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Published October 20, 2010 | Published
Journal Article Open

A multi-color optical survey of the orion nebula cluster. II. The H-R diagram


We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy.We study the color–color diagrams in BVI, plus a narrowband filter centered at 6200 Å, finding evidence that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M spectral-type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main-sequence isochrone.We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrowband photometry; in this way, we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BVI color–color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice of also the spectral-type–temperature transformation, we produce the new Hertzsprung–Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ~2 to ~3 Myr depending on the model. We study the distribution of the members in the mass–age plane and find that taking into account selection effects due to incompleteness, removes an apparent correlation between mass and age.We derive the initial mass function for low- and intermediate mass members of the ONC, which turns out to be model dependent and shows a turnover at M ≲ 0.2 M_⊙.

Additional Information

© 2010 American Astronomical Society. Received 2009 November 19; accepted 2010 August 5; published 2010 September 27. N.D.R. kindly acknowledges financial support from the German Aerospace Center (DLR) through grant 50 OR 0401. K.G.S. acknowledges support from NSF Career grant AST- 0349075 and a Cottrell Scholar award from the Research Corporation. This work was made possible in part by GO program 10246 of the Hubble Space Telescope, which is operated by the Space Telescope Science Institute. This work was made possible through the Summer Student Program of the Space Telescope Science Institute. Facilities: Max Planck:2.2m, HST.

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August 19, 2023
October 20, 2023