Institute for Computational Astrophysics

Previous Images of the Month - 2012

January February March April May June July August September October November

December 2012

click on image to enlarge

The Case of the Distorted Pulsating Star

This simple plot shows something that, as of this writing, we do not understand. In the terminology of the field, the plot is an echelle diagram of a pre-main-sequence star that compares observations, black symbols and red crosses, with a stellar model fit, the open green circles. Pre-main-sequence stars are young stars whose cores are not yet hot enough to initiate the nuclear burning that powers most stars. Their energy is derived almost entirely from gravitational collapse energy. Some pre-main-sequence stars vibrate (called pulsating PMS stars). With sensitive telescopes placed above the earth’s atmosphere, like Canada’s MOST telescope, we can observe the minute variations in the luminosity of the star as it pulsates. Asteroseismologists commonly represent the spectrum of vibrations (called oscillations) in an echelle diagram like that shown. In such a diagram the oscillation spectrum of most pulsating stars will align along a vertical sequence of “ridges.” Here the model fit shows two sequences labeled l = 0 and l = 1, representing radial and first harmonic nonradial oscillations.

The mystery is related to the blue symbols. It was discovered by Mike Casey, Ph.D. graduate (2012, Saint Mary’s University), that for stars similar to the one shown, averaging the nearby black symbols, which, again, are the actual observations of oscillations on the PMS star, yields a strong fit to the stellar model.

The star shown here is HD 261771, which was observed (Dr. Konstanze Zwintz, University of Vienna) with the space telescope CoRoT in 2008 and again in 2011 (yielding identical results (the red crosses represent 2008 data and the black symbols 2011 data)). The blue symbols locate the averaging of nearby observed points. We do not know why the averaged points fit the model. We do not know what is causing the spread in frequencies within each group. And we do not know why the spread itself decreases with increasing frequency.

A Metaphor: A struck guitar string sounds a single note. Actually, the single note is made up of a fundamental vibration and a series of harmonics. The harmonics give the guitar its characteristic sound. Most pulsating stars act like the guitar string vibrating at not just a single frequency but the fundamental and all its harmonics. All the frequencies appear along ridges in an echelle diagram. The spread in frequencies that exists in the pulsating PMS star HD 261771 is similar to connecting a distortion box and pedal to an electric guitar. The pure sound of the guitar is distorted. For HD 261771 some unknown process is distorting its vibrations.

David Guenther (Saint Mary’s University), Mike Casey (Saint Mary’s University), Konstanze Zwintz (University of Vienna).

November 2012

click on image to enlarge

The interaction of radial pulsation and convection has been a long standing problem in stellar astrophysics. This interaction becomes especially important as convection strengthens as the effective temperature of RR Lyrae stars approaches the effective temperature of the "red edge" of the RR Lyrae instability strip from the hotter side. The "red edge" is the cool side of the region in temperature and luminosity space where RR Lyrae stars pulsate. Recent work by Marconi et al. (2007) showed that while a 1D time dependent mixing length theory approach works reasonably well for matching observed light curves of RR Lyrae much hotter than the red edge, it has difficulty reproducing light curve shape and amplitude simultaneously near the red edge, in particular for variable star v120 in M 3 (See figure 17 of Marconi et al. 2007, A&A, 474, 557 for their fit to this star's light curve).

The image above shows the phased, observed light curve from Cacciari et al. (2005) for v120, doubled to show two cycles for easier comparison. It is matched by the small cyan points for a 6300 K effective temperature model. The model is two dimensional with 20 horizontal zones covering 6 degrees of the simulated star's surface. The model light curve points are from seven consecutive periods at fully amplitude which have been phased and doubled in the same manner as the observations. Bolometric corrections from Bessell et al. (1998) have been used to convert the models absolute bolometric magnitude to absolute visual magnitude and a distance modulus of 15.34 has been chosen to allow comparison with the apparent visual magnitude of the observed light curve. The small spread in the model light curve points indicates how well the pulsation cycles reproduce. In fitting the light curve only the phase shift and distance modulus were adjusted. We also did not perform a detailed parameter search to find the best fundamental model parameters for matching this light curve but rather used values representative of the cluster as a whole. The fit is a noticeable improvement over the 1D time dependent mixing length approach for this cool RR Lyrae star.

These results are based on a 2D hydrodynamic simulation with the SPHERLS code, which is being developed in the ICA by Ph.D. student Chris Geroux under the guidance of Dr. Robert Deupree, ICA Director.

October 2012

click on image to enlarge

One of the main challenges in observational cosmology is trying to understand the formation and evolution of galaxies and to test theoretical models of galaxy formation at every redshift. Building large statistical samples of galaxies allows us to explore galaxy populations at early epochs and to develop a more complete view of the evolution of structure in the universe.

This image, made by ICA graduate student Liz Arcila-Osejo and her supervisor, Dr. Marcin Sawicki, is called “Rest-frame R Luminosity Function of passive and star-forming galaxies” and it basically represents the frequency of galaxies at a given luminosity per Mpc3 for two different populations at redshift z ~ 2: Star-forming (blue) and Passive (red) galaxies as a function of the absolute magnitude of the galaxy in the rest-frame R band.

Using a modified version of the BzKs selection technique; the gzHKs selection criteria allowed us to classify and discriminate between these two populations at z ~ 2.

Data for this graph were obtained from the Canada France Hawaii Telescope Legacy Survey (CFHTLS), which represent four independent fields in the sky (called D1, D2, D3 and D4), with a large effective area of ~ 2.5 deg2.

Photometry was performed thanks to the computational capabilities of Mahone (one of six available ACEnet clusters) in collaboration with Dr. Taro Sato who performed PSF- matching on the images to ensure accurate colors.

One important result of this graph is that it is consistent with recent findings that reflect the so-called “Downsizing Scenario” of star formation proposed by Cowie et al. (1996). In the downsizing scenario, star formation ceases in massive systems at higher redshifts while lower mass systems continue to form stars at lower redshfits.

Most faint galaxies at high reshift are star-forming and only very few are quiescent.

September 2012

click on image to enlarge

In asteroseismology, researchers use stellar pulsations to probe the structure and evolution of stars. The Sun, but also other stars which are very similar in luminosity and temperature, oscillates in many of the so-called pulsation modes which can be compared to what our theoretical models predict. This comparison then allows us to answer important questions about stellar astrophysics and to make the models more realistic.

One of the most obvious problems with current models of solar-like stars is the inadequate modelling of their outer layers, where convection is very important for transporting the energy coming from fusion in the stellar core. These model shortcomings imprint themselves onto the so-called "high radial-order" pulsation modes. Consequently, even our best models differ significantly from the observations at these higher radial orders. Previously, these differences could not be studied in a reliable way, until Ph.D. candidate Michael Gruberbauer and Dr. David Guenther from the ICA developed a new way of comparing their models to observations by using probability theory.

The graph above shows the difference of the model frequencies and observed frequencies for 10 solar-like stars observed with the NASA Kepler satellite, as measured by their probability method. The Y-axis gives the magnitude of the frequency difference (normalized to a specific quantity called "the large frequency separation") and the X-axis is a proxy for the radial order of the mode. It can clearly be seen that for every star the frequency difference becomes larger in magnitude as we go to higher radial orders. The colour of the lines shows the effective temperature of the stars as measured from spectroscopy. The intriguing result suggests that stars with lower temperatures have smaller deviations than stars with higher temperatures. This could help to more closely pinpoint the modelling deficiencies in the future.

August 2012

click on image to enlarge

In this plot of the density of gas in a galaxy simulation, we can see a number of dense clumps of gas (yellow blobs, top and bottom) being pushed out from the galaxy's disc, which is now starting to run out of gas (dark area, centre). This gas is mostly pushed out by a large number of powerful supernovae - the violent explosions that mark the deaths of massive stars all across the galaxy. This heats the gas to extremely high temperatures. However, when we observe these galaxies with advanced telescopes, we can see cold gas is also flowing out of the galaxy. David Williamson, under the supervision of Dr. Rob Thacker, has performed simulations to attempt to understand where this cold gas comes from. The above simulation was performed using the FLASH simulation code on the Cerberus computing cluster at the ICA, making use of 192 processors for about a week. This plot was generated with a raytracing code of David Williamson's own design.

July 2012

click on image to enlarge

Values of the effective temperatures ("Teff" values) of solar metallicity red giant stars as inferred by modelling the stars' spectra with the PHOENIX atmospheric modelling program, overplotted with the results of some other methods for comparison.  (See the September 2011 ICA Image of the Month for a sample of what PHOENIX synthetic stellar spectra looks like.)
X-axis: The spectral class of the star on the Morgan-Keenan (MK) classification system (these giant stars are of MK luminosity class III).  Y-axis: The inferred Teff value of the star. 
Squares: The Teff value inferred from PHOENIX models with a simpler treatment of the thermodynamic equilibrium of the gas and radiation in the star's atmosphere (Local Thermodynamic Equilibrium (LTE)).
Plus signs: The Teff value inferred from PHOENIX models with a more realistic treatment of the thermodynamic equilibrium (Non-LTE).
Red and Blue symbols: Models fitted to the "red" and "blue" bands of the visible spectrum (wavelength greater than and less than 460 nm, respectively).  For spectral classes with no red symbol visible, the red symbol lies underneath the blue symbol - ie. the red- and blue-band values agree perfectly.
Green line with triangles: Teff values measured by Baines et al. (2010) by measuring the angular diameters of red giants with the cutting-edge CHARA optical interferometric array of telescopes.
Dotted line: Authoritative Teff calibration of Ramirez & Melendez (2005) using a more empirical, less model-dependent method (the "Infrared Flux Method" (IRFM)).

These PHOENIX calculations were made by Dr. Ian Short, ICA Faculty member, using the ACEnet computer cluster.

June 2012

click on image to enlarge

The interaction of radial pulsation and convection has been a long standing problem in stellar astrophysics. In particular the interaction of convection with radial pulsation can stop the pulsation of a star altogether by transporting heat through the hydrogen ionization region at the critical phase and removing this as a driving region for pulsation. This is what happens at the “red edge” of the RR Lyrae instability strip, the cool side of the region in temperature and luminosity space where RR Lyrae stars pulsate.

Purely radiative (heat transported only through radiation) models of radial pulsation in RR Lyrae stars has been unable to match the observed location of the red edge of the RR Lyrae instability strip (the lower effective temperature at which RR Lyrae stars cease pulsating). The image shows how the convective luminosity (the amount of energy transported via convection) varies over the pulsation cycle. As a reference for the phase of pulsation the radial velocity is plotted (expansion of the star is positive) in the bottom panel. As can be seen, as the star expands and is reaching maximum extent (positive radial velocity approaching zero) the convective luminosity is at a minimum causing energy to be dammed up (time a in figure). Conversely as the star contracts, the convective luminosity is at a maximum releasing energy (time b in figure). The action of convection works in the opposite way to the familiar kappa-mechanism for driving pulsation, and instead damps pulsation.

These results are based on a 3D hydrodynamic simulation with the SPHERLS code, which is being developed in the ICA by Ph. D. student Chris Geroux under the guidance of Dr. Robert Deupree, ICA Director. The short term spikes and troughs occur when the maximum convective luminosity shifts from one radial shell to another.

Chris Geroux (PhD Student, ICA)

May 2012

click on image to enlarge

Computer simulations are a useful tool when comparing theories to reality.  It is very frequent that several, often mutually exclusive, theories are developed when trying to describe a phenomenon, and these theories often predict how the phenomenon formed and/or evolves.  One useful method of testing the theory is to incorporate it into a numerical simulation, which can then be run over the period of interest - in astronomy, this can sometimes be several million to a few billion years.  If there are multiple theories, then they can be compared by running each of them in a numerical simulation, where everything is kept constant amongst the simulations except for the specific theory itself.

At the centre of every massive galaxy is a supermassive black hole, and the black hole returns feedback energy to its environment; there are currently many theories about how to calculate this feedback energy and how it is returned to its environment. We have tested six of these theories by incorporating them into a simulation of a merger of two Milky Way-sized galaxies. Four of the models contain feedback algorithms from the literature (SDH05: Springel, et al., 2005, MNRAS, 361, 776; BS09: Booth, Schaye, 2009, MNRAS, 398, 53; ONB08: Okamoto, et al., 2008, MNRAS,385, 161; DQM11: Debuhr, et al., 2011, MNRAS, 412,1341), the fifth (DQM11(modified)) is a slightly modified version of DQM11, and the sixth model (WT12: Wurster & Thacker) is a composite model I created.  Each image shows the gas density of one of the two galaxies, and is taken 480Myr after the simulation started.  By inspecting these images, we can already see qualitative similarities and differences amongst the six models; by extracting quantitative data from these simulations, we can compare and contrast the models to a higher extent.  We find that there are some strong differences amongst the models, but at this point a ranking is not realistic.  While we find components of each theory that work well or work poorly, we must be aware that these conclusions are reached within the framework of the numerical code that we are using.  Once this comparison is complete, the next step is to compare the data to observational data to further constrain the likelihood of each theory.  This analysis is ongoing. 

I incorporated all of the algorithms into the SPH-AP3M code HYDRA, which was co-written by my supervisor, Dr. Rob Thacker.  The simulations were then run on Dr Thacker's cluster at the ICA. 

James Wurster (PhD Student, ICA)

April 2012

click on image to enlarge

Density greyscale of an axisymmetric simulation (top half only shown) of a propagating jet (Mach number M=10, jet-to-ambient density ratio η=0.1) from left to right with: (top) passive magnetic field; (middle) strong toroidal field (plasma beta β=0.2); and (bottom) strong poloidal field (β=0.2) all after the same propagation time. In 2-D, a toroidal field enhances propagation speed, a poloidal field impedes it.

Credits: D.A. Clarke and N.R. MacDonald

March 2012

click on image to enlarge

The radiation from the Big Bang must travel almost all the way through the Universe before it reaches the Earth. While during early research much effort was spent on calculating the precise properties of this radiation, which is known as the Cosmic Microwave Background, today experimental measurements are sufficiently precise to actually measure the impact of all the material in the universe the radiation passes by and through. Aside from interacting with hot gas in clusters of galaxies, the radiation also feels the impact of the gravitational fields of galaxies and clusters of galaxies. This produces a gravitational lensing effect on the CMB, although as these two images above show, indicating the temperature of radiation, the effect is exceptionally subtle and one cannot expect to see the differences with the eye alone. Research carried out by Dr. Rob Thacker, in collaboration with Dr. Diego Saez at the University of Valencia and other collaborators in Spain, is looking to combine a number of different foreground effects after this initial lensing study. These simulations require not only cosmological evolution calculations, but must also trace the history of each individual ray of radiation as it traverses through the simulated virtual universe!

February 2012

click on image to enlarge

ICA graduate student Diego Castañeda is extending his work on matching the observed spectrum of the rapidly rotating star α Oph (see December, 2011 Image of the Month) by beginning to compare individual lines. The results of the comparison for the Mg II h and k lines are shown in this month’s image. The green curve is the IUE data, while the blue line is the computed flux profile. The computed flux comes from a weighted intgration of the intensity of the radiation emitted in the direction of the observed over the visible surface of the 2D stellar model with rotation. The model is the one used previously to match the entire spectral energy distribution for α Oph. The integration includes the Doppler broadening produced by the (assumed uniform) surface rotation with a surface equatorial velocity of 236 km/s. While not perfect, the agreement between model and data in this wavelength interval is encouraging.

January 2012

click on image to enlarge

Computing Oscillation Frequencies of Red Giants

The space observatories, Kepler, CoRoT, and MOST are being used to observe the natural frequencies of vibration on stars. The seismic data can be used by stellar modelers to study the interior structure of stars. Red giants are evolved stars with dense cores surrounded by a diffuse and extended envelope. Their highly concentrated cores pose a computational challenge to asteroseismologists trying to model the oscillation spectrum of these stars. The upper figure (normalized radial amplitude variation, Y1, versus radius fraction ) shows the eigenfunction of a typical oscillation mode in a red giant and the lower figure shows a blow up of just the inner 1% of the star. The rapid variations are real and, needless to say, difficult to resolve numerically. The oscillations in the core couple to oscillations near the surface of the star. Since it is the surface oscillations that are observable, there is the additional challenge of computing the observable manifestation of these mode couplings.

David Guenther, Professor