy (together with many others). Other galaxies contain nebulae, too, which can be detected with considerably sensitive instruments within the images of these galaxies.


Radio relics

Galaxy clusters are among the largest known structures in the Universe, extending over several megaparsecs. Galaxy clusters are filled with an intracluster medium, consisting of hot gas, magnetic fields, cosmic-ray particles, dark matter, and many interspersed galaxies. Most galaxy clusters are visible as extended radio sources that emit synchrotron radiation. As the magnetic fields in the intracluster medium are turbulent, the radio emission is mostly unpolarized, but with a prominent exception:

Huge streams of gas are continuously stirring the structures in the Universe. When a galaxy cluster collides with another cluster or an intergalactic gas cloud, a shock forms that compresses the gas and the magnetic fields and accelerates cosmic-ray particles. These shocks become visible as huge arcs in radio synchrotron emission, called “radio relics”. In more than 70 clusters relics were found so far, but probably many more exist that are too faint to be observed with present-day radio telescopes. Shock fronts amplify and order the turbulent magnetic fields of the intracluster medium, resulting in enhanced radio synchrotron emission with a high degree of linear polarization. Polarized radio waves are ideal to track relics.