SCHWARZ, Hugo E.; MONTEIRO, Hektor. 3D photoionization structure and distances of Planetary Nebulae III. NGC 6781. Exacta, [S. l.], v. 4, n. 2, p. 281–288, 2008. DOI: 10.5585/exacta.v4i2.756. Disponível em: https://periodicos.uninove.br/exacta/article/view/756. Acesso em: 17 jul. 2024.