Poly-Web: A web-based polytrope calculator Nov 18 2009, 09:36

To help the students in my course develop better insights into simple stellar models, I've created a web-based code that constructs polytropes by solving the Lane-Emden equation for arbitrary polytropic index n. This Poly-Web tool provides a useful companion to EZ-Web, my web-based stellar evolution code.

Mad Star now has caching Oct 13 2009, 12:43

To address concerns I've had over the time taken to load pages on mad star (esp. the EZ-Web page, which has large tables), I've spent the morning implementing caching in the parsing engine. It all seems to work OK, and the speed-up was well worth the effort. But I really should do some science sometime...

I'm hiring a new Post-Doc Oct 9 2009, 15:51

Over the coming year I'll be hiring a Post-Doctoral Research Assistant, to form the core of the team for my NSF project modeling wave transport of angular momentum. Full details will be appearing in the November 2009 issue of the AAS Job Register, but here is the general description:

Interested applicants needn't wait until the advert appears in the Job Register; I'm accepting applications right now.

New paper: 'Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration' Oct 1 2009, 23:27

This letter, just been accepted for publication in Monthly Notices of the RAS, reports the discovery of a ~1.6 kG dipole magnetic field in HD 57682. The detection was made using the ESPaDOnS spectropolarimeter mounted on the Canada-France-Hawaii Telescope. The observations comprise part of MiMeS (Magnetism in Massive Stars) project that I'm involved in, one of the Large Programs currently being undertaken on the CFHT.

Personally, I can take very little credit for the paper — it's thanks to Jason Grunhut and the other observers that we obtained this great detection. But of course, I'm excited by the prospects for furthering our theoretical understanding of magnetism in massive stars; we now know of fields in four O-type stars, indicating that they are not so rare as once believed.

EZ-Web is working again Aug 25 2009, 11:10

After a few days of heavy hacking with PHP, I've managed to get EZ-Web working on my server here at Madison. EZ-Web is a simple, web-based interface to a stellar evolution code, that can be used to calculate stellar models over a wide range of masses and metallicities. In this updated version, I've included the ability to evolve low-mass stars beyond the Helium flash; in fact, these stars can now be followed all the way up the AGB. I'll be making extensive use of EZ-Web when I teach Astronomy 310 this fall.

The GRASSY is greener... Aug 10 2009, 19:37

Looks like luck is smiling on me right now; the other NSF proposal I submitted last November has also been approved for funding. This project falls under the aegis of NSF's Advanced Technologies & Instrumentation program. The plan is to build a computer platform for very fast spectral synthesis, to be used in analysis of observations of pulsating stars. The platform, GRASSY (GRaphics processing unit-Accelerated Spectral SYnthesis), will be build using graphics processors such as the Tesla unit in tedesco. These specialized pieces of hardware are extremely fast at interpolating values in pre-calculated tables, and have many (hundreds) of cores for parallel execution — just what is needed for spectral synthesis.

This is a joint project with Karu Sankaralingam from Computer Sciences, and will be involving two Grad Students (one from each department). It's a little unusual in its nature, but promises to be a lot of fun!

New paper: 'Toward self-consistent angular momentum transport in pulsating massive stars' Jul 20 2009, 14:53

I've just completed the write-up for the Santa Fe pulsation meeting. This short conference paper reports on progress with HEIMDALL, my prototype code for simulating wave transport of angular momentum inside pulsating massive stars. There's a potentially interesting finding that the sub-surface shear layer formed by unstable g modes actually serves to stabilize these modes, in a negative feedback loop. This may help shed some light on the mechanism(s) responsible for selecting which unstable modes grow to observationally detectable amplitudes.

MOST detects pulsation modes in Rasalhague Jun 10 2009, 11:00

During last week's pulsation meeting in Santa Fe, I received news from John Monnier that our MOST observations of Rasalhague (α Ophiuci) have detected δ Scuti-like pulsations. With just one day of monitoring, 30 or so modes have already popped up, and the expectation is that there will be 100 or so detected during the full run. This is a very welcome discovery, since we now have a good chance of undertaking detailed modeling of the effects of rapid rotation on the star's oscillation spectrum — a first for a δ Scuti pulsator.

'Wave-Transport' project gets NSF support Jun 7 2009, 21:14

I found out recently that one of the NSF proposals I submitted last November has been approved for funding. This project will look into wave transport of angular momentum — a phenomenon I've been hoping to explore for some time now. Such transport happens in the Sun and other low mass stars, mediated by stochastically excited gravity waves. I'm interested in whether the same thing can happen in massive stars, but in this case with global g modes driven by the iron opacity bump. (By coincidence, I've just returned from a pulsation meeting in Santa Fe, NM, where I presented some preliminary calculations along these lines).

The NSF award brings with it full support for a PostDoc, and partial support for a Grad Student; so I hope to be hiring come this fall. Watch this space...

MiMeS 2 Meeting May 11 2009, 19:36

I'm currently attending the MiMeS 2 meeting, which is taking place at the beautiful Observatoire de Paris. This meeting brings together a large group of collaborators, working on both observational and theoretical aspects of magnetic massive stars.

We've only had one day of the meeting so far, but I've already learned a lot about crosstalk on spectropolarimeters (and why I shouldn't worry about it); seen convincing reports of a magnetic field in a Be star (a first, I believe); and enjoyed a very nice talk by Mary Oksala, presenting new surface abundance maps of σ Ori E. My spirits are being kept high both by the lovely Paris ambience, and by sharing a room with someone who likes good food as much as I do. Bon apetit, David Cohen!

Suzaku proposal accepted Mar 8 2009, 21:22

David Morris informs me that our Suzaku proposal to study wind structure in high-mass X-ray binaries (HMXBs) has been accepted. (Full disclosure: I was very much a johnny-come-lately on this proposal, joining the team the day before the deadline!). The idea is to look for fluctuations in the X-ray brightness of the accreting compact object (neutron star or black hole) that arise when wind clumps pass through the line of sight. From measurements of the characteristic fluctuation timescale and amplitude, the hope is that we'll be able to obtain constraints on the size and density of the clumps — information that is sorely needed to unlock the mysterious origins of massive-star wind clumping.

My contribution to the project will be at the modeling end. A few months ago, some interesting conversations with Maurice Leutenegger (who is also on the Suzaku proposal, and in fact responsible for roping me in) led me to develop a clump variability code. It's nice to see that there are some real data on the horizon that the code can be used to interpret!

RRM movies uploaded Mar 6 2009, 08:11

One of the reasons I created mad star was to collect together the various movies I create in the course of my research. As an initial step in this direction, I've uploaded a grid of movies based on my Rigidly Rotating Magnetosphere (RRM) model, spanning a range of rotation rates ω/ω_c, observer inclinations i and magnetic obliquities β. Each movie shows the star and surrounding magneotspheric matter distribution, viewed over a complete rotation cycle. The movies can be downloaded individually or in groups, from here.

Supernova sweep-up of a magnetized wind Feb 21 2009, 09:17

At the suggestion of Bryan Gaensler, I've been exploring what happens when a supernova (SN) explodes into a pre-existing, magnetized wind. If the progenitor star is rotating, then we can expect the magnetic field threading the wind to be wound up into an Archimedean spiral — much as happens for our own Sun. When the SN occurs, the wind will be swept up into a shell, and inside this shell the magnetic topology should be largely toroidal. This toroidal shell may be able to explain the unusual distribution of Faraday rotation seen in radio observations of some SN remnants (see Harvey-Smith et al. 2009).

To investigate this idea, I've run some MHD simulations on Medusa using the freely available Athena3D code. The initial state consists of a constant-velocity wind threaded by a 'split-monopole' radial/azimuthal field, which is outward in the northern hemisphere and inward in the southern hemisphere. When this initial state is disrupted by dumping a large quantity of thermal energy into a central sphere (basically, an SN explosion), the wind is indeed swept up into a shell, and the field in this shell is toroidal. However, as can be seen from this PostScript contour plot of the azimuthal field, there are north-south sign reversals in the direction of the field, which aren't seen in the observations. So, the jury is still out on this one...

New paper: 'Gamma-ray Variability from Wind Clumping in HMXBs with Jets' Feb 8 2009, 00:58

The paper has just been accepted for publication in the Astrophysical Journal; it discusses γ-ray variability in so-called 'microquasars'. These are a subclass of high-mass X-ray binaries where jets from the compact object hit the wind of the companion star, producing pions that quickly decay into γ rays. If the wind is clumpy — as most massive-star winds are — then the γ ray production is expected to be variable, and observations of this variability can in principle help us to constrain the nature of the clumping.

My small contribution to the paper was figuring out that if we know the statistical properties of how a single wind clump interacts with the jet, then we can easily figure out how a whole ensemble of clumps behaves. The real work, however, was done by Stan Owocki and Gustavo Romero, and they deserve most of the credit for the paper.

Back from the mountain Feb 3 2009, 23:23

Fernando and I are now back from Kitt Peak, after a largely successful observing run. In addition to monitoring V* NU Ori and V* LP Ori (the two magnetic stars discovered by Petit et al. 2008), we had a look at FN CMa for a night, to see if we could get a handle on some weird variability that Thomas Rivinius told me about recently. We'll have to see what the data show.

Because our science targets were in Orion and Canis Majoris, they set around 2:30am, three and a half hours before sunrise. We used some of this 'spare' time to undertake service observations, but for the rest we went after some pretty galaxies. The image to the right is a composite of M81, based on the Johnson BVR filters plus an Hα filter. The latter very nicely picks out the star-forming regions in the spiral arms of the galaxy. A full-sized version of the image can be grabbed here; kudos to John Barentine for assembling the image from its constituent parts while we were up on the mountain. Stay tuned for similar images of M82 and M104.

RSS 2.0 feed added Jan 31 2009, 17:49

At the suggestion of Ryan Fox, one of my former Astronomy 310 students, I've put together an RSS 2.0 feed for the news component of the website. I'm a bit of a n00b with RSS, and I found these instructions extremely useful in figuring out what I had to do.

The feed can be accessed via the item on the right-hand menu. I'd appreciate hearing any thoughts on whether the feed might benefit from a bit more content — for instance, a brief synopsis of each news story.

In other news: mad star is now using a completely rewritten parser. Internally, text is stored in a format very similar to that used by Wikipedia; the parser has the job of converting this text into XHTML, suitable for display on a web browser.

A theorist goes observing Jan 31 2009, 07:39

Right now, I'm sat with Fernando Cardoso in the control room of the WIYN 0.9m telescope at Kitt Peak National Observatory. Although most of my research is theoretically oriented, I try to remain in touch with the experimental side of things — which leads me now and then to go off on an observing trip. This particular adventure came about due to 6 unscheduled nights on the 0.9m.

Fernando and I have mostly been undertaking differential CCD photometry of a couple of massive magnetic stars in Orion: V* NU Ori and V* LP Ori. The fields in these stars were only recently detected (see Petit et al. 2008), and we're looking to see whether the stars show the same sorts of periodic photometric variations as other, similar objects.

First light on Medusa Jan 24 2009, 10:42

I've just completed my first successful set of calculations on Medusa, the new computer cluster that the Theory Group recently purchased. Medusa comprises 72 nodes of 8 AMD Opteron processors each, linked together with an InfiniBand networking fabric. There have been a few teething problems, but everything seems to be working OK now.

The test calculations involve Monte-Carlo simulation of radiative transfer through the electron-scattering magnetosphere of σ Ori E. I'm trying to reproduce the temporal variations in the star's linear polarization, originally observed by Kemp & Herman (1977). I've made some Stokes IQU images of the simulation results, and strung them together as a movie (AVI; MOV; movie help).

New paper: 'An Exact Integration Scheme for Radiative Cooling in Hydrodynamical Simulations' Jan 20 2009, 15:19

The paper has just been accepted for publication in the Astrophysical Journal Supplement Series, and will be appearing in the August 2009 issue. It discusses a new method for implementing optically thin radiative cooling in hydro/MHD codes, which constitutes a big improvement over previous approaches. The method came to me when I was figuring out how to speed up cooling in VH-1; I had recently been reading a computer graphics article on speeding up Phong shading using look-up tables, and I realized that a similar approach could be used for the cooling calculation. The beauty of using look-ups is that they're not only fast, they're also exact.

MOST proposal accepted Jan 19 2009, 12:16

John Monnier informs me that our MOST proposal to observe Altair and Rasalhague has been accepted. These are both rapidly rotating A stars, known from interferometry to be oblate due to the centrifugal force. The objective of the observations is to look for δ Scuti-like pulsations (Altair is already know to pulsate), and to use the pulsations to place constraints on the internal rotation.

This will involve my developing a new pulsation code that can handle differential rotation — quite an undertaking, but something I've been meaning to do for nearly a decade now. The new computer recently purchased by the Theory Group will be the perfect platform on which to run the code; so, all that's required now is a few months of furious programming.

Getting dirty with CUDA Jan 18 2009, 09:48

I've finally been getting around to analyzing the MOST observations of σ Ori E. These span 3 weeks and comprise around 25,000 individual photometric measurements — a very impressive dataset. I'm using the CLEAN algorithm (Roberts et al. 1987) to derive the Fourier spectrum; the usual Fast Fourier Transform (FFT) approach can't be used, because the time sampling isn't uniform. The first part of the algorithm involves calculating the dirty spectrum, and can be quite computationally expensive — typically scaling as N^2. So, partly as a learning exercise, I've coded up a CUDA version to run on the new Tesla unit inside Tedesco.

Initial results seem quite impressive; it takes around 50 milliseconds 60 seconds to calculate the dirty spectrum for the whole MOST dataset. There are still a few accuracy issues to be addressed, but using the Tesla looks like a very promising approach to analyzing large time-series observations efficiently.

David Cohen is visiting Jan 13 2009, 11:41

David Cohen is visiting for a few days, having flown over from Swarthmore College on Sunday. We're working on a variety of topics, including modeling the X-ray line profiles of porous massive-star winds, and investigating the two new magnetic B stars discovered by Petit et al (2008). It seems one of these stars may be a helium-strong star, like σ Ori E; and both of them show measurable X-ray emission.

Geochron ported over Jan 7 2009, 13:03

I've ported my geochron over from my old (and soon to be deleted) UCL website. With a little poking, it seems to be working fine; but any reports of breakage would be welcomed.

Tesla C1060 installed Jan 6 2009, 23:00

Through their Professor Partnership Program, NVIDIA have very kindly lent me a Tesla C1060 Computing Processor This is basically a graphics card without the usual monitor socket; it contains 240 separate cores optimized for SIMT parallel computation.

My plan is to use the Tesla to speed up interpolation of specific intensities in pre-computed grids (although there are other potential applications that are catching my eye). After some creativity with power cables (the beast requires 2 extra PCI Express power connections), it is now installed in tedesco, the 8-core workstation under my desk. Preliminary benchmarks indicate a bilinear interpolation throughput of around 19 billion interpolations per second. Which is nice.

Website goes live Jan 6 2009, 22:00

After a week or so spent hacking around with PHP and MySQL, I've finally got this website in a state where it can go live and I can start adding content. Voila, you are reading the very first post in the news/home page section.