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    Distance of MisV1181    

October 12, 2003
Yoshida, S.; Greaves, J.

We estimated the rough distance to MisV1181 (V709 Cep), a possible nova discovered by the MISAO Project from unfiltered CCD images taken in 2001.

The absolute magnitude of a nova can be estimated as:

M = -7.92 - 0.81 * arctan ( ( 1.32 - log(t2) ) / 0.23 )

where t2 indicates the days the nova fades two mag (della Valle and Livio 1995).

MisV1181 was 12.7 mag on Sept. 19, 2001, and 14.3 mag on Nov. 10, 2001. It faded 1.6 mag during 52 days, which suggests t2 = 65 days. Then, the absolute magnitude of MisV1181 is derived as -7 mag.

The distance modulus, apparent minus absolute magnitudes, is given as:

m - M = 5 * log(D) - 5 + A

where m is the apparent magnitude, M is the absolute magnitude, D is the distance in parsec, and A is the absorption.

Assuming the absorption is normal, the absorption A is roughly equal to the distance in kiloparsec. For instance, A is 5 mag for distance of 5 kpc.

Assuming the maximum magnitude of MisV1181 is 12.5 mag, and the absolute magnitude is -7 mag, the left side value (m - M) is 19.5. If it was 5 kpc distant, the right side value becomes 18.5. If it was 6 kpc distant, the right side value becomes 19.9. Therefore, the distance to MisV1181 is derived between 5 and 6 kpc.

The object lies towards the Cepheus Spiral Arm. From the Cepheus OB1 and OB2 associations which lie in the Cepheus Spiral Arm, this Spiral Arm is about 3.5 kpc distant.

The derived distance implies that MisV1181 is beyond the Cepheus Spiral Arm.

If it was inside the Cepheus Spiral Arm, i.e. 3.5 kpc distant, the right side value would be 16.2, so the absolute magnitude would be about -3.5 or -4 mag. It is too faint for a common nova.

In conclusion, MisV1181 is about 5 or 6 kpc distant, just inside the Galaxy disk outer edge in that direction. Although it lies towards the Cepheus Spiral Arm, MisV1181 is not inside the arm.

The exact distance is highly sensitive to the number of assumptions that have to be followed on these matters, especially as some of the fundamental parameters are not necessarily well known.

della Valle, M., Livio, M., 1995, ApJ, 452, 704

MisV1181: Our First Nova Discovery
Yoshida, S.

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