Info

7.2.3.3 Molecular lines

Another important difference between the IR and optical wavebands is that in the IR we can see important molecular features in emission. Some features, particularly hydrides and H2O "steam bands," are important in absorption in the atmospheres of late-type stars and brown dwarfs in the sub-micrometer region. However, these are seldom seen in emission. Therefore, the most-prominent emission features are typically from molecular hydrogen (H2) and carbon monoxide (CO).

7.3. Galactic nebular sources of emission lines

As noted in the introduction to this chapter, the "fundamental unit" for emission-line studies is typically the HII region. Thus we also begin the discussion of Galactic sources with HII regions (which in any case are fundamentally Galactic in nature). However, due to the emphasis of other chapters on these, I will simply emphasize how Galactic studies of distant HII regions can differ from "standard" H Il-region studies. After that, I will move on to planetary nebulae - which can be simply envisioned as HII regions where the affected interstellar medium (ISM) has the compositional and kinematic imprint of the progenitor star. Note of course that this imprint has non-trivial implications for the emission lines arising from planetary nebulae as opposed to H II regions. Similar analogies apply to supernova remnants - though here the "kinematic imprint" of the progenitor star includes a shock wave moving as fast as at ~0.1c through the circumstellar medium and the ISM, which is of course highly non-trivial for the emission-line properties, and typically dominates line excitation from the hot (or, for old remnants, already-cooled) interior.

In the subsections below, I address each of these source classes in turn.

7.3.1 HII regions

Once again, I assume that the detailed properties of H II regions (both Galactic and extragalactic) are covered by other chapters in this volume. However, I also assume that few/none of these will emphasize the infrared properties of HII regions. If they are available, observations of optical (or better yet, UV) emission lines such as the Balmer (Lyman) series provide superior physical probes of HII regions. However, since much of our Galaxy's star-forming mass lies in regions with AV > 10 mag, IR observations are often the only practical choice available.

As their name implies, hydrogen lines often provide the most diagnostic insights into HII regions in the optical, and that truism extends to the IR region as well. Furthermore, even for relatively nearby HII regions, differential extinction (both along the line of sight

Figure 7.4. Multi-wavelength views of the Cepheus A star-forming HII region: POSS B band (top left); POSS "IR" band (top right); and 2MASS J band (bottom).

and within the immediate vicinity of the HII region itself) may make IR observations even more powerful than optical ones in some aspects (see Figure 7.4).

However, the fact that the "a" transitions for the Paschen and Brackett series are not available for high-sensitivity observations from the ground presents a non-trivial drawback. For instance, one of the most-powerful basic diagnostics for H II regions is measurement of the "Balmer decrement" (from Ha to Hß). One could try to measure a "Paschen decrement," but Paa lies in the middle of a telluric "band of avoidance" between the atmospheric H and K windows, generally requiring space-based observations for this measurement (though some novel approaches being developed may alleviate, if not necessarily eliminate, this problem). Similarly, a "Brackett decrement" using Bra and Bry is feasible from the ground - but Bra is well into the thermal-background-dominated regime, and lies at the ragged edge of an atmospheric window (thus requiring space-based observations, and a cooled telescope even then, for truly high sensitivity).

One can use an alternative approach combining Bry and Paß - both located well within "good" atmospheric windows - to create an "IR hydrogen decrement" analogous to the Balmer decrement in the optical. However, this line pair has no common energy levels (Bry is a 7-4 transition, and Paß is a 5-3 transition - see Table 1.1), requiring additional assumptions about the observed system and thus introducing significant uncertainties into the physical interpretation of the measurement.

7.3.2 Planetary nebulae Planetary nebulae (PNe) are a second class of "Galactic" nebular emission lines. They are interesting as the (near-)final evolutionary phase of the majority of stars in the Universe. They are also responsible for the return of significant amounts of chemically enriched material to the ISM. In addition, they are truly beautiful objects to behold, as demonstrated wonderfully by the Hubble Space Telescope.

7.3.2.1 PNe emission-line basics

The emission lines from PNe share with lines from H ii regions a similar basic idea, in that they arise from a hot stellar central source of ionizing radiation illuminating the "ISM." In this case, however, the "ISM" is dominated by the ejected circumstellar envelope, which has a different composition, density structure, and kinematic structure than the "standard" ISM. Nevertheless, detailed modeling of the PNe can provide similar diagnostics to those of H ii regions with respect to electron temperature, electron density, and ionic abundances. In addition, IR line features such as H2 and Fe provide insight into shocks and diagnostics to distinguish them from radiative excitation. The emission lines also show the kinematics of outflows and morphological features in the planetary nebula.

For electron-density diagnostics, key line pairs include [O ii] and [S ii] and, at slightly higher densities, [Cl iii] and [Ar iv] - similar to H ii regions. Analogous diagnostics for electron temperature also parallel H Il-region diagnostics. In addition, combining these with models for the PNe can provide insights into ionic abundances. As we will see below, though, the fact that the structure of PNe is more complex than that of many HII regions can complicate these models significantly. Major additional sources of uncertainty for PNe include their distance and internal differential extinction due to dust formation.

Figure 7.5 shows typical near-IR spectra of PNe. As one might expect, in the K-band window, dominant features tend to be Bry (2.165 |xm) and He I (2.058 |xm). However, in some of these spectra, molecular H2 emission features approach the strength of, and even dominate over, Bry and He I. Other notable features include He II as well as higher-ionization forbidden lines of Fe and Kr.

The H2 lines, as noted above, can provide diagnostics of shock excitation versus radiative excitation of emission lines. This is due to the fact that H2 can be excited by both fluorescence and thermal (collisional/shock) mechanisms. Thus, at low densities, we find that UV excitation of cool (T ~ 100 K) molecular gas causes the ratio of the 2.12-^m and 2.25-^m features to be fairly steady at 1.7. These are 1-0 S(1) and 2-1 S(1) transitions, respectively. At higher densities (> 104 cm~3), this ratio increases and becomes a useful diagnostic of temperature (up to T ~ 1000 K).

Another useful diagnostic IR feature in PNe is [Fe II]. This is because Fe in the ISM is usually depleted onto dust grains, producing very low abundances in the gas phase of this species. However, shocks (particularly slow shocks such as those commonly found in PNe) break up these dust grains as they pass through the ISM. Thus, shocks will greatly increase the local abundance of gas-phase Fe (at least temporarily), and their occurrence is revealed by the presence of [Fe II] lines. In the case of PNe, the shocks are so slow (in comparison with, for instance, supernova remnants), that only the fastest-moving PNe shocks seem to lead to Fe emission lines.

7.3.2.2 Morphology and outflow kinematics

Contrary to simple expectations, spherical stars tend NOT to produce spherical planetary nebulae when they die. Rather, most PNe are highly spherically asymmetric.

Figure 7.5. The near-IR spectrum of a planetary nebula. Reproduced from

However, PNe also tend not to be irregular - rather they are very symmetric in an aspheric way. In addition, in some cases there is eye-catching evidence for collimated outflows (see Figure 7.6).

The collimation in these PNe ranges from relatively mild, via "medium," to a high degree of collimation (see Figure 7.7). Furthermore, point symmetry seems to be pervasive in these objects (see Figure 7.7). This point symmetry is usually associated with bipolarity, a progressive variation in the direction of the outflows, and episodic events of (collimated) mass loss. Thus, point symmetry in PNe indicates the presence of a bipolar rotating episodic jet or collimated outflow (BRET). In a true BRET, the morphology is also reflected in the kinematics, and we see that this is the case in many PNe (Figure 7.8). Of course, emission lines are critical for making these diagnoses.

Figure 7.6. Morphologically evident outflows in PNe (figures reproduced from the Hubble Space Telescope website).

Figure 7.7. Examples of different degrees of collimation in PNe (figure courtesy of J. A. Lopez).

Figure 7.7. Examples of different degrees of collimation in PNe (figure courtesy of J. A. Lopez).

7.3.3 Supernova remnants

Supernovae are among the most-interesting phenomena in the astrophysics of recent decades, and their remnants are also critical for our understanding of the cosmos. Supernovae are the final events in the lives of the massive stars in the Universe. Their remnants are critical for chemical enrichment of the Universe as well as the return of kinetic energy

%

Was this article helpful?

0 0

Post a comment