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Figure 7.8. Morphological/kinematic connection in BRETs (figure courtesy of J. A. Lopez).

to the ISM - they are the delivery phase for "feedback" of stellar populations on galaxy evolution. They are also the birthplaces of neutron stars and black holes, and can provide important insights into the physics of these exotic objects.

There are two basic types of supernova remnants (SNRs): shell-like and center-filled. The young (<5000 yr) shell-like SNRs are typically dominated by material from the progenitor star, whereas older shell SNRs (5000—20000 yr) are typically dominated by ISM material entrained by the SNR expansion. As their name implies, they typically appear to be hollow "shells" in their structural properties (see Figure 7.9).

Center-filled SNRs, on the other hand, have (as their name also indicates) a "filled" appearance, rather than being hollow. These SNRs, also known as "plerions," are typically powered by pulsars and/or pulsar-wind nebulae. The archetypal example is the Crab Nebula (Figure 7.10), where we can clearly see that the relativistic particle wind from the pulsar is filling the SNR center with synchrotron-emitting particles. Plerions are almost uniformly young (<5000 yr).

A third class of SNR is constituted by the "composite" remnants, such as the Vela SNR, which have both a pronounced shell-like structure and some (typically small/faint) center-filling from a pulsar. Owing to their morphology and pulsar power, these are often considered to be old plerions, in which the pulsar's center-filling is fading away and the expanding SNR shell is clearly separated from the center.

The optical emission spectrum of most SNRs contains very little continuum emission -most of the light comes out in the form of emission lines (a partial exception of course being the blue synchrotron glow from the center of the Crab Nebula, for instance). The spectrum of CTB 1, for instance, has strong lines of H, [O ii], [O iii], [S ii], and [N ii] (Figure 7.11). We can also see fainter lines of many other species, including He I, He ii, [OI], [NI], [NeIII], [FeII], [FeIII], [CaII], and [Ariii].

Figure 7.12 shows a near-IR spectrum of a fast-moving knot (FMK) in Cas A. We can see very similar species to the optical spectrum, indicating a close link between these wavebands. However, we can also see high-ionization "coronal" emission lines of [Si vi] and [Si x].

Supemcva Remnant Cassiopeia A

Supemcva Remnant Cassiopeia A

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Figure 7.9. An example of a shell SNR - Cas A (figure reproduced from the Hubble Heritage archive).

.Hubble Hcri:jijc

Figure 7.9. An example of a shell SNR - Cas A (figure reproduced from the Hubble Heritage archive).

Figure 7.10. The Crab Nebula - an example of a plerion SNR (figure reproduced from the Chandra X-ray Observatory website).
Figure 7.11. The optical spectrum of the CTB1 SNR, reproduced from Fesen et al. (1985).

If we then turn to another feature in Cas A - a "quasi-stellar floccule" - we also see similar species to the optical (Figure 7.13). However, here we see that the He I and [Fe II] features are very strong - in fact, much stronger than even the hydrogen lines! If we then turn to a quasi-stellar floccule feature in the Kelper SNR, we see very similar spectral features. Thus, similar features in different SNR can resemble one another even more closely than different features from the same SNR!

Figure 7.12. The IR spectrum of a fast-moving knot in Cas A. Adapted from

Gerardy & Fesen (2001).

Figure 7.12. The IR spectrum of a fast-moving knot in Cas A. Adapted from

Gerardy & Fesen (2001).

These emission lines hold a wealth of information on the SNR, including its identity/nature, electron density/temperature, ionic abundance, shocks, and even the late evolutionary stages of the SNR progenitor star.

Owing to the high extinction in the Galactic Plane, most SNR have been identified by their shell-lock morphology in the radio. However, HII regions are a significant source of confusion, insofar as their "sculpting" by massive-star-forming regions can produce similar morphologies. Fortunately, SNR emission lines in the optical (and IR) can resolve this problem. Figure 7.14 shows a plot of various SNR and HII regions in a parameter space defined by line ratios of [Oi], [Oii], and Hp. As we can see, the SNRs are cleanly separated from the HII regions on such a plot. Similar diagnostics are also available using [S ii].

Emission lines can also provide important insights into the progenitor star's evolution, particularly in the case of very young (<100 yr) SNRs, such as SNR 1987A (Figure 7.15). As the supernova blast wave propagates through the ISM, it excites a wide variety of emission lines. For massive stars such as these, of course, the local ISM is in fact dominated by ejecta from the progenitor star (prior to the supernova event). This allows us to probe the composition, density, and kinematics of these ejecta, and thus tie them to the progenitor star's evolution.

I will wrap up SNRs with a quick note regarding [Fe II] emission. As noted above, Fe in the ISM is usually depleted onto dust grains, producing very low abundances in the gas phase of this species. However, shocks break up these dust grains as they pass through the ISM and greatly increase the local abundance of gas-phase Fe, as revealed by the presence of [Fe II] lines. Thus the [Fe II] lines in SNR provide excellent shock diagnostics, and can be used effectively to identify SNRs in contrast to confusing HII

Figure 7.13. The IR spectra of quasi-stellar flocculi in Cas A and the Kepler SNR. Adapted from Gerardy & Fesen (2001).

Figure 7.13. The IR spectra of quasi-stellar flocculi in Cas A and the Kepler SNR. Adapted from Gerardy & Fesen (2001).

Figure 7.14. Separation of SNRs from H ii regions using emission-line ratios. Adapted from

Figure 7.14. Separation of SNRs from H ii regions using emission-line ratios. Adapted from

Figure 7.15. Remnant of SN 1987A (figure reproduced from the Hubble Space Telescope website).

regions. This is particularly important given that the optical diagnostics noted above are often observationally unavailable for most SNRs due to extinction.

7.4. Galactic stellar sources of emission lines

Now that we have covered the "nebular" Galactic sources of emission lines, I will turn to "stellar" sources of the same. By "stellar," I mean relatively compact (i.e. nearly pointlike at typical angular resolution) objects with emission lines driven by the photospheric output of a star.

A recurring theme for stellar emission-line sources is the idea that "emission equals extension." That is, stellar emission-line activity essentially requires some sort of extended atmosphere beyond a "normal" photosphere - chromosphere arrangement. This is a fundamental feature of radiative transfer in stellar atmospheres. In a "standard" stellar atmosphere, any "piece" of the atmosphere is roughly in local thermodynamic equilibrium (LTE). This means that (at least approximately) thermal collisional excitation and de-excitation of atomic energy levels occur in equilibrium with the local blackbody radiation field. Hence the number of "line" photons emitted due to a "downward" shift in electron energy levels is exactly balanced by the number of line photons absorbed due to "upward" shifts in energy levels. Thus, no lines can be formed. As any basic text on stellar atmospheres explains, the ubiquitous absorption features found in most stellar spectra are due to non-LTE variations in the atmosphere.

Emission lines also require non-LTE conditions to hold for their formation. More specifically, they require that some process produce a population inversion of electron energy levels, so that an excess of electrons at high energy levels (relative to the number produced by absorption of incident "line" photons) produces a corresponding excess of emitted "line" photons via downward energy-level shifts. Exactly such an inversion is produced when a portion of the stellar atmosphere "sees" a radiation field that is "hard" (i.e. of higher energy or "hotter") relative to the local temperature - we adopt that as our working concept for an "extended" atmosphere. A standard atmosphere avoids such a situation because temperature changes are small over distance scales for which the optical depth has values t ~ 1.

In the following subsections, I will review prominent stellar sources of emission lines in the Galaxy, including young stellar objects, massive stars, magnetically active stars, and compact-object binary systems.

7.4.1 Young stellar objects By "young stellar objects" here, I primarily refer to objects making the transition from accreting protostars to hydrogen-burning main-sequence stars. I include in this class T Tauri stars, Herbig-Haro objects, and massive protostars. Note that these are not necessarily exclusive classifications - it may be possible for individual objects to belong to more than one of these classes simultaneously. However, as we shall see below, each class has its own defining characteristics.

7.4.1.1 T Tauri stars

T Tauri (or "TT") stars represent the final formation phase for low-mass stars - the most numerous stars in the Universe. As such, they are a critical part of the star-formation process. TT stars are generally thought to be low-mass protostars near the end of their accretion phase (in fact, some are only weakly accreting or perhaps even non-accreting). They are thought to consist of a central object (the protostar, or perhaps even a zero-age main-sequence object) surrounded by an accretion disk. (In the non-accreting case, this

Figure 7.16. A schematic diagram of a T Tauri star system, including the central protostar, accretion disk, disk central hole with protostar magnetosphere, wind region, and possible central jet outflow. Adapted from Kurosawa et al. (2006).

may be a "remnant disk" - the last vestiges of material left in the disk after accretion has halted.) TT objects typically have significant outflows as well. The accretion through the inner disk is typically thought to be slowed (or halted) by the magnetic field of the central object, and magnetospheric/disk interactions are likely to power parts of the outflow as well (particularly jets). This can be seen schematically in Figure 7.16.

TT stars have a continuum spectrum usually accompanied by strong emission lines -typically very strong hydrogen series. They can also exhibit strong He I (1.083-^m) emission, which has been tied to windy outflows. Somewhat surprisingly, TT stars can also exhibit strong CIII and O iv emission lines. These high ionization states are typical for temperatures T > 15 000 K, which is significantly higher than the main-sequence photo-spheric temperatures for such low-mass objects. This is probably indicative of accretion-powered "hot spots" in the system, indicating that accretion is a significant energy source in these systems.

Much can be deduced about TT stars by investigating the profiles of the strong H emission lines. The complex line profiles indicate highly non-trivial disk geometries (see Figure 7.17). These profiles also exhibit significant time variability, sometimes on timescales as fast as a few days. This indicates significant variations in the accretion flow in the TT-star system - in geometry, in accretion rate dM/dt, or in both. The He I (1.083-^m) line profiles (Figure 7.18) are also good diagnostics for the TT outflow. They are known to be very sensitive tracers of wind activity. More recently, they have been shown to trace mass infall (accretion) as well. Thus, we see that TT-star emission lines can provide good insights into accretion and outflow in the system.

7.4.1.2 Herbig-Haro Objects

Herbig-Haro ("HH") objects are another important formation phase for some low-mass stars, and are not necessarily unrelated to T Tauri stars. The hallmark of HH

Figure 7.17. T Tauri-star Ha line profiles, showing complex geometry and time variability.

Adapted from Edwards et al. (2006).

Figure 7.17. T Tauri-star Ha line profiles, showing complex geometry and time variability.

Adapted from Edwards et al. (2006).

Figure 7.18. T Tauri-star He I 1.083-|im line profiles, showing complex geometry in the disk wind. Adapted from Edwards et al. (2006).

objects is collimated outflows. Such outflows are known to carry away significant angular momentum, which is of course the critical activity for enabling accretion. Figure 7.19 shows pictures of the prototypical objects HH1 and HH2, where the outflows can be clearly seen. As can be seen in Figure 7.19, HH objects are also quite common in many star-forming regions.

The outflows from HH objects can have very-well-collimated jets, with opening angles of a few degrees or even smaller. The example shown in Figure 7.20, for instance, illustrates a projected opening half-angle of 0ij2 <0.5°. In addition, the jet outflows often have complicated structures of bright spots or knots along the flow. These are typically

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