jsandse

Thought I would take the opportunity to provide some notes of my experiences in astronomical spectroscopy to date with current projects carried out and others I would like to do in the hope it may help/motivate other people. My simple aim when I started out on astronomical spectroscopy was to take spectra and to understand how the spectral lines in stars are formed. So thats two objectives the practical side of doing spectroscopy and the theoretical side of finding out why things are the way they are. Sounds simple enough but when you start delving into the subject there is a lot to learn - however taking the spectra is far easier than understanding why the lines are formed. So I started with a couple of projects: - before going near the sky calibrate the spectroscope using the internal calibration lamp on my L200 and also some external calibration bulbs - I used the ones from Habitat detailed on Buils website at http://astrosurf.com/buil/calibration/lamp1.htm - take spectra of a bright star across its continuum and the sun/moon and learn a spectra data reduction process to process the spectra - take spectra of a spectroscopic binary to see the radial motion of the stars - my results are here for this task: http://f1.grp.yahoofs.com/v1/QHP_TEwOlSOe0oy6uHI3PfjBV0-yD2UBS3dWed50Kqlz8CIAbID\ 3q6UT9y4lCnNvjP3LxcdcPxtdwnEi7phTlstrhkgGiw/John_s/mizargrouporiginal.jpg - take spectra of epsilon aurigae and submit to the current campaign that Robin is coordinating What I can say about the projects is that I have managed to complete them all. However there is room for improvement for what I have done: increasing the resolution of the spectra I have taken by going to second order on my L200 or using a higher resolution grating - would get me to R ~ 15000 and would give me more a chance to get reasonably accurate measurements of periods of binaries and give me more accurate wavelength measurements for taking high resolution spectra of bright stars suchas epsilon aurigae. These are projects for me next year. As well as improving on the above there are other projects which I have been thinking about: Project A - measure continua of stars using ultra-low resolution spectroscopy from say 2 to 50 Angstroms resolution. Key stellar paramaters can be calculated from these type of measurements - temperature, gravity and metallicity. Two significant surveys have been carried out which provide reference field stars and data for doing these surveys: the LICK survey and more recently the MILES survey. Under 1000 stars has been carried out in each of these surveys and it would be interesting to carry out a similar analysis on stars that were not covered and which are in the range of amateurs such as myself Project B - using data from above to practice classifying star spectral types would also be interesting and perhaps looking at rules for classifying stars where our cameras have highest quantum efficiency as currently most of the classification rules are at the blue end (3800-4500A)for historical reasons or up at infra-red or beyond. So plenty of projects to carry out then even at low resolution :0) Lets talk about the second objective - the theory behind how spectra are formed. Basically what I want to do is before I go out in the field to take detailed spectra of a star I want to know what I should expect to see and why so that if I do see something unexpected I can talk about it to other amateurs. Finding out what I should expect to see is easier than understanding why its that way so I will cover that first. There are several ways of doing this but I use VOSPEC which is a great tool and it can be found at http://www.sciops.esa.int/index.php?project=ESAVO&page=vospec along with some great flash videos showing you how to use the tool. Benefit of this tool is it accesses loads of archives for stars and you can also look at spectra of synthetic stars built out of stellar models as well. So thats great as long as the star is there but if you want to know things like what elements are for what lines. You can look at a library of elements such as you do in VSPEC but then you normally have options as several elements have absorption lines around a specific wavelength. So this is where stellar modelling comes in. There is available on the net free software which will allow you to produce your own stellar models :0) There are several available out there but I use Kurucz stellar modelling software called ATLAS which has been updated by Castelli to run on linux. This models the main sequence stars which are not too hot eg O stars or the ones that are too cool). Before you go - oh no not linux I need another PC and how much is this going to cost I will say that -to use Atlas you can run it on your own PC and all the software is free providing you have an internet connection and enough spare disk space on your PC) How you do it is you download Virtual PC (if you have good old XP like me or you can use the Windows 7 equivalent which is also free)and install it on your machine. You then install linux on your machine - I use fedora - you can download the software from their site. Next you install two compilers a free c one the GNU version and the intel fortran compiler details of how to do this are on the intel web site. Next you follow the instructions linked to the Atlas cookbook and you download the atlas code and data and compile it. Link to Atlas cookbook is here: http://wwwuser.oat.ts.astro.it/atmos/atlas_cookbook/Atlas_Cookbook.html There are two versions of Atlas - Atlas9 and Atlas12. Atlas9 is what I have used up to now and it appears to work - The main atlas program generates the stellar model then you have two other programs in the Atlas9 sweet called Width and Synthe which let you specify the resolution and then generate the spectra. The data file which has the spectra in it has the great advantage of telling you what elements are contributing to the spectra :0) Although as input into Atlas you need to have a starting model for your star - you can access grids at kurucz web site http://kurucz.harvard.edu/ However to choose a grid you need to know the temperature, gravity and metallicity of the star you are studying. So how do you know that? Well you can get the data from simbad at http://simbad.u-strasbg.fr/simbad/ But if you can't find the data online you'll have to work it out for yourself - that means probably doing Project A which I mentioned above. Thats enough on Atlas. If you want to look at hot stars and stars with accretion disks then you could try TLUSTY which can be found at http://nova.astro.umd.edu/ although I have not tried to get that program working on linux yet. Ok so we are now in a position to produce synthetic spectra for a star and compare it against the spectra we see. Now for the tricky question why does the spectra appear in the way it does. I'll now list a set of books which will help you on your way to answering this question...and will comment on them Non-technical books: - Keith Robinsons books appear to be an excellent non-technical introduction to the theory behind spectroscopy and starlight. I have only flicked through them myself but only some vey basic school algebra is all you require - his books are called Starlight and Spectroscopy: Key to the stars and can be found on amazon. - Obviously there will be ken's book when it comes out but I believe it focusses on the practical aspects of spectroscopy and not the theory behind why stars have the spectra they have Technical books: To get the most out of the following books you will need to understand quite a bit of calculus - Schaums outline in calculus and advanced calculus should suffice for these. Here are two undergraduate books on astronomy: An Introduction to stellar Astrophysics - Francis Le blanc - this is an excellent introduction and the chapter on stellar atmospheres is excellent An introduction to Modern Astrophysics - Carroll/Ostle - also called the big orange book is excellent in its encyclopaedic coverage of astrophysics although in my opinion some bits could be more simply explained Now onto the postgraduate level stuff Stellar Photospheres - David Gray - I have only just bought this and in my opinion this is THE book on stellar spectroscopy if you want to see how the professionals do it - it covers a lot of ground and is quite readable and has plenty references out to the professional literature. I am ploughing through it and this book represents about how far my undertanding has got to in why spectra are the way they are. Now for the advanced stuff: Gray covers only slightly more than Local Thermodynamic Equilibrium models so where the non-local thermodynamic equilibrium models are described have to head to Rutten: Radiative Transfer in Stellar Atmospheres - this is freely available on the web and will be the next book I want to study after completing Gray And finally the book we have all been waiting for which is the new edition of Stellar Atmospheres which is written by Mihalas and Hubeny which is due out on amazon any month now....this one is not for the light hearted... I think thats enough for now any comments from people out there welcome I am interested to see who out there is trying to climb this slippery pole of finding out what the spectra of stars is all about John