Welcome back to our five-part examination of Webb’s Cycle 4 General Observations program. In the first and second installments, we examined how some of Webb’s 8,500 hours of prime observing time this cycle will be dedicated to exoplanet characterization, the study of galaxies at “Cosmic Dawn,” and the period known as “Cosmic Noon.”
Today, we’ll look at programs that will leverage Webb’s unique abilities to study stellar populations and the interstellar medium in galaxies.
On March 11th, the Space Telescope Science Institute (STScI) announced the science objectives for the fourth cycle of the James Webb Space Telescope‘s (JWST) General Observations program. Cycle 4 GO. This latest cycle includes 274 programs broken down into eight categories encompassing Webb’s capabilities. These range from exoplanet study and characterization and observations of the earliest galaxies in the Universe to stellar science and Solar System Astronomy.
As we covered in previous installments, Webb’s unique capabilities at imaging exoplanets are allowing scientists to refine their measurements of exoplanet habitability. These same capabilities have allowed astronomers to view some of the earliest galaxies in the Universe to trace their evolution from “Cosmic Dawn” to the present more recent cosmological epochs. These observations are leading to a more comprehensive picture of how the Universe has changed throughout cosmic history.
In keeping with these objectives, astronomers also want to leverage Webb’s unique abilities to study star formation in galaxies throughout cosmic history. By imaging stellar populations in distant galaxies, they hope to learn more about the different types and generations of stars and the mechanisms that allow new ones to form.
Gas, Dust, and the ISM
According to accepted stellar formation models, the first population in the Universe were massive, very hot, and very bright Population III stars. These stars formed almost entirely from the neutral hydrogen that permeated the Universe during the “Cosmic Dark Ages,” between 380,000 and 1 billion years after the Big Bang. These stars led to the “Epoch of Reionization” (EoR), where the intense ultraviolet (UV) radiation of the Pop III stars ionized the neutral hydrogen, causing the Universe to become “transparent” – i.e., visible to modern instruments.
When Pop III stars reached the end of their lifespan (just a few hundred million years), they went supernova and blew off their outer layers. This allowed heavier elements that formed in their interiors (carbon, oxygen, silica, iron, etc.) to be dispersed throughout galaxies that existed at the time. Those generations that followed, Population II and I, included these elements as part of their composition, as well as hydrogen and helium gas in the ISM.
Understanding stellar formation across generations is vital to understanding cosmic evolution and the mechanisms influencing the birth of new stars. Astronomers also want to learn more about Brown Dwarfs, stellar remnants, and other lesser-understood objects in our Universe to better understand what these mechanisms can produce.
For instance, there’s the GO 7800 program, “An Empirical Benchmark for H2 and PAHs at Extremely Low Metallicity.” Dr. Elizabeth Tarantino, a postdoctoral fellow with the STScI, is this program’s principal investigator (PI). Along with fellow team members Dr. Julia Roman-Duval and Dr. Martha Boyer, Tarantino is part of the Interstellar Medium Group – a collaborative research group focused on the study of interstellar, circumstellar, and circumgalactic media, mainly in nearby galaxies.
The team will rely on the Near-Infrared Spectronomer (NIRSpec) to conduct deep observations of the metal-poor galaxy Leo P. This small, star-forming irregular galaxy is 5.284 billion light-years distant in the constellation Leo. The purpose is to address Webb’s previous discovery of many “impossibly large galaxies” that existed during the early Universe. Since the distances involved prohibit detailed study of these galaxies, the team will examine nearby, metal-poor galaxies that mimic their pristine gas conditions.
As they state in their proposal, these studies will allow astronomers to place tighter constraints on models of star formation and dust content:
“Until recently, both the observations of small dust grains known as polycyclic aromatic hydrocarbons (PAHs) and the primary tracer of molecular gas, CO, have been difficult or impossible to detect in low metallicity galaxies. Now, with the resolution and sensitivity of JWST, we can detect and characterize this faint ISM emission. Recent MIRI-MRS observations of the closest, extremely metal-poor (3% Solar) galaxy Leo P have directly detected molecular hydrogen through the MIR rotationally excited transition S.”
Astronomers will also dedicate observation time to studying what becomes of stars once they reach the end of their life cycle. This includes GO 7806, “The infancy of a supernova remnant,” which will investigate the physics and evolution of Type II supernovae. Led by PI and ESA member Dr. Patrick Kavanagh, an Associate Physics Professor at the National University of Ireland Maynooth, this program will observe SN1987A using Webb’s NIRSpec and Mid-Infrared Imager (MIRI).
Located in the Large Magellanic Cloud (LMC), SN 1987A is the closest remnant of a supernova observed since Kepler’s supernova in 1604. Since it was first detected in 1987, this supernova remnant has undergone significant changes that have provided new insights and discoveries into the evolution of supernovae. Its proximity, alongside Webb’s capabilities, provides astronomers with a unique opportunity to study various aspects of SN physics and evolution.
Type II supernovae result from the collapse and violent explosion of massive stars that have exhausted their hydrogen and helium fuel. In the process, they blow off their outer layers, filling the space around them with metal-rich clouds. Previous JWST observations revealed interactions between this rapidly expanding cloud and the circumstellar disk. They also provided the first clear evidence of the remnant itself. Per their proposal paper:
“The goals of this proposal are to determine the metal-rich ejecta structure and composition in the interaction regions, and monitor the emerging compact object and evolution of dust in the outer ejecta. By Cycle 4, the ejecta and the interaction regions will have evolved substantially in the two years since the last JWST observations. The proposed observations also hold great legacy value for the community. SN 1987A has been observed regularly by all major telescopes since the very beginning, which has created a unique observational record of the evolution of a SN into a SN remnant across the electromagnetic spectrum.”
Webb will also dedicate observing time to 26 Milky Way stars using the MIRI Medium-Resolution Spectrometer (MIRI MRS) as part of the GO 7715 program, “Unique insights into the chemical composition of interstellar silicate dust grains in the Milky Way.” Led by PI Dr. Marjorie Decleir, an ESA Research Fellow with the Space Telescope Science Institute, this program will study the abundance of silicate minerals in these stars.
“Variations in the feature properties (such as its strength, width, and peak wavelength) will reveal variations in the properties of interstellar silicate grains between different sightlines,” the state. “Combining these measurements with existing elemental abundance measurements of Mg, Fe, Si, and O for the same sample of sightlines will provide unprecedented insights into the detailed chemical composition of silicate dust grains in the interstellar medium.”
Stars and Stellar Populations
In addition, Webb’s observation time will be dedicated to addressing outstanding questions about stellar formation and evolution. For example, GO 6821, “Dust, gas, and ice formed before and after stellar mergers,” will investigate the mystery of red novae. These transient objects are created when some non-compact binary stars end their lives in a cataclysmic merger. To date, six red novae have been discovered in the Milky Way, while over a dozen have been spotted in extragalactic space.
Led by PI Tomasz Kaminski, an ESA member and a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center, the team will use MIRI and NIRSpec data to characterize the environment around three red nova remnants that formed from merging giant stars: V1309 Scorpii (Nova 2008), V4332 Sagittarii (Nova 1994) and BLG360 (outburst 2002). This, they say, will shed light on how gas, dust, and minerals coalesce to form stars and systems of planets:
“Silicate and alumina dust features will be used to study dusty disks and dusty ejecta surrounding the coalesced star. Ice mantles, mainly of H2O, will be analyzed to investigate whether they might originated from mass loss preceding the merger. Multiple atomic and ro-vibrational lines (e.g. of CO, H2, OH, FeII, and H2O) will be used to probe shocks in the remnants, which either are a signature of material falling back on the star or manifest interaction of merger ejecta with an older outflow. These JWST observations will provide the most comprehensive view on remnants of stars which merged 1-3 decades ago, providing a benchmark for the study of extragalactic red nova analogs which are expected to be abundant in the Rubin-LSST survey.”
Another interesting program is GO 8872, “The Dark Side of the Force: Unraveling Protostellar Jet Asymmetry by Probing TMC1A’s Fainter Red-shifted Outflow with JWST.” Outflows and winds from young stars play a vital role in the evolution of protostars and the early stages of planet formation. However, the mechanisms behind these phenomena are still debated, including how they affect the protoplanetary disk structure.
To this end, a team led by PI Korash Assani – an Astronomy PhD Candidate at the University of Virginia – will use JWST’s NIRSpec and MIRI MRS to observe the atomic jet and molecular outflow of TMC1A (IRAS 04365+2535), a Class I protostar located 430 light-years away in the Taurus Molecular Cloud (TMC-1). Previously, Webb conducted observations of the blue-shifted jet emanating from one side, where the outflow is moving towards us. They aim to examine the red-shifted jet coming from the opposite side, which is moving away.
These observations will lead to a complete map of the red-shifted jet and outflow, which will provide new insights into bipolar outflow and the role it plays. Per their proposal:
“This will help distinguish between intrinsic (e.g., differential mass loss) and extrinsic (e.g., ambient medium interactions, extinction) explanations for the observed bipolar asymmetries. Leveraging JWST’s sensitivity to probe this deeply embedded system, our study will offer insights into protostellar outflow asymmetries and help constrain launching mechanisms. These findings have implications beyond star formation, as the same launching mechanisms and similar asymmetries in jets/outflows are observed around a wide range of astrophysical systems.”
Then there’s GO 7035, “Brown Dwarfs in NGC 602 in the SMC – An opportunity to characterize a substellar IMF at low metallicity.” Led by PI Peter Zeidler, an STScI Postdoctoral Fellow, the team proposes using NIRCam and NIRSpec to characterize NGC 602, a low-metallicity young star cluster in the Small Magellanic Cloud (SMC). Previous observations by Webb revealed tens of young, low-metallicity Brown Dwarfs, making NGC 602 the only known cluster to contain this class of object.
By obtaining spectra from tens of low-metallicity Brown Dwarfs in this cluster, the team hopes to assemble a unique dataset that will inform future studies of Brown Dwarf formation. As they indicated:
“Only by combining deep NIRCam photometry and NIRSpec MOS is it possible to derive accurate spectral-type-based masses and measure ages better than 0.5 Myr using evolutionary models and spectral templates. This dataset will add an invaluable piece to the puzzle of the formation of sub stellar objects at conditions that are different than Solar, and will address the environmental condition impact on the young star cluster IMF.”
These programs and their goals exemplify the objectives of the James Webb Space Telescope and its mission to investigate the most profound mysteries facing astronomers, astrophysics, astrobiologists, and cosmologists today. This includes tracking the evolution of the Universe since the earliest cosmological epochs but also completing the census of astronomical objects, extrasolar planets, and the physical processes governing them.
Stay tuned for our final installment in the series, where we will examine Cycle 4 GO programs that observe objects in our very own Solar System!
Further Reading: STScI