Difference between revisions of "Working with the HG-WELS data"
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*[[SED plots]] | *[[SED plots]] | ||
*[[Central wavelengths and zero points]] | *[[Central wavelengths and zero points]] | ||
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| − | We will ultimately need to make SEDs for everything, but | + | We will ultimately need to make SEDs for everything, for all bands, but to make this tractable, let's work with just some bands (WHAT) and just a few sources (WHICH? PICK EXTREME SOURCES AND FLESHED OUT ONES). 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 more or less 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, circle back to fix photometry if necessary. Discuss with the others what to do and why. Make sure to keep careful track of those things that are limits rather than detections. |
Another try at explaining: | Another try at explaining: | ||
| − | *''What do you have?'' | + | *''What do you have?'' UBVRI, JHK, WISE data in Vega mags. IRAS data in Janskys. |
*''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). | *''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. | *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 | + | *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 stars. This is a judgement call, and your judgement will improve with time as you gain some experience. |
'''Questions for you''': | '''Questions for you''': | ||
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#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? | #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? | ||
| − | + | #UPDATE THIS WITH BETTER SED EXAMPLE 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.) | |
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[[file:chauhan109sed.png]] | [[file:chauhan109sed.png]] | ||
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. | 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. | ||
Revision as of 23:32, 1 July 2014
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.
Contents
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:
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:
We will ultimately need to make SEDs for everything, for all bands, but to make this tractable, let's work with just some bands (WHAT) and just a few sources (WHICH? PICK EXTREME SOURCES AND FLESHED OUT ONES). 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 more or less 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, circle back to fix photometry if necessary. Discuss with the others what to do and why. Make sure to keep careful track of those things that are limits rather than detections.
Another try at explaining:
- What do you have? UBVRI, JHK, WISE data in Vega mags. IRAS data in Janskys.
- 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 stars. This is a judgement call, and your judgement will improve with time as you gain some experience.
Questions for you:
- What do the IR excesses look like in your plots? Do they look like you expected? Like objects in Monday's ppt or elsewhere?
- 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?
- UPDATE THIS WITH BETTER SED EXAMPLE 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.
