Extinction mapping through broad band photometry

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Contents 1 Purpose 2 Overview 3 Science Description 4 Specific Steps

[edit] Purpose

The purpose of this project is to derive information about point sources from broad band photometry.

[edit] Overview

The best way to get information about astronomical objects is to look at their spectra, the intensity versus the wavelength. Unfortunately, it is difficult to get spectra of every object of interest because the light is dispersed over a number of pixels and hence more exposure time is required for each object.

However, there are now surveys of large areas of the sky in several bands spread over the electromagnetic spectrum. The hope is that these may be used as low resolution spectra which will allow some discrimination between objects.

In its most general form, this can be a daunting task because of the vast amount of data and the combinations of parameters, especially as one may know which parameters are interesting. In this sense it is analogous to the data mining techniques in demand in industry to forecast trends and patterns in disparate data sources.

What I propose here is a smaller subset of the problem: simply to look at an extinction map of the sky.

[edit] Science Description

Stars are divided into several classes based on their temperature. These spectral classes range from the very hottest stars (O stars) to the coolest M stars as per the well known sequence OBAFGKM. At a first level, these stars are essentially black body emitters with a characteristic surface temperature given by the spectral type. Zombeck is an excellent reference for spectral information on stars as well as on many other astronomical topics.

Thus, we can distinguish stars of different spectral types by using their brightness in different wavelength regions. For example, a K star with a surface temperature of about 5000K will be much brighter in red than in blue while the reverse will be true for a B star with a surface temperature of 30000K. If we look at the B and V magnitudes of the two stars, we would see that the (B - V) of the K star would be greater than that for the B star (remember that brighter stars have smaller magnitudes). If we can observe in the UV, the contrast will be even greater as the maximum light from an O or B star will be in the UV.

The situation is complicated by extinction. Interstellar dust will preferentially absorb or scatter the starlight in the UV and blue reddening the star; ie., increasing the B magnitude more than the V. This is commonly measured as E(B - V) = (B_o - V_o) - (B_I - V_I) where the first term is the observed (B - V) and the second term is the (B - V) that the star would have if there were no interstellar absorption - the intrinsic (B- V).

In principle, spectra of the individual stars would allow us to deconvolve the spectral type from the extinction. Without spectra, we will have to use broad band photometry. It is hoped that this will also allow a unique determination of both from observations of unknown stars.

[edit] Specific Steps

1. Find point source data. A good source would be GALEX point source files available from the Space Telescope Archive page. This contains data from the two GALEX bands.
2. Get B and V magnitudes using Simbad.
3. Plot colour-colour plots. A spreadsheet will do for this.
4. Calculate theoretical colours. Can be calculated from flux calculator on the TAUVEX web page.
5. Compare theoretical colours with observed colours.
6. Add E(B-V) to see what difference there is.