' * ■ ■ ■ 1 ■ ■ ■ ■ *

L vvis

KXJ1038.4+4B31 3

6S50 8600 6650 6700

6S50 8600 6650 6700

Wavelength (A)

FIGURE 7.32. Optical spectra of dMe stars. Reproduced from Mochnacki et al. (2002).

7.4.4 Galactic compact-object sources of emission lines As the final section of this work, we turn to Galactic compact-object systems as sources of emission lines. These include cataclysmic variables (CVs) as well as X-ray binaries -both neutron-star and black-hole binaries. Cataclysmic variables

Cataclysmic variables (CVs) are accretion-powered binary systems with a white dwarf as the compact object. This general class includes a wide variety of systems, such as novae, dwarf novae, polars, and intermediate polars - a veritable bestiary of CVs. As was seen with the T Tauri stars above, accretion can produce much higher temperatures in a system than one would expect from "standard" photospheres. In the case of CVs, these temperatures are in the range of ~105 K and higher. Thus, photoionization often dominates the emission-line spectrum in these systems, up to and including very highly-ionized states of metals. The emission lines arise from all surfaces irradiated by "hard" radiation fields - the accretion disk, the donor-star face, the disk "hot spot" at its outer edge, and the accretion column where matter is funneled onto the white dwarf's surface by its magnetic field (if there is one).

The emission lines from CVs can provide important probes of physical conditions over a broad swath of the system's emitting regions. In particular, we expect different lines/ionization states to arise from different regions, so that we can separate them (though typically this is quite model-dependent, of course). By measuring the line strengths, we can diagnose the characteristics of the radiation field. Furthermore, by measuring line wavelengths and widths, we can constrain the motions of material in the system.

Perhaps most interestingly, for CVs we can carry out "Doppler tomography" to construct detailed pictures of the system's geometry and physics. This is because line wavelengths map line velocities, which in turn can be mapped to positions via a model of the system (Figure 7.33). Thus, line profiles can give some idea of the system's structure. Furthermore, as the CV system rotates orbitally, our line of sight changes, shifting the mapping and providing complementary information. By following through a complete revolution of the system orbit, we can use the line profiles to create a "tomogram" of the system. This can provide detailed insight into the structure of a system that is spatially completely unresolved! X-ray binaries

X-ray binaries (XRBs) are analogous to CVs in that they are also accretion-powered binary systems with a compact object. However, rather than the white dwarfs in CVs, XRBs typically contain either a neutron star or a black hole as their compact object. Again, the accretion process produces unusually high temperatures in these systems, and the deeper potential wells provided by these compact objects mean that the temperatures can and do reach higher levels (~107 K or higher) - resulting in the significant X-ray emission that earns their name. As with CVs, XRBs often display high ionization states of some metals, up to and including He-like Fe ions! Emission lines again arise from essentially all surfaces irradiated by the hard field produced here: the accretion disk, donor-star face, hot spot, and accretion column. In the case of XRBs, jet outflows are also frequently seen with emission lines. As with CVs, then, XRB emission lines provide diagnostics of all of these regions, including properties of the irradiating field and

Doppler Tomography Astronomy

Figure 7.33. An illustration of the concept of Doppler tomography. Adapted from

Figure 7.33. An illustration of the concept of Doppler tomography. Adapted from

motions within the system. In the case of XRBs, knowledge of these motions can allow measurement of the mass function, which thus provides a lower limit on the mass of the compact object, Mx.

The fundamental division in XRBs is between black-hole binaries and neutron-star binaries. However, it is observationally non-trivial to distinguish between these types. In the absence of a mass measurement, there is no currently accepted reliable diagnostic for this. Even for the small-ish subset with mass functions, the lower mass limit may be inconclusive as a diagnostic between neutron stars and black holes. Thus, we


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