Difference between revisions of "Finding cluster members"
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* Need Spitzer (that is, if we didn’t already have the data, as it would be in the general case of cluster membership, not specifically in IC 2118) | * Need Spitzer (that is, if we didn’t already have the data, as it would be in the general case of cluster membership, not specifically in IC 2118) | ||
* Will only find those stars which still have enough disk left to make an IR excess – will be unable to distinguish young stars without disks (Class IIIs) from the interlopers. | * Will only find those stars which still have enough disk left to make an IR excess – will be unable to distinguish young stars without disks (Class IIIs) from the interlopers. | ||
+ | * Background galaxies (many of which are forming stars) can have the same IR colors as stars with disks, so need additional data to distinguish stars from galaxies. | ||
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|(Flaring) X-rays | |(Flaring) X-rays | ||
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* Real life examples of people using this method as a primary method for finding young stars: Song et al., “[http://adsabs.harvard.edu/abs/2003ApJ...599..342S New Members of the TW Hydrae Association, Beta Pictoris Moving Group, and Tucana/Horologium Association],” 2003, ApJ, 599, 342; Mamajek et al., “[http://adsabs.harvard.edu/abs/1999ApJ...516L..77M The eta Chamaeleontis Cluster: A Remarkable New Nearby Young Open Cluster],” 1999, ApJL, 516, 77 (they use X-rays to also make the case, because this was such a surprising result, people wouldn’t have bought it just based on spatial motions alone.) | * Real life examples of people using this method as a primary method for finding young stars: Song et al., “[http://adsabs.harvard.edu/abs/2003ApJ...599..342S New Members of the TW Hydrae Association, Beta Pictoris Moving Group, and Tucana/Horologium Association],” 2003, ApJ, 599, 342; Mamajek et al., “[http://adsabs.harvard.edu/abs/1999ApJ...516L..77M The eta Chamaeleontis Cluster: A Remarkable New Nearby Young Open Cluster],” 1999, ApJL, 516, 77 (they use X-rays to also make the case, because this was such a surprising result, people wouldn’t have bought it just based on spatial motions alone.) | ||
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− | * Takes a long time; have to wait for star to move (units of proper motion are commonly arcseconds per century. Old telescopes like Palomar or Yerkes are best for doing these kinds of studies because they have such a long baseline of observation | + | * Takes a long time; have to wait for star to move (units of proper motion are commonly arcseconds per century). Old telescopes like Palomar or Yerkes are best for doing these kinds of studies because they have such a long baseline of observation. An ESA mission called [http://www.rssd.esa.int/index.php?project=HIPPARCOS Hipparcos] was designed for determining proper motions of things all over the sky. They are planning a future mission called Gaia to do the same thing but for fainter stars. |
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Revision as of 18:54, 19 December 2007
This document is also known as "Luisa’s Table of Characteristics of Young Stars for Determining Cluster Members".
Contents
Introduction
Whenever we study stellar clusters the question is: Which objects are the cluster members? This is easier with young clusters than old because the young stars are noticeably different than older stars, so it is easier to distinguish the young cluster members from the surrounding interloper stars (foreground and background populations). This process has a nice analogy with people too... when the IC 2118 teacher team came to visit the SSC, we all went out to lunch at a local Mexican place. If someone who didn't know any of us walked into the restaurant while we were eating lunch, as a group of astronomers, we are (for the most part! ;) ) not distinctly different than the rest of the adults in there, so we’d be difficult to pick out as a distinct ‘cluster’ of people, especially while we weren’t all physically co-located -- some of us were in line, getting salsa, and/or at the table. But, if a group from a day care center had been there, it would have been immediately clearly obvious that the children were a group that was different than the rest of the people in the restaurant. Moreover, the amount of time a human spends as a child is short compared to their entire lifetime, and so it is with stars. You have to seek out the group of young stars/humans in order to study their development.
Astronomers use as many of the following characteristics of young stars as possible to determine cluster membership, and we will do the same.
After reading this table, if you now go back and look at Maria Kun’s original IC2118 papers, see how many of these items she’s listing in making her case that she’s found young stars in IC 2118. I haven’t done this. Have I missed any in the list below?
Anatomy of a young star system (for reference) is to the right.
The Table
Characteristics | Pros | Cons |
IR Excess
(IR is emitted by circumstellar matter) |
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(Flaring) X-rays
(young stars emit lots of X-rays because they are completely convective and fast-rotating, so they have lots of starspots and therefore lots of flares, big and small) |
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Outflows
(only present for the very youngest objects, Class Os and Is) |
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Emission lines and other line shapes
(emitted/absorbed by accreting matter and technically disks too, though I wasn’t thinking of that at the time) |
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Variability
(because so much is happening in and around young stars, they are highly variable. In all cases here, I’m thinking of photometry, but as mentioned above, temporal studies using spectroscopy are also possible.) |
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Rotation rate
(a special case of ‘variability’ above) |
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UV
(due to shocks as accretion material hits star) |
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Spatial location
(localized in area of gas and dust) |
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Similar brightness (similar age)
(can also think of this as placing them on a color-magnitude diagram [CMD] or HR diagram [HRD]) |
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Spatial motion
(Vradial = radial velocity, AND motion across the sky = proper motion, often abbreviated with the greek letter “mu”) |
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Additional questions asked at the time
Can you have a disk without accretion? – yes, because the disk could just be sitting there, not actively dumping stuff onto the star; that’s how you get stars with an IR excess but no UV excess. (Cindy originally had: “yes, because you have Av extinction in the visible” .. the problem with that is that the Av could come from the general ISM, not just the circumstellar disk.
Can you have accretion without a disk? – seems awfully hard to imagine how this could happen, but we have a handful of stars that appear to be doing it. We don’t know what’s going on there. Since we are sensitive to DUSTY disks, maybe it is GAS that is still accreting onto the star.
Questions to think about and things to try
What happens if a star satisfies some but not all of the criteria above? What if it has only one of the properties? How many properties does it need to have before you could stand up and claim to have found a young star, and have no one argue with you about it?
Find a paper in the literature about finding young stars, not necessarily using Spitzer data, and see how many of these characteristics they use. Did we miss any?