isophotal wavelength

Hi guys!

I have a question about isophotal wavelength for a star. I have clear
the idea that given a stellar spectra F(l), the spectral response of a
filter T(l), I can compute the average in-band flux using

in-band-flux=\frac{\int l F(l) T(l) dl}{\int l T(l)dl} eq
(1)

where l is the wavelength.

Then the isophotal wavelength l_iso is the wavelength for which it
holds:

F(l_iso)=in-band-flux eq (2)

However real spectra can lead to a multiple solution for the above (eq
2) relation. Therefore, in principle, I can have more than one
isophotal wavelength. This is obviously wrong!

Do I have to search the wavelength on the continum of the stellar
spectra that satisty the relation written in eq (2)?
But if this is the case, I compute the in-band-flux using the stellar
spectra, while I find the isophotal wavelength using the continum
spectra. Is that ok ?

Thanks to any kind guy who wants to teach me a bit more about this! By
the way, if anyone knows some paper where I can find more informations
I would be really grateful if you could tell me about. Up to now, it
looks to me that the only complete source of information is a book by
Golay (1974) which I cannot find

Thanks !!
S.

wrote:

I have a question about isophotal wavelength for a star.
.......
Thanks to any kind guy who wants to teach me a bit more about this! By
the way, if anyone knows some paper where I can find more informations
I would be really grateful if you could tell me about. Up to now, it
looks to me that the only complete source of information is a book by
Golay (1974) which I cannot find

Go to the Astrophysics Data Service Abstract search site:

http://adsabs.harvard.edu/abstract_service.html

Type into the "Abstract Words" box the terms of interest:
I typed

isophotal wavelength filter spectrum

Then click the "Submit" button. You will receive a list
of references on the given topics. Many of the papers
are scanned and fully available to you. I noticed one
in a series which you might check out in particular:

http://adsabs.harvard.edu/cgi-bin/np...f6510b0d806586

(PASP 117, 421, 2005).

By the way, although you wrote that "it must be wrong"
that different spectra could lead to different isophotal
wavelengths for a given filter, I don't see why.

Michael Richmond

Thanks a lot for the advices, I'll make a search in ADS asap.

I already went throught the PASP paper you suggested me, it's clearly
explained, but they also rise the problem of multiple solution for
isophotal wavelength.

I probably explained myself in a bad way, but the problem is that, for
a given (stellar) spectra, you have absorprion lines and
discontinuites. Therefore, the spectra has not a monotonic trend. Thus
there can be more than one wavelength were the monochromatic flux equal
the in-band flux.

This is the reason why I wonder wheather I should consider the continum
of a spectra for searching the correct isophotal wavelength...

wrote:

......
This is the reason why I wonder wheather I should consider the continum
of a spectra for searching the correct isophotal wavelength...

Why exactly do you want to know the isophotal
wavelength? Perhaps there is some other
quantity which is better suited to your needs.
What is your goal?

Michael Richmond

Why exactly do you want to know the isophotal
wavelength? Perhaps there is some other
quantity which is better suited to your needs.
What is your goal?

Hi,

I'm currently working with the infrared flux method (irfm). This
method, is based on the idea that the ratio between the bolometric and
the monochromatic flux (in the infrared) gives you and estimate of the
stellar temperature.
There are quite a lot of paper on that, from the pioneering work of
Blackwell (1980), to the most recent one by Ramirez & Melendez (2005).
The most extensively and clearly explained is that of Alonso et al.
(1996)

So, to get back to my problems, I need to make the ratio between the
bolometric and the monochromatic flux at a chosen wavelength (in the
IR). To have the monochromatic flux I use a model atmosphere (Kurucz)
-- NOTE : ok, you probably say: if you use a model atmosphere you
already know the temperature of the star. Right, but I start with an
estimate (photometric) of the temperature of the star, I pick up the
corresponding model atmosphere, plug it into the irfm, get a new
temperature and iterate so on until temperature converges --

So, the Kurucz model tell me about the monochromatic flux. In first
approximation the idea is ok, but the risky thing is that a
monochromatic flux at my chosen wavelength can be dominated by
absorption lines. My thinking is therefore to compute the in-band flux
(thus avoiding trouble with absorption lines), and then pick up the
isophotal wavelength corresponding to the in-band flux.
In order to do this, I was therefore wondering if I can calculate the
isophotal wavelength directly on the continuum (that is also provided
by the model atmosphere). But then I have an in-band flux computed on
the spectra and a isophotal wavelength found on the continuum. So I'm
not sure this is the right procedure to follow (also because for cooler
(4-5000) K stars there is a quite large diffencence between continuum
and spectra in the IR. For higher temperatures, the sun already, a
polinomial fit to the spcetra is a quite good approximation of the
continuum).

Anyway, I was thinking to use the effective wavelength as a good
approximation of the isophotal wavelength. Indeed the problems are not
finnished, but then I make the post awfully long! If you are interested
I'll be glad to tell you more...

Thanks!
S.

wrote:
I'm currently working with the infrared flux method (irfm). This
method, is based on the idea that the ratio between the bolometric and
the monochromatic flux (in the infrared) gives you and estimate of the
stellar temperature.

What are you using for the bolometric flux?

So, to get back to my problems, I need to make the ratio between the
bolometric and the monochromatic flux at a chosen wavelength (in the
IR). To have the monochromatic flux I use a model atmosphere (Kurucz)
.....
So, the Kurucz model tell me about the monochromatic flux. In first
approximation the idea is ok, but the risky thing is that a
monochromatic flux at my chosen wavelength can be dominated by
absorption lines. My thinking is therefore to compute the in-band flux
(thus avoiding trouble with absorption lines), and then pick up the
isophotal wavelength corresponding to the in-band flux.

I am not certain, but it looks to me as though you are going about this
backwards. You have some photometric measurement you are trying to
match, right? That measurement is done with a known bandpass. So it
seems to me you want to integrate the model spectrum over the
measurement bandpass and ask what temperature for the model best
matches the data.

Steve Willner (who is not sure why Google won't put his last name in
the From header line)