Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Water vapour and hydrogen in the terrestrial-planet-forming region of a protoplanetary disk
Planetary systems (ours included) formed in disks of dust and gas aroundyoung stars. Disks are an integral part of the star and planet formationprocess, and knowledge of the distribution and temperature of inner-diskmaterial is crucial for understanding terrestrial planet formation,giant planet migration, and accretion onto the central star. Althoughthe inner regions of protoplanetary disks in nearby star-forming regionssubtend only a few nano-radians, near-infrared interferometry hasrecently enabled the spatial resolution of these terrestrial zones. Mostobservations have probed only dust, which typically dominates thenear-infrared emission. Here I report spectrally dispersed near-infraredinterferometric observations that probe the gas (which dominates themass and dynamics of the inner disk), in addition to the dust, withinone astronomical unit (1AU, the Sun-Earth distance) of the young starMWC480. I resolve gas, including water vapour and atomic hydrogen,interior to the edge of the dust disk; this contrasts with results ofprevious spectrally dispersed interferometry observations. Interactionsof this accreting gas with migrating planets may lead to short-periodexoplanets like those detected around main-sequence stars. The observedwater vapour is probably produced by the sublimation of migrating icybodies, and provides a potential reservoir of water for terrestrialplanets.
| 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.
| 2-16μm spectroscopy of micron-sized enstatite (Mg,Fe)2Si2O6 silicates from primitive chondritic meteorites
We present mid-infrared spectra from individual enstatite silicategrains separated from primitive type 3 chondritic meteorites. The2-16μm transmission spectra were taken with microspectroscopicFourier-transform infrared (FT-IR) techniques as part of a project toproduce a data base of infrared spectra from minerals of primitivemeteorites for comparison with astronomical spectra. In general, thewavelength of enstatite bands increases with the proportion of Fe.However, the wavelengths of the strong En100 bands at 10.67and 11.67 decrease with increasing Fe content. The 11.67-μm bandexhibits the largest compositional wavelength shift (twice as large asany other). Our fits of the linear dependence of the pyroxene peaksindicate that crystalline silicate peaks in the 10-μm spectra ofHerbig AeBe stars, HD 179218 and 104237, are matched by pyroxenes ofEn90-92 and En78-80, respectively. If thesesimplistic comparisons with the astronomical grains are correct, thenthe enstatite pyroxenes seen in these environments are more Fe-rich thanare the forsterite (Fo100) grains identified in thefar-infrared which are found to be Mg end-member grains. This differsfrom the general composition of type 3 chondritic meteoritic grains inwhich the pyroxenes are more Mg-rich than are the olivines from the samemeteorite.
| Observations of Herbig Ae Disks with Nulling Interferometry
We present the results of 10 μm nulling interferometric observationsof 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 mtelescopes. A portion of the observations was completed with theadaptive secondary at the MMT. We have conclusively spatially resolved 3of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter tworecently resolved using adaptive optics in combination with nullinginterferometry. For the resolved objects we find that the 10 μmemitting regions have a spatial extent of 15-30 AU in diameter. We alsohave some evidence for resolved emission surrounding an additional twostars (V892 Tau and R CrA). For those objects in our study with mid-IRSEDs in the classification of Meeus and coworkers, we find that thegroup I objects (those with constant to increasing mid-IR flux) are morelikely to be resolved, within our limited sample. This trend is evidentin correlations in the inferred disk sizes versus the submillimeter SEDslope and disk size versus fractional infrared luminosity of thesystems. We explore the spatial distribution and orientation of the warmdust in the resolved systems and constrain physical models that areconsistent with their observational signatures.The results presented here made use of the of MMT Observatory, a jointlyoperated facility of the University of Arizona and the SmithsonianInstitution. This paper also includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile.
| Constraining the wind launching region in Herbig Ae stars: AMBER/VLTI spectroscopy of HD 104237
Aims.We investigate the origin of the Brγ emission of the HerbigAe star HD 104237 on Astronomical Unit (AU) scales. Methods:.Using AMBER/VLTI at a spectral resolution R=1500 we spatially resolvethe emission in both the Brγ line and the adjacent continuum. Results: .The visibility does not vary between the continuum and theBrγ line, even though the line is strongly detected in thespectrum, with a peak intensity 35% above the continuum. Thisdemonstrates that the line and continuum emission have similar sizescales. We assume that the K-band continuum excess originates in a“puffed-up” inner rim of the circumstellar disk, and discussthe likely origin of Brγ. Conclusions: .We conclude thatthis emission most likely arises from a compact disk wind, launched froma region 0.2-0.5 AU from the star, with a spatial extent similar to thatof the near infrared continuum emission region, i.e., very close to theinner rim location.Based on observations collected at the European Southern Observatory,Paranal, Chile, within the guaranteed time programme 074.A-9025(A).
| AMBER, the near-infrared spectro-interferometric three-telescope VLTI instrument
Context: .Optical long-baseline interferometry is moving a crucial stepforward with the advent of general-user scientific instruments thatequip large aperture and hectometric baseline facilities, such as theVery Large Telescope Interferometer (VLTI). Aims: .AMBER is one ofthe VLTI instruments that combines up to three beams with low, moderateand high spectral resolutions in order to provide milli-arcsecondspatial resolution for compact astrophysical sources in thenear-infrared wavelength domain. Its main specifications are based onthree key programs on young stellar objects, active galactic nucleicentral regions, masses, and spectra of hot extra-solar planets.Methods: .These key science goals led to scientific specifications,which were used to propose and then validate the instrument concept.AMBER uses single-mode fibers to filter the entrance signal and to reachhighly accurate, multiaxial three-beam combination, yielding threebaselines and a closure phase, three spectral dispersive elements, andspecific self-calibration procedures. Results: .The AMBERmeasurements yield spectrally dispersed calibrated visibilities,color-differential complex visibilities, and a closure phase allowsastronomers to contemplate rudimentary imaging and highly accuratevisibility and phase differential measurements. AMBER was installed in2004 at the Paranal Observatory. We describe here the presentimplementation of the instrument in the configuration with which theastronomical community can access it. Conclusions: .After twoyears of commissioning tests and preliminary observations, AMBER hasproduced its first refereed publications, allowing assessment of itsscientific potential.
| Discovery of δ Scuti pulsation in the Herbig Ae star VV Serpentis
Context: .The study of pulsation in pre-main-sequence intermediate massstars represents an important tool for deriving information on thestellar parameters and structure, as well as for testing the validity ofcurrent theoretical models. The interest in this class of variable starshas significantly increased during the last decade and about 30 membersare presently known in the literature. Aims: .A new observationalstudy of the Herbig Ae star VV Ser has been performed to detect andaccurately measure pulsation frequencies in the δ Scuti range,thus enlarging the sample of known pulsators and contributing to theempirical definition of the pre-main-sequence instability strip. As itbelongs to the continuous field of view of the asteroseismologicalsatellite COROT, this study also aims at characterizing the propertiesof VV Ser as a potential "COROT additional program" candidate.Methods: .CCD time series photometry in the Johnson V filter has beenobtained for three consecutive years. The resulting light curves havebeen subject to detailed frequency analysis and the derived frequencieshave been compared to model predictions. Results: .Seven pulsationfrequencies have been measured on the basis of the best data setobtained in 2004, ranging from ~31 to ~118 μHz, with an accuracy ofthe order of 0.5 μHz. The comparison with an extensive set ofasteroseismological models shows that all the observed periodicities canbe reproduced if the stellar mass is close to 4~ Mȯ.Conversely, the measured frequencies can be associated with p modes onlyif the effective temperature is significantly lower than that obtainedfrom the spectral type conversion. Conclusions: .The presentresults seem to suggest that more accurate spectral type determinationis necessary to discriminate the best-fit model solution. In any case,the stellar mass of VV Ser is close to the upper mass limit (˜ 4~Mȯ) for this class of pulsators.Tables 3, 5 and 6 are only available in electronic form athttp://www.aanda.org
| SparSpec: a new method for fitting multiple sinusoids with irregularly sampled data
Context: .The location of pure frequencies in the spectrum of anirregularly sampled time series is an important topic in astrophysicaldata analysis. Especially in the domain of asteroseismology, a highlyprecise and unambiguous study of frequencies in photometric light orradial velocity curves is required. Aims: .Due to samplingirregularities and large observational gaps, the classic methods forfrequency estimation (prewhitening techniques, clean, cleanest, etc.)sometimes suffer false detections. We propose a new framework for thisproblem that allows a more precise and unambiguous frequencylocation. Methods: .Multisine fitting is addressed as the sparserepresentation of the data in an overcomplete dictionary of frequencies,hence the name SparSpec for the method. We model the data as the sum ofan arbitrarily large number of pure frequencies, discretised on a fixedgrid. Among all the many representations fitting the data, we seek theone with the fewest non-zero amplitudes. This solution can be computedby minimising a convex criterion with no local minima. A computationallyefficient and convergent optimisation strategy is derived and auser-friendly software implementing SparSpec is provided online athttp://www.ast.obs-mip.fr/Softwares. Results: . The method isfirst illustrated on a simple test example where SparSpec correctlylocates the frequencies while classic methods fail. Then, simulations onmore realistic artificial time series reveal the interest of this newmethodology in terms of robustness toward sampling aliases. Anapplication to the radial velocity curve of the pre-main sequence HerbigAe star HD 104237 is finally presented, where the method is able todetermine oscillation frequencies even in the presence of stronglow-frequency perturbations such as orbital movements. While SparSpecmainly confirms previously published studies for the four more importantfrequencies, it suggests some ambiguity about the position of a fifthfrequency. Additional simulations show that the SparSpec results aremore plausible.
| The magnetic field of the pre-main sequence Herbig Ae star HD 190073
Context: .The general context of this paper is the study of magneticfields in the pre-main sequence intermediate mass Herbig Ae/Be stars.Magnetic fields are likely to play an important role in pre-mainsequence evolution at these masses, in particular in controlling thegains and losses of stellar angular momentum. Aims: .Theparticular aim of this paper is to announce the detection of astructured magnetic field in the Herbig Ae star HD 190073, and todiscuss various scenarii for the geometry of the star, its environmentand its magnetic field. Methods: .We have used the ESPaDOnSspectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution,high signal-to-noise circular polarization spectra which demonstrateunambiguously the presence of a magnetic field in the photosphere ofthis star. Results: .Nine circular polarization spectra wereobtained, each one showing a clear Zeeman signature. This signature issuggestive of a magnetic field structured on large scales. Thesignature, which corresponds to a longitudinal magnetic field of74± 10 G, does not vary detectably on a one-year timeframe,indicating either an azimuthally symmetric field, a zero inclinationangle between the rotation axis and the line of sight, or a very longrotation period. The optical spectrum of HD 190073 exhibits a largenumber of emission lines. We discuss the formation of these emissionlines in the framework of a model involving a turbulent heated region atthe base of the stellar wind, possibly powered by magnetic accretion. Conclusions: .This magnetic detection contributes an important newobservational discovery which will aid our understanding of stellarmagnetism at intermediate masses.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| An Asymmetric Outflow from the Herbig Ae Star HD 163296
We present an observational picture of the HH 409 bipolar outflowincluding the detection of six previously unreported Herbig-Haro knotsfrom the Herbig Ae star HD 163296. This study combines seven years ofdata from ground-based Fabry-Pérot and HST coronagraphic imagery,as well as HST long-slit spectral imagery. The redshifted counterjetincludes a chain of six Herbig-Haro knots spanning >27" to thenortheast (P.A.~42deg) of the source and has been active for>80 yr. The brightest knot in the counterjet is HH 409 C, alow-excitation bow shock with a shock velocity Vs~50 kms-1 and total space motion Vjet~260 kms-1. The presence of additional knots in the counterjetbeyond the bow shock may indicate precession of the jet axis. Theblueshifted jet includes two closely spaced knots within 10" and adistant bow shock (~21") southwest (P.A.~223deg) of thesource. The brightest knot in the jet is HH 409 A, a higher excitationmore bullet-like shock with Vs~90 km s-1 andVjet~360 km s-1. The average opening angles forboth the jet and counterjet are similar,αa~2deg, and consistent with opening anglesof lower mass T Tauri stars. The mass-loss rates in both lobes of theflow, despite the asymmetry of the knots, are also comparable,M˙out~1.0×10-8 Msolaryr-1. This suggests that variations in the mass outflow rateare not more than a factor of ~2.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin
A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on theultrafast rotating yellow giant FK Comae Berenices (HD 117555; vsini~163km s-1) recorded emission profiles of C III λ977(T~8×104 K) and O VI λ1031(T~3×105 K) that are exceptionally broad andasymmetric, but nearly identical in shape, aside from a bluewardabsorption component in the latter (identified as interstellar O I,rather than, say, a C III outflow feature). The FWHMs exceed 500 kms-1, twice the broadest far-UV line shape of any normallate-type star observed to date, but similar to the Hα profiles ofFK Com, and following the trend of other fast spinning early G giantsthat often display ``superrotational'' broadening of their UV ``hot''lines. Although the red-asymmetric O VI λ1031 profile issuggestive of an outflow at ~3×105 K, the weaker memberof the doublet, λ1037, does not display the differentialabsorption pattern expected from a warm wind. Furthermore, at times thechromospheric Mg II λ2796 + λ2803 composite profile, froma collection of International Ultraviolet Explorer (IUE) echellegramsobtained two decades earlier, is nearly identical in shape tored-asymmetric O VI λ1031. A contemporaneous optical Doppler mapplaces the photospheric dark spots mainly in the polar regions of theapproaching hemisphere. The dominantly redward biased profiles of C IIIand O VI could be explained if the associated emission zones wereleading the starspots in phase and partially rooted in lower latitudes.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| A multisite photometric campaign on the pre-main-sequence δ Scuti pulsator IP Persei
We present the results of a photometric multisite campaign on theδ Scuti Pre-Main-Sequence star IP Per. Nine telescopes have beeninvolved in the observations, with a total of about 190 h ofobservations over 38 nights. Present data confirms the multiperiodicnature of this star and leads to the identification of at least ninepulsational frequencies. Comparison with the predictions of linearnon-adiabatic radial pulsation models allowed us to identify only fiveof the nine observed frequencies, and to constrain the position of IPPer in the HR diagram. The latter is in good agreement with theempirical determination of the stellar parameters obtained byMiroshnichenko et al. (2001, A&A, 377, 854). An initialinterpretation of the observed frequencies using the Aarhus non-radialpulsation code suggests that three frequencies could be associated withnon-radial (l=2) modes. Finally, we present new evolutionary andpulsation models at lower metallicity (Z=0.008) to take into account thepossibility that IP Per is metal deficient, as indicated byMiroshnichenko et al. (2001, A&A, 377, 854).
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| 51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.
| Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297. A revision of the X-ray properties of MWC 297
We present a Chandra ACIS-I X-ray observation of the region near theHerbig early-Be star MWC 297, where we detect a tight group of pointX-ray sources. These are probably physically associated to MWC 297,because of their obvious clustering with respect to the more scatteredfield-source population. These data are compared to earlier ASCA datawith much poorer spatial resolution, from which the detection of strongquiescent and flaring emission from MWC 297 itself was claimed. We arguethat this star, contributing only 5% to the total X-ray emission of thegroup, was probably not the dominant contributor to the observed ASCAemission, while the X-ray brightest star in the group is a much bettercandidate. This is also supported by the spectral analysis of theChandra data, with reference to the ASCA spectra. We conclude that noneof the X-ray data available for MWC 297 justify the earlier claim ofstrong magnetic activity in this star. The X-ray emission of MWC 297during the Chandra observation is even weaker than that found in otherHerbig stars with the same spectral type, even accounting for its largeline-of-sight absorption.
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| The Darkest Bright Star: Chandra X-Ray Observations of Vega
We present X-ray observations of Vega obtained with the Chandra HighResolution Camera and Advanced CCD Imaging Spectrometer. After a totalof 29 ks of observation with Chandra, X-rays from Vega remainundetected. We derive upper limits to the X-ray luminosity of Vega as afunction of temperature over the range of 105-107K and find a 99.7% upper limit as low as ~2×1025 ergss-1 at T=106.2 K. We also compare these new deeperobservations with the limit derived from a reanalysis of ROSAT PSPCdata. Our X-ray luminosity limit for Vega is still greater thanpredictions of post-Herbig Ae phase X-rays from the shear dynamo modelproposed by Tout & Pringle for a Vega age of 350 Myr. If the age ofVega is closer to 100 Myr, as suggested by some indicators, our X-raylimit is then similar to Tout-Pringle model predictions. Current X-rayobservations of Vega are therefore unable to discriminate betweendifferent scenarios explaining the X-ray activity of the convectivelystable Herbig Ae/Be stars. Further progress is more likely to beachieved through X-ray observations of younger main-sequence early-typeA stars, whose conjectured residual post-Herbig Ae phase X-ray activitywould be significantly higher.
| Near-Infrared and the Inner Regions of Protoplanetary Disks
We examine the ``puffed-up inner disk'' model proposed by Dullemond,Dominik, & Natta for explaining the near-IR excess radiation fromHerbig Ae/Be stars. Detailed model computations show that the observednear-IR excess requires more hot dust than is contained in the puffed-updisk rim. The rim can produce the observed near-IR excess only if itsdust has perfectly gray opacity, but such dust is in conflict with theobserved 10 μm spectral feature. We find that a compact (~10 AU),tenuous (τV<~0.4), dusty halo around the disk innerregions contains enough dust to readily explain the observations.Furthermore, this model also resolves the puzzling relationship noted byMonnier & Millan-Gabet between luminosity and the interferometricinner radii of disks.
| Seismology and rotation of the Herbig Ae star HD 104237 .
HD 104237 is the first pulsating Herbig Ae star for which very recentlya significant number of pulsation frequencies has been detected by meansof high-resolution spectroscopy. The high quality radial velocity curvebased on 1888 individual echelle spectra obtained in 42 nights in 1999and 2000 revealed for the first time by spectroscopic meansmultiperiodic oscillations in a pre-main sequence star: a total of 8frequencies have been detected so far, 5 amongst them were detected at avery high confidence level Böhm et al. \cite{pulsation}. Thisresult encouraged us to develop asterosismological models for such youngpre-main sequence pulsator (Dupret et al. \cite{dupret}), but thedramatic lack of a precise knowledge of the fundamental stellarparameters appear still to be critical. A summary of the main results ofthe pulsation study are presented and first indications for a stellarrotational modulation are exhibited.
| Recent results on Pre-main sequence delta Scuti stars. .
Intermediate mass Pre-main sequence stars (1.5 Mȯ< M< 5Mȯ) cross the instability strip on their way tothe main sequence. They are therefore expected to be pulsating in asimilar way as the delta Scuti stars. In this contribution we presentthe status of the observational studies of pulsations in these starswith special emphasis on recent results from our group. The prospectsfor future investigations of these objects from the ground and fromspace are discussed.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| The Herbig Ae Star HD 163296 in X-Rays
Chandra X-ray imaging spectroscopy of the nearby Herbig Ae star HD163296 at 100 AU angular resolution is reported. A pointlike, soft(kT~0.5 keV), emission-line source is detected at the location of thestar with an X-ray luminosity of 4×1029 ergss-1 (logLX/Lbol=-5.48). In addition,faint emission along the direction of a previously detectedLyα-emitting jet and Herbig-Haro outflow may be present. Therelatively low luminosity, lack of a hard spectral component, andabsence of strong X-ray variability in HD 163296 can be explained asoriginating from optically thin shock-heated gas accreting onto thestellar surface along magnetic field lines. This would require a(dipole) magnetic field strength at the surface of HD 163296 of at least~100 G and perhaps as high as several kG. HD 163296 joins the T Tauristar TW Hya in being the only examples known to date ofpre-main-sequence stars whose quiescent X-ray emission appears to becompletely dominated by accretion.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| Asteroseismology of δ Scuti Stars: Problems and Prospects
We briefly outline the state-of-the-art seismology of δ Scutistars from a theoretical point of view: why is it so difficult a task?The recent theoretical advances in the field that these difficultieshave influenced are also discussed.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Камелеон |
| Ректацензија: | 12h00m05.08s |
| Deклинација: | -78°11'34.6" |
| Apparent магнитуда: | 6.599 |
| Даљина: | 116.144 parsecs |
| Proper motion RA: | -39.8 |
| Proper motion Dec: | -5.2 |
| B-T magnitude: | 6.862 |
| V-T magnitude: | 6.621 |
Каталог и designations:
|
