Nicola da rio hst orion treasury science meeting ii baltimore september 12 13 2011
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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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A Multi-color optical survey of the Orion Nebula Cluster

Nicola Da Rio

HST OrionTreasury Science Meeting II

Baltimore, September 12-13, 2011

The OrionHST Treasuryprogram

Orion HST Treasury Program


Ground based simultaneous follow-up

2.2 ESO-MPG/WFI UBVI+Ha+620nm



Goal of the ground-based observations

  • Cover the bright end of the population

  • Account for stellar variability

  • 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

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

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


2Myr isoch

Av = 2

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

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.


[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”

[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

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

  • 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)


    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


    • 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.

    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


    • 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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