Working with the HG-WELS data

This page is similar in concept to the summer visit pages for my prior teams (Working with the C-CWEL data; Working with the C-WAYS data page; Working with the BRCs; Working with CG4+SA101 page; Working with L1688) HOWEVER, this page was developed and updated specifically for the 2014 HG-WELS team visit. Because this team has a very different science goal, it is very different, at least in places, than these other pages.

Please note: NONE of these pages are meant to be used without applying your brain! They are NOT cookbooks! This is presented as a linear progression because of the nature of this page, but we have already done some things "out of order", and moreover, chances are excellent that you will go back and redo different pieces of this at different stages of your work.

Assembling our initial catalog

DONE but kept here for reference because it is easy to forget.

We assembled our catalog in the spring from basically three sources:

• de la Reza's published catalog - biased towards sources bright in the IR
• Carlberg's published catalog - much less biased set of giants assembled without regard to IR or Li, spanning range of vsini
• Carlberg's private communication set of objects mentioned in the literature as Li rich (some of which subsequently vanished from de la reza's papers)

We have a list of 196 unique objects that we assembled, keeping track of where the source was listed. Sometimes it is listed in more than one of those three places.

Big picture goal: Understand which sources have been studied for these three samples, and what has been measured for them.

Relevant links:

• How can I find out what scientists already know about a particular astronomy topic or object?
• I'm ready to go on to the "Advanced" Literature Searching section

Assembling other data from large catalogs

PARTIALLY DONE - Luisa did this in their full glory but we need to do a few as a check and so you understand what I did.

Big picture goal: We are ultimately trying to get an understanding of whether or not these stars have excesses. It will further that goal if we accumulate as much data as we can from a variety of sources.

More specific shorter term goals: Use IRSA's catalog search to start assembling multi-wavelength information about these sources.

Relevant links:

• How can I get data from other wavelengths to compare with infrared data from Spitzer? - though this focuses more on imaging. We need photometry.
• FinderChart at IRSA
• IRSA in general
• Catalog search at IRSA IMPORTANT AND NEW TO YOU
• Resolution - spatial resolution matters!
• HG-WELS Resolution Worksheet - the worksheet we did in the Spring
• Vizier

FLESH THIS OUT one to one matching. copy-paste in excel. bookkeeping. phot quality flags. sometimes called 'bandmerging'.

Checking that the coordinates and photometry make sense, part 1 - image inspection

DONE at least a first pass.

Big picture goal: Just because the computer says it, does not make it right. Always check to make sure that the computer is correct. (AKA "count your change.")

More specific shorter term goals: Investigate the images for each source. Do we have the coordinates right? Is it just one point source? This is one of the major goals of our work, to determine if there is "source confusion" at these locations.

Relevant links:

• FinderChart at IRSA

FLESH THIS OUT

you may need to loop back to the prior step after doing this. i did.

Making SEDs

PARTIALLY DONE - Luisa made full SEDs in their full glory but we need to do a few as a check and so you understand what I did. We may skip this initially and circle back.

WARNING: lots of math and programming spreadsheets here too... you WILL do this more than once to get the units right!

Big picture goal: Understand how to convert magnitudes back and forth to flux densities. Understand what an SED is and why it matters.

More specific shorter term goals: Program a spreadsheet to convert between mags and flux densities. Make at least one SED yourself.

Make sure you understand how to get the fluxes from the magnitudes. This is not easy to do right the first time, so you will get the wrong answer the first few times you try.

Relevant links:

• Units
• SED plots
• Central wavelengths and zero points
• Studying Young Stars

We will ultimately need to make SEDs for everything, but for purposes of this example, let's work with the same test set of four as we worked with for Spitzer photometry above: 5384, 3704, 4009, and 3896. Start with just one. You will ultimately plot log (lambda*F(lambda)) vs log (lambda) -- see the Units page. It will take time to get the units right, but once you do it right the first time, all the rest come along for free (if you're working in a spreadsheet). Spend some time looking at the SEDs. Look at their similarities and differences. Identify the bad ones, and discuss with the others why/whether to drop them off the list of YSO candidates. See also stuff above about data at other wavelengths, and include literature/archival data from other sources where appropriate and possible. Make sure to keep careful track of those things that are limits rather than detections.

Another try at explaining:

• What do you have? UBVRIriHalpha, JHK, WISE data in Vega mags. IRAC, MIPS data in microJanskys.
• What do you need to get? everything into Jy, which are units of Fnu. Then convert your Fnu in Jy into Fnu in cgs units, ergs/s/cm2/Hz, so multiply by 10^-23. Then convert your Fnu into Flambda in cgs units, so multiply by c/lambda^2, with c=2.99d10 cm/s and lambda in cm (not microns!). Then get lambda*Flambda by multiplying by lambda in cm. Plot log (lambda*Flambda) vs. log (lambda).
• Once you make your first SED correctly, the rest are easy. But that first one is hard.
• Then you need to look through each of the SEDs and decide which look like you expect, which need photometry to be checked, and which seem unlikely to be legitimate YSOs. This is a judgement call, and your judgement will improve with time as you gain some experience.

Questions for you:

1. What do the IR excesses look like in your plots? Do they look like you expected? Like objects in Monday's ppt or elsewhere?
2. Find some SEDs of things you know are not young stars for comparison - pick some with zero IR color. (You may be able to find some sources with zero color among the previously identified YSOs). What do they look like?

Assessing SEDs

Now that you've made some SEDs, we need to next look at them with a critical eye.

Big picture goal: Understand what to expect in a YSO SED and how to discard objects for having questionable SEDs (or put them on the list for checking source matching, photometry, etc).

More specific goals: Examine the SEDs for all of our candidate objects. Use them to further evaluate the quality of the YSO candidates from the YSO candidate list.

Relevant links:

• Studying Young Stars
• the detailed object-by-object discussion in the appendix of the cg4 paper.

Test set: This list represents a set of things not in BRC 38 (they actually are all BRC27) that I deliberately picked so as to show you the range of things you might find in BRC 38, but are of a wider variety than we are likely to find in BRC 38. Some are deliberately tricky. After you do these, feel free to start on the BRC 38 set using the SEDs you made in the prior step. File:Sedtestsetblank.xlsx and File:Brc27trainingjustSEDs.pptx

For the BRC 38 set, after you make all the SEDs, you will need to spend some time really looking at the SEDs -- all of them! -- but for now, return to the set of four test SEDs you made above. Look at their similarities and differences. Identify the bad points, and discuss with the others why/whether to drop them off the list of YSO candidates. See also stuff above about data at other wavelengths, and include literature/archival data from other sources where appropriate and possible. Make sure to keep careful track of those things that are limits rather than detections. Compare your notes on SEDs with notes on images (e.g., if it's tagged "iffy" in images and "iffy" in SEDs, chances are excellent that it is not a good candidate).

Questions for you:

1. Which objects look like clear YSO SEDs? Which objects do not? Keep good notes on this throughout BRC 38!
2. What's the deal with this one (why does it look like this)? (In my SED, the y-axis units are cgs units [sorry], *=FTN data, +=optical literature data, diamonds=2mass, circles=irac, stars=WISE, arrows=limits, and boxes=MIPS if they exist, which they don't here.)

Answer: This source is near a bright nebulous patch in the WISE images that either is being inappropriately tagged as a point source (with its flux densities attached to this source) or whose brightness is contaminating the photometry beyond recovery. The Spitzer data are critical for sorting out what is going on here. And I still don't know what is going on with the optical data - it's apparently wrong for this source, but this is the best possible match given the information we have in the literature, so maybe the people who wrote the paper with the optical data screwed something up either in bandmerging or in their photometry.