arXiv subjects

P. Dufour

Publications and source records attributed to P. Dufour.

At least 55 records · Page 3Linked to original sources

arXiv2013

Orbital and Evolutionary Constraints on the Planet Hosting Binary GJ 86 from the Hubble Space Telescope

This paper presents new observations of the planet-hosting, visual binary GJ 86 (HR 637) using the Hubble Space Telescope. Ultraviolet and optical imaging with WFC3 confirms the stellar companion is a degenerate star and indicates the binary semimajor axis is larger than previous estimates, with a > 28 AU. Optical STIS spectroscopy of the secondary reveals a helium-rich white dwarf with C2 absorption bands and Teff = 8180 K, thus making the binary system rather similar to Procyon. Based on the 10.8 pc distance, the companion has 0.59 Msun and descended from a main-sequence A star of 1.9 Msun with an original orbital separation a > 14 AU. If the giant planet is coplanar with the binary, the mass of GJ 86Ab is between 4.4 and 4.7 MJup. The similarity of GJ 86 and Procyon prompted a re-analysis of the white dwarf in the latter system, with the tentative conclusion that Procyon hosts a planetesimal population. The periastron distance in Procyon is 20% smaller than in alpha Cen AB, but the metal-enriched atmosphere of Procyon B indicates that the planet formation process minimally attained 25 km bodies, if not small planets as in alpha Cen.

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arXiv2012

SDSS DR7 White Dwarf Catalog

We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital Sky Survey Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent a more than factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalog based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log(g) if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

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arXiv2012

The Frequency of Debris Disks at White Dwarfs

We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3+2.7-1.2% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7Msol stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of white dwarf cooling ages; white dwarfs accrete debris from several generations of disks for ~10Myr. The average total mass accreted by these stars ranges from that of 200km asteroids to Ceres-sized objects, indicating that white dwarfs accrete moons and dwarf planets as well as Solar System asteroid analogues.

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arXiv2012

Gaseous Material Orbiting the Polluted, Dusty White Dwarf HE1349-2305

We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE1349-2305. Optical spectroscopy reveals weak CaII infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE1349-2305 is the first gas disk-hosting white dwarf star identified amongst previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

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arXiv2012

A Multi-Survey Approach to White Dwarf Discovery

By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{é}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be a pulsating ZZ Ceti white dwarf.

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arXiv2012

Detailed compositional analysis of the heavily polluted DBZ white dwarf SDSS J073842.56+183509.06: A window on planet formation?

We present a new model atmosphere analysis of the most metal contaminated white dwarf known, the DBZ SDSS J073842.56+183509.06. Using new high resolution spectroscopic observations taken with Keck and Magellan, we determine precise atmospheric parameters and measure abundances of 14 elements heavier than helium. We also report new Spitzer mid-infrared photometric data that are used to better constrain the properties of the debris disk orbiting this star. Our detailed analysis, which combines data taken from 7 different observational facilities (GALEX, Gemini, Keck, Magellan, MMT, SDSS and Spitzer) clearly demonstrate that J0738+1835 is accreting large amounts of rocky terrestrial-like material that has been tidally disrupted into a debris disk. We estimate that the body responsible for the photospheric metal contamination was at least as large Ceres, but was much drier, with less than 1% of the mass contained in the form of water ice, indicating that it formed interior to the snow line around its parent star. We also find a correlation between the abundances (relative to Mg and bulk Earth) and the condensation temperature; refractory species are clearly depleted while the more volatile elements are possibly enhanced. This could be the signature of a body that formed in a lower temperature environment than where Earth formed. Alternatively, we could be witnessing the remains of a differentiated body that lost a large part of its outer layers.

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arXiv2011

The Discovery of a Debris Disk Around the DAV White Dwarf PG 1541+651

To search for circumstellar disks around evolved stars, we targeted roughly 100 DA white dwarfs from the Palomar Green survey with the Peters Automated Infrared Imaging Telescope (PAIRITEL). Here we report the discovery of a debris disk around one of these targets, the pulsating white dwarf PG 1541+651 (KX Draconis, hereafter PG1541). We detect a significant flux excess around PG1541 in the K-band. Follow-up near-infrared spectroscopic observations obtained at the NASA Infrared Telescope Facility (IRTF) and photometric observations with the warm Spitzer Space Telescope confirm the presence of a warm debris disk within 0.13-0.36 Rsun (11-32x the stellar radius) at an inclination angle of 60deg. At Teff = 11880 K, PG1541 is almost a twin of the DAV white dwarf G29-38, which also hosts a debris disk. All previously known dusty white dwarfs are of the DAZ/DBZ spectral type due to accretion of metals from the disk. High-resolution optical spectroscopy is needed to search for metal absorption lines in PG1541 and to constrain the accretion rate from the disk. PG1541 is only 55 pc away from the Sun and the discovery of its disk in our survey demonstrates that our knowledge of the nearby dusty white dwarf population is far from complete.

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arXiv2011

Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarf Stars

I present a broad historical review on cool white dwarf stars with carbon and/or oxygen detected at the photosphere. This book chapter covers the observational signatures, physical properties, and evolution of DQ, DBQ and Hot DQ white dwarfs, and also present an overview of the main challenges that future investigations of these types of object should try to address.

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arXiv2011

Stark Broadening of Carbon and Oxygen Lines in Hot DQ White Dwarf Stars: Recent Results and Applications

White dwarf stars are traditionally found to have surface compositions made primarily of hydrogen or helium. However, a new family has recently been uncovered, the so-called Hot DQ white dwarfs, which have surface compositions dominated by carbon and oxygen with little or no trace of hydrogen and helium (Dufour et al. 2007, 2008, 2010). Deriving precise atmospheric parameters for these objects (such as the effective temperature and the surface gravity) requires detailed modeling of spectral line profiles. Stark broadening parameters are of crucial importance in that context. We present preliminary results from our new generation of model atmosphere including the latest Stark broadening calculations for CII lines and discuss the implications as well as future work that remains to be done.

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arXiv2011

An aluminum/calcium-rich, iron-poor, white dwarf star: evidence for an extrasolar planetary lithosphere?

The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. Best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers.

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arXiv2011

Cool DZ white dwarfs in the SDSS

We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) u-g vs. g-r two-color diagram; 21 of these stars are new discoveries. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of log g = 8. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around Teff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, probably because of our selection procedures. The interpretation is complicated in terms of the accretion/diffusion scenario, because we do not know if accretion is still occurring or has ended long ago. Independent of that uncertainty, the masses of the metals currently present in the convection zones - and thus an absolute lower limit of the total accreted masses - of these stars are similar to the largest asteroids in our solar system.

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arXiv2011

Taking Advantage of the COS Time-Tag Capability: Observations of Pulsating Hot DQ White Dwarfs and Discovery of a New One

We present an analysis of the ultraviolet light curves of five Hot DQ white dwarfs recently observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). These light curves were constructed by extracting the time-tag information from the FUV and NUV spectroscopic data. Single-color light curves were thus produced in 60 s time bins. The Fourier analysis of these data successfully recovers the main pulsation modes of the three stars previously known to be variable from ground-based observations. We also report the discovery of pulsations in another object, SDSS J1153+0056, making it only the fifth member of the new class of variable Hot DQ stars, and the first pulsating white dwarf to be discovered from space-based observations. The relatively high amplitudes of the modes observed in the FUV -- 2 to 4 times that observed in the optical -- as well as the high fraction of stars variable in our sample suggest that most, if not all, Hot DQ white dwarfs might be pulsating at some level when observed at high enough sensitivity. Our results also underline the vast potential of the time-tag capability of the HST/COS combination.

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arXiv2011

Accretion of a Terrestrial-Like Minor Planet by a White Dwarf

We present optical and infrared characterization of the polluted DAZ white dwarf GALEX J193156.8+011745. Imaging and spectroscopy from the ultraviolet to the thermal infrared indicates that the white dwarf hosts excess infrared emission consistent with the presence of an orbiting dusty debris disk. In addition to the five elements previously identified, our optical echelle spectroscopy reveals chromium and manganese and enables restrictive upper limits on several other elements. Synthesis of all detections and upper limits suggests that the white dwarf has accreted a differentiated parent body. We compare the inferred bulk elemental composition of the accreted parent body to expectations for the bulk composition of an Earth-like planet stripped of its crust and mantle and find relatively good agreement. At least two processes could be important in shaping the final bulk elemental composition of rocky bodies during the late phases of stellar evolution: irradiation and interaction with the dense stellar wind.

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arXiv2011

The Magnetic and Metallic Degenerate G77-50

An accumulation of multi-epoch, high-resolution, optical spectra reveal that the nearby star G77-50 is a very cool DAZ white dwarf externally polluted by Mg, Fe, Al, Ca, and possibly Na, Cr, Mn. The metallic and hydrogen absorption features all exhibit multiple components consistent with Zeeman splitting in a B~120kG magnetic field. Ultraviolet through infrared photometry combined with trigonometric parallaxes yield Teff=5310K, M=0.60Msun, and a cooling age of 5.2Gyr. The space velocity of the white dwarf suggests possible membership in the Galactic thick disk, consistent with an estimated total age of 8.6Gyr. G77-50 is spectrally similar to G165-7 and LHS 2534; these three cool white dwarfs comprise a small group exhibiting both metals and magnetism. The photospheric metals indicate accretion of rocky debris similar to that contained in asteroids, but the cooling age implies a remnant planetary system should be stable. A possibility for G77-50 and similarly old, polluted white dwarfs is a recent stellar encounter that dynamically rejuvenated the system from the outside-in. Metal abundance measurements for these cooler white dwarfs have the potential to distinguish material originating in outer region planetesimals injected via fly-by. If common envelope evolution can generate magnetic fields in white dwarfs, then G77-50 and its classmates may have cannibalized an inner giant planet during prior evolution, with their metals originating in terrestrial bodies formed further out. Although speculative, this scenario can be ruled out if terrestrial planet formation is prohibited in systems where a giant planet has migrated to the inner region nominally engulfed during the post-main sequence.

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arXiv2010

The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet

Cool white dwarf stars are usually found to have an outer atmosphere that is practically pure in hydrogen or helium. However, a small fraction have traces of heavy elements that must originate from the accretion of extrinsic material, most probably circumstellar matter. Upon examining thousands of Sloan Digital Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS J073842.56+183509.6 shows the most severe metal pollution ever seen in the outermost layers of such stars. We present here a quantitative analysis of this exciting star by combining high S/N follow-up spectroscopic and photometric observations with model atmospheres and evolutionary models. We determine the global structural properties of our target star, as well as the abundances of the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca, and Fe. The relative abundances of these elements imply that the source of the accreted material has a composition similar to that of Bulk Earth. We also report the signature of a circumstellar disk revealed through a large infrared excess in JHK photometry. Combined with our inferred estimate of the mass of the accreted material, this strongly suggests that we are witnessing the remains of a tidally disrupted extrasolar body that was as large as Ceres.

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arXiv2010

Multiwavelength Observations of the Hot DB Star PG 0112+104

We present a comprehensive multiwavelength analysis of the hot DB white dwarf PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on medium-resolution FOS and GHRS data, on archival high-resolution GHRS observations, on optical spectrophotometry both in the blue and around Halpha, as well as on time-resolved photometry. From the optical data, we derive a self-consistent effective temperature of 31,300+-500 K, a surface gravity of log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) < -4.0. The FUSE spectra reveal the presence of CII and CIII lines that complement the previous detection of CII transitions with the GHRS. The improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23. No photospheric features associated with other heavy elements are detected. We reconsider the role of PG 0112+104 in the definition of the blue edge of the V777 Her instability strip in light of our high-speed photometry, and contrast our results with those of previous observations carried out at the McDonald Observatory.

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arXiv2010

Rocky Planetesimals as the Origin of Metals in DZ Stars

{Abridged}. An analysis of the calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ stars from the Sloan Digital Sky Survey demonstrates that interaction with the interstellar medium cannot account for their externally polluted atmospheres. The calcium-to-hydrogen ratios for the 37 DZA stars are dominated by super-solar values, as are the lower limits for the remaining 109 DZ stars. All together their metal-contaminated convective envelopes contain 10^{20+-2} g of calcium, commensurate with the masses of calcium inferred for large asteroids. It is probable that these stars are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge: 1) at least 3.5% of all main sequence A- and F-type stars build terrestrial planets; and 2) the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water-rich, extrasolar planetesimals.

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arXiv2009

Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs

We present follow-up time-series photometric observations that confirm and extend the results of the significant discovery made by Barlow et al.(2008) that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS J234843.30-094245.3 are luminosity variable. These are the second and third known members of a new class of pulsating white dwarfs, after the prototype SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s, and that the light pulse folded on that period is highly nonlinear due to the presence of the first and second harmonic of the main pulsation. We also present evidence for the possible detection of two additional pulsation modes with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3 is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no sign of harmonic components. A new oscillation, with a low amplitude and a period of 416.919+-0.004 s, is also probably detected in that second star. We argue, on the basis of the very different folded pulse shapes, that SDSS J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is probably not.

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