AAS Annual Report, 1992-1993

ISU Astronomy Group


This report covers the period from September, 1992 through August, 1993.

Contents

Personnel

Faculty during 1992-93 were Philip N. Appleton, George H. Bowen, Steven D. Kawaler, Richard C. Lamb, Russell Lavery, David A. Lewis, Curtis Struck-Marcell, and Lee Anne Willson. Joe Eitter continues as Observatory Manager.

Struck-Marcell served as Coordinator during 1992-3. Bowen retired from the faculty at the end of December, 1992, but continues to be active in research as an emeritus professor. Lavery joined the faculty as an Assistant Professor on 8/20/93. Kawaler was promoted to Associate Professor with tenure effective July 1, 1993. L. A. Willson received the title University Professor in May, 1993. Dr. Elizabeth J. Zita, assistant professor at Grinnel College, was a visiting assistant professor with our department for the months of July and August 1993. This visit was made possible through the ROA program of the NSF.

Don Luttermoser continued as post- doctoral associate and temporary assistant professor during this year; he left to take up another position in August '93. J. Christopher Clemens was awarded a Hubble Postdoctoral Fellowship for study at ISU, and will be arriving in the Fall of 1993 to begin working with S. Kawaler and others in the group. Alan Kerrick and Ping Wai Kwok were postdoctoral associates with the Very High Energy Gamma Ray project. Pnina Luban-Lotan continued to carry out postdoctoral research in modeling colliding galaxies in collaboration with Struck-Marcell.

Lewis Hainlin received an M.S. in Astronomy in August 1992. Continuing graduate students include Mark Bransford, Vassilios Charmandaris, Ben Dehner, Ralph Haefner, Gora Mohanty, M. Sean O'Brien, Mike Sindberg, Charles Winrich, and Jeff Zweerink.

Undergraduates involved in research included Ginger Alachniewicz, Shawna Helleur, Mark Smith, and Chad Uecker. S. Helleur has been awarded the Department of Physics and Astronomy's Bernice Black Durand Research Scholarship and G. Alachniewicz received a Margaret B. Weatherspoon Scholarship.

Facilities

Macintosh computer laboratory was constructed for use in teaching, with scheduled times for use by the astro courses (particularly the large-enrollment general course on the sky and the solar system), and graduate students having free access at all other times. The lab currently has 8 Mac LCIII's and is expected to have 12 by the end of the year. These are networked to laser printers, to the mainframe, and to the distributed computing network of Dec Workstations, Project Vincent.

Research at the E. W. Fick Observatory was conducted in two main areas during 1992/3. These were continued observations of the radial velocity of late-type stars, and wide-field CCD observations of faint galaxies and nebula. Because of unusually cloudy weather, significantly fewer nights of observation were obtained during this period than in previous years. For the Fick Radial Velocity Survey 1700 stellar radial velocities were obtained over 35 clear nights and approximately 43 nights were devoted to CCD observation.

Supernova : A high-point in this flood-ridden year in Iowa was the observations by Appleton and Eitter of SN1993J in the nearby M81 galaxy. Using the CCD camera, many epochs of observation were made in the first few weeks after the discovery of the supernova through BVR and I filters, and the results were made available to the astronomical community in an IAU telegram.

Image Processing

Appleton, with J. P. Basart (EECE) and J. Pedelty (NASA/Goddard) have begun work refining the filter they have developed to remove Galactic "cirrus" emission from the IRAS 100 micron Survey Data. The work is funded under the High Performance Computing and Communications Program of NASA and involves the development of the filter for use with massively parallel computers such as the MasPar and Intel machines. The paper describing our initial results has now been published.

Circumstellar/Interstellar Matter:

Using the Fick Observatory wide-field CCD camera, P. N. Appleton with S. D. Kawaler and J. J. Eitter have discovered a planetary nebula with the largest linear size yet known. The nebula, which is detected through its [OIII] and Ha emission, has a maximum extent of 13 arcminutes which corresponds to a linear size of 5.3pc at the published distance of 1.4 kpc. The expansion age of the nebula is estimated to be 1.5 x 105 years. The central star, RX J2117+34, was discovered by ROSAT and is a member of the PG 1159 spectroscopic class. Such stars have properties intermediate between planetary nebula nuclei (PNN's) and white dwarfs. The discovery of an evolved PN around RX J2117+34 confirms that PG1159 stars are transition objects between PNN's and helium-rich white dwarfs.

ROSAT X-ray Bright Galaxies

P. N. Appleton, with D. H. Hughes and M. Ward (University of Oxford, UK) made CCD observations of a small sample of the most luminous X-ray galaxies detected in the ROSAT sky survey. Deep Ha images were obtained with the Fick Observatory CCD Camera under conditions of good seeing. Unlike the super- luminous IR galaxies, it appears that the bright ROSAT galaxies do not form a well defined group of objects but appear to be a mixture of Seyfert galaxies and starburst systems. The Fick observatory images were combined with optical spectra (KPNO 4-meter) and IR photometry (UKIRT) to estimate star formation rates in the galaxies.

Interacting Galaxies -- Observations

P. N. Appleton, with A. P. Marston (Drake University), V. Charmandaris and C. Struck- Marcell have obtained optical and IR images and spectra of a sample of colliding ring galaxies. The optical and IR study of the Cartwheel ring galaxy (with graduate student P. Marcum and collaborator J. Higdon (NRAO)) has now been published. The results show major color gradients in the wake of the expanding ring, strongly supporting the collisional picture for the formation of ring galaxies. In a similar in- depth study of another ring system, Arp 10, Appleton, Marston and Charmandaris discovered three concurrent rings of star formation. By analyzing the dependence of the star formation rates on the ring over-densities, evidence was presented for a "threshold" star formation law in Arp 10. In regions of the ring which are above a certain ring amplitude, the star formation rates appears to jump to a high value. The results suggest that star formation rates can be driven over the threshold by global dynamics and may provide some new insight into the triggering process that leads to major "starburst" activity in galaxies. A preliminary analysis of the global properties of the sample of ring systems indicates that in many cases the global V-K colors are strongly influenced by the evolution behind the ring. For example, large ring galaxies are systematically redder in V-K color than small systems, suggesting the evolution of a burst population is responsible for much of the light from ring galaxies.

Appleton, with F. Ghigo (NRAO, Greenbank) has made multi-frequency radio continuum observations of a subset of the ring galaxy sample described above with the D-array of the VLA. During an 8 hr observing period, 11 of the ring systems were observed in snap-shot mode at 21, 6 and 3.6 cm wavelength. An analysis of the observations is still in progress at the time of this writing.

In June 1993 Appleton, with ISU graduate student C. Winrich and collaborator G. Fabbiano (Harvard/Smithsonian Center for Astrophysics) made X-ray observations of the interacting galaxy pair Arp 270 with the ROSAT PSPC instrument. The two merging galaxies were detected and the X-ray spectra are consistent with a two component thermal model for the emission. A major contribution to the X-ray emission appears to come from X-ray binaries, but a soft component is also detected which may have a different origin. The soft component may result from the shock-heated gas produced at the interface between the two galaxies or may come from hot winds from a young O/B star population. These possibilities are being tested by critically comparing the results with optical/IR imaging and radio observations of the galaxies made by Appleton. An interesting result of the ROSAT observations is that a number of small dwarf galaxies were also detected in the same group as the interacting pair.

Interacting Galaxies: Models and Theory

Struck-Marcell (in collaboration with J. L. Higdon, U. Texas) completed and published a modeling study of the Cartwheel ring galaxy. An extensive set of two and three dimensional smoothed particle hydrodynamics (SPH) simulations of the predominantly gas disk of the Cartwheel were performed. These simulations use fixed gravitational potentials, including one for the Cartwheel based on Higdon's 21 cm rotation curve. Local self-gravity within the disk was also calculated. The models indicate that the outer ring is a relatively weak density wave, while the inner ring is strong. The models with local self-gravity are able to reproduce the clumpy structure of the outer ring and the "spokes", but only if the conditions in the precollision disk were close to the threshold for gravitational instability (e.g. Toomre Q about unity). An unpublished comparison to the recently published ring galaxy simulations of Hernquist and Weil was also carried out. It was found, after accounting for the different gravitational potentials, that both studies agreed on the conditions needed for spoke formation. Thus, the models seem to be very successful in accounting for this phenomenon, and a useful aid in interpreting the observations. Ongoing work will includes modeling of the 21 cm kinematics, more study of the distribution of star formation in rings and spokes, and the effects of feedbacks from young star activity.

Struck-Marcell and J. Wallin (George Mason) undertook an optical and IR surface photometry, 21 cm. radio, and modeling study of the "Sacred Mushroom" ring AM1724-622. Previous imagery showed no blue knots or other evidence of enhanced star formation in the ring wave, although the companion is very disturbed and may be experiencing a starburst. Because this galaxy is located near the galactic plane, there are many foreground stars in the field and it proved impossible to obtain accurate colors of the ring. Nonetheless, there is no evidence for a young stellar population in the UBVRI data. Preliminary analysis of the 21 cm data indicates little or no gas in the ring galaxy, but gas is detected in the companion. Thus, the ring galaxy is an excellent place for testing the assertion of Struck-Marcell and Lotan (ApJ 358, 99) that nonlinear, old star rings are caustic structures bounded by sharp edges. Analytic caustics models, semi-analytic kinematic orbit models, and restricted three-body simulations were produced, and all agreed on the basic collision parameter values needed to produce this system. They also indicate that the rotation curve within the ring is rising, so the potential out to that radius is probably dominated by the bulge rather than a dark halo. The precursor galaxy was probably an S0 type. Intensity profiles in radial cuts across several sectors of the ring were derived, and these agree with the predictions of the caustics theory. However, definitive results await future observations with higher spatial resolution, and if possible, data on the stellar kinematics in the ring.

Distant Clusters of Galaxies

Russell J. Lavery, Michael J. Pierce (KPNO) and Robert D. McClure (DAO) have continued their program of high resolution imaging of distant clusters of galaxies using the Canada-France- Hawaii Telescope of the Mauna Kea Observatory. Their most recent result was the discovery of a very high surface brightness arclet associated with the cluster GHO 2154+0508. The spectrum of the arclet shows it to be at a redshift of 0.721, whereas the cluster redshift is only 0.33. From lensing models, the presence of this high surface brightness, yet low magnification, arclet suggests the mass responsible for producing the lensing is not as centrally concentrated as other known lensing clusters. GHO 2154+0508 may be a rich cluster of galaxies in the early stages of formation. While their original program concentrated on clusters with a redshift of 0.4, this program has now been extended to observe clusters out to redshifts of 0.7.

Russell J. Lavery and J. Patrick Henry (IfA/Hawaii) are working on a detailed study of the rich cluster of galaxies A963 at a redshift of 0.2. They have obtained deep B and R band images of the cluster, redshifts for 75 galaxies in the field of the cluster and a detailed x-ray image with the ROSAT telescope. The goal of the program is to combine these data to produce a mass distribution model for this cluster that is consistent with all the above data as well as the gravitational lensing produced by this cluster. Analysis of these data is now in progress.

Structure of the Gas in Galaxy Disks

Struck-Marcell continued work on two and three dimensional SPH simulations of the long-term relaxation of isolated gas galaxy disks. A simple, but realistic, cooling curve was incorporated into the SPH program, as well as a routine to impulsively heat gas elements that find themselves in high density clumps. These high density clumps form in the cool gas as a result of local self-gravity, which is included in these simulations. When the disk begins with a gas surface density near the gravitational instability threshold the gas clumps, heats locally and produces local fountains, and later holes or bubbles. A multi-phase medium is set-up with the cool gas located in a thin disk relative to the hot gas. The bubble and hole morphology is very reminiscent of published 21 cm. observations of M33 and M101. In the near future simulations with bar and collision-driven density waves will be carried out.

Very-high Energy Gamma Ray Astronomy

The Iowa State program in Very High Energy (1012 eV) gamma-ray astronomy involves two faculty members (Lamb and Lewis), two post- doctoral associates (Kerrick and Kwok), two graduate students (Mohanty and Zweerink) and three undergraduates (Pederson, Smith, and Smucker). Iowa State continues its participation, begun in 1982, in the Whipple Observatory Collaboration. Other member institutions in the collaboration are: the Harvard-Smithsonian Center for Astrophysics (responsible for the operation of the Whipple Observatory), University College Dublin, Leeds University, St. Patrick's College - Maynooth, the University of Michigan, and Purdue University.

The principle instruments used by the Collaboration are located at the Whipple Observatory and consist of a 10 meter diameter reflector, instrumented with a focal-plane camera of 109 photomultipliers (pixel size 0.25o) and an 11 meter diameter reflector, which saw first light in late 1991 with a medium resolution camera of 37 photomultipliers (pixel size 0.25o).

During the first 18 months of operation of the 11 meter reflector, several technical problems were encountered and were solved. The stability and the absolute positioning of the telescope mount were significantly improved. A CCD camera was installed as a check on its pointing direction. Its medium resolution camera was replaced during the summer 1993 with a 109 pixel, high resolution camera, and the data acquisition electronics for both reflectors was significantly upgraded.

A technique for extracting the energy spectrum of a source has recently been worked out by Lewis and Mohanty ("Energy Spectra from Cherenkov Telescopes: Application to the Crab Nebula", see publication listing), using Monte Carlo calculations of the behavior of gamma-ray induced showers expected in the gamma-ray telescope. The energy resolution of a single reflector is about 40%. Over the region from 400 GeV to 10 TeV the spectrum is well fit by a simple power law. However, that power law is significantly higher than the extrapolation of lower energy satellite measurements, and is consistent with recent calculations by de Jager and Harding (ApJ, 396, 161, 1992) based on a sychrotron-self Compton model for the Nebula.

This same technique was applied to the Whipple Observatory's 1992 observations of the active galactic nucleus, Markarian421. The spectrum is consistent with a single power law over the energy region 400 GeV to 5 TeV, and consistent with an extrapolation of lower energy observations obtained by the EGRET instrument of the Compton Gamma Ray Observatory.

One of the interesting questions associated with these observations of Mrk421 is whether or not there is any evidence for intergalactic absorption of the TeV photons due to pair production on visible and infrared starlight photons. The observations are consistent with little or no absorption; however, the failure to detect as yet any of the other EGRET AGN's may be related to such absorption since they are at distances which are at least 5 times greater than Mrk421.

In 1992 the previously unidentified high energy source, Geminga, was found to be an x-ray and high energy gamma ray pulsar with a spin period of 0.237 seconds. Using the accurate period ephemeris, the Whipple Collaboration was able to reduce our previous upper limits by nearly an order of magnitude and significantly below other claims of detection at TeV energies.

In July 1993, the Iowa State group acted as the local organizing committee for a workshop on the atmospheric Cherenkov technique. The workshop "Towards a Major Atmospheric Cherenkov Detector" was the second such workshop on this theme, the first of which was held in June 1992 at Palaiseau, France. Because the International Cosmic Ray Conference was held in Calgary, it was decided to hold the "Ames" workshop there. Sixty-five participants from 14 countries took part. Proceedings of the workshop are to be published by Iowa State in December 1993.

Research in very high energy gamma ray astronomy is supported by the Department of Energy and NASA. Papers on this subject published during the period of the report:

Stellar Evolution and Seismology

Kawaler continued theoretical efforts to model the pulsating hot pre-white dwarf star PG 1159- 035. Successful modeling provided the first determination of the subsurface structure in these young white dwarf stars, showing that their surfaces layers are compositionally stratified, with an outer helium-rich layer containing 0.2 percent of the mass of the star, a number smaller than predicted by standard stellar evolution theory. This work, in collaboration with P. Bradley (LANL) has been submitted for publication in the Astrophysical Journal.

Ph.D. student Ben Dehner continues his theoretical efforts concerning white dwarf evolution by including the effects of diffusion into the evolutionary models. Such time- dependent models will provide new insights into the compositional structure of the outer layers of white dwarfs which can be directly confronted with the results of seismological observations.

With support from the NSF ROA program, E. J. Zita (Grinnel) began a collaboration with Kawaler to investigate the role that magnetic fields may play in the phenomena associated with the rapidly oscillating Ap stars. Together, they plan to investigate various magnetic instabilities present in stellar plasmas to explore the connection between magnetic and vibrational modes of oscillation.

In September 1992, Kawaler acted as Co-PI (with Dr. R. E. Nather of the University of Texas) in a two-week observing run of the Whole Earth Telescope (WET), a world-wide network of telescopes of the 1 to 2 meter class outfitted with multichannel photoelectric photometers. The target was PG 2131, a pulsating GW Vir star. Data from this run is currently being analyzed by Kawaler and graduate student M. S. O'Brien. O'Brien also assisted with the WET in the most recent WET run, with PG 1159 itself as the target, in March 1993.

Kawaler, along with Howard Bond (STScI) obtained data with the Hubble Space Telescope on 4 white dwarf stars to search for evidence of radial pulsations in these objects. No evidence of such pulsations were found, with upper limits of 0.001 magnitude obtained for the DB white dwarf GD358; a paper describing some of the observations is now in press. Further data were obtained and are currently being analyzed. Bond and Kawaler also observed several white dwarfs with the 4 meter reflector at CTIO to look for these short period pulsations, and that data is also being analyzed.

Mass Loss and Evolution for LPV models

Bowen explored the effects of radiation pressure on molecules (e.g. H2O) on the structure and mass loss for a range of stellar masses, using realistic temperature- and density-dependent rates for the formation and destruction of the molecules in his dynamic models. Such molecules, which can form at higher temperatures (hence at smaller radii) than dust grains, can increase the mass loss rate substantially. However the molecules rapidly dissociate at the very high temperatures behind the strong shock that always forms fairly close to the photosphere, and they reform so slowly in the cooler, very low-density outer atmosphere that their amount there is generally far below equilibrium. The principal effect is therefore to increase the scale height in the inner atmosphere, which leads to an increased density at all greater radii, hence to an increased mass loss rate. (The primary driving mechanism for the wind is still radiation pressure on grains.) The effect is rather small for stars of one solar mass, but increases with increasing mass; it should have significant consequences for the evolution of stars above about two solar masses.

Struck-Marcell and Willson have undertaken a new effort to analytically model the extended stellar atmospheres and winds of pulsating stars. The relative simplicity of the mean density, temperature and flow velocity profiles in Bowen's numerical simulations for AGB stars suggests that analytic approximation should be possible. Preliminary quasi-steady models for cases where radiation pressure effects are negligible (e.g. no dust or strong molecular absorbers) are yielding promising agreement with the self-consistent simulations, and make the importance of the treatment of thermal relaxation on the structure of the atmosphere clearly evident.

Return to
Graduate
Astronomy
Please send your comments to the Astro Group web-manager

Last modified: Wed May 20 1998