The Program --
Prof. Bruce J. Hrivnak
Prof. Hrivnak has been carrying out observational studies of stars, using both ground-based and space-based telescopes. His work has focused on the study of proto-planetary nebulae, which are objects in a late stage of the evolutionary cycle of stars. It is known that stars like our sun eventually evolve from a stable configuration (called a main-sequence star) to become large and cool as a red giant, and then to become even larger as a so-called asymptotic giant branch (AGB) star. During this last phase, a star is unstable and pulsates, and the gas in the outer layers of this very large star expands away from the star. Eventually the outer layers of the star become detached, leaving a gaseous expanding envelope around the core of the star. This central star increases in temperature; meanwhile dust forms in the expanding, cooling envelope. A few thousand years after the envelope detaches, the central star is of such a high temperature that it ionizes the surrounding gas, causing it to glow by fluourescence. We call such objects, consisting of a hot central star surrounded by a glowing shell of gas, planetary nebulae (PNs). Over tens of thousands of years, the gas shell will disperse into space, and the hot central star will cool as a white dwarf.
Proto-planetary nebula are objects in the relatively short-lived phase when the mass loss of the AGB star has ended and the envelope of gas and dust is expanding away from the star, but the star is not yet hot enough to cause the gas to shine. This research will be described in detail below. Much of this research has been carried out in collaboration with Dr. Sun Kwok of the University of Calgary.
Proto-planetary nebulae (PPNs) are objects in transition between the AGB and PN phases of stellar evolution. Prior to the IRAS mission (1981), only a few PPN candidates, such as AFGL 618 and 2688 (Egg Nebula), were known. These were objects which were found in early infrared surveys to have bipolar nebulae.
PPNs were difficult to identify because (a) they have short lifetimes (a few thousand years) and (b) they are partially obscured by their circumstellar envelope (CSE). Their characteristics include the following: central star - luminous (supergiant spectra), spectral type B-G, chemical signature of post-AGB; CSE expanding molecular shell (CO or OH emission), circumstellar dust (mid-infrared emission), small size (inner radius of shell still close to star).
IRAS data allowed us to select PPN candidates based upon their color temperature, which is between that of AGB stars (T>300 K) and PNs (T=50-150 K); thus for PPNs, T=150-300 K. From this, an IRAS color-color region for PPNs was identified, which was later extended to include all IRAS 25 micrometer peakers. Since PPNs are expected to have small atmospheres and detached CSEs, they are not expected to show large flux variations, and so we also constrained candidates to have IRAS variability index VAR < 50. (General reviews of the properties of PPNs can be found in references 1, 2.)
Optical counterparts to these IRAS PPN candidates have been discovered by searching the sky around the somewhat uncertain IRAS positions, using a bolometer at 10 micrometer on the 3.6-m CFHT and the 3.8-m UKIRT (3-6). Ground-based follow-up has been carried out at visible, near-infrared, and mid-infrared wavelengths, using additional telescopes at KPNO and CTIO. We have found the PPN candidates to have counterparts with a variety of visible brightnesses some are bright (V<14 mag) and others are faint (V>20 mag). The spectral energy distributions (SEDs) of the PPNs are found to be double-peaked. The peak in the visible and near-infrared is due to the light from the reddened photosphere of the star and the peak in the mid-infrared is due to emission from the circumstellar dust. These spectral energy distributions have been modeled using a spherically symmetric dust radiative-transfer code, and from these we have determined the size and age of the dust shell and the luminosity of the object, as a function of their distance (4,7).
Low-resolution visible spectroscopy has been carried out for the identified PPN candidates. These are found to typically show the spectra of F-G supergiants, some with evidence of molecular carbon (C2 & C3) and s-process overabundance. These give qualitative agreement with what is expected for PPNs (8,9).
High-resolution spectroscopy (R= ~50,000) has been carried out for a sample of carbon-rich PPN (21 micrometer sources), to derive the elemental abundances. Results show them to be metal poor ([Fe/H]= -0.5), s-process overabundant ([s/Fe]=+1.5), and in some cases Li overabundant (10,11). This abundance pattern fits with theoretical modeling of AGB nucleosynthesis and the third dredge-up.
We are carrying out a program at the VU Observatory to monitor light variability in 50 PPNs. These are being observed intensively by VU undergraduate students in the summer and monitored less frequently during the school year. Over the six years of study thus far, we find that almost all are variable in light, typical with DeltaV=0.2-0.3 mag. Velocity curves of nine of the brighter ones obtained over a five-year period at the DAO show them all to vary in velocity, with a typical peak-to-peak range of 10 km/s. The evidence indicates that these objects are pulsating, with periods up to 150 d (12).
Imaging of these PPN has been carried out, both from the ground and space. Imaging from the CFHT with a resolution of <1" has revealed 11 of them to be resolved, including two clearly bipolar nebulae (13,14).
Hubble Space Telescope (HST) WFPC2 observations of four of these show stunning bipolar morphologies, with a series of concentric arcs visible in two, similar to those seen in the Egg Nebula. The aspherical shapes indicate that asymmetric mass loss begins early in the PPN stage, rather than later as the object becomes a PN (15-17). New HST NICMOS observations of these reveal the central star, and will add valuable constraints on the modeling of the CSE.
Near-infrared spectroscopy can give information on both molecular and dust features. A medium-resolution survey showed CO (2.3 micrometer) in absorption and in three cases in emission; the emission indicates some excitation source such as shocks from mass outflow. Also seen in a few carbon-rich PPN were 3.3 and 3.4 micrometer emission features. However, these had the unusual property that in some cases the 3.4 micrometer feature was comparable in strength to that at 3.3 micrometer, whereas in all other celestial sources observed the 3.3 micrometer feature is much stronger (18,19). In a recent study, we detected H2 emission from several PPNs.
Mid-infrared spectroscopy allows the chemistry of O-rich (9.8, 18 micrometer) and Crich PPNs to be distinguished. An exciting discovery to come out of the IRAS spectroscopic study was the detection of a new, broad emission feature at ,21 micrometer (20). The origin of this feature is not yet known, but it has been found only in carbon-rich PPN, not in the preceding AGB or the succeeding PN stages of evolution. It has been found in 12 sources to date. They have been discovered from observations made with the IRAS LRS, UKIRT CGS3, and ISO spectrometers (21).
Recent HST images have revealed additional PPNs at a variety of orientations to the line of sight (22). A bipolar structure appears to be quite common. In addition, two additional cases of circumstellar arcs and one case of pairs of "searchlight beams" were found (23). A spectacular example of a circumstellar disk and collimated bipolar outflow lobes was discovered (24).
Recent mid-IR spectra have been obtained of PPNs using ISO. With this higher signal-to-noise data, we have studied the shape of the "21 micrometer" feature, and find it to have the same intrinsic shape in all of the sources (25). With the extended wavelength coverage of ISO, one new 21 micrometer source has been discovered, the others confirmed, and several new 30 micrometer sources found. We have resolved the "30 micrometer" feature into two separate components, one at 26 micrometer ("26 micrometer" feature) and one longward of 30 micrometer ("30 micrometer" feature) (26).
From a study of the strength of the Unidentified Infared Bands (UIBs) in our spectra of PPNs, we have found that aliphatic bands are particularly strong in this phase, but in the following PN phase the aromatic bands are much stronger. This implies that within the few thousand years that it takes to evolve from a PPN to a PN, significant chemical evolution can take place in these organic molecules (27; also ISO and CNN news releases.)
Prof. Hrivnak's studies of PPNs continue at a variety of wavelengths using ground-based and satellite-based telescopes. The goal is to understand the chemistry and mass-loss processes in this short-lived but important stage in the evolution of stars.
"Many are the works of the LORD, studied by those who delight in them." Ps 110:2
Support for this research has been provided to Prof. Hrivnak by the National Science Foundation, NASA, and the NASA-funded Indiana Space Grant Consortium, in addition to Valparaiso University. All of this is very much appreciated.
(1) Kwok, S. 1993, An. Rev. A&A, 31, 63 (2) Hrivnak, B.J. 1997, IAU Symp. 180: Planetary Nebulae, 303 (3) Hrivnak, B.J., Kwok, S., & Boreiko, R.T. 1985, ApJ, 294, L113 (4) Hrivnak, B.J., Kwok, S., & Volk, K. 1988, ApJ, 331, 832 (5) Hrivnak, B.J., & Kwok, S., 1991, ApJ, 368, 564 (6) Hrivnak, B.J., & Kwok, S., 1991, ApJ, 371, 631 (7) Hrivnak, B.J., Kwok, S., & Volk, K. 1989, ApJ, 346, 265 (8) Hrivnak, B.J., & Kwok, S. 1991, ApJ, 371, 631 (9) Hrivnak, B.J. 1995, ApJ, 438, 341 (10) Reddy, B.E., Bakker, E.J., & Hrivnak, B.J. 1999, ApJ, 524, 831 (11) Hrivnak, B.J., & Reddy, B.E. 2000, ApJ, in preparation (12) Hrivnak, B.J., & Lu, W. 2000, IAU Symp. 177: Carbon Stars, in press (13) Kwok, Hrivnak, Zhang, & Langill 1996, ApJ, 472, 287 (14) Hrivnak, B.J., Langill, P.P., Su, K.Y.L., & Kwok, S. 1998, ApJ, 513, 421 (15) Kwok, S., Su, K.Y.L., & Hrivnak, B.J. 1998, ApJ, 501, L117 (16) Su, K.Y.L., Volk, K., Kwok, S., & Hrivnak, B.J. 1998, ApJ, 508, 744 (17) Hrivnak, B.J., Kwok, S., & Su, K.Y.L. 1998, ApJ, 524, 849 (18) Geballe, T.R., Tielens, A.G.G.M., Kwok, S., & Hrivnak, B.J. 1992, ApJ, 387, L89 (19) Hrivnak, B.J., Kwok, S., & Geballe, T.R. 1994, ApJ, 420, 783 (20) Kwok, S., Volk, K., & Hrivnak, B.J. 1989, ApJ, 360, L29 (21) Kwok, S., Hrivnak, B.J., & Geballe, T.R. 1995, ApJ, 454, 394 (22) Su, K.Y.L., Kwok, S., & Hrivnak, B.J. 2000, in preparation (23) Hrivnak, B.J., Kwok, S., & Su, K.Y.L. 2000, in preparation (24) in preparation (25) Volk, K., Kwok, S., & Hrivnak, B.J. 1999, ApJ, 516, L99 (26) Hrivnak, B.J., Volk, K., & Kwok, S. 2000, ApJ, 535, in press (27) Kwok, S., Volk, K., & Hrivnak, B.J. 1999, A&A, 350, L35
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