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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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1

Standard Photometric Systems

Michael S. Bessell

Research School of Astronomy and Astrophysics, The Australian National University, Weston,

ACT 2611, Australia

KEYWORDS: methods:data analysis techniques: photometric,spectroscopic catalogs.

ABSTRACT: Standard star photometry dominated the second part of the 20th century

reaching its zenith in the 1980s. It was introduced to take advantage of the high sensitivity

and large dynamic range of photomultiplier tubes compared to photographic plates. As the quantum e ciency of photodetectors improved and the wavelength range extended further to the red, standard systems were modi ed and re ned and deviations from the original systems proliferated. The revolutionary shift to area detectors for all optical and IR observations forced further changes to standard systems and the precision and accuracy of much broad and intermediate band photometry su ered until more suitable observational techniques and standard reduction procedures were adopted. But the biggest revolution occurred with the production of all-sky photometric surveys. Hipparcos/Tycho was space based, but most, like 2MASS, were ground-based dedicated survey telescopes. It is very likely that in the future, rather than making a measurement of an object in some standard photometric system, one will simply look up the magnitudes and colors of most objects in catalogs accessed from the Virtual Observatory.

In this review the history of standard star photometry will be outlined and the calibration and realization of standard systems will be examined. Finally, model atmosphere uxes are now very realistic and synthetic photometry o ers the best prospects for calibrating all photometric systems. Synthetic photometry from observed spectrophotometry should also be used as a matter of course to provide colors within standard systems and to gain insights into the spectra and colors of unusual stars, star clusters and distant galaxies.

CONTENTS Introduction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 2

Background ::::::::::::::::::::::::::::::::::::::::::

The worldwide web resources ::::::::::::::::::::::::::::::::

Basic astronomical photometry assumptions ::::::::::::::::::::::::

The nature of standard systems :::::::::::::::::::::::::::::::

Development of standard systems - changes in detectors and wavelength coverage :::

Synthetic photometry :::::::::::::::::::::::::::::::::::::

Broad-band photometric systems : : : : : : : : : : : : : : : : : : : : : : : : : : : : 10

The Johnson-Cousins UBVRI system ::::::::::::::::::::::::::::

The Washington CMT1 T2 system ::::::::::::::::::::::::::::::

The Sloan Digital Sky Survey ugriz system :::::::::::::::::::::::::

The Hipparcos-Tycho Hp BT VT system :::::::::::::::::::::::::::

The HST WFPC2 160w, 336, 439, 450, 555, 675, 814 system ::::::::::::::

Intermediate band photometric systems : : : : : : : : : : : : : : : : : : : : : : : : 15

The Stromgren 4-color uvby system :::::::::::::::::::::::::::::

The DDO 35,38,41,42,45,48 system ::::::::::::::::::::::::::::

Annu. Rev. Astron. Astrophys. 2005 43

The Geneva (UBB1 B2 VV1 G) system ::::::::::::::::::::::::::::

The Vilnius (UPXYZVS) system ::::::::::::::::::::::::::::::

The Walraven WULBV system :::::::::::::::::::::::::::::::

Narrow-band photometric systems : : : : : : : : : : : : : : : : : : : : : : : : : : : 20

The Oke AB magnitude system :::::::::::::::::::::::::::::::

The Wing 8-color system ::::::::::::::::::::::::::::::::::

Infrared photometric systems : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 21

Introduction::::::::::::::::::::::::::::::::::::::::::

Infrared spectrophotometry ::::::::::::::::::::::::::::::::::

The Johnson/Glass JHKLMN system :::::::::::::::::::::::::::

The MKO JHKL M system :::::::::::::::::::::::::::::::::

The 2MASS JHK System ::::::::::::::::::::::::::::::::::

The DENIS iJK System :::::::::::::::::::::::::::::::::::

Space Based IR Photometric and Spectrophotometric Systems ::::::::::::::

Observational complications with standard photometry : : : : : : : : : : : : : : : 25 Complications to the realisation of standard systems : : : : : : : : : : : : : : : : : 26

Standard systems may no longer represent real natural systems. :::::::::::::

–  –  –

1 Introduction

1.1 Background One of the major achievements of 20th century astrophysics has been the unravelling of the evolution of stars and the understanding of the creation of elements in stars and in supernova explosions. Another highlight has been work on the formation and evolution of galaxies stimulated by cosmological n-body simulations and large telescope observations of distant galaxies. Central to all this work is the quantitative photometry and spectroscopy of stars and stellar systems. For historical reasons, these brightness measurements are normally given in magnitudes (an inverse logarithmic scale) and magnitude di erences between di erent wavelength regions are called colors.





Much of astrophysics revolves around the position occupied by stars in the luminosity versus temperature plane known as the Hertzsprung-Russell (HR) diagram. The life history of a star is traditionally described by the path it traces in the HR diagram. The observational version of the HR diagram is the Color-Magnitude diagram. The integrated magnitudes and colors of star clusters and galaxies is often analysed in similar diagrams for integrated systems. Much e ort, observational and theoretical goes into converting, for example, the visual magnitude and color of a star into its total absolute ux (bolometric magnitude) and e ective temperature (related to the surface temperature) in order to derive the mass, composition and age of the stars. This involves empirical relations derived from stars with known luminosities and measured radii together with theoretical model atmosphere computations.

The integrated magnitudes and colors of star clusters are also of great interest as they can be used to derive the age and composition of the constituent stars. Similarly, the integrated magnitudes and colors of galaxies can be analysed in order to determine the nature and proportion of their constituent stars and gas and study galaxy evolution.

Such population synthesis is a powerful tool in the study of the distant universe as well as the local system (for example, Lancon & Rocca-Volmerange 1996 Ng 1998).

Common to all these endeavours is the necessity to place the measurements onto a standard physical ux scale by removal of the absorption by the Earth's atmosphere Standard Photometric Systems 3 and the calibration of the sensitivity of the photometric/spectroscopic equipment at di erent wavebands. For mainly historical reasons, but also for good practical reasons, astronomical photometric observations are calibrated through the use of networks of constant brightness stars, rather than laboratory based calibration lamps of constant temperature. These networks of stars comprise the various standard star systems that this review will examine.

Three signi cant papers on photometry have been published in this journal. Oke (1965) outlined the AB (absolute) photometric system of pseudo monochromatic photometry in which the uxes are on an absolute ux scale and the magnitude di erences (colors) are directly related to ux ratios. This system is the basis of all spectrophotometric calibrations.

In a very in uential and forward looking paper Johnson (1966) described the UBVRIJHKLMN system of broad band photometry extending from 300nm to 10 that he had established. This system forms the basis of all subsequent broad-band systems and initiated IR astronomical research. Fluxes in the Johnson system are normalized to that of Vega, so conversion of Johnson magnitudes to absolute uxes require multiplication by the ux of Vega. Johnson provided tables of intrinsic colors for dwarf and giant stars, temperature-color calibrations and bolometric corrections for stars with di erent color with which to derive total ux from their measured V magnitude.

In the same volume, Stromgren (1966) outlined the intermediate-band uvby system that he had devised to better measure the temperature, gravity and reddening of earlytype stars. This system revolutionised quantitative photometry by providing more precise estimates of temperature, gravity, metallicity and interstellar reddening for stars hotter than the sun. It also stimulated the establishing of other systems better suited for studying cooler stars.

Wider discussions of photometry and photometric practice are provided in the following selected books. Problems of Calibration of Multicolor Photometric Systems: Philip (1979) High Speed Astronomical Photometry: Warner (1988) Astronomical photometry

- A guide: Sterken & Manfroid (1992) Multicolor Stellar Photometry: Straizys (1992) Handbook of Infrared Astronomy: Glass (1999) Handbook of CCD Photometry: Howell (2000). The web also provides access to course notes and other useful photometric information some of which is described below.

Since the last reviews, there have been great changes in detectors and a proliferation of photometric systems, some related, some di erent. There are at least three outstanding

photometric questions for astronomers that this review will help answer. These are:

What are the precisions of the di erent photometric systems? What are the conversions between the magnitudes and colors in the di erent photometric systems and how reliable are they? How well are the passbands known and can the extant standard system be theoretically realised using model atmosphere uxes and observed spectra?

1.2 The worldwide web resources The advent of the worldwide web has opened up the way for ready access to standard star and standard bandpass databases. As with much on the web it is necessary to have a reasonable appreciation of what one is viewing, in this case standard star systems, their limitations and strengths, because most database providers are collectors rather than censors and the quality of the content is mixed.

There is also the problem with referring to WWW sites that the address may subsequently change, however the advantages of ready access to digital data outweighs the frustration of the non-existent address and often a similar or updated document can be found from higher in the tree or with the help of a search engine.

The Asiago data base of photometric systems has recently been established http://ulisse.pd.astro.it/Astro/ADPS/.

Moro & Munari (2000) provided a compilation of basic information and reference data for 201 photometric systems. Fiorucci & Munari (2003) added a further 17 systems and 4 Bessell provided homogeneous band and reddening parameters for all systems with known band transmission pro les. Future papers will deal with calibration of the systems in terms of the basic physical stellar parameters (temperature, gravity, metallicity, reddening) and transformations between the systems.

The Lausanne Photometric Database http://obswww.unige.ch/gcpd/gcpd.html also provides catalogs and details of the UBV, uvby, Geneva, Vilnius, Walraven, DDO, Washington and gnkmfu systems. Photometric calibrations are referenced in http://obswww.unige.ch/gcpd Standard Objects for Astronomy (http://sofa.astro.utoledo.edu/SOFA/photometry.html) has links to the other two databases and provides lists of standard star photometry in the optical and IR for many systems.

All the major observatories also provide web access to lists of standard stars, zeropoint calibrations and transformation coe cients for their instrumental systems.

Space Telescope Science Data Analysis System (STSDAS) synthetic photometry (synphot) package (http://stsdas.stsci.edu/documents/SyG 95/SG 1.html) is an IRAF (Image Reduction and Analysis Facility) based suite of tasks designed to simulate photometric and spectrophotometric data resulting from Hubble Space Telescope (HST) observations. In addition to the HST instrument lters, it also contains throughput data for several conventional photometric systems, such as Johnson-Cousins UBVRI, Stromgren uvby and Walraven VBLUW.

1.3 Basic astronomical photometry assumptions To assist those new to the arcane study of astronomical photometry, it is useful to mention a few of the usual assumptions and some of the vocabulary. If NB is the total number of electrons detected from a source above the background sky in t seconds through a lter B, the measured instrumental B i magnitude is B i = const - 2.5 log10(NB /t). That is, for two stars with uxes N1and N2 the magnitude di erence between them is B1 - B2 = -2.5log10(N1/N2 ).



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