Nicola Da RioNicola Da RioHST Orion Treasury Science Meeting IIHST Orion Treasury Science Meeting II

Baltimore, September 12-13, 2011Baltimore, September 12-13, 2011

A Multi-color optical survey of the Orion

Nebula Cluster

The Orion HST Treasury programOrion HST Treasury Program104 HST orbits, ACS+WFPC2+NIC3 UBVIZJHK

Ground based simultaneous follow-up 2.2 ESO-MPG/WFI UBVI+H+620nm 4m CTIO/ISPI JHK 0.9m CTIO UBVRI

Goal of the ground-based observations1) Cover the bright end of the

population2) Account for stellar variability

3) Characterize the ONC stellar population

Da Rio et al. (2009), ApJS, 183, 261

Da Rio et al. (2010), ApJ, 722, 1092

A multi-band, large field optical catalog

2612 stars detected in I,2612 stars detected in I, 58% in V,58% in V, 43% in B,43% in B, 17% U 17% U ~1000 sources in H~1000 sources in H

Wide-Field Imager (WFI)Wide-Field Imager (WFI)2.2m ESO/MPG telescope2.2m ESO/MPG telescopeLa Silla (Chile)La Silla (Chile)

33’x34’ Field of view33’x34’ Field of view

•1 pointing1 pointing•6 filters6 filters•2 nights2 nights

Analysis of the stellar colorsQuestion: what are the intrinsic (photospheric) colors of young PMS stars?Question: what are the intrinsic (photospheric) colors of young PMS stars? do the intrinsic colors depend on age? do the intrinsic colors depend on age?

WFI photometric system: The computed WFI photometric system: The computed ZAMS locus in incompatible with the ZAMS locus in incompatible with the observed sequence, which is bluer as observed sequence, which is bluer as predicted by atmosphere modelspredicted by atmosphere models

The synthetic ZAMS is in agreement The synthetic ZAMS is in agreement empirical colors of dwarfsempirical colors of dwarfs

For young M-type stars photospheric colors are significantly different than dwarf colorsFor young M-type stars photospheric colors are significantly different than dwarf colors

MSMS2Myr isoch2Myr isoch

Av = 2

Av = 2

Accretion DOES alter the broad-band optical colors

Colors are shifted to the blue Colors are shifted to the blue due to accretion hot spots on due to accretion hot spots on the stellar surfacethe stellar surface

Disentangling accretion and Disentangling accretion and extinction for individual stars:extinction for individual stars:1)1)Simulation of an accretion Simulation of an accretion spectrum, considering optically spectrum, considering optically thick + thin emission.thick + thin emission.2)2)Computation of the shifts in Computation of the shifts in the colors adding Lthe colors adding Laccraccr to a star to a star of given temperatureof given temperature3)3)Solution for LSolution for Laccraccr and A and AVV from from multi-band photometry and multi-band photometry and known spectral type.known spectral type.

UNBIASED ESTIMATE OF UNBIASED ESTIMATE OF INDIVIDUAL EXTINTIONSINDIVIDUAL EXTINTIONS

[TiO]: a spectro-photometric index to classify M stars•The 620nm medium band filter used is centered on a TiO spectral feature of M starsThe 620nm medium band filter used is centered on a TiO spectral feature of M stars

•Theoretical atmosphere models still show remarkable uncertainties in modeling such Theoretical atmosphere models still show remarkable uncertainties in modeling such featurefeature

•Can we use such information to constrain the stellar parameter for the ONC members?Can we use such information to constrain the stellar parameter for the ONC members?

•Definition of a Spectro-Definition of a Spectro-photometric [TiO] photometric [TiO] index:index:

•““the difference, in the difference, in magnitudes, between magnitudes, between the the measuredmeasured flux at flux at 620nm and the 620nm and the linear linear interpolationinterpolation between between V- and I-band fluxes”V- and I-band fluxes”

[TiO]: a spectro-photometric index to classify M stars

Comparison with synthetic photometry:

Poor agreement

Comparison with known spectral classification:Determination of a [TiO] – Spt correlation for M-Type stars

Classification of 217 new M-type stars in the ONC

Causes of spread: dependence of [TiO] on log(g)The lower the gravity, the higher the predicted Spt

Spectral Type – vs Temperature relation

For the M-type star, the measured colors For the M-type star, the measured colors are bluer than the intrinsic ones: are bluer than the intrinsic ones: Negative extinction?Negative extinction?

• Solution: change the SpT vs TSolution: change the SpT vs Teffeff relation.relation.

• Luhman (‘99) – semi empirical Luhman (‘99) – semi empirical relation imposing the members of the relation imposing the members of the GC Tau quadruplet to lie on a single GC Tau quadruplet to lie on a single BCAH isochroneBCAH isochrone

Spectral catalog of Hillenbrand (1997)

65 new types from unpublished spectroscopy

~200 new M-type stars from [TiO] index

Wide field, deep sample of ~1000 members with known spectral type

+

+

The revised H-R diagram for the ONCcorrected Teff, best determined AV - new intrinsic colors and BCs relevant for the ONC - correction for veiling - new revised distance

MC simulation of the propagation

of known uncertainties to

Teff and Ltot

Simulated completeness

(probabilistic) in the HRD

accounting for differential AV and selection

effects

Differences in the derived Ltot

compared to previous works:Higher L for M-

types stars (best intrinsic colors), lower L for early types (distance

+ BCs)

Conclusions:Orion is older: 2-3 Myr, The large age spread is still there: ~0.3-0.4 dex.

No evidence for a mass-age dependence:-In part it is a bias from selection effects and incompleteness- In part is a model dependent bias of the isochrones

IMF-Model dependent flattening at ~0.2 M

A catalog of calibrated H excesses

HH fluxes for ~1000 ONC fluxes for ~1000 ONC members.members.

Determination of the Determination of the photospheric continuum using photospheric continuum using 3 bands (3 bands (V,I, TiOV,I, TiO) improves ) improves the continuum subtraction the continuum subtraction accounting for color and Taccounting for color and Teffeff dependencedependence..

Absolute flux calibration – Absolute flux calibration – erg/s/cmerg/s/cm22/Å/Å

~1/3 of the sources shows H~1/3 of the sources shows H excess of excess of E.W.>50ÅE.W.>50Å

Data will be used for Data will be used for determiningdetermining ddM/M/dd

Conclusions1.1. New H-R diagram for Orion: ~1000 sourcesNew H-R diagram for Orion: ~1000 sources

1.1. Older age than previously estimated (2Myr)Older age than previously estimated (2Myr)

2.2. Mass-age correlation not confirmed: selection effect due to source detectionMass-age correlation not confirmed: selection effect due to source detection

3.3. IMF peaks at ~0.2-0.3Msun. Shape down to the H-burning limit strongly IMF peaks at ~0.2-0.3Msun. Shape down to the H-burning limit strongly model dependent.model dependent.

2.2. Photospheric colors of young stars differ from those of MS dwarfs (and from Photospheric colors of young stars differ from those of MS dwarfs (and from predictions of atmosphere modeling). Empirical calibration in BVI.predictions of atmosphere modeling). Empirical calibration in BVI.

3.3. Accretion excess affects the observed colors in the entire optical wavelength Accretion excess affects the observed colors in the entire optical wavelength range. A modeling in a 2-color diagram can be used to disentangle Lrange. A modeling in a 2-color diagram can be used to disentangle Laccacc and A and AV.V.

4.4. Derivation of spectral types from medium band photometry.Derivation of spectral types from medium band photometry.