Archive for the 'school' Category

HD 98800 – the polar circumbinary disk

As outlined in the paper and the press release (free PDF here), we have discovered that the HD 98800 quadruple star system hosts a disk that orbits at right angles to the binary it surrounds.

Here is Fig. 1 from the paper, which shows the setup. If the geometry of the disk around the inner binary doesn’t make sense, try taking a look at this simulation.

The HD 98800 system
The HD 98800 system

What does this show? The system has two binaries, A and B, which orbit each other at a distance of about the Pluto-Sun distance. Since both A and B are binaries, the individual stars are called Aa and Ab, and Ba and Bb. The “inner binary” BaBb is surrounded by a ring of gas and dust about the size of the Asteroid belt, this is shown in the right panel. All that is then orbiting the other binary, AaAb. The curious thing is that none of the orbital planes are aligned, and the BaBb plane is at right angles to the plane of the disk.

This system should prove very valuable to people that are interested in disk physics – here we can quantify the effects of the stars on the disk so compare the data with models. Of course, it’s possible that the disk is forming planets, and there is evidence that suggests this is underway, meaning that polar circumbinary planets may exist and be found in the future!




Paranal sunset


Paranal sunset




Discovery of the Fomalhaut C debris disc

This isn’t a photo for a change. It’s a graphic from a press release about my discovery of a debris disk around Fomalhaut C.

Artists illustration of Fomalhaut C, with Fomalhaut A in the background. Credit: Amanda Smith

Artist’s illustration of Fomalhaut C, with Fomalhaut A in the background. Credit: Amanda Smith

The story has travelled far, including (but not limited to) ABCastrobites, Centauri Dreams, CNES & CNRS (French), Discovery News, Natureslashdot & Universe Today.

Mauna Kea Frost




I’m in Hawaii up Mauna Kea doing some observing on the JCMT. Here’s the Keck laser guide star, click the image for a quick movie of the Earth turning.

Keck Laser

Keck Laser


Here’s the real reason I signed up for I’m a scientist, I needed a new mug!


Grant wins!

Very fun event where UK school kids get to quiz scientists in real time. Crazy two weeks of intense chats!

Herschel SPIRE time

16.5hrs of Herschel SPIRE time: check(0)

Thesis submission

Though I printed it a while ago, my PhD thesis was submitted yesterday. Thanks for three years of tax-free employment Australia!(0)

Final PhD talk

I gave my final PhD talk at Mt Stromlo the other day. It covered pretty much everything I’ve done during the last four years, which is documented on the PhD page.

I was exceptionally clever and used two laptops so I could have two slides up at a time. Of course I ended up with an extra 20 slides to explain… but it worked well for the double screen mode in our lecture theatre. The slides are here (main) and here (side).

This is secret…



Thesis printing

I printed my thesis last night, there was a nice sunset behind the AITC building.

stromlo sunset

stromlo sunset

ASA talk done: 3

Monday was the first day of the conference. I gave my talk and didn’t pay attention to much else until I did so…

The talk slides are here, for more info on my PhD go here.

Warrumbungle National Park

I’m at Siding Spring Observatory near Coonabarabran in NSW this week. I’m looking for planets via the transit technique for my friend Dan. The observatory is situated in a national park, with great views of the park and surrounding terrain.


There are more photos here.

Hot Super-Earths: accepted!

We have received the excellent news that my third paper has been accepted to ApJ. The paper is about possible mechanisms by which Earth-Neptune (low-)mass planets can reach very close orbits. Using standard models we find trends that might be found by future discoveries, and think about what we can learn from them.

Discovery of low-mass planets (which don’t have large Juipter-like atmospheres) will be particularly interesting, because they may be habitable due to (maybe) having solid surfaces. Unfortunately planets are much easier to find when they’re orbiting very close to their parent stars, and too hot to be habitable. Therefore, if they exist, the first decent sample of low mass planets will be discovered in short-period, close-in orbits.

It’s unlikely that planets in these orbits would form there, because it’s hard to form anything at all close to the star. Therefore, from a formation point of view, there are two main ways these planets
could get to close-in orbits after forming further out: by scattering off other planets, or by migrating through the disk out of which they form. In our paper, we show that planets that scatter will be hard to detect, and that migration is a better mechanism.

At present, very little is known about migration of planets in the “super-Earth” mass range, so discovery of these planets should tell us something about how migration works. Alternatively, we might not find any low-mass planets in short-period orbits, which would tell us that migration doesn’t work how current wisdom says. So either way we learn something!

The paper is posted on astro-ph for now, until the journal publishes it.

Third paper resubmitted

Thanks to a nice referees report, and some simulations finally finishing, I submitted a revised version of our “Hot super-Earths” paper this morning. Hopefully all will be well and that will be number three!(0)

Hot Super-Earths: submitted

So any complaints from referees aside, I’ve submitted a paper today, which gives me something to show for my time here in Boston!(0)

Paper accepted!

After four revisions and seven months of extra (part time) work, my second planet formation paper has been accepted, finally! To sum up the paper in a few sentences…

We think that the likelihood of a star forming at least one gas giant planet increases with the mass of the star. There are two reasons for this: Firstly, the mass of the disks surrounding these stars seems to increase with the mass of the star, and therefore so do the masses of protoplanets that form. Secondly, there is probably a threshold protoplanet mass at which gas accretion occurs and giant planets form. Higher mass stars can therefore form protoplanets above the gas giant forming threshold (and therefore gas giants) more easily, because on average they have more material to do so.

The preprint can be found here.

Planet formation around stars of various masses: The snow line and the frequency of gas giants

This paper was just accepted to the Astrophysical Journal. It considers a simple disk and snow line model, and uses this to predict the likelihood of stars of different masses harbouring gas giants. We think gas giant frequency increases with stellar mass, which is consistent with observations at present.

In summary, observations indicate that disk mass changes roughly with stellar mass, with a wide distribution of masses at a given stellar mass. Therefore more massive stars on average have more material available for planet formation. In addition, there appears to be a threshold protoplanet mass for forming gas giants of about ten Earth masses. Around more massive stars, a higher fraction of the disks can form protoplanets greater than ten Earth masses, so these stars are more likely to form gas giants.

Recent observations by John Johnson indicate a trend of increasing planet frequency with stellar mass, as our paper predicts. Future observations will improve statistics to solidify this result. In the more distant future, planet discoveries will find how processes like migration and scattering have influenced the distribution of planets we can see now.

The paper can be found through the NASA Astrophysics Data System. For more information on my PhD have a look at the main PhD page.

Planet formation around M Dwarfs: The moving snow line and super-Earths

This paper, just accepted to the 5SS conference proceedings, is similar to the ApJ letter here. It uses a smaller increase in surface density at the snow line, based on modern Solar abundance figures for oxygen. A brief comment on type I migration has also been added.

The paper can be found through the NASA Astrophysics Data System. For more information on my PhD have a look at the main PhD page.

A typical day at the CfA…

So I imagine people wondering, what is a typical day at Harvard-CfA like for Grant? Here is my ‘typical’ day…

So after wandering down to school through a crisp morning with patches of snow, I find there are some people I know passing through. I go and meet Anna who lives in Texas, and also see Martin and Ken from Australia. I then go to a talk by a guy from Caltech who does cool theoretical planet work, and later to a talk by another guy who does cool observational planet work. Finally I go out to dinner with more people who do things I’m interested in. And the best thing: dinner was free, thanks ITC!

Meeting three or four people in a day who do research directly related to what I do is very cool, and hasn’t even come close to happening in two years in Australia.

Of course some days I just sit at my desk at play with my computer…

Aus-VO Summer School

I’ve just spent three days in Sydney at the Aus-VO Summer School learning about the programming language python, and the capabilities of the ‘virtual observatory’. About 50 people sat in a room, all with laptops in front of them and tapped away, with breaks for nice food in between!

the rest of the geeks

Mt Stromlo on the rebuild III

Three years later the burnt out domes are getting repainted, they look very white!

very white indeed

Mt Stromlo on the rebuild II

Work is well underway on rebuilding the Commonwealth Solar Observatory (CSO), where people here used to have bbq’s every second week. The painted silver domes have been unveiled recently, and look very shiny!


Stromlo Student Seminars

The 4th annual Stromlo Student Seminars were held Thursday and Friday last week, and were a resounding success.

Thanks to all those who made the effort to come from elsewhere and contribute to a great couple of days of student science.

See this post for notes on the social side of things…

Mt Stromlo on the rebuild!

We have recently had our new Advanced Instrumentation and Technology Center (AITC) opened. I’ve been watching it come together from my office window, and put together a little time lapse animation of photos taken with my phone.

Have a look at the animation here (19Mb).

my room's view

more press release madness

On Monday an ANU press release went out and all hell broke loose. Well I did three radio interviews and made it onto PM, Radio National’s evening news which is quite an achievement aparrently.

My 15 minutes of fame is now over. There are a bunch of links to how my story got pasted over the internet on my PhD publications page.

It even made it into Nature’s research highlights!

Planet formation around low-mass stars: The moving snow line and super-Earths

Our first paper is about how the snow-line in a protoplanetary disk can move due to evolution of the central star, and how it helps the formation of super-Earth mass planets around M Dwarfs. Kennedy, Kenyon & Bromley (2006) uses a simple disk evolution model, with which we are starting to link formation of planets with that of their parent stars.

As a low-mass red dwarf is born it contracts to its main sequence size over hundreds of millions of years. This contraction limits the radiating area of the star, and so it becomes fainter, and the surrounding disk in which planets form becomes cooler.

The cooling of the star means that the distance where ices condense (known as the ‘snow line’) moves inward. In the same way that it is easier to build a snowman above the snow line on a mountain, the presence of ices in the planet forming disk makes it easier to build planets. These planets can be several times larger than our Earth, and are largely made up of ice, roughly similar in structure to Neptune.

The paper had an associated press release, and subsequently generated a bunch of media attention

ANU Press release | CfA Press Release | Uni of Utah press release | Universe Today | Spaceflight Now | Space Daily | Centauri Dreams | PhysOrg | Unexplained Mysteries | Astrobiology Magazine | SciFi Source Book | | Science Alert | ABC Radio National | ABC Canberra | Nature

The paper can be found through the NASA Astrophysics Data System. For more information on my PhD have a look at the main PhD page.

Is doing a PhD really that bad?

I can’t help but wonder whether all the worry about doing a PhD (like this, and this for example) is self perpetuating. Everyone thinks it’s going to be a killer from day one, and reads comics that promote this, rather than ones that are just naturally awesome

I don’t have to sit at my computer all the time (though I do sometimes).
I get paid plenty.
My supervisor is really nice.
I like what I’m doing.
Everyone at my school is nice too.
I have time to go to the beach in summer.
I also have time to do silly things.

What is everyone so worried about? So far it’s been a fun time where I get paid to learn stuff, I even have a few bucks left over to buy iPod, digital SLR, cellphone, car, holidays, bling etc…

My office

For those who heard me say ‘If I ever get a desk job you can kick my ass’ now may be the time to do it. This is my desk…

my desk

Note the cool tray thing for papers at the left, the high tech laptop support, old SIRTF poster, and the essential Sony Pro headphones…

Stromlo BBQ raffle winner!

To those who question whether it’s worth buying raffle tickets at the Stromlo barbeques I say:

490g Cadbury milk chocolate egg with assorted milk & dark chocolates

–thanks Jose, I’ll pay you later…

Fearless team leader

After only a year at Mt Stromlo I find myself in a position of real power. I have become team leader of BBQ Team 2. I have eight chances to inspire my humble group of astronomers to new levels of greatness and supply the punters here with a BBQ they have never seen the likes of before.

And I get to waste most of my eighth Fridays organising it…


Today I have officially started my doctorate. Well it feels like it anyway since I now have a whole new body of work that isn’t related to my comet work in any way.

I’m looking at stellar evolutionary tracks with the aim of tying these to planet formation.


if you’ve seen the news about there being a lot less comets these days and wondered what all the fuss was about, you can go have a look at Paul’s web page.

my own work will be along before xmas (or else!)