### ASTRONOMY 3303   HW#2     due Wed Sep 13, 2017

Print a copy of this page and enter it in your portfolio. Answer the questions (preferably type them) separately; you will include the graded answers in your portfolio after they are handed back.

For this homework, we assume you have reviewed Chapter 1, Chapter 2-2.4, 3.5 and Appendices A and B of the textbook "Extragalactic Astronomy and Cosmology" by Peter Schneider and the lecture notes through Sep 6th.

Part I: Quick calculations.   Make sure you take the quickest route, but be clear about what you do.

1.   Suppose you observe an emission line of [OII] from an HII region at l = 100o, b = 0o. The rest wavelength of the line is λ = 372.78 nm; the observed wavelength is 372.7625 nm.

a.   Draw a diagram (like Figure 2.25 in the textbook) which shows the location of the HII region in the Galactic plane.

b.   Based on the Doppler shift of the line, give an estimate of the heliocentric distance of the HII region.

c.   What is the distance of the HII region from the Galactic Center?

2.   Consider the case of two stars (1 and 2) initially at the same galactic longitude but 0.2 kpc apart, with R1 = 10 kpc and R2 = 10.2 kpc, where R is their galactocentric distance. Assume a galactic rotation curve Θ(R) for 4 < R < 12 kpc of the form:     Θ(R) = 65. + 42 R - 3.6 R2 + 0.1 R3    , where R is in kpc and Theta in km/s, and that these two stars have circular orbits in the Galactic plane. Find the time at which Star #1 will have gained half a lap in its orbit around the Galactic Center on Star #2.

Part II: The Structure of the Milky Way from Observations of Nearby Stars.

In HW #1, we looked at a public data set containing the properties and distances of stars contained in several public catalogs (the 2nd datafile from HW#1). The stars contained in those catalogs are generally nearby, with distances less than 1 kpc in general. Nonetheless, we can learn something about the Galactic structure from them.

For this exercise, we can make use of a different compilation with includes the observed radial velocities of some of the stars which are also contained in the catalog we used last time. Download the file and save it in the same location where you have stored the file from last week. Then use TOPCAT to explore what the two catalogs can reveal about the structure of the Milky Way.

a.   Load both CSV files into "TOPCAT". Let's call them file1 (the one from last week) and file2 (the new one). Review the columns in the new dataset. Notice that it contains the Right Ascension (RA) in decimal degrees (radeg) whereas the one on the file from last week contains the RA in decimal hours. Create a new column in file1 with the RA in decimal degrees. We will need this later.

b.   Not all of the stars in file2 actually have radial velocities vradial; we only need the ones that do. Make a row subset (called e.g., "withRV") containing only the stars with measured radial velocities assuming the vradial is entered as 0 where it has not been measured. What percentage of the entries make it into the row subset by this criterion?   Hint: the available mathematical operators in TOPCAT are documented at http://www.star.bris.ac.uk/~mbt/topcat/sun253/sec7.5.html.

c.   Join the two catalogs in TOPCAT using the Join option, performing a pair match, matching by the Sky algorithm with a maximum error of 5 arcsec. Be sure to select the correct row subset back in the main TOPCAT window (where you loaded the files: you should see two subsets (e.g. "all" and "withRV") now for file2). Save the resultant "matched" file. How many matches do you find?

d.   Generate two row subsets of the matched file according to color and absolute magnitude, corresponding separately to luminous blue and red stars:
Blue:                   BminusV < 0.0 & Absmag < 4
RedGiants:         BminusV > 1.2 & BminusV < 2.0 & Absmag < 1
How many stars are in each of the two subsets?

e.   Separately, look at the mean and standard deviations of the radial velocity distribution of the two subsets using Column statistics (under Views). Although this is admittedly crude, argue that the red stars are older than the blue ones based on this result alone.

f.   Using the Sky Plot (under Graphics), look at the distribution on the sky of the two subsets. Though likewise crude, argue that the blue stars are younger than the red ones based on this result alone.

Part III: In-class presentation:
You are each assigned a known-to-be-exceptional star in the Milky Way. Your assignment is to prepare a single slide that summarizes why the star is "exceptional" in the broad sense. Bulleted points are fine; a diagram from a scholarly source could be placed on a second slide (but it must be from a scholarly source!) if you want.

• Don't leave this for the last minute; you may have questions about where to look, on what to focus, or how to interpret what you read. Questions are welcome in person or by email (if they are posed sufficiently in advance).

• Send your slide(s) preferably in PDF format (unless an animation is really necessary) to Dominik (riechers_AT_astro.cornell.edu) by noon on Wed. Sep 13th. He will show all the slides on his laptop.

• Print your slide and include it with the rest of this assignment.

• Provide a list of all sources which you used to research your star. You must include at least one scholarly reference (publication in a refereed journal or conference proceedings) that you actually used.

• Be prepared to speak for not more than 3 minutes(!) on why you classmates should find this star as exceptional as theirs.

Exceptional Milky Way Stars
Who Topic
Christopher R136a1
Clare SDSS J102915+172927, "Caffau's star"
David T Tauri (T Tau)
Hortense SDSS J110217.48+411315.4
Isabel KIC 8462852, "Tabby's star"
Julia V4647 Sgr, the "Pistol"
Peter PSR B1957+20, the "Black Widow"
Ruixin SDSS J090745.0+024507, the "Outcast"
Sam Eta Carinae (η Car)
Tim SS 433

Hint: Professional astronomical research articles are catalogued by the Astrophysics Data System which provides useful tools to search its database. We suggest that you try out the "Astronomy query form" at http://adsabs.harvard.edu/abstract_service.html. Enter your star's name under "Enter Abstract Words" and click "Require text for selection". Then hit "Send Query" to get a list of references, starting with the most recent first. You can then click on the link to view the paper's abstract and get further links to the paper itself (nearly all of which should be available via the CU library). Ask if you have trouble with this (and keep the ADS in mind for future assignments).

You may find a lot of references! Keep in mind the intended brevity of your in-class presentation. You are not expected to read through all the references! We suggest that you look at some that are very recent, but you may also want to look at the earliest ones which may relate to the star's discovery. If you need help zeroing in on what to include, please email or ask Martha (mph6).