2mass vega system
Steve....@libero.it
I got a doubt about the 2mass magnitude system. It's known that 2mass
is in Vega system, but what does this actually mean??
I know that according to the definition "in the Vega system I assume
Vega having 0.00 mag in any band", but here is my doubt:
in Johonson-Cousins UBVRI system, the Vega magnitude is 0.03 in V and
all other colors are equal to 0, that means U=B=V=R=I=0.03
Now, let's use the Johonson-Cousins UBVRI + 2MASS JHK_s. What do I
have,now?
U=B=V=R=I=0.03
J=H=K_s=0.00
Is it correct??
Or should I have also J=H=K_s=0.03 ??? Using this last version I
preserve the definition of the colors being zero in all bands, but the
2mass measurement ultimately depends on the Johonson-Cousins system
that I have been using, and this sounds a bit weird ...
If I have correctly understood the paper by Cohen et al. (2003), Vega
should have J=H=K_s=0.00 .
So the Vega system mean : Vega in UBVRI having whatsoever values (0.03
in Johonson-Cousins, 0.whatsoever in HST system etc....) but the 2MASS
has been calibrated imposing EXACTLY J=H=K_s=0.00 for Vega.
Any help/confirmation on this?
Thanxx!
Cheers, S.
Steve Willner
Steve....@libero.it writes:
> I got a doubt about the 2mass magnitude system. It's known that 2mass
> is in Vega system, but what does this actually mean??
2MASS is tied to the photometric network of Cohen et al. (2003 AJ
117, 1864 -- see also 1993 AJ 104, 1650), which is supposedly based
on Vega=0. However, based on the Elias et al. (1982 AJ 87, 1029)
photometry, J(Vega)=-0.04 in the 2MASS system, H=+0.01, and K=-0.02.
See
http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec6_4b.html
and in particular the transformation to the CIT system, which is
based on Vega=0 at J, H, and K.
> I know that according to the definition "in the Vega system I assume
> Vega having 0.00 mag in any band", but here is my doubt:
> in Johonson-Cousins UBVRI system, the Vega magnitude is 0.03 in V and
> all other colors are equal to 0, that means U=B=V=R=I=0.03
Are you sure about that last? The Johnson-Cousins system is based on
colors being zero for "an average unreddened A0 star," but that
doesn't mean Vega itself. I agree about V=0.03, but I thought not
all the colors were zero. Anyway, the important point is that this
is an entirely separate magnitude system.
> Now, let's use the Johonson-Cousins UBVRI + 2MASS JHK_s. What do I
> have,now?
A mistake? These are not the same systems. If you really need
precision at this level, you need to go back to the original
literature and figure out what's what. The long series of Cohen et
al. papers in AJ is probably the place to start.
> Or should I have also J=H=K_s=0.03 ???
Photometry done at University of Arizona through about the mid-1980's
was based on this zero point for Vega. Most everyone else used
something consistent with the CIT/Elias system with Vega=0. There
are lots of references in the link above, and there used to be good
material on the IRTF web site. (I haven't checked to see whether
it's still there or not.) In any case, the 2MASS transformations
given at the link above probably supersede all earlier work.
--
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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Steve....@libero.it
> 117, 1864 -- see also 1993 AJ 104, 1650), which is supposedly based
> on Vega=0. However, based on the Elias et al. (1982 AJ 87, 1029)
> photometry, J(Vega)=-0.04 in the 2MASS system, H=+0.01, and K=-0.02.
> See
> http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec6_4b.html
> and in particular the transformation to the CIT system, which is
> based on Vega=0 at J, H, and K.
Thanks a lot for the reply!!
So, if I have correctly understood, the 2MASS system is defined in
accordance with the extensive work of Cohen, where J=H=K=0 for Vega.
Nevertheless, if you compare common stars between 2MASS and CIT system
(that is also based on a definition J=H=K=0 for Vega) you find out a
small offset.
So if you belive in the CIT system, the 2MASS Vega magnitudes come out
to be J=-0.04, H=+0.01 and K=-0.02.
So, here I come with a doubt. The 2MASS system as defined by Cohen
looks a carefully done job. But at the same time I would bet the CIT
system is well defined as well. The small offset found in the work of
Carpenter, could be done to observational errors? I mean, both 2MASS
and CIT are placed on a system where Vega J=H=K=0.
Then you select a common set of stars to the two systems and you find
small offset. Such offset however is at the level of a few hundreds of
mag, so likely to be a noisy scatter due to observational error. Is it
a reasonable guess?
However, I have another question. In the 2mass paper of Cohen, he
states "in the IR we adopt the synthetic Vega spectrum as a definition
of zero magnitude". The synthetic spectra is that of Kurucz that is
know to work well (actually, how much well???).
Anyway, there is lot of rumor about a possible IR variabilty of Vega
due to dusty circumstellar envelope (there was a paper on that couple
of month ago).
Cohen in his paper (1992 AJ 104, 1650) reccomended the use of Sirius as
a standard in the IR, but in his 2003 work he doesn't mention Sirius
anymore.
Is that because the 2mass system is defined on the Kurucz spectra for
Vega where they impose J=H=K=0 end of the story.
Then the real Vega out there can show variability or whatsoever, but
this does't affect at all the 2MASS system. Is it correct??
> > I know that according to the definition "in the Vega system I assume
> > Vega having 0.00 mag in any band", but here is my doubt:
> > in Johonson-Cousins UBVRI system, the Vega magnitude is 0.03 in V and
> > all other colors are equal to 0, that means U=B=V=R=I=0.03
>
> Are you sure about that last? The Johnson-Cousins system is based on
> colors being zero for "an average unreddened A0 star," but that
> doesn't mean Vega itself. I agree about V=0.03, but I thought not
> all the colors were zero. Anyway, the important point is that this
> is an entirely separate magnitude system.
Yes, now everything looks much more clear to me. The Johnson-Cousins is
defined to have V(Vega)=0.03, B(Vega)=0.02, R(Vega)=0.039 and
I(Vega)=0.035 -at least using the values from Bessell 1998-. Still,
observational error for Vega are in the orther of one hundreds of a
magn, so much more precision than this is not achievable
Still, thanks so much for the suggestions!
Steve
> > 2MASS is tied to the photometric network of Cohen et al. (2003 AJ
> > 117, 1864 -- see also 1993 AJ 104, 1650), which is supposedly based
> > on Vega=0. However, based on the Elias et al. (1982 AJ 87, 1029)
> > photometry, J(Vega)=-0.04 in the 2MASS system, H=+0.01, and K=-0.02.
> > See
> > http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec6_4b.html
> > and in particular the transformation to the CIT system, which is
> > based on Vega=0 at J, H, and K.
> So, if I have correctly understood, the 2MASS system is defined in
> accordance with the extensive work of Cohen, where J=H=K=0 for Vega.
That's what I thought before, but further research (and some
consultation with Martin Cohen) shows I was wrong. See Cohen et al.
2003, AJ, 126, 109. Near the end, you will find that if Vega could
have been measured by 2MASS, its JHK magnitudes would have been -0.001,
+0.019, -0.017. I think the original intention was to have Vega=0, but
the final result didn't quite come out that way.
> Nevertheless, if you compare common stars between 2MASS and CIT system
> (that is also based on a definition J=H=K=0 for Vega) you find out a
> small offset.
> So if you belive in the CIT system, the 2MASS Vega magnitudes come out
> to be J=-0.04, H=+0.01 and K=-0.02.
These are based on the transformation curves and may reflect problems
with the CIT system J. (H and K agree very well with the zero point
offset in the Cohen et al. paper.) I am surprised the error is as big
as 0.04, but it may reflect small differences in exact filter curves
and in atmospheric water vapor at the time measurements were made.
Also, the CIT J filter differed from nearly everybody else's. The most
accurate measurements were made at CTIO (with a different filter) and
transformed to the CIT system.
All in all, I think we understand H and K pretty well, but I don't know
what to make of J at the 0.04 level. For modern use, I think the best
one can do is use the 2MASS system with the Cohen et al. zero points.
> Then you select a common set of stars to the two systems and you find
> small offset. Such offset however is at the level of a few hundreds of
> mag, so likely to be a noisy scatter due to observational error. Is it
> a reasonable guess?
No, I don't think it's observational scatter. H and K just reflect how
the 2MASS magnitudes were set up, but I don't understand J.
> However, I have another question. In the 2mass paper of Cohen, he
> states "in the IR we adopt the synthetic Vega spectrum as a definition
> of zero magnitude". The synthetic spectra is that of Kurucz that is
> know to work well (actually, how much well???).
Probably OK at the 10% level, but you should check what Martin and his
collaborators have written. They have tied together large amounts of
data on A and K stars and absolute measurements both from the ground
and from satellites (mainly MSX). Nevertheless, it's a hard problem.
> Anyway, there is lot of rumor about a possible IR variabilty of Vega
> due to dusty circumstellar envelope (there was a paper on that couple
> of month ago).
Didn't see that, but I'd be astonished if there's much effect short of
20 microns wavelength. (Be warned: I've been wrong before!)
> Cohen in his paper (1992 AJ 104, 1650) reccomended the use of Sirius as
> a standard in the IR, but in his 2003 work he doesn't mention Sirius
> anymore.
> Is that because the 2mass system is defined on the Kurucz spectra for
> Vega where they impose J=H=K=0 end of the story.
> Then the real Vega out there can show variability or whatsoever, but
> this does't affect at all the 2MASS system. Is it correct??
Any photometric system is defined by some ensemble of the stars in it.
For 2MASS, there were a few fields observed over and over again. Those
in effect define the instrumental magnitudes for the survey. As noted,
Vega comes out close to but not exactly zero in this system.
The next step is converting instrument magnitudes to flux density in
physical units. For that purpose, the Kurucz Vega spectrum is the best
bet, but uncertainties are probably around 10%.