by Rex Parker, PhD email@example.com
Meet on Campus Feb 14
We’ll be back at Princeton Feb 14 for the monthly meeting with hopes that the Peyton Hall auditorium AV upgrade is finished so we can run this as hybrid-virtual. If you absolutely cannot make it physically there will be a Zoom link for the live meeting sent by email and on the website. The massive construction project across the street continues and the old parking lots are gone forever. So the University wants us to park (free) in the new garage at 148 Fitzrandolph Rd, off of Faculty Rd. That means a 15 minute walk around the football stadium to Peyton once you park your car. Our guest speaker will be John Church of AAAP. For more information on John’s presentation and for the walking route map to Peyton Hall, see Victor’s article below
We’re looking for additional members to give an Un-journal Club, a brief informal presentation for the second half of the meeting. “Un-journal” means this is not grad school, you don’t need scholarly journal-like topics, just what you care about in astronomy. You can use PowerPoint slides, JPEG’s, astro-images, travel pictures, book reviews, whatever you want (you can bring a USB memory stick to use on my laptop, or your own laptop). To get onto the schedule for an upcoming meeting, please contact us.
AAAP Astro-Imaging Interest Group Formed
In response to growing interest in the club and technical innovations in the field, we have created a new special interest group dedicated to hands-on astro-photography in its many forms. Michael DiMario is appointed chair (thanks Michael!) and will be coordinating future sessions. Members will have received emails last month describing the proposal and an invitation to join the group. If you are interested but didn’t respond yet please contact me.
Astro Geo Connections to Extraterrestrial Objects
The distinctions between astronomy and geology blur when the object of study goes from interplanetary to earthbound. Comets and asteroids, meteors and meteorites, all have their own geochemistry story to tell. The stakes and interest have never been higher for transient near-earth objects, especially after the book “Extraterrestrial” by Avi Loeb (AAAP’s guest speaker and topic last October) threw down the gauntlet for bolder interpretation of certain unusual interplanetary objects. Dr. Loeb of Harvard, head of the Galileo Project, recently described on his blog how he is going to lead an expedition to collect fragments of the first interstellar meteor which crashed into the south Pacific in 2014. The Galileo Project expedition received more than a million dollars in funding for this and they have a boat and team of professionals experienced in ocean expeditions. They are designing and manufacturing the required sled, magnets, collection nets and mass spectrometer to find an interstellar object.
In Jan 2014 an object from interstellar space (labeled IM1) hit Earth at high speed, and the fireball disintegrated into fragments off the coast of Papua New Guinea. From the observations available Loeb identified this as the first interstellar meteor ever discovered, confirmed in 2022 by the US Space Command under the Dept of Defense and NASA. The data on the path and energy released by the fireball suggested its composition was unlike other meteors in the near earth object catalogs. This has inspired the Galileo Project looking for the meteor fragments on the ocean floor. Analyzing the fragments’ composition could indicate whether the object is natural or artificial. For more insight into the nature of this object see a recent paper by Loeb https://lweb.cfa.harvard.edu/~loeb/ALS.pdf.
The Legacy of Astronomer Fritz Zwicky
An otherwordly news item keeps popping up in the media these days. Dubbed by the media as “the green comet”, this interplanetary visitor from the Oort Cloud is C/2022 E3 (ZTF). The name translates as the 3rd comet discovered in the 5th two-week period in 2022 by the Zwicky Transient Facility (see https://www.ztf.caltech.edu/. Named for the brilliant though apparently difficult Cal Tech astronomer Fritz Zwicky, today’s ZTF project is the offspring of the National Geographic-Palomar Sky Survey conducted in the 1950s – emulsion astrophotography days. This was the first advanced photo-telescopic survey of the deep sky, reaching 22nd magnitude. The new ZTF survey is built around the same 48” Oschin Schmidt camera telescope constructed by Cal Tech in the 1940s. The original Palomar survey photographic plates were digitized decades ago and distributed as a 102-CD disk collection. It survives today as one of the optional databases in TheSkyX,software running at the club observatory (and my own). Now the Oschin Schmidt is adapted for a very large CCD sensor to provide a 47 square degree field of view and covers the whole northern sky every two nights sequentially repeating. The fast cadence is part of the innovation of time-domain astronomy, looking for fast-movers and transients including near-Earth asteroids, comets, and distant supernovae.
An Appreciation of Green Chemistry
The green color in a comet’s core but not tail is an intriguing puzzle only recently solved. It is not the same photochemistry as other astrophysical processes, for example the blue-green fluorescence emission from doubly-ionized oxygen (OII) in planetary nebulae. The nucleus of a comet is an agglomeration of rock, dust, and frozen gases. As it gets closer to the sun and larger in our skies, heat increases and causes sublimation of gases to form a nebulous envelope around the nucleus, the coma. The tail is an extension of the coma’s molecules drawn out by the solar wind. Yet the green around the nucleus disappears in the tail which instead displays a distinct reddish brown color.
It was long speculated that a comet’s green comes from the breakdown of the reactive molecule dicarbon (diatomic carbon, C2). Dicarbon is abundant in the galaxy and the solar system but kinetically unstable on earth; in flames it quickly polymerizes to carbon soot. The multiple valence states occupied by electrons of dicarbon give rise to a colorful optical spectroscopy. The famous British physicist and chemist Wollaston, member of the Royal Society, analyzed blue-green flames in 1802 — the first study of dicarbon.
A recent photochemistry lab experiment provides deeper evidence supporting a mechanism for cometary green glow. UV-laser irradiated dicarbon dichloride was analyzed spectroscopically (Borsovszky. Proc Natl Acad Sci USA 2021, Vol 118, No 52). The sample was exposed to UV plus longer wavelength radiation to generate a metastable state (a radical) of the C2 molecule in a cuvette. The radical then decays and emits a characteristic green photon. The unique emission spectrum of photo-activated dicarbon is known as the Swan band, after the famed Scots physicist Swan in the 1850s. Swan bands are characteristic of carbon stars and some nebulae as well as comets. The emission wavelength (color) is very sensitive to the environment because the chemical species producing it is short-lived. Based on the study cited above, the half-life of the dicarbon radical is only ~2 days when the comet is about 1AU from the Sun. This is the strongest physical chemistry data available explaining why a comet’s head but not tail glows green, as the coma’s dicarbon radical with short half-life dissipates in material streaming out to the tail.
Green color in the coma fades away in the tail in comet C/2022 E3 (ZTF). The image is from my home observatory before sunrise on Jan 10 using a 12.5” reflector, tracking on the comet so the stars are trailing. The image is made from 15×2 min subframes with an ASI071MC CMOS camera.