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

Francesco DamianiYoung Clusters in the Gaia-ESO Survey, Palermo, 20 May 2014

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 starsOB stars (and A-F supergiants)Chemically peculiar stars (e.g. Carbon stars, Barium stars, Ap stars)


Some representativeHR15n spectrafrom 2 Vel dataset.

Colored bands indicatenon-molecular featuressampled by our indices.

M

K

G

F


Part 1: Fast-rotating late-type stars

The problem: narrow-band indices are sensitive to v sin i as 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 artificiallybroadened, low-v sin i template spectra (blackcircles) from 2 Vel, up to v sin i = 200 km/s.Blue crosses:Blue crosses: cluster members.

H flux from line core (e.g. strongchromosphere) may leak into line wings (mimicking a CTTS) for large enough v sin i:

→ this effect can be corrected!

WTTS CTTS


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


Metallicity estimates (index ) not badly affected by v sin i.

Gravity estimates (index ) are crucially affected (esp. above ~ 110 km/s).

Lowmetallicity

Low gravity


Some good news:

the M dwarf/giant discriminationprovided by indices 6-7is unaffected by fast rotation!

Giant

s

Dwarfs


Example of procedure:

measured index (and inferred Teff) vs. v sin i,and its analytical modeling(black dots).

Green triangles: actual data for2 Vel fast-rotating cluster stars.


Results of rotation correction for 2 Vel dataset:

A few (weak) CTTS become WTTS. The cluster sequence becomes narrower!

CTTSWTTS


More results for 2 Vel:

Veiling (estimated from both diagrams) becomes lower for several CTTS.


More results for 2 Vel:Less evidence for differential reddening in (V-I, ). Less apparent age spread in (Teff, log g).


A check of results:

Black: actual spectraof fast rotating 2 Vel stars.

Red: rotation-broadenedtemplates (2 Vel WTTS).

Teff

were derivedonly from spectral indices+ rotation correction.

Matching templatesselected from T

eff only,

not from spectrum best-fit!

Teff

change as a result of rotation correction


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 (RV) = 200 km/s) from real single-star spectra from 2 Vel, and built index-index diagrams.


Binarity has a similar effect to fast rotation!


Its adverse effect is almost irrelevant for Teff determinationfor (RV) ≤ 50-70 km/s (i.e., 50-70% of SB2 in 2 Vel)...


...while gravity determinations are compromised at (RV) ≥ 25 km/s!


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.


CTTS veiling r is determined from each of these diagrams separately:


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:

r2

r1


...then other indices are un-veiled accordingly.


Example of results for Cha I: Lithium EW vs. (~ Teff), all veiling-corrected.

Some spectra over-corrected? (also note the correlated EW – Teff errors).


Check of veiling-corrected Cha I CTTS spectra (black)

and same-Teff

zero-veilingtemplate spectraof 2 Vel members(red).

Teff was derived onlyfrom indices.

Again, these arenot spectral fits!


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 linesFast rotationFrequently, strong DIBsEmission lines

Luckily, the HR15n range includes “hot” lines from He I, He II, C II, Ca II.


Why our temperature index fails for OB stars?

On the MS, since H changesbehaviour when crossingtype A0!

For supergiants, this changeoccurs at colder Teff.

UVES-POP spectral library


UVES-POPtemplates

in HR15nrange,and

spectralfeaturesused fornew indices.


→ a “hot” temperature index h:

(not valid later than ~A5, where we have the “old” )

Emission-line OBA stars (and some peculiar stars) remain problematic even for h...

However, to discriminate F-G supergiants from higher-gravity stars we still need...


… a “hot” gravity index h:

using (mostly) H wings andCa II lines, selects wellA-F supergiants.

Earlier than ~A0, H emission may render h unusable as a gravity indicator.

Class V stars form a regularsequence near the bottom.

Class III OBA stars notdistinguishable from Class V...


A h, h) diagramfor selection ofstars requiring use of the “hot” indices:

(all except in the grey zone)


With this selection, thediagram (h, 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 !)


Part 5: chemically peculiar stars

The problem: our gravity index for late-type stars relies largely on the 6497 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 2.a s-element index , 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 index does not substitute our older index for young, faster-rotating stars.


Examples of Ba-rich (bottom) and Ba-normal (top) giants (from 2 Vel dataset):

Green regions:Ba I/II, Y I/II,Sr I, Zr I linesin HR15n,used to defineindex .

Red regions:gravity sensitivelines, used todefine index 2.


Indices 2 and vs. Teff (for 2 Vel dataset):

s-element-excess (field) stars are found above a limiting line in (, Teff). Black: known chemically peculiar stars from Tomasella et al. (2010).


...but what about NGC6705?A very narrow sequence in 2 between 7000-8500 K; instead, many stars scatteredabove the “regular” sequence in , where known Ap & SrCrEu stars lie...(yet WEBDA lists only one Ap star in NGC6705): What are these s-rich stars?


...are they field stars, like the Ba stars in 2 Vel?Probably not, their RVs being too much clustered!

...are they just noisy data?No: their S/N is on average larger than typicalcluster stars!

...are they evolved clusterstars, e.g. AGB stars?Unlikely, given their CMDposition.

One possibility: second-generation cluster stars,formed from s-element enriched material?(Cantat-Gaudin et al., submitted, find no enrichment...)


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 (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 h index; supergiant/dwarf distinction using new index h.

S-element excess stars identifiable thanks to new index: some s-element-enrighed stars in NGC6705?


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!

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

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