Trevor PicardGraduate Student NMSUAstronomy
As an undergraduate at the University of Michigan, I studied the chemistry and other properties of the interstellar medium under the tutelage of the honorable Dr. Nick Indriolo. I analyzed mid-infrared absorption spectra from the Very Large Telescope (among other sources) looking for signs of CO molecules in diffuse clouds. The basic process to find CO is to point the telescope at the bright star, record its spectrum, and identify the particular wavelengths where the star’s light was absorbed due to quantum transitions of the electrons in the cloud’s molecules. I specifically observed ro-vibrational transitions of CO due to collisions on very tiny scales. CO is the second most abundant component of molecular clouds, but the vast majority of molecules in the universe are two bonded hydrogen atoms. Imagine just a few CO molecules bouncing around within the clouds and colliding with the vast quantities of molecular hydrogen. Electrons in the CO absorb the light’s energy from the background star and move up in energy levels, which creates the absorption in the spectrum that we see.
Since so much of the interstellar medium is molecular hydrogen, we need to estimate the density of molecular hydrogen in these clouds in order to properly understand interstellar chemistry. However, hydrogen is a homonuclear molecule, making it very difficult to directly observe directly. Instead, my job was to relate how much light was absorbed by CO collisions with molecular hydrogen in each cloud, which tells us something how dense each cloud is. The estimated densities I derived are useful for other astronomers who need them to study the various other properties of the interstellar medium, how clouds collapse to form stars, and many other applications.
I have expanded my research as a graduate student at New Mexico State University to include the study the chemistry and kinematics of stellar populations within our galaxy, which will potentially reveal the details of how galaxies form and evolve. In particular, I am studying the stellar spectra of RR Lyrae variable stars from the NMSU 1-meter telescope at Apache Point Observatory. RR Lyrae are old, metal poor stars on the horizontal branch of the HR diagram that also occupy the instability strip, where instabilities in the stars cause them to pulsate in size and in brightness; each full cycle of pulsation is called a phase. The central bulge of the Milky Way is a very old structure that is composed of many RR Lyrae stars, so observing the dynamics of these stars gives us a glimpse into the ancient beginnings of our galaxy. Studying where theses stars are and how they move allows us to trace their movements into the past, revealing how the galaxy looked in its infancy.
The ultimate goal is to determine the 3D positions and 3D motions of the RR Lyrae stars in the bulge. We already have most of this information from photometric surveys like the Optical Gravitational Lensing Experiment (OGLE), including accurate positions, distances, and proper motions. However, radial velocities can only be determined spectroscopically, so to derive the final component of RR Lyrae dynamics we need large scale spectroscopic surveys like the Sloan Digital Sky Survey. The second generation of the Apache Point Observatory Galactic Evolution Experiment at Las Campanas Observatory (APOGEE-2S) will do just this; we will soon have spectra for thousands of RR Lyrae in the bulge. The radial velocity of stars at the time they were observed can then be derived for a large fraction of RR Lyrae in the bulge.
Accounting for the pulsations of RR Lyrae is a big challenge when studying the dynamics of the bulge. The radial velocities at the times of observation are not indicative of the systemic radial velocities; first, the motion of pulsation must be removed. I have expanded upon the work of other colleagues by creating radial velocity templates as a function of phase, which can be stretched and shifted to match the radial velocity derived from each spectrum. The template can then be used to easily determine how the star is actually moving along the line of sight, regardless of how it is pulsating. These templates will be powerful tools for studying the dynamics of the oldest structure in the Milky Way.
- 227th AAS Meeting (Kissimmee, FL)
- SDSS-IV Collaboration Meeting (Madison, WI)
- APOGEE-2 Team Meeting (Pasadena, CA)
- APOGEE-2 Splinter Meeting (remote participation; Santiago, Chile)