HOT AND COOL: BRIDGINGGAPS IN MASSIVE-STAR EVOLUTION ASP Conference Series, Vol. 425, c 2010 C. Leitherer, P. D. Bennett, P. W. Morris, and J. Th. van Loon, eds. Infrared Interferometry of the Upper Hertzsprung-Russell Diagram John D. Monnier University of Michigan, Astronomy Department, Ann Arbor, MI 48109, USA Abstract. The new generation of optical and infrared interferometers have unprecedented angular resolution and sensitivity. I will review recent advances in studies of massive objects in the upper portion of the Hertzsprung-Russell dia- gram, including fundamental parameters of individual red and blue supergiants, first interferometric imaging of interacting binaries, and new views of dust and gas in strong stellar winds. We note that massive stellar evolution is still usually discussed in the context of single star evolution, despite mounting evidence that binarity is commonplace and likely lies at the root of many observed phenomena. 1. Introduction Massive stars present many challenges to our understanding of stellar evolution. These challenges lie in three major areas: fundamental stellar parameters, mass transfer in interacting binaries, and mass-loss mechanisms. All three areas are still poorly understood, thus the theory of massive stellar evolution rests on a shaky foundation. One way to make progress is to study individual objects using our best techniques in order to elucidate the physical mechanisms at play. For this, infrared and optical interferometry is potentially powerful to measure and image stars and their surroundings with amazing angular resolution. Consider the following examples: • A nearby hot star (10 R⊙) at 200 pc is only 0.2 milliarcseconds across. • A nearby cool (massive) star (diameter 5 AU) at 200 pc is 25 milliarcsec- onds across. • The 50 AU dust shell around a close-by Wolf-Rayet star at 1500 pc is 33 milliarcseconds. • The inner wind at 10 AU around Eta Carina (2300 pc) is about 4 milliarc- seconds. You can see from this list that even the closest (and thereby prototypical) mas- sive stars present serious imaging challenges for single telescopes. The required angular resolution demands a 50+m-diameter telescope operating at the diffrac- tion limit—a prospect beyond even our most ambitious projects today. Fortu- nately, this level of angular resolution is routine for existing infrared interfer- ometers and I will be reporting a wide range of important work from today’s facilities in this review. 257 258 Monnier I will end each section of this review with a provocative question or two suggested by recent interferometry work. 2. Interferometry This review will not discuss the technical details of optical and infrared inter- ferometry. Please see review in Monnier (2003) for more information on this topic. Here, I did want to list and briefly describe the facilities that will be men- tioned in this review. In order of appearance: 1. COAST (Cambridge Optical Aperture Synthesis Telescope). This facil- ity was based in Cambridge, England, and was a pioneer in developing long-baseline interferometric techniques, most noted for the first aperture synthesis image (Baldwin et al. 1996). The array used four telescopes with baselines typically less than 50 m, and observed binaries and evolved stars. This facility is now closed, and the principals are working on the next generation facility: the Magdalena Ridge Optical Interferometer (MROI, Creech-Eakman et al. 2008). 2. IOTA (Infrared-Optical Telescope Array) was located at Mt. Hopkins, AZ, and was operated by the Smithsonian Astrophysical Observatory with ma- jor contributions from University of Massachusetts at Amherst (Traub et al. 2003). This system observed mainly in the near-infrared (H and K bands) and had a longest baseline of 38 m. First imaging with the three-telescope system was reported in 2004 (Monnier et al.). This facil- ity is now closed. 3. CHARA (Center for High Angular Resolution Astronomy) array is a six- telescope interferometer located on Mt. Wilson, CA. The array was built and is operated by Georgia State University (McAlister et al. 2005; ten Brummelaar et al. 2005). CHARA boasts the largest baselines in the world, maximum of 330 m baselines, and has instrumentation that work from visible wavelengths to K band. 4. ISI (Infrared Spatial Interferometer) is a mid-infrared, heterodyne inter- ferometer also located on Mt. Wilson, CA (Hale et al. 2000). ISI has three telescopes with commissioned baselines up to about 85 m. 5. NPOI (Navy Prototype Optical Interferometer) is located in Flagstaff, AZ, and is operated by the Naval Research Lab and the Naval Observatory (Armstrong et al. 1998) NPOI specializes in visible-wavelength observa- tions and has used six telescopes simultaneously for imaging. 6. SUSI (Sydney University Stellar Interferometer) is the only visible-light interferometer in the southern hemisphere (Davis et al. 1999). SUSI uses two telescopes at a time, with baselines up to ∼100 meters. IR Interferometry of the Upper HRD 259 7. VLTI is the Very Large Telescope interferometer (Haguenauer et al. 2008), located on Paranal, Chile, and is famously part of the European South- ern Observatory. The VLTI can be used with any of the four 8-m unit telescopes or any of three 1.8 m auxiliary telescopes. VLT currently has a near-infrared three-beam combiner with spectral capabilities (AMBER) and a mid-infrared two-beam combiner (MIDI). The longest baseline is usually around ∼120 m, although longer baselines are possible with the ATs. 8. PTI was the Palomar Testbed Interferometer (Colavita et al. 1999), located at Palomar Observatory, CA. PTI worked primarily at K band, observing a single baseline with any two of three telescopes in the array. The longest baseline was around 110 m. This facility closed in early 2009. 3. Red Supergiants Pioneering observations with single telescopes using speckle, rotation-shearing, and aperture masking (e.g., Lynds et al. 1976; Roddier & Roddier 1983; Buscher et al. 1990) have found that the surfaces of red supergiants are mottled with hotspots which vary on timescale of many months (Tuthill et al. 1997). These changing hotspots are now ascribed to the motions large convective elements on the surface (Schwarzschild 1975; Freytag et al. 2002). Using near-simultaneous, multi-wavelength data, the COAST interferome- ter found that the hotspots on Betelguese were markedly more pronounced at wavelengths probing the cooler molecular layers (Young et al. 2000). This led to the idea that the spots were generated by opacity structures (“holes”) in upper molecular layers. While opacity variations in the upper layers may be crucial in the optical regime, more recent work has found unmistakable spot structures in the H band likely resulting from true temperature differences on the continuum photosphere (with IOTA: Haubois et al. 2006, with CHARA: Kiss et al., these proceedings). Another major advance has been the discovery of an optically-thick water layer around cool stars (miras and red supergiants). Observers using IOTA (Per- rin et al. 1999, 2004), Keck Masking (Tuthill et al. 1999b; Woodruff et al. 2009), and ISI (Weiner et al. 2000) have reported very large diameter variations in and out of the near-infrared molecular bands and also at 11.15 µm. Essentially, at wavelengths where the water vapor absorbs infrared light, the measured angu- lar diameter is much larger than in the stellar continuum. Modeling this effect reveals a thick layer of warm molecular gas at about 1.5 R⋆. This idea was first suggested by Tsuji (2000) based on ISO data and water line lists have been used to self-consistently explain the wavelength-dependent diameter data and some line profiles (Ohnaka 2004; Weiner et al. 2003). These major results have yet to be incorporated in modern atmsopheric models. Another interesting result was that the diameter of the extreme red super- giant VY CMa was measured using a combination of IOTA + Keck data, which allowed for the correction of the dust emission. VY CMa has a K-band diam- eter of 21 AU, making it the largest known star (Monnier et al. 2004b). This large size could be affected by the aforementioned water layer, and more work is needed to understand if the continuum size is smaller. 260 Monnier Open Questions: • Is spot intensity on red supergiant photospheres related to a star’s propensity for peculiar mass ejections seen as detached dust shells (e.g., α Ori, α Sco; Danchi et al. 1994)? • Does an interferometric temperature scale for red supergiants (corrected for molecular blanketing) match up with the recent hotter scale of Levesque et al. (2005)? 4. Blue Supergiants Not surprisingly, blue supergiants are much smaller in physical size than red su- pergiants; they are nearly impossible to resolve with even today’s most powerful interferometers. While there are few results to report, we note that the NPOI optical interferometer will soon debut longer baselines, and the CHARA inter- ferometer recently commissioned two new visible instruments (VEGA, Mourard et al. 2008, and PAVO, Ireland et al. 2008). At visible wavelengths, CHARA will have approximately 0.2 mas angular resolution sufficient to resolve a handful of blue supergiants. Here we report mainly one interesting result. The nearby blue supergiant Deneb was resolved by the CHARA interferometer by Aufdenberg et al. (2006). They found the object was slightly elongated, implying a rotation rate of 65% of breakup. This model suggests the pole is 10% hotter than the equator, imply- ing that the apparent luminosity and temperature is viewing-angle dependent. These issues lead to an open question: Open Question: • How important is rapid rotation to consider for Blue Supergiants (e.g., latitude-dependence of winds, true location on H-R dia- gram)? 5. Transitioning Supergiants from Red to Blue While finding nearby examples of red or blue supergiants is difficult enough, we have to search even farther away to find the rare specimen of a transitioning object.