A pilot search for mm-wavelength recombination lines from emerging ionized winds in pre-planetary nebulae candidates

Abstract

We report the results from a pilot search for radio recombination line (RRL) emission at millimeter wavelengths in a small sample of pre-planetary nebulae (pPNe) and young PNe (yPNe) with emerging central ionized regions. Observations of the H30α, H31α, H39α, H41α, H48β, H49β, H51β, and H55γ lines at ∼1 and ∼3 mm have been performed with the IRAM 30 m radio telescope. These lines are excellent probes of the dense inner ( 150 au) and heavily obscured regions of these objects, where the yet unknown agents for PN-shaping originate. We detected mm-RRLs in three objects: CRL 618, MWC 922, and M 2-9. For CRL 618, the only pPN with previous published detections of H41α, H35α, and H30α emission, we find significant changes in the line profiles indicating that current observations are probing regions of the ionized wind with larger expansion velocities and mass-loss rate than ∼29 yr ago. In the case of MWC 922, we observe a drastic transition from single-peaked profiles at 3 mm (H39α and H41α) to double-peaked profiles at 1 mm (H31α and H30α), which is consistent with maser amplification of the highest frequency lines; the observed line profiles are compatible with rotation and expansion of the ionized gas, probably arranged in a disk+wind system around a ∼5-10 M central mass. In M 2-9, the mm-RRL emission appears to be tracing a recent mass outburst by one of the stars of the central binary system. We present the results from non-LTE line and continuum radiative transfer models, which enables us to constrain the structure, kinematics, and physical conditions (electron temperature and density) of the ionized cores of our sample. We find temperatures T e ∼ 6000-17 000 K, mean densities n e ∼ 10 5 -10 8 cm −3 , radial density gradients n e ∝ r −αn with α n ∼ 2-3.5, and motions with velocities of ∼10-30 km s −1 in the ionized wind regions traced by these mm-wavelength observations. We deduce mass-loss rates ofṀ pAGB ≈ 10 −6 -10 −7 M yr −1 , which are significantly higher than the values adopted by stellar evolution models currently in use and would result in a transition from the asymptotic giant branch to the PN phase faster than hitherto assumed.

Accounting for planet-shaped planetary nebulae

“…Observations and their interpretation strongly support the notion that most planetary nebulae (PNe) are shaped by a stellar companion that strongly interacts with the asymptotic giant branch (AGB) progenitor (e.g., limiting the list to some recent papers , Chiotellis et al 2016;García-Rojas et al 2016;Harvey et al 2016;Heo et al 2016;Hillwig et al 2016a,b;Jones 2016;Jones et al 2016;Chen et al 2017;Macdonald et al 2017;Sánchez Contreras et al 2017;Miszalski et al 2017;Hillwig et al 2017;Sowicka et al 2017). PN catalogs (e.g., Balick 1987;Chu et al 1987;Corradi & Schwarz 1995;Manchado et al 1996;Sahai & Trauger 1998;Sahai et al 2011;Parker et al 2016) show that most PNe are not spherical, but rather bipolar or ellipticals, suggesting that most AGB stars that formed bright enough PNe to be observed have interacted with a companion.…”

The influence of continuum radiation fields on hydrogen radio recombination lines

“…Goldberg (1966) showed that at low frequencies where stimulated processes are important the accuracy of the calculation of atomic level populations has a substantial influence on the theoretical intensities of recombination lines. This sentiment is echoed 50 years later by Sánchez Contreras et al (2017) who also point out that there is disagreement between the results of various authors without one set of E-mail: prozea@unisa.ac.za (AP) Figure 1. The black line shows the frequencies (wavelengths) of Hnα transitions as a function of n for hydrogen.…”

“…The results of Storey & Hummer (1995) (hereafter SH95) are considered to be the definitive values for optical/IR recombination lines, but stimulated processes are not included in their model. However, the values of SH95 are being used to study low frequency lines (see for example Fujiyoshi et al 2006;Bendo et al 2017;Sánchez Contreras et al 2017). In this paper we present results using a model that includes stimulated and absorption processes in the boundbound and bound-free transitions in an nl-model, and use updated numerical methods.…”

Radio Stars: From kHz to THz

“…For example, Cerrigone et al (2017) identified a few sources evolving toward PNe that showed either increases or decreases in the radio flux over the course of ∼10-20 years, but it is unclear if these mirror the ionization changes expected from source evolution or simply variability of the central star. For the pre-PN CRL 618 (see also below), Sánchez-Contreras et al (2017) recently found evidence that the long-term variability may be periodic.…”