COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2003

                                 Konkoly Observatory
                                 Budapest
                                 1981 August 19

                                 HU ISSN 0374-0676




        ON THE LOCATION OF NOVA CORONAE AUSTRINAE 1981




   In a recent communication Caldwell (1981) proposed, on the basis of
U, B, V, R, I photometry of the outburst, that the distance to this nova
ought to be of the order of 8 kpc., putting the object on the near side
of the galactic bulge. This note points out that this outburst could well
have happened on the far side of the bulge with respect to the Sun.
   Two additional pieces of evidence should be considered when trying
to understand the location of the present outburst. Firstly, stars from
Klare and Neckel (1977) within 5d from the nova position, have A_v~= 0.3 while
being at an average distance to 500 pc. Secondly, at the accurate position
given by Gilmore (1981) there is no image brighter that m_r~=19 on the red
extension of the Palomar Sky Survey.
   Brosch (1981) has shown, from spectrophotometry of the Balmer lines
in Nova CrA 1981 one week after its discovery, that the extinction towards
the object is A_v = 1.7 mag. The line of sight towards the nova leaves
the extinction layer of dust in the galactic plane rather quickly, the
nova being at b~=-14.4d. With this angle a distance of 500 pc. paces out
just about one scale height of extinction (Allen, 1973). Therefore most
of the extinction towards the nova (1.4 mag. at V) should be produced by
material in its "immediate" vicinity. Should the nova be located on the near
side of the bulge, the way to produce this effect is by having dust grains
near the pre-nova system. This would however have become evident in the
measurements carried out by Vrba and Rydgren (1981), as an infra-red signature
of the outburst. The second obvious possibility is that the nova was located
beyond the galactic bulge, and then material in the bulge would be responsible 
for the additional extinction.
   A simple exercise is characterizing the outburst points towards a
remarkable similarity to Nova Cygni 1975 (which has been remarked also by
Busko et al., 1981), and to CP Pup. The points in common are,among others
the velocity of ejection (2.2. 10^3 km/s, from the width of the Balmer lines),
the smooth and fast decline (see Figure 1 and compare with Duerbeck, 1981)
and the lower limit of outburst range (m_prenova - m_max>~ 12). The absolute
magnitude for both these objects at maximum was brighter than -10. A
bright maximum magnitude is obtained also when comparing with data in
Duerbeck (1981). The light curve of the present outburst is clearly of his type
A, implying that the nova belongs to his group I. The objects in this group
have an absolute magnitude at maximum of M_v = -9.4 +- 1.0, implying a distance
of 9 +- 5 kpc.

                              [FIGURE 1]

  Adopting an absolute magnitude of -10. at maximum for Nova CrA 1981
produces the following distance estimate:

       D = dex((m-M)/5 - (A_v / 5) + 1) = 12 kpc

This puts the object in the galactic bulge, on the far side from the Sun
and makes it simpler to obtain the required amount of extinction, in this
case from bulge material.
   Thus a location behind the galactic bulge is not inconceivable and at
least as easy to explain as the nova being in front of the bulge. The final
settlement of this question shall however rest until a nebular parallax to
the nova will be measured.





                 N. BROSCH
                 Laboratory Astrophysics
                 Huygens Laboratorium
                 Postbus 9504
                 2300 RA Leiden
                 The Netherlands




References:

Allen, C.W. 1973, Astrophysical Quantities (University of London; The
       Athlone Press). [BIBCODE 1973asqu.book.....A ]
Brosch, N. 1981, I.A.U. Circular No. 3601. [BIBCODE 1981IAUC.3601....2B ]
Busko, I., Jablonski, F., Quast, G. and Torres, C.A.O. 1981, I.A.U. Circular
          No. 3591. [BIBCODE 1981IAUC.3591....1G ]
Caldwell, J.A.R. 1981, I.B.V.S. No. 1981.
Duerbeck, H.W. 1981, Pub.Astron.Soc.Pacific 175, 165. [BIBCODE 1981PASP...93..165D ]
Gilmore, A.C. 1981, I.A.U. Circular No. 3591. [BIBCODE 1981IAUC.3591....1G ]
Klare, G. and Neckel, Th. 1977, Astron.Astrophys.Suppl. 27, 215. [BIBCODE 1977A&AS...27..215K ]
Vrba, F.J. and Rydgren, A.E. 1981, I.A.U. Circular No. 3604. [BIBCODE 1981IAUC.3604....1V ]