LICK OBSERVATORY
LICK OBSERVATORY
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In October 1995 the Lick Observatory Planet Search confirmed the first extrasolar planet, 51 Peg, two weeks after the intial announcement by Michel Mayor and Didier Queloz. Over the next 9 months the Lick Planet Search found the next five planets.
In October 1995 the Lick Observatory Planet Search confirmed the first extrasolar planet, 51 Peg, two weeks after the intial announcement by Michel Mayor and Didier Queloz. Over the next 9 months the Lick Planet Search found the next five planets.
President’s Day Weekend: From Copernicus Peak looking southwest 25 minutes before sunrise, the saffron-colored street lights of south Silicon Valley glow curiously pink through the fog. Small cumulous clouds scattered along the horizon reflect the approaching light of dawn. Coyotes bark and sing in the canyons below, piercing the frosty silence. Seven of the Observatory’s ten telescopes are visible from left to right: Katzman Automatic Imaging Telescope, Crossley 36” Reflector, Shane 120” Reflector, Tauchmann 22” Reflector, Crocker Dome (partially hidden in shrubs), and the Main Building with Lick 36” Refractor on the left, and Nickel 40” Reflector on the right.© 2006 Laurie Hatch / image and text.
At the heart of all modern precision Doppler surveys is a high resolution echelle spectrometer. Historically astronomers have either been able to work at high resolution but only covering a few angstroms of spectrum, or at low resolution covering hundreds or thousands of angstroms of spectrum. For comparison, the human eye can see from 4000 angstroms (violet) to 7000 angstroms (red).
The advent of CCD detectors and improving computer speed and storage led to development of modern echelle spectrometers in the early 1980s. Arguably the first modern echelle spectrometer, the Hamilton, was designed by Steve Vogt and built in the Lick Observatory optical shop in the early 1980s. This spectrometer remains in use on the 3-m Shane telescope at Lick, and can also be fed by the 24-inch CAT (coude-auxillary-telescope).
Schematic of the Hamilton echelle spectrometer. Starlight from the telescope is brought to a focus at the entrance slit to the spectrometer (upper left). The diverging beam of light is collected by the collimating mirror (bottom right) and converted to a parallel beam of light and sent onto the echelle grating (rainbow to the left). The light then passes through the two cross-dispersing prisms, and then into the CCD camera.
Hamilton echelle spectrum. Each colored stripe is about 50 angstroms of spectrum. The top is red light, the bottom is blue light. The dark patches in the colored stripes are stellar spectral lines from the absorption of atoms in the atmosphere of the star.
Geoff Marcy was Steve Vogt’s graduate student during much of the time that Vogt was designing and building the Hamilton spectrometer. After a postdoctoral fellowship at the Carnegie Observatories in Pasadena, Marcy became a tenure track professor at San Francisco State University in 1984. The Lick Observatory Planet Search began at San Francisco State in September 1986. Paul Butler was Marcy’s first graduate student. Butler’s undergraduate degree was in Chemistry. Marcy was aware of the great strides in precision velocities made by Bruce Campbell and Gordon Walker in Canada.
Butler followed Campbell and Walker’s idea of observing stars through an absorption cell. The spectrum of the absorption cell is embedded in the starlight and serves as the measuring stick for Doppler (wavelength) shifts in the stellar spectrum. Since the measuring stick is embedded in the starlight priory to entering the spectrometer, it is affected by the spectrometer in exactly the same manner as the starlight. This allows the effects of the spectrometer to be calibrated and removed.
After 6 months in chemistry libraries, chemistry laboratories, and day-time tests at the Hamilton spectrometer, Butler and Marcy settled on using molecular Iodine (I2). Butler and Mylan Healy, the SFSU chemistry glass blower, constructed the first precision velocity Iodine cell in May 1987.
The Iodine Cell at Lick Observatory. White starlight enters, but emerges with thousands of specific colors (wavelengths) absorbed by the iodine molecules. The iodine absorbs mostly green light, leaving the red and blue to pass through. Thus the lower part of the beam appears somewhat pink/violet in color. That absorption sets the wavelength scale for the stellar spectrum, like tick marks on a ruler, allowing the Doppler shift to be measured with a precision of one meter per second. © 2006 Laurie Hatch, image and text.
The Iodine cell was first used to take stellar data with the Hamilton spectrometer on the evening of June 10 1987. Butler completed his physics Masters thesis at SFSU in August 1987. He then moved to the University of Maryland to pursue his Ph.D.
Marcy and Butler ran into a myriad of problems along the path to producing precision velocities, many of them the same problems that Campbell and Walker had faced. Spectromers are composed of real stuff, lenses, mirrors, gratings made of different types of glass, separated and held in place with components made of different metals and other materials. Each of these materials expands and contracts at different rates with changes in temperature. Imperfections and jitter of the telescope drive cause the starlight to wander on the entrance slit to the spectrometer. The smearing function of the spectrometer varies with changes in temperature, air pressure, and telescope guiding. This is why prior to Campbell and Walker, Doppler measurement precision had been stalled at 300 m/s for decades.
Over short timescales, less than an hour, Butler and Marcy were quickly able to achieve precision 5 m/s. But night-to-night and month-to-month, the precision was 100 m or worse. It took 5 years to achieve long term precision better than 20 m/s. The key breakthrough was suggested by Jeff Valenti, a PhD student at Berkeley. Valenti was also using the Hamilton spectrometer, with the goal of measuring magnetic signatures in the spectrum of stars, a subtle effect. Valenti suffered from many of the same problems, in particular the variable smearing function of the spectrometer.
Valenti suggested that the spectrometer smearing function could be directly determined by observing a stable known spectrum. He suggested making observations of the Sun, either during the day, or by observing the moon or an asteroid, which reflect sunlight.
Marcy and Butler realized that they had a known spectrum embedded in every observations they took, the molecular Iodine spectrum from the Iodine absorption cell. In 1991 they took the Lick Observatory Iodine cell to the McMath Solar telescope on Kitt Peak in Arizona. The McMath had a very special type of spectometer, a Fourier Transform Spectrometer (FTS). FTS spectrometers provide extraordinarily high resolution, a factor of twenty or better than high resolution astronomical echelle spectrometers. Astronomers don’t use FTS spectrometers at the telescope because they require more light than telescopes can provide. FTS spectrometers are typically used by physcists in atomic spectroscopy labs, and at solar telescopes.
The NIST Fourier Transform Spectrometer. Dr. Gillian Nave setting up an Iodine absorption cell at the NIST atomic spectroscopy lab.
Iodine absorption cells are typically 4 inches long and 2 inches in diameter. The interior of the pyrex cell is evacuated and filled with a tiny about of Iodine. At temperatures above 40C the Iodine is entirely in gas phase. The Iodine primarily absorbs green light. The purple color is the result of red and blue light passing through the cell.
A detailed comparison of the FTS spectrum of the Iodine absorption cell to the Iodine cell as observed with the Hamilton echelle spectrometer allows for the Hamilton smearing function to be modeled and accounted for. Butler wrote the first software that could model and account for the spectrometer smearing function in early 1992. A major problem was the speed of early 1990s computers. Observations that took 5 minutes at the telescope requred more than 6 hours to analyze on the computer.
Though they could now achieve precision of 15 to 20 m/s, Butler and Marcy continued to use all their limited computer power in an effort to improve their nascent Doppler velocity reduction software. This was motivated by the results of the Canadian program, which stopped taking data in 1992. With 12 years of data covering 21 stars at a precision of 15 m/s, they did not find any planets (Walker et al. 1995). Based on this result it was decided that precision of 5 m/s or better was needed to make progress.
The Shane 120″ Reflector was the second largest telescope in the world when it was completed in 1959. It bears the name of former Lick Observatory director and astronomer Donald Shane, who spearheaded its development. The mirror was originally a test blank for the Palomar 200″ Reflector, then the world’s largest telescope. (Pyrex glass was invented specifically for use in these mirrors.) Although the Shane is modest in size by current standards, state-of-the-art research progresses in several fields, including adaptive optics and laser guide-star programs. Using the incomparable Hamilton Spectrograph, the Shane is a leader in discovering planets orbiting nearby stars. © 2003 Laurie Hatch, image and text.
The next big breakthrough in the project was led by Steve Vogt. Vogt had completed the design and construction of the HIRES echelle spectrometer for the Keck 10-m telescope. Based on a number of advances made over the previous decade, he went back to work on the Hamilton echelle. In November 1994 he replaced the spectrometer camera with a new design that he and Harland Epps invented. The new design dramatically improved the resolution of the Hamilton. In addition he replaced the old CCD detector with a next generation detector that was 6 times larger, significantly increasing the amount of spectrum that could be analyzed.
In January 1993 Butler completed his PhD at the University of Maryland, and moved back to California where he began a posdoctoral position at SFSU and UC Berkeley. Over most of the next 3 years he worked on improving the velocity reduction software package. After the Hamilton spectrometer upgrade in November 1994, his effort was focused on the newly emerging higher quality data. By May 1995 the upgraded software was producing 3 m/s precision with the new data.
Computer speed continued to be a problem. Marcy and Butler had two computers between them. The 8 years of data they had collected would require several years of computer time to process. Butler continued to work on improving the data reduction software, especially to speed it up.
At a meeting in Italy during the first week of October 1995 Michel Mayor and Didier Queloz announced the discovery of very strange planet. The planet “51 Peg b” has a mass similar to Jupiter, but orbits its host star in 4 days. While these “hot jupiters” are now known to be common, at the time nobody had suggested that such planets could exist. Much of the astronomical community as well as the press were skeptical of the claim.
Marcy and Butler had already been assigned 4 nights of precious time on the Lick Observatory 3-m telescope beginning on the evening of October 11. They observed 51 Peg multiple times each night. Butler reduced just the 51 Peg data each day. The observing run concluded on the morning of Sunday October 15. The first 3 nights of data were consistent with the discovery announcement from Mayor and Queloz, but Marcy and Butler wanted to see the final night of data before they went public. It took their two computers all day to reduce the four observations of 51 Peg from final night. They met back at their office in the Berkeley astronomy department at midnight. Within a half hour they were able to confirm the discovery of the first extrasolar planet. They put a plot of their 4 nights of data on the then brand new World Wide Web.
The discovery of 51 Peg b marked two major changes for the Lick Planet Search Program. No longer was the primary target jupiter-analogs with 12 year orbital periods. Planets could be found at any orbital period, and could already be embedded in the raw data taken over the previous 8 years. The second change was that the field of extrasolar planets had suddenly become very hot. In the wake of the newspaper and TV publicy that followed the discovery of 51 Peg b, several research groups at UC Berkeley offered the loan of research computers. Shortly thereafter SUN Microsystems made a grant of additional research computers the Lick Planet Search Program.
The first month of observations of 51 Peg from the Lick Observatory Planet Search, October-November 1995.
In late October 1995 Butler finalized the Doppler velocity reduction analysis, and began analyzing the 8 year backlog of data on an armada of computers that finally topped out at more than 20 machines. Clearing out the backlog of 8 years of data took until June 1996. Analyzing all the observations of a single star would take from half a day to several weeks, depending on how many observations had been taken.
Observations taken after Steve Vogt’s upgrade in November 1994 are internally referred to as “post-fix”. The data from the first 7 years is “pre-fix”. These are two separate data sets. Upgrading the camera and the CCD detector made the Hamilton a completely new spectrometer, requiring a completely new Doppler analysis package. Stitching together the “pre-fix” and “post-fix” data sets was a major problem. This problem would re-emerge on most of the subsequent Doppler surveys.
By mid-December 1995 hints of planet signals were emerging from the data. At 8 a.m. on the morning of Sunday December 31, Butler walked into the deserted Berkeley astronomy department to check on the armada of computers. A few jobs had finished, so Butler loaded the available computers with new stars and looked at the latest results. The bright nearby star 70 Vir had a whopping signal, the star was being tugged back-and-forth by several hundred meters per second. Within 5 minutes Butler had fit the data with a Keplerian planetary orbit indicating a 7 jupiter-mass planet in an 116 day orbit. The signal was so overwhelming that there could be no doubt. This was the first definitive planet to be discovered by the Lick Planet Search Program.
Discovery data for the planet orbiting 70 Vir. The orbital period is 116 days, and the minimum mass of the planet is 7 jupiter-masses! The shape of the velocity curve reveals that the planet is in an elliptical orbit.
After 9 years of working toward this moment, Butler was silent. He closed his eyes for several minutes, then looked back at the computer screen. The signal was still there. He did this several times to make sure that the signal did not vanish. In the absolute quiet of a New Years eve Sunday morning he sat for the next hour looking at the signal. For a long time he had the sense that Johannes Kepler was standing over his shoulder, looking at the same signal.
Over the next two weeks the case for a planet around 47 UMa firmed up. Butler solved the problem for putting together the pre-fix and post-fix data. The improved precision of the post-fix data sat on the pre-fix planet prediction like pearls on a string.
Marcy and Butler announed the planets around 70 Vir and 47 UMa at the winter meeting of the American Astronomical Society San Antonio Texas on January 17, 1996. The story received significant press coverage, including front page stories in the NY Times, Washington Post, and the cover of Time magazine.
The 70 Vir and 47 UMa papers were submitted to the Astrophysical Journal Letters on January 22 and February 15 respectively.
Discovery data for the planet orbiting 47 UMa. The orbital period is 3 years, and the minimum mass is 2.4 jupiter-masses. The dramatic improvement in precision after November 1994 is obvious.
The Lick radial velocities for Gliese 876 obtained from 1994 to 1998.6 vs orbital phase. The solid line is the radial velocity curve from the best–fit orbital solution from the Lick data alone.
Over the next 5 months the armada of computers ground through the 9 years of Lick observations. The next 4 planets quickly emerged, including the planets around rho 1 Cnc, tau Boo, nu And, and 16 Cyg B.
M (red) dwarf stars have a mass range from one-tenth to one-half of the Sun. They compose about 70% of all stars. They are intrinsically faint, less than 2% of the Sun’s luminosity. The lifetime of these stars are many times longer than the age of the universe, making them effectively immortal. The Lick planet search program announced the first planet to orbit an M dwarf star in 1998. The first planet discovered to orbit GJ 876 has a mass of 2.1 Mjup and an orbital period of 61 d.
In 1999 the Lick Planet Search Program announced the discovery of the first multiple planet system orbing HD9236 (nu Andromedae). This remarkable system includes gas giant planets in 4.6d, 242d, and 1269d orbits.
The inner planet orbiting nu Andromedae.
The outer two planets orbiting nu Andromedae. Their masses are 2 and 4 jupiter-masses, and their orbital periods of 242 and 1269 days respectively (Butler et al. 1999).
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