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On the Binary Nature of Dust-encircled BD +20 307
Three epochs of high-resolution spectra of the star BD +20 307 show thatit is a short-period (~3.4 day) spectroscopic binary of two nearlyidentical stars. Surprisingly, the two stars, although differing ineffective temperature by only ~250 K and having a mass ratio of 0.91,show very different Li line equivalent widths. A Li 6707 Å line isdetected from only the primary star, and it is weak. This star istherefore likely to be older than 1 Gyr. If so, the large amount of hotcircumbinary dust must be from a very large and recent, but very lateevolutionarily, collision of planetesimals.
| Spitzer Observations of the Hyades: Circumstellar Debris Disks at 625 Myr of Age
We use the Spitzer Space Telescope to search for infrared excess at 24,70, and 160 μm due to debris disks around a sample of 45 FGK-typemembers of the Hyades. We supplement our observations with archival 24and 70 μm Spitzer data of an additional 22 FGK-type and 11 A-typeHyades members in order to provide robust statistics on the incidence ofdebris disks at 625 Myr of age, an era corresponding to the late heavybombardment in the solar system. We find that none of the 67 FGK-typestars in our sample show evidence for a debris disk, while 2 out of the11 A-type stars do. This difference in debris disk detection rate islikely to be due to a sensitivity bias in favor of early-type stars. Thefractional disk luminosity, Ldust/L*, of the disksaround the two A-type stars is ~4×10-5, a level that isbelow the sensitivity of our observations toward the FGK-type stars.However, our sensitivity limits for FGK-type stars are able to exclude,at the 2 σ level, frequencies higher than 12% and 5% of disks withLdust/L*>1×10-4 andLdust/L*>5×10-4, respectively.We also use our sensitivity limits and debris disk models to constrainthe maximum mass of dust, as a function of distance from the stars, thatcould remain undetected around our targets.
| Warm Dust in the Terrestrial Planet Zone of a Sun-like Pleiades Star: Collisions between Planetary Embryos?
Only a few solar-type main-sequence stars are known to be orbited bywarm dust particles; the most extreme is the G0 field star BD +20 307that emits ~4% of its energy at mid-infrared wavelengths. We report theidentification of a similarly dusty star HD 23514, an F6-type member ofthe Pleiades. A strong mid-IR silicate emission feature indicates thepresence of small warm dust particles, but with the primary flux densitypeak at the nonstandard wavelength of ~9 μm. The existence of so muchdust within an AU or so of these stars is not easily accounted for giventhe very brief lifetime in orbit of small particles. The apparentabsence of very hot (>~1000 K) dust at both stars suggests thepossible presence of a planet closer to the stars than the dust. Theobserved frequency of the BD +20 307/HD 23514 phenomenon indicates thatthe mass equivalent of Earth's Moon must be converted, via collisions ofmassive bodies, to tiny dust particles that find their way to theterrestrial planet zone during the first few hundred million years ofthe life of many (most?) Sun-like stars. Identification of these twodusty systems among youthful nearby solar-type stars suggests thatterrestrial planet formation is common.
| The Chemical Composition of an Extrasolar Minor Planet
We report the relative abundances of 17 elements in the atmosphere ofthe white dwarf star GD 362, material that, very probably, was containedpreviously in a large asteroid or asteroids with composition similar tothe Earth-Moon system. The asteroid may have once been part of a largerparent body not unlike one of the terrestrial planets of our solarsystem.
| EF Chamaeleontis: Warm Dust Orbiting a Nearby 10 Myr Old Star
Most Vega-like stars have far-infrared excess (60 μm or longward inIRAS, ISO, or Spitzer MIPS bands) and contain cold dust (<~150 K)analogous to the Sun's Kuiper Belt region. However, dust in a regionmore akin to our asteroid belt and thus relevant to the terrestrialplanet building process is warm and produces excess emission inmid-infrared wavelengths. By cross-correlating Hipparcos dwarfs with theMSX catalog, we found that EF Cha, a member of the recently identified,~10 Myr old, ``Cha-Near'' moving group, possesses prominent mid-infraredexcess. N-band spectroscopy reveals a strong emission featurecharacterized by a mixture of small, warm, amorphous, and possiblycrystalline silicate grains. Survival time of warm dust grains aroundthis A9 star is <~105 yr, much less than the age of thestar. Thus, grains in this extrasolar terrestrial planetary zone must beof a ``second generation'' and not a remnant of primordial dust and aresuggestive of substantial planet formation activity. Such secondgeneration warm excess occurs around ~13% of the early-type stars innearby young stellar associations.
| Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| Infrared Emission from the Dusty Disk Orbiting GD 362, an Externally Polluted White Dwarf
We report Spitzer Space Telescope photometry between 3.6 and 24 μmand spectroscopy between 5 and 15 μm of GD 362, a white dwarf with aneffective temperature near 10,000 K that displays a remarkably highconcentration of metals in its photosphere and a thermal infraredexcess. We approximately reproduce both the infrared continuum and thevery strong 10 μm silicate emission feature with a model of anorbiting dusty disk that is flat out to 50 stellar radii and warpedbetween 50 and 70 stellar radii. The relatively small amount of coldmaterial implied by the weak 24 μm flux argues that the disk lieswithin the Roche radius of the star, and we may be witnessing a systemwhere an asteroidal-size body has been tidally destroyed. If so,determination of the photospheric metal abundances may measure the bulkcomposition of an extrasolar minor planet.
| Transience of Hot Dust around Sun-like Stars
In this paper a simple model for the steady state evolution of debrisdisks due to collisions is developed and confronted with the propertiesof the emerging population of seven Sun-like stars that have hot dust at<10 AU. The model shows that there is a maximum possible disk mass ata given age, since more massive primordial disks process their massfaster. The corresponding maximum dust luminosity isfmax=0.16×10-3r7/3t-1age,where r is disk radius in AU and tage is system age in Myr.The majority (4/7) of the hot disks exceed this limit by >>1000and so cannot be the products of massive asteroid belts; rather, thefollowing systems must be undergoing transient events characterized byan unusually high dust content near the star: η Corvi, HD 69830, HD72905, and BD +20 307. It is also shown that the hot dust cannotoriginate in a recent collision in an asteroid belt, since there is alsoa maximum rate at which collisions of sufficient magnitude to reproducea given dust luminosity can occur. The planetesimal belt feeding thedust in these systems must be located farther from the star than thedust, typically at >>2 AU. Other notable properties of the fourhot dust systems are as follows: two also have a planetesimal belt at>10 AU (η Corvi and HD 72905); one has three Neptune mass planetsat <1 AU (HD 69830); all exhibit strong mid-IR silicate features. Weconsider the most likely origin for this transient dust to be adynamical instability that scattered planetesimals inward from a moredistant planetesimal belt in an event akin to the late heavy bombardmentin our own system, the dust being released from such planetesimals incollisions and sublimation.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039
We report the discovery of a debris system associated with the ~30 Myrold G3/5V star HD 12039 using Spitzer Space Telescope observations from3.6-160 μm. An observed infrared excess(LIR/L*=1×10-4) above theexpected photosphere for λ>~14 μm is fit by thermallyemitting material with a color temperature of T~110 K, warmer than themajority of debris disks identified to date around Sun-like stars. Theobject is not detected at 70 μm with a 3 σ upper limit 6 timesthe expected photospheric flux. The spectrum of the infrared excess canbe explained by warm, optically thin material comprised ofblackbody-like grains of size >~7 μm that reside in a beltorbiting the star at 4-6 AU. An alternate model dominated by smallergrains, near the blowout size a~0.5 μm, located at 30-40 AU is alsopossible but requires the dust to have been produced recently, sincesuch small grains will be expelled from the system by radiation pressurein approximately a few times 102 yr.
| Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star
The slow but persistent collisions between asteroids in our Solar Systemgenerate a tenuous cloud of dust known as the zodiacal light (because ofthe light the dust reflects). In the young Solar System, such collisionswere more common and the dust production rate should have been manytimes larger. Yet copious dust in the zodiacal region around stars muchyounger than the Sun has rarely been found. Dust is known to orbitaround several hundred main-sequence stars, but this dust is cold andcomes from a Kuiper-belt analogous region out beyond the orbit ofNeptune. Despite many searches, only a few main-sequence stars revealwarm (> 120K) dust analogous to zodiacal dust near the Earth. Signsof planet formation (in the form of collisions between bodies) in theregions of stars corresponding to the orbits of the terrestrial planetsin our Solar System have therefore been elusive. Here we report anexceptionally large amount of warm, small, silicate dust particlesaround the solar-type star BD+20 307 (HIP8920, SAO75016). Thecomposition and quantity of dust could be explained by recent frequentor huge collisions between asteroids or other `planetesimals' whoseorbits are being perturbed by a nearby planet.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| South galactic CAP G and K stars with infrared excesses
Near-infrared and IRAS photometry is discussed for 13 G and K stars inthe south galactic cap. Five of them show a strong flux excess at 12microns as would be expected from dust shells emitting at temperaturesof several hundred Kelvin. One of them (XY Tau) has previously beenclassified as a 206-d Mira, but the new observations suggest it isactually a variable F or G star. The most likely explanation is that theoriginal classification was wrong and that it is in fact apremain-sequence star. The other four stars are either in very unusualevolutionary states or they are binary systems. The presence oflong-term variations in the near-infrared flux of the flare star CC Eriis reported. The colors are redder when the star is fainter, and thevariations might be associated with the star spot cycle.
| A survey for infrared excesses among high galactic latitude SAO stars
This project involves extending the previous analysis of infraredexcesses among a volume-limited sample of 134 nearby A-K main-sequencestars to a magnitude-limited sample of stars, culled from the SAOCatalog, with excesses determined from the IRAS Point Source Catalogflux density ratios. This new sample includes 5706 B-M type stars, 379of which have infrared excesses. The objective involved use of astatistically complete survey of objects in a standard catalog in orderto assess the frequency with which different physical processes canaffect the infrared output of stars. These processes include, but arenot limited to, orbiting cold particle clouds and the onset of rapidmass loss. It is concluded that cold disks are consistent with theinfrared excesses found among A-G dwarfs and G-K giants in the sample.
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