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«1 Standard Photometric Systems Michael S. Bessell Research School of Astronomy and Astrophysics, The Australian National University, Weston, ACT ...»

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Much information on standards and other information relevant for photometry with HST can be found on ftp://ftp.stsci.edu/cdbs/cdbs8/synphot tables/. WPFC2 calibration data is regularly updated on the WFPC2 website www.stsci.edu/instruments/wfpc2/wfpc2 resources.html.

Details of the photometric zeropoints for the STSCI speci ed WFPC2 bands are given at http://www.stsci.edu/instruments/wfpc2/Wfpc2 dhb/wfpc2 ch52.html#1902184. It is suggested that the passbands and zeropoints for F336W should be reassessed and the UBVRI passbands of Bessell (1990) be used in place of the synphot recommended UBVRI bands.

2.5.1 The HST STIS LP band An arbitrary shift in the synphot LP bandpass was also needed to get agreement between synthetic photometry and globular cluster giant observations in F814-LP and F606-LP.

This was described by Houdashelt, Wyse & Gilmore (2001) in an appendix. Bessell (2000)(unpublished) trialled variations in the bandpass and an acceptable t was obtained by suppressing the extended red tail of the band, thus e ectively shifting the synphot LP passband beyond 8000 A by about 80 A to the blue. These synphot passband and arbitrarily modi ed LP passband (light line) are also shown in Fig. 2. Table 2 lists the UX90, U3 and modi ed U, F336 and LP passbands.

3 Intermediate band photometric systems Real and perceived problems with the placement and de nition of the bands in the UBV system led to the development of intermediate band photometric systems. Figure 3 illustrates schematically the passbands for ve of the most widely used of these systems.

Table 3 lists the nominal central wavelengths and FWHM (full-width-half-maximum) of the passbands.

3.1 The Stromgren 4-color uvby system The system was speci cally devised by Stromgren (1951) for B, A and F stars, although it was extended later to other kinds of stars. Several indices are derived: m1 =(v-b)-(b-y) to measure the depression owing to metal lines around 4100A, c1 =(u-v)-(v-b), which measures the Balmer discontinuity and = w - n to measure the strength of the H line. Reddening independent indices m1 ] and c1 ] are also often used (Stromgren 1966).

The y magnitude is de ned to be essentially the same as V for non-M stars and b-y is a color like B-V but less sensitive to metallicity. Surprisingly, b-y also transforms extremely well to V-I. From all the E-region stars Cousins (1987) derives 16 Bessell

Table 2: Modi ed Passbands Wave UX90 Umod F336mod U3 Wave LPmod 3050 0.016 0.000 0.000 0.000 5200 0.000 3100 0.068 0.016 0.059 0.020 5400 0.105 3150 0.167 0.068 0.182 0.077 5600 0.822 3200 0.287 0.167 0.582 0.135 5800 0.899 3250 0.423 0.287 0.732 0.204 6000 0.974 3300 0.560 0.423 0.844 0.282 6200 1.000 3350 0.673 0.560 0.896 0.385 6400 0.990 3400 0.772 0.673 0.917 0.493 6600 0.980 3450 0.841 0.772 0.918 0.600 6800 0.966 3500 0.905 0.841 0.948 0.705 7000 0.893 3550 0.943 0.905 1.000 0.820 7200 0.886 3600 0.981 0.943 0.912 0.900 7400 0.885 3650 0.993 0.981 0.658 0.959 7600 0.807 3700 1.000 1.000 0.395 0.993 7800 0.710 3750 0.989 0.989 0.185 1.000 8000 0.653 3800 0.916 0.916 0.074 0.975 8200 0.517 3850 0.804 0.804 0.029 0.850 8400 0.499 3900 0.625 0.625 0.009 0.645 8600 0.482 3950 0.420 0.420 0.000 0.400 8800 0.432 4000 0.238 0.238 0.223 9000 0.356 4050 0.114 0.114 0.125 9200 0.264 4100 0.051 0.051 0.057 9400 0.162 4150 0.019 0.019 0.005 9600 0.090 4200 0.000 0.000 0.000 9800 0.045 10000 0.023 10200 0.000

Table 3: Wavelengths (A) and Widths (A) of Intermediate Systems Stromgren DDO Geneva Vilnius Walraven e e e e e u 3520 314 35 3460 383 U 3438 170 U 3450 400 W 3255 143 v 4100 170 38 3815 330 B 4248 283 P 3740 260 U 3633 239 b 4688 185 41 4166 83 B1 4022 171 X 4050 220 L 3838 227 y 5480 226 42 4257 73 B2 4480 164 Y 4660 260 B 4325 449 w 4890 150 45 4517 76 V 5508 298 Z 5160 210 V 5467 719 n 4860 30 48 4886 186 V1 5408 202 V 5440 260 51 5132 162 G 5814 206 S 6560 200 Standard Photometric Systems 17 b-y =0.000 +0.481(V-I) +0.161(V-I)2 -0.029(V-I)3 V-I = -0.002 +2.070(b-y) -1.113(b-y)2 +0.667(b-y)3.

The system has been extended to white dwarfs, K giants and dwarfs and FGK supergiants but problems have occurred in the standardization owing to di erences in lters (especially the v lter) and to the restricted color, reddening and spectral-type range of the original standards. Manfroid & Sterken (1987, 1992), Eggen (1976) and Olsen (1995) discuss some of these problems. Extensive photometry has been published by Crawford, Olsen and collaborators (Gronbech & Olsen 1977 Olsen 1983, 1984, 1988, 1994 Olsen & Perry 1984 Perry et al. 1987). The Hauck & Mermilliod (1998) homogenized catalog contains data for 63313 stars and is available through the VizieR interface as are other uvby catalogs.

Some of the transformation problems have also arisen from the initial prescriptions of photometric reduction procedures. Rather than standardizing the basic bands u,v,b,y or the colors u-v, v-b, users were instructed to standardize the derived quantities m1 and c1 along with b-y. Because few standard stars have extreme values for m1, it was impossible to properly determine secondary color terms. In the case of c1, non-standard v lters resulted in systematic di erences between B and F stars with the same standard c1 value. Cousins (1986, 1987, 1989, 1990) established excellent secondary standards in the E-regions by standardizing the intermediate colors u-v, v-b and b-y directly. Ironically, CCD observations and data reductions are forcing more scienti c standardizing procedures based on the individual bands (Grundahl et al. 2002) so better standardized Stromgren photometry can be expected in the future.

Although the transformation problems discussed above have made it di cult to dene the uvby system precisely for all spectral types and luminosity classes, a subset of the published photometry has been made with tightly controlled instrumental systems and a restricted set of photometric standards by Schuster, Nissen and their collaborators (Schuster & Nissen 1988 eg Schuster et al. 2004). These observations have been made on the 1.5 m telescope at the Observatorio Astronomico Nacional at San Pedro Martir, Baja California, Mexico, and on the Danish 1.5 m telescope, at La Silla, Chile using simultaneous uvby/H photometers with the bandpasses de ned by spectrograph exit slots combined with interference lters (Nielsen 1983). From these data it should be possible to realize the current standard uvby system and compute theoretical uvby colors for all stars. The precise Stromgren CCD photometry of Grundahl et al. (2002), indicates that good standard uvby photometry is possible using CCDs and good standards. However, they note from their standard star comparisons an increased scatter for the u and v lters to the bands of CN and NH in some stars and the slightly di erent passbands of the Schuster & Nissen photoelectric system and the CCD system.

Theoretical Stromgren colors for a grid of model atmospheres have been calculated by Clem et al. (2004) using the original lter passbands of Crawford & Barnes (1970). No adjustments to the raw colors were made. Rather than use these passbands, I suggest that it is better to use the passbands of Helt, Florentin Nielsen & Franco (1987) which seem identical to those used by Schuster at San Pedro Martir and on which the uvby system for metal-de cient stars is now based. The normalized and smoothed response functions, including atmospheric extinction are given in Table 4. These passbands need small color terms to realize the uvby system, similar to those given by Schuster & Nissen (1988) (Table 1). The exact values are currently being assessed using MARCS models.

Empirical calibrations for color excesses, E(b-y), photometric metallicities, Fe/H], and absolute magnitudes, MV, are provided by Schuster & Nissen (1989) and Nissen & Schuster (1991) and reviewed by Nissen (1994). Arellano Ferro et al. (1990) provide a catalog of F-K supergiants photometry. Gray & Olsen (1991) also discuss the calibration of the Stromgren system for A, F and G supergiants. Reddening is often better measured through uvby photometry than UBV photometry, although for O and B stars the reddening-free Q index (Q = (U-B) - 0.82(B-V) (B-V)0 = Q/3) is still a powerful tool.

Although the uvby system is well established and well calibrated empirically, it does 18 Bessell Table 4: Passbands of uvby System (including atmosphere) Wave u Wave v Wave b Wave y 3320 0.000 4000 0.000 4560 0.000 5340 0.000 3340 0.349 4020 0.427 4580 0.304 5360 0.389 3360 0.492 4040 0.754 4600 0.576 5380 0.672 3380 0.581 4060 0.884 4620 0.906 5400 0.912 3400 0.687 4080 0.979 4640 0.994 5420 1.000 3420 0.789 4100 0.995 4660 1.000 5440 0.982 3440 0.835 4120 1.000 4680 0.969 5460 0.953 3460 0.879 4140 0.996 4700 0.921 5480 0.913 3480 0.926 4160 0.942 4720 0.871 5500 0.850 3500 0.960 4180 0.742 4740 0.864 5520 0.798 3520 0.982 4200 0.440 4760 0.804 5540 0.759 3540 0.992 4220 0.219 4780 0.522 5560 0.725 3560 1.000 4240 0.000 4800 0.186 5580 0.631 3580 0.996 4820 0.000 5600 0.424 3600 0.991 5620 0.213 3620 0.973 5640 0.000 3640 0.934 3660 0.816 3680 0.356 3700 0.000 require high precision photometry, generally better than 0.01 mag, to deliver precision in the derived temperatures gravities and metallicities. Broad-band V-I or V-K can provide temperatures with 3 or 6 times (repectively) greater sensitivity in the color than b-y, and for the metallicity indicator in GK stars, the Washington C-M or C-V index or the DDO/Cousins 38-V color is also more sensitive than m1. For that reason, as well as for reaching fainter magnitudes in a given time, when CCD observations are to be undertaken, it is often advantageous to measure broad band and wider baseline colors rather than uvby.

3.2 The DDO 35,38,41,42,45,48 system Another very useful intermediate band system is the DDO system that was devised to work principally with GK giants and dwarfs. Because it has not been used as a general photometric system, the standard system is robust and well established (McClure 1976 McClure & Forrester 1981 Cousins 1993 Kilkenny & Cousins 1993 Cousins & Caldwell 1996). An homogenized catalog of 6139 stars has been prepared by Mermilliod & Nitschelm (1989). The 35 lter is the same as the Stromgren u the 38 lter is more semsitive to metal-line blanketing than the Stromgren v lter the 41 lter measures the CN band 42, 45 and 48 are pseudo-continuum lters the 51 lter measures the MgH feature in KM dwarfs. The color 35-38 (the 3538 index) measures the Balmer jump, 3842 the metallicity, 4245 and 4548 are used for gravity and temperature measurements, 4851 separates K dwarfs from K giants. Combining some of the DDO magnitudes or indices with broadband magnitudes such as R-I or M-51 (Geisler 1984) has proved very e ective for metallicity estimates and halo GK giant selection (Norris et al. 1985 Morrison et al.

2001), especially for faint stars.

Recent calibrations of temperatures and metallicity are given by Piatti et al. (1993), Claria et al. (1994a, 1994b, 1996) and Melendez & Ramirez (2003)(IRFM temperatures).

Absolute magnitude calibrations are discussed by H g & Flynn (1998).

Standard Photometric Systems 19

3.3 The Geneva (UBB1 B2VV1 G) system The best known of the closed photometric systems, the Geneva system, is supervised by a small group at the Geneva Observatory. VizieR contains the updated version of the 29,397 stars in the Geneva Catalog (Rufener 1999) the Lausanne photometric database contains data for 43931 stars http://obswww.unige.ch/gcpd/ph13.html. In practice, three colors U-B2, B2 -V1, V1 -G are used together with linear combinations which are reddening free for a standard extinction law and E(B-V) 0.4 mag (Golay 1972). The combination indices are d = (U-B1 ) - 1.430(B1;B2), F = (U-B2 ) - 0.832(B2G), g = (B1 -B2 ) - 1.357(V1-G) and m2 = (U-B1 ) - 1.357(V1-G). The indices d and m2 are closely related to the c1 ] and m1 ] indices of the Stromgren system. Nicolet (1996) has derived the natural system's passbands. Kunzli et al. (1997) provide a model atmosphere calibration for Geneva photometry. Melendez & Ramirez (2003) have provided an IRFM temperature calibration for the Geneva system.

3.4 The Vilnius (UPXYZVS) system The Vilnius system was developed independently from the Geneva system but for similar reasons, namely to derive temperatures, luminosities and peculiarities in reddening and composition from photometry alone (eg Straizys 1979). The colors are normalized by the condition U-P = P-X = X-Y = Y-Z = Z-V = V-S for unreddened O-type stars.

Therefore all colors for normal stars are positive. Reddening free indices are constructed as for the Geneva and Stromgren systems.

Straizys, Boyle & Kuriliene (1992) and Boyle et al. (1996) discuss transformations between the CCD and standard Vilnius system. Standard stars for CCD photometry are given by Cernis et al. (1997) and a photometry catalog of 7445 stars observed in the Vilnius system by Straizys & Kazlauskas (1993). Straizys et al. (1993b) provide a new calibration for temperature and gravity of B stars updated by Straizys, Kazlauskas & Bartasiute (1999). Melendez & Ramirez (2003) provide IRFM temperatures for the Vilnius system. Forbes, Dodd & Sullivan (1993, 1997) have established Southern Hemisphere E-region standards. Philip et al. (2003) and Kazlauskas et al. (2003) discuss the 7-color Stromvil system.

3.5 The Walraven WULBV system The most interesting of the closed photometric systems is the Walraven system. Theodore Walraven and Gerry Kron are two of the most under appreciated contributors to accurate astronomical photometry who made innovative instruments and ground breaking experiments with detectors. The Walraven photometer (Walraven & Walraven 1960) is a specially built simultaneous photometer with the passbands de ned by a special lter of crystalline quartz and Iceland spar (calcite) polarization optics and separated geometrically by a quartz prism spectrograph. The L band is taken from a beam that does not pass through the spectrograph. This innovative technique provides great stability of passbands which enabled Pel (1991) to discover small systematic errors in four other photometric systems a feat that was not possible with any other ground based systems.

Used at the Leiden Southern Station in South Africa until 1979, the photometer was moved to the Dutch Telescope at La Silla in 1979 (Pel et al. 1988). Much of the work in the VLBUW system has been on cepheids and RR Lyrae stars (eg Pel 1985) and halo stars (Pel et al. 1988, Trefzger et al. 1995).

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