arXiv subjects

M. Jura

Publications and source records attributed to M. Jura.

At least 19 recordsLinked to original sources

arXiv2022

Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44

Ultraviolet and optical spectra of the hydrogen-dominated atmosphere white dwarf star G238-44 obtained with FUSE, Keck/HIRES, HST/COS, and HST/STIS reveal ten elements heavier than helium: C, N, O, Mg, Al, Si, P, S, Ca, and Fe. G238-44 is only the third white dwarf with nitrogen detected in its atmosphere from polluting planetary system material. Keck/HIRES data taken on eleven nights over 24 years show no evidence for variation in the equivalent width of measured absorption lines, suggesting stable and continuous accretion from a circumstellar reservoir. From measured abundances and limits on other elements we find an anomalous abundance pattern and evidence for the presence of metallic iron. If the pollution is from a single parent body, then it would have no known counterpart within the solar system. If we allow for two distinct parent bodies, then we can reproduce the observed abundances with a mix of iron-rich Mercury-like material and an analog of an icy Kuiper Belt object with a respective mass ratio of 1.7:1. Such compositionally disparate objects would provide chemical evidence for both rocky and icy bodies in an exoplanetary system and would be indicative of a planetary system so strongly perturbed that G238-44 is able to capture both asteroid- and Kuiper Belt-analog bodies near-simultaneously within its $<$100 Myr cooling age.

astro-ph.EP↗

arXiv2021

Monte Carlo computer investigations of higher generation ideal dendrimers

The properties of ideal tri-functional dendrimers with forty-five, ninety-three and one hundred and eighty-nine branches are investigated. Three methods are employed to calculate the mean-square radius of gyration, $g$-ratios, asphericity, shape parameters and form factor. These methods include a Kirchhoff matrix eigenvalue technique, the graph theory approach of Benhamou et al. (2004), and Monte Carlo simulations using a growth algorithm. A novel technique for counting paths in the graph representation of the dendrimers is presented. All the methods are in excellent agreement with each other and with available theoretical predictions. Dendrimers become more symmetrical as the generation and the number of branches increase.

cond-mat.stat-mech↗

arXiv2020

Atmospheric Temperature Inversions and He I 5876 Core Profile Structure in White Dwarfs

We report distinctive core profiles in the strongest optical helium line, He I 5876, from high-resolution high-sensitivity observations of spectral type DB white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES, we find the core appearance to be related to the degree of hydrogen and heavy element content in the atmosphere. New Ca K-line measurements or upper limits are reported for about half the sample stars. He I 5876 emission cores with a self-reversed central component are present for those stars with relatively low hydrogen abundance, as well as relatively low atmospheric heavy element pollution. This self-reversed structure disappears for stars with higher degrees of pollution and/or hydrogen abundance, giving way to a single absorption core. From our model atmospheres, we show that the self-reversed emission cores can be explained by temperature inversions in the upper atmosphere. We propose that the transition to a single absorption core is due to the additional opacity from hydrogen and heavy elements that inhibits the temperature inversions. Our current models do not exactly match the effective temperature range of the phenomenon nor the amplitude of the self-reversed structure, which is possibly a result of missing physics such as 3D treatment, convective overshoot, and/or non-LTE effects. The He I 5876 line structure may prove to be a useful new diagnostic for calibrating temperature profiles in DB atmosphere models.

astro-ph.SR↗

arXiv2017

A dearth of small particles in the transiting material around the white dwarf WD 1145+017

White dwarf WD 1145+017 is orbited by several clouds of dust, possibly emanating from actively disintegrating bodies. These dust clouds reveal themselves through deep, broad, and evolving transits in the star's light curve. Here, we report two epochs of multi-wavelength photometric observations of WD 1145+017, including several filters in the optical, K$_\mathrm{s}$ and 4.5 $μ$m bands in 2016 and 2017. The observed transit depths are different at these wavelengths. However, after correcting for excess dust emission at K$_\mathrm{s}$ and 4.5 $μ$m, we find the transit depths for the white dwarf itself are the same at all wavelengths, at least to within the observational uncertainties of $\sim$5%-10%. From this surprising result, and under the assumption of low optical depth dust clouds, we conclude that there is a deficit of small particles (with radii $s \lesssim$ 1.5 $μ$m) in the transiting material. We propose a model wherein only large particles can survive the high equilibrium temperature environment corresponding to 4.5 hr orbital periods around WD 1145+017, while small particles sublimate rapidly. In addition, we evaluate dust models that are permitted by our measurements of infrared emission.

astro-ph.EP↗

arXiv2017

A SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust in a sample of post-asymptotic giant branch yellow supergiants that are believed to be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon rich and oxygen rich dust species-contrary to simple dredge-up models-with a majority of the dust in the form of amorphous carbon and graphite. The oxygen rich dust is primarily in the form of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests that the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. These results help to determine the physical properties of the post-AGB circumstellar environment and to constrain models of post-AGB mass loss and planetary nebula formation.

astro-ph.SR↗

arXiv2017

The Chemical Composition of an Extrasolar Kuiper-Belt-Object

The Kuiper Belt of our solar system is a source of short-period comets that may have delivered water and other volatiles to Earth and the other terrestrial planets. However, the distribution of water and other volatiles in extrasolar planetary systems is largely unknown. We report the discovery of an accretion of a Kuiper-Belt-Object analog onto the atmosphere of the white dwarf WD 1425+540. The heavy elements C, N, O, Mg, Si, S, Ca, Fe, and Ni are detected, with nitrogen observed for the first time in extrasolar planetary debris. The nitrogen mass fraction is $\sim$ 2\%, comparable to that in comet Halley and higher than in any other known solar system object. The lower limit to the accreted mass is $\sim$ 10$^{22}$ g, which is about one hundred thousand times the typical mass of a short-period comet. In addition, WD 1425+540 has a wide binary companion, which could facilitate perturbing a Kuiper-Belt-Object analog into the white dwarf's tidal radius. This finding shows that analogs to objects in our Kuiper Belt exist around other stars and could be responsible for the delivery of volatiles to terrestrial planets beyond

astro-ph.EP↗

arXiv2015

Evidence for Gas from a Disintegrating Extrasolar Asteroid

We report high-resolution spectroscopic observations of WD 1145+017 -- a white dwarf that recently has been found to be transitted by multiple asteroid-sized objects within its tidal radius. We have discovered numerous circumstellar absorption lines with linewidths of $\sim$ 300 km s$^{-1}$ from Mg, Ca, Ti, Cr, Mn, Fe and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe and Ni. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

astro-ph.EP↗

arXiv2015

Lambda Boo Abundance Patterns: Accretion from Orbiting Sources

The abundance anomalies in lambda Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically-inferred bounds for interstellar accretion. Therefore, a lambda Boo star's thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt Objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some lambda Boo stars accrete from the winds of hot Jupiters.

astro-ph.SR↗

arXiv2015

A Young White Dwarf with an Infrared Excess

Using observations of Spitzer/IRAC, we report the serendipitous discovery of excess infrared emission from a single white dwarf PG 0010+280. At a temperature of 27,220 K and a cooling age of 16 Myr, it is the hottest and youngest white dwarf to display an excess at 3-8 $μ$m. The infrared excess can be fit by either an opaque dust disk within the tidal radius of the white dwarf or a 1300 K blackbody, possibly from an irradiated substellar object or a re-heated giant planet. PG 0010+280 has two unique properties that are different from white dwarfs with a dust disk: (i) relatively low emission at 8 $μ$m and (ii) non-detection of heavy elements in its atmosphere from high-resolution spectroscopic observations with Keck/HIRES. The origin of the infrared excess remains unclear.

astro-ph.SR↗

arXiv2014

Evidence for an Anhydrous Carbonaceous Extrasolar Minor Planet

Using Keck/HIRES, we report abundances of 11 different elements heavier than helium in the spectrum of Ton 345, a white dwarf that has accreted one of its own minor planets. This particular extrasolar planetesimal which was at least 60% as massive as Vesta appears to have been carbon-rich and water-poor; we suggest it was compositionally similar to those Kuiper Belt Objects with relatively little ice.

astro-ph.EP↗

arXiv2014

The Drop during Less than 300 days of a Dusty White Dwarf's Infrared Luminosity

We report Spitzer/IRAC photometry of WD J0959-0200, a white dwarf that displays excess infrared radiation from a disk, likely produced by a tidally disrupted planetesimal. We find that in 2010, the fluxes in both 3.6 $μ$m and 4.5 $μ$m decreased ~ 35% in less than 300 days. The drop in the infrared luminosity is likely due to an increase of the inner disk radius from one of two scenarios: (i) a recent planetesimal impact; (ii) instability in the circumstellar disk. The current situation is tantalizing; high sensitivity, high cadence infrared studies will be a new tool to study the interplay between a disk and its host white dwarf star.

astro-ph.SR↗

arXiv2014

A Pilot Search for Evidence of Extrasolar Earth-analog Plate Tectonics

Relative to calcium, both strontium and barium are markedly enriched in Earth's continental crust compared to the basaltic crusts of other differentiated rocky bodies within the solar system. Here, we both re-examine available archived Keck spectra to place upper bounds on n(Ba)/n(Ca) and revisit published results for n(Sr)/n(Ca) in two white dwarfs that have accreted rocky planetesimals. We find that at most only a small fraction of the pollution is from crustal material that has experienced the distinctive elemental enhancements induced by Earth-analog plate tectonics. In view of the intense theoretical interest in the physical structure of extrasolar rocky planets, this search should be extended to additional targets.

astro-ph.EP↗

arXiv2014

Effect of carrier concentration on magnetism and magnetic order in the pyrochlore iridates

We present resistivity, magnetization, and zero field muon spin relaxation ($μ$SR) data for the pyrochlore iridate materials Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ ($x = 0, 0.06$, and $0.10$) and Sm$_2$Ir$_2$O$_7$. While Nd$_{2}$Ir$_{2}$O$_{7}$ (Nd227) is weakly conducting, Sm$_{2}$Ir$_{2}$O$_{7}$ (Sm227) has slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at $T_{M} = 105 K$ and $137 K$, respectively. However, zero-field $μ$SR measurements show that long-range magnetic order of the Ir$^{4+}$ sublattice sets in at much lower temperatures ($T_{LRO} \sim 8 K$ for Nd227 and $70 K$ for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at $T_{M}$ in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ samples, but the muon precession is fully suppressed for both.

cond-mat.str-el↗

arXiv2014

Elemental Compositions of Two Extrasolar Rocky Planetesimals

We report Keck/HIRES and HST/COS spectroscopic studies of extrasolar rocky planetesimals accreted onto two hydrogen atmosphere white dwarfs, G29-38 and GD 133. In G29-38, 8 elements are detected, including C, O, Mg, Si, Ca, Ti, Cr and Fe while in GD 133, O, Si, Ca and marginally Mg are seen. These two extrasolar planetesimals show a pattern of refractory enhancement and volatile depletion. For G29-38, the observed composition can be best interpreted as a blend of a chondritic object with some refractory-rich material, a result from post-nebular processing. Water is very depleted in the parent body accreted onto G29-38, based on the derived oxygen abundance. The inferred total mass accretion rate in GD 133 is the lowest of all known dusty white dwarfs, possibly due to non-steady state accretion. We continue to find that a variety of extrasolar planetesimals all resemble to zeroth order the elemental composition of bulk Earth.

astro-ph.EP↗

arXiv2013

26Al in the Early Solar System: Not so Unusual After All

Recently acquired evidence shows that extrasolar asteroids exhibit over a factor of 100 variation in the iron to aluminum abundance ratio. This large range likely is a consequence of igneous differentiation that resulted from heating produced by radioactive decay of 26Al with an abundance comparable to that in the solar system's protoplanetary disk at birth. If so, the conventional view that our solar system began with an unusually high amount of 26Al should be discarded.

astro-ph.EP↗

arXiv2013

The Hyades Cluster: Identification of a Planetary System and Escaping White Dwarfs

Recently, some hot DA-type white dwarfs have been proposed to plausibly be escaping members of the Hyades. We used hydrogen Balmer lines to measure the radial velocities of seven such stars and confirm that three, and perhaps two others, are/were indeed cluster members and one is not. The other candidate Hyad is strongly magnetic and its membership status remains uncertain. The photospheres of at least one quarter of field white dwarf stars are "polluted" by elements heavier than helium that have been accreted. These stars are orbited by extended planetary systems that contain both debris belts and major planets. We surveyed the seven classical single Hyades white dwarfs and the newly identified (escaping) Hyades white dwarfs and found calcium in the photosphere of LP 475-242 of type DBA (now DBAZ), thus implying the presence of an orbiting planetary system. The spectrum of white dwarf GD 31, which may be, but probably is not, an escaping member of the Hyades, displays calcium absorption lines; these originate either from the interstellar medium or, less likely, from a gaseous circumstellar disk. If GD 31 was once a Hyades member, then it would be the first identified white dwarf Hyad with a cooling age >340 Myr.

astro-ph.SR↗

arXiv2013

Two Beyond-Primitive Extrasolar Planetesimals

Using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we have obtained high-resolution ultraviolet observations of GD 362 and PG 1225-079, two helium-dominated, externally-polluted white dwarfs. We determined or placed useful upper limits on the abundances of two key volatile elements, carbon and sulfur, in both stars; we also constrained the zinc abundance in PG 1225-079. In combination with previous optical data, we find strong evidence that each of these two white dwarfs has accreted a parent body that has evolved beyond primitive nebular condensation. The planetesimal accreted onto GD 362 had a bulk composition roughly similar to that of a mesosiderite meteorite based on a reduced chi-squared comparison with solar system objects; however, additional material is required to fully reproduce the observed mid-infrared spectrum for GD 362. No single meteorite can reproduce the unique abundance pattern observed in PG 1225-079; the best fit model requires a blend of ureilite and mesosiderite material. From a compiled sample of 9 well-studied polluted white dwarfs, we find evidence for both primitive planetesimals, which are a direct product from nebular condensation, as well as beyond-primitive planetesimals, whose final compositions were mainly determined by post-nebular processing.

astro-ph.EP↗

arXiv2013

Discovery of Molecular Hydrogen in White Dwarf Atmospheres

With the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we have detected molecular hydrogen in the atmospheres of three white dwarfs with effective temperatures below 14,000 K, G29-38, GD 133 and GD 31. This discovery provides new independent constraints on the stellar temperature and surface gravity of white dwarfs.

astro-ph.SR↗