Radial and expansion velocities of compact planetary nebulae

Abstract

Optical spectra centered near 5000 A have been obtained of the compact planetary nebulae Ml-6, IC 2149, Ml-78, NGC 7027, and CN3-1. The expansion velocities of the ionized outflow and the systemic velocity of each nebula have been determined from the [O III] lines at 5007 A and 4958 Â.

M1–78: a nitrogen-rich Galactic compact H II region beyond the Perseus arm

“…We performed a single Gaussian fit of the profiles of the brightest lines of the blue range of the spectra (Hβ and [O iii] 4959 and 5007 Å) for determining the radial velocity of the nebula with respect to the local standard of rest (LSR), finding a value of −74 ± 1 and −70 ± 2 km s −1 for the SW arc and the NE blob, respectively. These values are in good agreement with the v LSR = −76 ± 2 km s −1 found by Gussie & Taylor (1989) also from optical spectroscopic observations and with measurements of hydrogen radio recombination lines obtained by Terzian et al (1974) and Churchwell et al (1976 Grandi (1975Grandi ( , 1976, all these lines come from upper levels of their respective ions that can be excited from the ground state by the absorption of stellar photons longward of 912 Å. Also, it is important to note that all these lines are precisely the brightest permitted ones of O i, N i and Si ii in the Orion Nebula (see Esteban et al 2004), although their line ratios with respect to Hβ are larger in M1−78 than in the Orion Nebula.…”

“…Another remarkable feature of M1−78 is the expansion velocity of the ionized gas of about 25 km s −1 measured by Gussie & Taylor (1989). Although this fact was considered by Gussie (1995) as an indication of the PN nature of the object, such supersonic expansion velocities are also common in WR ring nebulae (e.g.…”

M1-78: a nitrogen-rich Galactic compact HII region beyond the Perseus arm

“…We performed a single Gaussian fit of the profiles of the brightest lines of the blue range of the spectra (Hβ and [O iii] 4959 and 5007 Å) for determining the radial velocity of the nebula with respect to the local standard of rest (LSR), finding a value of −74±1 and −70±2 km s −1 for the SW arc and the NE blob, respectively. These values are in good agreement with the v LSR = −76±2 km s −1 found by Gussie & Taylor (1989) also from optical spectroscopic observations and with measurements of hydrogen radio recombination lines obtained by Terzian et al (1974) and Churchwell et al (1976).…”

The Classification of M 1-78

“…Planetary nebulae as a group have somewhat higher measured ionized-gas expansion velocities, averaging v exp ≈ 20 km s -1 (Weinberger 1989), and as such typically expand supersonically, even those with a regular shell-type morphologies. The measured expansion velocity of M 1-78 is v exp = (25.3 ± 2.0) km s -1 (Gussie and Taylor 1989). This supersonic expansion velocity favours classification as a PN since the morphology of the nebula is not cometary, and displays no other manifestations of a bow shock created by a supersonic central star.…”

“…Therefore, if M 1-78 were shown to have a distance significantly different from its kinematic value, identification as a PN would be indicated. The systemic velocity of the ionized nebula of M 1-78 (Gussie and Taylor 1989) is found to be v LSR = (-76 ± 2) km s -1 as measured by optical spectroscopic observations of the [OIII] lines.…”