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ProfileNathan
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Message 3521 - Posted: 27 May 2008, 18:43:23 UTC

First off I'd like to apologize for my horrendous lapse in checking these boards and posting: I've been working hard at getting a paper out and we finally got it accepted into the Astrophysical Journal. I know this is not an excuse, but I hope you can all understand.

You can view/download the the paper here.

For anyone that is interested in a complete discussion of what is going on here at Milkyway@home and how the results can be used, you can find a complete discussion in there. I do warn that it's pretty dense, though.


This publication proves that what we're doing here works and is the best means we have to do these types of studies. This is good and means that we can move on to doing science as opposed to proving that the method we've set up is valid.

In the coming days/weeks I'm going to start running searches using actual astronomical data taken by the SDSS. Up to this point Milkyway@home has been running on simulated data in order to get the infrastructure up and running and to determine the means and methods to make it work.

I'll keep you all updated as to when I start these searches and the results of them. Also, I'll try to post pictures that highlight the area that I'm actually looking at with a given search.

Again, I apologize for my lapse earlier, and I'll gladly answer any questions anyone has about the paper or the work that's going to start showing up.
~Nate~
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Message 3541 - Posted: 28 May 2008, 10:40:56 UTC

Nate,

Please consider adding the paper to the [trac]wiki:ProjectPapers[/trac] page in the Wiki.

If you don't have an account, I can add it for you. Just let me know.
Kathryn :o)
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Message 3552 - Posted: 28 May 2008, 16:47:57 UTC

Thanks for the update, but I don't think this paper qualifies to be there. Our paper wasn't to the point of using the Milkyway@home data yet as we were still in the process of getting it up and running while we did the work in the paper on a cluster. The next paper would definitely work though as a of the work should come from you guys.

Thanks again, and if I am misunderstanding what the qualifications for the papers page is please let me know.
~Nate~
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Message 3555 - Posted: 28 May 2008, 20:50:41 UTC - in response to Message 3521.  


For anyone that is interested in a complete discussion of what is going on here at Milkyway@home and how the results can be used, you can find a complete discussion in there. I do warn that it's pretty dense, though.


Even though the paper is technical, I suggest you post a summary here that crunchers and the general public can understand.
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Message 3567 - Posted: 29 May 2008, 9:13:15 UTC - in response to Message 3552.  

Thanks for the update, but I don't think this paper qualifies to be there. Our paper wasn't to the point of using the Milkyway@home data yet as we were still in the process of getting it up and running while we did the work in the paper on a cluster. The next paper would definitely work though as a of the work should come from you guys.

Thanks again, and if I am misunderstanding what the qualifications for the papers page is please let me know.


Sorry Nate, I misunderstood what you said. Once you get the MW@H data published, definitely put the paper(s) on that page.

:-)
Kathryn :o)
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Message 3814 - Posted: 17 Jun 2008, 16:30:23 UTC - in response to Message 3555.  


For anyone that is interested in a complete discussion of what is going on here at Milkyway@home and how the results can be used, you can find a complete discussion in there. I do warn that it's pretty dense, though.


Even though the paper is technical, I suggest you post a summary here that crunchers and the general public can understand.



The paper describes in rigorous detail (and in mathematical detail) the method and models in which we do all of our calculations: a very brief summary is that we use a very small volume to study spatial distributions of stars. This allows us to model the stream debris as a cylinder within some background distribution which are described by a number of parameters. We then use a technique called maximum likelihood to "search" the likelihood "space" and find the values of the parameters that have the highest likelihood. These correspond to the "correct" values for those parameters.

Using this method we were able to reproduce the parameters for a simulated data set, and closely reproduce earlier results on the stripe 82 data that we used. However, we were able to greatly reduce the errors regarding these values and determine new quantities as well. All total we were able to present values for the position, orientation, and size (spatial and number) of the stream and the values that best define the background distribution.

If there's any questions I'll answer them, and there is much more detailed explanation in the paper.
~Nate~
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Message 4120 - Posted: 15 Jul 2008, 2:49:45 UTC - in response to Message 3814.  

If there's any questions I'll answer them, and there is much more detailed explanation in the paper.


I'm working my way down through the paper, but please bear in mind that I'm only an amateur astronomer and have spent most of my "serious" research in astrometry of NEOs. I've thusfar left anything stellar to you pros! ;) I'm starting to dabble in stellar photometry, but my understanding is still on the "low" side.

You refer to F turnoff stars as "not a very good standard candle", but I assume you mean in respect to the precise candles like cepheid and rr lyrae variables. Is it that F turnoff stars have a relatively flat absolute magnitude or is it just that they are far more abundant in the Sgr dwarf tidal stream? Is it that you expect them to more clearly define the tidal stream in relation to the galactic spheroid?

In short, I think I get your reasoning for limiting your study to one type, but why F turnoffs?
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Message 4142 - Posted: 15 Jul 2008, 16:25:13 UTC - in response to Message 4120.  

I'm working my way down through the paper, but please bear in mind that I'm only an amateur astronomer and have spent most of my "serious" research in astrometry of NEOs. I've thusfar left anything stellar to you pros! ;) I'm starting to dabble in stellar photometry, but my understanding is still on the "low" side.


Happy to hear your trying to work you way through it and understanding it so well it sounds. I'll answer your questions below.

You refer to F turnoff stars as "not a very good standard candle", but I assume you mean in respect to the precise candles like cepheid and rr lyrae variables.


Exactly!

Is it that F turnoff stars have a relatively flat absolute magnitude or is it just that they are far more abundant in the Sgr dwarf tidal stream?


The reason they are not a good "standard candle" can be seen if you look at this picture
.
It is an H-R diagram of the galaxy M55, made by NASA, and it gives a relation of the Luminosity or Magnitude (on the y-axis) versus the Temperature or Color (on the x-axis) of stars. This one being that of all the stars we can see in M55. If you look between the green bars you see what would be considered F turnoff stars. You should also easily see that the magnitude range that they populate on the diagram is relatively spread out but seems to follow a somewhat Gaussian shape. This Gaussian is what we attempt to model with the Absolute Magnitude distribution convolution you see defined in the paper.

To add to this a commonly used standard candle is Blue Horizontal Branch (BHB) stars, as they have a relatively constant magnitude. You can see these on the image at Mv ~ 1 and in the range of 0 < B-V < .25. You can see how different these distributions are.

Is it that you expect them to more clearly define the tidal stream in relation to the galactic spheroid?

In short, I think I get your reasoning for limiting your study to one type, but why F turnoffs?


Using more than one type of star would be very difficult as it would mean that we need to be able to clearly define what type of star each star is in our data set and be able to dynamically determine what its absolute magnitude is while simultaneously fitting it with our algorithm. Quite frankly this is not possible yet. One of the hardest parts to doing this is that you cannot simply look at the color and determine the type of star. Notice on the image that the farther right you go you get two distinct branches: the main sequence stars on the lower and the giants on the upper. Given just the color of the star how do you then determine which branch it is on? There is some work being done to try and characterize the distributions of stars within an H-R diagram, but it is slow going. Eventually, we would like to give it an entire data set of all stars and fit that way, but not yet.

Why then, you may ask, do we use F turnoff stars instead of the better candle BHB stars? This is quite simply numbers. There are a LOT of F turnoff stars compared to the relatively few BHB stars, and given we are doing probabilistic searches over these stars, the more stars we have the better (and more accurate) our results. Therefore, we use the F turnoff stars. However, BHB stars are visible at a much larger distance than F turnoff stars, so in the future we may need to use the BHB stars to fit the more distance pieces of the Sgr stream, as well as other debris.

One final reason we use F-turnoff stars is that the spheroid of the Milkyway is comprised of an older metal-poorer population than the disks. This means that the stars in the halo are bluer (shifted to the right of an H-R diagram) than the Milkyway. Thus, by being clever with the color range of stars we fit, we are able to create a sample that has very little contamination (if any) from the disks of the Milkyway and therefore are able to avoid modeling them. This is because the spheroid's F turnoff stars are shifted further to the left than the rest of the Milkyway's F turnoff stars.

I hope I answered all your questions, and please don't be afraid to keep asking more!
~Nate~
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Message 4174 - Posted: 16 Jul 2008, 4:03:02 UTC - in response to Message 4142.  

I hope I answered all your questions, and please don't be afraid to keep asking more!


Wow! You certainly did! Heck, I'm so happy to get such a fast and thorough response from a BOINC project scientist that I got off my rear-end and attached 3 more computers and raised my resource share for MW! :)

You also caused me to go out looking for more information about BHB stars. If I'm interpreting what I read and the H-R diagram correctly, then they make a better standard candle since their absolute magnitude tends to very tightly correlate to their color temperature? (I know this is a bit off topic since you're not actually using them)

I'm now much clearer on the reason for F turnoffs. I sort of suspected that one reason might be how numerous they are, but the diagram really helped me see the gaussian distribution of their magnitudes, so I'm sure clearer on that aspect of the algorithm. I found the differences in the metallicity of the spheroid v disks very interesting. You alluded to that difference on page 11, but I didn't make the connection of WHY the spheroid stars were bluer.

Now I'm wondering all sorts of things about evolutionary differences between spirals and other galaxies.... why more metal rich in the disk? any connection to past mergers in the formation of the disk v spheroid, etc. ? (don't feel the need to answer this stuff... I'm just musing in type) :)

I did want to make one clarification for anyone else reading, lest they think M55 is a galaxy. (You did make me reach for my lone Messier book to make sure my memory hadn't failed me) M55 is a visually wonderful globular cluster in Sagittarius. One of the few that really look like an obvious star cluster in even a pair of binoculars! Tonight would be a terrible night to see it since the moon is too near, but anyone with any decent pair of binoculars can find it pretty easily in about a week. Go out before the moon rises and find the "teapot" shape of Sagittarius (for help visualizing it without spending any $$'s I recommend a free open source program called stellarium (www.stellarium.org). Look for the two stars that make the 'handle' of the teapot and look in a line going south from them about 2x the distance between them and you should see a fuzzy patch. Spend some time looking at it and you'll start to make out the fact that the "grainy" appearance is actually clumps of stars within a large cluster of them. You won't see any of the color variations that the diagram Nathan provided indicates without a decent sized telescope (sadly our eyes need a LOT of light to distinguish color), but it's still pretty cool! Enjoy!
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Message 4194 - Posted: 16 Jul 2008, 19:03:42 UTC - in response to Message 4174.  

Wow! You certainly did! Heck, I'm so happy to get such a fast and thorough response from a BOINC project scientist that I got off my rear-end and attached 3 more computers and raised my resource share for MW! :)


Thank you!

You also caused me to go out looking for more information about BHB stars. If I'm interpreting what I read and the H-R diagram correctly, then they make a better standard candle since their absolute magnitude tends to very tightly correlate to their color temperature? (I know this is a bit off topic since you're not actually using them)


Yes

I'm now much clearer on the reason for F turnoffs. I sort of suspected that one reason might be how numerous they are, but the diagram really helped me see the gaussian distribution of their magnitudes, so I'm sure clearer on that aspect of the algorithm. I found the differences in the metallicity of the spheroid v disks very interesting. You alluded to that difference on page 11, but I didn't make the connection of WHY the spheroid stars were bluer.


Glad to have cleared that up. :-)

Now I'm wondering all sorts of things about evolutionary differences between spirals and other galaxies.... why more metal rich in the disk? any connection to past mergers in the formation of the disk v spheroid, etc. ? (don't feel the need to answer this stuff... I'm just musing in type) :)


The disk is more metal rich because that's where all (read that all means most) the star formation happens because it's where all the dust/gas is. Therefore stars are born and die within the disk and when they die (and to some extent during their lifetimes) they eject material (most notably during supernovae) which is comprised of all of the heavier elements they made during the fusion process. Therefore when a new star is formed using this material they have a higher initial metalicity. Lather, rinse, repeat. :-)

The formation of the spheroid is a topic of hot debate. Some say that the spheroid is entirely composed of past mergers, but at any rate there is ton of debris in the spheroid from many different things (3 that have been found are assumed to be dwarf galaxy mergers).

As for the evolutionary differences in galaxies, I can't help you much. I am a Galactic astronomer, not an extra-Galactic astronomer; so my knowledge of the subject is not very verbose.

I did want to make one clarification for anyone else reading, lest they think M55 is a galaxy. (You did make me reach for my lone Messier book to make sure my memory hadn't failed me) M55 is a visually wonderful globular cluster in Sagittarius. One of the few that really look like an obvious star cluster in even a pair of binoculars! Tonight would be a terrible night to see it since the moon is too near, but anyone with any decent pair of binoculars can find it pretty easily in about a week. Go out before the moon rises and find the "teapot" shape of Sagittarius (for help visualizing it without spending any $$'s I recommend a free open source program called stellarium (www.stellarium.org). Look for the two stars that make the 'handle' of the teapot and look in a line going south from them about 2x the distance between them and you should see a fuzzy patch. Spend some time looking at it and you'll start to make out the fact that the "grainy" appearance is actually clumps of stars within a large cluster of them. You won't see any of the color variations that the diagram Nathan provided indicates without a decent sized telescope (sadly our eyes need a LOT of light to distinguish color), but it's still pretty cool! Enjoy!


I apologize for saying it was a galaxy, it is indeed a globular cluster. Thunder gives a good description of how to find it in the sky. I would like to add another nice little trick you can perform: if you are able to see Sagittarius (the teapot described above) and you put your arm out and with a closed fist over the teapot, the amount of area taken up by your fist is approximately the amount of area on the sky that the core of the Sagittarius dwarf galaxy takes up on the sky. That's how close it is to us!!! The only reason that you can't "see" it and that it took until 1994 to discover is that you are also looking straight through the disk of the Milky Way, as the dwarf is on the opposing side from us.
~Nate~
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