Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| Inner Rim of a Molecular Disk Spatially Resolved in Infrared CO Emission Lines
We present high-resolution infrared spectroscopy of the Herbig Ae starHD 141569A in the CO v=2-1 transition. With the angular resolutionattained by the adaptive optics system, the gas disk around HD 141569Ais spatially resolved down to its inner-rim truncation. The size of theinner clearing is 11 +/- 2 AU in radius, close to the gravitationalradius of the star. The rough coincidence with the gravitational radiusindicates that the viscous accretion working together withphotoevaporation by stellar radiation has cleared the inner part of thedisk.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features
Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) featurestowards young low-mass (T Tauri) stars and compare them with surveys ofintermediate mass (Herbig Ae/Be) stars. The presence and strength of thePAH features are interpreted with disk radiative transfer modelsexploring the PAH feature dependence on the incident UV radiation, PAHabundance and disk parameters. Methods: .Spitzer Space Telescope5-35 μm spectra of 54 pre-main sequence stars with disks wereobtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars withunknown spectral type. Results: .Compact PAH emission is detectedtowards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2μm PAH feature is detected in all of these sources, as is the 6.2μm PAH feature for the 4 sources for which short wavelength data areavailable. However, the 7.7 and 8.6 μm features appear strongly inonly 1 of these 4 sources. Based on the 11.2 μm feature, PAH emissionis observed towards at least 3 T Tauri stars, with 14 tentativedetections, resulting in a lower limit to the PAH detection rate of 8%.The lowest mass source with PAH emission in our sample is TCha with a spectral type G8. All 4 sources in our sample withevidence for dust holes in their inner disk show PAH emission,increasing the feature/continuum ratio. Typical 11.2 μm lineintensities are an order of magnitude lower than those observed for themore massive Herbig Ae/Be stars. Measured line fluxes indicate PAHabundances that are factors of 10-100 lower than standard interstellarvalues. Conversely, PAH features from disks exposed to stars withT_eff≤ 4200 K without enhanced UV are predicted to be below thecurrent detection limit, even for high PAH abundances. Disk modelingshows that the 6.2 and 11.2 μm features are the best PAH tracers forT Tauri stars, whereas the 7.7 and 8.6 μm bands have low feature overcontinuum ratios due to the strongly rising silicate emission.
| Discovery of an 86 AU Radius Debris Ring around HD 181327
HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 haverevealed the presence of a ringlike disk of circumstellar debris seen in1.1 μm light scattered by the disk grains, surrounded by a diffuseouter region of lower surface brightness. The annular disk appears to beinclined by 31.7d+/-1.6d from face-on, with the disk major-axis P.A. at107deg+/-2deg. The total 1.1 μm flux density ofthe light scattered by the disk (at 1.2"
| Stellar parameters and evidence of circumstellar activity for a sample of Herbig Ae/Be stars
Aims.We investigate evidence of accretion in a sample of 15 Herbig Ae/Bestars to determine whether these events originate in a remnant gaseousstructure from the primordial cloud (rich in hydrogen) or in ametal-rich body (like comets in our Solar System). During such analysiswe also determine precise stellar parameters for this sample ofstars. Methods: .The stars were observed using high resolutionspectroscopy (R = 48 000). A synthetic photospheric spectrum wasconstructed and then subtracted from the observed one to obtain thecircumstellar component. An iterative procedure was applied to find thestellar parameters that were used to build the synthetic photosphericspectrum. Results: .Evidence of circumstellar activity were foundin four stars: HD 100546, HD 142666, HD 144432 and HD 145718. Thepresence of redshifted absorption features only in the Balmer linesimplies that the accreting material is hydrogen-rich, excluding thepossibility that the accretion events might have been created bycomet-like bodies. We determined effective temperature, surface gravity,metallicity and the projected rotational velocity for the stars in oursample.
| 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.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| No Detectable H+3 Emission from the Atmospheres of Hot Jupiters
H+3 emission is the dominant cooling mechanism inJupiter's thermosphere and a useful probe of temperature and iondensities. The H+3 ion is predicted to form in thethermospheres of close-in ``hot Jupiters,'' where its emission would bea significant factor in the thermal energy budget, affecting temperatureand the rate of hydrogen escape from the exosphere. Hot Jupiters arepredicted to have up to 105 times Jupiter'sH+3 emission because they experience extremestellar irradiation and enhanced interactions may occur between theplanetary magnetosphere and the stellar wind. Direct (but unresolved)detection of an extrasolar planet, or the establishment of useful upperlimits, may be possible because a small but significant fraction of thetotal energy received by the planet is reradiated in a few narrow linesof H+3 within which the flux from the star islimited. We present the observing strategy and results of our search foremission from the Q(1,0) transition of H+3 (3.953μm) from extrasolar planets orbiting six late-type dwarfs usingCSHELL, the high-resolution echelle spectrograph on NASA's InfraredTelescope Facility. We exploited the time-dependent Doppler shift of theplanet, which can be as large as 150 km s-1, by differencingspectra between nights, thereby removing the stellar photospheric signaland telluric lines. We set limits on the H+3emission from each of these systems and compare them with models in theliterature. Ideal candidates for future searches are intrinsically faintstars, such as M dwarfs, at very close distances.
| Modeling the gas-phase chemistry of the transitional disk around HD 141569A
Aims.The chemistry, distribution and mass of the gas in the transitionaldisk around the 5 Myr old B9.5 V star HD 141569A are constrained.Methods: .A quasi 2-dimensional (2D) chemistry code for photon dominatedregions (PDR) is used to calculate the chemistry and gas temperatures inthe disk. The calculations are performed for several gas distributions,PAH abundances and values of the total gas mass. The resulting COJ=2{-}1 and J=3{-}2 emission lines are computed with a 2D radiativetransfer code and are compared to observations. Results: .The COabundance is very sensitive to the total disk mass because the disk isin a regime where self-shielding just sets in. The observed CO emissionlines are best fit by a power-law gas distribution of 80 M_⊕starting at 80 AU from the central star, indicating that there is somegas in the inner hole. Predictions are made for intensities of atomicfine-structure lines. [C I], which is the dominant form of carbon inlarge parts of the disk, is found to be a good alternative tracer of thegas mass.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris
We present F435W (B), F606W (broad V), and F814W (broad I) coronagraphicimages of the debris disk around β Pictoris obtained with theHubble Space Telescope's Advanced Camera for Surveys. These imagesprovide the most photometrically accurate and morphologically detailedviews of the disk between 30 and 300 AU from the star ever recorded inscattered light. We confirm that the previously reported warp in theinner disk is a distinct secondary disk inclined by ~5° from themain disk. The projected spine of the secondary disk coincides with theisophotal inflections, or ``butterfly asymmetry,'' previously seen atlarge distances from the star. We also confirm that the opposingextensions of the main disk have different position angles, but we findthat this ``wing-tilt asymmetry'' is centered on the star rather thanoffset from it, as previously reported. The main disk's northeastextension is linear from 80 to 250 AU, but the southwest extension isdistinctly bowed with an amplitude of ~1 AU over the same region. Bothextensions of the secondary disk appear linear, but not collinear, from80 to 150 AU. Within ~120 AU of the star, the main disk is ~50% thinnerthan previously reported. The surface brightness profiles along thespine of the main disk are fitted with four distinct radial power lawsbetween 40 and 250 AU, while those of the secondary disk between 80 and150 AU are fitted with single power laws. These discrepancies suggestthat the two disks have different grain compositions or sizedistributions. The F606W/F435W and F814W/F435W flux ratios of thecomposite disk are nonuniform and asymmetric about both projected axesof the disk. The disk's northwest region appears 20%-30% redder than itssoutheast region, which is inconsistent with the notion that forwardscattering from the nearer northwest side of the disk should diminishwith increasing wavelength. Within ~120 AU, themF435W-mF606W andmF435W-mF814W colors along the spine of the maindisk are ~10% and ~20% redder, respectively, than those of β Pic.These colors increasingly redden beyond ~120 AU, becoming 25% and 40%redder, respectively, than the star at 250 AU. These measurementsoverrule previous determinations that the disk is composed of neutrallyscattering grains. The change in color gradient at ~120 AU nearlycoincides with the prominent inflection in the surface brightnessprofile at ~115 AU and the expected water-ice sublimation boundary. Wecompare the observed red colors within ~120 AU with the simulated colorsof nonicy grains having a radial number density ~r-3 anddifferent compositions, porosities, and minimum grain sizes. Theobserved colors are consistent with those of compact or moderatelyporous grains of astronomical silicate and/or graphite with sizes>~0.15-0.20 μm, but the colors are inconsistent with the bluecolors expected from grains with porosities >~90%. The increasinglyred colors beyond the ice sublimation zone may indicate the condensationof icy mantles on the refractory grains, or they may reflect anincreasing minimum grain size caused by the cessation of cometaryactivity.Based on guaranteed observing time awarded by the National Aeronauticsand Space Administration (NASA) to the ACS Investigation Definition Team(HST program 9987).
| Dust in the solar system and in extra-solar planetary systems
Among the observed circumstellar dust envelopes a certain population,planetary debris disks, is ascribed to systems with optically thin dustdisks and low gas content. These systems contain planetesimals andpossibly planets and are believed to be systems that are most similar toour solar system in an early evolutionary stage. Planetary debris diskshave been identified in large numbers by a brightness excess in thenear-infrared, mid-infrared and/or submillimetre range of their stellarspectral energy distributions. In some cases, spatially resolvedobservations are possible and reveal complex spatial structures. Actingforces and physical processes are similar to those in the solar systemdust cloud, but the observational approach is obviously quite different:overall spatial distributions for systems of different ages for theplanetary debris disks, as opposed to detailed local information in thecase of the solar system. Comparison with the processes of dustformation and evolution observed in the solar system therefore helpsunderstand the planetary debris disks. In this paper, we review ourpresent knowledge of observations, acting forces, and major physicalinteractions of the dust in the solar system and in similar extra-solarplanetary systems.
| Gas Flow across Gaps in Protoplanetary Disks
We analyze the gas accretion flow through a planet-produced gap in aprotoplanetary disk. We adopt the α-disk model and ignore effectsof planetary migration. We develop a semianalytic, one-dimensional modelthat accounts for the effects of the planet as a mass sink and alsocarry out two-dimensional hydrodynamic simulations of a planet embeddedin a disk. The predictions of the mass flow rate through the gap basedon the semianalytic model generally agree with the hydrodynamicsimulations at the 25% level. Through these models, we are able toexplore steady state disk structures and over large spatial ranges. Thepresence of an accreting ~1MJ planet significantly lowers thedensity of the disk within a region of several times the planet'sorbital radius. The mass flow rate across the gap (and onto the centralstar) is typically 10%-25% of the mass accretion rate outside the orbitof the planet, for planet-to-star mass ratios that range from5×10-5 to 1×10-3.
| The Warped Circumstellar Disk of HD 100546
We propose that the two-armed spiral features seen in visible HubbleSpace Telescope images of scattered light in HD 100546's circumstellardisk are caused by the illumination of a warped outer disk. A tilt of6°-15° from the symmetry plane can cause the observed surfacebrightness variations, providing the disk is very twisted (highlywarped) at radii greater than 200 AU where the spiral features are seen.Dust lanes are due in part to shadowing in the equatorial plane from theinner disk within a radius of 100 AU. HD 100546's outer disk, if viewededge-on, would appear similar to that of Beta Pictoris. A disk initiallymisaligned with a planetary system becomes warped due to precessioninduced by planetesimal bodies and planets. However, the twistedness ofHD 100546's disk cannot be explained by precession during the lifetimeof the system induced by a single Jovian-mass planet within the clearingat ~13 AU. One possible explanation for the corrugated disk is thatprecession was induced by massive bodies embedded in the disk at largerradius. This would require approximately a Jupiter mass of bodies welloutside the central clearing at 13 AU and within the location of thespiral features or at radii approximately between 50 and 200 AU.
| Evidence for Companion-induced Secular Changes in the Turbulent Disk of a Be Star in the Large Magellanic Cloud MACHO Database
The light curve of a blue variable in the MACHO LMC database (FTS ID78.5979.72) appeared nearly unvarying for about 4 yr (the quasi-flatsegment) but then rapidly changed to become periodic with noisy minimafor the remaining 4 yr (the periodic segment); there are no antecedentindications of a gradual approach to this change. Lomb periodogramanalyses indicate the presence of two distinct periods of ~61 and 8 daysin both the quasi-flat and the periodic segments. Minima of the periodicsegment cover at least 50% of the orbital period and contain spikes oflight with the 8 day period; maxima do not show this short period. Thesystem typically shows maxima to be redder than minima. The most recentOGLE-III light curve shows only a 30 day periodicity. The variable's Vand R magnitudes and color are those of a Be star, and recent sets ofnear-infrared spectra 4 days apart, secured during the time of theOGLE-III data, show Hα emission near and at a maximum, confirmingits Be star characteristics. The model that best fits the photometricbehavior consists of a thin ringlike circumstellar disk of low mass withfour obscuring sectors orbiting the central B star in unison at the 61day period. The central star peers through the three equispacedseparations between the four sectors producing the 8 day period. Thesesectors could be dusty vortices comprised of particles larger thantypical interstellar dust grains that dim but selectively scatter thecentral star's light, while the remainder of the disk contains hydrogenin emission, making maxima appear redder. A companion star of lower massin an inclined and highly eccentric orbit produces an impulsiveperturbation near its periastron to change the disk's orientation,changing eclipses from partial to complete within ~10 days. The mostrecent change to a 30 day period observed in the OGLE-III data may becaused by obscuring sectors that have coalesced into larger ones andspread out along the disk.
| Near-infrared imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| Results from the adaptive optics coronagraph at the William Herschel Telescope
Described here is the design and commissioning of a coronagraph facilityfor the 4.2-m William Herschel Telescope (WHT) and its Nasmyth AdaptiveOptics for Multi-purpose Instrumentation (NAOMI). The use of the NAOMIsystem gives an improved image resolution of ~0.15 arcsec at awavelength of 2.2μm. This enables the Optimised Stellar Coronagraphfor Adaptive optics (OSCA) to suppress stellar light using smallerocculting masks and thus allows regions closer to bright astronomicalobjects to be imaged. OSCA provides a selection of 10 differentocculting masks with sizes of 0.25-2.0arcsec in diameter, including twowith full grey-scale Gaussian profiles. There is also a choice ofdifferent sized and shaped Lyot stops (pupil plane masks). Computersimulations of the different coronagraphic options with the NAOMIsegmented mirror have relevance for the next generation of highlysegmented extremely large telescopes.
| An 850 μm Survey for Dust around Solar-Mass Stars
We present the results of an 850 μm JCMT/SCUBA survey for dust around13 nearby solar-mass stars. The dust mass sensitivity ranged from5×10-3 to 0.16 M⊕. Three sources weredetected in the survey, one of which (HD 107146) has been previouslyreported. One of the other two submillimeter sources, HD 104860, was notdetected by IRAS and is surrounded by a cold, massive dust disk with adust temperature and mass of Tdust=33 K andMdust=0.16 M⊕, respectively. The thirdsource, HD 8907, was detected by IRAS and ISO at 60-87 μm and has adust temperature and mass of Tdust=48 K andMdust=0.036 M⊕, respectively. We find thatthe deduced masses and radii of the dust disks in our sample are roughlyconsistent with models for the collisional evolution of planetesimaldisks with embedded planets. We also searched for residual gas in two ofthe three systems with detected submillimeter excesses and place limitson the mass of gas residing in these systems. When the propertiesmeasured for the detected excess sources are combined with the largerpopulation of submillimeter excess sources from the literature, we findstrong evidence that the mass in small grains declines significantly ona ~200 Myr timescale, approximately inversely with age. However, we alsofind that the characteristic dust radii of the population, obtained fromthe dust temperature of the excess and assuming blackbody grains, isuncorrelated with age. This is in contrast to self-stirred collisionalmodels for debris disk evolution, which predict a trend of radiusincreasing with age tage~R3d. The lackof agreement suggests that processes beyond self-stirring, such as giantplanet formation, play a role in the evolutionary histories ofplanetesimal disks.
| Extrasolar planets and brown dwarfs around A-F type stars. I. Performances of radial velocity measurements, first analyses of variations
We present the performances of a radial velocity measurement method thatwe developed for A-F type stars. These perfomances are evaluated throughan extensive set of simulations, together with actual radial velocityobservations of such stars using the ELODIE and HARPS spectrographs. Wereport the case of stars constant in radial velocity, the example of abinary detection on HD 48097 (an A2V star, with v sin{i} equal to 90 kms^-1) and a confirmation of the existence of a 3.9 M_Jup planet orbitingaround HD 120136 (Tau Boo). The instability strip problem is alsodiscussed. We show that with this method, it is in principle possible todetect planets and brown dwarfs around A-F type stars, thus allowingfurther study of the impact of stellar masses on planetary systemformation over a wider range of stellar masses than is currently done.
| Dust Distribution in Gas Disks. II. Self-induced Ring Formation through a Clumping Instability
Debris rings of dust are found around young luminous stars such as HR4796A and HD 141569. Some of these entities have sharp edges and gaps,which have been interpreted as evidence for the presence of shepherdingand embedded planets. Here we show that gaps and sharp edges in thedebris disks of dust can also be spontaneously self-generated if theyare embedded in optically thin regions of gaseous disks. This clumpinginstability arises in regions where an enhancement in the dust densityleads to local gas temperature and pressure increases. Consequently, therelative motion between the gas and the dust is modified. The subsequenthydrodynamic drag on the dust particles leads to further enhancement oftheir concentration. We show that this process is linearly unstable andleads to the formation of ringlike structures within the estimatedlifetime of such young objects. Once the gas is removed (e.g., byphotoevaporation), the structures are ``frozen'' and will persist, evenwhen the gas might not be observable anymore.
| Mid-Infrared Spectra of Polycyclic Aromatic Hydrocarbon Emission in Herbig Ae/Be stars
We present spectra of four Herbig Ae/Be stars obtained with the InfraredSpectrograph (IRS) on the Spitzer Space Telescope. All four of thesources show strong emission from polycyclic aromatic hydrocarbons(PAHs), with the 6.2 μm emission feature shifted to 6.3 μm and thestrongest CC skeletal-mode feature occurring at 7.9 μm instead of at7.7 μm, as is often seen. Remarkably, none of the four stars hassilicate emission. The strength of the 7.9 μm feature varies withrespect to the 11.3 μm feature among the sources, indicating that wehave observed PAHs with a range of ionization fractions. The ionizationfraction is higher for systems with hotter and brighter central stars.Two sources, HD 34282 and HD 169142, show emission features fromaliphatic hydrocarbons at 6.85 and 7.25 μm. The spectrum of HD 141569shows a previously undetected emission feature at 12.4 μm that may berelated to the 12.7 μm PAH feature. The spectrum of HD 135344, thecoolest star in our sample, shows an unusual profile in the 7-9 μmregion, with the peak emission to the red of 8.0 μm and no 8.6 μmPAH feature.
| An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 μm) and the coronagraph on theNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) on theHubble Space Telescope to survey 45 single stars with a median age of0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7mag. For the median age we were capable of detecting a 30MJcompanion at separations between 15 and 200 AU. A 5MJ objectcould have been detected at 30 AU around 36% of our primaries. Forseveral of our targets that were less than 30 Myr old, the lower masslimit was as low as 1MJ, well into the high mass planetregion. Results of the entire survey include the proper-motionverification of five low-mass stellar companions, two brown dwarfs(HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).
| Photophoresis and the Pile-up of Dust in Young Circumstellar Disks
A rapidly growing number of observations reveal ever more structure inyoung circumstellar disks that are presumed to be forming planetarysystems. Prominent features observed are ring-shaped dust distributionswith sharp inner edges around stars like the young, main-sequence starHR 4796A. Models aiming to explain the formation of these dust rings bygrain migration incorporate radiation pressure of the central star asone shaping force in radial direction. However, the radiometric effectof photophoresis has been ignored, so far, in this context. This effectis based on a radiation-induced temperature gradient on the surface of aparticle and the consequential nonuniform interaction with surroundinggas. The resulting force is able to effectively influence the motion ofparticles in gaseous environments, but so far photophoresis has beenlimited to applications in the field of aerosol science. Here we presentcalculations that underline the relevance of the photophoretic force forthe dynamics of particles in gas-rich, optically thin circumstellardisks. Depending on the gas pressure, photophoresis can be stronger thanradiation pressure, gas drag, and gravity by orders of magnitude. Thenthe motion of particles ranging in size from 1 μm to 10 m will bedominated by photophoresis. Since the photophoretic force is a functionof the gas density, it provides an efficient mechanism for fast radialmigration of particles to a definite distance from the star where thegas density reaches a value at which photophoresis is in equilibriumwith all other forces at work. By this effect, material is swept outfrom the inner region of the disk and piled-up in a more or lessconfined belt around the star. Thus, the formation of ringlikestructures of the dust distribution can most naturally be explainedwithout any further assumptions. Since photophoretic pile-up also worksfor larger bodies, it might even trigger the formation of Kuiper belts.
| 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.
| Cool Customers in the Stellar Graveyard. II. Limits to Substellar Objects around Nearby DAZ White Dwarfs
Results from a concerted Hubble Space Telescope (HST) survey of nearbywhite dwarfs for substellar objects are presented. A total of seven DAZwhite dwarfs with distances of <50 pc had high-contrast and highspatial resolution NICMOS coronagraphic images taken to search forcandidate substellar objects at separations <~10" away. Limits tounresolved companions are derived through analysis of Two Micron All SkySurvey photometry of the white dwarfs compared to expected fluxes basedon the white dwarf's effective temperature, distance, and gravity. OurHST survey of seven DAZ white dwarfs identified candidate companions forfour of the white dwarfs. For three of these, HST and ground-basedsecond-epoch observations showed the candidates to be background stars.The fourth white dwarf, which is close to the Galactic plane, has sevencandidate companions at distances of 2"-4", which remain to be followedup. We find that for four of the white dwarfs we are sensitive toplanetary companions of >~10MJ. For all the targets, weare sensitive to companions of >18MJ. The lack ofsignificant near-IR excesses for our targets limits any kind ofunresolved companions present to be substellar. In light of theseresults we make several comments on the possibility of determining theorigin of metals in the atmospheres of these white dwarfs.
| Spiral structure when setting up pericentre glow: possible giant planets at hundreds of AU in the HD 141569 disk
This paper discusses the impact of introducing a planet on an eccentricorbit into a dynamically cold planetesimal disk. That planet's secularperturbations cause the orbits of the planetesimals to evolve in such away that at any one time planetesimals at the same distance from thestar have common pericentres and eccentricities. This causes the surfacedensity distribution of an extended planetesimal disk to exhibit twospirals, one exterior the other interior to the planet's orbit. Thesetwo spirals unwind in different directions and their structure isdescribed by just two parameters: the time since the planet wasintroduced relative to the characteristic secular timescale,tsec(3:2) =0.651sqrt{apl^3/M_star}(M_star/Mpl); and theplanet's eccentricity, epl. At late times the spirals becometightly wound and the offset centre of symmetry of the pericentre glowapproximation is recovered. Comparison with spiral structure seen in theHD 141569 disk shows that its spiral at 325 AU is similar to thestructure that would be caused by introducing a planet into the disk 5Myr ago with a mass in the range 0.2{-}2 MJup orbiting at235-250 AU with an eccentricity of 0.05-0.2; likewise a Saturn massplanet at 150 AU would cause structure like that seen at 200 AU. Moredefinitive statements about any planets orbiting HD 141569 from thismodel could be made once the effect of the binary companion on the diskis known (e.g., from knowledge of its orbit), and once the disk'sstructure has been better characterised down to 100 AU, including thelocation of the star within the disk. The relatively young age of thissystem ( 5 Myr) means that if giant planets really do exist at hundredsof AU from HD 141569, this provides a unique opportunity to setconstraints on the mechanism by which those planets came to be at suchlarge distances, especially since the structure of the disk out of whichthose planets would have formed can be imaged.
| FUSE observations of molecular hydrogen on the line of sight towards HD 141569A
We present an analysis of the Far Ultraviolet Spectroscopic Explorer(FUSE) spectrum of HD 141569A, a transitional object known to possess acircumstellar disk. We observe two components of gas at widely differenttemperatures along the line of sight. We detect cold H2, which isthermalized up to J=2 at a kinetic temperature of ~51 K. Such lowtemperatures are typical of the diffuse interstellar medium. Since theline of sight to HD 141569A does not pass through its disk, it appearsthat we are observing the cold H2 in a low extinction envelopeassociated with the high Galactic latitude dark cloud complex L134N,which is in the same direction and at nearly the same distance as HD141569A. The column densities of the higher J-levels of H2 suggest thepresence of warm gas along the line of sight. The excitation conditionsdo not seem to be consistent with what is generally observed in diffuseinterstellar clouds. The observed radial velocity of the gas impliesthat the UV spectral lines we observe are likely interstellar in originrather than circumstellar, although our absorption line study does notdefinitely rule out the possibility that the warm gas is close to thestar. The discovery of such warm gas along the line of sight may provideevidence for turbulent phenomena in the dark cloud L134N.
| Discovery of a Nearly Edge-on Disk around HD 32297
We report the discovery of a nearly edge-on disk about the A0 star HD32297 seen in light scattered by the disk grains revealed in NICMOSPSF-subtracted coronagraphic images. The disk extends to a distance ofat least 400 AU (3.3") along its major axis with a 1.1 μm fluxdensity of 4.81 +/- 0.57 mJy beyond a radius of 0.3" from thecoronagraphically occulted star. The fraction of 1.1 μm starlightscattered by the disk, 0.0033 +/- 0.0004, is comparable to itsfractional excess emission at 25 + 60 μm of ~0.0027 as measured fromIRAS data. The disk appears to be inclined 10.5d +/- 2.5d from anedge-on viewing geometry, with its major axis oriented 236.5d +/- 1°eastward of north. The disk exhibits unequal brightness on opposingsides and a break in the surface brightness profile along the NE-sidedisk major axis. Such asymmetries might implicate the existence of oneor more (unseen) planetary mass companions.
| Search for H+3 in HD 141569A
A search for H+3 line emission, reported to havebeen detected toward the young star HD 141569A and possibly originatingin a clump of planet-forming gas orbiting the star, has yielded negativeresults. Observations made at the United Kingdom Infrared Telescope andthe Subaru Telescope during 2001-2005 covered 11 major transitions ofH+3 from 3.42 to 3.99 μm. NoH+3 emission lines were detected; one marginaldetection at 3.9855 μm in 2002 June was not confirmed in laterspectra. The upper limits to the line strengths were significantly lowerthan the previously reported detections. Supplemental slit-scanningspectroscopy using adaptive optics was performed within 0.38" of HD141569A to search for extended emission from H+3,but no emission was detected. We compare our upper limit to theluminosity in H+3 from HD 141569A with thatpossible from a gas giant protoplanet and also from a jovian massexoplanet in close orbit about its central star.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.Based on data collected at the United Kingdom Infrared Telescope, whichis operated by the Joint Astronomy Centre on behalf of the UK ParticlePhysics and Astronomy Research Council.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Libra |
| Ascensió Recta: | 15h49m57.75s |
| Declinació: | -03°55'16.4" |
| Magnitud Aparent: | 7.12 |
| Distancia: | 99.01 parsecs |
| Moviment propi RA: | -18.3 |
| Moviment propi Dec: | -20.5 |
| B-T magnitude: | 7.228 |
| V-T magnitude: | 7.129 |
Catàlegs i designacions:
|
INHABITED SKY
