Continuum Radio Emission from Stars

Since the construction of large arrays like the WSRT and VLA in the 1970's, the field of stellar radio astronomy has advanced tremendously. Radio emission has been detected from all stages of stellar evolution, from birth to death, and in all these stages has shown us astrophysical phenomena and stellar activity not detectable by any other means. However, further major advances in stellar radio astronomy are limited by sensitivity. Only a few hundred stellar radio sources are now known. The SKA will increase this number by over four orders of magnitude to at least 10⁶ stars (Seaquist 1996). This phenomenal increase will bring about a new era in the field of stellar radio astronomy. New classes of stars and previously unknown phenomena related to stellar radio emission are certain to be discovered. The discipline will literally be reborn.

The loci of 441 radio-detected stellar systems on a Hertzsprung-Russell Diagram (HRD, brightness $M_{\rm V}$ vs. color B-V) is shown in Fig. 2.1. We clearly see the main sequence, a clump in the G-K subgiant/giant area, mostly due to RS CVn binary systems and PMS, and the concentration in the red giant/supergiant area, mostly associated with the cool components of symbiotic stars (to which the radio emission has been attributed). Despite the apparent plenitude of radio detections across the HR diagram, many features result from strong selection bias and detection limitations. Obviously, the observed radio luminosity increases drastically with increasing absolute brightness (decreasing $M_{\rm V}$). But since the space density at the same time steeply decreases, optically luminous stars are on average distant objects, with only very radio-luminous examples being detected by radio telescopes.

  

Figure 2.1: The Radio HR Diagram showing the loci of 441 radio detected stellar systems. The stellar classes are color-coded, and the symbol sizes indicate the radio luminosity. Only detections within 1-10 GHz have been considered. The radio star catalog by Wendker (1995; 1998) has been used, complemented by (mostly Hipparcos) distances and B-V colors, with some color corrections due to emission lines (WR stars) or to extinction.

  

Figure 2.2: The distribution of radio luminosity with position in the Hertzsprung-Russell Diagram. The envelope of peak radio luminosities is shown as a 2-D surface. The left panel includes all stars (see Fig. 2.1). The left includes only single stars (or components in binary systems that are not tidally interacting).

The radio luminosity distribution is graphically illustrated in two panels of Fig. 2.2. The 2-D surfaces represent an approximation to the envelope of the peak radio luminosity detectable as a function of $M_{\rm V}$ and B-V (binning, peak search, and median smoothing was applied for this representation). The right panel represents only single stars (or components in binaries that are not tidally interacting and which are the likely sources of the observed emission; PMS are excluded).

With very rare exception, normal stellar photospheric emission cannot be detected with current sensitivities. Detectable radio emission is virtually always associated with active stellar phenomena, such as energetic outbursts or mass loss. Radio emission is generated in different atmospheric environments characterizing different areas in the HRD. In hot stars, radio emission is believed to stem from electrons accelerated in shocks that form in strong stellar winds. In the cool-giant area, many sources are binaries, some of which produce radio emission in shocks of colliding winds; also, chromospheric emission is detected from red giant stars. These range of phenomena can be broadly categorized based on the characteristic power law of the radio continuum spectrum, ranging from:

• normal stellar photospheres ($\nu^{+2}$)
• bremsstrahlung emission from circumstellar environments and stellar ejecta in the form of winds (WR and OB stars), shells (novae, planetary nebulae), and jets (symbiotic stars, class 0 PMS stars) ($\nu^{+1}$)
• synchrotron and gyrosynchrotron emission from flare stars, active binary systems, pre-main sequence (PMS) stars, and x-ray binaries ($\nu^{-1}$)
• pulsars ($\nu^{-2}$)

For all these stellar types we have only a very poor census of properties. Our current understanding of these phenomena is based on investigations of a few bright members of the class. For pulsars, non-thermal stars, and thermal circumstellar emission (type 1, 2 and 3 listed above), the SKA will uniquely afford the sensitivity to provide measurements of radio properties for whole stellar populations, allowing meaningful studies of the relationship between radio phenomena and other stellar properties and evolutionary states. Many classes will be detected over the entire volume of the Galaxy and in the Magellanic Clouds. Pulsars will be detected throughout the Local Group of galaxies.

  

Figure 2.3: The flux density expected for a typical Mira variable as a function of frequency and distance compared to the continuum sensitivity of the SKA after 8 hours integration (dashed line). Also shown is the sensitivities of the VLA and LSA/MMA for the same integration time.

  

Figure 2.4: Flux density at 5 GHz versus distance for a number of non-thermally emitting stars compared with the continuum sensitivity of the upgraded VLA and the SKA for an integration time of 8 hours. The luminosities used are typical values. (after Seaquist 1996).

In the first class of phenomena, stellar photospheres, emission rises rapidly toward shorter wavelengths, and planned millimeter arrays will detect a large number. However, at centimeter wavelengths, the SKA will study a similar number of these stars and the high angular resolution at the shortest wavelengths will allow direct imaging of the stellar surfaces. Figure 2.3 shows the flux densities expected from the photosphere of a Mira variable star at various distances. Also shown is the continuum sensitivity of the SKA as a function of frequency. Presently, isolated Miras can be detected at radio wavelengths out to a distance of only a few hundred pc. With the SKA this will be increased to more than 10 kpc, making possible the detection of emission from Mira stars throughout most of the Galaxy.

The expected flux densities for several classes of non-thermal stars versus distance are shown in Fig. 2.4, along with the continuum sensitivity of the SKA at 5 GHz after 8 hours integration. With the SKA the quiescent radio emission from the Sun would be detectable to a distance of $\sim$100 pc. This volume contains $\sim10^4$ G-dwarfs. It would be possible for the first time to explore the Solar-stellar connection in the radio, and place the radio phenomena of the Sun within the context of the properties of stars such as rotation, mass, magnetic field and chemical abundance. Even the nearest stars with solar radio luminosity are undetectable at present.

Quiescent radio emission from flare stars is detectable out to several kpc, and typical RS CVn systems, PMS stars, peculiar magnetic stars, and non-thermal components of WR winds would be detectable over the entire Galaxy and the Magellanic Clouds. X-ray binary systems and brighter WR stars would be detectable in M31. For most of these classes, only a handful of objects are now known. With the SKA, in addition to studying large and complete samples of these populations, we will be able to trace their spatial distributions in star formation regions and clusters and within the Galaxy as a whole, thereby relating radio properties to characteristics such as age and environment.