Rehearsals aren't just for the theatre

We all know that actors rehearse, but did you know that telescopes rehearse as well? Large telescopes and surveys often rehearse the process of choosing targets, observing, and analysing the data so that they can be well prepared for the real thing.

Its a process that is particularly relevant with large surveys where efficient scheduling of observations is necessary to observe as many targets as possible in the time available. Modern telescopes also produce an incredibly large amount of data, sometimes many terabytes per night, and transferring this data from the telescope to different universities, and then processing and storing it all can be quite a mammoth task! Practising this process in advance allows potential issues to be identified and ensures that when the telescope is up and running the process proceeds smoothly.

The William Herschel Telescope
(Credit: Wikimedia Commons)

Most of the last month of my time has been spent preparing for an "Operational Rehearsal" for the WEAVE spectrograph being built for the William Herschel Telescope in the Canary Islands. This is quite an endeavour, as we're simulating over a week of observations on the telescope, which requires everyone involved to produce simulated target lists and spectra for them.

The goal of this work isn't just that the telescope operators and survey team will be able to experience the day-to-day running of the survey, its also so that the survey science teams become familiar with the process of supplying targets for the observations.

And so for the last month I've been busy simulating "fake" target lists, preparing "fake" spectra for those targets, and assembling everything into the format needed by the survey organisers and telescope operators. Its been quite a task, but hopefully it'll be useful for everyone involved in the survey to have gone through this rehearsal and learnt from it.

How to make a star cluster

In my last two posts I've been discussing how stars are distributed at the time they're born. This is an important question because many of our theories for how stars form suggest that their environment can play a crucial role in how they accrete material. Our current observations suggest that stars form directly out of the dense gas that is found in filamentary structures across molecular clouds. But if stars form with an elongated and filamentary distribution, how do spherical star clusters form?

The answer to this question has eluded astronomers for many decades, and though there is still considerable debate in the community, a picture is emerging whereby star clusters arise when filaments of dense gas merge.

Star clusters forming in the Rosette Molecular Cloud
(Credit: Schneider et al. 2012)

The figure to the right shows an image of filaments and star clusters in a star forming region known as the Rosette Molecular Cloud (so-called because its very close to the famous Rosette Nebula). The background image shows the distribution of dense gas in the cloud, with the density of the gas ranging from low-density (black) to high-density (green and red).

On top of this are marked (in white) the positions of the filaments that make up the molecular cloud, and on top of that (the turquoise stars) are the positions of known star clusters.

If you inspect the image closely you'll see that the majority of the star clusters (which were known about well in advance of this study) sit at the intersections between the filaments. In fact out of the 14 star clusters in this molecular cloud, 13 of them are found at these intersections. This is unlikely to be a coincidence, so it appears that the formation of star clusters is closely linked to overlapping or merging filaments.

Over the last decade astronomers have seen various strands of evidence pointing towards this picture (a good summary of the early evidence can be found here). However it wasn't until the launch of the far-infrared Herschel Space Observatory in 2009 that the filamentary structure of molecular clouds became so apparent, and soon after that the relation between clusters and filaments began to emerge.

So if stars clusters are found where filaments overlap, this suggests that the collision between the filaments might create the necessary conditions for a star cluster to form. The question this then poses is whether the filament collision occurs before, after, or even during the star formation process.

If the filament collision occurs before star formation then the collision is effectively bringing together large volumes of dense gas into a small space. This would allow star formation to proceed very rapidly in a very dense cluster of gas, leading to the formation of stars in a highly clustered distribution. This has sometimes been referred to as clustered star formation or in-situ cluster formation.

Alternatively, the filament collision might occur after star formation has begun, in which case the filament collision would be bringing together stars that have already formed, depositing them in a highly clustered distribution. This is usually referred to as conveyor-belt cluster formation.

Which of these two scenarios is right has big implications for how stars form and how the environment affects the star formation process. There are strands of evidence in favour of both scenarios, though neither has been conclusively shown to be true yet. Of course its possible that both scenarios might occur, perhaps in different environments, in which case it would be interesting to understand which process occurs more often, and whether the clusters that form from the two processes differ in some way. Hopefully that's a question we can answer soon!

What sort of environment do stars form in?

Last week we talked about the initial spatial distribution of young stars and how their distribution follows that of the dense gas in molecular clouds. But we also know that stars form in groups with a wide variety of sizes and densities, which astronomers think is really important for determining the type and sizes of the star clusters that form.

Distribution of young stars in the Perseus Molecular Cloud
(red, green and blue dots) projected against the gas
distribution (Credit: Evans et al. 2009)

The image on the right shows the distribution of young stars across the Perseus Molecular Cloud. These young stars were all detected by the Spitzer Space Telescope, an infrared telescope that was particularly effective in detecting young stars due to the copious amounts of infrared light they emit.

The molecular cloud is very elongated, as the image clearly shows, but even within that elongated structure the young stars are not evenly distributed, they're clumped into groups. Many of these groups represent the well-studied embedded star clusters typically found in molecular clouds, such as IC 348 and NGC 1333.

In addition to these dense and compact clusters there are also smaller groups, such as the clumps of young stars labelled B1 and B5, as well as numerous young stars that appear relatively isolated.

It appears that while young stars do like to form in groups, there are almost as many young stars that form alone - so is there a typical group size and density that stars form in? And if so, what is it?

One way that astronomers have attempted to tackle this problem is to study the distribution of densities that stars are forming at. To do this astronomers have measured the density of stars surrounding each young star. The distribution of densities is usually referred to as the surface density distribution of young stars.

The figure below shows such a distribution compiled from Spitzer Space Telescope observations of numerous nearby star forming regions. Mid-infrared observations from the Spitzer Space Telescope were chosen for this because it allows astronomers to peer deep within molecular clouds and hopefully identify all the young stars that are present. Hopefully this means no stars were missed!

The surface density distribution of young stars (both Class I and Class II young stars) identified from
Spitzer Space Telescope observations (Credit: Bressert et al. 2010)

The figure shows the fraction of stars born at various densities, from low densities on the left (surface densities of 1 star per square parsec) to high densities on the right (hundreds to thousands of stars per square parsec). The former represent stars that have formed in relative isolation, while the latter represent stars that have formed in dense groups or clusters.

Most notable in this figure is the fact that there is a smooth distribution from low to high densities, which suggests that stars don't just form at low and high densities (in isolation and in clusters), but at a wide range of densities, with groups and clusters existing over a variety of densities.

This is important for our understanding of star formation because it tells us about the conditions under which stars form, as well as the sort of environment where planetary systems form. A planet forming in a dense cluster faces very different conditions compared to one born around a relatively isolated star. In a dense cluster there could be multiple interactions or collisions between stars and planets, as well as a very powerful radiation field due to the close proximity of so many other stars, which could damage a forming planet's atmosphere.

Hopefully as we start to learn more about the various types of planetary system that exist, and especially once we start studying the atmospheres of these planets, we can hopefully address the question of what impact the birth environment has on a forming planetary system.

Star formation, filaments, and the initial distribution of young stars

I've talked before about molecular clouds, the birthplaces of stars, but only in very general terms. Today I want to go a little deeper to look at how stars are distributed within molecular clouds when they form, and how this can lead to the formation of star clusters.

Molecular clouds aren't just big fluffy clouds of gas that uniformly form stars, they have considerable structure. This is clear if you look at molecular clouds using an infrared telescope, particularly one that's tuned to the far-infrared, such as the Herschel Space Observatory (the further into the infrared you go, the cooler the material you can study because cooler things radiate light at longer wavelengths, i.e. the far infrared).

A far-infrared image of the Taurus molecular cloud, showing the filamentary structure of the gas
(Credit: Herschel Space Observatory)

This image from the Herschel Space Observatory shows the incredibly filamentary structure of the gas in the Taurus molecular cloud. This structure is thought to arise due to a combination of shock compression (due to collisions between material) and self-gravity (meaning the filaments can form gravitationally-stable structures by themselves). These filaments can be seen on all spatial scales, from the large filament spanning the entire image, through to small filaments emanating from the larger ones.

Many of these filaments are dense, containing many times the mass of our Sun in molecular gas. This high density means that they can become gravitationally unstable, which can lead them to collapse and potentially form stars. Simulations of filaments suggest that a single filament may actually fragment into multiple stars distributed along it's length.

The NGC 1333 molecular cloud. Contours
show the distribution of dense gas, while
 the red dots show the very young stars that
are still forming (Credit: Gutermuth et al. 2008)

One of the predictions of this model of filamentary star formation is that the densest gas in star forming regions should be distributed in filaments and that the youngest stars - those that are actually still forming - should be found within these filaments.

The image to the right shows that the distribution of very young stars (red dots) in the star forming molecular cloud NGC 1333 are almost entirely projected against (and therefore likely to be within) the filamentary dense gas shown by the contours.

This provides strong evidence that stars form directly out of the dense, filamentary gas in molecular clouds.

Not only do the young stars trace the spatial distribution of the dense gas, but they also appear to have very similar motions. Numerous studies have measured the radial velocities (the speed of an object along the line of sight - i.e. towards or away from us) of stars such as these and found that they are almost identical to the velocities of the gas (e.g., Walsh et al. 2004, Hacar et al. 2016).

Figure showing the Orion A molecular cloud. The
background greyscale image show the dense gas
while the red dots show very young stars
(Credit: Megeath et al. 2012)

This picture where star formation occurs in dense gas is seen in all the star forming regions we've studied, and not just on small scales, but also on the much larger scales of giant molecular clouds such as the Orion A cloud, shown on the left.

This is important for understanding star formation because it not only tells us where stars will form, but also provides clues as to how stars form (i.e. the process requires very dense gas) and what must occur to set up the necessary conditions (i.e. the gas in molecular clouds must be compressed in dense filamentary structures that become self-gravitating).

It is also important for understanding star clusters because it tells us not just the initial spatial distribution of groups of stars that may go on to form clusters, but also their kinematics, i.e. how those stars are moving.

The next question to ask is how do these elongated and substructured groups of stars evolve into the classic centrally-concentrated images of star clusters we see so often? But that's a question for another day!

Gaia's first data release is here!

Last wednesday the Gaia satellite made its first major data release, exactly 1000 days after the satellite was launched back in 2013.

Gaia's view of 1 billion stars in the Milky Way (the strange arcs across the image are due to the way Gaia
scans across the sky and are not real) (Credit: ESA)

This first data release contains over 2 million stars with positions, distances and proper motions, as well as positions for over 1 billion stars. The astronomical community has been pouring over this data for the last week, and there are already many exciting results and revelations that have been announced, including the discovery of over 3000 variable stars that can be used to independently calculate distances, and over 2000 background quasars.

There's sure to be many more exciting results from this mission and its future data releases, so stay tuned!