Astronomy 61: Current Problems in Astronomy and Astrophysics
Prof. David Cohen
Friday 11:30 to 12:20
SC 113 (seminar room)
Basic class information is posted on the right-hand side of this page.
We will be offering the half-credit, CR/NC class, Astro 61, in the spring, as we do almost every year. Normally, I'd be telling you how the class is about learning to read and discuss research papers in the current literature, and learning about some subject area – with a different area chosen each year. Last year, it was compact objects (neutron stars, white dwarfs, black holes). From that page: "The goals of this class are to learn how to search and use the astronomical literature, to read papers critically, to apply your knowledge of physics and astronomy to research problems of current interest, and to learn about new developments in astrophysics." And our weekly process would be: "Each week, one of us will chose a paper for us all to read. We'll read it, and then the person who chose the paper will send a summary of it to the class by 9PM on Thursday. And each (other) student will post at least one question on the discussion board forum, also by 9PM on Thursday. The discussion board is available on the class Moodle page."
But this year, we're going to do something different. Or really something extra. We will still read and discuss papers, and we'll still have a particular topic to focus on. But we will also work with real data – from the Chandra X-ray Observatory – in order to try to answer an outstanding research question – in the field of massive star winds – namely, what are the mass-loss rates of O star winds?
We will use a large, homogeneous dataset of over fifty medium-resolution X-ray spectra of O stars taken in a large Chandra campaign to map out and study the physics of a giant star formation region known as the Carina Complex (associated with the Carina Nebula and comprising also several individual star clusters). And we will use a new technique for modeling the attenuation of X-rays by the winds of these O stars. The X-rays are produced within those winds by a mechanism called the line deshadowing instability (LDI) but some of them are absorbed by the wind material before they can escape, and the degree of this absorption tells us about the wind mass-loss rate of the star. We can determine the degree of attenuation because its effect is different on X-rays of different energies. And as a bonus, by modeling the wind absorption, we are able to better determine the intrinsic properties of the LDI-generated hot wind plasma, and thus provide information to constrain the models of X-ray production in massive stars.
My extended research group does the modeling of the X-ray production via the LDI and we also developed the technique for measuring the wind absorption via X-ray spectroscopy. And I have been heavily involved in the Chandra Carina Complex Project, which was led by Leisa Townsley at Penn State. I have published a paper (with James MacArthur (class of 2011) and Emma Wollman (class of 2009)) applying this technique to one particular star in Carina. And Leisa and her colleagues have extracted and calibrated the spectra of the other O stars in the data set. But the X-ray modeling of these spectra is yet to be done.
So, in Astro 61 this spring, we will read papers about O star X-ray emission, stellar winds, the Carina Complex, and X-ray spectral modeling. But we will also learn how to fit models to Chandra data and each student will be responsible for modeling the X-ray spectra of a subset of the O star sample and thereby derive wind mass-loss rates for their stars. As a group, we will thus analyze the entire dataset and together we will draw conclusions about the overall strength of O star winds and how it varies with other properties of these stars (presumably their luminosities). Our ultimate goal will be to write a research paper together on our findings.
In terms of the workload for the class, most weeks we will still read and discuss one relevant paper. But we will also do some work with data each week, too (outside of class – students will work on their own or with each other). And occasionally we will spend our hour of class discussing the data modeling issues rather than discussing a new paper. Toward the end of the semester, we will outline and write a paper. If our results are not complete, we will still write a document that can serve as an interim report for our research group, and that will form the basis of an eventual paper.
Update (11/23): students should expect to work five to seven hours per week outside of class.
If you think you might be interested in taking this class, explore the links embedded in the announcement text above, and also the ones collected on the right-hand side of this page. Keep in mind that much of the information in these documents will be too unfamiliar for you to understand now, but they can still give you a flavor of the material we'll be learning. And by the end of the semester, you will be able to understand much of this material.
It is likely that the first paper we'll read and discuss will be Townsley et al., "An Introduction to the Chandra Carina Complex Project," ApJS, 194, 1 (2011).
We will read one paper per week, and each student will post a question about that paper on a Moodle discussion forum by Thursday evening each week. I will maintain an archive here of the papers we have read.
We will be analyzing Chandra X-ray spectra using a software package called XSPEC. See the link to the manual, below.
Weekly data assignments as well as reading assignments will be posted on this page, in the assignments section on the left.
There are quite a few valuable resources you'll be consulting frequently, and I have collected them below, and will add new ones as it becomes apparent that they're useful.
The class is CR/NC, meets just one hour per week, and is a half-credit class. A student may take Astro 61 for credit more than once. Note also that it is a requirement for the astronomy major and the astro minor, though not for the astrophysics major. Taking it at least once is recommended if you're planning on going to graduate school. And we do generally offer it each spring.
general astro research information
Astrobetter – lots of useful information about the nitty-gritty of astronomical research and many aspects of being an astronomy student and astronomer
Astrobites – very short summaries of interesting research papers, written for undergraduates by graduate students.
This page is maintained by David Cohen
cohen -at- astro.swarthmore.edu
Last modified: November 23, 2014