Contains reports on seven research projects.M. I. T. Sloan Fund for Basic ResearchNational Science Foundation (Grant GP-8415)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NGL 22-009-016

Barrett, Alan H.

Burke, Bernard F.

Ewing, M. S.

Hinteregger, H. F.

Knight, C. A.

Papadopoulos, George D.

Price, R. M.

Reifenstein, E. C., III

Rosenkranz, P. W.

Schwartz, P. R.

Shapiro, I. I.

Staelin, David H.

Waters, J. W.

Whitney, A. R.

Wilson, W. J.

Zapata, C. A.

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III. RADIO ASTRONOMYAcademic and Research StaffProf. A. H. Barrett Prof. R. M. Price D. C. PapaProf. B. F. Burke J. W. Barrett C. A. ZapataGraduate StudentsM. S. Ewing P. C. Myers J. W. WatersW. W. Gebhart G. D. Papadopoulos A. R. WhitneyL. P. A. Henckels P. W. Rosenkranz T. T. Wilheit, Jr.H. F. Hinteregger P. R. Schwartz T. L. WilsonC. A. Knight W. J. WilsonA. VERY LONG BASELINE INTERFEROMETRYVery long baseline interferometry (VLBI) techniques (using independent atomic clocksas local oscillators and separate recording of the video signals at the participating radiotelescopes) have been extended to extremely accurate measurement of time delay. Fromsuch delay measurements both source positions and baseline coordinates, including rel-ative time, can be determined several orders of magnitude more accurately than withany other existing technique. The scientific applications of such measurements arenumerous, including such diverse effects as the gravitational bending test of relativity,continental drift, and possibly proper motions of quasars. The basic modification inVLBI methods, which have been used primarily for size and structure measurementsfor which the inherently small recordable fractional bandwidth and hence very largedelay ambiguity was of no serious consequence, was to provide for coherent multifre-quency switching of the receivers over a wide band. If the frequency spacings areproperly chosen and adequate phase calibration is performed, the phase may be unambig-uously extrapolated from one sampled frequency to the next throughout a range ofreceiving frequencies so that for the purpose of the delay measurement a large effectivebandwidth is obtained.The first significant attempt to use a long baseline delay interferometer (LBDI) wasmade early in October 1968 at the time of the close approach of the sun to the strongquasar source 3C279. The Haystack 120-ft - NRAO 140-ft interferometer was success-fully operated at both X- and L-bands. Fringes have been observed for both bands inthe last three days of the experiment at least. The X-band VLB represents the shortestwavelength, thus far, at which successful measurements have been made. The X-bandThis work was supported principally by the National Aeronautics and Space Admin-istration (Grant NGL 22-009-016) and the National Science Foundation (Grant GP-8415);and in part by the Joint Services Electronics Programs (U. S. Army, U. S. Navy, andU. S. Air Force) under Contract DA 28-043-AMC-02536(E), and the M. I. T. Sloan Fundfor Basic Research.QPR No. 93(III. RADIO ASTRONOMY)receivers had been modified and bench-tested to allow frequency switching over 900 MHz,but a flaw in one paramplifier limited the range of receivable frequencies. An adequatephase calibrator for the large frequency steps had also not yet been developed. BothX- and L-band receivers were switched over approximately 10 MHz in the second localoscillator and should yield delay resolutions of the order of 10-100 nsec. Evaluationof the effects of bending in the sun's gravitational field and of refraction by the solarcorona must await reductions that are now in progress. Instrumental problems occurredduring the first half of the October experiment, but a substantial quantity of good datawere obtained during the second half of the program.We decided to try a simpler experiment in January 1969, operating only at L-band.Preparation for this experiment included additions to the NRAO and Haystack receiversto allow them to be synchronously returned over approximately 150 MHz, the develop-ment of a phase calibrator for the larger frequency steps, additions to and modificationsof the video conversion scheme, and a mock switched interferometer system check inwhich coherent noise was fed into both the Haystack and NRAO front ends at Haystack.Eight of the strongest quasar sources at L-band were chosen to be looked at repeatedly(approximately every hour) while they were at relatively high elevations. Roughly50 pairs of tapes switched over 150 MHz were taken on each of two days. A preliminaryanalysis of some of these data shows that a range of frequencies from 1600 MHz to1710 MHz could be covered consistently. Fringes have been obtained on the weaker, aswell as stronger, sources. We have every reason to believe that delay resolution of1- 10 nsec was achieved on most of these data. Reduction program improvements andmodifications are being implemented, at present, to allow more consistent and efficientreduction of the data, rapid fringe notation, and combination of the multifrequencyresults into a best estimate of the delay for a given run. A postprocessing program totake the various delay measurements and convert them to source and baseline coordi-nates, synchronization, and instrumental delay errors is being developed. Another tryat the gravitational bending experiment (October 1968) is being planned, probably with atranscontinental baseline.B. F. Burke, I. I. Shapiro, H. F. Hinteregger,A. R. Whitney, C. A. Knight[Professor Irwin I. Shapiro is in the Department of Geology and Geophysics, M. I. T.]B. PULSAR STUDIESA search for new pulsars and a program of study of known pulsed sources wasstarted by Professor D. H. Staelin (now on leave) and Dr. E. C. Reifenstein III of theNational Radio Astronomy Observatory (NRAO) during 1968. This search led to the dis-covery of two pulsars in the vicinity of the Crab Nebula.1, 2 Subsequent studies by othergroups have shown two new properties of one of these pulsating sources: the pulsarQPR No. 93(III. RADIO ASTRONOMY)located in the direction of the optical center of the Crab Nebula is slowing down, that is,its period is increasing approximately 40 nsec per day; this same pulsar has been foundto have an optical counterpart, also pulsating with the same period.An enlarged M. I. T.-NRAO effort is now under way to study the properties of thosepulsars that are observable with the 300-ft transit telescope at the National RadioAstronomy Observatory. These studies are being carried out by using a 50-channelreceiver to obtain frequency resolution, and sampling at rates up to 200 times persecond (each 5 msec). Observing frequencies have included a 5-MHz band centered at112, 234, and 405 MHz. All data are recorded on magnetic tape and are being sorted toenable us to study individual pulses and the time variation of the amplitude of the pulsesas a function of time.D. H. Staelin, R. M. Price, R. F. Burke,M. S. Ewing, E. C. Reifenstein III[Dr. E. C. Reifenstein III is at the National Radio Astronomy Observatory.]References1. D. H. Staelin and E. C. Reifenstein III, Science 162, 1481 (1968).2. E. C. Reifenstein III, W. D. Brundage, and D. H. Staelin, Phys. Rev. Letters 22,311 (1969).C. PULSAR FACILITYThe components of the feeds for the sixteen 50-ft dishes of the pulsar facility arecompleted. Assembly of 4 antennas is completed and the remaining feeds will beassembled and mounted as soon as weather permits.Installation of the open wire transmission lines is essentially completed. Asmall shed has been built at the center of the array for the transmission lines to feedinto. Electronics apparatus of the receiver front end will be housed at that point.One feed has been erected and tests have been carried out, using the 10-channelreceiver, which has also been completed. Further tests will be carried out, as soonas weather permits, to determine the operating frequency necessary to minimize RFI.B. F. Burke, R. M. Price, M. S. EwingD. KU-BAND INTERFEROMETERThe interconnections among the constituent blocks of the interferometer have beencompleted.During the months of January and part of February, 1969, several system checkswere carried out. To calibrate the index positions of the two dishes, we scanned theQPR No. 93(III. RADIO ASTRONOMY)sun in declination and hour angle. Preliminary calculations indicate an antenna effi-ciency of approximately 40%. This efficiency will probably improve when we furtheradjust the focusing of the radiometer.G. Papadopoulos, B. F. BurkeE.- OBSERVATIONS OF OH EMISSION ASSOCIATED WITHINFRARED STARSDuring the past year we have discovered and studied a new class of OH emissionsources associated with infrared (IR) stars. These OH sources are characterized byhaving their strongest emission at the OH satellite line frequency of 1612 MHz, withmuch weaker emission (if present at all) at the main line frequencies (1665, 1667 MHz)and no emission at the other satellite line frequency 1720 MHz. The strong 1612 MHzemission is largely unpolarized, while the main line emission has a large percentageof polarization (circular and linear).In July 1968, we made observations of twenty IR stars, using the 140-ft radio tele-scope of the National Radio Astronomy Observatory (NRAO), searching for OH emission.We found that four of the IR stars had OH emission associated with them (see Wilsonand Barrettl). Among the four OH sources detected, the IR star NML Cygni is thestrongest OH source with a peak flux of 600 flux units (So). A spectrum of NML Cygni OHemission is shown in Fig. III-1.NML Cygni was discovered in 1965 by Neugebauer, Martz, and Leighton,2 during200 - NML Cyg1612 MHzJULY I, 1968160 RESOLUTION 2KHz=0.36 km/secwS120-wa-LUw 80zzw 4040 20 0 -20 -40RADIAL VELOCITY (km/sec)Fig. III-1. The 1612-MHz spectrum of the infrared (IR) star NML Cygni. Antennapolarization was left circular, and integration time was 12 min.QPR No. 93(III. RADIO ASTRONOMY)an infrared survey of the sky. It is one of the strongest IR stars known, andhas an effective black-body temperature of approximately 700 0 K. The angularsize of the IR emission from NML Cygni was computed by Johnson, Low, andSteinmetz 3 to be 0. 2 second of arc. There have been some suggestions that itcould be a protostar.To gain further information on the size of NML Cygni, we made a seriesof very long baseline interferometer measurements in October 1968. These mea-surements were made in cooperation with Dr. James M. Moran of Lincoln Lab-oratory, M. I. T. The two antennas used in the interferometer measurements werethe NRAO 140-ft antenna, in Greenbank, West Virginia, and the 120-ft Haystackantenna, at Lincoln Laboratory, M. I. T. The baseline of this interferometer isapproximately 5 X 106 wavelengths. The OH features of NML Cygni were detected,and their angular size was measured to be ~0. 1 second of arc. (At a distanceof 100 parsecs, this corresponds to a diameter of 10 astronomical units.) Wefound that the NML Cygni source is very complex, with many emission featurescoming from different locations within a region of radius of 1 second of arc.This could correspond to a region of space containing dense clouds of OH mol-ecules and moving with respect to one another.In addition to the July 1968 infrared star observations, we made a further survey of46 infrared sources, in October 1968. In these observations, using the 140-ft antennaof NRAO at a frequency of 1612 MHz, we found 4 more OH emission sources. InDecember 1968, we again used the NRAO 140-ft antenna to make detailed measurementsof the properties of the IR stars previously detected. We are now in the final stages ofthe reduction of these data. It is possible, however, to summarize some of the prelim-inary results.1. Sixty-six IR stars have been observed at 1612-MHz frequency. Eight of theseIR stars have detectable OH emission.2. The strongest OH emission is at 1612 MHz, and is mainly unpolarized - within10%. Line emission at 1665 or 1667 MHz is much weaker; there is no detectable1720-MHz emission. The fact of 1612-MHz emission and no 1720-MHz emission canbe explained by recent theories of near IR pumping of the OH maser, as proposed byLitvak. 43. The OH sources associated with IR stars have two separate emission velocities,thereby suggesting a contracting or expanding model for the OH cloud.Further research plans in this field include measurement of the properties of thesources thus detected, looking for time variations in these properties, and extendingthis OH survey to many additional IR stars to obtain an idea of the statistics involvedin this kind of emission.W. J. Wilson, A. H. BarrettQPR No. 93(III. RADIO ASTRONOMY)References1. W. J. Wilson and A. H. Barrett, "Discovery of Hydroxyl Radio Emission fromInfrared Stars," Science 161, 778 (23 August 1968).2. G. Neugebauer, D. E. Martz, and R. B. Leighton, "Observations of ExtremelyCool Stars," Astrophys. J. 142, 399 (1965).3. H. L. Johnson, F. J. Low, and D. Steinmetz, "Infrared Observations of theNeugebauer-Martz-Leighton 'Infrared Star' in Cygnus," Astrophys. J. 142, 808(1965).4. M. M. Litvak, "Infrared Pumping of Interstellar OH," Astrophys. J. (in press).QPR No. 93(III. RADIO ASTRONOMY)F. OBSERVATIONS OF RADIO EMISSION FROMGALACTIC H20The Haystack Research Facility of Lincoln Laboratory, M. I. T., has been used toobserve interstellar sources of 1. 35-cm microwave emission from the 616 - 523 rota-tional transition of water vapor, following the recent discovery of such emission. H20emission was observed from the sources NGC 6334(N), Orion A, W3 (two sources), W24(Sgr B2), W49, W51, W75(S), and VY Canis Majoris. Spectra, uncorrected for atmo-spheric absorption, taken with the Haystack correlator are shown in Fig. 11-2. Withthe exception of NGC 6334(N), H2 0 emission has been previously observed in thesesources.2 VY Canis Majoris is an infrared nebula, while the other sources are HIIregions. All except one of the W3 sources are associated with 18-cm OH emission.The range of Doppler velocities in which H20 emission is observed generally coincideswith that for OH emission from a given source, although there are exceptions.The antenna temperature on Haystack, because of the largest feature in the W49spectrum, is greater than 1500 0 K, which makes this line by far the strongest of anyobserved in radio astronomy before the discovery of interstellar H 20. In addition tothe complex spectrum centered around 0. O0 km/sec, spectral features not shown inFig. III-2 were found in W49 at Doppler velocities of -130, -98, and +140 km/sec. OHemission at these velocities has not yet been observed. Significant time variation inthe W49 spectrum over a period of a few weeks has also been reported. 2 Our obser-vations have not been fully analyzed for time variations but the high-velocity line at+140 km/sec in W49 seems to have disappeared between 23 February and 9 March,1969.The sources were not spatially resolved with the Haystack antenna beamwidth of0. 0250 and an upper limit of approximately 0. 010 is placed on the angular extent of theH2 0 emitting regions. In Orion A, two emission regions separated by approximately0. 0050 were found. The polarization of the H20 emission does not appear to beappreciable, in contrast to the OH emission. Some of the spectral features in theOrion spectrum may be strongly polarized, however, and more careful analysis ofour data is required.The H2 0 emission provides new puzzles for radio astronomers. The molecular-1transition giving rise to this emission is between two excited levels 445 cm abovethe ground state. Any theory of the emission mechanism must explain the populationof these levels and the intense signals received. The upper limit of 0. 0100 on the sizeof W49 gives a lower limit of 30, 0000K to the brightness temperature of this source.A maser type of amplification is suggested because H20 would dissociate at thistemperature. The occurrence of H20 emission in regions where no OH emission hasbeen observed and vice versa defies a simple explanation of the chemical relationsQPR No. 93-74.05 -60.57 -47.09 -33.601000800300 CVY CANIS MAJORIS2/21/69ORION A2/16/69200 -11.72 15.;Fig. III- 2.77 19.81 23.85kmise;-23.99-10.51 2.96Interstellar H 2 0 Spectra. Antenna temperatures are uncorrectedfor atmospheric absorption and velocities are with respect to thelocal standard of rest.QPR No. 931600 -W492/15/69W32/15/69800 --13.47200 -W512/10/69100 -55.61 63.71W24(Sgr B2)2/16/69-43.14Fig. III-2.NGC 6334(N)2/29/69(concluded).51.5763.36 -29.65 16.17 -2.69QPR No. 93I 1 /300w75(S)2/16/6959.66(III. RADIO ASTRONOMY)between interstellar OH and H20. The apparent difference in the polarization proper-ties of the H20 and OH radiation suggests that different emission mechanisms may beoperating in the two cases.These observations were made jointly with M. L. Meeks and J. C. Carter, of Lin-coln Laboratory, M. I. T. , and W. E. Brown III, of NASA Electronics Research Center.A. H. Barrett, P. Rosenkranz, P. R. Schwartz,J. W. Waters, W. J. Wilson, C. A. ZapataReferences1. A. C. Cheung, D. M. Rank, C. H. Townes, D. D. Thornton, and W. J. Welch,"Detection of Water in Interstellar Regions by Its Microwave Radiation," Nature 221,626 (February 15, 1969).2. S. H. Knowles, C. H. Mayer, A. C. Cheung, D. M. Rank, and C. H. Townes,"Spectra, Variability, Size, and Polarization of H20 Microwave Emission Sourcesin the Galaxy" Science 163, 1055 (1969).G. 20-CHANNEL DIGITAL SYNCHRONOUS DETECTORA set of 20 filters with 500-kHz bandwidth covering the frequency range 25-35 MHzwere constructed for the 20-channel synchronous detector described in a previousreport.1 This system was taken to Haystack and operated in parallel with the correlatorduring the interstellar observations described in Section III-F. Although its outputagreed qualitatively with that of the correlator, the noise in the 20-channel system wasan order of magnitude larger because two command signals were asynchronous. Thisproblem has since been solved.J. W. WatersReferences1. L. P. A. Henckels and D. H. Staelin, "Digital Synchronous Detector," QuarterlyProgress Report No. 90, Research Laboratory of Electronics, M. I. T. , July 15,1968, p. 14.QPR No. 93

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