JAMES E. NEFF
One of the most fundamental questions in solar/stellar physics is the physical nature of the hot, thin plasma forming outer atmospheres (i.e. chromospheres and coronae) of late-type stars. Why do they exist at all? What is their energy source? How is the energy transferred from the stellar interior? How is it deposited into discrete magnetically active regions, and how are these regions confined?
Stellar outer atmospheres, like the Sun's, are spatially inhomogenous and highly dynamic. Nevertheless, nearly all atmospheric models have been based on the assumptions of plane-parallel, homogeneous, static atmospheres. Such models are essentially useless, because these spatially-averaged properties do not represent any regions that are actually present in the atmospheres. In order to derive physically meaningful atmospheric models, stars must first be spatially resolved. Only by measuring the atmospheric structure and the conditions within magnetically active regions can we uncover the true mechanisms responsible for structuring and heating outer stellar atmospheres.
In my Ph.D. thesis and in subsequent work I developed a technique to image stellar chromospheres using a series of high-resolution ultraviolet emission line spectra of active stars. These techniques permit us to simultaneously probe the spatial structure and the dynamical properties of stellar atmospheres. I applied this technique to IUE spectra of AR Lacertae obtained in 1981, 1983, 1985, 1987, 1989, 1991, and 1994. The analysis of the 1983 and 1985 data sets formed the bulk of my Ph.D. thesis. I have combined these results into a movie showing the characteristics and spatial distribution of active regions, and I am pulling together all of these data into a definitive series of papers describing the evolution of activity in this system.
New data from the Hubble Space Telescope permits this imaging techniqe to be applied to other lines, emitted from different atmospheric regions. For example, we observed AR Lac with HST in 1991 both at Mg II (for intercomparison with the IUE data) and at C IV, which arises from the top of the transition region. In September 1993, I obtained (in collaboration with R. Dempsey, STScI) HST/GHRS spectra of density-sensitive lines (Si III and C III), along with C IV and Mg II, in order to map the transition region plasma density across the surface of HR 1099. We recently observed V824 Ara for 26 HST orbits while it was in HST's ``continuous viewing zone'', permitting nearly continuous observations throughout the system's 1.8 day cycle.
I have also organized multiwavelength observing campaigns in order to apply this technique to other stars (HR 1099, TY Pyx, EI Eri, and HD 199178). All of these observations were obtained in conjunction with other ground- and space-based observations. Such data sets, when combined, will permit us to study the structure of stellar atmospheres in three-dimensions, from the photosphere through the corona.
I also have been developing a spectroscopic technique to directly measure the temperature and area of magnetic ``spots'' in stellar photospheres. Such information, when combined with visual-band ``Doppler'' imaging, will yield unique and highly detailed images of stellar photospheres.
In collaboration with Steve Saar (CfA) and Penn State graduate student Doug O'Neal, I used the McMath-Pierce (NSO/Kitt Peak) telescope to obtain high-resolution, high S/N spectra of the TiO bands at 7100 and 8860 Å. Through detailed analysis of these spectra, we determined simultaneously photospheric starspot temperatures and total areas, even on slowly rotating stars. We expanded this program to use echelle spectrographs: the Penn State Fiber-Optic Echelle and the NICMASS (near-infrared camera) with the Coude Feed telescope at Kitt Peak, the Nordic Optical Telescope at La Palma, the 2.1m cassegrain echelle at McDonald Observatory, and at Penn State's Black Moshannon Observatory. We are finding larger spot filling factors than previously believed (e.g, even at its historical maximum brightness, II Peg was over 30% covered with spots). This work formed the basis of O'Neal's Ph.D. thesis at Penn State. We recently began a new phase of this program, extending it into the infrared using the PHOENIX echelle spectrograph at Kitt Peak.
The emission from chromospheres and coronae occasionally brightens by many orders of magnitude in a very short time (minutes). Most suggested mechanisms involve magnetic field reconnection, perhaps within a close-binary magnetosphere, but also from single stars. The observed properties of solar, dMe star, and RS CVn system flares are vastly different, either because the physical conditions are different or because the physical processes are different. My spectral imaging procedures permit the location, size, and most importantly, dynamics within individual flaring regions to be measured. These are the best data available for the study of the dynamics of RS CVn-star flares, and I wrote a review paper about ultraviolet flares from RS CVn systems. Since that paper was written, I have observed flares that defy easy interpretation. Several white-light flares have now been observed from RS CVn systems, for example. We also observed a flare in the Paschen-q and i lines from II Peg in October 1989. Flares in HR 1099 in December 1992 and again in September 1993 produced an unprecedented enhancement in all chromospheric and transition region lines along with extremely broad, symmetric line wings in Mg II.
I am deeply involved in an exciting program to study the characteristics of proto-planetary systems. In collaboration with Dr. K.-P. Cheng (California State University at Fullerton), I obtained with the NSO McMath-Pierce telescope at Kitt Peak high-S/N ( > 300) and high-resolution (l/Dl > 125,000) spectra of the Ca II K and Na I D lines from candidate proto-planetary systems identified from the IRAS database. In nine 6-night McMath-Pierce observing runs, I completed a northern-hemisphere survey of 42 targets and began monitoring the most interesting targets. We have observed the 20 southern stars using the 1.5m telescope at CTIO (Chile), and with the 74'' telescope on Mt. Stromlo (Australia). We have observed all of these systems with IUE, and our initial HST observations were obtained in August 1995. We are monitoring with the McDonald Observatory 2.7m telescope a smaller sample having both dust and gas to study the dynamical properties of the circumstellar gas. Similar studies of the only system with a directly-imaged dust disk (b Pictoris) show variable (in intensity and velocity), non-periodic absorption features that have been interpreted as due to the evaporation of accreting, comet-like bodies.
Separating stellar chromospheres (T ~ 104) from stellar coronae (T ~ 106) lies a very thin transition region. This region corresponds to the ionization of neutral hydrogen and the consequent loss of H I Lyman alpha (Ly-a) as an effective cooling channel. H I Ly-a (1216 Å) is thus the most crucial diagnostic of this critical layer. Nevertheless, it has been neglected in most far-ultraviolet studies of late-type star. By using care to remove the geocoronal emission background, and by studying stars with a high radial velocity so that their Ly-a emission line is relatively unattenuated, I am able to probe the intrinsic emission profile of late-type stars. This work has led me to a parallel interest in probing the H I distribution of the local interstellar medium.
I have several ongoing programs to study high-resolution IUE and HST spectra of stars with high radial velocities. Two of these stars, AR Lac and TY Pyx have high radial velocities due to their binary orbital motion. Two others, Delta Lep and HD 41312, have high space velocities. We observed HD 41312 during the first GO cycle with the Hubble Space Telescope (Delta Lep observations were approved for Cycle 1 but never obtained). I recently observed two more high-velocity stars (HD 6755 and HD 6833) with HST. The analysis of all of these data is complete, and we are working on a single paper that includes and compares all of the results.
My spectral imaging programs carried out with space-based observatories demonstrated the requirement that such observations be conducted continuously for several days, which is not possible from a single ground-based site (except the poles). I have been a Principal Investigator in the MUSICOS campaigns (a consortium formed to obtain high-resolution spectra of a single target for several days continuously). To do this, identical spectrographs are placed on several 2m-class telescopes around the world, and supporting observations are arranged at many others. Because I study variability timescales of hours to days, I have had to become an expert on coordinating observations at many different observatories. I continue to be involved with actively promoting this mode of observing.
I have been upgrading and enhancing the capabilities of the observatory at the College of Charleston (16" DFM telescope) and at the University of the Virgin Islands (15" telescope in St. Thomas). We are beginning a program to use these two telescopes simultaneously to measure orbits (and perhaps parallaxes) of near-earth asteroids. I am also evaluating the potential of the St. Thomas site for a larger telescope (it looks very promising!).
I served as the local system manager and software consultant for IDL at Penn State. Several large software libraries were developed in-house (e.g. a ROSAT data analysis package, the XMM Optical Monitor software package), and I developed software for astronomy laboratory courses. At the College of Charleston, I am serving as a unix system administrator for 8 Sun workstations (we also have a few linux machines thrown into the mix). We are installing all of the astronomical packages as well as the McIdas-X weather system.
1 February 1999