Transcript My power point presentation on spectroscopy of stars (ppt file)

Studying stars with spectroscopy
John Landstreet
Department of Physics and Astronomy
University of Western Ontario
21 October 2004
UWO
How can we learn anything about a star?
• No star has ever been visited by humans
• No space probe has gone to any star
• We can hardly see any surface details from
the solar system, except for our own Sun
• The interior of a star is even more hidden
than the surface layers
• Essentially the only information a star sends
to us is its electromagnetic radiation
• Can we dissect the radiation from a star to
find out anything interesting about the star?
21 October 2004
UWO
What does star’s surface look like?
• Our Sun gives
us hints of what
we might see
on other stars
• The surface is
marked with
sunspots
• These are
regions where a
strong magnetic
field emerges
through the
atmosphere
21 October 2004
UWO
Above the visible atmosphere
• Above the visible
surface is the hot
gas of the corona
• It is visible in Xray
images taken from
satellites
• The bright regions
are at a
temperature of
about 1,000,000 K
21 October 2004
UWO
What could we see at a distance?
• We could
measure the
Sun’s energy
distribution
• This indicates a
body at about
6,000K
• The small
excess of
radiation in the
UV is from the
hot outer layers
21 October 2004
UWO
We could observe the spectrum
• The spectrum is
filled with
absorption lines due
to various elements
• These lines tell us
about temperature,
pressure, chemical
composition,
rotation velocity,
magnetic fields,
binary companions
….
21 October 2004
UWO
I am a stellar spectroscopist
• I take spectra of distant stars and then try to discover
what they are telling us about those stars
• I do this by modelling the spectra, using a computer
programme that computes what the spectrum would look
like for a “model star”
• The parameters of the model are changed until good
agreement is reached, or until the process fails
• A successful model tells me many facts about the star,
such as its chemical makeup, and where in its life history
it is at present
• Failed modelling give me hints on missing physics in my
models
21 October 2004
UWO
A lab-rat’s view of stellar
spectroscopy
• What is actually involved in carrying through
a research project in this branch of
astrophysics?
• Let me give you a guided tour through some
of the steps in a research project, from
thinking it up to publishing the paper
21 October 2004
UWO
Step one: get telescope time
• Start by thinking of a good project or question and the
observations that it needs
• Apply for telescope time at an appropriate telescope,
many months in advance
• This is a competition, usually with 3 or more nights
requested for every 1 night assigned
• IF time is granted, we will be assigned a few specific
nights
• If the nights are cloudy, or the equipment fails, it’s our
tough luck. We start again.
21 October 2004
UWO
Step two: get the observations
• My choice is often the
Canada-France-Hawaii
telescope on Mauna
Kea, Hawaii
• The telescope has a
superb high-resolution
spectrograph, and a
brand new
spectropolarimeter,
“Espadons”, as well as
wonderful imaging
capabilities
21 October 2004
UWO
The CFHT
• The telescope is
located at 4200 m
altitude
• It has a 3.6-m diameter
primary mirror
• The spectrograph has
resolving power to
resolve lines 3 km/s in
width
• A spectrum covers
about 100 A (10 nm),
3% of visible range
21 October 2004
UWO
Step three: “reduce” the spectra
• The raw spectra are 2D
images on a CCD
detector.
• They must be extracted
to 1D, have “artifacts”
removed, be
normalized, and have a
wavelength calibration
applied
• We use a big
programme called IRAF
for this
21 October 2004
UWO
Step four: identify spectral lines
• Once the spectra have
been reduced, analysis
begins
• We need to determine
some stellar parameters
(temperature, gravity)
from photometric
measurements
• Then spectral lines
must be identified and
necessary atomic data
obtained from database
21 October 2004
UWO
Step five: modelling
• I have written a large computer programme
that can compute the spectrum of a model star
• The programme takes hydrostatic and thermal
structure of stellar atmosphere as input
• The programme reads in atomic line (and
other) data, solves equation of radiative
transfer for many wavelengths, and compares
resulting spectrum with observed one
• If desired, the programme can iterate
parameters to improve fit to observations
21 October 2004
UWO
Some modelling results
• Sometimes modelling is extremely successful
• An example is the star Sirius, a main
sequence star about 2.28 times as massive
as Sun, with a white dwarf companion
• For this star, the excellent fit to this spectrum
allows us to determine relative abundances
of Fe, Cr, Si, and Zr
• We also get rotation velocity (16.5 km/s) and
atmospheric turbulent velocity (2 km/s)
21 October 2004
UWO
Some modelling results
• Sometime fitting works reasonably well,
but not perfectly
• In this case we can often obtain
approximate values of parameters such
as chemical abundances, rotation, …
• The remaining discrepancies give us
information about physics missing from
the model
• For the supergiant omicron Scorpii, the
discrepancy shows that I have not
correctly modelled the atmosphere’s
convective motions
21 October 2004
UWO
Some modelling results
• For some stars the discrepancies are quite
severe, and we get very uncertain parameter
values
• An example is the magnetic star HD 66318 in
the open cluster NGC 2516
• This star shows strong Zeeman splitting in
almost every spectral line, due to a magnetic
field of 14.5 kG (1.45 Tesla)
• The trace elements in the atmosphere are
stratified vertically and horizontally patchy. I
have not included enough of these effects in my
model to reproduce the observed spectrum
21 October 2004
UWO
So what?
• Why is this kind of work interesting to anyone
but me?
• Establishing facts about stars is central to
being able to understand how they form,
develop through their lives, and finally die
• As a result we now know a lot about how our
Sun, our solar system, our stellar neighbours,
our Milky Way galaxy, and the universe have
developed through their histories
• But there are still MANY missing pieces for
some of you to work on
21 October 2004
UWO
Thanks to the team
•
•
•
•
Tim Officer
Jessie Silaj
Anna Townshend
Many previous undergraduate helpers
(including UWO Prof Aaron Sigut…)
• And to NSERC and HIA-NRC for research
funding (e.g. $20,000 per assigned night at
CFHT)!
21 October 2004
UWO