Physics 728
Radio Astronomy:  Lecture #12
Prof. Dale E. Gary

Astronomical Radio Emission

Solar System Objects

Planets and other solar system objects are general only thermal emitters (producing radio emission only due to black body radiation from their surfaces).  Since they are generally cold (700 K for Mercury down to 30 K or so for Pluto), they are weak emitters.  Here is Saturn's blackbody radio imageJupiter is the main exception, since it has a very large magnetosphere (would be larger angular size than the Moon if we could see it in visible light), which traps high-energy electrons that then emit synchrotron radiation.  Still, we can image the thermal radiation of the planet at higher frequencies. With ALMA in mm wavelengths, we can even make a thermal map of Europa. We can also bounce radar signals off the nearby planets (Venus and Mars), and image the echos.
A good place to start with stellar radio emission is to look at an H-R diagram.
This diagram is from Stephen White, when at University of Maryland (now at Air Force Research Lab).  The placement of the symbols is according to the star's classical visual magnitude vs. color (B-V), but the symbols themselves encode the information about the radio emission.  Most of the main sequence and subgiant objects are nonthermal emitters (filled circles), while most of the giants and many of the O-B stars are thermal emitters (simply because they are big).  The blue circles near the center of the diagram are RS CVn binaries.  These have a late-type subgiant "revved up" by tidal interactions with its close binary companion, as shown in the two figures below.  The open circles in the above figure, just above the RS CVn ones, are symbiotic stars, which again are binaries, but now with a compact companion (perhaps a black hole).
Left: Model of an RS CVn binary, showing interactions between stars due to their high field strengths and close proximity. Right: Star spot mapping of the surface of one component of an RS CVn binary, showing that 10-20% of the stary may be covered with spots. The spot size and location is determined from eclipses and rotational modulation.

Note that the G, K and M dwarfs (red dots) are weak emitters.  These objects are also flare stars, meaning that occasionally they have strong radio outbursts.  Why should red dwarfs have large flares?  You can calculate how strong the Sun's radio outbursts would appear if observed from the distance of the nearest stars, and you will find that they are barely detectable.  Flare stars, on the other hand, have both optical and radio flares that are giant in comparison to solar flares.  There is good evidence that such stars have a large fraction of their surfaces covered with "sunspots."  This is probably due to their having fast rotation coupled with a fully convective interior, so that dynamo generation of magnetic fields is much larger than for the Sun.  A few stars have so much activity that they can be said to have detectable "quiescent" radio emission all of the time (e.g. the star UV Cet). Here is a report of an intense flare on the flare star AD Leo, reaching a probable brightness temperature of 1013 K. ALMA provides extremely high spatial resolution images of protoplanetary disks like this icy dust ring around the star Fomalhaut. Here is a gallery of images from ALMA.

When a star's mass at the end of its life is M > 1.4 Mo, electron degeneracy is no longer enough to keep gravity at bay, and matter is crushed to force inverse β decay
p+ + e- ---> n + ν.        (ν is a neutrino)
so the protons and electrons are combined to form neutrons--a neutron star.  The state of matter is a neutron degenerate gas.  Degenerate objects have the peculiar property that with greater M they have smaller radii, up to ~ 3 Mo.  Radii are typically 10-30 km.

Pulsars are rapidly rotating neutron stars that have a very high magnetic field (concentrated during the collapse of the core of a star into a neutron star) whose poles happens to be offset from the direction of the spin axis.  If the spinning happens to bring the poles around to point at Earth, we will see these bright poles briefly as an intense radio emitting source.  The emission is due to synchrotron emission of electrons in the high magnetic fields.  The pulses, of course, are repeated on each spin.  The pulses suffer dispersion as they travel through the interstellar medium. The properties of the dispersion allow the column density of electrons between us and the pulsar to be deduced.

Pulsars spin at periods ranging from 4 s to 1.6 ms.  Here is how they sound:

pulsar sounds

You can imagine the forces on the fastest pulsar.  The surface speed is v = 2πR / P, where P is the pulsar period and R is the pulsar radius (about 10 km = 104 m):

v = 2πR / P = 2π(104 m) / 0.0016 s = 3.9 x 107 m > 0.1 c.
Its centripetal acceleration is ac = v2/R, and the star will rip itself apart if this is greater than the gravitational acceleration holding the star together, ag = GM / R2, that is, if the surface velocity v is greater than v = [GM/R]1/2.  What is the smallest possible period P that a pulsar could have?
P = 2πR / v = 2πR / [GM/R]1/2 = 0.3 ms
We can combine mass and radius and write this in terms of only one quantity, the density, as
P = 3.8 x 105ρ-1/2.
With the fastest periods of about 2 ms, one can see that the density must be high.

Binary Pulsars and pulsar evolution

Pulsar Evolution

Galactic Objects
Cassiopeia A is a supernova remnant that has been studied with detailed maps over a couple of decades, now.  One can actually see the expansion by watching the detailed images as a movie (unfortunately, the movies seem to be not working).  What we are seeing is the outer envelope of an exploded star that is moving outward into the interstellar medium with high velocity, surrounded by a shock wave that is still heating material to emit X-rays.

SiO maser emission in found in stellar atmospheres, and water maser emission comes from H II and star formation regions, so there is a surprising variety of radio emission mechanisms.  The SiO molecules surrounding some stars with extended, cool atmospheres is preferentially in the J = 1 spin state, and as radio emission at the right frequency (43 GHz) stimulates the transition from J =1 to 0, they emit another photon.  This photon, along with the original one, proceed into the cloud of molecules and stimulates more transitions, giving rise to a very bright line emission in small regions. The direction of the magnetic field can be deduced from the direction of linear polarization of the emission.  Likewise for the water (H2O) maser, operating at 22 GHz.

Mapping H I In Galaxies
Atomic hydrogen (also called neutral hydrogen, and H I), shows the distribution of relatively cool gas in a galaxy.  Comparing two different types of galaxy shows the extreme differences.

Nearby Spiral Galaxy
"Our 21 cm mosaic provides the most detailed view yet attained of neutral hydrogen in a spiral galaxy (other than the Milky Way). The observations are characterized by spatial resolution of 20 pc (5" at 840 kpc) and velocity sampling
of 1.3 km/s. For this reason, our database compares straightforwardly with the recent ATCA+Parkes surveys of the Large and Small Magellanic Clouds (Staveley-Smith et al. 1997, Stanimirovic et al. 1999, Kim et al. 1998).  At the VLA, M33 was observed using six mosaic pointings in both the B (48 hr) and CS (6 hr) configurations. Our interferometric data has recently been complemented by ultra-sensitive total power observations obtained at WSRT, using the Dutch instrument in an auto-correlation mode whereby all 14 elements are employed as incoherent single-dishes.

"Figure 1 shows a color representation of our peak brightness temperature image, in which the hue has been assigned on the basis of velocity at peak ?B in each of the spectra. The pattern of galactic rotation dominates one's visual impression, but doesn't obscure significant localized motions, perhaps most apparent as abrupt color changes within the spiral arms. For this preliminary image, no masking of the cube has been applied. Instead, we preserved sensitivity by tapering to 40 pc resolution (10" FWHM). We are now developing methods to create a “multiresolution” version of this map, in which the beam size is position dependent and broadens to maintain signal-to-noise in faint regions such as the outer disk and interarm gaps."

Elliptical Galaxy
"VLA atomic hydrogen observations of the shell galaxy NGC 2865. The gas is shown as yellow contours on an optical image from the Digital Sky Survey. The main body of the NGC 2865 is typical of early type galaxies, but at fainter light levels the galaxy exhibits a peculiar morphology, with many shells, ripples and loops. The VLA spectral line observations shows gas within the  main body of the elliptical, but also distributed in an extended ring around it."

Why do spirals have gas and dust in them, and ellipticals do not?  The answer lies in our new understanding of how ellipticals form, through galaxy collisions and mergers.  The stars in such a "collision" do not collide, but merely pass through each other.  The gas and dust, however, does collide and ends up outside the galaxy.

Quasars and Lobe Radio Sources

Quasars are Active Galactic Nuclei (AGN), i.e. the centers of extremely active galaxies.  For a time they were mysterious objects because they appear only like a faint star optically (the term quasar is short for quasi-stellar object), but recent observations show that they do have faint "nebulosity" around them, which is actually the light from the rest of the galaxy in which they are embedded.  We now know that these are powered by supermassive black holes.  They have extremely well collimated jets, seen both optically and in the radio (as below).  The jets culminate in giant radio lobes (sometimes on only one side), which are many, many times the size of the parent galaxy.

Image courtesy of NRAO/AUI"

This image shows the radio emission from relativistic streams of high energy particles generated by the quasar. This is a classic double-lobed radio source. Astronomers believe that the jets are fueled by material accreting onto a super-massive black hole at the center of the host galaxy (not shown in this image). The high energy particles are confined to remarkably well collimated jets, and are shot into extragalactic space at speeds approaching the speed of light, where they eventually balloon into massive radio lobes. The overall linear size is of the radio structure is 212 kpc (for a Hubble constant of 100 km/s/Mpc), which can be compared to a typical galaxy diameter of about 30 kpc. The quasar has double lobes with prominent hot spots, and has a narrow jet, but no counter-jet. Its possible that we only see the jet that is pointing toward us, which may be "Doppler boosted" in brightness when the particles emitting the radio radiation are moving toward us at close to the speed of light. The counter-jet would be moving away from us, and would thus not experience Doppler boosting. The jet brightens and bends as it enters its lobe."

One interesting phenomenon that one can observe in the jets is the presence of "superluminal" sources.  These are sources that appear to move at velocities as much as 45 times the speed of light!  This is just an apparent speed, caused by the source moving very close to our line of sight at nearly the speed of light.  In effect, we see time compressed, and so the source appears to be moving faster than c.