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In memoriam Harold V.J. Linnartz 1965 – 2023: Unlocking the Chemistry of the Heavens

With great sadness we share the news that Prof. Harold Linnartz passed away suddenly and unexpectedly on Sunday 31 December 2023. We are all in shock, and our thoughts are with his wife and children, other family, and friends. Harold was at the heart of our institute, as a researcher, as a supervisor, as education director and as head of the astrophysics lab. His passing will have an enormous impact on us all.

On 31 December 2023 Prof. dr. Harold Linnartz passed away unexpectedly at the age of 58. He leaves behind his wife Helga  and their children Anne and Thomas.  Linnartz held the Chair for Laboratory Astrophysics at Leiden Observatory, Leiden University, the Netherlands, where he was appointed in 2005. He was a world leader in laboratory studies of gaseous and icy molecules in interstellar space. Linnartz also served as Education Director at Leiden Observatory.  He guided with passion the very successful education program in astronomy

Linnartz was born on October 16, 1965 in Heerlen, in the province of Limburg of The Netherlands, where he finished high school (Gymnasium beta ‘Rolduc’ in Kerkrade) in 1984. He subsequently went to study experimental physics at the University of Nijmegen, where he also did his PhD research following a one year stay in Göttingen. He graduated in 1994 at Nijmegen on the thesis Infrared and Far Infrared Spectroscopy of Transient Species with Prof. dr. J. Reuss as promotor.

In the following decade, Harold Linnartz continued his career abroad, first at Bonn and then for six years at the University of Basel in the group of Prof. dr. John P. Maier, where he also did his Habilitation in 2002. In that period, he developed new laboratory techniques to measure the electronic spectra of gaseous ions and radicals, i.e., transient molecules that react very quickly under Earth-like conditions but can be stable in the vacuum of space. A central theme in Basel was the study of the so-called Diffuse Interstellar Bands (DIBs), a set of (by now) 500 bands that are seen in clouds between the stars at visible wavelengths but that have defied identification for more than a century. Long carbon-chain molecules have been postulated as possible carriers and Harold had a leading role in systematically measuring the spectra of these and other candidate molecules in the laboratory. Solving the DIB mystery continued to be a major theme during his career, also when he moved in 2003 back to the Netherlands to the Vrije  Universiteit (VU) at Amsterdam as a senior researcher at the VU Laser Centre and subsequently at Leiden. He kept his close connections with the VU through a special chair for molecular laboratory astrophysics in the 2008-2013 period. One of his last papers was on DIBs as part of the ESO-EDIBLES consortium, conducting an observational survey and highlighting that high resolution optical spectroscopy in space and in the laboratory go hand-in-hand.

After his move to Leiden in 2005, Harold Linnartz became Director of the Leiden Laboratory for Astrophysics. This entailed a broadening of his scientific focus to include, besides gas-phase spectroscopy, also solid-state chemistry and, besides optical, also infrared studies in which the Leiden laboratory was specialized. Thanks to his in-depth expertise in designing, building, and commissioning equipment, the laboratory started soon after his arrival to harvest scientific results from a number of new experiments in ultra-high vacuum surface science.

Stimulated by his passionate leadership, together with excellent postdocs and PhD students, the laboratory was rapidly thriving and became world leading in the study of the chemistry and spectroscopy of interstellar ices under Harold’s leadership.  It grew from three to seven set-ups, including also experiments dedicated to Polycyclic Aromatic Hydrocarbons (PAHs) and gas-phase molecules, using state-of-the-art techniques in experimental physics. His overall publication list of more than 260 papers is a testimony to the many excellent results. He became a co-leader of the NWO Dutch Astrochemistry network and highly active internationally in the organization of conferences and workshops, and more generally in steering the interdisciplinary field of laboratory astrophysics worldwide.

When the Atacama Large Millimeter/submillimeter Array (ALMA) became operational in 2013, observational studies of complex organic molecules in star-forming regions took a new turn.  He shifted the laboratory experiments to unraveling the chemistry in ices and how these molecules can form under the coldest conditions when species hardly move on surfaces. Meet and greet was Linnartz’ answer: he and his team showed that reactions between neighboring molecules could take place on cold icy grains and form the sugars, alcohols, aldehydes, and other ingredients of the cosmic cocktail that were being observed with ALMA. Even amino acids, the starting blocks for prebiotic chemistry, could be produced in the laboratory. These cosmic recipes are now an integral part of the latest astrochemical models.

The advent of the James Webb Space Telescope, including the Netherlands’ investment in the MidInfraRed Instrument (MIRI), meant a new challenge for the laboratory: infrared spectroscopy of complex molecules. The databases from the 1990’s are highly incomplete on the types of molecules that JWST could search for. A large number of spectra were collected by his team prior to launch and made publically available through the Leiden Database for Ice, LIDA in 2022, a telephone directory for ices in his words. Harold Linnartz was co-PI on the JWST IceAge program and thoroughly enjoyed the arrival of the first high-quality JWST data. Just last month, the culmination of this work happened with the publication of the first detection of more complex molecules in interstellar ices, demonstrating that they indeed have a solid-state origin. A beautiful synergy between laboratory and observational studies that he was deservedly very proud of.

Harold Linnartz was not only an extremely active researcher but also a great educator who was convinced of the utmost importance of education. He became Director of Education in 2018, a role he was performing with much passion. He shaped the education office into an  effective team capable of organizing one of the largest bachelor and master programs in astronomy in the world. The Covid period was challenging, but with Harold’s overview and attention to detail, the education program continued successfully. Moreover, Harold  always kept a vivid eye on the interests of individual students: he realized that a University is a big place but that he could play crucial roles in the lives of individual students. Many of them are still grateful for his help.

Harold Linnartz was an excellent and dedicated mentor to his many students, PhD candidates and postdocs. With his warm and engaging personality, he inspired them in many ways and always had good advice and active support, from science and writing of papers to career paths. Accepted papers were celebrated with a famous paper cake, and the annual laboratory outings were a social highlight of the year. Harold had a great sense of humor and was a jovial companion  at meetings and during travel. He was also an excellent popular writer with a long string of Dutch and English press releases, radio interviews and newspaper contributions, always with just the right quote. He loved astronomy so much that he shared his passion in regular articles in his local hometown paper about the night sky.

Throughout his illustrious career, Harold Linnartz focused on unlocking the chemistry of the heavens. As a spectroscopist pur sang, shining light on molecules in space was one of his guiding principles. Fathom the behavior of molecules under the extreme conditions of space was another. Harold had many ideas to continue his research in these areas over the coming years, but it is not to be. We will miss him dearly.

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