An accomplished observer

Shrinivas Kulkarni, who has to his credit several seminal astronomical discoveries, is elected a Fellow of the Royal Society, London.

PROFESSOR SHRINIVAS KULKARNI, a distinguished astronomer at the California Institute of Technology (Caltech), Pasadena, California, was recently elected a Fellow of the Royal Society, London. To receive a fellowship from the society is thought to be a singular honour for a scientist. The society counts among its fellows some of the finest minds of the last few centuries. Currently, India has about 10 Fellows, distributed over different scientific disciplines.

Kulkarni is an accomplished observer credited with several seminal astronomical discoveries. He has worked in several areas of astronomy, including the nature of matter between the stars in the Milky Way galaxy, pulsars, brown dwarfs and gamma-ray bursts. Kulkarni is also an expert in developing complex instruments to detect signals gathered by the largest telescopes in the world.

Kulkarni was born in the small town of Kurundwad in southern Maharashtra and had his early education in Hubli, Karnataka. He obtained a Master's in Physics from the Indian Institute of Technology (IIT), Delhi, and left for the United States for a doctorate in astronomy. It is ironic that while Kulkarni is one of the leading astronomers at Caltech now, and was Chairman of the Astronomy Department, he did not succeed in gaining admission to Caltech to do his Ph.D. Instead, he joined the fine Astronomy Department of the University of California at Berkeley, and obtained a Ph.D. from there in 1983. Kulkarni says that the experimental project he had done at IIT Delhi helped him to get admission to Berkeley. When he started on his course work in Berkeley, Kulkarni had virtually no background in astronomy. However, he had no fears about asking the simplest questions, even at the cost of appearing naive. His seniors and peers soon realised that he had an extraordinarily sharp and productive mind, and that he was a person with great determination.

Kulkarni first worked on the nature and distribution of hydrogen gas in the Milky Way galaxy. This involved observations with a large radio telescope located in Puerto Rico. On one of his trips to Puerto Rico, Kulkarni was asked to observe a specific radio source which was thought to be a pulsar. It is common practice among astronomers to ask colleagues or students to help out with observing. In this case the results were spectacular. The radio source turned out to be a pulsar with the shortest known period and its discovery in 1982 led to much observational and theoretical work. This was just the first famous discovery that Kulkarni was involved in, all of which he attributes to "uncommon good luck for an astronomer".

WHEN a star several times bigger than the sun exhausts all its nuclear fuel, its central region collapses to form what is known as a neutron star, while the outer parts are expelled in an explosion known as a supernova. The neutron star, as its name suggests, is made up mostly of neutrons, has a mass of about 1.5 times the mass of the sun, and a radius of only about 10 km. The large mass and small size make the object very dense with a cubic centimetre of it containing a billion tonnes of matter. The neutron star has a strong magnetic field, spins rapidly and emits energy. When the neutron star is relatively young, a fraction of the energy is emitted as radio waves directed in a narrow beam. When such an object is observed from the earth, a series of radio pulses are seen, and the object is known as a pulsar. Following the discovery of the first pulsar in 1967, many hundreds of pulsars have been observed.

The novel thing about the pulsar, called PSR 1937+21, found by Kulkarni and his collaborators, is that it has very high spin.

The pulsar takes just 1.6 milliseconds to complete a single rotation. This means that the object, with a mass comparable to the mass of the sun, completes about 625 rotations every second. Such a spin rate is just about what a neutron star can endure: if it spun any faster, it would break up because of the centrifugal forces generated. Normal pulsars have spin rates which range from a few tens of milliseconds to a few seconds. Pulsars are born spinning rapidly and slow down as they age because they lose energy due to the radiation that they emit. The rapid spin of the millisecond pulsar should mean that it is young, but the observational evidence says that it is at least 100 million years old. Explaining this discrepancy has led to much theoretical work, and it is now believed that the millisecond pulsar is an old pulsar which has been spun up by matter which has flown into it from a companion star and which has since been lost.

Having become involved with work on pulsars, Kulkarni has continued to work on them for years. He has made important contributions to our understanding of how millisecond pulsars and pulsars in binary systems are formed, and their relation to binary stellar systems which produce X-rays. In this context, he has studied the way magnetic fields in neutron stars behave and how pulsars could be formed in systems of stars known as globular clusters.

ANOTHER first to Kulkarni's credit is his discovery, in collaboration with colleagues from Caltech and the Johns Hopkins University, of a brown dwarf. Brown dwarfs are objects which have too little mass to become a star and too much mass to be a planet. Stars and brown dwarfs are both formed from collapsing clouds of inter-stellar matter. When the cloud has a mass which is at least a tenth as large as the mass of the sun, the temperature at the centre increases so much that a nuclear reactor is set off there. In this fusion reactor, hydrogen gas is converted to helium and energy is released in the process. This energy escapes from the surface of the object and makes it appear very bright, and one calls it a star. When the mass of the collapsing cloud is less than about a tenth of the mass of the sun, the temperature at the centre never reaches the value of several million degree celsius which is required to begin the fusion reaction. In such a case, the object can only emit gravitational energy released in contraction and remains relatively cool with the temperature at the surface being about 1,500 C. Such an object emits gravitational energy mostly at the near infrared wavelengths, and sophisticated detectors are needed to observe it.

Kulkarni and his collaborators have made the first unambiguous detection of a brown dwarf, which is named Gliese 229B. This is a companion to a cool red star which is about 19 light years from the earth. The brown dwarf has a mass which is a few tens of times the mass of Jupiter, the biggest planet in the solar system. It is about a 100,000 times fainter than the sun, which makes it very difficult to spot at its distance from the earth. Kulkarni and his collaborators first detected the object with a 60-inch telescope on Mount Palomar, using a technique known as adaptive optics that helps to detect small and dim objects near other stars. The spectrum of the object was obtained with the famous 200-inch telescope on Palomar. The presence of methane confirmed that it was is not a star as even a cool star would be too hot for methane to exist on it. The brown dwarf was later observed with the Hubble Space Telescope, which provided further information on it.

INTENSE, short duration bursts of Gamma rays were discovered in the late 1960s by the Vela spy satellites. These had been launched by the United States to monitor possible nuclear tests conducted in the atmosphere, and made the serendipitous discovery of Gamma-ray bursts (GRBs). This was announced in 1973 after it became clear that the bursts were cosmic in origin and had no strategic significance of any kind.

The most systematic observations of the GRBs has come from the Compton Gamma-ray Observatory, launched in early 1991. Observations from this satellite have shown that a GRB occurs approximately once a day and can last from a small fraction of a second to about 1,000 seconds. A bursting source is an extremely bright source in a Gamma-ray sky which can otherwise be considered to be rather dark. The many hundreds of bursts now known are uniformly distributed around the earth, and there is a relative scarcity of very faint sources.

In spite of the great number of GRBs which were observed, their basic nature remained an enigma. The main reason for this was the uncertainty about the position of GRBs in the sky. The satellite-based instruments used in detecting GRBs can pinpoint the direction in which any given source is located. Hence it is not possible to identify GRBs with any known class of astronomical objects, like X-ray sources in the Milky Way galaxy, other galaxies, quasars or some new kind of source. A great debate continued on this for several years, with two competing ideas emerging: either the GRBs were objects in the Milky Way galaxy, distributed to large distances in a spherical halo around the visible galaxy, or they were located in external galaxies. While arguments in favour of the two models were advanced by their proponents, the debate remained inconclusive because of a lack of hard observational evidence.

The breakthrough in the study of GRBs came with the launch of a Dutch-Italian satellite called BeppoSAX. This satellite could detect GRBs and determine their position with good accuracy because it had the capability to observe X-rays emitted by the GRBs. A GRB is a fleeting event and any X-ray, optical, radio or other emission associated with it has to be observed soon after the GRB is spotted. Using the capabilities of BeppoSAX, a team of Dutch astronomers succeeded in finding the optical counterpart of a GRB which occurred on February 28, 1997, and is known as GRB970228. The GRB was observed to be located in a fuzzy patch of light which could be presumed to be a galaxy. Later observations with the Hubble Space Telescope showed that the GRB was located not at the centre of the galaxy but significantly offset from it. These observations established that the GRB was in an external galaxy, and that is was not associated with a supermassive black hole since such a black hole is expected to be found at the centres of galaxies. In spite of these exciting observations, some uncertainty remained because the distance to GRB970228 was not known, and only knowledge of that would clinch the issue.

Progress after the identification of GRB970228 was rapid, and much of it was owing to work done by Kulkarni and his collaborators from Caltech. To determine the distance, it is necessary to obtain the spectrum of the optical counterparts of GRBs, and this can be done only with large telescopes since the objects are very faint. The astronomers at Caltech have access to the Keck telescopes - a pair of optical telescopes, each with a mirror of 10 m diameter. These are the biggest telescopes in the world. Using one of the Kecks, a feature was found in the spectrum of GRB970508 which was produced by absorption in a galaxy somewhere between the Milky Way and the GRB. This observation established that the GRB was at a distance of at least 15 billion light years. The huge distance means that the energy emitted by the GRB in just a few seconds is comparable to the total energy emitted by the sun during its entire lifetime of about 10 billion years.

Kulkarni and his collaborators have observed from several GRBs the optical and radio radiation which is released after the initial Gamma-ray burst. This radiation, known as the afterglow, is crucial to determine the physical mechanism for the burst. The favoured model for a GRB is that it starts off as a fireball of energy which expands with speed approaching the speed of light, and shocks in the expanding clouds as well as in the matter surrounding it produce the observed radiation. The radio observations in particular have revealed a wealth of detail about the expanding source, including the possibility that the radiation is emitted in a beam, rather than equally in all directions from the GRB. It is believed that the fireball is triggered by two neutron stars coalescing into each other, or by a hypernova, an implosion of the central region of a star. The hypernova model is the favoured one at the present time.

THE examples of Kulkarni's work discussed clearly show that he always involves himself with the most challenging observational problems. He brings the full force of modern technology to seek successfully what may have proved to be very elusive to others for a long time. He follows up his discoveries with insightful observations which go far in establishing the properties of the discovered objects, as well as the physical mechanisms which drive them. Kulkarni firmly believes that the answers to astronomical questions and enigmas are to be found in well-thought-out observations and not in theoretical arguments alone.

Kulkarni's approach to astronomy has brought him much success. He has been honoured with many awards and prizes, and is particularly proud of the Alan T. Waterman award. The award is given by the National Science Foundation every year to the most accomplished scientist/doctor/engineer, under the age of 35 years, working in any field in the U.S.

Kulkarni is known for saying bluntly and emphatically what he believes is correct. His ideas and opinions are often right and inspiring to his students and colleagues. Kulkarni has been very successful in working with large groups of highly talented and motivated astronomers of all ages, producing in the process much more work than would ever be possible for any one person.

Kulkarni has before him many years of research and development. His areas of interest in the future will include interferometry for obtaining high resolution images of stars at optical and infrared wavelengths. He is a member of several important working groups in the U.S. which will be setting the strategies for research in this area. He is also involved in important projects that search for and study planets outside the solar system - one of the key subjects of astronomical research in the coming years.

Ajit Kembhavi is with the Inter University Centre for Astronomy and Astrophysics, Pune.