Calendar

Sep
12
Wed
Colloquium PhD Thesis Defense: Alexander Thelen (Host: Nancy Chanover)
Sep 12 @ 3:00 pm – 4:00 pm
Colloquium PhD Thesis Defense: Alexander Thelen (Host: Nancy Chanover) @ Domenici Hall Room 102

The Chemical Composition and Dynamics of Titan’s Atmosphere as Revealed by ALMA

Alexander Thelen, NMSU

Over the last century, remarkable advances in our understanding of Titan’s atmosphere have been accomplished by a campaign of ground- and space-based observations revealing a wealth of complex, organic species in the moon’s upper atmosphere. Many of Titan’s atmospheric constituents produced through the photochemistry and ionospheric interactions of N2 and CH4 exhibit significant variations with latitude and time, particularly towards the poles and within the winter circumpolar vortex. The measurement of spatial and temporal variations in Titan’s atmosphere enables us to elucidate connections between its dynamics, photochemistry, and the influence of seasonal changes. At the end of the Cassini mission in 2017, we can employ the Atacama Large Millimeter/submillimeter Array (ALMA) for future observations of Titan’s atmosphere. Here we detail the analysis of numerous short integration (~3 minute) ALMA observations from 2012 to 2015 to investigate Titan’s stratospheric composition, temporal variations, and search for new molecular species. Using the Non-linear optimal Estimator for MultivariatE spectral analySIS (NEMESIS) radiative transfer code, we retrieved vertical profiles of temperature and abundance in Titan’s lower stratosphere through mesosphere (~50–550 km) from three spatially independent regions. We modeled CO emission lines to obtain temperature measurements, and retrieved abundance profiles for HCN, HC3N, C3H4, and CH3CN. The combination of integrated flux maps and vertical atmospheric profiles from spatially resolved observations allowed us to study the circulation of Titan’s middle atmosphere during northern spring. We observed increased temperatures in Titan’s stratopause at high northern latitudes and a persistent northern enrichment of HCN, C3H4, and CH3CN during this epoch; however, increased abundances of all molecules in the southern mesosphere, particularly HCN, and spatial maps of HC3N also show evidence for subsidence at the south pole. We validated these measurements through direct comparisons with contemporaneous Cassini data, previous ground-based observations, and photochemical model results. While no new trace species were detected, ALMA has proven to be a highly capable asset to enhance the data from the final few years of the Cassini mission, and for the continued study of Titan’s atmospheric dynamics, composition, and chemistry into Titan’s northern summer.

Oct
6
Sat
Cume #427
Oct 6 @ 9:30 am – 12:00 pm
Oct
27
Sat
Cume #428
Oct 27 @ 9:30 am – 12:00 pm
Nov
17
Sat
Cume #429
Nov 17 @ 9:30 am – 12:00 pm
Jan
19
Sat
Cume #430
Jan 19 @ 9:30 am – 12:00 pm
Jan
23
Wed
Colloquium Thesis Defense: Lauren Kahre
Jan 23 @ 3:00 pm – 4:00 pm
Colloquium Thesis Defense: Lauren Kahre

Extinction Mapping and Dust-to-Gas Ratios of Nearby Galaxies

Lauren Kahre, NMSU

We present a study of the dust{to{gas ratios in 31 nearby (D >
10 Mpc) galaxies. Using Hubble Space Telescope broad band WFC3/UVIS UV and
optical images from the Treasury program LEGUS (Legacy ExtraGalactic UV
Survey) combined with archival HST/ACS data, we correct thousands of
individual stars for extinction across these galaxies using an
isochrone-matching (reddening-free Q) method. We generate extinction maps
for each galaxy from the individual stellar extinctions using both
adaptive and fixed resolution techniques, and correlate these maps with
neutral HI and CO gas maps from literature, including The HI Nearby Galaxy
Survey (THINGS) and the HERA CO-Line ExtraGalactic Survey (HERACLES). We
calculate dust-to-gas ratios and investigate variations in the dust-to-gas
ratio with galaxy metallicity. We find a power law relationship between
dust-to-gas ratio and metallicity. The single power law is consistent with
other studies of dust-to-gas ratio compared to metallicity, while the
broken power law shows a significantly shallower slope for low metallicity
galaxies than previously observed. We find a change in the relation when
H_2 is not included. This implies that underestimation of N_H2 in
low-metallicity dwarfs from a too-low CO-to-H2 conversion factor X_CO
could have produced too low a slope in the derived relationship between
dust-to-gas ratio and metallicity. We also
compare our extinctions to those derived from fitting the spectral energy
distribution (SED) using the Bayesian Extinction and Stellar Tool (BEAST)
for NGC 7793 and and systematically lower extinctions from SED-fitting as
compared to isochrone matching. Finally, we compare our extinction maps of
NGC 628 to maps of the dust obtained via IR emission from Aniano et al.
(2012) and find a factor of 2 difference in dust-to-gas ratios determined
from the two maps, consistent with previous work.

Feb
16
Sat
Cume #431
Feb 16 @ 9:30 am – 12:00 pm
Mar
16
Sat
Cume #432
Mar 16 @ 9:30 am – 12:00 pm
Apr
6
Sat
Cume #433
Apr 6 @ 9:30 am – 12:00 pm
Apr
27
Sat
Cume #433
Apr 27 @ 9:30 am – 12:00 pm