The early Palomar program (1950-1955) for the discovery of classical novaein M81: Analysis of the spatial distribution, magnitude distribution, and distance suggestion

Data obtained in the 1950-1955 Palomar campaign for the discovery of classi cal novae in M81 are set out in detail. Positions and apparent B magnitudes are listed for the 23 novae that were found. There is modest evidence that the spatial distribution of the novae does not track the B brightness dist ribution of either the total light or the light beyond an isophotal radius that is 70 " from the center of M81. The nova distribution is more extended than the aforementioned light, with a significant fraction of the sample a ppearing in the outer disk/spiral arm region. We suggest that many (perhaps a majority) of the M81 novae that are observed at any given epoch (compare d with, say, 10(10) years ago) are daughters of Population I interacting bi naries. The conclusion that the present-day novae are drawn from two popula tion groups-one from low-mass white dwarf secondaries of close binaries ide ntified with the bulge/thick disk population, and the other from massive wh ite dwarf secondaries identified with the outer thin disk/spiral arm popula tion-is discussed. We conclude that the M81 data are consistent with the tw o population division as argued previously from (1) observational studies o n other grounds of nearby galaxies, (2) Monte Carlo simulations of novae in M31 and in the Galaxy, and (3) population synthesis modeling of nova binar ies. Two different methods of using M81 novae as distance indicators give a nova distance modulus for M81 as (m - M)(0) = 27.75, consistent with the C epheid modulus that is the same value.