Download Composition of the Sun - Indiana University Astronomy

Astronomy A305 - Modern Observational Techniques
Lab Assignment 9
Determining the Composition of the Sun
Goal: Continue to improve your IRAF and Linux skills. Become familiar with the
identification of spectral lines and the measurement of equivalent widths. Use your
measurements from a solar spectrum to determine the composition of the Sun.
What to hand in - Hand in a write-up of your work, including:
• An introduction describing the measurement you are trying to make
• A description of your procedure and observations, including a list of the lines you
measured, and their lab wavelengths, excitation potentials, the "gf-values," and
equivalent widths of each.
• A brief summary of what problems you encountered in trying to identify lines,
measure the equivalent widths.
• A description of your analysis procedures and plots, including error analysis
o How accurately can you determine the iron abundance based on your
measurements?
o How accurately can you determine the Sun's temperature based on your
measurements?
o How uncertain is the abundance of each of these species based on uncertainties in
equivalent width and temperature? Be quantitative.
• A copy of the MOOG summary output file
• A copy of the MOOG plots for any species with at least 6 lines measured.
• The iron abundance you derive for the Sun, in bracket notation (see below)
• A table of the abundances of the other elements you included, also in bracket notation
in the form [m/H]. The table should include the mean abundance and standard
deviation and the number of lines used of each element.
An appropriate length is 3-5 typed pages, single spaced, 12-point font, with 1" margins,
plus figures and tables. You may work with partners to obtain the data for the lab, but
your analysis and writeup should be entirely your own. Be sure to review the general lab
instructions on the "lab options" sheet.
The Solar Spectrum
In the solar subdirectory of the A305 directory, you will find a high signal-to-noise ratio,
moderate resolution spectrum of the daytime sky taken with the Hydra fiber spectrograph
on WIYN. Copy that spectrum into a new subdirectory of your own home directory and
examine it using the IRAF task splot. In addition to the FITS file of the spectrum are
three additional files you will also need (params, model.sun, and linelist); copy them into
your new directory, too. Several additional models are also available to assist with the
determination of the sensitivity of your derived abundance to temperature.
In Rm 246 are a copy of the solar spectrum, and of a line list of solar lines prepared by F.
Thevenin (Astronomy and Astrophysics Supplement Series, Vol. 82, pp. 179-188, 1990).
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Use the solar spectrum and the line list to identify 15 relatively unblended spectral
lines of neutral iron (Fe I) in the spectrum and up to 20 lines of various other atomic
species.
Verify your line identifications by comparing the pattern of line strengths in the
Hydra spectrum with the published solar spectrum. The measured wavelengths of the
lines you select should also agree very closely (within 0.05 Angstroms) with their
laboratory wavelengths.
Make sure the Fe lines you select have a range of excitation potential (0 to 5 eV).
Use the splot task to measure the equivalent widths of the lines (try splot keys e, d, or
k). Make sure that the lines you select include a range of equivalent widths from
about 5 to 150 mA.
Once you have selected the lines and measured their equivalent widths, enter the
wavelengths, equivalent widths, excitation potentials, and log gf values into a text file.
The file linelist is an example of the format you must follow in entering the data. Be sure
NOT to use tabs in the file.
The next step is to use the MOOG spectrum synthesis program to derive the solar
composition. To execute the program, simply type "moog" in a terminal window. The
program will request a parameter file (params), which includes the name of the model
atmosphere and the name of the line list file. If you use different filenames, be sure to
edit the params file to include the new names. Documentation for MOOG is available in
Rm 311. The physics behind what MOOG is doing is covered in Astronomy 451.
MOOG will produce a plot of the iron abundance vs. excitation potential, vs. line
strength, and vs. wavelength. You may save this file for printing by using the "f" key in
MOOG at the appropriate time. MOOG also writes a summary of its output to the file
"out2."
The solar sub-directory includes additional model atmospheres to allow you to determine
how sensitively your derived iron abundance depends on temperature and gravity.
Derive the solar abundance using several different models to determine how changing the
model atmosphere parameters affects the derivation of the solar composition. For
example if you use a temperature that is 100K hotter, does the abundance of iron come
out the same or is it different? By how much?
Note that the standard deviation of the Fe I lines also applies to other lines. The standard
deviation for Fe I gives a typical uncertainty for the uncertainty of the abundance
measurement from any individual line.
Notation
In stellar astrophysics, the abundance of an element is given in two forms, the "bracket
notation" and the "log ε" notation.
In the bracket notation, abundances are expressed relative to a standard star, typically the
Sun.
⎛ N ( Fe) / N ( H ) Star ⎞
⎟⎟ = log( N ( Fe) / N ( H ) Star ) − log( N ( Fe) / N ( H ) Sun )
[ Fe / H ] = log⎜⎜
⎝ N ( Fe) / N ( H ) Sun ⎠
In this notation [Fe/H] = -1 means that the star has 1/10 of the iron-to-hydrogen ratio of
the Sun.
In the log ε notation, abundances are expressed relative to 1012 hydrogen atoms.
Log ε(H) = 12
For every H atom, there are 10-4.5 iron atoms in the Sun, so
Log ε(Fe) = 7.5
MOOG presents the iron abundance in the log ε notation.
Bracket notation is also used for the abundances of elements relative to iron. If you know
[Ca/H], you can determine [Ca/Fe] by simply taking the difference
[Ca/Fe] = [Ca/H] - [Fe/H]
You may want to demonstrate to yourself mathematically why this works using the
definitions give in Exercise 5.
Values of log ε for each element that Moog uses are given on the back side.
Splot hotkeys:
? - This display
/ - Cycle thru short help on stat line
a - Autoexpand between cursors
b - Toggle base plot level to 0.0
c - Clear and redraw full spectrum
d - Deblend lines using profile models
e - Equiv. width, integ flux, center
f - Arithmetic functions: log, sqrt...
g - Get new image and plot
h - Equivalent widths(*)
i - Write current image as new image
j - Fudge a point to Y-cursor value
k - Profile fit to single line(*)
l - Convert to F-lambda
m - Mean, RMS, snr in marked region
n - Convert to F-nu
o - Toggle overplot of following plot
p - Convert to wavelength scale
q - Quit and exit
r - Redraw the current window
s - Smooth (boxcar)
t - Fit continuum(*)
u - Adjust coordinate scale(*)
v - Velocity scale (toggle)
w - Window the graph
x - Connects 2 cursor positions
y - Plot std star flux from calib file
z - Expand x range by factor of 2
) - Go to next spectrum in image
( - Go to previous spectrum in image
# - Select new line/aperture
% - Select new band
$ - Toggle wavelength/pixel scale
- - Subtract deblended fit
, - Down slide spectrum
. - Up slide spectrum
I - Interrupt task immediately
<space> - Cursor position and flux