extinction. Extinction must be taken into account for distances greater than about 100
pc.
[37]
Over much of the spectrum, A
(the number of magnitudes of extinction at wavelength
) falls
sharply with increasing wavelength.
[38]
In the visual band, we have, approximately,
(6)
A
V
=
R $ E
(
B
-
V
)
where E(B-V) is the observed color excess and R is a proportionality constant (the ratio of total
to selective absorption), R ~ 3.1 ± 0.2.
[39]
There is some disagreement over the exact form of
eq.
(6)
[40]
(that is, how to account for the variation of R with spectral type).
Burstein and Heiles
[41]
provide contour maps of E(B-V). Apparently, the most accurate form of the extinction relation is
one due to Fisher and Tully,
[42]
(7)
A
B
=
0
-
0.149
+
6.41 $ N
H
N
H
[ 232
N
H
>
232
N
H
=
323 csc b
323 csc b
+
[105
-
3.8 b
-
89 cos(k
-
140)]
b
<
0
b
>
0
A
B
=
(R
+
1) E
(
B
-
V
)
and
R
=
A
V
E
(
B
-
V
) =
3.1
+
0.3 (B
-
V)
1
-
0.02 A
B
where b is the galactic latitude,
is the galactic longitude, and N
H
is the neutral hydrogen column
density (in units of 10
18
atoms cm
-2
). There is some dependence of R on spectral type, due to the
finite width of the spectral bands. This is (at least partially) taken into account in
eqs. (7).
POINTS could measure trigonometric distances directly to massive stars such as
OB stars, Wolf-
Rayet stars, and Cepheids, sidestepping the uncertainties in the amount of reddening and in the
relation between absolute magnitude and spectral type. In addition, we could assess the extinc
-
tion relation eq. (6) through different paths through the ISM.
d:\points\proposal\sz1\scifull.lwp
POINTS Science
16:29 Wednesday, January 03, 1996
page 11 of 20
[42]
Fisher, J.R., and Tully, R.B., 1981, Astrophys J Supp 47, 139, "Neutral Hydrogen Observations of a Large Sample
of Galaxies"; see also Rowan-Robinson (1985)
[41]
Burstein, D., and Heiles, C., 1982, Astron J 87, 1165, "Reddening Derived from HII and Galaxy Counts: Accu
-
racy and Maps"
[40]
Blanco, V.M., 1956, Astrophys J 123, 64, "Some Remarks on the UBV System";
Lee, T.A., 1970, Astrophys J 162, 217, "Photometry of High-Luminosity M-Type Stars";
Humphreys, R.M., 1979, Astrophys J Supp 39, 389, "Studies of Luminous Stars in Nearby Galaxies. II. M
Super-
giants in the Large Magellanic Cloud";
Morgan, D.H., and Nandy, K., 1982, MNRAS 199, 979, "Infrared Interstellar Extinction in the LMC"
[39]
Mihalas, D., and Binney, J., 1981, Galactic Astronomy, W.H. Freeman, San Francisco
[38]
see, e.g., Savage, B.D., and Mathis, J.S., 1979, Ann Rev Astron Astrophys 17, 73, "Observed Properties of Inter-
stellar Dust"
[37]
Rowan-Robinson 1985
