Kepler Bandpass

Kepler Bandpass
Originally uploaded by Astro Guy

The transmission functions for the Johnson B,V,R,I filters are shown from left to right in blue, green, red and magenta respectively and have been scaled to peak at 100% transmission. The Kepler bandpass is shown in black which peaks at approximately 70% throughput. The CoRoT whitelight bandpass is shown by the dot-dashed line. The MOST bandpass is shown by the dashed line. The spectrum for an A2V star is shown in cyan, which peaks in the UV and the spectrum for a M2V star is shown in orange which peaks in the infrared. The two spectra have been scaled to have equal flux in the Johnson V filter.

If two stars of two different spectral types have equal brightness in the V filter, then one would like to know what is the difference in flux over the Kepler bandpass for the two stars. One starts by defining that a G2V star will have equal brightness in the V filter and Kepler bandpass. This definition provides a natural way to scale the artificial spectra and to compute the flux difference in the V filter and Kepler bandpass as a function of spectral type. The Figure demonstrates how a hot A2V and cool M2V star with equal brightness in the V filter can have quite different brightnesses through the Kepler bandpass with extends to the near infrared part of the spectrum.

Don't Touch the Ash.

Don't Touch the Ash.
Originally uploaded by Astro Guy

umm.. warning to the people of Prince George not to touch the ash, so CBC shows a picture of a resident holding up ash and posing for a nice photo. I hope his hand is okay.

Original URL: http://www.cbc.ca/canada/british-columbia/story/2008/05/28/bc-prince-george-fire-ash-health.html

Hockey, Hockey, Hockey

HP Pavilion - "The Shark Tank"
Originally uploaded by Astro Guy

With the Leafs not making the playoffs, I've had an open mind concerning who to cheer for in the playoffs (except for Ottawa, very happy to watch Ottawa loose!). This has changed in round 2. My picks are:

San Jose, Montreal, Pittsburgh and Colorado.

So far, I'm not doing so well. I can't stand Philly and their rude fans. Who boos O'Canada and chants U.S.A. over and over? No respect. Considering a large portion of the team are Canadian Natives - including goaltender Biron - the fans should give their head a shake.

Picked San Jose, as that's where I live now, and the team has good talent. They just need to learn how to perform in the playoffs. Don't really care for Detroit or Dallas.

If trends continue. I guess I'll be cheering for Sid the Kid.

Planetary Models.

I've been working on a planetary atmosphere dynamics model for the last while and I've finally made some real progress. Here's a video showing the local vorticity.



Vorticity is a measure of the spin potential.

Finding planets by timing transits.

Finding planets by timing transits.
Originally uploaded by Astro Guy

There are about a dozen exosolar planets that transit their host star (a transit is when the planet moves in front of the star blocking out light). Measuring the when transits occur can be a powerful tool to determine if other planets are present in that system.

The Figure above shows a simulation of transit timings if additional planets are present. The primary planet has a mass of 0.69 times the Jupiter and an orbital period of approximately 3.52 days. The colours indicate the mass of secondary planet, ranging from 1 to 100 times the mass of the Earth. The scale on the bottom shows the period of the secondary planet ranging from 1 to 17 days. The scale on the left indicates the change in the occurrence of the transit event. It is a log scale (0 -> 1 sec, 1 -> 10 sec, 2-> 100 sec and so on..).

Long term monitoring of transiting systems gives the opportunity to possibly discover other Earth-sized planets outside out Solar System.

Frequency Analysis...

Frequency Analysis Diagram
Originally uploaded by Astro Guy

The first process usually done on the time series of a variable star is the frequency analysis. For a star with periods on the order of a day, such as with gamma Doradus types, ground based data can be quite troublesome to work with due to aliasing. The aliasing comes from the day-night cycle of the Earth where a star can only be observed at night. With a spacebased telescope in a proper orbit this problem disappears.

For HR 1613 we obtained 8.5 days of data that is nearly continuous. The result is that we can resolve individual frequencies that compose the lightcurve shape. Mathematically, we want to fit a function consisting of sine curves that have different frequencies, amplitudes and phases. Fitting an equation of this form in a non-linear problem. This means that finding a set of parameters that best fits the data is tricky business.

One method of estimating the best fit parameters is to use a Fourier Transform (in this case, discrete=DFT) to estimate parameters. The method is simple to describe:

1. Perform the DFT of dataset
2. Record the frequency and phase with the highest amplitude
3. use DFT info as guess for fit solution. Then optimize solution.
4. remove solution from original data set to produce residuals.
5. Perform DFT on residuals.
6. Find next highest amplitude.
7. add new frequency to solution and optimize again.
8. repeat 4-7 until there are no significant frequencies remaining in the DFT.

There are some hiccups along the way. The above method does not guarantee that you have found the best solution. Also, your solution parameters may become degenerate if two frequencies are inserted in the solution that are seperated less than the resolution afforded by your observations.

The best way around resolution problems, is to gather more observations! Otherwise, one must be quite careful about claims one may make based on a non-linear model as you may not have found the best solution. The Figure above shows a solution for HR 1613. So far I am not convinced that is a unique solution and it will be very difficult to get new observations. So I'm working on methods to test the uniqueness by efficiently exploring the local parameter space for the model parameters to find other local-minimum/maximum.

A year later and HR 1613

hr1613
Originally uploaded by Astro Guy

Argh. I had just finished a lengthly blog entry and then found out that google has changed blogger since the last time I posted (1 year ago!). I guess that will teach me to wait so long between posts.

Anyways, I lost track of blogging as I was busy finishing my thesis, defending, finding a job and moving. Well that's all done now. I successfully defended and submitted the thesis. I am no longer a student. Being a poor grad student was being to really suck, now I can be a not-so-poor poctdoc. As a result, I've moved from Vancouver to San Jose and now work at NASA-Ames with the Kepler project. That alone should give me plenty to chat about in the future.

I've been working recently on trying to publish some results on the binary system system HR 1613. The figure above shows an 8.5 days observing campaign from the MOST satellite. The magenta lightcurve at the bottom shows the variability of the stellar system.

The primary, more massive, member of the system is a 1.5 solar mass main-sequency star (A9 V) and the secondary is a less massive and intrinsically less bright K or M-dwarf star. The A9V star shows variability on the time scale of a day. It appears to be gamma Doradus type pulsations. I've discussed these type of pulsations before in my study of the star BD+18 4914. The interpretation is that these are g-modes which may be driven by the star itself or excited by tidal interaction with the companion star.

The other 4 lightcurves show a wide range of stellar activity for different temperatures and mass. The top (red) curve shows a hotter, more massive A-type star which shows variability on the timescale of an hour. These are interpreted as p-mode pulsations and the star is likely a delta Scuti type pulsator. The next (green) curve shows a star with a very similar temperature, but does not show any intrinsic variability. It's evolutionary stage is unknown. The 1 day modulation that you can see by eye in the light curve is an instrumental artifact. It was purposely left in the lightcurve to show its effect on the photometry.

The middle curve (blue) shows a relatively hotter B star. The variability pattern and stellar classification are consistent with being a beta Cephei type star. The next curve (cyan) shows a cooler K-type star. The erratic behaviour in the lightcurve is likely caused by spots on the surface of the star which rotate out of view as the star rotates and change shape as influenced by the stars magnetic field.

Each star shows a unique lightcurve which we can use to probe the internal structure of star, which inturn allows us to infer the star's past and future. It's a very interesting science, to understand the points of light that we see on a clear night.

Wiener the Puck

Wiener the Puck
Originally uploaded by Astro Guy.

Went to a hockey game while in Vienna. It was too much fun. The fans really make the game. Lots of locals with war drums, flags, banners and sparklers continuous cheering! The Vienna Captials won 4-1 which made the game even better. The announcer would say, "and the score is, Vienna Capitals" and we'd all say "4" and the announcer would say, "Linz" and we say "1, they suck!". Talk about home team advantage!
There where also two Canadian flags up, Canadian players on the ice (including former NHLers) and a few NHL jerseys in the stands. I had my Maple Leafs cap on (Go Leafs Go!)

Riesenrad Prater.

Riesenrad Prater.
Originally uploaded by Astro Guy.

It's been a while since I blogged an entry, and this is going to be a short one. I'm currently in Vienna to work with the science team over here. It's been a productive time so far as I slowly get through my backlog of papers to write. A new location far away at a clean desk definitely helps!

Layla Barbara Rowe

Layla Barbara Rowe
Originally uploaded by Astro Guy.

My brother is now a dad with a beautiful little girl! The photo on the left is Layla and the photo on the right is my brothers baby photo. It's amazing how much they look alike!

I can wait to get back to Toronto!

dDor/dSct hybrid spectra.

dDor/dSct hybrid spectra.
Originally uploaded by Astro Guy.

Been awhile since I blogged last, been quite busy and haven't had much to say. I did manage to submit a paper on the discovery of a hybrid gamma Doradus/ delta Scuti pulsator. I've talked about this really cool star in the past. I was very interested to get a spectra of the star and get some basic stellar parameters. From a literature search I could only find a sketchy effective temperature of 6500K which was too cool to be placed properely in the observed instability strips.

I contacted DAO and a week later I received the spectra above. The spectra has a resolution of about 20000 centred on H alpha (the big absorption feature in the middle). Even at this medium resolution one can see the rotational broadening of the lines. It's about 35 km/s, well about the instrumental broadening of about 18 km/s. So it's got a significant rotation (this is important!)

Next I computed a grid of Atlas 9 model spectra (Kurucz models) to get an estimate of the effective temperature and surface gravity. The strength of H alpha points to an effective temperature of 7250K and log g = 3.7 cgs. This is much better as it moves this star into the observed delta Scuti and gamma Doradus instability strips. The star is also a bit evolved off the ZAMS. ZAMS stars have log g=4.3.

The next finding was to check the abundance patterns. I've labeled some of the strong elemental lines such as iron. In general the iron lines are observed to be a bit stronger than compared to a solar abundance. The Ca line is also very weak which is an indicator of an Am star. The physical process at play here is diffusion. Some elements are good light absorbers and get pushed to the surface of the star, others are pulled down by gravity. Only elements at the surface of the star (in the photosphere) show up in the spectra as stellar material becomes complete ionized towards the center of the star.

So whats the punchline? Well, the role of diffusion is Am stars is the number or reason for muted pulsations. One needs partial ionization zones to drive the star. Add a bit of rotation and you can counter the effects of diffusion. On top of that there are only 4 hybrid pulsators currently known or which 3 are Am stars and the 4th is binary. Think I've found a neat star to study for a while! Can't wait to learn more.

Problem solved...

M27 - The Dumbell Nebula
Originally uploaded by Astro Guy.

The photo above is of M27, the dumbbell Nebula. The blue dot in the centre used to like our Sun, but it's now running out of fuel in its core to burn. As this happens outer layers of the star are becoming gravitationally unstable and leave the star driven away by photon pressure and treats us to beautiful sites. The bluish-green gas is due to forbidden transition lines of Oxygen (absorption and emission of photos) and the red gas is due to Hydrogen. One day our Sun will probably do this as well, but don't work thats billions of years away.

The image itself was obtained at the Dominion Astrophysical Observatory (DAO) in Victoria British Columbia. I acquired 3 60 second exposures in B,V and R and then assembled the image you see in front of you.

Now on to my current work:
Today started off lousy, but ended up with a good ending (well, the day isn't over yet).

When I extract photometry from the raw CCD MOST satellite images I usually do not have any dark/flat calibration frames. The dark frame measures both the bias offset and identifies hot pixels on the frame. The flat image is used to measure the differential gain from pixel to pixel. Both of these are important if your target of interest changes position in each image. The MOST satellite seems to have a crack in the side of it. It was probably created during the launch process. This has been a nuisance as it allows stray light (mostly from the Earth itself) to reach the CCD detectors and peaks once per orbit (about 14 times per day). This causes all sorts of problems that I theories to explain, but I'll leave that for another day. The point today, is that I can abuse the stray light to generate local dark/flat fields for each subraster on the CCD. If I assume that the straylight is a uniform source over a 20x20 subraster (good approximation) then I can compare the individual pixel counts compared to the mean. If any pixel increases in brightness faster than the others, this is interepted as a gain difference. Likewise I can measure a zero point (the value of the pixel with no light) which is the dark/bias value.

In practice this works well, especially for dim stars. The problem I've encountered is that the amplitudes of variable stars gets distorted. Initially I thought this was because I was using PSF fitting photometry opposed to pure aperture photometry, but that is not the case. In fact, it's the gain variations that are causing this effect. I never made sure to check that the overall gain corrections come out to 1. In fact they usually come out to about 1.2 or something like that. In other words, I forgot to normalize my flatfield to 1. So simple, Oh well, at least I caught it and have time to make corrections to the datasets for which it is critical! Luckily, absolute amplitudes are usually useless as MOST uses a unique filter (300-700 nm broadband filter!). If anything, it's the relative amplitudes that are important and this is perserved. Plus I have all the raw photometry to compare to as well, so this looks like a bookkeeping exercise.

Another day, another sunset.

M33 Spiral Galaxy - Scaled!
Originally uploaded by Astro Guy.

The image above was probably the first really cool astronomy picture that I made. My masters research was a carbon star survery of the nearby galaxy M33. Carbon stars are evolved stars that are burning Helium near their cores (shell burning) instead of hydrogen core burning (like our Sun does). Normally these stars are called AGB stars have have atmospheres that are chemically dominated by an abundance of oxygen after the formation of carbon-monoxide. Sometimes, a AGB star will form a double shell energy source with the addition of a hydrogen burning shell. Having to heat sources at different radii from the centre of the star is thermally imbalanced. The star comes almost complete convective and carbon rich material from the core of the star (formed by fusing helium) is transported to the surface of the star. This excess of carbon alters the chemistry of the atmosphere to a carbon dominated state. So in an Oxygen dominated atmosphere (M-star) one can see the formation of molecules such as TiO, whereas in a Carbon dominated atmosphere molecular species such as CN are present. Each of these molecules absorps light at different wavelenghts cause observationally different spectra to be observed.

So, one can build a set of narrow band filters to distinguish between an M-star and C-star. My masters project was to map this ratio around the disc of the nearby (800 Mpc) galaxy M33. It's about the same distance away from us as the Andromedia galaxy (M31). The ratio of C-stars to M-stars (C/M ratio) is a good tracer of the initial metal content of the star. A population of stars formed from metal poor gas is more efficient at forming C-stars than metal rich gas. So we have a tracer of the metallicity of the galaxy and a peak at the history of star formation in a galaxy. All in all, I found about ten thousand C-stars out of 1.3 million stars. It was a lot of work, but we got our result, and I got to make this cool picture from all the data in the end!

and now.. another garbled song...

Time, time, time, sees, what from me it became during I am around
approximate after my possibilities that I was hard therefore with
please however volatile view around, the pages brown looked and the
sky is nebulous nuance of the winter

Listening to the stars.

M92 - Globular Cluster
Originally uploaded by Astro Guy.

One always seems to apply that space is quiet. Well, that's true in the sense that sounds needs a medium to travel through (eg. air!). However, sound is also our brains interpretation of the frequencies that we hear. Image that you could interept the light traveling to you from stars as sounds, what would you hear? Normally nothing. A star such as the Sun vibrates with a period of 5 minutes. If you interpret this as a sound, that puts it at a frequency of 0.003 Hz. The average person hears sounds from about 20 to 20000 Hz. We can hear vibrations at a much higher rate.

What makes a star sound interesting, is that there is usually more than one note. This is evident in the fourier transforms of light curve data that I've shown in previous posts where many individual frequencies are present. A star is more like a chord. So lets ramp up the frequencies and listen!

I've done this with some data from the MOST satellite that I've been working with. You can find some of the mp3s here for your listening pleasure. Some of the stars sound like ring sheets of metal, others sound like diseased cat making love (not so nice). You be the judge. If you do download any of the sound files for use be sure to reference the MOST satellite.

In each case I've matched the largest amplitude frequency to 500 Hz and allows the other frequencies to fall as they will.

Milky Way from San Pedro Martir

Milky Way
Originally uploaded by Astro Guy.

This has been a week of CPU busy time. Running lots of different models and generating lots of text output. That make a very boring blog entry. So I searched through some of my older photographs and I found this one I took while on an observing run in Mexico. I used 200 ISO film and strapped my camera to the side of the telescope and left the shutter open for about 10 minutes. I was quite excited to see this photo when I developed the film. This is a picture of the Milky Way - our own galaxy. So we are sorta looking at it inside out. The brightest patch in the photo is the direction towards the galactic centre. The dark patches that seem devoid of the stars is actually due to dust that obscures light in the optical part of the spectrum. It's not that there are not any stars there, we just can see them.

I was observing at San Pedro Martir, which is on the Baja California region of Mexico and I spent about 30 days at the summit. It's probably the best site I've been to - better than Hilo in my opinion. You can watch stars rise and set. Not just fade away into the hazy of the atmosphere low on the horizon, but disappear behind land. It was spectacual. You almost feel like the MilkyWay is the only light around you (which is not true..). Can't wait to go back.

The purpose of the observing run was to survey northern Ap stars for variability. To see if they are roAp (rapidilly oscillating) or noAp stars (no oscillations). Currently, almost all roAp stars are found at southern declinations. Everyone seems to believe this is just an observational bias, but every northern survey comes up blank. It's very odd.

roAp stars are known to have pulsation modulation (in FT space you see a spacing of very close frequencies). The model to explain this is called the oblique rotator, where the rotation axis is different from the pulsation axis. Since the non-radial modes are sequencial, the pulsation pattern appears skewed to one pole. As the star rotates this pole will go in and out of view causing the amplitudes of the pulsations to change. I made a movie to show this effect:

roAp Star Movie

The sound is actually from light curve data from an observed roAp star that I've tried to sync up with the movie. It's close, but not bad.

Anyways, with rotation (and magnetic fields!!) one could easily observing stars during pulsation minimum and not pickup the small photometric variations (tenths of a millimag and smaller!). It's a project that just takes patience and lots of telescope time. If it turns out that roAp stars are preferentially found in the South, then the fun begins of explain why!

Lost in Translation.

Having fun with the google translator today, I started to insert lyrics and translate them to various languages and then back to English to see what the final result is.

Here's an easy one to figure out:

The lamp is burnin which is weak according to my table the upper surface which snows it gently tomb the air is still in the rest of my sector than I hear your voice gently naming if I could have you only narrowly a sigh or two to breathe I were fortunately right to hold that with loves of hand I after this precaution of winter with you

(song answer: Song for a Winters Night - Gordon Lightfoot)

This one is a bit more challenging:

For here, I am not it having sat in a box on the world planet mass am far blue
and gives only I can make well that I would be beyond the miles miles hundred
me smells very still and me thinks, my spacecraft know, which manner with my
wife I, they explain outward journey like much them white

(song answer: Space Oddsity - David Bowie)

This could easily entertain me all day, but work to be done.. so back to it.

Gamma Doradus Star

Gamma Doradus Star
Originally uploaded by Astro Guy.

Another day, another neat light curve. Here is 44 days of continuous photometry of a gamma Doradus star. The top panel shows the DFT (amplitude, not power!) and the bottom panel shows the light curve binned by 40 minute intervals. The gamma Doradus stars are a relativity new class of variable stars found in the classical instability strip. They commonly have low amplitudes (4 mmag for this star!) and periods around 1 day, which makes them difficult to pick up from the ground. The low frequency comment (~1 c/d) are thought to be high-n, low-l g-mode (gravity is the restoring force). This star also shows delta-Scuti like pulsations around 9 c/d. Quite and interesting star and will serve well to test current ideas, such as mode identification schemes!

Spherical harmonic animation.

Spherical harmonic animation.
Originally uploaded by Astro Guy.

So I'm still working on models to explain some extraneous frequencies in the echelle diagram for AQ Leo. The application of non-radial modes has been suggested - although I'm not really a fan. I think the science is more interesting just in explaining the existence of combination frequencies.

Well, to investigate the non-radial suggestion, one has to work with spherical harmonics, which are a set of solutions to Laplace's equation and are handy for decomposing pulsation modes in stars into a series of numbers. If you click on the photo above you'll be linked to my flickr account. Click on the "all sizes" button above the picture, and you'll get to see the animation of a pulsation mode described by the spherical harmonic of l=5, m=2. Of course a star does not pulsate with such large variations, but only a fraction of a percent of the radius, but it's much cooler to see the deformations amplified.

So now I'm inserting non-radial modes by brute force to see what kind of light curve is produced with the addition of rotation and radial pulsations. I don't expect to see anything dramatic, because the important physics is missing.. namely, boundaries and assymetrics within the star. Since an RR Lyrae is a helium burning horizontal branch star the pulsation should travel through convective and radiative zones. You dump a non-radial mode at a transition zone and the radial pulsation becomes preterbed, possibly setting up a standing wave from interference. In photometry this will show up as combination frequencies.

This is definitely work in progress.

AQ Leo Echelle Diagram.

AQ Leo Echelle Diagram.
Originally uploaded by Astro Guy.

A beautiful weekend in Vancouver! Temperatures went above 30C! I've got a nice tan now. Let just hope this lasts a while!

I started fiddling around with echelle diagrams on the weekend. An echelle diagram is a method of displaying evenly spaced frequencies. For instance, with the double mode RR Lyrae star AQ Leo you can extract about 60 significant frequencies from a discrete fourier transform. The y-axis on the diagram plots the frequency, the xaxis shows the phase of the frequency which depends on the folding frequency. For the case of AQ Leo I folded at the frequency of the fundamental pulsation mode (marked as F1 in the diagram). The cyan and magenta lines show where conbination frequencies are expected to occur. Almost all frequencies can be accounted for, except the magenta lines are not quite satifactory. I'm going to investigate if a second overtone pulsation will help matters.

M51 - Spiral Galaxy

M51 - Spiral Galaxy
Originally uploaded by Astro Guy.

I made this photo a couple years back now. I got the observations at the Dominion Astrophysical Observatory (DAO) in Victoria British Columbia with 60 second exposure times in B,V and R filters. I'm in a rush out the door, so I'll comment more on this tomorrow.