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Nicola Da Rio HST Orion Treasury Science Meeting II Baltimore, September 12-13, 2011

A Multi-color optical survey of the Orion Nebula Cluster. Nicola Da Rio HST Orion Treasury Science Meeting II Baltimore, September 12-13, 2011. The Orion HST Treasury program. Orion HST Treasury Program 104 HST orbits, ACS+WFPC2+NIC3 UBVIZJHK Ground based simultaneous follow-up

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Nicola Da Rio HST Orion Treasury Science Meeting II Baltimore, September 12-13, 2011

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  1. A Multi-color optical survey of the Orion Nebula Cluster Nicola Da Rio HST OrionTreasury Science Meeting II Baltimore, September 12-13, 2011

  2. The OrionHST Treasuryprogram Orion HST Treasury Program 104 HST orbits, ACS+WFPC2+NIC3 UBVIZJHK Ground based simultaneous follow-up 2.2 ESO-MPG/WFI UBVI+Ha+620nm 4m CTIO/ISPI JHK 0.9m CTIO UBVRI Goal of the ground-based observations • Cover the bright end of the population • Account for stellar variability • Characterize the ONC stellar population

  3. Da Rio et al. (2009), ApJS, 183, 261 Da Rio et al. (2010), ApJ, 722, 1092

  4. A multi-band, large field optical catalog Wide-Field Imager (WFI) 2.2m ESO/MPG telescope La Silla (Chile) 33’x34’ Field of view • 1 pointing • 6 filters • 2 nights 2612 stars detected in I, 58% in V, 43% in B, 17% U ~1000 sources in Ha

  5. Analysis of the stellar colors Question: what are the intrinsic (photospheric) colors of young PMS stars? do the intrinsic colors depend on age? WFI photometric system: The computed ZAMS locus in incompatible with the observed sequence, which is bluer as predicted by atmosphere models The synthetic ZAMS is in agreement empirical colors of dwarfs MS 2Myr isoch Av = 2 For young M-type stars photospheric colors are significantly different than dwarf colors

  6. Accretion DOES alter the broad-band optical colors Colors are shifted to the blue due to accretion hot spots on the stellar surface Disentangling accretion and extinction for individual stars: • Simulation of an accretion spectrum, considering optically thick + thin emission. • Computation of the shifts in the colors adding Laccr to a star of given temperature • Solution for Laccr and AV from multi-band photometry and known spectral type. UNBIASED ESTIMATE OF INDIVIDUAL EXTINTIONS

  7. [TiO]: a spectro-photometric index to classify M stars The 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 feature Can we use such information to constrain the stellar parameter for the ONC members? Definition of a Spectro-photometric [TiO] index: “the difference, in magnitudes, between the measured flux at 620nm and the linear interpolation between V- and I-band fluxes”

  8. [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

  9. Spectral Type – vs Temperature relation 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 For the M-type star, the measured colors are bluer than the intrinsic ones: Negative extinction? • Solution: change the SpTvs Teff relation. • Luhman (‘99) – semi empirical relation imposing the members of the GC Tau quadruplet to lie on a single BCAH isochrone

  10. The revised H-R diagram for the ONC corrected 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)

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

  12. A catalog of calibrated Ha excesses Ha fluxes for ~1000 ONC members. Determination of the photospheric continuum using 3 bands (V,I, TiO) improves the continuum subtraction accounting for color and Teff dependence. Absolute flux calibration – erg/s/cm2/Å ~1/3 of the sources shows Ha excess of E.W.>50Å Data will be used for determiningdM/dt

  13. Conclusions • New H-R diagram for Orion: ~1000 sources • Older age than previously estimated (2Myr) • Mass-age correlation not confirmed: selection effect due to source detection • IMF peaks at ~0.2-0.3Msun. Shape down to the H-burning limit strongly model dependent. • Photospheric colors of young stars differ from those of MS dwarfs (and from predictions of atmosphere modeling). Empirical calibration in BVI. • Accretion excess affects the observed colors in the entire optical wavelength range. A modeling in a 2-color diagram can be used to disentangle Lacc and AV. • Derivation of spectral types from medium band photometry.

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