John H. Debes

An intermediate luminosity transient in NGC 300: The eruption of a dust-enshrouded massive star

We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discovered in the nearby galaxy NGC 300. We find that the transient (NGC 300 OT2008-1) has a peak absolute magnitude of M-bol approximate to -11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85 OT2006-1 and SN 2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (similar to 200-1000 km s(-1)) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect asymmetric Ca II H&K absorption with a broad red wing extending to similar to 10(3) km s(-1), indicative of gas inflow at high velocity (possibly the wind of a massive binary companion). The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points to a dust-enshrouded star with a luminosity of about 6 x 10(4) L-circle dot, indicative of a similar to 10-20 M-circle dot progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation. (Less)

STIS IMAGING OF THE HR 4796A CIRCUMSTELLAR DEBRIS RING

We have obtained high spatial resolution imaging observations of the HR 4796A circumstellar debris dust ring using the broad optical response of the Hubble Space Telescope Imaging Spectrograph (STIS) in coronagraphic mode. We use our visual wavelength observations to improve upon the earlier measured geometrical parameters of the ring-like disk. Two significant flux density asymmetries are noted: (1) preferential forward scattering by the disk grains and (2) an azimuthal surface brightness anisotropy about the morphological minor axis of the disk with corresponding differential ansal brightness. We find the debris ring offset from the location of the star by ~1.4 AU, a shift insufficient to explain the differing brightnesses of the northeast and southwest ansae simply by the 1/r 2 dimmunition of starlight. The STIS data also better quantify the radial confinement of the starlight-scattering circumstellar debris, to a characteristic region less than 14 AU in photometric half-width, with a significantly steeper inner truncation than outward falloff in radial surface brightness. The inferred spatial distribution of the disk grains is consistent with the possibility of one or more unseen co-orbital planetary-mass perturbers, and the colors of the disk grains are consistent with a collisionally evolved population of debris, possibly including ices reddened by radiation exposure to the central star.

COMPLEX ORGANIC MATERIALS IN THE CIRCUMSTELLAR DISK OF HR 4796A

We combine HST NICMOS imaging photometry of the HR 4796A disk at previously unobserved wavelengths between 1.71-2.22 μm with reprocessed archival observations to produce a measure of the dusts scattering efficiency as a function of wavelength. The spectrum of the dust, synthesized from the seven photometric measures, is characterized by a steep red slope increasing from 0.5 to 1.6 μm followed by a flattening of the spectrum at wavelengths >1.6 μm. We fit the spectrum with a model population of dust grains made of tholins, materials composed of complex organic materials seen throughout the outer parts of our solar system. The presence of organic material around a star that may be in the later stages of giant planet formation implies that the basic building blocks for life may be common in planetary systems.

Measuring M Dwarf Winds with DAZ White Dwarfs

Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, show evidence for ongoing accretion of material onto their surfaces. Some DAZs are known to have unresolved M dwarf companions, which could account for the observed accretion through a stellar wind. I combine observed Ca abundances of the DAZs with information on the orbital separation of their M dwarf companions to infer the mass-loss rate of the M dwarfs. I find that for three of the six known DAZs with M dwarf companions, a stellar wind can plausibly explain the observed accretion on the white dwarfs assuming Bondi-Hoyle accretion of solar abundance stellar winds on the order of 10-14 to 10-16 M☉ yr-1. The rest of the sample have companions with orbits 1 AU and require companion mass-loss rates of >10-11 M☉ yr-1. I conclude that there must be an alternative explanation for accretion of material onto DAZs with widely separated companions. The inferred winds for two of the close binaries are orders of magnitude smaller than typically assumed for the angular momentum loss of red dwarf-white dwarf pairs due to magnetic braking from a stellar wind and may seriously affect predictions for the formation rate of CVs with low-mass companions.

THE SURVIVAL RATE OF EJECTED TERRESTRIAL PLANETS WITH MOONS

During planet formation, a gas giant will interact with smaller protoplanets that stray within its sphere of gravitational influence. We investigate the outcome of interactions between gas giants and terrestrial-sized protoplanets with lunar-sized companions. An interaction between a giant planet and a protoplanet binary may have one of several consequences, including the delivery of volatiles to the inner system, the capture of retrograde moons by the giant planet, and the ejection of one or both of the protoplanets. We show that an interesting fraction of terrestrial-sized planets with lunar-sized companions will likely be ejected into interstellar space with the companion bound to the planet. The companion provides an additional source of heating for the planet from tidal dissipation of orbital and spin angular momentum. This heat flux typically is larger than the current radiogenic heating of the Earth for up to the first few hundred million years of evolution. In combination with an atmosphere of sufficient thickness and composition, the heating can provide the conditions necessary for liquid water to persist on the surface of the terrestrial-mass planet, making it a potential site for life. We also determine the possibility for directly detecting such systems through all-sky infrared surveys or microlensing surveys. Microlensing surveys in particular will directly measure the frequency of this phenomenon.

Color Gradients Detected in the HD 15115 Circumstellar Disk

We report HST NICMOS coronagraphic images of the HD 15115 circumstellar disk at 1.1 μm. We find a similar morphology to that seen in the visible and at H band—an edge-on disk that is asymmetric in surface brightness. Several aspects of the 1.1 μm data are different, highlighting the need for multiwavelength images of each circumstellar disk. We find a flattening to the western surface brightness profile at 1.1 μm interior to 2 (90 AU) and a warp in the western half of the disk. We measure the surface brightness profiles of the two disk lobes and create a measure of the dust scattering efficiency between 0.55 and 1.65 μm at 1, 2, and 3. At 2 the western lobe has a neutral spectrum up to 1.1 μm and a strong absorption or blue spectrum >1.1 μm, while a blue trend is seen in the eastern lobe. At 1 the disk has a red F 110W − H color in both lobes.

Detection of a variable infrared excess around SDSS J121209.31+013627.7

We present near-infrared JHKs photometry and light curves of the candidate magnetic white dwarf+brown dwarf binary SDSS J121209.31+013627.7 and report on the detection of near-infrared excess and variability in the Ks band. The observed near-infrared excess can be explained by the presence of an L7 brown dwarf and an extra emission source. The J and H light curves appear flat, which rules out eclipses deeper than 0.2 mag and the presence of an accretion hot spot on the white dwarf. From the variable Ks light curve, we get a refined period for the system of 88 ± 1 minutes. We show that the observed variability in the Ks band can be explained by cyclotron emission, which can be modeled by a small spot on the surface of the white dwarf. SDSS J121209.31+013627.7 exhibits similarities to the ultra-short-period Polar EF Eridani; however, the lack of evidence for Roche lobe overflow accretion suggests that it may be a pre-Polar.

A SECOND LOOK AT THE METAL LINE VARIABILITY OF G29-38

The pulsating white dwarf G29-38 possesses a dust disk and metal lines attributed to the accretion of its disk material. Von Hippel and Thompson have reported variability in the equivalent width of G29-38s Ca II K line on a timescale of days. We use high-resolution optical spectroscopy of G29-38s Ca II K line to test this observation. Over 6 days spanning 2007 June to 2007 October we see no evidence for variability in the equivalent width of the Ca II K line. We also sample the variability of the Ca II K line over integrated timescales of ~100-500 s, where errors from incomplete coverage of pulsation modes are predicted to be ~8%-15%. We find that the scatter of the equivalent widths over this time period is consistent with measurement errors at the 7% level, slightly weaker than predicted but within the uncertainties of predictions. Weaker Ca and Mg lines observed show no significant variability on yearly timescales over 10 years based on our data and other high-resolution spectra. We conclude that further study is warranted to verify whether the accretion onto G29-38 is variable.

Cool Customers in the Stellar Graveyard. IV. Spitzer Search for Mid-IR excesses Around Five DAs

Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, require external accretion of material to explain the presence of weak metal-line absorption in their photospheres. The source of this material is currently unknown, but could come from the interstellar medium, unseen companions, or relic planetesimals from asteroid belt or Kuiper Belt analogs. Accurate mid-infrared photometry of these white dwarfs provides additional information to solve the mystery of this accretion and to look for evidence of planetary systems that have survived post-main-sequence evolution. We present Spitzer Space Telescope IRAC photometry accurate to ~3% for four DAZs and one DA with circumstellar absorption lines in the UV. We search for excesses due to unseen companions or circumstellar dust disks. We use Hubble Space Telescope NICMOS imaging of these white dwarfs to gauge the level of background contamination to our targets, as well as to rule out common proper motion companions to WD 1620-391. All of our targets show no excesses due to companions >20 MJ, ruling out all but very low mass companions to these white dwarfs at all separations. No excesses due to circumstellar disks are observed, and we place limits on what types of disks may still be present.

High Contrast Imaging with Gaussian Aperture Pupil Masks

Gaussian aperture pupil masks (GAPMs) can, in theory, achieve the requisite contrast for directly imaging an extrasolar planet around a nearby solar-type star. We outline the process of designing, fabricating, and testing a GAPM for use on current telescopes, and specifically the Penn State near-IR Imager and Spectrograph (PIRIS) at the Mount Wilson 100 inch (2.54 m) telescope. We find that the initial prototype observations are quite successful, achieving a contrast similar to a traditional Lyot coronagraph without blocking any light from a central object, and useful for finding faint companions to nearby young solar analogues. In the lab, we can reproduce the expected point-spread function reasonably well and with a single-aperture design that achieves ~4 × 10-5 contrast at 10λ/D. We find that small inaccuracies in the mask fabrication process and insufficient correction of the atmosphere contribute the most degradation to contrast at these levels.