Posted by Vytautas Straizys on July 06, 1999 at 10:22:33:
To:
J. MacKenty, WFC3 Deputy Instrument Scientist
R. O'Connell, Chair, WFC3 Scientific Oversight Committee Dear colleagues, One of the most important purposes of the WFC3 is a possibility of
determining temperatures, luminosities, metallicities and peculiarity
types for very faint stars affected by various amounts of interstellar
reddening, inaccessible for the surface telescopes. For this aim, a photometric system capable to classify all types of
stars in the presence of interstellar reddening is essential. Such a
system is the Stromvil system described by Straizys, Crawford
& Philip (1996), Straizys & Hoeg (1995), Straizys, Hoeg & Philip (1997)
and Straizys (1999). The system consists of 7 passbands of medium width
(20-40 nm) with the following mean wavelengths:
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Designation Mean lambda Half-width Spectral feature
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u 350 nm 30 nm Ultraviolet intensity P 374 nm 26 nm Crowding of Balmer lines v 411 nm 19 nm Intensity before the Balmer jump b 467 nm 18 nm Break-point of the interstellar
extinction law Z 516 nm 21 nm Absorption by Mg I triplet lines
and MgH band y 547 nm 23 nm Standard continuum magnitude S 656 nm 20 nm H alpha line
----------------------------------------------------------------------- This system makes possible to identify completely photometrically in the
presence of interstellar reddening the following types of stars:
(1) O-B-A-F-G-K-M stars of various luminosities of solar metallicity,
(2) metal-deficient F-G-K subdwarfs, (3) metal-deficient G-K giants,
(4) chemically peculiar B-A stars, (5) emission-line stars,
(6) carbon-rich stars, (7) white dwarfs, (8) many types of unresolved
binaries. If color indices in the Stromvil system are measured with
the accuracy of +/- 0.01 mag, then the following accuracy of the
determined parameters is expected with the available calibration: -- spectral class: +/- 0.8 decimal subclass,
-- temperature: from +/- 2000 K for hot stars to +/- 200 K for cool
stars,
-- absolute magnitude: +/- 0.4--0.6 mag for luminosity V--III stars,
+/- 0.8--0.9 mag for supergiants,
-- surface gravity log g: +/- (0.2--0.5) dex,
-- metallicity [Fe/H]: +/- (0.15--0.20) dex,
-- color excess E(B-V): +/- (0.02--0.03) mag,
-- interstellar extinction A(V): +/- 0.1 mag. The classification accuracy in various places of the HR diagram is shown
by Straizys et al. (1998). Three passbands of the Stromvil system, namely v,b and y can be set up
by the filters F410M, F467M and F547M present in the WFC3 Strawman list.
The filter F336W is suitable to be used instead of the Stromvil u
filter. Additionally, we suggest to place in the WFC3 the following
three filters: P at 374 nm, Z at 516 nm and S at 656 nm. The
justification of these filters is given below: The filter P at 374 nm (with a half-width of about 30 nm) will measure
the integrated effect of crowding of higher members of the Balmer lines
near the Balmer limit. The resulting P magnitude is very sensitive to
luminosity for B-A-F type stars. The filter Z at 516 nm (with a half-width of about 20-25 nm) is placed
on a wide and deep absorption feature in the spectra of G-K-M stars made
by a crowding of metallic lines, of which the Mg I triplet is the
strongest. Additionally, in K and M dwarfs the Z passband contains
strong MgH molecular bands. Both the Mg I lines and MgH bands show
strong negative luminosity effect, making color indices, containing the
Z magnitude, to be luminosity discriminants for G, K and M-stars. The filter S is placed on the H alpha line and for all types of stars it
indicates the presence of emission in this line. This makes possible to
identify Be, Herbig Ae/Be, T Tauri, WR and other emission-line stars. The Stromgren system uvby which is planned to set up in the WFC3 without
the P, Z and S filters will not be able to classify late-type stars and
to identify peculiar stars in the presence of interstellar reddening.
The accomplishment of the complete seven-color system will help to
classify photometrically very faint and reddened stars both in our
Galaxy and in other nearby galaxies. References: Straizys V. 1999, Baltic Astronomy, 8, 109 Straizys V., Crawford D.L., Philip A.G.D. 1996, Baltic Astronomy, 5, 83 Straizys V., Hoeg E. 1995, in Proceed. of RGO-ESA Workshop on Future
Possibilities for Astrometry in Space, Cambridge, ESA SP-379, p. 191 Straizys V., Hoeg E., Philip A.G.D. 1997, in Proceed. ESA Symposium
Hipparcos-Venice '97, ESA SP-402, p. 761 Straizys V., Liubertas R., Lazauskaite R. 1998, Baltic Astronomy, 7, 529
V. Straizys
Institute of Theoretical Physics and Astronomy
Gostauto 12, Vilnius 2600
Lithuania email: straizys@itpa.lt