DSS Images Other Images
|Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations|
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
|Discovery of a Brown Dwarf Very Close to the Sun: A Methane-rich Brown Dwarf Companion to the Low-Mass Star SCR 1845-6357|
We present VLT/NACO SDI images of the very nearby star SCR 1845-6357(hereafter SCR 1845). SCR 1845 is a recently discovered M8.5 star just3.85 pc from the Sun. Using the capabilities of the unique SDI device,we discovered a substellar companion to SCR 1845 at a separation of 4.5AU (1.170"+/-0.003" on the sky) and fainter by 3.57+/-0.057 mag in the1.575 μm SDI filter. This substellar companion has an H magnitude of13.16+0.31-0.26 (absolute H magnitude of15.30+0.31-0.26), making it likely the brightestmid-T dwarf known. The Simultaneous Differential Imager (SDI) consistsof three narrowband filters placed around the 1.6 μm methaneabsorption feature characteristic of T dwarfs (Teff<1200K). The flux of the substellar companion drops by a factor of 2.7+/-0.1between the SDI F1 (1.575 μm) and F3 (1.625 μm) filters,consistent with strong methane absorption in a substellar companion. Weestimate a spectral type of T5.5+/-1 for the companion based on thestrength of this methane break. The chances that this object is abackground T dwarf are vanishingly small-and there is no isolatedbackground T dwarf in this part of the sky, according to 2MASS. Thus, itis a bound companion, hereafter SCR 1845B. For an age range of 100 Myrto 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of(9-65)MJup for SCR 1845B from the Baraffe et al. ``COND''models. SCR 1845AB is the 24th-closest stellar system to the Sun (at3.85 pc); the only brown dwarf system closer to the Sun is the binarybrown dwarf ɛ Indi Ba-Bb (at 3.626 pc). In addition, this is thefirst T dwarf companion discovered around a low-mass star.Based on observations collected at the European Southern Observatory,Paranal, Chile, through proposal 075.C-0357(A).
|IRS Spectra of Solar-Type Stars: A Search for Asteroid Belt Analogs|
We report the results of a spectroscopic search for debris diskssurrounding 41 nearby solar-type stars, including eight planet-bearingstars, using the Infrared Spectrometer (IRS) on the Spitzer SpaceTelescope. With the accurate relative photometry of the IRS between 7and 34 μm we are able to look for excesses as small as ~2% ofphotospheric levels, with particular sensitivity to weak spectralfeatures. For stars with no excess, the 3 σ upper limit in a bandat 30-34 μm corresponds to ~75 times the brightness of our zodiacaldust cloud. Comparable limits at 8.5-13 μm correspond to ~1400 timesthe brightness of our zodiacal dust cloud. These limits correspond tomaterial located within the <1 to ~5 AU region that, in our solarsystem, originates predominantly from debris associated with theasteroid belt. We find excess emission longward of ~25 μm from fivestars, of which four also show excess emission at 70 μm. Thisemitting dust must be located in a region starting around 5-10 AU. Onestar has 70 μm emission but no IRS excess. In this case, the emittingregion must begin outside 10 AU; this star has a known radial velocityplanet. Only two stars of the five show emission shortward of 25 μm,where spectral features reveal the presence of a population of small,hot dust grains emitting in the 7-20 μm band. One of these stars, HD72905, is quite young (300 Myr), while the other, HD 69830, is olderthan 2 Gyr. The data presented here strengthen the results of previousstudies to show that excesses at 25 μm and shorter are rare: only 1out of 40 stars older than 1 Gyr or ~2.5% shows an excess. Asteroidbelts 10-30 times more massive than our own appear are rare amongmature, solar-type stars.
|A Method for Determining the Physical Properties of the Coldest Known Brown Dwarfs|
We present a method for measuring the physical parameters of the coldestT-type brown dwarfs using low-resolution near-infrared spectra. Bycomparing H2O and H2-sensitive spectral ratiosbetween empirical data and theoretical atmosphere models, andcalibrating these ratios to measurements for the well-characterized 2-5Gyr companion brown dwarf Gliese 570D, we derive estimates of theeffective temperatures and surface gravities for 13 mid- and late-typefield T dwarfs. We also deduce the first quantitative estimate ofsubsolar metallicity for the peculiar T dwarf 2MASS 0937+2931. Derivedtemperatures are consistent with prior estimates based on parallax andbolometric luminosity measurements, and examination of possiblesystematic effects indicate that the results are robust. Two recentlydiscovered late-type T dwarfs, 2MASS 0939-2448 and 2MASS 1114-2618, bothappear to be >~50 K cooler than the latest type T dwarf, 2MASS0415-0935, and are potentially the coldest and least luminous browndwarfs currently known. We find that, in general, higher surface gravityT dwarfs have lower effective temperatures and luminosities for a givenspectral type, explaining previously observed scatter in theTeff/spectral type relation for these objects. Masses, radii,and ages are estimated for the T dwarfs in our sample using theevolutionary models of Burrows et al.; we also determine masses andradii independently for eight T dwarfs with measured luminosities. Thesetwo determinations are largely consistent, lending support to thevalidity of evolutionary models at late ages. Our method is well suitedto large samples of faint brown dwarfs and can ultimately be used todirectly measure the substellar mass function and formation history inthe Galaxy.
|Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer MIPS Survey|
We have searched for infrared excesses around a well-defined sample of69 FGK main-sequence field stars. These stars were selected withoutregard to their age, metallicity, or any previous detection of IRexcess; they have a median age of ~4 Gyr. We have detected 70 μmexcesses around seven stars at the 3 σ confidence level. Thisextra emission is produced by cool material (<100 K) located beyond10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Only one star,HD 69830, shows excess emission at 24 μm, corresponding to dust withtemperatures >~300 K located inside of 1 AU. While debris disks withLdust/L*>=10-3 are rare around oldFGK stars, we find that the disk frequency increases from 2%+/-2% forLdust/L*>=10-4 to 12%+/-5% forLdust/L*>=10-5. This trend in thedisk luminosity distribution is consistent with the estimated dust inour solar system being within an order of magnitude greater or less thanthe typical level around similar nearby stars. Although there is nocorrelation of IR excess with metallicity or spectral type, there is aweak correlation with stellar age, with stars younger than a gigayearmore likely to have excess emission.
|The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an elongated polar envelope|
Context: .Be stars show evidence of mass loss and circumstellarenvelopes (CSE) from UV resonance lines, near-IR excesses, and thepresence of episodic hydrogen emission lines. The geometry of theseenvelopes is still uncertain, although it is often assumed that they areformed by a disk around the stellar equator and a hot polar wind.Aims: .We probe the close environment of the fast rotating Be starAchernar at angular scales of a few milliarcseconds (mas) in theinfrared, in order to constrain the geometry of a possible polar CSE. Methods: .We obtained long-baseline interferometric observations ofAchernar with the VINCI/VLTI beam combiner in the H and K bands, usingvarious telescope configurations and baseline lengths with a wideazimuthal coverage. Results: .The observed visibility measurementsalong the polar direction are significantly lower than the visibilityfunction of the photosphere of the star alone, in particular at lowspatial frequencies. This points to the presence of an asymmetricdiffuse CSE elongated along the polar direction of the star. To ourdata, we fit a simple model consisting of two components: a 2Delliptical Gaussian superimposed on a uniform ellipse representing thedistorted photosphere of the fast rotating star. Conclusions: .Weclearly detected a CSE elongated along the polar axis of the star, aswell as rotational flattening of the stellar photosphere. For theuniform-ellipse photosphere we derive a major axis of θ_eq = 2.13± 0.05 mas and a minor axis of θ_pol = 1.51 ± 0.02mas. The relative near-IR flux measured for the CSE compared to thestellar photosphere is f = 4.7 ± 0.3%. Its angular dimensions areloosely constrained by the available data at ρ_eq = 2.7 ± 1.3mas and ρ_pol = 17.6 ± 4.9 mas. This CSE could be linked tofree-free emission from the radiative pressure driven wind originatingfrom the hot polar caps of the star.
|Abundances of refractory elements in the atmospheres of stars with extrasolar planets|
Aims.This work presents a uniform and homogeneous study of chemicalabundances of refractory elements in 101 stars with and 93 without knownplanetary companions. We carry out an in-depth investigation of theabundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg and Al. The newcomparison sample, spanning the metallicity range -0.70< [Fe/H]<0.50, fills the gap that previously existed, mainly at highmetallicities, in the number of stars without known planets.Methods.Weused an enlarged set of data including new observations, especially forthe field "single" comparison stars . The line list previously studiedby other authors was improved: on average we analysed 90 spectral linesin every spectrum and carefully measured more than 16 600 equivalentwidths (EW) to calculate the abundances.Results.We investigate possibledifferences between the chemical abundances of the two groups of stars,both with and without planets. The results are globally comparable tothose obtained by other authors, and in most cases the abundance trendsof planet-host stars are very similar to those of the comparison sample.Conclusions.This work represents a step towards the comprehension ofrecently discovered planetary systems. These results could also beuseful for verifying galactic models at high metallicities andconsequently improve our knowledge of stellar nucleosynthesis andgalactic chemical evolution.
|X-ray emission of brown dwarfs: towards constraining the dependence on age, luminosity, and temperature|
Aims.We observed brown dwarfs in different evolutionary stages with theChandra X-ray Observatory with the aim to disentangle the influence ofdifferent stellar parameters on the X-ray emission of substellarobjects. The ages of our three targets (HR 7329 B, Gl 569 Bab, and HD130948 BC) are constrained by them being companions to main-sequencestars of known age. With both known age and effective temperature orbolometric luminosity, the mass can be derived from evolutionary models.Methods.Combining the new observations with previous studies presentedin the literature yields a brown dwarf sample that covers the age rangefrom ~1 Myr to ~1 Gyr. Since the atmospheric temperature of brown dwarfsis approximately constant at young ages, a sample with a large agespread is essential for investigating the possible influence ofeffective temperature on X-ray activity. Results.Two out of three browndwarfs are detected with Chandra, with variable lightcurves andcomparatively soft spectra. Combining our results with published dataallows us to consider a subsample of high-mass brown dwarfs (with0.05-0.07 M_ȯ), thus eliminating mass from the list of freeparameters. We find evidence that X-ray luminosity declines withdecreasing bolometric luminosity steeper than expected from thecanonical relation for late-type stars (L_x/L_bol =10-3...-5). Effective temperature is identified as a likelyparameter responsible for the additional decline of X-ray activity inthe more evolved (and therefore cooler) brown dwarfs of the "high-mass"sample. In another subsample of brown dwarfs characterized by similareffective temperature, the X-ray luminosity scales with the bolometricluminosity without indications for a deviation from the canonical rangeof 10-3...-5 observed for late-type stars.Conclusions.Ourfindings support the idea that effective temperature plays a criticalrole for the X-ray activity in brown dwarfs. This underlines an earliersuggestion based on observations of chromospheric Hα emission inultracool dwarfs that the low ionization fraction in the cool browndwarf atmospheres may suppress magnetic activity.
|Astrophysics in 2004|
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
|Spectroscopic mass ratios for two visual binaries: HD 206804 & HD 217166|
|VISIR, a taste of scientific potential.|
|Predicting accurate stellar angular diameters by the near-infrared surface brightness technique|
I report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids.
|Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres|
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
|New Mass-Loss Measurements from Astrospheric Lyα Absorption|
Measurements of stellar mass-loss rates are used to assess how windstrength varies with coronal activity and age for solar-like stars. Massloss generally increases with activity, but we find evidence that windssuddenly weaken at a certain activity threshold. Very active stars areoften observed to have polar starspots, and we speculate that themagnetic field geometry associated with these spots may be inhibitingthe winds. Our inferred mass-loss/age relation represents an empiricalestimate of the history of the solar wind. This result is important forplanetary studies as well as solar/stellar astronomy, since solar winderosion may have played an important role in the evolution of planetaryatmospheres.
|The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations|
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
|The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method|
The infrared flux method (IRFM) has been applied to a sample of 135dwarf and 36 giant stars covering the following regions of theatmospheric parameter space: (1) the metal-rich ([Fe/H]>~0) end(consisting mostly of planet-hosting stars), (2) the cool(Teff<~5000 K) metal-poor (-1<~[Fe/H]<~-3) dwarfregion, and (3) the very metal-poor ([Fe/H]<~-2.5) end. These starswere especially selected to cover gaps in previous works onTeff versus color relations, particularly the IRFMTeff scale of A. Alonso and collaborators. Our IRFMimplementation was largely based on the Alonso et al. study (absoluteinfrared flux calibration, bolometric flux calibration, etc.) with theaim of extending the ranges of applicability of their Teffversus color calibrations. In addition, in order to improve the internalaccuracy of the IRFM Teff scale, we recomputed thetemperatures of almost all stars from the Alonso et al. work usingupdated input data. The updated temperatures do not significantly differfrom the original ones, with few exceptions, leaving the Teffscale of Alonso et al. mostly unchanged. Including the stars withupdated temperatures, a large sample of 580 dwarf and 470 giant stars(in the field and in clusters), which cover the ranges3600K<~Teff<~8000K and -4.0<~[Fe/H]<~+0.5, haveTeff homogeneously determined with the IRFM. The meanuncertainty of the temperatures derived is 75 K for dwarfs and 60 K forgiants, which is about 1.3% at solar temperature and 4500 K,respectively. It is shown that the IRFM temperatures are reliable in anabsolute scale given the consistency of the angular diameters resultingfrom the IRFM with those measured by long baseline interferometry, lunaroccultation, and transit observations. Using the measured angulardiameters and bolometric fluxes, a comparison is made between IRFM anddirect temperatures, which shows excellent agreement, with the meandifference being less than 10 K for giants and about 20 K for dwarfstars (the IRFM temperatures being larger in both cases). This resultwas obtained for giants in the ranges 3800K
|A Deep Look at the T-Type Brown Dwarf Binary ɛ Indi Bab with Chandra and the Australia Telescope Compact Array|
We present deep observations of the nearby T-type brown dwarf binaryɛ Indi Bab in radio with the Australia Telescope Compact Arrayand in X-rays with the Chandra X-Ray Observatory. Despite longintegration times, the binary (composed of T1 and T6 dwarfs) was notdetected in either wavelength regime. We reached 3 σ upper limitsof 1.23×1012 and 1.74×1012 ergss-1 Hz-1 for the radio luminosity at 4.8 and 8.64GHz, respectively; in X-rays, the upper limit in the 0.1-10 keV band was3.16×1023 ergs s-1. We discuss the aboveresults in the framework of magnetic activity in ultracool, low-massdwarfs.
|Single-Visit Photometric and Obscurational Completeness|
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
|Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)|
We report on the results of the Sun in Time multiwavelength program(X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chiefscience goals are to study the solar magnetic dynamo and to determinethe radiative and magnetic properties of the Sun during its evolutionacross the main sequence. The present paper focuses on the latter goal,which has the ultimate purpose of providing the spectral irradianceevolution of solar-type stars to be used in the study and modeling ofplanetary atmospheres. The results from the Sun in Time program suggestthat the coronal X-ray-EUV emissions of the young main-sequence Sun were~100-1000 times stronger than those of the present Sun. Similarly, thetransition region and chromospheric FUV-UV emissions of the young Sunare expected to be 20-60 and 10-20 times stronger, respectively, than atpresent. When we consider the integrated high-energy emission from 1 to1200 Å, the resulting relationship indicates that about 2.5 Gyrago the solar high-energy flux was about 2.5 times the present value andabout 3.5 Gyr ago was about 6 times the present value (when lifesupposedly arose on Earth). The strong radiation emissions inferredshould have had major influences on the thermal structure,photochemistry, and photoionization of planetary atmospheres and haveplayed an important role in the development of primitive life in thesolar system. Some examples of the application of the Sun in Timeresults on exoplanets and on early solar system planets are discussed.
|Structure in the ɛ Eridani Debris Disk|
New submillimeter images have been obtained of the dust disk around thenearby K2 V star ɛ Eridani, with the total data set now spanning 5yr. These images show the distribution of dusty debris generated bycomet collisions, reflecting clearing and perturbations by planets, andmay give insights to early conditions in the solar system. The structureseen around ɛ Eri at 850 μm and published in 1998 is confirmedin the new observations, and the same structure is also seen in an imageobtained for the first time at 450 μm. The disk is inclined by~25° to the sky plane, with emission peaking at 65 AU, a 105 AUradius outer edge, and an inner cavity fainter by a factor of ~2. Thestructure within the dust ring suggests perturbations by a planetorbiting at tens of AU, and long-term tracking of these features willconstrain its mass and location. A preliminary analysis shows that twoclumps and one arc appear to follow the stellar motion (i.e., are notbackground objects) and have tentative evidence of counterclockwiserotation of ~1° yr-1. Within the ring, the mass ofcolliding comets is estimated at 5-9 M⊕, similar to theprimordial Kuiper Belt, and so any inner terrestrial planets may beundergoing an epoch of heavy bombardment.
|Proper motion surveys in the infrared|
Current and future wide field surveys in infrared passbands presentopportunities for ultra low-mass stellar sample selection via propermotions, as well as or instead of purely photometric techniques.Furthermore, proper motion measurements yield additional clues as tocandidate membership of cluster or field (kinematic) populations. Inthis paper we review some recent results and speculate as to thepossibilities afforded by the next generation of wide-field infraredsurveys.
|Multiplicity among Widely Separated Brown Dwarf Companions to Nearby Stars: Gliese 337CD|
We present Lick Natural Guide Star Adaptive Optics observations of theL8 brown dwarf Gl 337C, which is resolved for the first time into twoclosely separated (0.53"+/-0.03"), nearly equal magnitude componentswith a Ks flux ratio of 0.93+/-0.10. Companionship isinferred from the absence of a 3.6" offset source in Two Micron All SkySurvey or photographic plate images, implying that the observedsecondary component is a comoving late-type dwarf. With a projectedseparation of 11 AU and nearly equal magnitude components, Gl 337CD hasproperties similar to those of other known companion and fieldsubstellar binaries. Its long orbital period (estimated to be ~140-180yr) inhibits short-term astrometric mass measurements, but the Gl 337CDsystem is ideal for studying the L-T transition at a fixed age andmetallicity. From a compilation of all known widely separated (>~100AU) stellar-brown dwarf multiple systems, we find evidence that thebinary fraction of brown dwarfs in these systems is notably higher thanthat of field brown dwarfs, 45+15-13% versus18+7-4% for analogous samples. We speculate onpossible reasons for this difference, including the possibility thatdynamic (ejection) interactions that may form such wide pairspreferentially retain binary secondaries because of their greatercombined mass and/or ability to absorb angular momentum.
|Disk populations from HIPPARCOS kinematic data. Discontinuities in the local velocity distribution|
The full space motions including radial velocities of a stellarsample drawn from HIPPARCOS catalogue are used to discriminatedifferentiated statistical behaviours that are associated with stellarpopulations in the solar neighbourhood. A sampling parameter is used tobuild a hierarchical set of nested samples, where a discontinuouspattern, based in a partition introduced by two normal distributions,scans the subsamples. Two quantities inform whether any subsample fitsproperly into the discontinuous model. A χ2 test measuresthe Gaussianity of both components, and the entropy of the mixtureprobability gives account of how informative the resulting segregationis. The less informative partition is the one with maximum populationentropy, which provides most representative kinematic parameters. Eachnew population merged to the cumulative subsample produces adiscontinuity in the plot entropy versus sampling parameter, that allowsto determine the number of populations contained in the whole sample.The resulting method has been named MEMPHIS, Maximum Entropy of theMixture Probability from HIerarchical Segregation. In addition to bothmain kinematic components, thin and thick disk, with respective velocitydispersions (28± 1, 16± 2, 13± 1) and (65±2, 39± 9, 41± 2) km s-1, two discretenon-Gaussian subcomponents are detected within the thin disk. Thesepopulations are identified with early-type and young disk stars.Moreover, a continuous old disk population is mixed with the foregoingsubcomponents composing all together the thin disk. Older thin diskstars have a velocity dispersion overlapping a wing of the thick disk.Although they could appear like an intermediate continuous population,nested subsamples distributions allow us to conclude that theydefinitively belong to the thin disk, and that a clear discontinuitydetaches thick from thin disk. Almost the same qualitative results, butwith less accuracy, are obtained whether MEMPHIS is applied tosubsamples from the Third Catalogue of Nearby Stars (CNS3). A dynamicmodel according to Chandrasekhar's approximation, under particularsymmetry hypotheses, allows to interpret the results. The non-vanishingvertex deviation lower for older stars of all Galactic components issuggesting that, at least, point-axial symmetry is required in order toexplain the local kinematic behaviour. According to this model, theoldest thick disk population, with no net radial movement, can beextrapolated, having heliocentric velocities of -76 ± 2 kms-1 in rotation, and -18 ± 1 km s-1 in theradial direction. Early-type stars show a worthy local singularity,nearly with no net radial motion, similarly to the oldest thick diskstars. Older populations half of the thin disk and the whole thickdisk share a common differential galactic movement, suggesting acommon dynamical origin for the rupture of the axial symmetry. Therelationship between the maximum stellar velocity of a sample and itsaverage age τ is discussed, finding an approximate relation|V|max ∝ τ. Local stellar populations can bedescribed from a Titius-Bode-like law for the radial velocitydispersion, σ1 = 6.6 (4/3)^x, so that for naturalvalues x=2,3,5,8 it determines average energy levels of discretepopulations, while for continuous intervals x≤ 5 and x≥ 7 itdescribes the velocity-age evolution of thin and thick disk components,according to x 1.5 ln τ.
|Abundances of Na, Mg and Al in stars with giant planets|
We present Na, Mg and Al abundances in a set of 98 stars with knowngiant planets, and in a comparison sample of 41 “single”stars. The results show that the [X/H] abundances (with X = Na, Mg andAl) are, on average, higher in stars with giant planets, a resultsimilar to the one found for iron. However, we did not find any strongdifference in the [X/Fe] ratios, for a fixed [Fe/H], between the twosamples of stars in the region where the samples overlap. The data wasused to study the Galactic chemical evolution trends for Na, Mg and Aland to discuss the possible influence of planets on this evolution. Theresults, similar to those obtained by other authors, show that the[X/Fe] ratios all decrease as a function of metallicity up to solarvalues. While for Mg and Al this trend then becomes relatively constant,for Na we find indications of an upturn up to [Fe/H] values close to0.25 dex. For metallicities above this value the [Na/Fe] becomesconstant.
|Search for radio emission from the nearby binary brown dwarf system ɛ Indi Bab|
We have observed the nearest known brown dwarfs, the binary systemɛ Indi Bab (d= 3.626 pc), for 8.6- and 4.8-GHz radio emissionwith the Australia Telescope Compact Array. If either brown dwarf emitsradio flares, then they are respectively at least 5.5 and 1.2 timesweaker than LP 944-20, the nearest brown dwarf with detected radioemission. We associate the ROSAT source 1WGA J2203.9 - 5647 withɛ Indi Bab since the separation was about 30 arcsec at the timeof the ROSAT observation. Assuming the association, then ɛ IndiBab has an L0.4-2.4keV X-ray luminosity of 5.6 ×1025 erg s-1 which makes it roughly a factor of 2less luminous than LP 944-20. The radio non-detections imply thatɛ Indi Bab does not violate, or at least does not violate asstrongly as LP 944-20, the Güdel-Benz relationship of X-ray andradio emission.
|A search for short time-scale JHK variability in ultracool dwarfs|
The results of continuous monitoring, with typical durations of 2-4 h,of 18 L and T dwarfs are presented. Measurements were madesimultaneously in JHKS, with a typical time resolution ofabout 10 min. Limits on the variability are well below 0.02 mag in allcases except for the faintest object. There are none the less a fewdwarfs in which we may have observed variability: the same patterns ofvery low-level activity seemed to be present in the data for more thanone filter. Simultaneous frequency-domain analysis of the J, H andKS data furthermore hints at a very short period (0.8-1.6 h)in a few objects. We also present a brief assessment of some of thevariability test statistics that have been used in the field ofultracool dwarf variability.
|Improved Baade-Wesselink surface brightness relations|
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
|The debris disc around τ Ceti: a massive analogue to the Kuiper Belt|
An excess of far-infrared emission is seen towards the nearby G8V starτ Ceti, and this has been attributed to orbiting dust particlesgenerated in planetesimal collisions. A new 850-μm image shows thatthere is indeed such a debris disc, extending out to ~55 au (15 arcsec)radius. This is the first disc around a Sun-like star of latemain-sequence age to be confirmed by imaging. The dust mass is at leastan order of magnitude greater than in the Kuiper Belt, although thedimensions of the systems are very similar and the age of τ Cetiexceeds that of the Sun. Modelling shows that the mass in collidingbodies up to 10 km in size is around 1.2 Earth masses, compared with 0.1M⊕ in the Kuiper Belt, and hence the evolution aroundthe two stars has been different. One possibility is that τ Ceti haslost fewer comets from the outskirts of the system, compared with theSun. Alternatively, a greater number of comets could have been forcedout by a migrating planet, compared with the case of Neptune in theSolar system. Notably, the disc of τ Ceti fits the expected declinewith time compared to that of the younger nearby star ɛ Eridani.Among these three stars, the Sun would then be the case with the leastdust and a `minimal Kuiper Belt' - a situation which may be beneficialin terms of less bombardment and better stability for life.
|Spitzer Infrared Spectrograph (IRS) Observations of M, L, and T Dwarfs|
We present the first mid-infrared spectra of brown dwarfs, together withobservations of a low-mass star. Our targets are the M3.5 dwarf GJ1001A, the L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary systemɛ Indi Ba/Bb. As expected, the mid-infrared spectral morphology ofthese objects changes rapidly with spectral class because of the changesin atmospheric chemistry resulting from their differing effectivetemperatures and atmospheric structures. By taking advantage of theunprecedented sensitivity of the Infrared Spectrograph on the SpitzerSpace Telescope, we have detected the 7.8 μm methane and 10 μmammonia bands for the first time in brown dwarf spectra.The IRS was a collaborative venture between Cornell University and BallAerospace Corporation funded by NASA through the Jet PropulsionLaboratory and the Ames Research Center.
|Classification of Spectra from the Infrared Space Observatory PHT-S Database|
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).