Working with the HG-WELS data

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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, for the most part, 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. AAAAND, LET'S DISCUSS THE QUESTIONS BELOW.

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

We assembled our catalog in the spring from 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. Some objects are listed in more than one of those three places.

Relevant links:

Questions for you

  • Why is it important to keep track of which stars came from which of these samples?
  • Why do we not need to assemble more stars from other places? (Both scientific and practical reasons!)

Assembling other data from large catalogs

Luisa did this in its full glory but we need to do a few as a check and so you understand what I did...and so you can do it yourself later on your own for other projects.

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. Especially since our sources are (on average) bright, we have more potential catalogs that we can draw on.

Relevant links:

More words: Several surveys with archived data covered the whole sky. There are other surveys that just covered part of the sky. We are trying ultimately to determine if these sources have infrared (IR) excesses. We would like to assemble data from as many places as we can to flesh out the SEDs between optical (V-band) and 100 microns (the longest IRAS wavelength). As we spelled out in the proposal, the meat of what we are likely to use is probably going to be WISE 1 and WISE 4, or possibly K and WISE 4. But, as we will see below, having additional data can REALLY help us to assess whether or not we believe the two bands we will use to determine whether or not our sources have IR excesses.

  1. Get from your email (or assemble yourself) an IPAC table file with all our targets and their positions in decimal ra/dec.
  2. Go to the catalog search at IRSA
  3. Ultimately, for this portion of the process, you will want to assemble source lists from 2MASS, WISE, and IRAS. (For the record, I did these plus many more -- those, plus Akari, Denis, both PSC and FSC from IRAS, MSX, SEIP, and certain bright objects by hand in Vizier.) Pick one of 2MASS, WISE, and IRAS to start with.
  4. Do a multi-object search using that IPAC table file. Make sure to use 1-to-1 matching -- this option finds the source closest to your search position within your given search radius, and returns one line per object, even one line for those things that did not find a match. This greatly helps with the next steps.
  5. Look at what it gives you in response to your search. It comes up with a plot of distance to your source as a function of source number. Why is this important? Is there a place in the list where it gets much worse? Why is this?
  6. Save the output of the search to a file. Rename it and put it someplace you can find it.
  7. Circle back and repeat for the rest of 2MASS, WISE, and IRAS. You will need a smallish radius for 2MASS and a largish radius for WISE and IRAS. (I used, I think, 5 arcsec for 2MASS and 20 arcsec for WISE and IRAS.)
  8. Note that, as long as you use the same input tbl file every time and choose 1-to-1 matching every time, there is always the same number of lines in the output file. This makes matching across catalogs very easy. Note that all catalogs return the same columns (source name, input ra/dec, matched source id, matched source ra/dec), as well as a wide variety of additional columns. Identify the columns out of these catalogs that you actually need. (Work with the group to identify which columns you need. Hint: the photometric measurements, the errors on them, and the phot quality flags.)
  9. Start an Excel file. Read in one of the search results tbl files. Delete the columns you don't need. Repeat for the other search results tbl files. Copy and paste very carefully to match the same source across all the catalogs into one Excel sheet, such that in the end you have one row per object with all the relevant resulting information you have discovered about these sources. Save often! This process is sometimes called "bandmerging" because it is merging across bands (wavelengths).
  10. Spot check some sources. Are there sources bright at all bands?

Questions for you (in addition to the ones embedded above):

  1. Why does resolution matter?
  2. How will this process fail, if/when it fails?

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

DONE -- at least a first pass.

Big picture goal: Check to make sure we have sensible matches. 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:

Minimal additional words:

  • You may need to loop back to the prior step after doing this. I did. (Note that we identified coordinate issues in this step, which would be one reason to go back!)
  • After doing the SED inspection below, you will probably need to loop back to this and the prior step to check things. I did.

Questions for you:

  1. One of our major scientific goals here is to identify sources that are not good single, red giant candidates. Which are the sources that need the most scrutiny for this?
  2. Locate the most recent version of the merged source list with all our comments combined. (You may need to check email.) Using that information, assemble a list of these sources that become more than one piece. Since this is a major goal of our work, we will need to report that "XX sources from YY list broke into pieces when viewed with WISE." Assemble what you need to write that sort of sentence.

Making SEDs

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, and so that you can make some of the CMDs we will get to below.

WARNING: lots of math and programming spreadsheets... 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 (or, possibly, "we") will ultimately need to make SEDs for everything, for all bands, but to make this tractable for your visit, let's work with just the bands you merged above (2MASS, IRAS, WISE) and just a few sources. Let's try these five:

  • Tyc3340-01195-1
  • HD6665
  • IRAS07227-1320
  • HIP36896
  • IRAS11044-6127

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 or patch 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? 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:

  1. Which objects look like they have excesses? Which don't?
  2. What do the IR excesses look like in your plots? Do they look like you expected? Like objects in Monday's ppt or elsewhere?
  3. Find the object in this list of five with zero IR color. What are the WISE magnitudes? How does this fold into the Vega-based definition of magnitudes and some of the talks on Monday?
  4. EXTRA CREDIT: add a Rayleigh-Jeans line to your SEDs, anchored at K-band (2.2 um). (Hint: answer to prior question!)

Checking that the coordinates and photometry make sense, part 2 - SED inspection

In an ideal world, you'd make all the SEDs for all the bands to which we have access, identify those with photometry issues, fix those, and make SEDs again. If you were programming in Python, you'd have a shot at making a first pass at fully-populated SEDs in less than an hour or two, but even for me, tracking down all the photometry issues was ~2-3 days. Let's not waste that time right here, right now; let's use my SEDs and jump into the next step.

Big Picture Goal: Go through and look at each and every SED. From the SEDs, you can look and see if the photometry assembled from all the catalogs above make sense, and see if there is an obvious IR excess you can see, or if the excess likely involves more than one band.

Relevant links:

Chauhan109sed.png

PRACTICE SED: What's the deal with this one (why does it look like this)? (In my SED, the y-axis units are cgs units [sorry], *=new optical data, +=optical literature data, diamonds=2mass, circles=irac, stars=WISE, arrows=limits, and boxes=MIPS if they exist, which they don't here.) (Note that this example comes form last year but is still good for us to look at. Then, they were worrying about Spitzer vs. WISE; now we are worrying about WISE vs. IRAS. ...Same idea!) THINK about your answer BEFORE READING ON!...

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. There is something 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.


More words: Obtain my set of SEDs from email. In my SEDs, I use the following symbols for particular surveys. Vertical black lines through any point is the error on the point; in many cases, the error bar is very small. Go through all of the SEDs. You will need to look for three things -- see the questions below. Keep notes on this.

symbol color survey
+ cyan literature optical UBVRI
+ black SDSS ugriz
diamond black 2MASS JHKs
square blue Denis IJKs
circle black Spitzer IRAC
stars black WISE
x yellow Akari IRC, FIS
triangle cyan MSX
square black Spitzer MIPS
upside down triangle red IRAS PSC, FSC
actual arrow black limits at any band

Questions or Tasks for you:

  1. Make a list of sources where there are things that seem wrong in the SED - things suggestive of a source mismatch (e.g., source seen at optical is NOT source seen at NIR, is NOT source seen at MIR, etc) - or things suggestive of a photometry problem. We can use this information to circle back and repeat the search for photometric matches above. (In fact, I did exactly this over the course of about 2-3 days.)
  2. Make a list of sources where IRAS fluxes are too bright given the new WISE information. This is another facet of one of our big science goals - seeing where WISE resolves source confusion means both where there are multiple sources and where there is just high surface brightness from the nebulosity contaminating the IRAS measurements.
  3. Make a list of sources whose SEDs suggest that they may be non-stellar. This is yet another facet of our science goals - identifying which objects are not likely red giants.
  4. As you are going along, can you tell at a glance whether or not any given object has an IR excess? (This may be easier if you managed to put an RJ (Rayleigh-Jeans) line on your SEDs, but still.) What constitutes an excess? Where are you looking to compare points to see whether or not there is an excess? (These are all leading questions, setting up the next several steps.)

Calculating excesses, part 1

Probably above, when I asked which points you were comparing to see whether or not there is an excess, you were comparing points near the peak of the photosphere portion of the SED to the longer wavelengths.

Sometimes it's really easy to decide whether or not a given object has an IR excess. By now, you should already have found some SEDs that have obvious excesses. But, how big of an excess does it have?

Now, we need to start moving towards formally, mathematically, calculating whether or not these stars have an excess. To do this, we need to compare measurements at a relatively short wavelength to a relatively long one. This will make the most sense for the most stars if we pick bands that are available for the largest fraction of stars out of our sample.

Pay attention to detections (not limits).

Questions or Tasks for you:

  1. What relatively short wavelength band do we have for the most stars?
  2. What relatively long wavelength band do we have for the most stars?
  3. Depending on what you got for the answers to the prior questions, try calculating K-[22] for the ensemble of stars, or [3.4]-[22], or some other combination you think is a good idea.
  4. What value of K-[22] or [3.4]-[22] (or your other combination) do you expect for stars without circumstellar dust? Why?

Calculating excesses, part 2: Making CMDs

Relevant links:

Words: In looking for stars with excesses, it will help to look at the distributions of K-[22] or [3.4]-[22] as a function of other parameters.

Make a color-magnitude diagram for the ensemble of sources. K vs K-[22] and/or [3.4] vs. [3.4]-[22] are good places to start. Pay attention to detections (not limits). You may want to use IRSA Viewer rather than Excel because then you can pick out individual sources that are outliers and see immediately which source they are. However, you need to get the catalog into IPAC Table format first in order to make that happen, so you may decide that Excel is easier.

You may want to start color-coding points based on the sample from which they come (de la Reza original? Joleen's unbiased sample?).

  1. Are there any sources that you can tell right now have large excesses?
  2. Are there any sources that you can tell right now have major photometry problems?
  3. Are there sources where you are undecided if they have excesses? (hint: yes.)
  4. BONUS: add some error bars to your diagram.

Calculating excesses, part 3: Making different color-color diagrams

In order to formally decide if a star has an IR excess, we need to define what is NOT an IR excess.

Doesithaveirx.png
  1. On Monday, what did I say should be the IR color of plain photospheres?
  2. On Monday, I also showed a movie of blackbody curves as a function of temperature. When might the IR color of plain photospheres change?
  3. One way to decide if we need to worry about this is to plot things as a function of temperature. Out of the data that we have, are there any color indices that are a sensitive function of temperature?
  4. Try plotting V-K against K-[22] or [3.4]-[22]. Are there any we need to worry about?
  5. What should the predicted [3.4]-[22] be in this equation? Obsminusexp.png Now you should have a better sense of what this equation might mean.
  6. Are there still objects for which you are unsure if they have IR excesses? (hint: yes)
  7. Look at this SED on the right hand side. Does it have an excess? That vertical bar is the error bar. It's big, at least comparatively. If you extend an RJ line from K band (2.2 um), what if that RJ line hits the lower portion of the plot symbol at 22 um? Is that an excess? Is that a significant excess? How would you know? (Hint: this is setting up the next steps!)

Calculating excesses, part 4: Significance of excesses

By now, you should already have found some IR excess objects. But, especially for this data set, we have a LOT more sources where the IR excess is ambiguous. You can't necessarily tell just by looking at the SED or the color. You need to calculate whether or not it has an excess, and you need to worry about the uncertainty on the measurements (the measurement errors).

Chicalc.png Here is the calculation from our proposal. You now know what to put in the numerator. The denominator is the uncertainty on the color. To get the uncertainty on the color, add the uncertainties from each point (that goes into the color) in quadrature. What this means: (error on x-y)^2 = (error on x)^2 + (error on y)^2.

This chi is an estimate of significance -- the signal divided by the noise, or the measurement divided by the error. Chi values that are greater than 3 have a 99.7% chance of being a real excess. Calculate chi values for various combinations that you have decided are important.

Make another color-mag or color-color diagram as you did above, but this time identify sources with excesses (with your chi greater than 3) by making them a different point shape or point color. Where do the stars with excesses fall in the diagram? You may also wish to make different plots for different subsamples or different symbols for the different subsamples.

Pick some of the objects that have small excesses and go check out their SEDs. Are their excesses based solely on one point or is there corroborating evidence for an excess from another wavelength?

Can you find objects where chi calculated for two bands does NOT agree with the chi calculated for another two bands? Go get the SEDs for them and see if you can figure out why the math is coming out the way that it is.

Calculating excesses, part 5: Further muddying the waters

Astronomical data are VERY hard to precisely calibrate. The 2MASS folks, the Denis folks, and the WISE folks (and for that matter, all the other teams too) all calibrated their instruments slightly differently. It could be that small excesses are not real dust around the star, but absolute calibration uncertainties in the way the data were reduced. Are there any sources for which you are worried that this might be the case? In this sense, since the WISE bands were all calibrated the same way by the same people, a comparison of two WISE bands may be the most sensitive measure we can have of IR excess.

These stars intrinsically vary. K measured at one time may not be at all the same as the K measured at another time, and not for source mismatch reasons, but real, astrophysical variations -- our Sun has sunspot cycles, and other stars do too. If one measurement was obtained at a low point in the starspot cycle, and another measurement was obtained at a high point in the starspot cycle, the measurements will be legitimately different. In that sense, since the WISE bands were obtained essentially simultaneously, WISE data should be the best data for a comparison of two bands, since the star itself will not have had time to significantly change between WISE measurements.

One way to constrain this is to look at the individual WISE exposures that go into the mean WISE phot we have used. One recent paper finds in some cases that the mean of the photometry of the object on the individual frames is significantly different than the photometry reported from the Atlas mosaic. If we are going to write this up in a journal article, we may need to worry about this.

Many of our stars are saturated in WISE 1 or 2. Patel et al. 2014 present an approach for 'correcting' the saturated WISE values to obtain viable estimates of the true brightness of the star at these bands. It is most badly needed for WISE 2, on which we've not been focusing. The WISE 1 corrections are relatively small between 4.5 and 8.4 mags. Does that help us include more sources in our work?

Comparison to the literature

reproduce funky color-color from dela reza. make updated version. color-color appropriate?

Answering the bigger question(s)

for each sub sample, what is IR excess fraction? at what wavelengths?

IRx vs. vsini, A(Li), Vmag?, carbon isotope ratio?

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