When the Stars Align: A 5 $\sigma$ Concordance of Planetary Nebulae Major Axes in the Centre of our Galaxy

The formation history of our Galactic centre, the bulge, is complex and remains under debate (see review by Kormendy & Kennicutt Jr, 2004, and references therein). Star formation here occurs within collapsing molecular clouds under an environment including gravity, collisionless dynamics, turbulence, relativistic particle interactions and magnetic fields (e.g., McKee & Ostriker, 2007). The bulge is also permeated by magnetically confined hot gas (Blitz et al., 1993). Interdependence between gravity, turbulence and magnetic fields could align a star’s angular momentum on formation (e.g. Crutcher, 2012; Gray et al., 2018). This may extend to binary system orbital parameters. For any coherence to be detectable such a process has to be strong, ordered and must endure. Proof of this has been lacking.

However, there is a key, touchstone population that forms a highly visible component for study. These are planetary nebulae (PNe), the short lived, ionised, gaseous ejecta from evolved low- to intermediate-mass stars of one to eight times the mass of our Sun (e.g. see review by Kwitter & Henry, 2022). This occurs near the end of their lives as the remnant stellar core contracts, heats up and evolves to the white dwarf (WD) stage. The PNe mass range covers progenitor stars that may have lived for billions of years at the low mass end to only a few tens of millions of years at the high mass end. This is compared with a PN phase that lasts for only a few tens of thousands of years. PNe hence effectively give an instantaneous snap-shot of stellar death in a complex population like the Galactic bulge.

PNe have key, basic shapes (morphologies) that are mostly elliptical, round, or bipolar, e.g. Parker et al. (2006, 2016), but with some very compact (young and/or distant) or in rare cases, irregular or asymmetric. Only bipolars and ellipticals can provide a reliable orientation measurement of their principal symmetry axis from 2-D projections of their 3-D shapes. These can be converted into Galactic coordinates to yield Galactic position angles (GPAs) after determining their major axis orientations measured relative to the PN’s equatorial coordinate system. This ranges from $0^{\circ}$ to $180^{\circ}$ and is referred to as the equatorial position angle (EPA). The orientation axis from the projected bulge PN 2-D image was determined visually as what best represents the long symmetry axis of each PN (a subjective process). For elliptical PNe, the ellipse major axis was taken. For bipolar examples the geometric direction of the lobes and low-density interior structures were taken to determine their principle axis for the EPA measurement (see Fig. 1). The EPAs of PNe were then converted to Galactic position angles (GPAs), measured from the direction of the Galactic north towards the east for further analysis following the formula given in Corradi et al. (1998) and reproduced in Section 3.2 for convenience. These may relate to preferred matter ejection directions during the PN phase.

Grinin & Zvereva (1968) was the first to claim that PNe major axes may have a preferential orientation along the Galactic plane. This was followed by contradictory results, e.g. Corradi et al. (1998). Weidmann & Díaz (2008) used a sample of 440 “elongated” PNe with measured axes and found evidence for an alignment at $\sim 100^{\circ}$. Rees & Zijlstra (2013) used high-resolution Hubble Space Telescope (HST) and European Southern Observatory (ESO) New Technology Telescope imagery for 130 compact PNe within the central $10\times 10$ degrees of our Galaxy (the Galactic bulge), and found evidence at the 3.7$\sigma$ level for alignment at a similar angle but only for a subset of bipolar PNe. Finally, Danehkar & Parker (2016) also examined the issue and found an effect for a smaller, independently observed PNe sample along the broader Galactic plane whose orientation measures were based on kinematic modelling from 3-D data cubes. The causal reasons for any alignment has not been clear: Rees & Zijlstra (2013) suggest strong magnetic fields during star formation in the Galactic bulge.

Here, we reduce our ESO 8 m Very Large Telescope (VLT) narrow-band imagery for a slightly larger sample of 136 compact, bulge PNe, supplemented by re-examination and re-measurement of multiple exposure, high-resolution, archival HST imagery for 40 of these. This is to investigate this putative alignment issue afresh given the significant implications the tentative earlier results would have for our understanding of the pervasiveness, longevity and effect of ordered magnetic fields in our Galaxy. This is especially if the origin of the signal could be identified and proven real.

The PNe sample used are considered Galactic bulge members from Rees & Zijlstra (2013). Our new high quality imaging data yielded new information on large and small scale morphological features including point-symmetric structures, internal nebular condensations, external jets etc. These enabled a more detailed morphological classification following the “ERBIAS sparm” scheme outlined in Parker et al. (2006). This scheme is somewhat different to the morphological classification scheme used in Rees & Zijlstra (2013), so the bipolar samples are not quite directly comparable. Furthermore, a more accurate determination of the principle nebula axis of orientation was made here. Our morphological classifications, kinematic ages and detected PNe central stars for this carefully vetted sample were previously presented in Tan et al. (2023), hereafter Paper I.

The bulge PNe currently going through the short-lived PNe phase, regardless of whether in a binary or even triple system with other companions (including planets), could have a range of progenitor masses, as suggested by Bensby et al. (2013, 2017). The progenitors will not have been born at the same time. Gesicki et al. (2014) derive an extended star formation history from bulge PNe. For example, a range of 1 solar mass at stellar birth would mean a difference of hundreds of millions of years for stars born with masses between 2 and 3 M_⊙ (see Fig. 3 in Buzzoni, 2002) based on evolutionary models of single stars. In a PNe hosting a binary or even multiple system, only one of the stars is currently going through the PNe phase as it reaches that particular stage of evolution.

By chance, two such PNe in our short-period binary sub-sample have had their CSPN ages previously estimated by Gesicki et al. (2014), namely Th 3-12 (PNG 356.8+03.3) and H 2-13 (PNG 357.2+02.2), at 7 and 10 Gigayears respectively (but based on a single-star model). Hence, if the observed GPAs of their major bipolar lobes do indeed reflect the inclinations of their binary orbits today, and if these orbital inclinations originate from system birth, then the causal influence acting on star formation has to have endured, effectively unchanged, for cosmologically significant time periods. The difference of about 3 Gigayears just for the two examples above suggests that the alignment process was not caused in a single star formation burst but acted for at least several Gyr.

Why do only the bipolars with short-period binaries demonstrate the astonishing GPA alignment shown across several kpc of the Galactic bulge? A possible explanation that other bipolars are not in (close) binary systems is contrary to currently accepted wisdom for how bipolars form e.g. Ondratschek et al. (2022). Our results imply that more than one type of systems forms bipolar nebulae, given only a modest fraction of observed bipolars fit the bill. Perhaps the observed alignment effect is also a function of the separation of the binary components at system birth? Note that close binary CSPN have isolated binary evolution and the angular momentum vector of the system remains constant after formation. The only other alternative is a physical process operating now in the Galactic bulge but only on bipolar PNe in short period binaries, that aligns their bipolar ejecta over the short timescales that these PNe are now visible (only a few tens of thousands of years).

PNe axes of symmetry are thought to trace the rotation of their CSPN which may themselves align with a very strong interstellar magnetic field at their time of formation from the collapsing disks of rotating molecular gas where the orbital inclinations, if also part of a binary or multiple system, are established (Rees & Zijlstra, 2013). An alternative is they may possibly be directly tilted later by the extant magnetic field itself (Falceta-Gonçalves & Monteiro, 2014). Binary star systems may have formed long ago in the presence of a strong Galactic magnetic field where their orbits align with this field due to their higher angular momenta compared with single-star systems (Rees & Zijlstra, 2013). Indeed, it is reported by Price & Bate (2007) that magnetic fields have important effects on molecular cloud core fragments and on their subsequent condensation and formation of binary systems. They report the scale of binary separation at birth as an important factor in the level of binary orbit to magnetic field alignment eventually achieved with the effect more pronounced at shorter separations.

Symmetry axes of PNe ejecta from one of the stars within binary systems take the axes of their angular momenta. The magnetic field in the Galactic bulge was proposed to counter the contraction of any star-forming cloud perpendicular to the direction of the Galactic plane during the early phases of bulge star formation (e.g. Price & Bate, 2007; Gray et al., 2018). However, we observe the preferential major axis orientation only in objects with confirmed or inferred short-period binaries that have also likely undergone a CEE phase. These objects may have been in tighter orbits to begin with before then spiralling in for the CEE phase and may have higher angular momentum. PNe that avoid a CEE would have much wider orbits at birth.

According to Falceta-Gonçalves & Monteiro (2014) PNe symmetry axes modification can occur if the local magnetic field is $>100$ $\mathrm{\mu G}$. From optical polarization measures, radio synchrotron data and Zeeman splitting seen in object spectra, the average total magnetic field strength in the bulge today is $\sim$20-40 $\mu$G (LaRosa et al., 2005). This is likely insufficient to modify binary orbital inclinations over the short timescales implied before the bulge PNe we see today are formed. It is more plausible the observed effect was “frozen in” in the past. The simulations of Falceta-Gonçalves & Monteiro (2014) show further orbit modification can occur at t $<$ 10⁴ years after PN envelope ejection, compared with the rapid CEE timescaled expected (a few years or even less, Ivanova et al., 2013). This might not happen to post-CEE PNe that form our short period binary sample. Only post-CEE PNe may maintain their ancient orientations resulting from their orbital angular momenta aligning with the initial (presumably historically stronger) magnetic field at their formation time along the Galactic plane. This assumes they originally had tighter orbits following the arguments of Price & Bate (2007).

PNe formed from wider binaries could have their symmetry axes change over time more randomly due to the ambient field where directions may evolve due to a Galactic wind or bulk motions of the ISM. This provides a possible explanation for the observed effect only being seen in post-CE binary PNe. If this is the process then it must remain ordered, stay potent and have endured for billions of years. The only alternative is a pervasive force (magnetic?) acting currently only on the bipolar lobe ejection mechanism in PNe hosting short period binaries that is sufficiently strong to align GPA’s over the entire Galactic bulge in a few thousand years.

We are grateful to the referee and statistics editor whose comments and suggestions have significantly improved the paper. S.T. thanks HKU and Q.A.P. for provision of an MPhil scholarship and subsequent Research Assistant position, Q.A.P. thanks the Hong Kong Research Grants Council for GRF research support under grants 17326116 and 17300417. A.R. thanks HKU for the provision of postdoctoral fellowship under Q.A.P. A.A.Z. thanks the Hung Hing Ying Foundation for a visiting HKU professorship and acknowledges UK STFC funding under grant ST/T000414/1. This work made use of the University of Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula (HASH PN) database, hosted by the Laboratory for Space Research at the University of Hong Kong. We acknowledge use of ESO observations under program IDs 095.D-0270(A), 097.D-0024(A), 099.D-0163(A), and 0101.D-0192(A) for PI Rees. We acknowledge use of data from the Wide Field and Planetary Camera 2 Instrument (WFPC2) of the HST through HST-SNAP-8345 (PI: Sahai), HST-SNAP-9356 (PI: Zijlstra) and HST-GO-11185 (PI: Rubin). Some data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed viahttps://doi.org/10.17909/pej3-ce46 (catalog doi:0.17909/pej3-ce46).