Contents
Images
Upload your image
DSS Images Other Images
Related articles
On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing EREin many dusty interstellar environments remains unidentified. Severalcompeting models are more or less able to match the observed broad,unstructured ERE band. We now constrain the character of the ERE carrierfurther by determining the wavelengths of the radiation that initiatesthe ERE. Using the imaging capabilities of the HST, we have resolved thewidth of narrow ERE filaments appearing on the surfaces of externallyilluminated molecular clouds in the bright reflection nebula NGC 7023and compared them with the depth of penetration of radiation of knownwavelengths into the same cloud surfaces. We identify photons withwavelengths shortward of 118 nm as the source of ERE initiation, not tobe confused with ERE excitation, however. There are strong indicationsfrom the well-studied ERE in the Red Rectangle Nebula and in thehigh-|b| Galactic cirrus that the photon flux with wavelengths shortwardof 118 nm is too small to actually excite the observed ERE, even with100% quantum efficiency. We conclude, therefore, that ERE excitationresults from a two-step process. The first, involving far-UV photonswith E>10.5 eV, leads to the creation of the ERE carrier, most likelythrough photoionization or photodissociation of an existing precursor.The second, involving more abundant near-UV/optical photons, consists ofthe optical pumping of the previously created carrier, followed bysubsequent deexcitation via photoluminescence. The latter process canoccur many times for a single particle, depending upon the lifetime ofthe ERE carrier in its active state. While none of the previouslyproposed ERE models can match these new constraints, we note that underinterstellar conditions most PAH molecules are ionized to the dicationstage by photons with E>10.5 eV and that the electronic energy levelstructure of PAH dications is consistent with fluorescence in thewavelength band of the ERE. Therefore, PAH dications deserve furtherstudy as potential carriers of the ERE.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555. These observations are associated with program9471.
| The Herbig B0e star HD 53367: circumstellar activity and evidence of binarity
Aims.We investigate the spectroscopic behaviour of the young B0e star HD53367 within a cooperative observing programme conducted from 1994 to2005. Methods: .The data include more than 100 high-resolutionspectra collected at the Crimean Astronomical Observatory (CrAO) nearHα, Hβ, He i λ 5876, 6678 Å, DNa i, and O iiλ 6641 Å lines. Two spectra obtained at theObservatório do Pico dos Dias (LNA), in the spectral bandsλλ 4575-4725 Å and λλ 5625-5775Å, were used for spectral classification of HD 53367. The temporalbehaviour of the circumstellar lines Hα and Hβ as well as thephotospheric lines O ii λ 6641 Å and He i λ 6678Å were investigated during different stages of the photometricactivity of this object. Results: .We confirm that the long-termphotometric variability of HD 53367 is related to the alternation of twostates of this object when the gaseous circumstellar envelope disappearsand rises again. Both these processes start near the star and spread tothe outlying parts of the envelope. We find that the radial velocitiesof He i and O ii photospheric lines demonstrate a cyclic variabilitywith a period of P=183.7 days and semi-amplitude K=19 km s-1.The radial velocity change is interpreted in the framework of a model inwhich the star is a component of an eccentric binary system. An orbitalsolution is derived and the system's parameters estimated. We find thatthe orbital eccentricity is e=0.28, and the mean companion separation is1.7 AU. Conclusions: .Based on the estimated parameters, weconclude that the system consists of a massive (~20 M_ȯ) mainsequence primary B0e star, and a secondary which is most likely a 5solar mass pre-main sequence object. We found evidence that the mainpart of the circumstellar gas in this system is concentrated near thesecondary companion. Although the young age of HD 53367, its evolvedprimary B0e star seems to have already became a classical Be starexhibiting a specific alternation of the B-Be stages.
| Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297. A revision of the X-ray properties of MWC 297
We present a Chandra ACIS-I X-ray observation of the region near theHerbig early-Be star MWC 297, where we detect a tight group of pointX-ray sources. These are probably physically associated to MWC 297,because of their obvious clustering with respect to the more scatteredfield-source population. These data are compared to earlier ASCA datawith much poorer spatial resolution, from which the detection of strongquiescent and flaring emission from MWC 297 itself was claimed. We arguethat this star, contributing only 5% to the total X-ray emission of thegroup, was probably not the dominant contributor to the observed ASCAemission, while the X-ray brightest star in the group is a much bettercandidate. This is also supported by the spectral analysis of theChandra data, with reference to the ASCA spectra. We conclude that noneof the X-ray data available for MWC 297 justify the earlier claim ofstrong magnetic activity in this star. The X-ray emission of MWC 297during the Chandra observation is even weaker than that found in otherHerbig stars with the same spectral type, even accounting for its largeline-of-sight absorption.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| Time-Resolved Halpha Monitoring of the Herbig Ae/Be Star HD 200775
Our time-resolved H_alpha monitoring of V380 Cep has revealed nosignificant rapid profile variations for the components of the H_alphaline. A slow shift to shorter wavelengths of individual line componentswas observed during 20 days. The results are difficult to interpret asresulting from the star's axial rotation.
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars?
We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at theCFHT, of magnetic fields in the young A-type stars HD 101412, V380 Oriand HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-mainsequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physicallyassociated with a HAeBe star. Remarkably, evidence of surface abundancespots is detected in the spectra of HD 72106A. The magnetic fields ofthese objects display intensities of order 1 kG at the photosphericlevel, are ordered on global scales, and appear in approximately 10% ofthe stars studied. Based on these properties, the detected stars maywell represent pre-main sequence progenitors of the magnetic Ap/Bpstars. The low masses inferred for these objects (2.6, 2.8 and 2.4M_ȯ) represent additional contradictions to the hypothesis ofHubrig et al. (2000, ApJ, 539, 352), who claim that magnetic fieldsappear in intermediate-mass stars only after 30% of their main sequenceevolution is complete. Finally, we fail to confirm claims by Hubrig etal. (2004, A&A, 428, L1) of magnetic fields in the Herbig Ae star HD139614.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions
We have coupled a singular value decomposition method with a Monte Carlosearch algorithm to analyse the mid-infrared ISOCAM spectral maps ofphotodissociation regions (PDRs) in NGC 7023 and ρ Oph-SR3.Threedifferent spectra and their associated spatial distribution wereextracted from this analysis. It is shown that they can be associatedwith polycyclic aromatic hydrocarbons (PAHs) in their cationic andneutral forms and a third population of carbonaceous very small grains(VSGs). The method allows for the first time (i) to separate thecontribution of neutral PAHs to the interstellar emission spectrum fromthat of cationic PAHs; (ii) to show that the 7.8 μm component of the``7.7 μm'' broad feature is carried by VSGs, whereas the 7.6 μmcomponent is due to PAHs; and (iii) to give evidence that free-flyingPAHs are produced in PDRs by photoevaporation of VSGs. It is proposedthat these carbonaceous VSGs are indeed PAH clusters. We derived aminimal size of 400 carbon atoms per cluster and estimated a UVabsorption power of the order of 10-24 W C-1 fortheir dissociation.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| New Infrared Emission Features and Spectral Variations in NGC 7023
We observed the reflection nebula NGC 7023, with the Short-High moduleand the long-slit Short-Low and Long-Low modules of the InfraredSpectrograph on the Spitzer Space Telescope. We also present InfraredArray Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS)images of NGC 7023 at 3.6, 4.5, 8.0, and 24 μm. We observe thearomatic emission features (AEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7μm, plus a wealth of weaker features. We find new unidentifiedinterstellar emission features at 6.7, 10.1, 15.8, 17.4, and 19.0 μm.Possible identifications include aromatic hydrocarbons or nanoparticlesof unknown mineralogy. We see variations in relative feature strengths,central wavelengths, and feature widths, in the AEFs and weaker emissionfeatures, depending on both distance from the star and nebular position(southeast vs. northwest).
| A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitudelight curve with a period near 540 days has recently been reported fromHipparcos photometry. This period satisfies all of the spectroscopysince at least 1982, including extensive new observations during 2003and 2004, and it has predicted the next transition during 2004September-October. Measurements of the Hα equivalent width reveala sharp emission peak in the phase diagram, in contrast to theapparently sinusoidal light curve. The He II absorption-line strength isessentially constant, while He I varies strongly, possibly filled in byemission in the O6 state, thus producing the apparent spectral typevariations. The O8 state appears to be the ``normal'' one. Twointermediate O7 observations have been obtained, which fall at theexpected phases, but these are the only modern observations of thetransitions so far. The period is too long for rotation or pulsation;although there is no direct evidence as yet for a companion, a model inwhich tidally induced oscillations drive an enhanced wind nearperiastron of an eccentric orbit appears promising. Further observationsduring the now predictable transitions may provide a critical test.Ultraviolet and X-ray observations during both states will likely alsoprove illuminating.
| Ion Chemistry in Photon-dominated Regions: Examining the [HCO+]/[HOC+]/[CO+] Chemical Network
HOC+ and CO+ have been detected toward twowell-known photon-dominated regions (PDRs), S140 and NGC 2023, using theArizona Radio Observatory 12 m telescope. The J=1-->0 transition ofHOC+ at 89 GHz and the two spin components of the N=2-->1line of CO+ near 236 GHz were observed, as well as theJ=1-->0 transitions of H13CO+ andHC18O+. The J=3-->2 line of HOC+ at268 GHz was also mapped across the Orion Bar. The[HCO+]/[HOC+] ratios determined in S140 and NGC2023 were ~12,408 and 1913, respectively, values indicative of quiescentmolecular gas rather than PDR sources, where the abundance ofHOC+ is thought to be enhanced. However, the beam in boththese measurements may contain material from the adjoining molecularcloud, favoring HCO+. Alternatively, the[HCO+]/[HOC+] ratio may vary with Av inPDR regions. The [CO+]/[HOC+] ratio in S140 andNGC 2023, at several positions in the Orion Bar, and in other PDRs, onthe other hand, falls uniformly in the range ~1-10. In addition, theline profiles of CO+ and HOC+ in the Orion Barlook remarkably similar. The syntheses of HOC+ andCO+ appear to be correlated in PDRs, most likely through thecommon precursor, C+. The reaction of C+ andH2O is thought to preferentially create HOC+, asopposed to HCO+, and C++OH or O2 leadsfavorably to CO+.
| Rocket and Far Ultraviolet Spectroscopic Explorer Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflectionnebula IC 405 obtained by a rocket-borne long-slit spectrograph and theFar Ultraviolet Spectroscopic Explorer (FUSE). Both data sets show arise in the ratio of the nebular surface brightness to stellar flux(S/F*) of approximately 2 orders of magnitude toward the blueend of the far-UV bandpass. Scattering models using simple dustgeometries fail to reproduce the observed S/F* for realisticgrain properties. The high spectral resolution of the FUSE data revealsa rich fluorescent molecular hydrogen spectrum ~1000" north of the starthat is clearly distinguished from the steady blue continuum. TheS/F* remains roughly constant at all nebular pointings,showing that fluorescent molecular hydrogen is not the dominant cause ofthe blue rise. We discuss three possible mechanisms for the ``bluedust'': differential extinction of the dominant star (HD 34078), unusualdust-grain properties, and emission from nebular dust. We conclude thatuncertainties in the nebular geometry and the degree of dust clumpingare most likely responsible for the blue rise. As an interestingconsequence of this result, we consider how IC 405 would appear in aspatially unresolved observation. If IC 405 were observed with a spatialresolution of less than 0.4 pc, for example, an observer would infer afar-UV flux that was 2.5 times the true value, giving the appearance ofa stellar continuum that was less extinguished than radiation from thesurrounding nebula, an effect that is reminiscent of the observedultraviolet properties of starburst galaxies.
| PV Cephei: Young Star Caught Speeding?
Three independent lines of evidence imply that the young star PV Cep ismoving at roughly 20 km s-1 through the interstellar medium.The first and strongest suggestion of motion comes from the geometry ofthe Herbig-Haro (HH) knots in the ``giant'' HH flow associated with PVCep. Bisectors of lines drawn between pairs of knots at nearly equaldistances from PV Cep imply an east-west motion of the source, and aplasmon model fitted to the knot positions gives a good fit of 22 kms-1 motion for the star. The second bit of damning evidencecomes from a redshifted trail of molecular gas pointing in the sameeast-west direction implied by the HH knot geometry. The third exhibitwe offer in accusing PV Cep of speeding involves the apparent tilt inthe high-velocity molecular jet now emanating from the star. This tiltis best explained if the true, current jet direction is north-south, asit is in Hubble Space Telescope WFPC2 images, and the star is moving,again at roughly 20 km s-1. Tracing the motion of PV Cepbackward in time to the nearest cluster from which it might have beenejected, we find that it is very likely to have been thrown out of themassive star-forming cluster NGC 7023, more than 10 pc away. PV Cep andNGC 7023 are at similar distances, and the backward trace of PV Cep'smotion is astonishingly well aligned with a dark, previously unexplainedrift in NGC 7023. We propose that PV Cep was ejected, at a speed largeenough to escape NGC 7023, at least 100,000 yr ago but that it did notenter the molecular cloud in which it now finds itself until more like35,000 yr ago. Our calculations show that the currently observablemolecular outflow associated with PV Cep is about 10,000 yr old, so theflow has had plenty of time to form while in its current molecularcloud. However, the question of what PV Cep was doing and what gas/diskit took along with it in the time it was traveling through thelow-density region between NGC 7023 and its current home is an open one.Recent numerical simulations have suggested that condensed objectsshould be ejected at high velocity before they have ``finished'' formingin a cluster. Prior to this work, a handful of pre-main-sequence starshave been shown to be moving at speeds greater than 10 kms-1. To the best of our knowledge, though, the analysis of PVCep and NGC 7023 described here is the first observational workassociating a speeding young star with a distant ancestral cluster.These high-speed ejections from clusters will create a class of rapidlymoving young stars in molecular clouds. If these ejections are at allcommon, their existence confounds both calculations of clouds'star-forming efficiency and theories of star formation that do not allowfor stars to move rapidly through a reservoir of star-forming materialwhile they form.
| Modeling the Continuum Emission from the Circumstellar Environment of Herbig Ae/Be Stars
This paper discusses a model for the continuum emission of the HerbigAe/Be stars in the light of an updated set of observational dataspanning 5 orders of magnitude in wavelength and including thelow-resolution spectra obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatory(ISO). The model is used to reproduce the continuum emission of the 36Herbig Ae/Be stars included in the list by Thé and coworkers andobserved by ISO. The circumstellar matter responsible for the observedspectral energy distributions has been investigated by comparing the setof the observations with the model spectra computed for differentpossible distributions of circumstellar matter. Cases have beenconsidered with the circumstellar regions partially evacuated along thepolar axis by the action of the stellar wind, a phenomenon that is quitecommon in these pre-main-sequence objects. The inclusion of the polarcavities indirectly allows geometries in which a small-scale disklikestructure around the central star is present. The possible coexistenceof two different density profiles, in the inner and the outer region ofthe envelope, respectively, has been also considered. The comparison ofthe computed models with the observed spectral energy distributionsselects the parameter values in such a way that the larger dust grainsare preferentially associated with the later spectral types. We findthat 17 objects are reasonably fitted, eight of which with a purelyspherical model and the remaining nine with the inclusion of the polarcavities. For 10 further objects the fit is worse, and for the remainingnine, almost all associated to IR companions, our model is clearlyinappropriate. A linear relationship is suggested between the logarithmof the initial density n0 and the exponent p of the power lawn(r)=n0(R*/r)p adopted for thecircumstellar density distribution.
| Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431
Spectral and photoelectric (ubvy, H, H) observations of the Herbig Ae/Bestar HD 259431 are reported. It is found that as its brightness fades,this star becomes bluer in the Paschen continuum and the intensity andequivalent width of the hydrogen emission lines increase. The spectralobservations reveal significant variations in the intensity of the Mg II4481 Å photospheric absorption line. A rise and fall in theluminosity by 0m.04 within a period of 5-7 minutes was recorded. Radicalvariations in the H lineshape ("double" "P Cyg") and flare activity arenot only observed in this star, but also in a number of HAEBE stars. Itis suggested that flare activity may initiate a change in the velocitygradient at the base of the wind and, thereby, induce "double P Cyg" or"P Cyg single" transitions. The nonradial pulsations of this star arealso discussed.
| On Rotation of an Isolated Globule
During CO observations of new Southern objects with the 15-m SEST mmtelescope (Cerro La Silla, Chile) we have found that the globuleconnected with the object CLN127-128 rotates with an angular velocity4.3 · 10-14 s-1, which corresponds to the velocity of extremelyfast rotating globules. The object CLN127-128 is a chain of three stars;two of them are connected with bright nebulae, and the third is asuspected Herbig Ae/Be star. All three stars are bright in near IR,which is specific for the existence of circumstellar shells (or disks)around them. The specific angular momentum of the globule confirms thatit is in virial equilibrium. Besides the finding of a rotating globule,CO observations suggest the presence of a blue-shifted outflow fromCLN127-128 with a velocity of -1.1 km/s (in the system connected withthe globule).
| Long-Term Spectral Variability of the Herbig Ae-Star HD 179218
The results of high-resolution long-term spectral monitoring of HerbigAe star HD 179218 in the region of emission H line and the sodiumresonance doublet Na I D are presented. The received data show theexistence of short-term variability (about 10 days) and long-termwave-like variability of equivalent width EW, intensity of circumstellar(CS) emission I and parameters of H emission profile. The analysis ofthese data allows us to suppose the existence of the global variabilityof parameters of accretion disk.
| Spectral Analysis and Classification of Herbig Ae/Be Stars
We present an analysis of the optical spectra of 75 early-typeemission-line stars, many of which have been classified previously asHerbig Ae/Be (HAeBe) stars. Accurate spectral types were derived for 58members of the sample; high continuum veiling, contamination bynonphotospheric absorption features, or a composite binary spectrumprevented accurate spectral typing for the rest. Approximately half ofour sample exhibited [O I] λ6300 forbidden-line emission down toour detection limit of 0.1 Å equivalent width; a third of thesample exhibited Fe II emission (multiplet 42). A subset of 11 of theHAeBe sample showed abnormally strong Fe II absorption; 75% of thissubset are confirmed UX Ori objects. Combining our spectral typingresults with photometry from the literature, we confirm previousfindings of high values of total-to-selective extinction(RV~5) in our larger sample, suggesting significant graingrowth in the environments of HAeBe stars. With this high value ofRV, the vast majority of HAeBe stars appear younger than withthe standard RV=3.1 extinction law and are more consistentwith being pre-main-sequence objects.
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| A new phase of activity of the Herbig Be star HD 200775 in 2001: Evidence for binarity
The results of high-resolution spectroscopy of the Herbig Be star HD200775 obtained within the framework of a cooperative observingprogramme in 2000-2002 are presented. A new high-activity phase of theobject's Hα line occurred in the middle of 2001 in full agreementwith a 3.68-year periodicity predicted by Miroshnichenko et al.(\cite{mirosh}). A complicated picture of the Hα line profilevariability near the activity maximum phase turned out to be verysimilar to that observed during the previous one in 1997. Variations ofthe radial velocity with the activity phase are detected in He I, Si II,and S II photospheric lines. The observed phenomena are interpreted inthe framework of a model in which the star, together with its gaseousenvelope, is a component of an eccentric binary system. A preliminaryorbital solution is derived, and the system's parameters are estimatedfrom the radial velocity curves of the Hα emission line. We findthat the orbital eccentricity is e 0.3, the mean companionseparation is 1000 Rȯ, and the secondary companionis most likely to be a 3.5 Mȯ pre-main sequenceobject. We emphasize the importance of coordinated spectroscopic andinterferometric observations at different phases of the object'sactivity for further understanding the properties of the system.Partially based on observations collected at the Canada-France-Hawaiitelescope (CFHT), operated by the National Research Council of Canada,the Centre National de la Recherche Scientifique, and University ofHawaii.
| Active phenomena in the circumstellar environmentof the Herbig Ae star HD 31648
We present the results of observations of the Herbig Ae star HD 31648including high-resolution spectroscopy near the Hα, He I λ5876 Å and DNa I lines, multi-color photometry and polarimetry.These observations have revealed many features of the observationalappearance of HD 31648 similar to those of the ``classical'' Herbig Aestars. Among them are spectral manifestations of both the mass loss andthe disk accretion onto the star and certain types of line profilevariations. The totality of the observed properties of HD 31648 providesevidence in favor of complicated structure in the stellar wind zone,exhibiting latitudinal stratification and containing loinhomogeneitiesin the form of outflowing streams and density condensations. Structuralvariations of the stellar wind have been found to correlate with changesin the accretion process in the envelope. The circumstellar environmentof HD 31648 is supposed to contain dust particles of different formsmanifesting themselves, in particular, in the variable multi-componentintrinsic polarization.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Κηφεύς |
| Right ascension: | 21h01m36.92s |
| Declination: | +68°09'47.8" |
| Apparent magnitude: | 7.435 |
| Distance: | 429.185 parsecs |
| Proper motion RA: | 8.2 |
| Proper motion Dec: | -0.6 |
| B-T magnitude: | 7.819 |
| V-T magnitude: | 7.467 |
Catalogs and designations:
|
