Difference between revisions of "Finding cluster members"
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* Photometry; need stars to cooperate -- another observing campaign on the same stars a year later will only recover about half(!) of the periodic stars, presumably due to changes in the stars themselves (star spot shape and coverage, disk ‘puffiness’, etc) | * Photometry; need stars to cooperate -- another observing campaign on the same stars a year later will only recover about half(!) of the periodic stars, presumably due to changes in the stars themselves (star spot shape and coverage, disk ‘puffiness’, etc) | ||
* Photometry: possible – though unlikely for fast rotation rates – to be fooled by binaries or disk occultations | * Photometry: possible – though unlikely for fast rotation rates – to be fooled by binaries or disk occultations | ||
+ | |- | ||
+ | |UV | ||
+ | ''(due to shocks as accretion material hits star)'' | ||
+ | | | ||
+ | * Lots more UV than expected is a dead give-away for mass accretion onto star (no clear way to create lots of UV any other way) | ||
+ | * Real life examples of people using this method as a primary method for finding young stars: Rebull et al., “Circumstellar Disk Candidates Identified from UV Excesses in the Orion Nebula Cluster Flanking Fields,” 2000, AJ, 119, 3026 | ||
+ | | | ||
+ | * long integration because star faint due to shorter wavelengths | ||
+ | * star needs to be accreting | ||
+ | * subtle accretion rates look like coronal activity in older stars (similar to Halpha “cons” above | ||
+ | |- | ||
+ | |Spatial location | ||
+ | ''(localized in area of gas and dust)'' | ||
+ | |- | ||
+ | * easy to measure – can do from just images | ||
+ | * we have Spitzer data already, and Spitzer easily finds dust. | ||
+ | * Real life examples of people using this method as a primary method for finding young stars: Padgett et al., “An Aggregate of Young Stellar Disks in Lynds 1228 South,” 2004, ApJS, 154, 433 [ok, so spatial location is a co-primary method with IR excess in this paper]; Kiss et al., “Star formation in the Cepheus Flare region: implications from morphology and infrared properties of optically selected clouds,” 2006, A&A, 453, 923 [again, morphology isn’t the only thing but it plays an important role] | ||
+ | |- | ||
+ | * Details of extinction not easy to measure | ||
+ | * Chance superposition of foreground or background stars (and galaxies) can easily fool you, so usually you need at least one other indicator of youth before you can write a paper. | ||
+ | |- | ||
+ | Similar brightness (similar age) | ||
+ | ''(can also think of this as placing them on a color-magnitude diagram [CMD] or HR diagram [HRD])'' | ||
+ | | | ||
+ | * Can do with photometry of any sort (we can do this with Spitzer data we have) | ||
+ | * To really put in CMD and get ages/masses, need optical data (photom and spec) | ||
+ | * Real life examples of people using this method as a primary method for finding young stars: Rebull et al., “Circumstellar Disk Candidates Identified from UV Excesses in the Orion Nebula Cluster Flanking Fields,” 2000, AJ, 119, 3026 [ok, so I found them first using UV, but the optical CMD is important for making the case that they’re really young]; Rebull et al., “Circumstellar Disk Candidates Identified in NGC 2264,” 2002, AJ, 123, 1528 [ditto!] | ||
+ | | | ||
+ | * need optical spectra to give us a spectral type (we had time to do this at Palomar) to help with placement in CMD/HRD (we need to get a handle on optical reddening, since reddening will make the stars appear fainter than they should, making it hard to see if they all have similar brightnesses) | ||
+ | |- | ||
+ | |Spatial motion | ||
+ | ''(Vradial = radial velocity, AND motion across the sky = proper motion, often abbreviated with the greek letter “mu”)'' | ||
+ | | | ||
+ | * A cluster will be moving through space together, and if we really know the motion of individual stars, we can determine which objects are part of the cluster. | ||
+ | * Real life examples of people using this method as a primary method for finding young stars: Song et al., “New Members of the TW Hydrae Association, Beta Pictoris Moving Group, and Tucana/Horologium Association,” 2003, ApJ, 599, 342; Mamajek et al., “The eta Chamaeleontis Cluster: A Remarkable New Nearby Young Open Cluster,” 1999, ApJL, 516, 77 [he uses 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.] | ||
+ | | | ||
+ | * 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 | ||
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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. |
Revision as of 23:27, 16 November 2007
This document is also known as "Luisa’s Table of Characteristics of Young Stars for Determining Cluster Members".
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
- easy to measure – can do from just images
- we have Spitzer data already, and Spitzer easily finds dust.
- Real life examples of people using this method as a primary method for finding young stars: Padgett et al., “An Aggregate of Young Stellar Disks in Lynds 1228 South,” 2004, ApJS, 154, 433 [ok, so spatial location is a co-primary method with IR excess in this paper]; Kiss et al., “Star formation in the Cepheus Flare region: implications from morphology and infrared properties of optically selected clouds,” 2006, A&A, 453, 923 [again, morphology isn’t the only thing but it plays an important role]
- Details of extinction not easy to measure
- Chance superposition of foreground or background stars (and galaxies) can easily fool you, so usually you need at least one other indicator of youth before you can write a paper.
Characteristics | Pros | Cons |
IR Excess
(IR is emitted by circumstellar matter) |
|
|
(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) | ||
|
| |
Spatial motion
(Vradial = radial velocity, AND motion across the sky = proper motion, often abbreviated with the greek letter “mu”) |
|
|
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.