Carina Nebula

Astronomy 61: Current Problems in Astronomy and Astrophysics

Prof. David Cohen

Spring 2015
Friday 11:30 to 12:20
SC 113 (seminar room)


Townsley image windtabs transmission windtabs fit HD93129





posted 3/25

We have job candidate talks this Wednesday, March 25 (4:30), and Friday, March 27 (12:30). Please try to attend both talks.

Archive of old announcements.



posted 3/25

This week's assignment is to bring all our modeling ingredients together and (1) reproduce the fit to the ACIS (zeroth order) spectrum of HD93129A, described on p. 3361 (right-hand column) of Cohen et al., 2011, MNRAS, 415, 3354 (ADS) – make a plot of your best-fit model and write a short summary of the model parameters and the fit statistic value. (2) Run the error task in XSPEC to find the confidence limits on the best-fit temperatures, normalizations, and column densities. Summarize those results. (3) We have been fitting (multiple) APEC components, allowing both the temperature and normalization of each component to be free parameters. That means our two-temperature models have four free parameters (just for the two APEC components). Let's explore a different type of multi-component APEC model – one where we have four temperature components, but we freeze the temperatures, each at a different value (to mimic a continuous temperature distribution; so we'll make them relatively evenly spaced in temperature) and allow only the normalization of each component to be a free parameter of the fit. Please do some exploratory model fitting of this sort to the HD93129A spectrum. Include TBABS and WINDTABS, as you did in task 1, here. I'll leave it up to you to decide what four temperatures to use. But inform yourself about the temperature ranges of the line emissivities, which are plotted in Fig. 1 of Cohen et al., "Measuring the shock-heating rate in the winds of O stars using X-ray line spectra," 2014, MNRAS, 444, 3729 (ADS) when you choose your four temperatures (you don't want to miss any of the line emissivity peaks by too much; and note that 12 million K is 1 keV – APEC likes its temperatures in keV). Our ultimate goals with this third task are to find an optimal set of four (or maybe just three) temperatures to use in all our multi-component APEC modeling for every star we analyze, and to see if we can achieve a better fit (to HD93129A) with the four-temperature APEC model than the two-temperature model (with free temperatures).

So, there are three tasks in this week's assignment. For tasks (1) and (2) you should hand something in (via email) by midnight on Thursday. For task three, please really do some exploratory model fitting, but there's nothing to hand in before class. Bring your computers to class on Friday. We will work on task (3) in class together. But please do get a start on it ahead of time. Let me know if you have any questions or run into any problems.

Archive of old assignments.



Class information

Class Moodle page

class information (posted before the semester)


Useful resources

Local computing information

ADS: for papers
astro-ph: for more papers
SIMBAD: for objects
Astronomical Catalogs: for catalogs

X-ray astronomy

Chandra X-ray Center
XSPEC manual: spectral fitting
ATOMDB webguide (line list)

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-

Last modified: March 25, 2015