Speaker: Robert H. Rubin ( NASA/Ames Research Center)

Time:  10:00 AM, Oct. 8th (This Wednesday)

Location:  middle conf. room, 3rd floor

Abstract:

    On Earth, neon is not very abundant and perhaps most of us will first associate it with the glitz of neon signs (think Las Vegas). Althouge neon is the fifth most abundant element in the Universe, it has been difficult getting a precise determination. The abundance of Ne in the Sun is particularly uncertain and the subject of an ongoing controversy. By the good fortune of excellent instrument planning, Spitzer's infrared spectrometer (IRS) can measure all the dominant forms of neon (Ne II and Ne III) in H II regions, at the exact same time and area. In the same spectrum, the IRS can do nearly as well measuring the sulfur abundance via its dominant forms in H II regions (S III and S IV).

    Here we discuss our recent work that has added significant new data regarding the Ne/S abundance ratio. We observed 25 H II regions in the substantially face-on spiral galaxy M33 and found that the Ne/S ratio was relatively constant with the median value ~16. In order to determine how much the Ne/S ratio can vary or whether or not there is a fairly 'universal' value, we applied precisely the same methodology to other recently published Spitzer observations. We compare with what is predicted from nucleosynthesis, galactic chemical evolution theory and add our 2 cents to the solar debate.

 Our observations were also used to:

1. find a correlation of higher ionization and lower metallicity with increasing galactocentric distance in M33.

2. test the predicted ionizing spectral energy distribution of various stellar atmosphere models. We compare the ratio of fractional ionizations of the observed Ne and S ions with predictions made from photoionization models using several of the state-of-the-art stellar atmosphere model grids.