Difference between 'Optical Spectral Type' and 'IR Spectral Type'?

In the list of L and T dwarfs at 'dwarfarchives.org' (
http://ldwarf.ipac.caltech.edu/archi...rf&format=html
) there are two columns named 'spectral_type_opt' and
'spectral_type_ir'. I guess this comes from the type of instrument
used in the measurements? Am I correct?

To my knowledge, the Spectral Type indicates the surface temperature
of stars. If so why would the optical and infrared spectra give two
different measurements? Which one is more accurate? Since the visual
spectra of L and T dwarfs are very faint, would the 'infrared spectral
type' be more accurate?

Thanks

On Sun, 27 Feb 2011 17:15:26 -0800 (PST), eric948470
wrote:

In the list of L and T dwarfs at 'dwarfarchives.org' (
http://ldwarf.ipac.caltech.edu/archi...rf&format=html
) there are two columns named 'spectral_type_opt' and
'spectral_type_ir'. I guess this comes from the type of instrument
used in the measurements? Am I correct?

To my knowledge, the Spectral Type indicates the surface temperature
of stars. If so why would the optical and infrared spectra give two
different measurements? Which one is more accurate? Since the visual
spectra of L and T dwarfs are very faint, would the 'infrared spectral
type' be more accurate?

Thanks

It isn't a new system of stellar classification. L, T and Y are just
additional stellar classes in addition to O, B, A, F, G, K, M, etc.
The L, T and Y objects do have most of their radiation in the
infrared.

Bud

On Feb 27, 9:19*pm, wrote:
On Sun, 27 Feb 2011 17:15:26 -0800 (PST), eric948470

wrote:
In the list of L and T dwarfs at 'dwarfarchives.org' (
http://ldwarf.ipac.caltech.edu/archi...t.php?table=lt....
) there are two columns named 'spectral_type_opt' and
'spectral_type_ir'. I guess this comes from the type of instrument
used in the measurements? Am I correct?

To my knowledge, the Spectral Type indicates the surface temperature
of stars. If so why would the optical and infrared spectra give two
different measurements? Which one is more accurate? Since the visual
spectra of L and T dwarfs are very faint, would the 'infrared spectral
type' be more accurate?

Thanks

It isn't a new system of stellar classification. *L, T and Y are just
additional stellar classes in addition to O, B, A, F, G, K, M, etc.
The L, T and Y objects do have most of their radiation in the
infrared.

Bud

Sorry about not making my question more clear. What I have trouble
understanding is that that the same star can have two different values
in "spectral_type_ir" and "spectral_type_opt". For example, the star
"GJ 1001B, LHS 102B" (4th from top) has a "spectral_type_opt" of L5
and a "spectral_type_ir" of L4.5. How can it be both at the same time?
And which one is more likely to be accurate?

eric948470 wrote:
On Feb 27, 9:19 pm, wrote:
On Sun, 27 Feb 2011 17:15:26 -0800 (PST), eric948470

wrote:
In the list of L and T dwarfs at 'dwarfarchives.org' (
http://ldwarf.ipac.caltech.edu/archi...t.php?table=lt...
) there are two columns named 'spectral_type_opt' and
'spectral_type_ir'. I guess this comes from the type of instrument
used in the measurements? Am I correct?

To my knowledge, the Spectral Type indicates the surface temperature
of stars. If so why would the optical and infrared spectra give two
different measurements? Which one is more accurate? Since the visual
spectra of L and T dwarfs are very faint, would the 'infrared
spectral type' be more accurate?

Thanks

It isn't a new system of stellar classification. L, T and Y are just
additional stellar classes in addition to O, B, A, F, G, K, M, etc.
The L, T and Y objects do have most of their radiation in the
infrared.

Bud

Sorry about not making my question more clear. What I have trouble
understanding is that that the same star can have two different values
in "spectral_type_ir" and "spectral_type_opt". For example, the star
"GJ 1001B, LHS 102B" (4th from top) has a "spectral_type_opt" of L5
and a "spectral_type_ir" of L4.5. How can it be both at the same time?
And which one is more likely to be accurate?

These actually sound pretty similar. A difference of 0.5 subclasses is not
remarkable, especially for two different wavelength regions. Probably the
IR type is more accurate, as most of the light is emitted there. You need
to remember that the spectral classes are "boxes" containing lots of similar
stars, with some up against the higher T side (L4) box boundary and others
the low side (e.g., box L6). Or there may be specific boxes for L4.5 and
L5.5. There would be very little difference between L5 near the boundary
and L4.5 near the boundary.

There is a similar discrepancy for hot stars observed optically and in the
ultraviolet.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

In article ,
eric948470 writes:
To my knowledge, the Spectral Type indicates the surface temperature
of stars.

Strictly speaking, it indicates the overall appearance of the
spectrum, but that appearance is mostly (though not entirely)
governed by temperature.

If so why would the optical and infrared spectra give two
different measurements?

I was hoping to look into this a bit but haven't had time. The most
likely reason is that the respective classification systems are based
on different (but overlapping) groups of stars. The stars probably
differ in surface gravity and metallicity, and these may not be
adequately taken into account when assigning spectral types. Or it
may be something as simple as low signal to noise in some of the
spectra used for establishing the classification systems. The L and
T types are relatively new, and no doubt the classification systems
will improve with time.

Which one is more accurate? Since the visual
spectra of L and T dwarfs are very faint, would the 'infrared spectral
type' be more accurate?

That last is probably the way to bet, but I don't think one could say
for sure without studying the spectra of the specific star in
question and of the respective classification stars. A difference of
one or two spectral subtypes is not surprising, given the various
uncertainties.

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