Ordinary Meeting, 2006 March 22

 

Low Mass Stars, Brown Dwarfs and Hot Jupiters

Dr Viti opened with an apology that her's was not a field which could yet produce nice images; the objects she would be talking about were too faint to be meaningfully imaged. Yet she would be arguing that even without images, a tremendous amount had been learnt from such objects in the past decade, and that there were exciting prospects for the future.

The primary subject of this talk would be Low Mass Stars (LMSs), defined to be those stars of around half the mass of the Sun or less. Apart from their mass, their next most obvious feature was that they were much cooler than larger stars, typically having surface temperatures no higher than 3,500 K. For comparison, the temperature of the Sun's surface was 6,000 K. Their core temperatures were also correspondingly lower, reducing the rates of nuclear fusion within them. This slowing of the nuclear reactions was so significant that it actually took LMSs longer to exhaust their fuel supplies than more massive stars, even though they had less of it to 'burn'. In other words, lower mass stars lived for much longer than more massive stars. In fact, lifetimes of LMSs were so long that they were invariably much longer than the present age of the Universe, and, to good approximation, it could be said that every LMS that had ever formed was still extant. At the same time, LMSs were very faint, on account of their low surface temperatures, and this explained why they were so notoriously difficult to detect.

Brown Dwarfs, the second class of objects in the speaker's title, were a subclass of LMSs – those which were insufficiently massive to fuse hydrogen nuclei at all – those whose core temperatures were too cool for nuclear reactions to take place. According to theories of stellar structure, stars with less than ~8% of the mass of the Sun (85 Jupiter masses) were expected to fall into this category. Their existence had been theorised since the 1960s, and some had even proposed that all of the dark matter in the Universe might be made up of these cold, faint stars. It had since become clear, though, that a population of brown dwarfs so numerous to explain all of the dark matter in the Universe would be quite conspicuous by its sheer size, and the observational fact was that there had been no confirmed detection of a brown dwarf until that of Gliesse 229B in 1994.

Some considered the use of the term 'star' to apply to brown dwarfs rather inappropriate, feeling that only hydrogen-fusing bodies should be called 'stars'. The use of the term 'dwarf' was fairly uncontroversial; the speaker thought that 'failed stars' was a fairly accurate label.

The third and final class of objects that the speaker would be talking about were so-called 'hot jupiters' – Jupiter-like gas giant planets in orbits around stars other than our own. Though they might appear somewhat unrelated to LMSs, the speaker explained that their relevance to this talk was that observationally they were very difficult to distinguish from brown dwarfs. Being planets, hot jupiters did not shine appreciably in their own light, and orbited around parent stars. But if a brown dwarf were to be found in orbit around a larger companion star, it too would share these characteristics. Brown dwarfs did shine a small amount of their own light, but this was also true of Jupiter. Moreover, the two classes of object were physically of near-identical size. Their surface temperatures were their only difference – 900 K and under for hot jupiters, as compared to 1,800 K or more for brown dwarfs – but in such cold, faint objects, temperature was difficult to measure.

Clearly there were many motivations for wanting to understand the processes of planet formation, central to the questions of how our solar system came to be, and what proportion of other stars might host similar planetary systems. This indirectly presented one reason for studying of LMSs: we needed to find ways of distinguishing them from planets. Lacking, as we did, any easy means of distinguishing these two classes of object at present, the number of extra-solar planet discoveries which had been reported to date – around 150 – was really an upper-limit; some of these objects might not be planets at all. And, whilst planets and brown dwarfs might look alike, we could not pretend that they were not fundamentally different objects. From a theorist's viewpoint, they were very different, because they formed in different ways. Whilst planets formed by the clumping together of material in planetary discs around stars, brown dwarfs formed like stars, from the collapse of clumps of inter-stellar gas. Theorists wanted to understand the workings of these two processes in separation.

The speaker did not want to suggest, however, that her field of study was only of interest as a source of contamination in another field: to the contrary, there were many reasons for wanting to understand the properties of LMSs. Because of their long lifetimes, and the slow nuclear reactions within them, any LMSs which had formed in the very early Universe would still exist today, and be composed of relatively pristine primordial material. Thus they had potential in the future to provide an insight into the evolution of the chemical makeup of the Universe.

Another consequence of their long lifetimes was that they were expected to be quite pervasive, and so lock up a considerable proportion of a typical galaxy's mass. As mentioned earlier, this could not account for all of the dark matter which was known to exist, but the gravitational contribution of a large population of brown dwarfs might still be sufficient to make a difference to a galaxy's dynamics.

Turning to the process of star formation – the collapse of inter-stellar gas clouds down to form stars – here also, the speaker believed LMSs to have a contribution to make. Within modern astrophysics, the so-called 'initial mass function' – the distribution of masses of newly formed stars – remained a matter of contention. No theory could yet predict its form, but observations broadly suggested that lower mass stars formed much more frequently than their higher mass counterparts. Given the comparatively small number of known brown dwarfs – none up until 1994 – it was not yet clear to what extent this trend continued down to brown dwarf masses. The speaker's suggestion was that the trend was likely to continue down to some critical minimum mass, below which stars could no longer form. If that prediction turned out to match observation, the critical switch-off mass would be a parameter for which theorists could hope to find some physical explanation, as a first step towards the greater question of why stars formed with the range of masses that they did.

As a final motivation for studying LMSs, Dr Viti added that the environment in their vicinities might be curiously well suited for the development of life. Although such stars were very cool, an Earth-like planet in a close orbit could still be warm enough to become habitable. Given the long lifetime of the parent star, there would be ample time for lifeforms to develop. With this in mind, it was proposed that when the European Space Agency came to launch its Darwin probe – a specialist instrument for searching for terrestrial planets, presently planned for 2015-20 –LMSs should be amongst the stars it should study.

The speaker then turned to discuss the challenges faced in trying to observe LMSs – essentially that of their sheer faintness, but compounded because such cool objects emitted the bulk of their light in the infrared, rather than at visible wavelengths. Though technology did now exist to observe in the infrared – for example, the United Kingdom InfraRed Telescope (UKIRT) in Hawaii – it was comparatively new, and poor atmospheric transparency remained a plague at these wavelengths. In any event, it was impossible to know anything about objects outside of the immediate neighbourhood of the Sun, as their faintness severely restricted the distance out to which they could be seen.

Measuring the surface temperatures of LMSs was vital, both to estimate their masses, and, as mentioned earlier, to distinguish them from planets. The blackbody spectra of such cool stars peaked well into the infrared, rendering their visible colours rather insensitive to temperature. Infrared spectra could only be taken from space, but here, astrochemistry came to the rescue. The atmospheres of these stars were sufficiently cool that some simple molecules did not dissociate, but could survive for long periods. These gave rise to a plethora of absorption lines in the spectra of LMSs, in contrast to the relatively featureless spectra of hotter stars. The exact details of which molecules were present, and which spectral lines were seen, was incredibly sensitive to temperature – typically a change of 50 K produced a complete change in the line features of a spectrum. Understanding how to relate this chemistry to temperature was a hugely difficult task, but potentially, a vast amount of information could be gleaned.

As an example, the speaker illustrated how titanium oxide (TiO) lines were seen in the hottest LMSs, meanwhile calcium hydroxide (CaOH) was seen in slightly cooler objects. Towards the lower end of the mass scale, methane and water began to dominate. In recent years a huge breakthrough had been made in understanding how this chemistry related to temperature, arising from the recognition that sunspots, being cooler parts of the Sun's material, had a great deal in common with the surfaces of LMSs. Being so much more nearby, they could be studied in much more detail, providing valuable insights.

In conclusion, the speaker summed up that low mass stars were probably very common in the Universe, but also exceptionally tricky to observe. In coming years, however, they might play a central role in our understanding of fields as disparate as star formation, planet formation, galaxy evolution and the search for extra-terrestrial intelligence.

Following the applause, a member asked how many confirmed brown dwarf discoveries had been made to date. Dr Viti replied that there had now been several hundred – many more than the number of known extra-solar planets, of which there were only around 150. The President asked whether amateurs could make any contribution to the field. The speaker suspected that this was one field where amateurs would have difficulty, as had professionals until recent times. Although some brown dwarfs were mag 16 in the V-band, they were often associated with more massive stars and very difficult to resolve from the glare. Moreover, most of the interesting science relied upon high-resolution spectra and infrared observations.

The President then adjourned the Meeting until the Out of London weekend, to be held at the University of Liverpool from April 21-23.

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Dominic Ford

Fairfield

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