My research is based on spectroscopic studies of the interstellar medium (ISM). Many atomic and molecular species can be studied in interstellar space due to their absorption of starlight in either the UV or optical. I focus on diffuse interstellar clouds which lie mostly in the plane of our Galaxy and are mainly composed of neutral hydrogen gas. These clouds have typical densities of 10-100 atoms per cubic centimeter and average temperatures of 30-100 K. By examining interstellar absorption lines, we can determine the physical conditions in individual clouds, their kinematics, and their chemical composition. A description of recent and ongoing projects is given below.

Continuing an investigation began by others (K. Pan, M. Martinez), I studied the interaction taking place between the interstellar gas in the vicinity of the Pleiades and the stars of the open cluster by analyzing optical absorption lines of Ca II, Ca I, CH⁺, CH, and CN in spectra taken at McDonald Observatory (S. R. Federman, D. L. Lambert). Our analysis revealed coherent velocity structure in the atomic and molecular gas likely resulting from the encounter with cluster stars. From our measured molecular abundances, we determined the density of the gas to be approximately 50 atoms (and molecules) per cubic centimeter. Our paper appeared in the October 1, 2006 issue of The Astrophysical Journal and can be viewed here.

In an ongoing project, I will measure the boron abundance in diffuse interstellar clouds in an effort to identify the source of light element synthesis. The resonance line of the B II ion lies in the UV and cannot be observed from earth. Thus, we will use data obtained by the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) in a survey of over fifty sight lines which span all 360^o on the sky in the plane of the Galaxy and along some directions above and below the Galactic plane. Some of our aims will be to examine the overall level of boron depletion onto grains as a function of gas density and to deduce the importance of neutrino-induced spallation in core-collapse supernovae (SNe II) in synthesizing boron in the interstellar medium.

Using recently acquired data from McDonald Observatory, I will be measuring the ambient carbon isotope ratio (¹²C/¹³C) toward stars in the Pleiades from the ratio of ¹²CH⁺/¹³CH⁺. The carbon isotope ratio in the ambient interstellar medium imposes an important observational constraint on models of Galactic chemical evolution because the abundance of secondary elements like ¹³C should increase with time relative to primary elements like ¹²C. This investigation will either confirm or refute previous measurements of the ¹²CH⁺/¹³CH⁺ ratio toward the Pleiades which suggested a lower value than that observed toward other local clouds, an important result which bears on the small-scale homogeneity of the ISM.

Observing Time

• McDonald Observatory (3 Nights)
• 2.7 m telescope, 2dcoude spectrograph, (R = 175,000)
• The Interstellar ¹²C/¹³C Ratio toward the Pleiades

Adam Ritchey / aritchey@physics.utoledo.edu

Last Modified: 18 Feb 2007