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The DDO photometric system

The DDO photometric system. It was developed at the D avid D unlap O bservatory near Toronto by McClure & van den Bergh (1968) Intermediate band system, filter defined: 48 (at λ4886), 45 (at λ4517), 42 (at λ4257), 41 (at λ4166), 38 b (at λ3815) and 35 (at λ3460). The DDO photometric system.

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The DDO photometric system

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  1. The DDO photometric system • It was developed at the David Dunlap Observatory near Toronto by McClure & van den Bergh (1968) • Intermediate band system, filter defined: 48 (at λ4886), 45 (at λ4517), 42 (at λ4257), 41 (at λ4166), 38b (at λ3815) and 35 (at λ3460) The DDO photometric system

  2. The DDO photometric system • 210 stars, 70 star clusters and 56 galaxies were originally observed • Purpose: measuring integrated light from galaxy nuclei and constructing stellar population models of galaxies and globular clusters  determining spectral types and luminosities of a wide range of reddened stars • These observations were not originally intended to be the basis for a standard photometric system, but to supplement UBV-photometry by segregating stellar temperature and luminosity effects from reddening and ultraviolet excess

  3. The DDO photometric system • System specifications and standard stars were revised by McClure (1975) because: • Many nights at Toronto were of dubious photometric quality (= large random errors), • Bandpass of the ultaviolet interference filter measuring line blanketing (38) was too narrow  limiting magnitude for observations of late-type stars was severely restricted • Only color indices were published (lack of a magnitude system difficults detection of variable stars) • Original Toronto filter set deteriorated shortly after observations were completed The DDO photometric system

  4. The DDO photometric system • New observations of 285 standard stars made on the 41-cm and 91-cm reflecting telescopes at Kitt Peak National Observatory (1971-1973) replaced the original ones for the calculations • Equipment used: conventional single channel photometers with dry-ice-cooled 1P21 photomultipliers • Magnitude system for the 48 filter was incorporated (48M = V + 0,375 (B-V) for A-K stars) The DDO photometric system

  5. The DDO Filters (acc. McClure 1975)Specifications The DDO photometric system

  6. The DDO Filters (acc. McClure 1975)Transmission curves The DDO photometric system

  7. The DDO Filters (acc. McClure 1975)Transmissions of the standard DDO set The DDO photometric system

  8. The DDO Filters (acc. McClure 1975) The following four filters have been used for Population I stars to provide measurementes of Teff, surface gravity and abundance of heavy elements: • The 48 filter (λ4886): • Similar to the Hβ filter of Crawford & Mander, but its use here is not connected with the hydrogen line • Situated in a region of absorption by MgH • The 45 filter bandpass (λ4517) : • Situated in a region relatively free of strong spectral features • Comparison bandpass for the 42 and 48 filters The DDO photometric system

  9. The DDO Filters (acc. McClure 1975) • The 42 filter bandpass (λ4257) : • Located shortward of the main CH absorption of the G band (λ4299) and longward of the violet cyanogen-band head (λ4216) • The 41 filter bandpass (λ4166) : • Located in the region of the violet cyanogen-band absorption The DDO photometric system

  10. The DDO Filters (acc. McClure 1975) For very metal-poor Population II stars (weak or non-existent CN band) or where more than the abundance of CNO elements is of interest, metallic-line blanketing index are necessary: • The 38b filter (λ3815) : • Located shortward of the break near λ4000 (due to metallic blanketing in UV) • The 35 filter (λ3460) : • Identical to the “u” filter of Crawford & Barnes (1970) • Located shortward of the Balmer discontinuity and longward of the region where the Earth´s atmospheric absorption becomes total The DDO photometric system

  11. The DDO Standard Stars (acc. McClure 1975) • Almost all stars observed by McClure and van den Bergh (1968) were retained (about 2/3). • They were originally chosen acc. to following criteria: • Stars as bright as possible (to provide an easily observed set of standards) • Spectral type range from O9 to M6, including all luminosity classes • Both metal-poor and metal-rich stars • A number of unusual stars should be included (e.g. strong cyanogen absorption, peculiar, WR, metallic-lines, etc) • Half were picked to be brighter than 6.0mag, and half between 6.0 and 8.0 mag The DDO photometric system

  12. The DDO Standard Stars (acc. McClure 1975) It is always wise to limit the temperature range of standard stars observed. Most standards should be similar to the program stars in spectral type and luminosity class The DDO photometric system

  13. The DDO Standard Stars (acc. McClure 1975) It is always wise to limit the temperature range of standard stars observed. Most standards should be similar to the program stars in spectral type and luminosity class The DDO photometric system

  14. The DDO color indices Succesive color indices are formed from measurements through each of the filter bandpasses. Example: for the intensity of light through filters 45 and 48  C(45-48) = 2,5 log (I48/I45) • The C(45-48) index: • Sensitivity to surface gravity for late-type stars (due to MgH absorption region for filter 48) The DDO photometric system

  15. The DDO color indices • The C(42-45) index: - Designed to measure the break in the continuum of late-type stars due to the G band of CH and other nearby lines -The G band break is primarily sensitive to spectral type or Teff - Affected by microturbulence (in the 42 band CH lines presumably makes it more effective). The effect is stronger in coolest models and decreases with increasing Teff The DDO photometric system

  16. The DDO color indices • The C(41-42) index: • Also called “cyanogen-index” • Sensitivity to luminosity (Lindblad, 1922) • Sensitivity to abundance of heavy elements (C, N) The DDO photometric system

  17. The DDO color indices • The C(38-42) index: • Defined as [C(38-42) = C(38-41)+C(41-42)] • Designed to be a blanketing index • The C(35-38) index: • Measures the Balmer discontinuity • Sensitive to both temperature and luminosity • Index of limited use for late stars (except for those too hot for C(45-48)) The DDO photometric system

  18. The DDO color-color diagrams • The C(45-48) vs C(42-45) diagramm: • The most basic and important in the DDO system • It enables two-dimensional spectral classification as accurate as the MK-system • Central role in the determination of absolute magnitudes and metallicities of late-type Pop I giants when combined with the C(41-42) cyanogen index

  19. The DDO color-color diagrams • The C(41-42) vs C(42-45) diagramm: Very sensitive to overall metallicity (insofar as C and N vary in lockstep with Fe and the rest of the metals) The DDO photometric system

  20. The DDO color-color diagrams • The C(38-42) vs C(45-48) diagramm: Allows luminosity classification

  21. Impact of molecular absorption on DDO photometry • DDO photometry measures a relatively narrow spectral window. It includes a number of strong molecular absorption features in late-type stars. It is interesting knowing the resulting color offsets relative to the original model for each molecule MgH, SiH, CH, CN The DDO photometric system

  22. References • McClure, Robert (1975), Standard Stars for DDO photometry, The Astronomical Journal, Vol 81, Nr. 3, p. 103 • Tripicco, M.J. and Bell, R.A. (1991), Exploring the properties of the DDO system using synthetic colors, The Astronomical Journal, Vol. 102, Nr. 2, p. 744 • McClure, R. and van den Bergh, S. (1968), Five-Color Intermediate-Band Photometry of Stars, Clusters and Galaxies, The Astronomical Journal, Vol 73, Nr. 5, Part 1, p. 313 The DDO photometric system

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