Annual Meeting of the Deep Sky Section, 2011 March 12

The Herschel Space Telescope and Star Formation

Prof. Ward-Thompson expressed his honour at having been invited to speak, recalling that he had been a member of the BAA himself in the 1970s. He explained that he would be talking about work he had recently been doing with data from the Herschel Space Telescope, a 3.5-metre telescope operated by the European Space Agency (ESA) which observed at far-infrared wavelengths. At these long wavelengths it was possible to see the faint thermal glow of cool objects which were only a few tens of Kelvin above absolute zero – i.e. below -200°C – but to do so required Herschel's cameras to have incredibly low noise levels – so low that they had to be cooled to a temperature below 2K using liquid helium. The speaker added that the supply of coolant needed to maintain these low temperatures would be the limiting factor that would set the telescope's useful operating lifetime.

Herschel had been launched aboard an Araine 5 heavy launch vehicle on 14th May 2009, and had initially been planned to operate for three years, though it now seemed likely that its helium supply would stretch to at least 3.5 years. It orbited the Earth at an altitude of 1.5 million km – four times more distant than the Moon – at a point known as the second Lagrangian point, where it took exactly a year for it to circle the Earth. This was an orbit that it shared with several other long-wavelength telescopes, including WMAP and Planck, and which was useful because it kept the Sun and Earth in close conjunction all the year round. As the Sun and Earth were both exceedingly bright infrared sources, which the telescope had to be directed well away from, it was useful to keep them together in the sky so that observers only had to be mindful of a single exclusion zone when pointing the telescope.

The speaker explained that Herschel was being used to observe many different kinds of objects, including AGNs and local group galaxies, but that he would be concentrating on star-forming regions in this talk, and in particular on Gould's Belt, a ring of star-forming regions around the Sun, measuring around 3,000 lightyears across, and which included a number of famous groupings of stars including the Pleiades (M45) and the Orion Nebula (M42). He added that Gould's Belt was inclined at around 20° to the plane of the Milky Way, crossing it in Perseus and Scorpius, and that this made it particularly attractive to study, as parts of it were well separated from the confusing mass of stars in the Milky Way.

He explained that stars began their lives in dense clouds of cool hydrogen gas known as molecular clouds. These were often also referred to as dark clouds, because condensations of dust grains within them blocked the light of background stars and made them appear visually as dark patches on the sky. Within these clouds, the process of forming a star could begin if a clump of gas, referred to as a prestellar core, became sufficiently dense that its gravitational self-attraction was sufficient to overcome the gas pressure pushing it outwards. Once such cores had formed and begun to collapse under their own gravity, they began to gravitationally draw in more gas from their surroundings, forming an accretion disc similar to those found around AGNs. The accretion disc was of especial interest, as it could later evolve into a protoplanetary disc, and perhaps eventually into a planetary system around the star. The core itself would go on to evolve into a T Tauri type star – a kind of variable star often studied by amateurs – before becoming a main sequence star.

Turning to an infrared image that he had taken of the horsehead nebula, a familiar example of such a dark cloud, in 2006 using the SCUBA camera on the James Clerk Maxwell Telescope (JCMT) in Hawaii, he pointed out that his infrared image showed clear evidence of a warm condensation of material around the horse's throat.¹² This was a prestellar core, whose solar wind and radiation might be expected to blow the horsehead nebula apart within the next few million years.

The speaker explained that a long-standing puzzle about the process of star formation was the distribution of the masses of the stars that it produced, a distribution known as the initial mass function. This distribution seemed to be universal for all molecular clouds, regardless of their environments – i.e. all molecular clouds seemed to produce low mass and higher mass stars in roughly the same relative proportions. This suggested a distribution that stemmed from a very fundamental and universal underlying mechanism, but it was not clear what this mechanism was.

He went on to explain that with the advent of infrared telescopes over the past 20 years, it had become possible to measure the masses of individual prestellar cores within molecular clouds. It seemed that these potential precursors to stars shared a very similar distribution of masses to the stars that the cloud might eventually go on to form. This suggested that the mass of star which each core was going to evolve into was already set at this very early stage. Thus, the problem had been pushed back a level: the shape of the initial mass function appeared to directly mimic the shape of the mass distribution of the prestellar cores. But what caused the cores to form with the mass distribution that they did?

One of the aims of the Herschel Space Telescope's survey of the Gould Belt was to observe large numbers of prestellar cores, in order to improve the statistical certainty with which their mass distribution was known, and to attempt to understand better the processes by which they evolved. Showing a few early images from the project, Prof. Ward-Thompson remarked that the sensitivity of Herschel's cameras was so great that the pixel-to-pixel noise was determined by the light of very distant background galaxies, rather than the camera itself.

Turning to two contrasting star forming regions, in Aquilla and around Polaris, he noted that the cores in Aquilla appeared to follow the stellar initial mass function as expected, but shifted such that the cores were on average three times more massive than newly-born stars. He added that this was relatively simple to explain: it was likely that two-thirds of the material within each core would be lost in the initial burst of energy as the star warmed up.

However, the distribution of the masses of the cores around Polaris was different. Here, the cores once again followed the stellar initial mass function, but this time shifted such that they were less massive than typical newly-born stars. This was harder to explain: where could these cores get extra mass from? This, he argued, was perhaps a failed star-forming region, that would actually never be able to form stars. If this was the case, he hoped that by studying it, determining how it was different from regions like Aquilla, and why it hadn't managed to form stars, some clues would be given about the initial conditions needed to form stars. The speaker concluded with a selection of further early images from Herschel, though he added that it would take several years to finish the analysis of them.

Following the applause, the Director thanked Prof. Ward-Thompson, as well as all of the day's speakers. In closing the meeting, he thanked all those who had helped ensure its smooth running, and especially Ann Davies of Newbury Astronomical Society for providing the catering for tea, coffee and lunch. He thanked Owen Brazell for bringing along a sales stand for the Webb Society, Ann Davies for bringing along a sales stand for the BAA, and Nick Hewitt for providing a polypin of ale at lunchtime.

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Dominic Ford