Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Subaru Near-Infrared Imaging of Herbig Ae Stars
We report results of H-band (? = 1.65?m) imaging observationsof young intermediate-mass stars using the Subaru 8.2-m Telescope withthe adaptive optics AO36 and the infrared coronagraph CIAO. The targetsconsist of 16 Herbig Ae/Be stars (15 Herbig Ae stars and one Herbig Bestar) and four additional main-sequence stars with infrared excesses.Five protoplanetary disks have been spatially resolved around the HerbigAe stars with ages of 2-8 Myr. The resolved disks have outer radii ofseveral 100 AU, and their surface brightnesses range from 10 magarcsec-2 to 18 mag arcsec-2. The images revealvarious morphologies in optically thick disks: spiral arms around ABAur, a banana-split structure with an outer arm for HD 142527, a compactcircumprimary disk of HD 150193, a faint discontinuous ring around HD163296, and an unstructured face-on disk of HD 169142. The detection ofan optically thick disk in scattered light implies that it is verticallyflared, and intercepts stellar light at least in the outer region wherethose images were obtained. However, the surface brightnessdistribution, the resolved structure, and other observationalcharacteristics suggest that the disks are unlikely to be continuouslyflared young disks with small grains well mixed with gas. The detectionrate and the disk brightness do not correlate with the stellar age andthe disk mass, but there is a tendency that the brightest disks arestill surrounded by long-lived envelopes (AB Aur, HD 100546, HD 142527).The significant diversity of the disk structure can be attributed to themultiplicity and the initial condition of the local star-formingenvironments. The detections of companion candidates around our targetedstars are also reported.
| The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems
We present B- and R-band spectroastrometry of a sample of 45 HerbigAe/Be (HAe/Be) stars in order to study their binary properties. All butone of the targets known to be binary systems with a separation of~0.1-2.0 arcsec are detected by a distinctive spectroastrometricsignature. Some objects in the sample exhibit spectroastrometricfeatures that do not appear attributable to a binary system. We findthat these may be due to light reflected from dusty haloes or materialentrained in winds. We present eight new binary detections and fourdetections of an unknown component in previously discovered binarysystems. The data confirm previous reports that HAe/Be stars have a highbinary fraction, 74 +/- 6 per cent in the sample presented here. We usea spectroastrometric deconvolution technique to separate the spatiallyunresolved binary spectra into the individual constituent spectra. Theseparated spectra allow us to ascertain the spectral type of theindividual binary components, which in turn allows the mass ratio ofthese systems to be determined. In addition, we appraise the method usedand the effects of contaminant sources of flux. We find that thedistribution of system mass ratios is inconsistent with random pairingfrom the initial mass function, and that this appears robust despite adetection bias. Instead, the mass ratio distribution is broadlyconsistent with the scenario of binary formation via disc fragmentation.Based on observations made with the William Herschel Telescope (WHT) andthe Isaac Newton Telescope (INT) operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias.E-mail: pyhew@leeds.ac.uk
| Analysis of the Pico Dos Dias Survey Herbig Ae/Be Candidates
A large sample of Herbig Ae/Be (HAeBe) candidates, distributed indifferent Galactic regions south to declination +30°, wereidentified by the Pico dos Dias Survey (a search for young stellarobjects based on IRAS colors). Most of the candidates are nearby orassociated with star-forming clouds, but several others are consideredisolated objects. Aiming to verify the young nature of 93 HAeBecandidates, we searched for additional information that could be usefulto confirm if they are pre-main-sequence (PMS) stars or evolved objects,which coincidentally show similar IRAS colors. By adopting a spectralindex that is related to the amount of infrared excess and the shape ofthe spectral energy distribution, we have classified the sampleaccording to three groups, which are analyzed on the basis of (1)circumstellar luminosity; (2) spatial distribution; (3) opticalpolarization; (4) near-infrared colors; (5) stellar parameters (mass,age, effective temperature); and (5) intensity of emission lines. Ouranalysis indicates that only 76% of the studied sample, mainly the groupwith intermediate to low levels of circumstellar emission, can be moreconfidently considered PMS stars. The nature of the remaining stars,which are in the other group that contains the highest levels ofinfrared excess, remains to be confirmed. They share the samecharacteristics of evolved objects, requiring complementary studies inorder to correctly classify them. At least seven objects showcharacteristics typical of post-asymptotic giant branch orproto-planetary nebulae.Based on observations made at the Observatório do Pico dosDias/LNA (Brazil).
| Characterising discs around Herbig Ae/Be stars through modelling of low-J 12CO lines
Context: While there has been extensive investigation of the dustemission in discs around young intermediate-mass or Herbig Ae/Be starsat a range of wavelengths, their gas content has been systematicallystudied mainly via spatially unresolved (sub)millimetre observations ofthe rotational lines of 12CO. Aims: We are interestedin how the available low-J 12CO spectra compare to the discproperties inferred from the dust emission, and to what extent the gas-and dust-emission approaches to disc modelling are complementary to eachother. Methods: First, we use the disc structure derived from thespectral energy distribution (SED) modelling to produce the synthetic12CO J = 3-2 spectra for a discrete sample of sources. Wethen compare these synthetic spectra to observations, to test theexisting disc models for each source. In our second approach, we studythe dependence of the 12CO J = 3-2 spectrum on disc size,inclination and temperature, for discs around Herbig Ae/Be stars ingeneral. We calculate the spectral line profiles for a grid ofparametric disc models. The calculated spectra are compared to thespectra observed towards a large sample of sources. Both methods use amolecular excitation and radiative transfer code for the calculation ofthe 12CO line emission. Results: SED models areinsensitive to the parameters that dominate the low-J 12COemission, i.e., the disc size and orientation. To minimise some of theimportant parameter degeneracies, it is necessary to model a disc's SEDwith prior knowledge of the disc size and inclination. We show how thespectral profile of low-J 12CO lines can be used to constrainthese parameters, and to obtain disc models that are good startingpoints for the outside-in SED modelling of discs from long to shorterwavelengths. Conclusions: For a disc gas mass of the order of 0.01M?, the optically thick J = 3-2 12CO line intensityshows that the majority of discs around Herbig Ae/Be stars are smallerthan 200 AU, and that the largest and brightest sources thoroughlystudied with submillimetre interferometry are not representative of thesample.
| The Nainital-Cape Survey. III. A search for pulsational variability in chemically peculiar stars
Context: The Nainital-Cape survey is a dedicated research programme tosearch for and study the pulsational variability in chemically peculiarstars in the Northern Hemisphere. Aims: The aim of thesurvey is to search for such chemically peculiar stars that arepulsationally unstable. Methods: The observations of thesample stars were carried out in high-speed photometric mode using athree-channel fast photometer attached to the 1.04-m Sampurnanandtelescope at ARIES. Results: The new photometricobservations confirmed that the pulsational period of star HD 25515 is2.78-hrs. The repeated time-series observations of HD 113878 and HD118660 revealed that previously known frequencies are indeed present inthe new data sets. We have estimated the distances, absolute magnitudes,effective temperatures and luminosities of these stars. Their positionsin the H-R diagram indicate that HD 25515 and HD 118660 lie near themain-sequence while HD 113878 is an evolved star. We also present acatalogue of 61 stars classified as null results, along with thecorresponding 87 frequency spectra taken from 2002-2008. A statisticalanalysis of these null results shows, by comparison with past data, thatthe power of the noise in the light curves has slightly increased duringthe last few years.
| Near-Infrared Photometric Survey of Herbig Ae/Be Candidate Stars
We report near-infrared photometric measurements of 35 Herbig Ae/Becandidate stars obtained with direct imaging and aperture photometry.Observations were made through the broadband J, H, and K' filters, witheach source imaged in at least one of the wavebands. We achievedsubarcsecond angular resolution for all observations, providing us withthe opportunity to search for close binary candidates and extendedstructure. The imaging revealed five newly identified binary candidatesand one previously resolved T Tauri binary among the target sources withseparations of lsim2farcs5. Separate photometry is provided for each ofthe binary candidate stars. We detect one extended source that has beenidentified as a protoplanetary nebula. Comparing our magnitudes to pastmeasurements yields significant differences for some sources, possiblyindicating photometric variability. H-band finding charts for all of oursources are provided to aid follow-up high-resolution imaging.
| On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars
Context: Based on their spectral energy distribution, Herbig stars havebeen categorized into two observational groups, reflecting their overalldisk structure: group I members have disks with a high degree of flaringas opposed to their group II counterparts. Literature results show thatthe structure of the disk is a strong function of the disk mass in?m-sized dust grains. Aims: We investigate the 5-35 ?mSpitzer IRS spectra of a sample of 13 group I sources and 20 group IIsources. We focus on the continuum emission to study the underlying diskgeometry. Methods: We have determined the [30/13.5] and [13.5/7]continuum flux ratios. The 7-?m flux excess with respect to thestellar photosphere is measured, as a marker for the strength of thenear-IR emission produced by the hot inner disk. We have compared ourdata to the spectra produced by self-consistent passive-disk models, forwhich the same quantities were derived. Results: We confirm theresults by Meijer et al. (2008, A&A, 492, 451) that the differencesin continuum emission between group I and II sources can largely beexplained by a difference in amount of small dust grains. However, wereport a strong correlation between the [30/13.5] and [13.5/7] fluxratios for Meeus group II sources. Moreover, the [30/13.5] flux ratiodecreases with increasing 7-?m excess for all targets in the sample.To explain these correlations with the models, we need to introduce anartificial scaling factor for the inner disk height. In roughly 50% ofthe Herbig Ae/Be stars in our sample, the inner disk must be inflated bya factor 2 to 3 beyond what hydrostatic calculations predict. Conclusions: The total disk mass in small dust grains determines thedegree of flaring. We conclude, however, that for any given disk mass insmall dust grains, the shadowing of the outer (tens of AU) disk isdetermined by the scale height of the inner disk (~1 AU). The inner diskpartially obscures the outer disk, reducing the disk surfacetemperature. Here, for the first time, we prove these effectsobservationally.
| The Alignment of the Polarization of Herbig Ae/Be Stars with the Interstellar Magnetic Field
We present a study of the correlation between the direction of thesymmetry axis of the circumstellar material around intermediate massyoung stellar objects and that of the interstellar magnetic field. Weuse CCD polarimetric data on 100 Herbig Ae/Be stars. A large numberof them show intrinsic polarization, which indicates that theircircumstellar envelopes are not spherical. The interstellar magneticfield direction is estimated from the polarization of field stars. Thereis an alignment between the position angle of the Herbig Ae/Be starpolarization and that of the field stars for the most polarized objects.This may be an evidence that the ambient interstellar magnetic fieldplays a role in shaping the circumstellar material around young stars ofintermediate mass and/or in defining their angular momentum axis.Based on observations made at the Observatório do Pico dos Dias,Brazil, operated by the Laboratório Nacional deAstrofísica.
| Circumstellar disks around Herbig Be stars
Aims. Our goal is to investigate the properties of the circumstellardisks around intermediate mass stars to determine their occurrence,lifetime and evolution. Methods: We completed a search forcircumstellar disks around Herbig Be stars using the NRAO Very LargeArray (VLA) and the IRAM Plateau de Bure (PdB) interferometers. Thusfar, we have observed 6 objects with 4 successful detections. Theresults towards 3 of these stars (R Mon, MWC 1080, MWC 137) werepresented elsewhere. We present our new VLA and PdBI data for the threeobjects MWC 297, Z CMa, and LKHα 215. We constructed the SED fromnear-IR to centimeter wavelengths by adding our millimeter andcentimeter data to the available data at other wavelengths, mainlySpitzer images. The entire SED was fitted using a disk+envelope model.In addition, we compiled all the disk millimeter observations in theliterature and completed a statistical analysis of all the data. Results: We show that the disk mass is usually only a small percentage(less than 10%) of the mass of the entire envelope in HBe stars. For thedisks, there are large source-to-source variations. Two disks in oursample, R Mon and Z CMa, have similar sizes and masses to those found inT Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are,however, smaller (r_out < 100 AU). We did not detect the diskstowards MWC 137 and LkHα 215 at millimeter wavelengths, whichlimits the mass and the size of the possible circumstellar disks. Conclusions: A comparison between our data and previous results for TTauri and Herbig Ae stars indicates that although massive disks (~0.1M_ȯ) are found in young objects (~104 yr), the masses ofthe disks around Herbig Be stars are usually 5-10 times lower than thosearound lower mass stars. We propose that disk photoevaporation isresponsible for this behavior. In Herbig Be stars, the UV radiationdisperses the gas in the outer disk on a timescale of a few105 yr. Once the outer part of the disk has vanished, theentire gaseous disk is photoevaporated on a very short timescale(~105 yr) and only a small, dusty disk consisting of largegrains remains.
| Parameters of Herbig Ae/Be and Vega-type stars
Context: This work presents the characterization of 27 young early-typestars, most of them in the age range 1-10 Myr, and three suspected hotcompanions of post-T Tauri stars belonging to the Lindroos binarysample. Most of these objects show IR excesses in their spectral energydistributions, which are indicative of the presence of disks. The workis relevant in the fields of stellar physics, physics of disks andformation of planetary systems. Aims: The aim of the work is thedetermination of the effective temperature, gravity, metallicity, mass,luminosity and age of these stars. An accurate modelling of their disksrequires the knowledge of most of these parameters, since they willdetermine the energy input received by the disk and hence, its geometryand global properties. Methods: Spectral energy distributions andmid-resolution spectra were used to estimate T_eff, the effectivetemperature. The comparison of the profiles of the Balmer lines withsynthetic profiles provides the value of the stellar gravity, g_*.High-resolution optical observations and synthetic spectra are used toestimate the metallicity, [M/H]. Once Teff, g* and[M/H] are known for each star, evolutionary tracks and isochronesprovide estimations of the mass, luminosity, age and distances (or upperlimits in some cases). The method is original in the sense that it isdistance-independent, i.e. the estimation of the stellar parameters doesnot require the knowledge of the distance to the object. Results:Stellar parameters (effective temperature, gravity, metallicity, mass,luminosity, age and distances - or upper limits) are obtained for thesample of stars mentioned above. A detailed discussion of someindividual objects, in particular VV Ser, RR Tau, 49 Cet and the threesuspected hot companions of post-T Tauris, is presented. Conclusions:These results, apart from their intrinsic interest, would be extremelyvaluable to model the disks surrounding the stars. The paper also showsthe difficulty posed by the morphology and behaviour of the systemstar+disk in the computation of the stellar parameters.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, jointly operated by the Max-Planck Institut fürAstronomie and the Instituto de Astrofísica de Andalucía(CSIC), and on observations made with the WHT telescope operated on theisland of La Palma by the Isaac Newton Group in the Spanish Observatoriodel Roque de los Muchachos of the Instituto de Astrofísica deCanarias.
| What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?
Context: The disks of gas and dust that form around young stars and canlead to planet formation contain polycyclic aromatic hydrocarbons (PAHs)and very small grains (VSGs). Aims: In this paper we analyze themid-infrared (mid-IR) emission of these very small dust particles in asample of 12 protoplanetary disks. Our goal is twofold: first we want tocharacterize the properties of these particles in disks and see how theyare connected to interstellar matter, and second we investigate thepossibility that their emission can be used as a probe of the physicalconditions and evolution of the disk. Methods: We define a basis madeof three mid-IR template spectra: PAH^0, PAH^+, and VSGs that werederived from the analysis of reflection nebulae, and an additionalPAHx spectrum that was introduced recently for analysis ofthe spectra of planetary nebulae. Results: From the optimization of thefit of 12 star+disk spectra, using a linear combination of the 4template spectra, we found that an additional small grain component witha broad feature at 8.3 μm is needed. We find that the fraction of VSGemission in disks decreases with increasing stellar temperature. VSGsappear to be destroyed by UV photons at the surface of disks, thusreleasing free PAH molecules, which are eventually ionized as observedin photodissociation regions. In contrast, we observe that the fractionof PAHx increases with increasing star temperature except inthe case of B stars where they are absent. We argue that this iscompatible with the identification of PAHx as large ionizedPAHs, most likely emitting in regions of the disk that are close to thestar. Finally, we provide a UV-dependent scheme to explain the evolutionof PAHs and VSGs in protoplanetary disks. These results allow us to putnew constraints on the properties of two sources: IRS 48 and“Gomez's Hamburger” which are poorly characterized. Conclusions: Very small dust particles incorporated into protoplanetarydisks are processed while exposed to the intense radiation field of thecentral star. The resulting shape of the mid-IR spectrum can reveal thisprocessing and be used as an efficient probe of the radiation field i.e.luminosity of central star.This work is based on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract with NASA. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, theNetherlands and the UK) and with the participation of ISAS and NASA.
| PAH Emission from Herbig Ae/Be Stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) starsobtained with the Infrared Spectrograph on Spitzer. All but one of theHerbig stars show emission from PAHs, and seven of the spectra show PAHemission, but no silicate emission at 10 μm. The central wavelengthsof the 6.2, 7.7-8.2, and 11.3 μm emission features decrease withstellar temperature, indicating that the PAHs are less photoprocessed incooler radiation fields. The apparent low level of photoprocessing inHAeBe stars, relative to other PAH emission sources, implies that thePAHs are newly exposed to the UV-optical radiation fields from theirhost stars. HAeBe stars show a variety of PAH emission intensities andionization fractions but a narrow range of PAH spectral classificationsbased on positions of major PAH feature centers. This may indicate that,regardless of their locations relative to the stars, the PAH moleculesare altered by the same physical processes in the protoplanetary disksof intermediate-mass stars. Analysis of the mid-IR SEDs indicates thatour sample likely includes both radially flared and moreflattened/settled disk systems, but we do not see the expectedcorrelation of overall PAH emission with disk geometry. We suggest thatthe strength of PAH emission from HAeBe stars may depend not only on thedegree of radial flaring but also on the abundance of PAHs inilluminated regions of the disks and possibly on the vertical structureof the inner disk as well.
| Disks around CQ Tauri and MWC 758: dense PDR or gas dispersal?
Context: The overall properties of disks surrounding intermediate PMSstars (HAe) are not yet well constrained by current observations. Thedisk inclination, which significantly affects spectral energydistribution modeling, is often unknown. Aims: We attempted toresolve the disks around CQ Tau and MWC 758 to provide accurateconstraints on the disk parameters, in particular the temperature andsurface density distribution. Methods: We report arcsecondresolution observations of dust and CO line emissions with the IRAMarray. We also searched for the HCO+ J = 1 → 0transition. The disk properties are derived using a standard disk model.We use the Meudon PDR code to study the chemistry. Results: Thetwo disks share some common properties. The mean CO abundance is lowdespite disk temperatures above the CO condensation temperature.Furthermore, the CO surface density and dust opacity have differentradial dependence. The CQ Tau disk appears warmer and perhaps less densethan that of MWC 758. Modeling the chemistry, we find thatphotodissociation of CO is a viable mechanism to explain its lowabundance. The photospheric flux is not sufficient for this: a strong UVexcess is required. In CQ Tau, the high temperature is consistent withthe expectation for a PDR. The PDR model has difficulty explaining themild temperatures obtained in MWC 758, for which a low gas-to-dust ratiois preferred. A yet, unexplored alternative could be that, despitecurrently high gas temperatures CO remains trapped in grains, as themodels suggest that large grains can be cold enough to prevent thermaldesorption of CO. The low inclination of the CQ Tau disk, ~30°,challenges previous interpretations given for UX Ori - like luminosityvariations of this star. Conclusions: We conclude that CO cannotbe used as a simple tracer of gas-to-dust ratio, the CO abundance beingaffected by photodissociation and grain growth.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain).Appendices A-C are only available in electronic form athttp://www.aanda.org
| ?Sct pulsation in magnetic Ap stars: the discovery of ?Sct pulsations in HD218994AB and measurement of the magnetic fields of HD218994A and HD21190
We have discovered that at least one component of the visual binary starHD218994AB is a ?Sct star. The primary star of this system is aknown rapidly oscillating Ap (roAp) star; therefore, it is possible thatit is both an roAp star and a ?Sct star - a hybrid. There are noconfirmed such hybrid roAp-?Sct stars, and they are predictedtheoretically not to exist. More probably, HD218994B is the ?Sctstar, as it is 0.54mag fainter than the primary and also lies in the?Sct instability strip. It then offers the potential to determineTeff and L for the primary which appears to be one of themore evolved roAp stars and an important test the theoreticallycalculated roAp star instability strip. In this case, the system also isimportant since the stars are a co-eval pair of very similar mass, yetone is a magnetic roAp star and the other is probably a non-magnetic?Sct star with a normal spectrum. We detect a significantlongitudinal magnetic field strength of 440 +/- 23G in the roAp starHD218994A. We also present the first measurement of the longitudinalmagnetic field strength for the ?Sct star HD21190, themost-evolved Ap star known. We obtain a formally significant value of 47+/- 13G. If confirmed, this will make HD21190 the first magnetic?Sct star known.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 079.D-0241.E-mail: dwkurtz@uclan.ac.uk
| A search for rapid pulsations among nine luminous Ap stars
The rapidly oscillating Ap (roAp) stars are of importance for studyingthe atmospheric structure of stars where the process of chemical elementdiffusion is significant. We have performed a survey for rapidoscillations in a sample of nine luminous Ap stars, selected from theirlocation in the colour-magnitude diagram as more evolved main-sequenceAp stars that are inside the instability strip for roAp stars. Untilrecently this region was devoid of stars with observed rapid pulsations.We used the Very Large Telescope UV-Visual Echelle Spectrograph toobtain high time resolution spectroscopy to make the first systematicspectroscopic search for rapid oscillations in this region of the roApinstability strip. We report nine null detections with upper limits forradial velocity amplitudes of 20-65ms-1 and precisions of? = 7-20ms-1 for combinations of Nd and Pr lines.Cross-correlations confirm these null results. At least six stars aremagnetic and we provide magnetic field measurements for four of them, ofwhich three are newly discovered magnetic stars. It is found that fourstars have magnetic fields smaller than ~2kG, which according totheoretical predictions might be insufficient for suppressing envelopeconvection around the magnetic poles for more evolved Ap stars.Suppression of convection is expected to be essential for the opacitymechanism acting in the hydrogen ionization zone to drive thehigh-overtone roAp pulsations efficiently. Our null results suggest thatthe more evolved roAp stars may require particularly strong magneticfields to pulsate. Three of the studied stars do, however, have magneticfields stronger than 5kG.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 075.D-0145 (A), 078.D-0080(A),072.D-0138(A) and 077.D-0150(A).E-mail: lmfreyhammer@uclan.ac.uk
| The SCUBA Legacy Catalogues: Submillimeter-Continuum Objects Detected by SCUBA
We present the SCUBA Legacy Catalogues, two comprehensive sets ofcontinuum maps (and catalogs) using data at 850 and 450 μm of thevarious astronomical objects obtained with the Submillimetre Common UserBolometer Array (SCUBA). The Fundamental Map Data Set contains data onlywhere superior atmospheric opacity calibration data were available. TheExtended Map Data Set contains data regardless of the quality of theopacity calibration. Each data set contains1.2deg×1.2deg maps at locations where dataexisted in the JCMT archive, imaged using the matrix inversion method.The Fundamental Data Set is composed of 1423 maps at 850 μm and 1357maps at 450 μm. The Extended Data Set is composed of 1547 maps at 850μm. Neither data set includes high sensitivity, single-chop SCUBAmaps of ``cosmological fields'' nor solar system objects. Each data setwas used to determine a respective object catalogue, consisting ofobjects identified within the respective 850 μm maps using anautomated identification algorithm. The Fundamental and Extended MapObject Catalogues contain 5061 and 6118 objects, respectively. Objectsare named based on their respective J2000.0 position of peak 850 μmintensity. The catalogues provide for each object the respective maximum850 μm intensity, estimates of total 850 μm flux and size, andtentative identifications from the SIMBAD Database. Where possible, thecatalogues also provide for each object its maximum 450 μm intensityand total 450 μm flux and flux ratios.
| Comparing the Observational Instability Regions for Pulsating Pre-Main-Sequence and Classical δ Scuti Stars
A comparison of the hot and cool boundaries of the classical instabilitystrip with observations has been an important test for stellar structureand evolution models of post- and main-sequence stars. Over the last fewyears, the number of pulsating pre-main-sequence (PMS) stars hasincreased significantly: 36 PMS pulsators and candidates are known as of2007 June. This number allows to investigate the location of theempirical PMS instability region and to compare its boundaries to thoseof the classical (post- and main-sequence) instability strip. Due to thestructural differences of PMS and (post-)main-sequence stars, thefrequency spacings for nonradial modes will be measurably different,thus challenging asteroseismology as a diagnostic tool.
| A search for mid-infrared molecular hydrogen emission from protoplanetary disks
We report on a sensitive search for mid-infrared molecular hydrogenemission from protoplanetary disks. We observed the Herbig Ae/Be starsUX Ori, HD 34282, HD 100453, HD 101412, HD 104237 and HD 142666, and theT Tauri star HD 319139, and searched for H _2~0-0~S(2)~(J=4-2) emissionat 12.278 micron and H _2~0-0~S(1)~(J=3-1) emission at 17.035 micronwith VISIR, ESO-VLT's high-resolution mid-infrared spectrograph. None ofthe sources present evidence for molecular hydrogen emission at thewavelengths observed. Stringent 3σ upper limits to the integratedline fluxes and the mass of optically thin warm gas (T= 150, 300 and1000 K) in the disks are derived. The disks contain less than a fewtenths of Jupiter mass of optically thin H2 gas at 150 K, and less thana few Earth masses of optically thin H2 gas at 300 K and highertemperatures. We compare our results to a Chiang & Goldreich (1997,ApJ, 490, 368, CG97) two-layer disk model of masses 0.02Mȯ and 0.11 Mȯ. The upper limits to thedisk's optically thin warm gas mass are smaller than the amount of warmgas in the interior layer of the disk, but they are much larger than theamount of molecular gas expected to be in the surface layer. If thetwo-layer approximation to the structure of the disk is correct, ournon-detections are consistent with the low flux levels expected from thesmall amount of H2 gas in the surface layer. We present a calculation ofthe expected thermal H2 emission from optically thick disks, assuming aCG97 disk structure, a gas-to-dust ratio of 100 and T_gas = T_dust. Weshow that the expected H2 thermal emission fluxes from typical disksaround Herbig Ae/Be stars are of the order of 10-16 to10-17 erg s-1 cm-2 for a distance of140 pc. This is much lower than the detection limits of our observations(5 × 10-15 erg s-1 cm-2). H2emission levels are very sensitive to departures from the thermalcoupling between the molecular gas and dust in the surface layer.Additional sources of heating of gas in the disk's surface layer couldhave a major impact on the expected H2 disk emission. Our resultssuggest that in the observed sources the molecular gas and dust in thesurface layer have not significantly departed from thermal coupling(T_gas/T_dust < 2) and that the gas-to-dust ratio in the surfacelayer is very likely lower than 1000.Based on observations collected at the European Southern Observatory,Chile (program ID 076.C-0129(A) and 078.C-0288(A)).
| Formation and evolution of Ae and Be stars
Several scenarios for the formation of accretion and decretion disks insingle and binary Ae and Be stars are proposed. It is shown that, inorder for a rapidly rotating main-sequence Be star to lose mass via adisk, the star’s rotation must be quasi-rigid-body. Estimates showthat such rotation can be maintained by the star’s magnetic field,which is probably a relict field. The evolution of single Bemain-sequence stars is numerically simulated allowing for mass loss viathe stellar wind and rotational mass loss assuming rigid-body rotation.The stellar wind is the factor that determines the maximum mass of Bestars, which is close to 30 M ?. The evolution of Bestars in close binaries is analyzed in the approximation adopted in ourscenario. Long gamma-ray bursts can be obtained as a result of thecollapse of rapidly rotating oxygen—neon degeneratedwarfs—the accreting companions of Be stars—into neutronstars.
| The Unusual Hydrocarbon Emission from the Early Carbon Star HD 100764: The Connection between Aromatics and Aliphatics
We have used the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope to obtain spectra of HD 100764, an apparently single carbonstar with a circumstellar disk. The spectrum shows emission featuresfrom polycyclic aromatic hydrocarbons (PAHs) that are shifted to longerwavelengths than normally seen, a characteristic of ``class C'' systemsin the classification scheme of Peeters et al. All seven of the knownclass C PAH sources are illuminated by radiation fields that are coolerthan those which typically excite PAH emission features. The observedwavelength shifts are consistent with hydrocarbon mixtures containingboth aromatic and aliphatic bonds. We propose that the class C PAHspectra are distinctive because the carbonaceous material has not beensubjected to a strong ultraviolet radiation field, allowing relativelyfragile aliphatic materials to survive.
| HD 97048's Circumstellar Environment as Revealed by a Hubble Space Telescope ACS Coronagraphic Study of Disk Candidate Stars
We present the results of a coronagraphic scattered-light imaging surveyof six young disk candidate stars using the Hubble Space Telescope Advanced Camera for Surveys. The observations made use of the 1.8"occulting spot through the F606W (broad V) filter. Circumstellarmaterial was imaged around HD 97048, a Herbig Ae/Be star located in theChamaeleon I dark cloud at a distance of 180 pc. The material is seenbetween ~2" (360 AU) and ~4" (720 AU) from the star in all directions. A V-band azimuthally averaged radial surface brightness profile peaks atr=2'' with a value of 19.6+/-0.2 mag arcsec-2 andsmoothly decreases with projected distance from the star asI~r-3.3+/-0.5. An integrated flux of 16.8+/-0.1 mag ismeasured between 2" and 4", corresponding to a scattered-lightfractional luminosity lower limit ofLsca/L*>8.4×10-4. Filamentarystructure resembling spiral arms similar to that seen in Herbig Ae/Bedisks is observed. Such structure has been attributed to the influenceof orbiting planets or stellar encounters. Average surface brightnessupper limits are determined for the five nondetections: HD 34282, HD139450, HD 158643, HD 159492, and HD 195627. Possible reasons for thenondetections are disks that are too faint or disks hidden by theocculter.
| Probing the structure of protoplanetary disks: a comparative study of DM Tau, LkCa 15, and MWC 480
Context: The physical structure of proto-planetary disks is not yet wellconstrained by current observations. Millimeter interferometry is anessential tool to investigate young disks. Aims: We study thevertical and radial temperature distribution in a few well-known disksfrom an observational perspective. The surface density distribution ofCO and HCO+ and the scale-height are also investigated. Methods: We report CO observations at sub-arcsecond resolution withthe IRAM array of the disks surrounding MWC 480, LkCa 15, and DM Tau,and simultaneous measurements of HCO+ J = 1? 0. Toderive the disk properties, we fit a standard disk model in which allparameters are power laws of the distance to the star to the data.Possible biases associated with the method are detailed and explained.We compare the properties of the observed disks with similar objects. Results: We find evidence for a vertical temperature gradient inthe disks of MWC 480 and DM Tau,as in AB Aur, but not in LkCa 15.The disk temperatures increase with stellar effective temperature.Except for AB Aur, the bulk of the CO gas is attemperatures smaller than 17 K, below the condensation temperature ongrains. We find the scale height of the CO distribution to be larger (by50%) than the expected hydrostatic scale height. The total amount of COand the isotopologue ratio depends globally on the star. The more UVluminous objects appear to have more CO, but there is no simpledependency. The [ 13CO] /[ HCO^+] ratio is ~600, withsubstantial variations between sources, and with radius. The temperaturebehavior is consistent with expectations, but published chemical modelshave difficulty reproducing the observed CO quantities. Changes in theslope of the surface density distribution of CO, compared to thecontinuum emission, suggest a more complex surface density distributionthan is usually assumed in models. Vertical mixing seems an importantchemical agent, as does photo-dissociation by the ambient UV radiationat the disk's outer edge.
| A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate- and high-mass stars isfundamentally a mystery. Clues towards solving this basic astrophysicalproblem can likely be found at the pre-main-sequence (PMS) evolutionarystage. With this work, we perform the largest and most sensitive searchfor magnetic fields in PMS Herbig Ae/Be (HAeBe) stars. We seek todetermine whether strong, ordered magnetic fields, similar to those ofmain-sequence Ap/Bp stars, can be detected in these objects, and if so,to determine the intensities, geometrical characteristics, andstatistical incidence of such fields. 68 observations of 50 HAeBe starshave been obtained in circularly polarized light using the FORS1spectropolarimeter at the ESO VLT. An analysis of both Balmer andmetallic lines reveals the possible presence of weak longitudinalmagnetic fields in photospheric lines of two HAeBe stars, HD 101412 andBF Ori. Results for two additional stars, CPD-53 295 and HD 36112, aresuggestive of the presence of magnetic fields, but no firm conclusionscan be drawn based on the available data. The intensity of thelongitudinal fields detected in HD 101412 and BF Ori suggest that theycorrespond to globally ordered magnetic fields with surface intensitiesof order 1 kG. On the other hand, no magnetic field is detected in 4other HAeBe stars in our sample in which magnetic fields had previouslybeen confirmed. Monte Carlo simulations of the longitudinal fieldmeasurements of the undetected stars allow us to place an upper limit ofabout 300 G on the general presence of aligned magnetic dipole magneticfields, and of about 500 G on perpendicular dipole fields. Taking intoaccount the results of our survey and other published results, we findthat the observed bulk incidence of magnetic HAeBe stars in our sampleis 8-12 per cent, in good agreement with that of magnetic main-sequencestars of similar masses. We also find that the rms longitudinal fieldintensity of magnetically detected HAeBe stars is about 200 G, similarto that of Ap stars and consistent with magnetic flux conservationduring stellar evolution. These results are all in agreement with thehypothesis that the magnetic fields of main-sequence Ap/Bp stars arefossils, which already exist within the stars at the PMS stage. Finally,we explore the ability of our new magnetic data to constrainmagnetospheric accretion in Herbig Ae/Be stars, showing that ourmagnetic data are not consistent with the general occurrence in HAeBestars of magnetospheric accretion as described by the theories ofKönigl and Shu et al..Based on observations from the ESO telescopes at the La Silla ParanalObservatory under programme ID 072.C-0447, DDT-272.C-5063, 074.C-0442.E-mail: wade-g@rmc.ca
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| 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.
| 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.
| 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.
| The Nainital-Cape Survey. II. Report for pulsation in five chemically peculiar A-type stars and presentation of 140 null results
Aims.We search for photometric variability in chemically peculiar A typestars in the northern hemisphere. Methods: .High-speed photometricobservations of Ap and Am star candidates have been carried out fromARIES (Manora Peak, Nainital) using a three-channel fast photometerattached to the ARIES 104-cm Sampurnanand telescope. Results:.This paper presents three new variables: HD 113878, HD 118660 and HD207561. During the time span of the survey (1999 December to 2004January) pulsations of the δ Sct type were also found for the twoevolved Am stars HD 102480 and HD 98851, as reported in Joshi et al.(2002, 2003). Additionally, we present 140 null results of the surveyfor this time span. Conclusions: .The star HD 113878 pulsates witha period of 2.31 h, which is typical of δ Sct stars. HD 118660exhibits multi-periodic variability with a prominent period of nearly 1h. These periods need to be investigated and make HD 118660 aparticularly interesting target for further observations. For HD 207561,a star classified as Am, a probable pulsation with a period of 6 min wasfound in the light curves obtained on two consecutive nights. Both HD102480 and HD 98851 exhibit unusual alternating high and low amplitudemaxima, with a period ratio of 2:1. The analysis of the null resultsconfirms the photometric quality of the Nainital site.
| Discovery of an Optically Thick, Edge-on Disk around the Herbig Ae Star PDS 144N
We have discovered an optically thick, edge-on circumstellar disk arounda Herbig Ae star in the binary system PDS 144, providing the firstintermediate-mass analog of HK Tau and similar T Tauri stars. Thissystem consists of a V~13 mag primary and a fainter companion, with thespectra of both stars showing evidence for circumstellar disks andaccretion; both stars were classified as Herbig Ae by the Pico dos DiasSurvey. In Lick adaptive optics polarimetry, we resolved extendedpolarized light scattered from dust around the northern star. Follow-upKeck adaptive optics and mid-infrared observations show that this staris entirely hidden by an optically thick disk at all wavelengths from1.2 to 11.7 μm. The disk major axis subtends ~0.8" on the sky,corresponding to ~800 AU at a distance of 1000 pc. Bright ``wings''extend 0.3" above and below the disk ansae, due most likely toscattering from the edges of an outflow cavity in a circumstellarenvelope. We discuss the morphology of the disk and the spectral energydistributions of the two PDS 144 stars, present preliminary disk models,and identify a number of open questions regarding this fascinatingsystem.Some of the data presented here were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| A Keplerian Disk around the Herbig Ae Star HD 169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in 1.3 mm continuum emission and 12CO J=2-1 lineemission at ~1.5" resolution that reveal a circumstellar disk. Thecontinuum emission is centered on the star position and resolved, and itprovides a mass estimate of ~0.02 Msolar for the disk. The COimages show patterns in position and velocity that are well matched by adisk in Keplerian rotation with low inclination to the line of sight. Weuse radiative transfer calculations based on a flared, passive diskmodel to constrain the disk parameters by comparison to the spectralline emission. The derived disk radius is 235 AU, and the inclination is13°. The model also necessitates modest depletion of the COmolecules, similar to that found in Keplerian disks around T Tauristars.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 05h16m00.47s |
| Declinazione: | -09°48'35.4" |
| Magnitudine apparente: | 9.847 |
| Moto proprio RA: | -0.7 |
| Moto proprio Dec: | -3.2 |
| B-T magnitude: | 10.226 |
| V-T magnitude: | 9.879 |
Cataloghi e designazioni:
|
