After an amount of time that I'd really rather not think about, my most recent astronomical paper has been published. This paper is particularly special because it is likely to be my last astronomical paper (at least as first author).
I thought that this paper would be good to blog about not only because the subject matter is interesting in itself but because I thought that the story of this paper's journey to publication might make an interesting one. In an effort to keep my posts at a readable length as well as milking a subject for all it's worth, I'm going to split the post into two, the first dealing with the subject of the paper, the second with the story of how the paper was published.
Part I
As this is a blog and intended to be at least partly accessible to the average member of the public, I am only going to write about the subject of my paper in fairly broad terms. If you're interested in the details of the experiments, analysis and so on, please go and read the paper for all the gritty details. It is hosted on the Monthly Notices of the Royal Astronomical Society site here.
A relatively succinct description of the paper might be that we were interested in the potential differences between the Perseus molecular cloud, seen below and the W3 molecular cloud seen, um... belower.
The reason we're interested in these differences is that both clouds are host to a lot of young new stars still in the process of forming. However, in Perseus these 'protostars' are generally quite small (around the size of our Sun or smaller) while in W3 they are generally considerably bigger (up to tens of times the size of our sun). Figuring out why one region make the big stars that can disrupt the Galaxy for light years around, while another is happy just churning out smaller, longer-lived stars, can give us insight into the way in which our own solar system formed as well as where we might expect to see future star systems possibly hosting Earth-like planets.
Do big stars form in the same way (more or less) that small stars do? Is it just coincidence and/or chance that big stars form where they do or is there some underlying difference about say, the Perseus cloud versus the W3 cloud that leads to them forming stars with different properties? This question is one that has been leading a large part of star formation research over recent years.
By investigating this question we found that the 'dusty clumps' that are the core or nugget of star formation in these clouds do have different appearances, which we suggest implies that they have come from different underlying structures. Our images of sites of star formation, like this -
are like looking at an island in a sea. Picture the island sticking up out of the sea in a way that the height of the island corresponds to the brightness of our star-forming region which, when you're looking at them in the way that we are, implies the density of the protostar.
You can see here that the surface of the island might continue below the sea so, let's say the sea level rises to the height shown by the dashed line in my cartoon. All of a sudden we can't see as much of the island, though the island could be said to be just as big as it was before. Star formation is much like this, the protostar is a big clump of dust and gas which sits in a large 'sea' of dust and gas. The larger 'sea' makes it hard for us to determine exactly how big our island is and what it looks like. In order to determine anything from our observations we have to go off what small part of the island we can see, as well as the properties of the sea which we might be able to determine from other observations.
Well, to torture this metaphor a little further, when we look at the stars that are forming in Perseus, we see small islands in large seas. When we look at W3, we see that the islands are quite a bit bigger, even though the seas are around the same level. This implies that the two regions may, in fact be forming stars in quite different ways. A model which might fit our observations has been suggested recently by Phil Myers, who suggests that smaller stars form by drawing in local material, while larger stars form by pulling in more material from the greater store available in the larger cloud.
While our study is quite speculative, it does tell us something important, which is that when we compare different types of observation, we have to be very careful in our assumptions about what we are looking at. The 'island' which we are looking at can often be assumed to bear some relation to the 'sea' in which it is sat. However, the relationship which exists between one set of islands in one sea cannot always be assumed to exist when looking elsewhere. Our paper implies is that there is a distinct difference between the underlying structures surrounding the protostars in each region. If this is the case then the ways in which big stars and little stars form is actually quite different. Maybe this is due to different conditions at the outset, or perhaps other factors come into play which affect the ways that stars form and these factors vary from place to place within our Galaxy. What is clear is that we must be careful when we perform our large scale surveys of the Galaxy, such as the Herschel Space Observatory's Galactic Plane Survey HiGAL. If we are not careful to take account of the underlying differences between star-forming regions in our Galaxy, then we risk misinterpreting results and potentially drawing false conclusions.
I think it's best to leave my convoluted island metaphor there for now. A bit more on the actual observations and physics that we used to make our observations is below, please feel free to read it if you have made it this far! Also, I'd be more than happy to answer any questions you might have in comments.
To investigate these two clouds we looked at them both in two quite different ways, both invisible to the human eye. Firstly we looked at the big clumps of 'dust' that host the protostars in the clouds using the SCUBA instrument on the James Clerk Maxwell Telescope in Hawaii, due to be decommissioned towards the end of this year. This dust is warm by comparison to most of interstellar space at about - 250 °C and at that temperature shows up well at submillimetre wavelengths, well outside of the range of human vision. Looking at this dust tells us a few things about the protostar (usually multiple ones) forming in the molecular clouds, typically we can determine the approximate temperature and mass of them, although some careful analysis and modelling can sometimes tell us more.
In addition to these dusty temperature maps that we made of the young stars in Perseus and W3, we looked at them using the Green Bank Telescope, another (ENORMOUS!) great telescope in danger of being shut down (visit http://www.savethegbt.org to learn more and help save it!). The GBT has a great array detector which is able to map out these young stars using molecular spectroscopy. In this case, the molecule in question is ammonia, found in abundance in many protostars. The places where stars form in our Galaxy may be hot and dense compared to what we call 'empty space' but they are still astonishingly cold compared to Earth and are at a density equivalent to the best ever vacuum achieved by mankind on Earth! These extraordinary conditions mean that ammonia (amongst other molecules) can exist in states that would never be found in normal human experience. This turns out to be very lucky for astronomers as careful analysis of the radiation emitted by ammonia in these states can tell us not only what the temperature of the forming stars is but also what the density is as well as how the gas is moving around within the clouds.
Not only are these two ways of examining protostars very informative on their own but by comparing them we can learn even more. Crudely speaking, by comparing the temperature we find using one method with the temperature we find from the other, we can infer how close a protostar is to becoming a fully-fledged star and what kind of size that star might be. As you can imagine this is a powerful technique that has been used in astronomy for some time in various guises (usually referred to as the virial mass or virial ratio).
Part I
As this is a blog and intended to be at least partly accessible to the average member of the public, I am only going to write about the subject of my paper in fairly broad terms. If you're interested in the details of the experiments, analysis and so on, please go and read the paper for all the gritty details. It is hosted on the Monthly Notices of the Royal Astronomical Society site here.
A relatively succinct description of the paper might be that we were interested in the potential differences between the Perseus molecular cloud, seen below and the W3 molecular cloud seen, um... belower.
 |
| Credit to APoD - http://apod.nasa.gov/apod/ap111021.html |
 |
| Credit to the Herschel Observatory - http://herschel.cf.ac.uk/results/w3-star-forming-region |
Do big stars form in the same way (more or less) that small stars do? Is it just coincidence and/or chance that big stars form where they do or is there some underlying difference about say, the Perseus cloud versus the W3 cloud that leads to them forming stars with different properties? This question is one that has been leading a large part of star formation research over recent years.
By investigating this question we found that the 'dusty clumps' that are the core or nugget of star formation in these clouds do have different appearances, which we suggest implies that they have come from different underlying structures. Our images of sites of star formation, like this -
Well, to torture this metaphor a little further, when we look at the stars that are forming in Perseus, we see small islands in large seas. When we look at W3, we see that the islands are quite a bit bigger, even though the seas are around the same level. This implies that the two regions may, in fact be forming stars in quite different ways. A model which might fit our observations has been suggested recently by Phil Myers, who suggests that smaller stars form by drawing in local material, while larger stars form by pulling in more material from the greater store available in the larger cloud.
While our study is quite speculative, it does tell us something important, which is that when we compare different types of observation, we have to be very careful in our assumptions about what we are looking at. The 'island' which we are looking at can often be assumed to bear some relation to the 'sea' in which it is sat. However, the relationship which exists between one set of islands in one sea cannot always be assumed to exist when looking elsewhere. Our paper implies is that there is a distinct difference between the underlying structures surrounding the protostars in each region. If this is the case then the ways in which big stars and little stars form is actually quite different. Maybe this is due to different conditions at the outset, or perhaps other factors come into play which affect the ways that stars form and these factors vary from place to place within our Galaxy. What is clear is that we must be careful when we perform our large scale surveys of the Galaxy, such as the Herschel Space Observatory's Galactic Plane Survey HiGAL. If we are not careful to take account of the underlying differences between star-forming regions in our Galaxy, then we risk misinterpreting results and potentially drawing false conclusions.
I think it's best to leave my convoluted island metaphor there for now. A bit more on the actual observations and physics that we used to make our observations is below, please feel free to read it if you have made it this far! Also, I'd be more than happy to answer any questions you might have in comments.
To investigate these two clouds we looked at them both in two quite different ways, both invisible to the human eye. Firstly we looked at the big clumps of 'dust' that host the protostars in the clouds using the SCUBA instrument on the James Clerk Maxwell Telescope in Hawaii, due to be decommissioned towards the end of this year. This dust is warm by comparison to most of interstellar space at about - 250 °C and at that temperature shows up well at submillimetre wavelengths, well outside of the range of human vision. Looking at this dust tells us a few things about the protostar (usually multiple ones) forming in the molecular clouds, typically we can determine the approximate temperature and mass of them, although some careful analysis and modelling can sometimes tell us more.
In addition to these dusty temperature maps that we made of the young stars in Perseus and W3, we looked at them using the Green Bank Telescope, another (ENORMOUS!) great telescope in danger of being shut down (visit http://www.savethegbt.org to learn more and help save it!). The GBT has a great array detector which is able to map out these young stars using molecular spectroscopy. In this case, the molecule in question is ammonia, found in abundance in many protostars. The places where stars form in our Galaxy may be hot and dense compared to what we call 'empty space' but they are still astonishingly cold compared to Earth and are at a density equivalent to the best ever vacuum achieved by mankind on Earth! These extraordinary conditions mean that ammonia (amongst other molecules) can exist in states that would never be found in normal human experience. This turns out to be very lucky for astronomers as careful analysis of the radiation emitted by ammonia in these states can tell us not only what the temperature of the forming stars is but also what the density is as well as how the gas is moving around within the clouds.
Not only are these two ways of examining protostars very informative on their own but by comparing them we can learn even more. Crudely speaking, by comparing the temperature we find using one method with the temperature we find from the other, we can infer how close a protostar is to becoming a fully-fledged star and what kind of size that star might be. As you can imagine this is a powerful technique that has been used in astronomy for some time in various guises (usually referred to as the virial mass or virial ratio).
