Wide Field Camera 3
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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:
-----------------------------------------------------------------------
Designation   Mean lambda  Half-width    Spectral feature
-----------------------------------------------------------------------
     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