From: Chris Lidman (clidman@eso.org)
Date: Sun May 16 2004 - 08:57:45 PDT
Hi Serena,
I've though a bit more about the issue of cross filter k-corrections
and I attach a short report. Alex, I'd like you to read this and to give
us your opinion.
Cheers, Chris.
IR to Optical k-corrections
===========================
The cross filter k-correction is defined in Kim, Goobar and Perlmutter
(1996)
and is applied to apparent magnitudes according to
m_y = m_x + K_xy(z)
where x and y are different filters.
Let's assume that z=0 and that Z=F. Let's further assume that we are
using
the Kurucz model for Vega. In this case K_xy(0) = 0 and, hence,
m_y = m_x
However this is not true for Vega for all x and y (i.e. all filters)
m_V=0.026 +/- 0.008 (Bohlin and Gilliland, 2004)
m_I=0.031 +/- 0.009 (Bessell, Castelli and Plez, 1998)
m_J=-0.001 +/ -0.005 (Cohen, Wheaton and Megeath, 2003)
where J is on the 2MASS system.
If we set x to I and y to J
Hence m_J=m_I-0.032.
or m_I - m_J = 0.032 = -K_IJ
I propose that we add an extra term to the cross filter k-correction as
defined by Alex which reflects the colours of Vega. Note that Alex's
definition is completely correct for an object in which all colours are
zero.
The new k correction is then
K'_xy(z) = K_xy(z) - (x-y),
where (x-y) is the colour of Vega and K_xy(z) is the k-correction
defined by Alex.
For the I band paper, we added this correction to the J band photometry.
I think we should change this by changing the way we do the
k-correction,
as I have described above.
Apart from the uncertainties described here, there is a systematic
uncertainty
in how well the Kurucz spectrum represents Vega. Bohlin and Gilliland
quote an uncertainty of 2% from optical to IR wavelengths. We should add
this systematic uncertainty to all IR to optical K corrections.
Bessell, M. S., Castelli, F. and Plez, B. 1998, AA, 333, 231
Bohlin, R. C. and Gilliland R. L. 2004, astro-ph/0403712
Cohen, M., Wheaton, WM. A. and Megeath, AJ, 2003, 126,1090
Kim, A, Goobar, A, and Perlmutter, S. 1996, PASP, 108, 190
On Fri, 2004-05-07 at 19:59, Chris Lidman wrote:
> Hi Serena,
> I'd like to draw your attention to some nice work that has been done
> on the absolute flux calibration of the Vega SED, which is central
> to the way we compute cross-filter k-corrections and, in particular,
> the corrections from IR to optical filters. See Bohlin and Gilliland
> (astro-ph/040371) and Cohen et al. (AJ 126, 1090).
>
> The core of their results is that Vega has V-J = 0.026 +/- 0.008.
> It is not 100% clear to me what this implies for IR -> optical
> k-corrections, but I'll read these papers carefully over the
> next week and I'll let you know what I think. It is interesting to note
> that this number is not too far away from the ensemble of A0 stars
> from the Hipparcos and 2MASS catalogs that were used to compute
> the offset between IR and optical photometric systems - V_J-J_2MASS =
> 0.043
>
> Cheers, Chris.
>
>
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