Empirical Studies of Star Formation: "Below Main Sequence" Stars, Age Spreads, and Photometric Variability

1. Empirical Studies of Star Formation: “Below Main Sequence” Stars, Age Spreads and Photometric Variability John R. Stauffer IPAC, Head of Science Staff

2. Steve’s 1st Paper on YSO’s • Strom, Strom et al. - 1972, ApJ 171, 267. • Optical and IR photometry in order to determine the time spread of star formation • Found IR excesses due to disks, blue excesses due to chromospheres(?), and “below main sequence” stars • Put limits on age spread, but because of the above “features”, learned that determining the age spread in star forming regions was hard. • We still debate these issues -

3. NGC 2264

4. NGC 2264 - age spread?

5. W90 - The Prototype BMS star • An A0 spectral type member of NGC2264 that falls about 2.5 mag below the ZAMS (in a V vs. B-V CMD) • E(B-V) determined to be small • Gravity consistent with PMS nature • L and 10 micron excesses • Can’t be just relatively old (since below ZAMS) • Best explained as an edge on disk • What would Steve and Karen have done if they had access to Spitzer?

6. NGC2264 - K band excesses?

7. N2264 - Disks better detected at L than K

8. 35 Years Later - IRAC & N2264

9. N2264 - IRAC 8 micron excesses

10. Steve’s stars in Lori’s Diagram

11. IRS Low Res Spectrum for W90

12. Robitaille Models for W90

13. W90 in 2008 • There is now a lot of new Spitzer (and probably other) data for W90, as well as much better modelling tools. • Someone should write a paper on W90 - is it now a “well-understood” star?

14. New BMS Stars in NGC2264 • THE INITIAL MASS FUNCTION AND YOUNG BROWN DWARF CANDIDATES IN NGC 2264. III. PHOTOMETRIC DATA Sung, Bessell, et al. AJ 135, 441, 2008 ABSTRACT. We have performed deep wide-field CCD photometry of the young open cluster NGC 2264 to study the extent of star-forming regions (SFRs) and the shape of the initial mass function. In this paper, we present VRI and Halpha photometry for more than 67,000 stars. From the spatial distribution of the selected H? emission stars, we identify two active SFRs and a less active halo region surrounding these two SFRs. There are several Halpha emission stars in the field region outside the halo region, and these may be newly formed stars in the Mon OB1 association surrounding the cluster. The locus of pre-main-sequence (PMS) stars in the IC versus V - IC diagram is revised from the distribution of Halpha and X-ray emission stars in the diagram. The mean reddening of late-type PMS stars is estimated to be E(B - V) ~ 0.2 mag using the distribution of X-ray emission stars in the 2MASS color-color diagram. We can confirm that the Halpha emission stars below the PMS locus (so-called BMS stars) are bona-fide members of NGC 2264 from their spatial distribution as well as from their near-IR excess in the 2MASS color-color diagram. In addition, four objects around IRS-2 detected with the Spitzer IRAC are also classified as BMS stars.

15. New BMS Stars in NGC2264

16. Age Spread in NGC2264?

17. Photometric Variability as a Signpost of Youth • One of the characteristics of youth used by the Strom’s in studying YSO’s in the 1970’s was photometric variability - particularly in the IR • One of Steve’s most recent papers was a chapter in the Spitzer Warm Mission workshop, where he advocated using IRAC for time series photometry in star-forming regions. • Steve’s paper was one impetus for Tom Soifer to request a DDT observation of a star-forming region to determine what could be learned from time series photometry with IRAC. Those data were obtained in early March - and this group is the first to see the results.

18. IC1396A - The Elephant Trunk Nebula

19. YSO’s in IC1396A

20. Mid-IR Variability in IC1396A

21. Mid-IR Variability in IC1396A

22. Short-timescale Variability – a PMS Delta Scuti Star?

23. Conclusions • We are still arguing over some of the same issues raised in Steve and Karen’s 1972 papers • We have made progress though – and Spitzer has served as a good tool to pursue this type of science