Research

Below, I describe the many facets of my research interests. There is great opportunity for students to become involved in any of these research projects, from observational work to detect or follow light echoes or dust production, to the use of radiative-transfer models to interpret our data. Projects could range from a short, one-month experience to dabble in research to a multi-year project with trips to present research at major conferences, take data at observatories, and publish papers.

Research Interests and Projects

I am interested in what stars do just before and after they die. To best explain this, I will start with a primer in stellar death. The end states of stars are like Wyle E. Coyote’s bombs in cartoons: they either go off with a bang, or fizzle away with a wisp of smoke. Low-mass stars like our sun evolve into extremely luminous but cool Asymptotic Giant-Branch (AGB) stars, whose sizes are larger than the earth’s orbital distance from the sun. Giants are powered by fusing helium into carbon and oxygen, which settle into a dense, inert core. Too cool to initiate fusion, the core is compressed by the outer layers until it is supported by a quantum-mechanical pressure known as “electron-degeneracy.” If this degenerate core reaches a critical mass (the “Chandrasekhar limit” at 1.4 solar masses), thermonuclear fusion of the carbon and oxygen ignites in a runaway process, ripping the star apart in a “Type Ia” supernova (SN) explosion, releasing more energy in a few days than over the star’s entire lifetime.

All stars have winds that, to varying degrees, strip material from the stellar surface. For stars under 8 solar masses, this mass loss is sufficient to keep the core from ever reaching the Chandrasekhar limit. Instead, the carbon-oxygen core is exposed as a “white dwarf,” its hot surface ionizing the expelled gas into a glowing “planetary nebula” (an unfortunate historic misnomer), then like a hot ember, it slowly fades away. More massive stars are able to burn increasingly-heavy elements, until the core consists of iron, for which fusion is not energetically feasible. With no central engine, the core can no longer support the star, and it collapses on itself, driving electrons into protons and creating a stiff neutron star, off of which the outer layers rebound in a catastrophic “Type II” supernova explosion.

My current interests are mainly in three categories: (1) understanding the mechanisms of asymmetric mass loss at the end of a star’s life; (2) probing the structure and composition of circumstellar and interstellar environments with light echoes of supernovae; and (3) addressing whether supernovae can produce the dust we see throughout the universe. I discuss these, and some of the techniques I use, below.

Asymmetric Mass Loss

Mass-loss is believed to primarily occur via a coupling in the star’s cool, outer layers between gas and condensing dust: the dust, accelerated outward via radiation pressure, drags the gas along with it. Most outflows show remarkable departures from the spherical symmetry that is expected of this process. Asymmetry is almost the norm for mass-loss late in a star’s life, yet the mechanisms causing it are very poorly understood, with great debates raging over the need for single or binary stars, magnetic fields, centrifugal launching, etc. It is becoming clear that in lower-mass stars, asymmetries develop during the transitional phases from the AGB to planetary nebulae, which we appropriately call “post-AGB” and “protoplanetary nebulae.” My own interests are in reconstructing the three-dimensional geometry and kinematics of these nebulae, which in turn yield mass-loss histories, critical to understanding how and when the asymmetries formed.


AFGL 618 is among the best-studied proto-planetary nebulae. Shown at right, bipolar jets are punching through a previously-expelled wind, while the hot, central source is completely obscured by a dusty torus. Two papers are currently in preparation that constrain the mass-loss history, first comparing optical, near-IR, and mm-band CO data to study the spatial extent of the asymmetries, then applying the two-dimensional radiative transfer code “2-Dust” (Ueta & Meixner 2003) to model the dust geometry and kinematics.

Radiative-transfer modeling is vital to understanding the mass loss of evolved stars. With 2-Dust, the user can tightly constrain an object’s dust properties (mass, composition, geometry, kinematics), the mass-loss history, and properties of the central source, all by modeling the object’s spectral energy distribution and appearance at multiple wavelengths. Although usually challenging, these projects are straightforward, well-defined in scope, and tractable for undergraduates.

Scattered-Light Echoes


Like the flat narrator in Edwin Abbott’s Flatland, astronomers struggle to understand a three-dimensional (3-D) universe observable only from the confines of a flat (2-D) plane. Consider the picture at left: only by assuming a model, i.e. this cube is viewed in perspective, can we infer its spatial orientation. But can we be certain? The single greatest challenge to an observational astronomer is determining the distances of objects, both relative and absolute. We reconstruct what we hope are complete 3-D models, pieced together from clues on the 2-D image planes of telescopes. Only one technique directly determines 3-D positions without any input assumptions: scattered light echoes.

Light echoes are analogous to radar or sonar echolocation, only the source and observer are distinct (see below). An echo occurs when the light pulse from some source (e.g. a supernova) is scattered toward Earth by intervening dust. Echoes lie on the set of points equidistant in light travel from the source and observer, i.e. an ellipsoid. This simple geometry directly yields the 3-D position of the dust, uncertain only by the distance to the source. Echoes appear to us as arcs or rings, with a one-to-one mapping between their 2-D image positions (right) and their 3-D locations in space (left).

Dust is believed to be made of silicate or carbonaceous compounds condensed into grains 1 nanometer to a few microns in size, with interstellar number densities of roughly one grain per cubic meter, and circumstellar densities of about one grain per cubic centimeter. Since the scattering properties of dust are functions of its chemical composition, size, and number density, multi-wavelength observations of light echoes constrain these quantities. In short, light echoes probe both the macroscopic (3-D distribution) and microscopic (composition, density) structure of dust. I have thoroughly reviewed the observation, analysis and modeling of light echoes in Sugerman (2003).


Light echoes have been reported around only 11 objects – seven supernovae and four novae – seven of which I study. One of the supernovae I study, SN 1993J, is shown to the right. The left panel shows a Hubble Space Telescope image taken in 2001 of the SN (image center), located in the galaxy M81, 6.3 Mpc from earth. Note the faint echo arc at the lower right. The right panel shows the same data using a sensitive reduction technique I employ to remove all constant flux from imaging, thereby highlighting otherwise undetectable variable sources. Two echoes appear (delineated by thick lines) as well as a variable star (arrowed). The echo at upper left is indistinguishable from the background in the left panel, and indeed, was missed by a competing group. The outer echo is from a sheet of dust 770 light years in front of the supernova, and its colors are consistent with carbon-rich dust having a similar size distribution as that of our own galaxy. This is the first direct measurement of dust properties in a galaxy at this distance, which I reported in Sugerman & Crotts (2002) and analyzed in Sugerman (2003). I received five Hubble orbits in 2005 to continue studying the SN 1993 echoes, and 19 additional orbits in 2006 to study echoes from SNe 1991T, 1998bu, and 2002hh, and I recently discovered an echo from SN 2003gd in archival Hubble data (Sugerman 2005)

Supernova 1987A

SN 1987A was a Type II supernova discovered on February 23, 1987 in the Large Magellanic Cloud (at 51 kpc or 166,000 light years, the nearest galaxy to the Milky Way). This is the closest observed supernova in 400 years, and the first supernova remnant (SNR) seen to form within a pre-existing environment. It serves as a vital test bed for the colliding winds model of mass-loss nebulae and the interaction of supernova ejecta with interstellar and circumstellar material; an important rung in the cosmological distance ladder; a valuable probe of the interstellar medium; and a unique laboratory for studying SNR formation and the final stages of massive-star evolution. With SN1987A, I have pushed the limits of modern observing technology to monitor, for the first time in history, the birth of a supernova remnant, and to build the first three-dimensional map of any star’s circumstellar environment.

Hot Spots in SNR 1987A


The distance of SN 1987A permits only the largest and most advanced of telescopes to monitor its detailed evolution. In the early 1990’s, Hubble images revealed the SN to be surrounded by three glowing rings of gas (right), which we believe formed via colliding stellar winds some 20,000 years before core collapse. For scale, the angular size of the innermost, equatorial ring (ER) is 1″.6, or that of a U.S. quarter seen 2.3 miles away. When the star exploded, it launched its envelope outward, forming a thick shell of ejecta that are driving a forward shock toward these rings. Traveling at up to 10% of the speed of light, this shock was expected to impact the ER symmetrically in its rest-frame, appearing to us as an initial brightening at the northern (closest) segment, and spreading to engulf the entire ring within roughly 1 year, due to light-travel delays. This collision will subsequently crush the ER material, heating the gas by millions of degrees, and giving birth to a supernova remnant.


The first evidence for this impact came in 1997, when a “hot spot” was detected on the ER (arrowed at left). Rather than spread as predicted above, no new emission was detected until early 2000, when my collaborators and I discovered new hot spot activity in ground-based infrared imaging Lawrence et al. (2000). I am a member of an international collaboration (SAIntS, PI: R. Kirshner) that studies SN 1987A with HST, and I have discovered new hot spots with each observation taken since 1999 (Sugerman et al. 2002), for a total of 21 as of January 2003. My detection techniques are the most sensitive of anyone monitoring this system, allowing me to trace the first spot’s appearance to 1995, and to uncover proper motions of the spots at an unprecedented level of 5 milliarcsec per year.

The appearance of spots, rather than extended bright regions, suggests that they mark the impact of the ejecta with protrusions pointing inward from the ring. The ER appears offset west of the SN, consistent with more spots appearing to the east. The order of appearance further suggests the forward-shock velocity is higher toward the west. I interpret these as indicative of an azimuthal asymmetry in the progenitor’s mass loss. The proper motion of spots translates into velocities between 1000-4000 km/s, and is interpreted as the motion of the flux-center of all post-shock gas on a protrusion. This “smoking gun” is the first direct measurement of the forward shock.

Light Echoes from SN 1987A

How did the azimuthal asymmetry reported above come about? SN 1987A provided the means to perform “stellar paleontology,” or reconstructing the progenitor’s mass loss history, by illuminating circumstellar material with scattered-light echoes. Much of the pioneering observational work in light echoes was performed by my thesis advisor, Dr. Arlin Crotts, who used echoes from SN 1987A to map out dust structures tens to thousands of light years from the SN. His work has revealed giant bubbles in the ISM of the Large Magellanic Cloud, as well as a fragmented picture of an extensive circumstellar environment. For my Ph.D. dissertation, I constructed and analyzed the most complete map ever made of the nebulae surrounding SN 1987A. Shown below, this is also the first directly-observed 3-D map of any star’s circumstellar environment (Sugerman et al. 2005).

Using 15 years of ground and space-based imaging, I identified known structures 1-28 light years (lt-yr) from the SN in significantly greater spatial detail than before, while discovering many unknown features. Extensive geometric analyses of these echoes (above, panels a-b) reveals a richly-structured bipolar nebula, as rendered above in panels c-d. This fossil remnant is comprised of an outer, double-lobed “peanut,” interior to which is a narrow cylindrical hourglass containing the famous three-ring optical nebula, and that connects to the peanut by a thick equatorial disk. Surprisingly, we find the nebulae are asymmetric: they are slightly elliptical in cross section, and marginally offset west of the SN, in agreement with my findings from hot spot appearance.

This geometry provides concrete constraints on the three rings, from which I measure their absolute sizes, geometry and positions. Soon after its discovery, unresolved UV fluorescence echoes of the SN were observed from the ER. These have been used with various techniques to estimate a geometric distance to the SN, which in turn is a calibrator for the cosmological distance scale. I have modeled that emission using the geometric information discussed here, and find a distance of 51.3 +/- 0.7 kpc, which decreases the value of the HST Key Project Hubble constant to 70 +/- 8 km/s/Mpc, in excellent agreement with the recent WMAP results.

Echo fluxes constrain the grain sizes, composition, and surrounding gas density. Of note, the nebulae have a total mass of 1.7 solar masses, providing the first observational measurement of the star’s mass-loss rate (5 millionths of a solar mass per year). We find all interacting-winds models (e.g. Martin & Arnett 1995) of SN 1987A fail to match the observed geometry. However, we do find an unorthodox fusion of extant models qualitatively explains the nebulae if the progenitor is allowed to experience more than one evolutionary loops between slow, dense winds and fast, tenuous ones. We anxiously await the results of new hydrodynamic models that aim to reproduce the full extent of the CSE presented here.

Can SNe Provide the Origin of Dust in the Universe?

Dust alters the spectra of objects, and hence our determination of their physical properties, from distances of single stars to Type Ia supernovae, and from the star-formation history of our Galaxy to that of the Universe. Understanding the cosmic origins of dust and its role in galaxies, including extinction, heavy element content, mass recycling, and feedback are all key goals of modern research with NASA’s Great Observatories.

The hypothesis that dust can condense within the cooling ejecta of supernovae has gained renewed interest now that SCUBA sub-mm observations have revealed that high-redshift galaxies contain large quantities of cool dust (about 100 million solar masses each, e.g. Ivison et al 2000). At high-redshift, the lifetimes of most giant-branch stars exceeded the age of the Universe, making it highly improbable that their winds formed the dust found in early galaxies. High-mass supernovae, whose progenitors evolve to core collapse in less than one billion years, therefore remain the most probable source of this dust. However, there is a frustrating discrepancy between the dust masses found in the early Universe, predicted by nucleation theory (about 0.1 solar masses per SN), and inferred from supernova observations (less than 0.001 solar masses each). Few supernovae have ever been probed for dust production, since it has been extremely difficult to observe. With the launch of the Spitzer Space Telescope and the commissioning of sensitive optical to mid-infrared detectors on large ground-based telescopes, the observational challenges are now largely reversed.

I have helped organize, and maintain an active role in the international collaboration “SEEDS” (Search for Evolution of Emission from Dust in Supernovae, P.I. Mike Barlow) to address the extent to which SNe produce dust, and whether they are a primary source of this dust, by performing a comprehensive, targeted optical to mid-infrared study of dust emission in young SNe. I am investigating why observations underestimate predicted dust masses by orders of magnitude, and am exploring new ways to to estimate dust mass based on different dust-formation indicators.

Dust is believed to condense in the expanding (i.e. cooling) ejecta when the gas reaches 1100-1800 K, or about 1-2 years after outburst, producing 0.08-1 solar masses of dust, depending on dust composition, progenitor mass, metallicity, and the explosion energetics. Two pieces of evidence indirectly link dust to SNe: the presence of significant dust at high-redshift, and pre-solar grains from the Murchison meteorite, which contain large excesses of Ca-44, the decay product of the isotope Ti-44 produced deep in SNe. Direct indications of grain condensation within ejecta include (1) IR excesses, appearing 1-3 years after outburst as the newly-condensed dust cools (below, left); (2) increase in optical extinction; and (3) asymmetric blue-shifted emission lines, as dust in the ejecta obscures emission from receding gas (below, right). Only SN 1987A has shown all three indicators, while a few others have displayed some subset: SN 1998S showed asymmetric lines and IR excess, and SN 1999em had asymmetric lines and a tenfold increase in extinction. All three SNe are estimated to have produced less than 0.001 solar masses of dust.

Left 2 panels: spectral-energy distributions (SEDs) of SN 1987A on days 60 and 615 (after maximum light) from Wooden et al (1993). A three-component continuum model is shown for hot gas (long-dashed lines), free-free and bound-free emission (dotted lines), and warm 300-400 K dust (solid lines). Dust emission first appears at day 260, was unambiguous by day 615 and faded rapidly 400 days later. Right panel: Evolution of H-alpha line profiles in SN 1998S, showing the preferential extinction of redshifted emission at late times, presumably caused by dust condensing in the ejecta.

 

Using my expertise in sensitive data reduction, we have established that nearby SNe can be reliably observed in the mid-IR. Thus far, we have reported the mid-IR detections of five SNe, including 1999bw (Sugerman et al. 2004), 2004dj (Sugerman et al, 2005), and 2004et (Fabbri & Sugerman 2005), all of which are being followed up in the optical and mid-IR. Our first detection was SN 2002hh in NGC 6946 (D=6 Mpc), which as we reported in Barlow et al. (2005), has fluxes that are consistent with pre-existing circumstellar material (CSM) warmed by the SN blast. However, the spectral-energy distribution is cooling similarly to that seen in SN 1987A; expecting that dust may also have been condensing within its ejecta, we have been closely monitoring SN 2002hh with HST and SST through two of my successful observing proposals.

Most recently, I detected faint emission from SN 2003gd in NGC 628 (D=9.4 Mpc) 500 and 678 days after outburst Sugerman et al. (2006). In addition to the mid-infrared excess, SN 2003gd also shows the two other signatures of dust formation: a fourfold increase in optical extinction (Sugerman 2005), and asymmetric shifts in its emission lines (Hendry et al. 2005). SN 2003gd is only the second SN to show all three dust-formation signatures. Using the MOCASSIN radiative-transfer models of Ercolano et al (2005), we were able to show that up to 0.02 solar masses of dust formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae could have been major dust producers throughout the history of the universe. This generated a lot of buzz, with Press Releases being picked up by USA Today, The Financial Times, NASA’s homepage, and many, many others.

Now that we have established that young SNe can be detected and do show signs of dust formation, our project is moving into its second phase, in which we will concentrate our efforts on quantifying the circumstances under which dust forms in SNe. This winter, we are requesting long-term, coordinated observations of a new sample of targets with Hubble, Spitzer, and ground-based telescopes, so that we can monitor all signatures of dust formation over its entire sequence. While our new targets are being observed, I will also investigate how to best estimate dust-masses by using radiative-transfer models to correlate IR excess with extinction, and to study the effects of cold or clumpy dust.