COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2147 Konkoly Observatory Budapest 1982 May 17 HU ISSN 0374-0676 REDISCUSSION OF NOVA CrA 1981 DISTANCE Caldwell (1981) estimated the distance to Nova CrA 1981 (l = 358d, b= -14.4d) but several developments have since indicated that a re-evaluation of its probable distance is called for. These are, firstly, that the author misconstrued how to apply the nova t_2 calibration to begin with, secondly, that Honda has revised his discovery m_V by half a magnitude (Kozai, private communication), thirdly, that Duerbeck (1981) has recalibrated the nova (M_V : Log t_3,V) relation, and fourthly, that Brosch (1981a-c) has also estimated the nova distance, using probably too large an intrinsic maximum brightness. [FIGURE 1] Figure 1. Lightcurve of Nova CrA 1981 Figure 1. gives the V (and partial B) light curve, with revised m_V,MAX of 6.5, from the sources Gilmore (1981), Kozai and Kosai (1981, revised in private comm.), Cragg (1981, confirmed in private comm.), Mattei (1981), Vrba and Rydgren (1981), and Caldwell. The error bars shown are +- 0.2 for visual and +- 0.1 for photoelectric photometry; the latter arises principally from the system transformation uncertainty, while the former is intended to cover the scale uncertainty but not the transformation error, which may well be larger. Brosch estimated the local Galactic extinction in the direction toward the nova as A_V (at 0.5 +- 0.12 kpc) = 0.32 +- 0.13 from Klare and Neckel's (1977) work. It may be noted that this extinction is slightly lower than that implied by the global absorption model used earlier by Caldwell, namely A_V (at 0.5 kpc) = 0.44. Rescaling that model to fit Brosch's information then yields A_V (at nova)= 0.45 +- 0.16 for the full path length through the disk towards the nova. In the most recent treatments of the nova absolute magnitude calibration, three methods appear, namely the Duerbeck (M_V : log t_3,v) plot, the Pfau (1976) (M_B : log t_3,B) regression, and the Pfau M_15,B standard magnitude. In Duerbeck's plot, Nova CrA with log t_3,V = 1.08 +- 0.08 would fall securely into the Group I (very fast or fast novae), which is characterized by log t_3,V = 1.08 +- 0.28 (disp.) From his tables one further obtains that M_V (Group I) = -9.2 +- 0.5 (disp.), excluding the two most extreme "outriders," or -9.2 +- 1.0 including them. The result follows that m_V - M_V - A_V = 15.25 +- 0.55 (from M_V : log t_3,V). Pfau's work leads two two rather smaller modulus estimates, but not significantly so given the errors. His (M_B : log t_3,B) regression, combined with Duerbeck's B-V_MAX = 0.35 +- 0.15 and our derived log t_3,B= 1.36 +- 0.07, implies m_B - M_B - A_B = 14.5 +- 0.55 (from M_B : log t_3,B). His M_15,B figure of -5.74 +- 0.60, combined with our derived m_15,B = 9.3 +- 0.1, implies m_B - M_B - A_B = 14.45 +- 0.65 (from M_15,B). Combining the three results and using the dispersion as a conservative estimate of the uncertainty, one obtains m - M - A = 14.8 +- 0.5, along with the further results for Nova CrA 1981 given in Table I. TABLE I : PARAMETERS OF NOVA CrA 1981 t_0 JD2444697.25 t_2,V 5.8 +- 0.5 d t_3,V 12 +- 2d m_V,MAX 6.5 +- 0.2 M_V,MAX -8.75 +- 0.5 A_V 0.45 +- 0.16 r 9 +- 2 kpc The almost exact agreement between this distance, 9+-2 kpc, and the Brosch result of 9.1 kpc (when m_V,MAX is updated to 6.5), is deceptive because he has adopted estimates for the nova intrinsic brightness and for the total interstellar extinction which we think are large overestimates that happen to cancel. Brosch adopted a total A_V (at nova) of 1.7 magnitudes, as inferred from the H_alpha:H_beta Balmer decrement, and attributed the extra 1.4 magnitudes (beyond the .3 in the local disk) to absorption by interstellar matter within the central bulge of the Galaxy (z~ -2.3 kpc). High absorption arising in circumstellar dust was not favored (Kunkel and Rydgren). One may object to Brosch's high extinction figure, firstly, because the Halpha:Hbeta decrement may well not yield a valid A_V for reasons discussed in Walker et al. (1979), Feast and Glass (1980), and Grinin (1980), and, secondly, because there is reason to believe the spheroidal population II to be nearly free from obscuring material. On the other hand Brosch adopted an absolute magnitude for the nova at maximum of M_V = -10, on the basis of the spectral similarity of Nova CrA to Nova Cygni 1975; Duerbeck had obtained M_V = -10.1 for the latter. Nova Cygni 1975, by all accounts the most extreme example of its class (fast novae) yet observed in the Galaxy, had t_2,V and t_3,V of 2.5 and 3.6 +- 0.1 d, respectively. This is to compared to t_2,V = 5.8 +- 0.5 and t_3 V = 12 +- 2 d for Nova CrA (Figure 1.), which suggests that Nova CrA is more plausibly an "average" Group I. nova or about 1 fainter than Nova Cygni 1975 at maximum. In summary, the similarity between our estimate and Brosch's, both placing Nova CrA in the central bulge of the Galaxy, comes down to the fortuitous circumstance that (M_V + A_V)_Cald=( -8.75 + .45)=(-10 + 1.7) = (M_V +A_V)_Brosch. We hope it has been useful to rediscuss the determination of a distance for Nova CrA 1981, firstly, to point out that it need not have been exceptionally luminous for its decay time, and, secondly, to give some indication of the uncertainties that can arise in a specific practical case of finding a nova distance. I wish to thank Noah Brosch for a preprint, Patricia Whitelock and A W J Cousins for useful discussions, and anonymous referees for helpful criticisms. JOHN A. R. CALDWELL South African Astronomical Observatory P O Box 9, Observatory 7935, R S A References: Brosch, N. 1981, I.A.U. Circ. 3601. [BIBCODE 1981IAUC.3601....2B ] Brosch, N. 1981, I.B.V.S. 2003. Brosch, N. 1981, "Spectrophotometry of Nova Coronae Austrinae 1981," Astron. Astrophys. in press. [BIBCODE 1982A&A...107..300B ] Caldwell, J.A.R. 1981, I.B.V.S. 1981. Cragg, T. 1981, I.A.U. Circ. 3590. [BIBCODE 1981IAUC.3590....1K ] Duerbeck, H.W. 1981, P.A.S.P. 93, 165. [BIBCODE 1981PASP...93..165D ] Feast, M.W. and Glass, I.S. 1980, Obs. 100, 208. [BIBCODE 1980Obs...100..208F ] Gilmore, A.C. 1981, I.A.U. Circ. 3591. [BIBCODE 1981IAUC.3591....1G ] Grinin, V.P. 1980, Bull. Crimean Ap. Obs. 62, 42. [BIBCODE 1980BCrAO..62...42G ] Kozai, Y. and Kosai, H. 1981, I.A.U. Circ. 3590. [BIBCODE 1981IAUC.3590....1K ] Mattei, J. 1981, I.A.U. Circ. 3594. [BIBCODE 1981IAUC.3594....1M ] Pfau, W. 1976, Astron. Astrophys. 50, 113. [BIBCODE 1976A&A....50..113P ] Vrba, F.J. and Rydgren, A.E. 1981, I.A.U. Circ. 3604. [BIBCODE 1981IAUC.3604....1V ] Walker, G.A.H., Speers, G.E., Pritchet, C., and Glaspey, J.W. 1979, P.A.S.P. 91, 446. [BIBCODE 1979PASP...91..446W ]