Calendar

Apr
3
Fri
Remote Colloquium Thesis Defense: Jeremy Emmett
Apr 3 @ 3:15 pm – 4:15 pm
Remote Colloquium Thesis Defense: Jeremy Emmett @ Online

Dependence upon Obliquity of the Formation of Martian PLD Vertical Structure

Jeremy Emmett, NMSU

Mars’ polar layered deposits (PLD) are comprised of layers of varying dust-to-water ice volume mixing ratios (VMR) that are thought to record astronomically-forced climatic variation over Mars’ recent orbital history. Retracing the formation history of these layers by quantifying the sensitivity of polar rates of deposition to astronomical forcing may be critical for the interpretation of this record. Using a Mars global climate model (GCM), we investigate the sensitivity of annual polar water ice and dust surface deposition to a variety of obliquity and surface water ice source configurations at zero eccentricity that may provide a reasonable characterization of the evolution of the PLD during recent low-eccentricity epochs. The GCM employs a fully interactive dust lifting/transport scheme and accounts for dust-and-water physics coupling effects on the transport and deposition of water ice and dust. GCM results suggest that snowfall in the form of water ice-nucleated dust particles generally provides the greatest contribution to both water ice and dust deposition on the polar surfaces, suggesting that dust-and-water physics coupling is an important consideration in the modelling of PLD layer formation processes. Under a range of tested obliquities (15° – 35°), predicted net annual accumulation rates range from -1 mm/yr to +14 mm/yr for water ice and from 0.005 – 0.57 mm/yr for dust. When these GCM-derived accumulation rates are ingested into an integration model that simulates polar accumulation of water ice and dust over five consecutive obliquity cycles (~700 thousand years) during a low eccentricity epoch, select integration model simulations predict combined north polar water and dust accumulation rates that correspond to the observationally-inferred average growth rate of the north PLD (0.5 mm/yr) over its ~5 million year formation history. These integration model simulation results are characterized by net water transfer from the south to the north polar region. In the north, a ~230 m-thick deposit is accumulated over ~700 thousand years. Three types of layers are produced per obliquity cycle: a ~30 m-thick dust-rich (20 – 30% dust volume mixing ratio) layer that forms at high obliquity when both water ice and dust deposition rates are large, a ~0.5 m-thick dust lag deposit (pure dust) that forms at low obliquity when net removal of water ice occurs, and two ~10 m-thick dust-poor (~3%) layers that separate the dust rich layers and form when obliquity is increasing or decreasing. The ~30 m-thick dust-rich layer is reminiscent of a ~30 m scale length feature derived from analysis of visible imagery of north PLD vertical structure. This work demonstrates the capability of obliquity variations to produce PLD stratigraphy reminiscent of observed PLD structure when water and dust deposition are interactively coupled.

May
29
Fri
Remote Colloquium Thesis Defense: Drew Chojnowski
May 29 @ 2:30 pm – 3:30 pm
Remote Colloquium Thesis Defense: Drew Chojnowski @ Online

H-Band Spectroscopy of Exotic, Massive Stars

Drew Chojnowski, NMSU

We report on spectroscopy of exotic B-type emission line (Be) stars and chemically peculiar (CP) stars in the near-infrared (NIR) H-band, using data provided by the Apache Point Observatory Galactic Evolution Experiment, one of the sub-surveys of the Sloan Digital Sky Survey (SDSS). Between 2011-2020, SDSS/APOGEE has observed more than a million stars in the Milky Way Galaxy (MW), with roughly 10% of the targets being hot, blue stars that serve as telluric absorption standard stars (TSS). The TSS are selected mostly on the basis of having blue raw J-K color indices with no preference for any particular spectral type that might be known from optical spectroscopy. This targeting strategy has led to the TSS being a mixed bag, with those observed in the MW Halo typically being F-type stars that are only slightly more massive than the Sun, and with those observed in the MW Disk and Bulge being OBA-type stars of a few up to 20 times the mass of the Sun. While the vast majority of the TSS are superficially normal main sequence stars, the inclusion of large numbers of Be and CP stars has serendipitously resulted in the largest ever homogeneous spectroscopic surveys of these stellar classes, both of which present observational anomalies that remain very poorly understand despite more than a hundred years of research. Prior to SDSS/APOGEE, the H-band spectra of Be and CP stars had only been discussed in a handful of studies, all of which used small numbers of spectra of considerably lower resolution than the R=22,500 of the APOGEE instruments. The material presented in this thesis therefore represents the first ever detailed studies of Be and CP stars in the H-band, while also greatly expanding the known samples through discovery of many hundreds of new examples.