21 October 2004 UWO

Studying stars with spectroscopy

John Landstreet

Department of Physics and Astronomy

University of Western Ontario

UWO21 October 2004

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?

UWO21 October 2004

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

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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

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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

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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 ….

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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

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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

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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.

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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)!