From Water Vapor on Venus to Neutron Stars in the Crab Nebula

Transcription

1 From Water Vapor on Venus to Neutron Stars in the Crab Nebula Jim Moran Harvard-Smithsonian Center for Astrophysics StaelinFest 2011 July 18, 2011 Figure 1

2 Dave Staelin Radio Astronomy Pioneer * * Venus and other planets (+1980s Jupiter) (Barrett) CMB (Burke) Pulsars (Reifenstein) Optical interferometry (Shao) Uranus (Eikenberry) Figure 2 *minor but interesting activities

3 Venus Water Vapor Radiometer Staelin and Barrett, ApJ, 144, 352 (1966) Figure 3

4 Could Dave s instrument have detected water vapor masers? (They were discovered by Townes in 1968 toward the Orion star-forming region.) I. Sensitivity okay ν = GHz T B K S up to 10 6 Jy (10-20 w m -2 hz -1 ) B 100 KHz Staelin s system T s = 22,000 K ( T = 4 K in 4 s) B = 8 MHz τ = 4 s A = 30 m 2 S = 2kT s /A Bτ = 360 Jy S maser = 10 6 (0.1/8) = 12,000 Jy SNR = 35 in 4 seconds II. Where to point? With a beam of 6 resolution, the sky has 4 x 10 6 pixels. Figure 4

5 Technical Progress in Radio Astronomy at 22 GHz factor D 8.5 m (28 ft.) (MIT LL) 100 m (GBT at NRAO) 14 T s 22,000 K 22 K 10-3 Figure 5 Ratio of sensitivities D 2 /T s 1.4 x 10 5 = 2 17 in 40 years Doubling time (aka Moore s law ) = 2.3 years

6 Figure 6 Crab Nebula (Supernovae of 1054 AD) Scale 3 or 2 parsecs

7 Discovery of Crab Pulsar GB 300-ft. Telescope, October 1968 Staelin and Reifenstein, Pulsating Radio Sources Near the Crab Nebula, Science, 62, 1481 (Dec. 27, 1968) Figure 7

8 Pulsars A Review by F. Graham-Smith Reports on Progress in Physics, 35, 399 (1972) The most significant discovery since the original Cambridge work then came from the National Radio Astronomy Observatory (NRAO) in USA. Staelin and Reifenstein (1968) found two pulsars close to the Crab nebula, the best known supernova remnant in the whole sky. They could not, however, measure the periods, and it was left to Comella et al. (1969) working with the 1,000-ft. telescope at Arecibo to show that one of these two had a period of only 33 ms and that it appeared to be within the nebula. This pulsar is also slowing down, but so much faster than the Vela pulsar that its age is of the order of only 1,000 years. This is in very good agreement with the known age of the Crab nebula itself, and the association is completely established. Figure 8

9 1 From Water Vapor on Venus to Neutron Stars in the Crab Nebula Talk at StaelinFest, July 18, 2011 James Moran I am delighted to have been invited to speak at this conference in honor of Dave Staelin. I had four key mentors at MIT who helped me get through graduate school, and Dave was one of them. I am immensely grateful for his help, both on a technical and personal level. I arrived at MIT in the fall of I came to MIT specifically because of the opportunity to work with Alan Barrett. He had another graduate student at the time, Dave Staelin, who was finishing up his PhD thesis involving a search for water vapor on Venus through its 22 GHz transition with the 28-ft. telescope (aka antenna) at Lincoln Laboratory in Lexington. Barrett (perhaps at Dave s suggestion) asked me to work with Dave and use his radiometer to study the brightness temperature variations of the moon. This resulted in my master s thesis. It was a very exciting time to be entering the field of radio astronomy. In the five-year period of 1963 to 1968, quasars (those stellar-looking objects that put out the power of whole galaxies of stars) were discovered (1963) along with masers (radiation with extremely high brightness temperatures from molecules with radio transitions), pulsars (pulsing radio sources produced by beamed emission for rotating neutron stars), and the first of many molecular transitions in the radio band marking the birth of astrochemistry. I always thought of Dave s work in astronomy as a sideline or hobby, but he made important contributions in three areas: planetary radio astronomy, pulsars, and the development of optical interferometry (figure 2). I ll talk today about the work on Venus and the Crab pulsar. Back to Dave s and my theses. Searching for water vapor from Venus was an exciting topic because of the theoretical controversy about whether it existed there. Carl Sagan was a prominent participant in these discussions. Although I did not appreciate it at the time, there was also considerable interest in the moon because of the concern raised by Tom Gold and others about whether the astronauts in the Apollo program would sink out of sight in the lunar dust when they landed on the moon. When I went to my first AAS meeting in 1965 in Ithaca, the session on the moon, in which I happened to present our measurements, was packed because of the interest in knowing the lunar surface properties.

10 2 I vividly recall my first trip to the 28-ft. telescope at Lincoln Lab. The antenna, although possessing a very precise surface for its diameter, was a rather primitive instrument. For example, it had no computer-controlled tracking system. The pointing was accomplished manually by a person moving two trackballs, one that controlled the telescope position in azimuth and elevation angle and the other that controlled the tracking velocities. The person at the controls looked at the image of Venus on an optical guidescope and tried to keep it in the crosshairs. Dave set things up and then handed over the tracking to me while he did something else. Hmmm. This is easy, I thought. I nudged the position ball to follow the target in azimuth and elevation, but Venus slowly started accelerating in each coordinate. Soon, I was frantically spinning the position ball as fast as I could make it go. Dave stepped in, and with his right hand on the position ball and his left on the velocity ball, he deftly kept Venus centered with quick and seemingly effortless flicks of the balls. Perhaps he had played pinball as a youth. Figure 3 shows a block diagram of Dave s radiometer. Note the output format: punched paper tape. Venus did not cooperate, and Dave detected no real evidence of water there, so his thesis result was an upper limit. Water vapor has since been detected on Venus, but in other transitions and chemical forms like HDO and at much lower column densities than the limits reached by Dave. To this day, no one has detected water vapor in the 22 GHz transition. OH masers were detected in 1965 and water vapor masers in A lot was learned between those years about the maser phenomenon and its occurrence in the molecular clouds that pervade the galaxy. It is interesting to consider whether Dave could have detected water vapor in the interstellar medium, had he known where to look. It turns out that Dave s radiometer easily had the needed sensitivity. The radiometer had five channels of 8 MHz bandwidth each that straddled the transition frequency and were widely spaced in frequency. The system temperature was about 22,000 K, so the sensitivity was 4 K in 4-second integration. This corresponds to a flux density sensitivity of 360 Jy in 4 seconds (see figure 4). Now the strongest water maser is the one originally found by Townes in 1969 toward the Orion nebula, which has a flux density that reaches 10 6 Jy and has a line width of about 100 KHz. This dilutes to an average flux density of about 12,000 Jy in the 8 MHz radiometer channel width. Hence, the SNR in only 4 seconds of integration could have been about 35! However, knowing where to point the telescope is another matter. The 28-ft. telescope had a beamwidth of about 6 (about six times worse than the resolution of the human eye). At this resolution, there are about 4 million independent pixels on the sky. Nothing was known in 1964 about masers or molecular clouds that formed stars and excited the masers. I am not sure

11 3 what lesson there is to be learned here, except that astronomy has a lot of parameter space to be explored, i.e., like the old saw, scientists are just like the child who picks up a pebble on the beach to examine, while the whole universe around remains to be discovered. It is interesting to use Dave s state-of-the-art system of 1964 as an example of the rapid progress of technology in radio astronomy. For a fixed bandwidth and integration time, the sensitivity of a radio telescope is proportional to telescope area (the square of its diameter D) divided by the receiver system temperature (see figure 5). In Dave s case, the telescope diameter was about 8 m and the system temperature was 22,000K. By 2004, the Green Bank 100-m telescope was running at 22 GHz with a system temperature as low as 22 K on cold days. The improvement in collecting area was a factor of 14 and in system temperature a factor of 1,000. Hence, the improvement in sensitivity over 40 years was about 1.4 x 10 5, or This corresponds to an average time for the doubling of capability or sensitivity of 2.3 years. Hence, radio astronomy has its Moore s law. It will be possible to maintain this improvement rate only by the commissioning of ever larger telescopes, such as the proposed Square Kilometer Array. In November 1967, Hewish et al. discovered the first pulsar at Cambridge University. The world learned about it in a Nature paper published in February It seemed like half the astronomical community turned its attention to pulsars, and many were quickly found and theories developed but no optical identifications made. Since the theoretical connection to white dwarfs or neutron stars was quickly postulated, an obvious place to search for pulsars was toward supernovae. Several groups looked at the Crab supernova remnant (see optical image in figure 6), the residue of the famous supernova of 1054 recorded by the Chinese and Native Americans, the latter through rock art in New Mexico. Dave happened to be taking a sabbatical at NRAO in the academic year He teamed up with former MIT student Ted Reifenstein and got time on the NRAO 300-ft. telescope to carefully search the Crab nebula for pulsars. Although most known pulsars had periods of about 1 second, Dave set the integration period on the radiometer to its smallest value, 50 ms. In October 1968, they saw pulses drifting in frequency (figure 7), as expected due to the refractive delay of the ionized ISM. The pulses seemed erratic, with pulses of dramatically different amplitude appearing about once every second. They could not determine a period but published their exciting discovery in Science in December The news hit the astronomical community like a bombshell. Arecibo tuned in and quickly identified the period as 33 ms. Dave and Ted had seen only the occasional giant pulses, for which the Crab pulsar is now famous. Optical pulses at 33-ms period were soon detected. Theoreticians could prove a firm link between pulsars and neutron stars. In his review paper on pulsars in 1972 (see figure 8), F. Graham-Smith called the discovery of Staelin and

12 4 Reifenstein the most important advance in the field during the first year of frantic observations following the initial discovery. Others have continued the kind of thinking outside the box work exemplified by Staelin and Reifenstein. In 1982, Backer and his colleagues identified the first millisecond pulsar by increasing the time resolution even more. These millisecond pulsars turn out to be extremely stable clocks. The search is now on to use them to try to detect directly gravitational radiation by measuring slight deviations in the pulse arrival times at the Earth caused by a dramatic event such as the merger of two highly massive black holes in the center of a distant galaxy. For such a measurement, the Earth itself becomes a key element in the detector as it responds to gravity waves. So we can see that Dave s work in astrophysics, although highly selective, has had an important lasting impact on the field. Discussion after the paper: 1. Before I arrived on the scene in 1963, the pointing of the 28-ft. telescope was even more primitive. Cosmo Papa and Jack Barrett, the legendary technicians of the radio astronomy group at MIT, each sat outside on the telescope mount, one axis each, with a hand crank to follow the motion of Venus. 2. While Dave was at NRAO in , he submitted a proposal with Lew Snyder and David Buell to search for water vapor in the newly identified molecular clouds that showed formaldehyde emission. The proposal was rejected by the proposal review committee. Townes and his group made the discovery of water vapor maser emission with the Hat Creek Observatory a year later. The rejection of the Snyder, Buell, and Staelin proposal is a prominent part of the folklore of radio astronomy. Burke pointed out that this event led to Staelin s rule for proposal writing : If you have a really good idea, bury it in a proposal for mundane observations that the proposal review committee can understand.