ANNEX IV THE FORMALDEHYDE LINES 1. Introduction Radio lines from clouds of formaldehyde (H2CO) in our galaxy have been detected on at least eight different frequencies. Lines from the carbon-13 isotope at 4593 MHz and the oxygen-18 isotope at 4389 MHz have also been detected. The most important line is at 4830 MHz which generally appears in absorption against bright radio sources, ionized hydrogen regions, and even the microwave background in nearby dust clouds. The next lines of importance after this line, are those at 14.488 GHz and at 140.840 GHz, 145.603 GHz, and 150.498 GHz. The last three lines appear in the high density cores of rapidly condensing galactic clouds. 2. Formaldehyde absorption at 4830 MHz The discovery in March, 1969, of the 4830 MHz absorption line of formaldehyde [Snyder et al., 1969] had followed the detection of ammonia and water in late 1968, and so, clearly demonstrated the existence of organic polyatomic molecules in interstellar space. None of the molecular lines subsequently discovered in 1970 and 1971 (with the possible exception of the CO line at 115.267 GHz) are as important to the study of dust clouds and galactic structure as the 4830 MHz line of formaldehyde. A large number of radio sources have formaldehyde absorption lines in them [Zuckerman et al., 1970; Whiteoak and Gardner, 1970] showing that formaldehyde, like hydrogen and hydroxyl, is a common constituent of galactic spiral arms. Hence, it can be used to study the structure of our Milky Way galaxy. High-resolution studies have been made of the formaldehyde lines in the galactic centre using lunar occultation techniques [Kerr and Sandqvist, 1970] and interferometry [Fomalont and Weliachew, 1973]. Absorption at 4830 MHz has been detected in external galaxies [Gardner and Whiteoak, 1974]. One of the most interesting aspects of formaldehyde absorption are the anomalous lines observed in a number of nearby dust clouds [Palmer et al., 1969], This aspect is quite anomalous, since only emission lines from other molecules are observed in the same cloud. The absorption must be produced against the 3 K background radiation, the microwave remnant of the primeval fireball. Accordingly many nearby dust clouds can be examined by means of the formaldehyde molecule without the need for a background radio source. This refrigerator mechanism is the inverse of a maser mechanism and it is only observed in dust clouds and possibly in the very dense cores of galactic sources [Townes and Cheung, 1969]. Hence the 4830 MHz transition is useful for studying the physical conditions inside dust clouds. Formaldehyde is separated into ortho and para levels and transitions between the ortho and para states are forbidden by electric dipole transition. This can be thought of as two species of the molecule that differ in energy: ortho formaldehyde and para formaldehyde. The lowest energy state for ortho formaldehyde will normally have a high population. Not only is this the optimum configuration for producing an absorption line, but also 4830 MHz is a good frequency for large telescopes. At lower frequencies, the spatial resolution deteriorates and at higher radio frequencies, the continuum background necessary for absorption becomes weaker. A similar situation occurs in the OH molecule. The formaldehyde line is actually split into six components covering about 30 kHz [Tucker et al., 1971], In most cases the Doppler broadening of the line is much greater than this. 3. Other formaldehyde absorption lines Absorption lines from two other pairs of levels of ortho formaldehyde have been found. The lines appear at 14.489 GHz, 28.975 GHz and 48.285 GHz, and have a large optical depth in the galactic centre [Evans et al., 1970; Welch, 1970]. They are rarely seen outside this region, which indicates that the higher energy levels are not as well populated as those levels that give rise to the 4830 MHz transition. In addition, the continuum sources are considerably weaker at the above 14, 29 and 48 GHz lines. Increases in receiver sensitivity may make these lines useful in the future for studying the excitation of formaldehyde. 4. The 140 to 150 GHz emission lines Three high frequency emission lines of formaldehyde have been found in the Orion Nebula. Lines of ortho formaldehyde have been detected in Orion A, the galactic centre source Sgr A (NH3A) and at least two other galactic sources [Kutner, et al., 1971; Thaddeus, et al., 1971]. The emission lines are the most intense in Orion, whereas the 4830 MHz absorption line is very weak [Kutner and Thaddeus, 1971]. This happens because the molecular cloud is located in the dusty region behind the bright nebulae and radio source. The angular diameter of the emission region is about 3' arc, and is generally centred on the OH maser [Thaddeus et al., 1971], a region where emission lines from other molecules have been found. Another line at a frequency of 72.838 GHz has been detected, and is important because it is the lowest ground state transition of the formaldehyde molecule. We know from the 4830 MHz absorption measurements that the formaldehyde cloud extends southward for at least 30' arc [Kutner and Thaddeus, 1971], The conclusion is that the densities required to produce the 140 to 150 GHz emission lines are quite high, of the order of I05 hydrogen molecules/cnr [Thaddeus et al., 1971], Such densities are only achieved in the central part of the nebula. It is generally assumed that these dense regions are highly collapsed clouds in the process of forming stars. Hence we can see the importance of both the 140 to 150 GHz emission, and 4830 MHz absorption lines, in studying this region which is at an important stage in the evolution of interstellar clouds.
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