WORKPLANS: Workshop on Planetary Nebula Observations

: This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The specific aims of the WORKPLANS workshop series are ( i ) to discuss and prioritize the most important topics to be investigated by the PN community in the following years; ( ii ) to establish a network of excellent researchers with complementary expertise; ( iii ) to formulate ambitious observing proposals for the most advanced telescopes and instrumentation presently available (ALMA, SOFIA, VLT, GTC, HST, etc.), addressing those topics; and ( iv ) to develop strategies for major proposals to future observatories (JWST, ELT, SPICA, Athena, etc.). To achieve these goals, WORKPLANS II brought together experts in all key sub-areas of the PNe research field, namely: analysis and interpretation of PNe observational data; theoretical modeling of gas and dust emission; evolution from Asymptotic Giant Branch stars (PNe progenitors) to PNe; and the instrumentation and technical characteristics of the relevant observatories.


Introduction
Planetary nebulae (PNe) are formed by the ejection of the outer layers of evolved low and intermediate mass stars (∼1-8 M ). These objects have important roles in stellar evolution, in the enrichment of the interstellar medium (ISM) and in the evolution of galaxies. PNe are unique astrophysical laboratories in which we can investigate a wide variety of physical conditions from highly-ionized plasma to low-temperature dusty molecular regions in a spatially resolved manner. The findings from PNe research are applicable not only within the field, but also to a broad range of astrophysical objects with similar characteristics. Solving the open issues in the field of PN would therefore have a great impact not only for the PN research community, but also for the astronomical community as a whole.
The IAU PN Working Group discusses several open problems in the field of PN in the white paper entitled "The present and future of planetary nebula research" [1]. According to the paper, the main problems to be investigated by the community are: • uncover PNe that cannot be identified in the optical spectral range • improve central star evolutionary models • improve our knowledge of the atmospheres and winds of central stars • investigate the role of binary central stars in the evolution/formation of PNe • understand the history and mechanisms of mass-loss and structure formation in the circumstellar nebulae • improve techniques to derive chemical abundances • understand the abundance discrepancies measured from collisional and recombination lines • understand the formation mechanisms and survival of dust and molecules in PNe The IAU PN working group white paper [1] also makes clear the need of further multi-wavelength observations in the PN research field and serves as a motivation for this workshop series. As in any topic in Astronomy, research on PNe is strongly driven by observations. New facilities built in the last few decades have improved the instruments and opened new spectral windows. Other new and forthcoming facilities, like ALMA and future telescopes such as JWST, will play a key role on the state-of-art astronomy research in the next years. Multiwavelength studies have become possible and, since different wavelength ranges probe different realms of chemistry and physics, the multiwavelength approach has already greatly improved our understanding of a wide range of astronomical objects, including PNe. This provides strong motivation for bringing together researchers with expertise in different wavelength ranges but with a common goal, namely, to understand the formation and evolution of PNe.
To fully explore the possibilities of the current and future instruments for the study of PNe, it is essential to aggregate different expertise-and the formation of an international network of scientists is then a natural step. Such a network provides a framework for the more efficient use of astronomical data and helps the PN community compete for time on the main telescopes, where a strong worldwide presence is an important factor.
The aim of the WORKPLANS series of workshops is to build a network of PN observers whose expertise in observations or theoretical analysis is spread across different sub-fields and spectral domains. The participants are organized in groups to formulate and draft proposal concepts for future observations maximizing the scientific output. During the workshop, we overview the key questions in the field, review the capabilities of observational facilities for (current and future) PN research, discuss which observations are best suited to solving the open problems in the field, and discuss the corresponding necessary analysis (in terms of tools and expertise).
Workshops with similar purposes have been organized with great success by the Asymptotic Giant Branch stars (AGB) research community, which has regular meetings strategically scheduled before each ALMA observing cycle. These meetings generated successful proposals that were granted observing time by this highly competitive observatory. The PNe community is relatively small, with a history of successful collaborations, as exemplified by the ChanPlaNS and HerPlaNS collaborations, both of which have produced significant results (e.g., [2][3][4][5][6][7][8]). These surveys were conceived during the Planetary Nebulae Workshop held in Rochester, NY, USA, in 2009 [9], with follow up at the Planetary Nebulae Workshop in Miraflores de la Sierra, Madrid, Spain in 2013; both meetings were organized with goals similar to those of WORKPLANS.

WORKPLANS I
In the first WORKPLANS (25-29 January 2016), 51 researchers gathered in the Lorentz Center 1 , Leiden, Netherlands, for what was a very fruitful week. We started the meeting with a broad review of potential relevant topics to be investigated in the next years in the PNe field (with talks mainly on Monday). From Tuesday to Thursday, we focused on the scientific and technical aspects of specific wavelength ranges. In the mornings, we reviewed the capabilities of the main telescopes and instruments from each spectral range. The afternoons of these three days were dedicated to group discussions, networking and drafting observing proposals. Brief summaries of those discussion and proposal were given at the end of the day. On Friday, a summary was given by the group leaders, together with the current status of the draft. At least eight proposals were drafted and submitted soon after the workshop.

WORKPLANS II
The format and goals of WORKPLANS II were similar to its previous edition. The main change was to reduce the time for the "X-ray observations" session, based on experience from WORKPLANS I and the increasing limitations of current facilities. The "X-rays" session was merged into the optical/ultraviolet session, which was made independent from the "infrared observations" session. These changes better balanced the current interests of the PN community and the facilities presently available and under construction. We also made small changes to the program in comparison to WORKPLANS I to add a little more time for plenary discussions. The workshop program is compiled in Table 1.
We organized the workshop to occur with ample time before deadlines for submission of observing proposals for various major facilities (in particular the JWST Cycle 1 proposal deadline), while attempting to avoid periods with classes in universities, to maximize attendance. The workshop was held during the week of 16-20 December 2019, in the Lorentz Center, the Netherlands. WORKPLANS II had short-term and long-term practical goals. In the short term, we intended to reinforce the previously formed network, inviting at least some of the discussion leaders who emerged during the last workshop while expanding the network by inviting new participants. During the workshop, we formed task forces to address the topics previously mentioned. The task forces focused on a specific spectral realm to strategize proposals designed to use the current forefront facilities that can complement extant data. We invited experts in observations to cover the whole radiative spectrum, from X-rays to radio. Teams were assembled that also included specialists with deep knowledge of the fundamental physics of PNe and hence were prepared to analyze and model the observed objects.
We also discussed preparations for the use of future instruments, in particular JWST, to be launched in 2022. This represents one of our key long-term goals. Another long-term goal is to keep WORKPLANS as a regular and productive tradition in the PNe community.
Fifty-five participants from all around the world, covering a wide range of expertise, gathered for WORKPLANS II. To maximize the workshop science output, in addition to having a wide range of expertise, in the preparation of our participants list and the program, we also considered the balance of gender, career stage, and geographical representation. The list of participants is compiled in Table 2, and the group picture appears in Figure 1.
A good measure of the success of WORKPLANS II was that, by the end of the workshop, the participants were already talking about a third installment. While gone, Stuart's research will have a long lasting impact in the study of PNe for many years to come, and a current and new generation of PN scientist will keep benefitting from his tremendous and productive career.
Author Contributions: All authors contributed equally to the workshop and this paper conceptualization, methodology, resources, writing-original draft preparation, writing-review and editing, visualization, and funding acquisition. Project administration and supervision by I.A. All authors have read and agreed to the published version of the manuscript. Workshop was organized with the financial support of the Lorentz Center, which was provided by the Netherlands Organization for Scientific Research (NWO) and the Leiden University.