Info

Regions of recent star formation the star is mostly by convection. The part of the evolutionary track at which the luminosity is decreasing quickly while the temperature increases slightly is called the Hayashi track. After this collapse slows, the star begins to approach the main sequence. Eventually, it reaches the luminosity of a main sequence star, though it may vary somewhat before settling down.

When we study star formation, we find that there are some very obvious signposts of recent or ongoing star formation. Regions of recent star formation are important for a number of reasons. First, they call our attention to places where star formation might still be taking place. Second, the newly formed stars have some effect on their immediate vicinity, which might promote or inhibit further star formation. In this section we will look at some of the most prominent: (a) HII regions, (b) masers, (c) energetic flows, and (d) protostellar cores. In each case the object becomes prominent either because of the unique conditions that accompany star formation or because of the effect of newly formed stars on the cloud out of which they were born.

15.6.1 HII regions

When a massive star forms it gives off visible and ultraviolet photons. Photons with wavelengths shorter than 91.2 nm, in the ultraviolet, have enough energy (> 13.6 eV) to ionize H. The stars that give off sufficient ultraviolet radiation to cause significant ionization are the O and early B stars. When most of the hydrogen is ionized, we call the resulting part of the cloud an HII region, as shown in Fig. 15.8.

In equilibrium in an HII region there is a balance between ionizations and recombinations.

10z*

Density (molecules/cm3)

Model for the collapse of an interstellar cloud into a protostar and a pre-main sequence star.

10z*

Density (molecules/cm3)

HII regions. (a) The Lagoon Nebula (M8), in Sagittarius, at a distance of 2 kpc. It is 20 pc across. Notice the cluster of bright blue-white stars, which produce ionizing radiation.The ionized gas glows red.The name comes from the dust lane that cuts across the front, blocking our view of the gas behind. (b) HST image view of M8. (c) The Eagle Nebula (M16), in Serpens. (d) HST image of the dust lanes in M16.The bright edges are regions of recent ionization. (e) HST image of the Omega Nebula (M17), in Sgr, at a distance of 2 kpc. Here the ionizing stars are not as obvious, and are embedded deep within the nebula.

(Continued) (j) The central region of the Eta Carina Nebula. (k) HST image of the immediate vicinity of Eta Carina.

[(a), (c), (f), (h)-(j) NOAO/AURA/NSF; (b), (d), (g), (k) STScI/NASA; (e) ESO]

(Continued) (j) The central region of the Eta Carina Nebula. (k) HST image of the immediate vicinity of Eta Carina.

[(a), (c), (f), (h)-(j) NOAO/AURA/NSF; (b), (d), (g), (k) STScI/NASA; (e) ESO]

Ri = Nu this reason, HII regions are often referred to as Stromgren spheres, and the radius of an HII region is called the Stromgren radius, rS.

We can see how the balance between ionizations and recombinations determines the Stromgren radius. If Nuv is the number of ultraviolet photons per second given off by the star capable of ionizing hydrogen, then this is the number of hydrogen atoms per second that can be ionized. That is, the rate of ionizations Ri is given by

The higher the density of protons and electrons, the greater the rate of recombinations. The recombination rate is given by

Free electrons and protons collide, forming neutral hydrogen atoms. However, the ultraviolet photons from the star are continuously breaking up those atoms to form proton-electron pairs. The balance between these two processes determines how large a particular HII region can be. Within the HII region, almost all of the hydrogen is ionized. There is a rapid transition at the edge, from almost entirely ionized gas to almost entirely neutral gas. The theoretical reasons for this sharp transition were first demonstrated by the Swedish astrophysicist, Bengt Stromgren. For where V is the volume of the HII region and a is a coefficient (which depends on temperature in a known way). For the volume, we can substitute the volume of a sphere with radius rS. If the only ionization is of hydrogen, the number density of electrons must equal that of protons, since both come from ionizations of hydrogen. Equation (15.24) then becomes

Equating the ionization and recombination rates gives

Solving for rS gives rs = (3/4™)1/3 (NUv)1/3 V2/3 (15.27)

From equation (15.27) we can see that the size of an HII region depends on the rate at which the star gives off ionizing photons and the density of the gas. If the gas density is high, the ionizing photons do not get very far before reaching their quota of atoms that can be ionized. The rate at which hydrogen ionizing photons are given off changes very rapidly with spectral type, as indicated in Table 15.2, so the HII region around an O7 star is very different from that around a B0 star. Often, O and early B stars are found in very small groupings. In these groupings, the HII regions from various stars overlap, and the region appear as one large HII region.

The ultraviolet radiation from stars can also ionize other elements. For example, after hydrogen, the next most abundant element is helium. However, the ionization energy of helium is so large that only the hottest stars produce significant numbers of photons capable of ionizing helium. On the other hand, the ionization energy of carbon (for removing one electron) is less than that of hydrogen. There are many photons that are capable of ionizing carbon that will not ionize hydrogen. This, combined with the lower abundance of C relative to H, means that CII regions are generally much larger than HII regions (see Problem 15.20).

There are actually two conditions under which the boundary for an HII region can exist. One is that which we have already discussed. The cloud continues beyond the range of the hydrogen-ionizing photons. When this happens, we say that the HII region is ionization bounded. The other possibility is that the cloud itself comes to an end while there is still hydrogen-ionizing radiation. In this case, we say that the HII region is density bounded, since its boundary is determined by the place where the density is so low that we no longer think of the cloud as existing. When an HII region is density bounded, hydrogen-ionizing radiation can slip out into the general interstellar radiation field. This is an important source of ionizing radiation in the general interstellar medium (i.e. not near HII regions).

The temperature of HII regions is quite high -about 104 K. HII regions are heated by the ionization

Table 15.2. I Rates of H-ionizing photons for main sequence stars.

Spectral type_Photons/s (X 1048)

05 51

06 17.4

of hydrogen. When an ultraviolet photon causes an ionization, some of the photon's energy shows up as the kinetic energy of the free proton and electron. Cooling in an HII region is inefficient, since there are no hydrogen atoms and no molecules. Cooling can only take place through trace constituents, such as oxygen. Transitions within these constituents are excited by collisions with protons and electrons. The collisions transfer kinetic energy from the gas to the internal energy of the oxygen. The oxygen then radiates that energy away. Since the heating is efficient and the cooling is inefficient, the temperature is high.

HII regions can give off continuous radiation, which can be detected in the radio part of the spectrum. This radiation results from collisions between electrons and protons in which the two do not recombine. Instead, the electron scatters off the proton. In the process the electron changes its velocity. When a charged particle changes it velocity, it can emit or absorb a photon. This radiation is called Bremsstrahlung (from the German for "stopping radiation"). It is also called free-free radiation, because the electron is free (not bound to the proton) both before and after the collision. The spectrum of free-free radiation (Fig. 15.9) is characterized by the temperature of a gas. The spectrum is not that of a blackbody because the gas is not optically thick. The spectrum is a blackbody curve multiplied by a frequency dependent opacity. Because the radiation can be described by the gas temperature, it is also known as thermal radiation. This radiation is strongest in the radio part of the spectrum. Therefore, we can use radio continuum observations to see HII regions anywhere in our i i

0 0

Post a comment