Stellar classification using spectral indices

1. Francesco Damiani INAF-Osservatorio Astronomico di Palermo, ITALY Stellar classification using spectral indices Outline A set of spectral indices was defined from the spectral range 6444-6818 Å (FLAMES/Giraffe HR15n), enabling derivation of stellar fundamental parameters (Teff, log g, [Fe/H]) for Teff ≤ 8000 K (Damiani et al., A&A, in press, astro-ph 1405.1205). Now, the technique is being refined to deal with: Fast-rotating late-type stars (>90 km/s for Teff, >30 km/s for log g) SB2 stars, and veiled PMS stars OB stars (and A-F supergiants) Chemically peculiar stars (e.g. Carbon stars, Barium stars, Ap stars) Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014

2. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Some representative HR15n spectra from g2 Vel dataset. Colored bands indicate non-molecular features sampled by our indices. M K G F

3. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Part 1: Fast-rotating late-type stars The problem: narrow-band indices are sensitive to v sin ias soon as spectral lines become wider than wavelength ranges used. Solution: the effect of rotation broadening was studied for each index, a correction was modeled analytically, and applied to recover “true” index values (assuming v sin i is accurately known). Example:index-index diagrams for artificially broadened, low-v sin i template spectra (black circles) from g2 Vel, up to v sin i = 200 km/s. Blue crosses: cluster members. Ha flux from line core (e.g. strong chromosphere) may leak into line wings (mimicking a CTTS) for large enough v sin i: →this effect can be corrected! WTTS CTTS

4. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Two diagrams used to compute PMS veiling. Fast rotation (>70 km/s) has however a similar effect to veiling: this must be corrected for before computing veiling. Veiled stars Veiled stars

5. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Metallicity estimates (index z) not badly affected by v sin i. Gravity estimates (index g) are crucially affected (esp. above ~ 110 km/s). Low gravity Low metallicity

6. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Some good news: the M dwarf/giant discrimination provided by indices m6-m7 is unaffected by fast rotation! Giants Dwarfs

7. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Example of procedure: measured index t (and inferred Teff) vs. v sin i, and its analytical modeling (black dots). Green triangles: actual data for g2 Vel fast-rotating cluster stars.

8. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Results of rotation correction for g2 Vel dataset: A few (weak) CTTS become WTTS. The cluster sequence becomes narrower! WTTS CTTS

9. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 More results for g2 Vel: Veiling (estimated from both diagrams) becomes lower for several CTTS.

10. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 More results for g2 Vel: Less evidence for differential reddening in (V-I, m). Less apparent age spread in (Teff, log g).

11. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 A check of results: Black: actual spectra of fast rotating g2 Vel stars. Red: rotation-broadened templates (g2 Vel WTTS). Teff were derived only from spectral indices + rotation correction. Matching templates selected from Teffonly, not from spectrum best-fit! Teffchange as a result of rotation correction

12. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Part 2: SB2 binaries The problem: similarly to fast-rotating stars, in SB2 systems part of the lines sampled fall outside the extraction regions. Solution: the effect of line doubling for equal-mass SB2 systems was studied for each index, and limits of applicability of the method were determined. No inversion procedure was established, as in general the mass ratio q is unknown. Procedure: we computed synthetic SB2 spectra (up to D(RV) = 200 km/s) from real single-star spectra from g2 Vel, and built index-index diagrams.

13. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Binarity has a similar effect to fast rotation!

14. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Its adverse effect is almost irrelevant for Teff determination for D(RV) ≤ 50-70 km/s (i.e., 50-70% of SB2 in g2 Vel)...

15. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 ...whilegravity determinations are compromised at D(RV) ≥ 25 km/s!

16. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Part 3: veiling in PMS stars The problem: disentangling the (qualitatively similar) effects of veiling and fast rotation on spectral indices. Both phenomena are frequent in CTTS. Solution: after applying the rotation correction to indices, veiling r can be determined as in Damiani et al. (2014). The Chamaeleon I cluster permits a good test of the procedure.

17. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 CTTS veiling r is determined from each of these diagrams separately:

18. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 The adopted veiling r is an average of the two values r1 and r2, (note their non-independent errors). Their correlation is a consistency check: r1 r2

19. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 ...then other indices are un-veiled accordingly.

20. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Example of results for Cha I: Lithium EW vs. t (~ Teff), all veiling-corrected. Some spectra over-corrected? (also note the correlated EW – Teff errors).

21. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Check of veiling-corrected Cha I CTTS spectra (black) and same-Teff zero-veiling template spectra of g2 Vel members (red). Teff was derived only from indices. Again, these are not spectral fits!

22. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Part 4: OBA stars • The problem: our spectral indices were originally developed for stars later than ~ A5. Occasionally, earlier-type stars are also observed with HR15n: • we should be able to classify them as well. • Solution: dominant spectral features of OBA stars in the range 6450-6800 Å were studied using UVES-POP library spectra, and a new set of indices was defined to recover their spectral type and (sometimes) luminosity class. • Additionally, the new indices are able to classify A-F supergiants,which were not satisfactorily dealt with using the original indices. • Specific issues for early-type stars in HR15n wavelength range: • Few spectral lines • Fast rotation • Frequently, strong DIBs • Emission lines Luckily, the HR15n range includes “hot” lines from He I, He II, C II, Ca II.

23. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Why our temperature index t fails for OB stars? On the MS, since Hachanges behaviour when crossing type A0! For supergiants, this change occurs at colder Teff. UVES-POP spectral library

24. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 UVES-POP templates in HR15n range, and spectral features used for new indices.

25. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014

26. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014

27. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 →a “hot” temperature indexth: (not valid later than ~A5, where we have the “old” t) Emission-line OBA stars (and some peculiar stars) remain problematic even for th... However, to discriminate F-G supergiants from higher-gravity stars we still need...

28. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 … a “hot” gravity indexgh: using (mostly) Ha wings and Ca II lines, selects well A-F supergiants. Earlier than ~A0, Ha emission may render gh unusable as a gravity indicator. Class V stars form a regular sequence near the bottom. Class III OBA stars not distinguishable from Class V...

29. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 A (gh, th) diagram for selection of stars requiring use of the “hot” indices: (all except in the grey zone)

30. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 With this selection, the diagram (th, spectral type) for OB stars and AF supergiants becomes: Best-fit residual rms deviation ~2.5 subclasses (excluding emission-line stars and extreme outliers, of luminosity class 0/I !)

31. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Part 5: chemically peculiar stars • The problem: our gravity index g for late-type stars relies largely on thel6497 Ba II line. Therefore, gravity estimates may be inaccurate in stars with a Ba overabundance (or s-element overabundance). • Solution: we defined: • a new s-element-free gravity index g2. • a s-element index s, to select stars rich of s-elements. Typically, the s-element-rich late-type stars are slowly-rotating giants: indices may thus be safely defined using very narrow and selective bands. For the same reason, the new gravity indexdoes not substitute our older index g for young, faster-rotating stars.

32. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Examples of Ba-rich (bottom) and Ba-normal (top) giants (from g2 Vel dataset): Green regions: Ba I/II, Y I/II, Sr I, Zr I lines in HR15n, used to define index s. Red regions: gravity sensitive lines, used to define index g2.

33. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Indices g2 and s vs. Teff(for g2 Vel dataset): s-element-excess (field) stars are found above a limiting line in (s, Teff). Black: known chemically peculiar stars from Tomasella et al. (2010).

34. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 ...but what about NGC6705? A very narrow sequence in g2 between 7000-8500 K; instead, many stars scattered above the “regular” sequence in s, where known Ap & SrCrEu stars lie... (yet WEBDA lists only one Ap star in NGC6705):What are these s-rich stars?

35. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 ...are they field stars, like the Ba stars in g2 Vel? Probably not, their RVs being too much clustered! ...are they just noisy data? No:their S/N is on average larger than typical cluster stars! ...are they evolved cluster stars, e.g. AGB stars? Unlikely,given their CMD position. One possibility: second-generation cluster stars, formed from s-element enriched material? (Cantat-Gaudin et al., submitted, find no enrichment...)

36. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Conclusions • We obtain good Teff and [Fe/H] for fast rotators (up to 200 km/s) and log g up to 100 km/s. • SB2 Teff and [Fe/H] obtainable for D(RV) up to 50-70 km/s; log g up to 25 km/s. • Good parameters for veiled CTTS, except a few over-corrected cases. • OB star Teff obtained using new th index; supergiant/dwarf distinction using new index gh. • S-element excess stars identifiable thanks to new s index: some s-element-enrighed stars in NGC6705?

37. Francesco Damiani Young Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014 Conclusions - 2 Problem: by now, you are maybe so (rightly) scared of all this complexity that you will never use this method... Solution: a forthcoming Web tool will help you: the StarClass stellar classification service (URL still TBD). You submit FITS spectra, and it provides stellar parameters: a first version (not including recent improvements) is ready right now!