COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1711

                                 Konkoly Observatory
                                 Budapest
                                 1979 December 3


  INTERSTELLAR LINES IN THE SPECTRUM OF NOVA DN GEMINORUM (1912)


Distance determinations of novae are fairly scarce and often inaccurate. A
rediscussion of existing material is especially rewarding for those old novae,
whose spectroscopic and photometric history is thoroughly investigated, whose
distances, however, could not be obtained because of lack of observable
expanding shells. Since the nature of the "stationary calcium lines" was not
yet clearly established, their distances were not (or only later) estimated
from the strength of the interstellar lines.

The intensity of the interstellar lines in nova DN Gem was obviously never
quantitatively studied, as noted by McLaughlin (1965), while extensive
discussions of the spectroscopic and photometric evolution were published by
Stratton (1920) and McLaughlin (1965).

We have used tracings of the spectra Nos. 1865, 1867, 1870, and 1873 obtained
by F. Kustner with the Bonn 0.30 m refractor and the Toepfer 3-prism
spectrograph in March and April, 1912. The photometric calibration was
established by measuring the strength of different iron arc comparison lines
belonging to the same multiplet, as proposed by Hogg (1929). While the Ca II K
line was always weakly exposed, the Ca II H line was clearly seen and measured
against the broad, diffuse H_epsilon + Ca II H emission. Since the distance
relations are always given for the Ca II K line strength only, a conversion
factor 1.78 (Binnendijk 1952) was applied:

W_K=1.78 W_H=0.165 A
            +-.022 m.e.

Using distance relations by Allen (1973), Beals and Oke (1953), and Binnendijk
(1952), a distance

d=450+-70 pc

is obtained. Taking into account the inaccuracies of the distance and the
plate calibrations, the error is probably twice as high.

The method of the residual radial velocity of the interstellar lines,
interpreted as being due to differential galactic rotation, and successfully
applied by McLaughlin (1941) to three novae, yields in the case of DN Gem (r.v.
helioc. of Ca II=+4.8 km/s; McLaughlin, 1965):

v_r=-14.7 1 km/s

This value indicates a distance of the order of 10 kpc, which cannot be
reconciled with the observed strength of the interstellar lines, and the known
absolute magnitude range of galactic novae - a fact which obviously prevented
McLaughlin from applying his 1941 method in his 1965 study on DN Gem. The
observed residual radial velocity must be interpreted as being due to the
peculiar motion of interstellar Ca II in the direction of DN Gem.

Thus we will rely only on the distance determination by the inferred strength
of the Ca II K line. Interstellar absorption may be estimated by using the
studies of Neckel (1967), Gottlieb and Upson (1969), and Deutschman et al.
(1976). For a distance of 450 pc,

A_V=0.27+-0.13

DN Gem reached m_pg=3.6 at maximum light. Assuming C_0=0.25 around maximum
light, an absolute visual magnitude

M_V=-5.3+-0.5

is derived. While it is possible that the absolute magnitude, due to the many
uncertainties entering the calculations, may be as high as -6.5, the estimates
of -7.2 ... -7.6 given by McLaughlin are certainly too high.


H.W. DUERBECK
Observatorium Hoher List
der Universitats-Sternwarte Bonn
D-5568 Daun, F.R.G.

B. LEMKE and E. WILLERDING
Astronomisches Institut der Universitat Munster
Steinfurter Str. 107
D-4400 Munster, F.R.G.

References :

Allen, C.W.; 1973, Astrophysical Quantities (3rd ed.), London: Athlone Press
Beals, C.S., Oke, J.B., 1953, Mon. Not. R. astr. Soc. 113, 530 [BIBCODE 1953MNRAS.113..530B ]
Binnendijk, L., 1952, Astrophys. J. 115, 428 [BIBCODE 1952ApJ...115..428B ]
Deutschman, W.A., Davis, R.J., Schild, R.E., 1076, Astrophys. J. Suppl. 30, 97 [BIBCODE 1976ApJS...30...97D ]
Gottlieb, D.M., Upson, W.L., 1969, Astrophys. J. 157, 611 [BIBCODE 1969ApJ...157..611G ]
Hogg, F.S., 1929, Harvard College Obs. Circ. 337
McLaughlin, D.B., 1941, Astrophys. J. 93, 417 [BIBCODE 1941ApJ....93..417M ]
McLaughlin, D.B., 1965, Publ. Obs. Univ. Michigan 9 (5), 113 [BIBCODE 1965POMic...9..113M ]
Neckel, Th., 1967, Veroff. Landessternwarte Heidelberg-Konigstuhl 19 [BIBCODE 1967VeHei..19.....N ]
Stratton, F.J.M., 1920, Ann. Solar Phys. Obs. Cambridge 4 (1)