[vsnet-id-2mass 136] re 2MASS data format ... also broad to narrow passband transformations

[no name <crawl@zoom.co.uk>]

Taichi Kato wrote:

> 2MASS data format
> 
>    I have received the following query:
> 
> >     I have been reading with interest the mails concerning 2mass
> > identifications. I will grateful if you could kindly clarify as to what
> > are the three numbers given after 2mass co-ordinates for epoch 2000. I
> > realise that they are most likely related to fluxes in the three bands JHK
> > and I would like to the units if they are fluxes or whether they are
> > directly the JHK magnitudes themselves.
> > With kind regards
> 
>     The values represent J, H, K magnitudes (Jm, Hm, Km in the 2MASS
> description).
> 
> Regards,
> Taichi Kato
> 

The K passband is actually Ks which means "K short" : basically the
Johnson-Cousins K passband covers areas at the long wavelength end where
atmospheric water vapour [and possibly others for all I know] can cause
problems unless the observing site & night are of perfect photometric
quality.

So it seems that both 2MASS and DENIS use Ks instead for more efficient
surveys, which is the same as K but not as wide a passband, ie not going
to those longer wavelengths.

It would be useful to know if there are significant IR molecular bands
in stars at the long end of the full K passband than can mean that the
flux/magnitude in Ks is not directly equatable to that in K (there is a
reasonable amount of literature on conclusions re LPVs via J-K versus
K-L colours for instance, etc).

Try the following for a quick abstract mentioning the passband ranges...
...not the same as a passband profile though [an all one line URL, the
email editor has unfortunately wrapped it!]:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995AAS...187.7507S&db_key=AST&high=390983001627709


When the SDSS gets fully underway, and later still data returns from the
USNO FAME mission to be launched in 2004 [????], we'll all have to
figure out how to use [modified] gunn u'g'r'i' photometry to recover
normal V and I etc., and given that the broader the passband [and
u'g'r'i' will be very broad!], the more the chance of some
absorption/emission line particular to a specific class of stars will
artificially lower/heighten the flux compared to other types of star, I
can't see how very broad bands can be safely transformed to narrow ones.
Even if you stick to one sort of star, we have for example [and
topically] B and Be stars which may or may not be showing varying levels
of Halpha emission compared to each other.

The general professional scientists will be okay, because they'll have
an internally consistent data set within which their work will
concentrate, but the variable star people [professional and amateur]
will perforce need to relate these new passbands to historical
observations from the UBVRI school of photometries [and possibly even
v!]

Cheers

John

JG, UK

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