Recent Presentations

Many of these presentations involve student research. You can get information about student research in the Department of Physics and Astronomy here at Swarthmore College. You can also see what else my research group is up to.

Refereed papers are archived.

I gave a talk at the 19th High Energy Astrophysics Division meeting of the AAS in Pittsburgh (though I delivered the talk remotely) in March 2022. This talk — "Chandra grating spectroscopy finds a surprising change in the wind mass-loss rate of the O supergiant zeta Puppis over 18 years" — presents recent results, obtained by Jiaming Wang ('22), on the line-profile derived mass-loss rate of zeta Pup (see MNRAS, 499, 6044 (2020)) along with new, preliminary results on a broadband X-ray absorption derived mass-loss rate, based on work done by Gwendolyn Rak ('22), which agrees well with the profile diagnostic.

I gave the department colloquium in November 2016, "The Radiation-Driven Winds and X-ray Emission of Massive Stars" [pdf].

In March 2016, I gave a talk on massive stars and their X-ray emission [pdf] to the Delaware Valley Amateur Astronomers.

I gave the physics colloquium at the Royal Military College of Canada, in Kingston, Ontario in February 2016. My talk was titled, "X-rays from the Winds of Massive Stars" [pdf].

In fall 2015, I gave a talk at the Department of Physics and Space Sciences at Florida Tech. The talk was an overview of massive star X-ray emission: X-ray production via embedded wind shocks, X-ray mass-loss diagnostics, and magnetic massive stars [pdf].

I gave an invited talk on X-ray spectroscopy of stellar winds [pdf] at the Universe in High-Resolution X-ray Spectra at Harvard in August 2015.

I gave a talk at the Harvard CfA Solar, Stellar, and Planetary Sciences Division's weekly seminar on the new work I did with Zack Li on using X-ray spectra to determine the O star wind shock heating rate [pdf].

For the festschrift of my friend Huib Henrichs, in Amsterdam in September 2014, I gave an invited talk on new X-ray observations of the prototype magnetic O star, theta1 Ori C [pdf].

I gave the department colloquium in February 2013 on the use of X-ray line profiles to diagnose X-ray production, wind clumping, and mass-loss rates in O stars [pdf].

I presented our group's results on using X-ray spectroscopy to constrain O star wind mass-loss rates, clumping, and shock physics at the Universities of Iowa [pdf] and Wisconsin [pdf] in March, 2012. In these talks, I showed the results from a 1-D radiation-hydro simulation done by my colleague Jon Sundqvist. I presented a similar talk at the Hamburg Sternwarte in October 2012 [pdf].

I gave an invited talk on X-rays production in massive star winds [pdf] at the meeting on Circumstellar Dynamics at High Resolution at Foz do Iguacu, Brazil in February 2012.

My colleagues, Stan, Jon, and Maurice and I have put together a new talk, on clumping and porosity [pdf]. I gave it in October, 2011, at the University of Liège, while visiting Yaël Nazé's group.

During October 2011, I gave a colloquium at the Institute Anton Pannekoek, the astronomy department, at the University of Amsterdam. The talk provided an overview of our X-ray based wind mass-loss rate measurements, using both individual X-ray emission line profile shapes and also the broadband X-ray spectral energy distributions [pdf].

At the meeting to honor Tony Moffat, in Lac Taureau, Quebec, I gave a talk on HD 93129A, one of the most massive stars in the Galaxy [pdf of talk and pdf of paper].

At the Winter 2011 American Astronomical Society meeting in Seattle, I gave an invited talk in the special session on High Energy Processes in Star Formation, entitled "X-rays from Massive Stars" [pdf or pptx].

IAU Symposium 272: Active OB Stars was held in Paris in July 2010. I gave a contributed talk entitled "X-ray Spectral Diagnostics of Activity in O and Early B Stars" [pdf or pptx]. This talk grew out of a longer presentation I gave at the Massive Stars and Starbursts journal club at the Space Telescope Science Institute in May.

I gave a solicited talk at the X-ray Spectroscopy: Past, Present, and Future meeting in Utrecht, The Netherlands, in March 2010. The title of the talk was "Line Shapes in Hot Stars: Hydrodynamics and Mass-Loss Rates" [pdf or pptx] and I discussed how the resolved X-ray emission lines of hot stars can be used to test wind shock theory and measure wind mass-loss rates. I also briefly discussed the lack of evidence for significant porosity in the winds of hot stars, and touched on resonance scattering, which was discussed in depth by Maurice Leutenegger in the talk following mine.

In November 2009, the Department of Energy sponsored a Research Needs Workshop (ReNeW) on High Energy Density Laboratory Plasmas in Rockville, Maryland. I was invited to give a half-hour plenary lecture on connections between laboratory and astrophysical plasmas, with an emphasis on photoionized plasmas. [pdf or ppt - note that slides 31, 35, and 36 contain animated gifs which will not play in the pdf file.]

Fall 2009: I gave colloquia at the University of Rochester [pdf] and Lehigh University on X-rays from massive stars [pdf].

I presented an overview of X-rays and magnetic massive stars, focusing on θ¹ Ori C, at the MiMeS2 meeting in Paris in May 2009: "X-ray Diagnostics and Their Relationship to Magnetic Fields." The simulation movie linked on the left, below, is courtesy of Rich Townsend and Asif ud-Doula.

I attended the Atomic Processes in Plasmas meeting in Monterey, where I gave a talk on "X-ray Spectroscopy of the Radiation-Driven Winds of Massive Stars: Line Profile and Line Ratio Diagnostics." My students Erin Martell and Emma Wollman presented a poster on "Photoelectric Absorption as an X-ray Spectral Diagnostic of Massive Star Winds."

In March 2009, I gave a short faculty lunch talk at Swarthmore, on our discovery of a binary companion to beta Crucis, touching on several different aspects of the connection between astronomy and culture. It might be the only talk given at the College this year that included a cocktail menu. [big pdf file]

I gave a "pre-colloquium" talk in the Physics and Astronomy Department at Swarthmore to provide some background for Gail Glendinning's upcomong colloquium in October 2008. This talk touched on inertial confinement fusion, laboratory astrophysics, the new National Ignition Facility, and included this nifty movie demonstrating the Rayleigh-Taylor instability.

In September 2008, I presented my and my students' research results on X-rays from Massive Stars at our departmental colloquium, at Swarthmore.

I gave a very brief presentation on my research interests at the Astro Philly 2008 meeting at Widener University.

IAU 250: Massive Stars as Cosmic Engines met in Kauai in December, 2007. I gave a talk entitled "X-ray Emission from O Stars." And in the 'specialist session' the day before the meeting, on Magnetic Hot Stars, I gave a talk on "X-rays from Magnetically Channeled Winds of OB Stars."