Searching for nascent planetary nebulae: OHPNe candidates in the SPLASH survey

Abstract

The evolution of asymptotic giant branch stars from the spherical symmetry into the diverse shapes of planetary nebulae (PNe) is a topic of intensive research. Young PNe provide a unique opportunity to characterize the onset of this transitional phase. In particular, OH maser-emitting PNe (OHPNe) are considered nascent PNe. In fact, only 6 OHPNe have been confirmed to date. In order to identify and characterize more OHPNe, we processed the unpublished continuum data of the interferometric follow-up of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). We then matched the interferometric positions of OH maser and radio continuum emission, considering the latter as a possible tracer of free-free emission from photoionized gas, characteristic of PNe. We report 8 objects with a positive coincidence, 4 of which are classified as candidate OHPNe here for the first time (IRAS 16372–4808, IRAS 17494–2645, IRAS 18019–2216 and OH 341.6811+00.2634). Available evidence strongly indicates that they are evolved stars, while the comparison with confirmed OHPNe indicates that they are likely to be PNe. Their final confirmation as bona fide PNe, however, requires optical/infrared spectroscopy. The obtained spectral indices of the radio continuum emission (between ≃ 0.4 – 1.3) are consistent with partially optically thick free-free emission from photoionized gas. Also, they cluster in the same region of a WISE colour-colour diagram as that of the confirmed OHPNe ($9.5\lesssim [3.4]-[22]\lesssim 13.5$, and $4.0\lesssim [4.6]-[12] \lesssim 7.0$), thus this diagram could help to identify more OHPNe candidates in the future.

Nascent planetary nebulae: new identifications and extraordinary evolution

“…We have processed all the continuum data associated with the interferometric follow-up of SPLASH (see Qiao et al 2020), which was carried out with Australia Telecope Compact Array. We found three previously identified bona fide OHPNe, and four new OHPNe candidates (Cala et al 2022). The new OHPNe candidates present characteristics that are similar to those observed in bona fide OHPNe (see Cala et al 2022).…”

“…We found three previously identified bona fide OHPNe, and four new OHPNe candidates (Cala et al 2022). The new OHPNe candidates present characteristics that are similar to those observed in bona fide OHPNe (see Cala et al 2022). We aim to confirm the new OHPNe candidates as bona fide PNe by means of optical/infrared spectroscopy.…”

Evolution of the Outflow in the Water Fountain Source IRAS 18043–2116*

“…An alternative explanation for the origin of the radio continuum emission is the onset of photoionization at which this source is entering the PN phase. An increase in the flux density at radio wavelengths has also been observed in several young PNe (Kwok et al 1981;Knapp et al 1995;Christianto & Seaquist 1998;Gómez et al 2005;Tafoya et al 2009;Cala et al 2022). Such a flux density increase has been interpreted as an expansion of the ionized region.…”