Massive Stars

My research interests are varied. However, I spend about 50% of my research time working on the problem of massive star formation. Over the past few years, I have devised a spectroscopic method (briefly illustrated here) which allows more precise measurements to be made of massive stars which are usually too heavily shrouded at optical wavelengths to be properly studied (Hanson 1995; Hanson, Conti & Rieke 1996, Hanson, Luhman & Rieke 1998a). This technique allows me to directly study and determine the physical characteristics of very young massive stars while they are still heavily embedded within the molecular cloud they just formed from (Hanson, Howarth & Conti 1997; Watson & Hanson 1998).

I am currently surveying over 200 galactic ultra-compact HII regions in our galaxy, to find central stellar sources which can be directly studied at 2 microns (Hanson, Luhman & Rieke 1998b). The paper on this first study in the Northern Hemisphere, using the Multiple Mirror Telescope, MMT), includes 63 UCHII regions and recently appeared in the Astrophysical Journal (Hanson, Luhman & Rieke, 2002). I am also studying the Southern Hemisphere, using the European Southern Observatory's (ESO) telescopes in the South (the 3.6-m New Technology Telescope, NTT). My collaborators, Lex Kaper (Univ. Amsterdam) and Fernando Comeron (ESO, Munich) and I have also been awarded time on the 8.2-meter diameter Very Large Telescope (VLT), the flagship telescope of ESO, for this project. With the VLT, we are obtaining unprecedented high resolution and signal-to-noise spectra, useful in modeling the physical characteristics of extremely young, heavily embedded massive stars.

Most recently, a vital addition to this program is the use of the non-LTE "Unified Model Atmosphere" code of the Munich University Observatory, obtained through a collaboration with Dr. Joachim Puls. If you can read German, you can learn about the code here. The program with Dr. Puls is far more encompassing than our initial pursuit, to model the physical characteristics of massive stars at the moment of birth. Most importantly, reliable quantitative models for hot stellar atmospheres at near-infrared wavelengths can be applied to any hot star through out the galaxy. Because of the enormous potential of such a method, I was recently given an NSF CAREER award for the development, testing and application of near-infrared atmospheric modeling.

Sub-mm Molecular Cloud Studies

In the Fall of 1999 I began a new research program using the Heinrich Hertz Sub-millimeter Telescope. This telescope, located at 10,600 ft elevation, on Mt. Graham, 75 miles NE of Tucson, AZ, is part of the Mt. Graham International Observatory, and is partially run by the University of Arizona. My graduate student, Ms. Hemamala Uswatte, worked closely with Dr. Tom Wilson, Director of the SMT, a fellow collaborator on the program. They observed the M17 Giant Molecular Cloud region to study very early processes of massive star formation. This program is funded through an NSF POWRE Grant. The millimeter CO and CS maps of the clouds have been obtained and their full reductions are complete. Our paper, presenting these results, Wilson, Hanson \& Muders (2002), is nearly complete.

Massive X-Ray Binary Studies

I have recently completed a long term project to study the highly variable, X-ray and gamma-ray binary source, Cyg X-3. This work is in collaboration with Drs. Rob Fender of the University of Amsterdam, and Guy Pooley of Cambridge University. In our first of two papers, we present 2 years worth of monitoring at X-ray and Radio fluxes set against near-infrared spectral characteristics of the system (Fender, Hanson & Pooley 1999). In this paper we propose a unique disk-wind model to explain the spectral variations seen (model). The near-infrared spectra were taken with the MMT on Mt. Hopkins, about an hour south of Tucson, AZ. In our second paper, Hanson, Still and Fender (2000), we apply tomographic techniques for devising the physical structures of the binary wind to test the predictions outlined in the first paper. This also allowed us to constrain, for the first time, the mass function for the Cyg X-3 binary system. This second paper on Cyg X-3 is in collaboration with Dr. Martin Still of Goddard's Space Flight Center.

A new collaboration with Dr. Robert Ogley (Saclay, France), intent on determining the nature of the high mass X-ray binary system, CI Cam, is underway using the University of Arizona's Bok Telescope. CI Cam, formerly known as nothing more than a mere B emission line star for more than 60 years, has recently been associated with a bright transient X-ray burst, known as XTE J0421+560. During the X-ray Burst, CI Cam brightened by several magnitudes: definite proof it is associated with the X-ray burster. It is assumed that CI Cam, a more or less normal Be star, is in orbit with either a black hole or neutron star, which causes the Be star to flare when they come in close contact. We began our near-infrared spectroscopic monitoring of the system in Fall 1999, and obtained additional data in Fall 2000, since there is evidence to suggest the system has suffered yet another outburst. My undergraduate research assistant, Lara Mercurio, is working on these data.

Extragalactic Starbursts

Working with researchers at Louisiana State (Dr. Geoff Clayton), and the University of Arizona (Drs. Karl Gordon and George Rieke), we have recently completed a study the stellar populations seen in starburst galaxies (Gordon, Hanson, et al. 1999). In this collaboration, I bring expertise in near-infrared spectroscopic observations, using the University of Arizona's Bok Telescope. My goal is to bridge together modeling projects that use optical techniques with those that use near-infrared techniques to better estimate ages, mass function, and the star formation history for stellar clusters found in distant galaxies.