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Recent results on Pre-main sequence delta Scuti stars. .
Intermediate mass Pre-main sequence stars (1.5 Mȯ< M< 5Mȯ) cross the instability strip on their way tothe main sequence. They are therefore expected to be pulsating in asimilar way as the delta Scuti stars. In this contribution we presentthe status of the observational studies of pulsations in these starswith special emphasis on recent results from our group. The prospectsfor future investigations of these objects from the ground and fromspace are discussed.
| 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.
| A compact dusty disk around the Herbig Ae star HR 5999 resolved with VLTI / MIDI
Aims.We have used mid-infrared long-baseline interferometry to resolvethe circumstellar material around the Herbig Ae star HR 5999, providingthe first direct measurement of its angular size, and to deriveconstraints on the spatial distribution of the dust. Methods:.MIDI at the VLTI was used to obtain a set of ten spectrally dispersed(8-13 μm) interferometric measurements of HR 5999 at differentprojected baseline lengths and position angles. To derive constraints onthe geometrical distribution of the dust, we compared ourinterferometric measurements to 2D, frequency-dependent radiationtransfer simulations of circumstellar disks and envelopes. Results: .The derived visibility values between 0.5 and 0.9 show thatthe mid-infrared emission from HR 5999 is clearly resolved. Thecharacteristic size of the emission region depends on the projectedbaseline length and position angle, and it ranges between 5-15milliarcsec (Gauss FWHM), corresponding to remarkably small physicalsizes of 1-3 AU. For disk models with radial power-law densitydistributions, the relatively weak but very extended emission from outerdisk regions ( 3 AU) leads to model visibilities that aresignificantly lower than the observed visibilities, making these modelsinconsistent with the MIDI data. Disk models in which the density istruncated at outer radii of 2 - 3 AU, on the other hand, provide goodagreement with the data. Conclusions: .A satisfactory fit to theobserved MIDI visibilities of HR 5999 is found with a model of ageometrically thin disk that is truncated at 2.6 AU and seen under aninclination angle of 58degr (i.e. closer to an edge-on view than to aface-on view). Neither models of a geometrically thin disk seen nearlyedge-on, nor models of spherical dust shells can achieve agreementbetween the observed and predicted visibilities. The reason why the diskis so compact remains unclear; we speculate that it has been truncatedby a close binary companion.
| Pulsating pre-main sequence stars in IC 4996 and NGC 6530
Context: .Asteroseismology of pulsating pre-main sequence (PMS) starshas the potential of testing the validity of current models of PMSstructure and evolution. As a first step, a sufficiently large sample ofpulsating PMS stars has to be established, which allows to selectcandidates optimally suited for a detailed asteroseismological analysisbased on photometry from space or ground based network data. Aims:.A search for pulsating PMS members in the young open clusters IC 4996and NGC 6530 has been performed to improve the sample of known PMSpulsators. As both clusters are younger than 10 million years, allmembers with spectral types later than A0 have not reached the zero-agemain sequence yet. Hence, IC 4996 and NGC 6530 are most suitable tosearch for PMS pulsation among their A- and F-type cluster stars.Methods: .CCD time series photometry in Johnson B and V filters has beenobtained for IC 4996 and NGC 6530. The resulting light curves for 113stars in IC 4996 and 194 stars in NGC 6530 have been subject to detailedfrequency analyses. Results: .2 δ Scuti-like PMS stars havebeen discovered in IC 4996 and 6 in NGC 6530. For another PMS star ineach cluster, pulsation can only be suspected. According to the computedpulsation constants, the newly detected PMS stars seem to prefer topulsate in a similar fashion to the classical δ Scuti stars, andwith higher overtone modes.
| 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.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| X-ray emission from T Tauri stars in the Lupus 3 star-forming region
Aims.In this paper, I present analysis results of an {XMM-Newton}observation of the Lupus 3 region that contains a high proportion ofyoung low mass (M < 0.3 Mȯ) T Tauri stars in theLupus star-forming complex. Methods: .The detection of X-raysources in 0.5 to 4.5 keV images of the Lupus 3 core was performed usingthe standard source detection method of the {XMM-Newton} ScienceAnalysis Software. The detected sources were correlated with a list ofHerbig-Haro objects and Hα emission stars that contains mainlyclassical T Tauri stars, with a catalogue of weak-line T Tauri Stars andwith a recent list of new low-mass members of the Lupus 3 dark cloudfound in a visible-light spectroscopic survey at the center of the Lupus3 star-forming core. The light curves and spectra of the brightest X-raysources with known T Tauri star counterparts were analysed.Results: .One hundred and two X-ray sources were detected in the30´ diameter field-of-view of the EPIC cameras, of which 25 havevisible or near-IR counterparts that are known as pre-main sequencestars. Their X-ray luminosity ranges from 3 × 1028 to 3× 1030 erg s-1. Two of these objects withmass estimates lower than 0.075 Mȯ have an X-rayluminosity of about 4-7 × 1028 erg s-1,comparable with that of flaring young brown dwarfs. A linear correlationis found between the X-ray luminosity and the mass or volume of thestars that is qualitatively expected from some models of distributedturbulent dynamos. The EPIC spectra of the X-ray brightest sources canbe fitted using optically thin plasma emission models with twocomponents at temperatures in the ranges 3-9 × 106 Kand 1-50 × 107 K, respectively. The large emissionmeasure of hot plasma may be caused by disruptions of magnetic fieldsassociated with an intense flaring activity, while the X-ray emissionfrom the "cool" plasma components may result from solar-type activeregions. The emission measures of the plasma components are of the orderof 1052 cm-3, typical of the values expected fromcoronal plasmas in T Tauri stars, post-T Tauri stars, and activelate-type dwarfs in close binary systems. One property of the X-raybrightest stars in Lupus 3 that seems common among pre-main sequencestars is the low abundance of Fe.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| A multisite photometric campaign on the pre-main-sequence δ Scuti pulsator IP Persei
We present the results of a photometric multisite campaign on theδ Scuti Pre-Main-Sequence star IP Per. Nine telescopes have beeninvolved in the observations, with a total of about 190 h ofobservations over 38 nights. Present data confirms the multiperiodicnature of this star and leads to the identification of at least ninepulsational frequencies. Comparison with the predictions of linearnon-adiabatic radial pulsation models allowed us to identify only fiveof the nine observed frequencies, and to constrain the position of IPPer in the HR diagram. The latter is in good agreement with theempirical determination of the stellar parameters obtained byMiroshnichenko et al. (2001, A&A, 377, 854). An initialinterpretation of the observed frequencies using the Aarhus non-radialpulsation code suggests that three frequencies could be associated withnon-radial (l=2) modes. Finally, we present new evolutionary andpulsation models at lower metallicity (Z=0.008) to take into account thepossibility that IP Per is metal deficient, as indicated byMiroshnichenko et al. (2001, A&A, 377, 854).
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| 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.
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| Asteroseismology of δ Scuti Stars: Problems and Prospects
We briefly outline the state-of-the-art seismology of δ Scutistars from a theoretical point of view: why is it so difficult a task?The recent theoretical advances in the field that these difficultieshave influenced are also discussed.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| From Dusty Filaments to Cores to Stars: An Infrared Extinction Study of Lupus 3
We present deep near-infrared observations of a dense region of theLupus 3 cloud obtained with the ESO NTT and VLT. Using the NICE method,we construct a detailed high angular resolution dust extinction map ofthe cloud. The dust extinction map reveals embedded globules, a densefilament, and a dense ring structure. We derive dust column densitiesand masses for the entire cloud and for the individual structurestherein. We construct radial extinction profiles for the embeddedglobules and find a range of profile shapes from relatively shallowprofiles for cores with low peak extinctions, to relatively steepprofiles for cores with high extinction. Overall, the profiles aresimilar to those of pressure-truncated isothermal spheres of varyingcenter-to-edge density contrast. We apply Bonnor-Ebert analysis tocompare the density profiles of the embedded cores in a quantitativemanner and derive physical parameters such as temperatures, centraldensities, and external pressures. We examine the stability of the coresand find that two cores are likely stable and two are likely unstable.One of these latter cores is known to harbor an active protostar.Finally, we discuss the relation between an emerging cluster in theLupus 3 cloud and the ring structure identified in our extinction map.Assuming that the ring is the remnant of the core within which thecluster originally formed, we estimate that a star formation efficiencyof ~30% characterized the formation of the small cluster. Ourobservations of the Lupus 3 cloud suggest an intimate link between thestructure of a dense core and its state of star-forming activity. Thedense cores in this cloud are found to span the entire range ofevolution from a stable, starless core of modest central concentration,to an unstable, star-forming core that is highly centrally concentrated,to a significantly disrupted core from which a cluster of young stars isemerging.Based on observations carried out at ESO, La Silla and Paranal, Chile.
| X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264
Images of the NGC 2264 star-forming region, which we have acquired withthe XMM-Newton spacecraft, reveal strong X-ray emission from threedeeply embedded (AV>=10 mag) young stellar objects in thevicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz& Grasdalen's infrared source (RNO-EW = IRS 2). Thermal plasmamodels for the brightest source in X-rays, located 11" southwest ofAllen's star, yield a quasi-steady luminosity ofLX=1033 ergs s-1 and an extraordinarilyhigh X-ray temperature of 100 MK. The high temperature is consistentwith the presence of emission lines of Fe XXV and Fe XXVI at photonenergies of 6.7 and 6.9 keV, respectively. An even higher temperature ofnearly ~140 MK was observed during the rise phase of a powerfulimpulsive X-ray flare of another young star in the IRS 2 region.Moderate-resolution near-infrared (1-4 μm) spectra of the embeddedobjects, obtained at the NASA Infrared Telescope Facility, exhibit deepwater ice absorption bands, as well as a variety of emission andabsorption features of H I, CO, and both neutral and ionized metals.
| 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.
| Very low-mass members of the Lupus 3 cloud
We report on a multi-band survey for very low-mass stars and browndwarfs in the Lupus 3 cloud with the Wide Field Imager (WFI) at theESO/MPG 2.2 m telescope on La Silla Observatory (Chile). Our multibandoptical photometry is combined with available 2MASS JHK photometry toidentify 19 new young stars and 3 brown dwarf candidates as probablemembers of this star forming region. Our objects are mostly clusteredaround the cloud core. Stars and brown dwarfs have similar levels ofHα emission, probably a signature of accretion. One object, abrown dwarf candidate, exhibits a near-infrared excess, which mayindicate the presence of a disk, but its Hα emission cannot beconfirmed due to its faintness in the optical passbands. We also findtwo visual pairs of probable Lupus 3 members that may be wide binaries.
| 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.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| [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.
| Pre-Main-Sequence A-type stars
Young A-type stars in the pre-main-sequence (PMS) evolutionary phase areparticularly interesting objects since they cover the mass range(1.5-4 Mȯ) which is most sensitive to the internalconditions inherited from the protostellar phase. In particular, theyundergo a process of thermal relaxation from which they emerge as fullyradiative objects contracting towards the Main Sequence. A-type starsalso show intense surface activity (including winds, accretion,pulsations) whose origin is still not completely understood, andinfrared excesses related to the presence of circumstellar disks andenvelopes. Disks display significant evolution in the dust properties,likely signalling the occurrence of protoplanetary growth. Finally,A-type stars are generally found in multiple systems and smallaggregates with lower mass companions.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| New Perspectives on the X-Ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra
The origin of the X-ray emission from Herbig Ae/Be stars is not yetknown. These intermediate-mass pre-main-sequence stars lie on radiativetracks and are not expected to emit X-rays via solar-like magneticprocesses, nor are their winds powerful enough to produce X-rays byradiative wind shocks as in more massive O-type stars. The emissioncould originate in unseen low-mass companions, or it may be intrinsic tothe Herbig stars themselves if they still have primordial magneticfields or can sustain magnetic activity via a nonsolar dynamo. Wepresent new X-ray observations of the nearby Herbig Ae star HD 104237(=DX Cha) with XMM-Newton, whose objective is to clarify the origin ofthe emission. Several X-ray emission lines are clearly visible in theCCD spectra, including the high-temperature Fe Kα complex. Theemission can be accurately modeled as a multitemperature thermal plasmawith cool (kT<1 keV) and hot (kT>~3 keV) components. The presenceof a hot component is compelling evidence that the X-rays originate inmagnetically confined plasma, either in the Herbig star itself or in thecorona of an as yet unseen late-type companion. The X-ray temperaturesand luminosity (logLX=30.5 ergs s-1) are withinthe range expected for a T Tauri companion, but high-resolution Chandraand Hubble Space Telescope images constrain the separation of a putativecompanion to less than 1". We place these new results into broadercontext by comparing the X-ray and bolometric luminosities of a sampleof nearby Herbig stars with those of T Tauri stars and classicalmain-sequence Be stars. We also test the predictions of a model thatattributes the X-ray emission of Herbig stars to magnetic activity thatis sustained by a shear-powered dynamo.
| 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.
| Magnetic fields in Herbig Ae stars
Herbig Ae stars are young A-type stars in the pre-main sequenceevolutionary phase with masses of 1.5-3 Mȯ. Theyshow rather intense surface activity (Dunkin et al. \cite{Du97}, MNRAS,290, 165) and infrared excess related to the presence of circumstellardisks. Because of their youth, primordial magnetic fields inherited fromthe parent molecular cloud may be expected, but no direct evidence forthe presence of magnetic fields on their surface, except in one case(Donati et al. \cite{Do97}, MNRAS, 291, 658), has been found until now.Here we report observations of optical circular polarization with FORS 1at the VLT in the three Herbig Ae stars HD 139614, HD 144432 and HD144668. A definite longitudinal magnetic field at 4.8 σ level, =-450±93 G, has been detected in the HerbigAe star HD 139614. This is the largest magnetic field ever diagnosed fora Herbig Ae star. A hint of a weak magnetic field is found in the othertwo Herbig Ae stars, HD 144432 and HD 144668, for which magnetic fieldsare measured at the 1.6 σ and 2.5 σ levelrespectively. Further, we report the presence of circular polarizationsignatures in the Ca II K line in the V Stokes spectra of HD 139614 andHD 144432, which appear unresolved at the low spectral resolutionachievable with FORS 1. We suggest that models involving accretion ofmatter from the disk to the star along a global stellar magnetic fieldof a specific geometry can account for the observed Zeeman signatures.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programme No. 072.D-0377).
| Fundamental parameters and evolutionary state of the Herbig Ae star candidate HD 35929
HD 35929 is a pre-main-sequence Herbig Ae star candidate that exhibitsemission in the Hα line and a weak IR excess. Although itsluminosity type was estimated as IV from low-resolution spectroscopy,the insignificant HIPPARCOS parallax (0.88±0.93 mas) contradictedthe low luminosity. To resolve this discrepancy, we took 3high-resolution optical spectra of the star (region 5200-6915 Å, R60 000) and used an archival one (4000-6800 Å, R 48000). Our analysis of the spectral and photometric information showsthat HD 35929 is an F2 III star with Teff=6880±100 K,log g = 3.3±0.1, log L/Lȯ=1.7±0.2, v sini =70±5 km s-1, M=2.3±0.2 Mȯ, amild metal deficit [Fe/H]=-0.2±0.1, and a weak interstellarreddening (AV 0.1 mag) at a distance D = 345±60pc. These results confirm an earlier suggestion by Marconi et al.(\cite{marc00}) that HD 35929 is located within the instability strip.At the same time, we argue that it is not a young object, but rather apost-main-sequence giant. The fundamental parameters and IR excess of HD35929 are similar to those of HD 19993, an emission-line late A-typegiant recently discovered by us (Miroshnichenko et al. \cite{mir03}).These objects might represent a group of evolved intermediate-mass starswith an enhanced mass loss.
| Spectroscopic monitoring of the Herbig Ae star HD 104237. I. Multiperiodic stellar oscillations
We present the results of multisite observations spanning two years ofthe pre-main-sequence Herbig Ae star HD 104237. The star was observed inhigh resolution spectroscopy (R ≥ 35 000) for a total of 157.3 h ofeffective exposure time, over 42 nights, corresponding to a data base of1888 individual echelle spectra. We confirm that the HD 104237 systemincludes a spectroscopic binary, whose primary component HD 104237 A ispulsating. The resulting high quality radial velocity curve allows us todetect for the first time by spectroscopic means multiperiodicoscillations in a pre-main sequence star. Five different modes aredetected with very high confidence, with frequencies ranging between28.5 and 35.6 d-1, typical of δ Scuti pulsations; anadditional 3 frequencies have been extracted from the data, but with alower level of confidence. The pattern of frequencies indicates that atleast some of the detected modes are non-radial. The precise orbitdetermination and the measurement of the double line spectroscopicbinary observed around periastron enabled us to determine a mass ratioof 1.29 ± 0.02 between the primary and the secondary; based onthe primary mass of 2.2 ± 0.1 Mȯ we conclude thatthe spectroscopic secondary HD 104237 b should have a mass of 1.7± 0.1 Mȯ and lie outside the pre-main sequenceinstability strip towards later spectral types. A search for pulsationsin the radial velocity curve of the much weaker secondary component wasnot conclusive at this stage. The location of the primary in the HRdiagram and its position with respect to recent pre-main sequenceevolutionary tracks and isochrones implies a location of the secondaryindicative of spectral type K3.Based on observations collected at the 1.9 m SAAO Radcliffe telescopeand the 74 inch MSSSO telescope.Table \ref{log} is only available in electronic form athttp://www.edpsciences.org
| 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
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Miembro de los siguientes grupos:
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Datos observacionales y astrométricos
| Constelación: | Escorpio |
| Ascensión Recta: | 16h08m34.20s |
| Declinación: | -39°06'19.0" |
| Magnitud Aparente: | 7.05 |
| Distancia: | 207.9 parsecs |
| Movimiento Propio en Ascensión Recta: | -12.5 |
| Movimiento Propio en Declinación: | -21.6 |
| B-T magnitude: | 7.457 |
| V-T magnitude: | 7.128 |
Catálogos y designaciones:
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