Special Talk

• Special Talks

  Speaker: Matthew Burger (STScI)
  Title: Exospheric Escape from a Bare-Rock Planet Orbiting a Solar-Type Star
  Abstract: Surface-bounded exospheres, atmospheres that are collisionless down to the planetary surface, are one of the most common types of atmospheres in this solar system, and are likely common in in exoplanetary systems. These atmospheres are in a constant state of production and loss due to the short time scales for loss. Common source processes in surface-bound exospheres include thermal desorption, photon-stimulated desorption, micrometeoroid impact vaporization, and sputtering by high-energy stellar wind or magnetospheric ions. Loss is dominated by photoionization, interactions with stellar wind or magnetospheric plasma (e.g., charge exchange or electron impact ionization), or radiation pressure which accelerates material radially away from the star out of the planet’s exosphere. 

  The sodium component of Mercury’s exosphere was discovered from the ground over forty years ago (Potter & Morgan 1985,1986), and was more recently observed on a near-daily cadence from Mercury orbit by the MESSENGER spacecraft. Sodium emission is due to resonant scattering where solar photons at the resonant wavelength in the rest frame of the atom are absorbed and immediately reemitted isotropically. This process also imparts momentum to the sodium pushing it back into a comet-like tail pointed away from the Sun. Because of Mercury’s elliptical orbit and Na absorption in the Sun’s atmosphere, the magnitude of radiation acceleration varies by an order of magnitude over a Mercury year. I will discuss observations and models of exospheric escape of sodium from Mercury, highlighting seasonal variations in the distribution of sodium around the planet, the sodium escape rate, and observed radiance.

  Speaker: Adarsh Ranjan (STScI)
  Title: From Metal-rich Spirals to Pristine Dwarfs: A Resolved View of Star-Forming Galaxies in the Local Universe
  Abstract: Interpreting the spectra of galaxies across cosmic time requires connecting the detailed physics of nearby resolved star-forming regions with the integrated light observed from distant galaxies. This has become increasingly important with JWST, where ultraviolet emission lines often provide the primary constraints on metallicity, ionization, and stellar populations.

  In this talk, I will present a multi-wavelength study of nearby star-forming galaxies spanning nearly two orders of magnitude in metallicity. In the metal-rich spiral galaxy M83, I combine HST far-UV absorption spectroscopy with spatially matched optical spectroscopy to investigate how young star clusters enrich the surrounding neutral and ionized interstellar medium, revealing the roles of stellar feedback, chemical mixing, and galactic environment. At the opposite extreme, I study multiple resolved H II regions in the extremely metal-poor star-forming galaxy DDO 68 to test ultraviolet emission-line diagnostics under conditions analogous to those in the early Universe.

  These complementary systems demonstrate how local variations in metallicity, feedback, and ionization shape the integrated spectra of galaxies and highlight the challenges of deriving reliable elemental abundances when individual H II regions cannot be resolved. Together, they provide a framework for improving the interpretation of galaxy spectra from the nearby Universe to cosmic dawn.

  Speaker: James Muzerolle Page (STScI)
  Title: Multi-epoch NIRSpec/IFU Observations of Periodic Protostellar Accretion Outbursts
  Abstract: The accretion history of young stars is a critical component of star and planet formation. Time-dependent accretion outbursts are thought to be an important if not dominant contributor, though the mechanisms responsible are not well-understood. I will present new JWST/NIRSpec IFU observations of a protostar that exhibits large periodic flux outbursts, which enable real-time characterization of the outbursts and their effect on the circumstellar material. Initial results suggest that the outbursts originate in the inner disk of the system, producing increased continuum flux and enhanced mass loss through disk winds, and then propagate inward onto the star where they manifest through stellar accretion signatures. Possible outburst mechanisms capable of explaining these features include magnetospheric corotational instability and companion-modulated accretion. The latter may indicate a connection with planet-mass triggering of disk instabilities, which has often been invoked to explain other types of outbursting objects such as FUOrs and EXOrs.