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Unexplained Brightening of Unusual Star

New HST Observations of the Southern Globular Cluster 47 Tucanae

15 stycznia 1997

Recent observations with the Hubble Space Telescope (HST) have documented an unexpected and rapid, seven-fold brightening of an unusual double star at the centre of the impressive 47 Tucanae globular cluster in the southern sky. This is the first HST observation of such a rare phenomenon. The astronomers [1] who are involved in this observational program find that this event cannot be explained by any of the common processes known to occur in such stars.

The cores of globular clusters

Globular clusters are like huge swarms of stars, containing about one million suns, which move around in their common gravitational field. Most galaxies contain globular clusters; around 150 are known within the confines of our Galaxy, the Milky Way.

Globular clusters change with time. In particular, at some stage in the life of a globular cluster, its central region will contract whereby the stars there move closer to each other. This phenomenon is referred to as core collapse [2]. Observations with the Hubble Space Telescope (HST) have revealed enormous central densities of the order of 30,000 stars per cubic light-year in clusters with fully collapsed cores; this is to be compared with the stellar density in the solar neighborhood of only 0.003 stars per cubic light-year [3].

Binary stars in globular clusters

Binary (i.e., double) stars play an important role in the evolution of globular clusters: they can delay, halt, or even reverse the process of core collapse. In this dense stellar environment, close encounters between passing stars and binaries are relatively frequent. Such events may leave the binary stars more tightly bound, and at the same time speed up the motion of the stars involved, thereby counteracting the contraction of the core.

The same close stellar encounters may also produce a diverse progeny of exotic objects. The centers of globular clusters contain blue stragglers (stars that "look'' younger than they really are), millisecond pulsars (rapidly rotating, very compact objects), both high- and low-luminosity X-ray sources, and cataclysmic variables (double stars whose light "flickers'). The kinds and numbers of these objects in cluster cores constrain the complex and as yet incompletely understood formation channels, most of which involve encounters with binaries. Many of the above exotic objects are strong emitters of ultraviolet light.

The globular cluster 47 Tucanae

47 Tucanae is an impressive globular cluster that is visible with the naked eye from the southern hemisphere. It is one of the closest (distance ~ 15,000 lightyears) and heaviest (total mass about 1 million solar masses) in our Galaxy. It contains about 1 million stars and the member stars have been intensively studied for decades. The observed structure of 47 Tucanae indicates that it is now approaching its ultimate fate during a core collapse phase.

There are five known low-luminosity X-ray sources in the core of this cluster, eleven millisecond pulsars, many blue stragglers, and a centrally concentrated population of eclipsing binary stars. The observations support the idea that the population of primordial binaries in this cluster has been heavily modified by stellar encounters.

The HST observations

In late 1996, the group of astronomers obtained time to observe the central area of 47 Tucanae with the Hubble Space Telescope and the second Wide Field and Planetary Camera (WFPC2). During a period of more than 4 hours, a total of 15 CCD exposures were obtained through an ultraviolet filter (transmission near 3000 A), showing the thousands of individual stars in this densely populated region.

When inspecting this material, it immediately became clear that one of the stars had undergone a substantial brightening in the course of these observations. In fact, its brightness increased by as much as 2.1 magnitudes, that is a factor of seven, in less than one hour; see the photos that accompany this Press Release. By the end of the observations, it had become the brightest star in the core of the cluster.

Earlier in 1996, this star - that carries the designation AKO 9 - has been found to be a double system of two stars that revolve around each other with an orbital period of just over 1 day. They are so close to each other that, at the distance of 47 Tucanae, they are observed as a single point of light. However, as seen from the Earth, one of the components moves in front of the other during eclipses once per revolution. When this happens, the brightness of the double star diminishes by about 1 magnitude for a short time.

What is the cause for the brightening?

The astronomers believe that the unexpected brightening of AKO 9 is connected to some kind of unusual event in this double system. There are in principle several possibilities, none of which, however, appears to be the true cause. In fact, it has not yet been possible to identify unambiguously the source of the observed phenomenon.

In some double stars that consist of a solar-type star with a cooler and heavier companion, magnetic activity has been observed in the companion's upper layers. This may lead to a rapid brightness increase, but never by a factor as large as that observed in AKO 9 .

Another possibility is that the system consists of a cool and large star together with a rather compact companion, a so-called white dwarf star. The latter is surrounded by a rotating accretion disc of matter and is no larger than the Earth although it weighs as much as the Sun. The accretion disc mainly contains matter that has been transferred from the cool star into an orbit around the white dwarf.

In such a system instabilities in the accretion disc may occur from time to time which give rise to X-ray emission and also leads a significant increase in the ultraviolet brightness of the system. Nevertheless, the other observed properties of AKO 9 do not indicate that it is a binary system of this type. In particular, with just two exceptions, its orbital period is longer than those of all 150 such systems known.

Yet another possibility would be the {\it nova} phenomenon which is due to a sudden nuclear explosion in the atmosphere of the white dwarf. But in such cases, the brightness increase is much larger than observed here.

Future investigations

Consequently, it is at this moment not yet possible to understand the nature of the observed brightening of the AKO 9 binary system. Although it is one of the best observed close binary systems within any globular cluster, the available observations will have to be complemented during future investigations before the responsible mechanism may be identified.

Uwagi

[1] The group consists of Georges Meylan (ESO, Garching, Germany), Dante Minniti (Lawrence Livermore National Laboratory, Livermore, USA), Carlton Pryor (Rutgers Univ., Piscataway, USA), E. Sterl Phinney (Caltech, Pasadena, USA), Bruce Sams (Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany), Chris G. Tinney (Anglo-Australian Observatory, Epping, Australia).

[2] The phenomenon of core collapse is reminiscent of the `red-giant phase' of stellar evolution when - towards the end of its life - the outer layers of a star begin to expand while its central regions contract.

[3] This corresponds to ~ 10 6 stars/pc 3 and ~ 0.1 stars/pc 3 , respectively.

Więcej informacji

A provisional report about this work will be presented on January 15, 1997, at the semi-annual meeting of the American Astronomical Association in Toronto, Canada.

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