And Then There's This
Now in 2011 it has been five years since my book was published on my 80th birthday. Rather than an end, it was a beginning.
Perhaps I could have anticipated that it would put me in touch with my old astronomical friends, but not that it would lead me to becoming an active astronomer again. I never imagined such a thing. A conference held at the NRAO in Socorro, New Mexico in May 2006 opened a new world to me. It had the formidable title of Small Ionized and Neutral Structures in the Diffuse Interstellar Medium, abbreviated SINS (!). I wanted very much to go to the conference because of my work in the late 1970s on very small clouds in the HI interstellar gas. The trip was impossible, but a birthday card signed by 40 of the 60 participants was a very pleasant surprise.
An even greater surprise came a year later with the publication of the proceedings of the SINS conference [ed. M. Haverkorn and W. M. Goss, ASP Conference Series 365, 2007]. I turned the first few pages of the book when it came and found this:
Dedication: This volume is dedicated to Nan Dieter Conklin for her pioneering work on early observations of the small-scale structure of the interstellar medium, and to Lyman Spitzer for his skeptical and enlightening interest in this topic during the last years of his life.
How fortunate I am! I had the fun of finding the first surprisingly small cloud, and now of seeing how that discovery has changed our view of interstellar gas. It is very rare that a scientist has such opportunities, and perhaps rarer that her colleagues are so generous. How happy Garret would have been for me.
I was certain that the succeeding pages of the book would be quite beyond my comprehension because 30 years would have yielded so much new instrumentation and so many new theories that I would be lost. But I should certainly try. As I did so, I needed to remember how my original observation came about. In 1975 the technique of VLBI, very long baseline interferometry, was in its early stages and, although filled with promise of immensely greater resolution, was also filled with unsolved problems every step of the way. A group from Caltech proposed using their telescope at Owens Valley and ours at Hat Creek as an interferometer with a baseline of 300 miles to observe 21 cm continuum radiation from discrete sources. Although I knew very little of the observing technique, I suggested that we assist them in their observations if they would do the same for us. My idea was to try looking at HI absorption in front of bright sources, ignoring the fact that no spectral-line observations had been made with the new technique. So, undaunted by the additional problems of observation and reduction, and skeptical of theories that said no small-scale structure in the HI could persist, I persuaded Jack Welch at Berkeley and Jon Romney at Caltech to show me how to make reliable observations in this new format.
In the end the observations were successful, but only now do I realize how difficult it is to explain the presence of such small condensations in the gas (on a scale of tens of AU). Lyman Spitzer was at first very skeptical about the observations, even those of much more recent times and greater dependability, and subsequently could not find a satisfactory theory to explain them. If I had been aware of all that, I might not have tried. Ignorance, sometimes, is bliss.
As I looked through the SINS book I was indeed astonished at the progress over these 30 years. However I rediscovered the fact that in order to read and comprehend published work I need some sort of personal stake in the subject, some hook to catch my full and critical attention. I found it in the Heiles and Stinebring review of observational studies that includes references to very small molecular clouds. As I was preparing the H2CO observations for my book, I heard that such clouds had been found, but I was unable to pursue the subject without concrete references. Now I had them, and as usual one thing led to another.
I found a series of papers by Alan Marscher and colleagues, (e.g. Observational Probes of the Small-Scale Structure of Molecular Clouds) that did indeed support the presence of very small scale structure in the formaldehyde clouds, but I found very little reaction to the results. Since their method depends on variation in the absorption profile with time, and the last observations had been in 1992, the observations should certainly be repeated. That would require time on the great instrument at Socorro, the VLA. I had never used it, but I did know that the scheduling and the reduction of the data were enormously complicated. Obviously I could not do this project alone, but Miller Goss and Estevan Araya (then PhD student) agreed that it was worthwhile. Clearly this project would take quite a long time to bring about, and I could not contribute much at this stage, so along with reading everything I could find. I embarked on another unrelated adventure. (The observations were finally made in March 2009, and I have yet to see the results.)
As I continued to read about small-scale structure in both HI and H2CO spectra, I realized that as the observed scales reached milliarcseconds the method used by Marscher and colleagues could no longer be ignored.
"... They found significant temporal variation, and that the variation could be explained by a factor never before considered in interpreting very high-resolution observations. Because we are moving with respect to the distant background source, our line of sight intersects the nearby cloud at slowly varying locations. As the resolution has reached the level of milliarcseconds their 'searchlight effect' (my term) should be considered in the interpretation of all absorption line profiles of interstellar gas, atomic as well as molecular. In fact, it can be decisive in determining whether secular variations in optical depth are caused by physical changes in the interstellar cloud or by our changing line of sight through it. Physical changes would support the idea that these structures are very short-lived."
The quotation is taken from a paper, Sub-parsec Structure of Interstellar H2CO Clouds, I published in the Astronomical Journal in 2010, 31 years since my last publication! What a joy it was. I learned several things in the process of publication. First, publication is easier and much, much faster with the speed of digital communication, and then there really are page charges that seemed irrelevant when there was a grant to pay them. (For this paper the journal waived the charges.) My principal purpose in writing the article was to show how useful this imaginative approach can be for observing HI clouds as well. Included is a table of eight extragalactic sources that have been observed with milliarcsecond resolution to examine small-scale structure in nearby absorbing hydrogen clouds. For each of them the predicted motion of our line of sight at the clouds should be detectable in the observations made in the next few years. A large advantage of this procedure is that one need not examine in exquisite detail a map of the cloud in order to detect very small features. If one finds a change from one time to another in the profile averaged over the whole source, that change is evidence of milliarcsecond size cells. For all these sources the first observation in the series has already been made.
I began examining the accumulated literature on interstellar H2CO. The enormous amount of original data published was a treasure to be explored from a point of view different from that of the original observers. For example, some of the observations of H2CO absorption in front of Galactic HII regions were made to resolve ambiguities in determination of the distance to the HII regions. Others were designed to search for H2CO emission thought to arise in areas of active star formation -- large HII regions. One aspect of the data that was not discussed was the width of each individual H2CO line, the velocity dispersion of the gas. It was a subject that had interested me for a long time beginning with my survey of H2CO in nearby dust clouds absorbing the cosmic microwave background. More recent observations have extended that survey to more dust clouds, observed with far higher angular resolution. When I compared them all I found confirmation of the fact that they were all exceedingly narrow, somewhat less than 1 km/s. In addition, the central velocities were also very similar, suggesting that a very large difference in angular resolution had virtually no effect, a conclusion without a reasonable explanation.
Examination of the H2CO absorption of radiation from HII regions presented even more puzzling values of the velocity dispersion. Throughout the large accumulation of data, hundreds of features, the line widths were consistently larger than those observed absorbing the CMB, averaging 2 km/s instead of 1 km/s. My effort to explain the difference was fruitless until I considered the distance to each H2CO cloud. The greater line width was apparent in all the clouds, both those near the illuminating HII region and those far from it. One might imagine that a H2CO cloud near an area of star formation might have higher velocity dispersion, but such dispersion in an area far away was puzzling. The answer lay in the linear size of the area sampled in each cloud.
Except in the case of extragalactic sources as background, where the resolution is effectively infinite, the resolution at the cloud depends on the telescope used to observe it and on the distance to the cloud. For the dust clouds, at about 100 parsecs, all beamwidths up to 10' yield resolution at the cloud less than about 0.4 parsec. That suggests the presence of cells about this size with velocity dispersion about 1 km/s. To sample such sub-parsec areas at a distance of 500 parsecs, one needs a beamwidth of 2.5'; for 1 kiloparsec, a beamwidth of 1'; and for 5 kiloparsecs, a beamwidth of a few seconds. Since almost all of the H2CO clouds absorbing HII region radiation lie at distances of 1 kpc or more, the average area sampled is larger than 0.4 pc, and the average velocity dispersion is 2 km/s. Altogether, the data suggest that a large molecular cloud commonly consists of cells of about half a parsec with internal velocity dispersion of about 1 km/s This conclusion should be considered in the search for the origin and life of interstellar clouds.