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What are cold cores?

In our project we call "cold cores" any dense and compact region of interstellar clouds where the dust temperature is measured to be around 14 Kelvin degrees or below. The temperature can be determined from the shape of the dust spectrum that is measured with Herschel or with Planck in combination with data from the earlier IRAS satellite. Usually one calls a core something that is held together by gravity. However, in the Planck survey, because of the relatively low spatial resolution of five arc minutes, many of the detected objects may be larger clumps. These are likely to contain one or several actual cold cores whose presence explains the observed low temperature.

Dust can become very cold only in regions that are not directly heated by star light. This is possible in very dense cores that are shielded from the external radiation by the dust in the outer cloud layers. These dense cores are interesting because they can eventually collapse under gravity. The collapse of a cloud core will lead to the formation of an extremely dense object that becomes hot because of the gravitational energy that is released during the contraction. If the mass of the object is sufficient, the temperature will rise enough to start fusion reactions. In other words, the end result is a new star.

Our main objective is to catalog and study cores that represent early stages of the star formation process before or at the beginning of the collapse phase. However, the samples also include some objects that will disperse without ever producing stars and, on the other hand, clouds where the star formation has already begun. These data will help us to understand under what conditions and how the star formation proceeds and how this may be different in different environments.

Above is an image of the isolated cloud B68. The image is composed of optical and near-infrared observations and shows how the dust in the cloud hides all the background stars leaving a "dark nebula". Because of the same shielding effect, the centre of B68 is cold qualifying it as a "cold core". (Image credit ESO and J. Alves)

Cold cores/clumps are usually found within more extended molecular clouds as in the case of the cloud shown above. The left hand frame shows the intensity of dust emission at 250µm wavelength as observed with Herschel. The intensity is roughly proportional to the amount of material along the line of sight and shows the complex structure of the region. The right hand frame displays a map of the estimated dust temperature for the same area. This shows the presence of three separate cold regions where the temperatures decreases below 14K (blue/black colours). The angular size of the field is about half a degree (similar to the apparent size of a full moon). In linear scale the cloud is is more than ten light years across and contains gas and dust equal to tens of solar masses.

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