Interstellar Molecules

The first interstellar molecules were discovered in 1937-1938, when molecular absorption lines were found in the spectra of some stars. Three simple diatomic molecules were detected: methylidyne CH, its positive ion CH+ and cyanogen CN. A few other molecules were later discovered by the same method in the ultraviolet. Thus molecular hydrogen H2 was discovered in the early 1970's, and carbon monoxide, which had been discovered by radio observations, was also detected in the ultraviolet. Molecular hydrogen is the most abundant interstellar molecule, followed by carbon monoxide.

Molecular Hydrogen. The detection and study of molecular hydrogen has been one of the most important achievements of UV astronomy. Molecular hydrogen has a strong absorption band at 105 nm, which was first observed in a rocket experiment in 1970 by George R. Carruthers, but more extensive observations could only be made with the Copernicus satellite. The observations showed that a significant fraction of interstellar hydrogen is molecular, and that this fraction increases strongly for denser clouds with higher extinction. In clouds with visual extinction larger than one magnitude essentially all the hydrogen is molecular.

Hydrogen molecules are formed on the surface of interstellar grains, which thus act as a chemical catalyst. Dust is also needed to shield the molecules from the stellar UV radiation, which would otherwise destroy them. Molecular hydrogen is thus found where dust is abundant. It is of interest to know whether gas and dust are well mixed or whether they form separate clouds and condensations.

UV observations have provided a reliable way of comparing the distribution of interstellar gas and dust. The amount of dust between the observer and a star is obtained from the extinction of the stellar light. Furthermore, the absorption lines of atomic and molecular hydrogen in the ultraviolet spectrum of the same star can be observed. Thus the total amount of hydrogen

(atomic + molecular) between the observer and the star can also be determined.

Observations indicate that the gas and dust are well mixed. The amount of dust giving rise to one magnitude visual extinction corresponds to 1.9 x 1021 hydrogen atoms (one molecule is counted as two atoms). The mass ratio of gas and dust obtained in this way is 100.

Radio Spectroscopy. Absorption lines can only be observed if there is a bright star behind the molecular cloud. Because of the large dust extinction, no observations of molecules in the densest clouds can be made in the optical and ultraviolet spectral regions. Thus only radio observations are possible for these objects, where molecules are especially abundant.

Radio spectroscopy signifies an immense step forward in the study of interstellar molecules. In the early 1960's, it was still not believed that there might be more complicated molecules than diatomic ones in interstellar space. It was thought that the gas was too diffuse for molecules to form and that any that formed would be destroyed by ultraviolet radiation. The first molecular

radio line, the hydroxyl radical OH, was discovered in 1963. Many other molecules have been discovered since then. By 2002, about 130 molecules had been detected, the heaviest one being the 13-atom molecule HCuN.

Molecular lines in the radio region may be observed either in absorption or in emission. Radiation from diatomic molecules like CO (see Fig. 15.20) may correspond to three kinds of transitions. (1) Electron transitions correspond to changes in the electron cloud of the molecule. These are like the transitions in single atoms, and their wavelengths lie in the optical or ultraviolet region. (2) Vibrational transitions correspond to changes in the vibrational energy of the molecule. Their energies are usually in the infrared region. (3) Most important for radio spectroscopy are the rotational transitions, which are changes in the rotational energy of the molecule. Molecules in their ground state do not rotate, i. e. their angular momentum is zero, but they may be excited and start rotating in collisions with other molecules. For example, carbon sulfide CS returns to its ground state in a few hours by emitting a millimetre region photon.

Fig. 15.20. Radio map of the distribution of carbon monoxide 13C16O in the molecular cloud near the Orion nebula. The curves are lines of constant intensity. (Kutner, M. L., Evans 11, N. J., Tucker K. D. (1976): Astrophys. J. 209, 452)
328

15. The Interstellar Medium

Table 15.4.

Some molecules observed

in the interstellar

medium

Molecule

Name

Year of discovery

Discovered in

the optical and ultraviolet region:

CH

methylidyne

1937

CH+

methylidyne ion

1937

CN

cyanogen

1938

H2

hydrogen molecule

1970

CO

carbon monoxide

1971

Discovered in

the radio region:

OH

hydroxyl radical

1963

CO

carbon monoxide

1970

CS

carbon monosulfide

1971

SiO

silicon monoxide

1971

SO

sulfur monoxide

1973

H2O

water

1969

HCN

hydrogen cyanide

1970

nh3

ammonia

1968

H2CO

formaldehyde

1969

HCOOH

formic acid

1975

HCCNC

isocyanoacetylene

1991

c2h4o

vinyl alcohol

2001

H2CCCC

cumulene carbene

1991

(CH3)2O

dimethyl ether

1974

C2H5OH

ethanol

1975

HC11N

cyanopentacetylene

1981

able conditions are thus found inside dust and molecular clouds near dense dark nebulae and HII regions.

Most of the molecules in Table 15.4 have only been detected in dense molecular clouds occurring in connection with HII regions. Almost every molecule yet discovered has been detected in Sagittarius B2 near the galactic centre. Another very rich molecular cloud has been observed near the HII region Orion A. In visible light this region has long been known as the Orion nebula M42 (Fig. 15.18). Inside the actual HII regions there are no molecules, since they would be rapidly dissociated by the high temperature and strong ultraviolet radiation. Three types of molecular sources have been found near HII regions (Fig. 15.21):

15"

A number of interstellar molecules are listed in Table 15.4. Many of them have only been detected in the densest clouds (mainly the Sagittarius B2 cloud at the galactic centre), but others are very common. The most abundant molecule H2 cannot be observed at radio wavelengths, because it has no suitable spectral lines. The next most abundant molecules are carbon monoxide CO, the hydroxyl radical OH and ammonia NH3, although their abundance is only a small fraction of that of hydrogen. However, the masses of interstellar clouds are so large that the number of molecules is still considerable. (The Sagittarius B2 cloud contains enough ethanol, C2H5OH, for 1028 bottles of vodka.)

Both the formation and survival of interstellar molecules requires a higher density than is common in interstellar clouds; thus they are most common in dense clouds. Molecules are formed in collisions of atoms or simpler molecules or catalysed on dust grains. Molecular clouds must also contain a lot of dust to absorb the ultraviolet radiation entering from outside that otherwise would disrupt the molecules. The most suit

45"

15"

15"

45"

15"

0IRS2

1RS s^rrny

. I i-1—

Right ascension 5 h 32 min

Right ascension 5 h 32 min

Fig. 15.21. Infrared map of the central part of the Orion nebula. In the lower part are the four Trapezium stars. Above is an infrared source of about 0.5" diameter, the Kleinmann-Low nebula (KL). BN is an infrared point source, the Becklin-Neugebauer object. Other infrared sources are denoted IRS. The large crosses indicate OH masers and the small crosses H2O masers. On the scale of Fig. 15.18 this region would only be a few millimetres in size. (Goudis, C. (1982): The Orion Complex: A Case Study of Interstellar Matter (Reidel, Dordrecht) p. 176)

15.4 The Formation of Protostars

Table 15.5. The five phases [K] [cnn-3] of interstellar gas

1.

Very cold molecular gas clouds (mostly hydrogen H2)

20

> 103

2.

Cold gas clouds (mostly atomic neutral hydrogen)

100

20

3.

Warm neutral gas enveloping the cooler clouds

6000

0.05-0.3

4.

Hot ionized gas (mainly HII regions around hot stars)

8000

> 0.5

5.

Very hot and diffuse ionized coronal gas, ionized and heated by supernova explosions

106

10-3

1. Large gas and dust envelopes around the HII region.

2. Small dense clouds inside these envelopes.

3. Very compact OH and H2O maser sources.

The large envelopes have been discovered primarily by CO observations. OH and H2CO have also been detected. Like in the dark nebulae the gas in these clouds is probably mainly molecular hydrogen. Because of the large size and density (n ~ 103-104 molecules/cm3) of these clouds, their masses are very large, 105 or even 106 solar masses (Sgr B2). They are among the most massive objects in the Milky Way. The dust in molecular clouds can be observed on the basis of its thermal radiation. Its peak falls at wavelengths of 10-100 ^m, corresponding to a dust temperature of 30-300 K.

Some interstellar clouds contain very small maser sources. In these the emission lines of OH, H2O and SiO may be many million times stronger than elsewhere. The diameter of the radiating regions is only about 5-10 AU. The conditions in these clouds are such that radiation in some spectral lines is amplified by stimulated emission as it propagates through the cloud. Hydroxyl and water masers occur in connection with dense H II regions and infrared sources, and appear to be related to the formation of protostars. In addition maser emission (OH, H2O and SiO) occurs in connection with Mira variables and some red supergiant stars. This maser emission comes from a molecule and dust envelope around the star, which also gives rise to an observable infrared excess.

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