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A Near-infrared Spectroscopic Survey of Class I Protostars
We present the results of a near-IR spectroscopic survey of 110 Class Iprotostars observed from 0.80 ?m to 2.43 ?m at a spectroscopicresolution of R = 1200. This survey is unique in its selection oftargets from the whole sky, its sample size, wavelength coverage, depth,and sample selection. We find that Class I objects exhibit a wide rangeof lines and the continuum spectroscopic features. Eighty-five percentof Class I protostars exhibit features indicative of mass accretion, andwe found that the veiling excess, CO emission, and Br ? emissionare closely related. We modeled the spectra to estimate the veilingexcess (rk ) and extinction to each target. We also usednear-IR colors and emission line ratios, when available, to alsoestimate extinction. In the course of this survey, we observed thespectra of 10 FU Orionis-like objects, including 2 new ones, as well as3 Herbig Ae-type stars among our Class I young stellar objects. We usedphotospheric absorption lines, when available, to estimate the spectraltype of each target. Although most targets are late-type stars, thereare several A- and F-type stars in our sample. Notably, we found no A orF class stars in the Taurus-Auriga or Perseus star-forming regions.There are several cases where the observed CO and/or water absorptionbands are deeper than expected from the photospheric spectral type. Wefind a correlation between the appearance of the reflection nebula,which traces the distribution of material on very large scales, and thenear-IR spectrum, which probes smaller scales. All of the FUOrionis-like objects are associated with reflection nebulae. The spectraof the components of spatially resolved protostellar binaries tend to bevery similar. In particular both components tend to have similar veilingand H2 emission, inconsistent with random selection from thesample as a whole. There is a strong correlation between [Fe II] andH2 emission, supporting previous results showing thatH2 emission in the spectra of young stars is usually shockexcited by stellar winds.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars. II. A Survey
We have used archival CCD data, Fourier analysis, and self-correlationanalysis to study the photometric time variability of 162 T Tauri starsand related objects, including Herbig Ae/Be stars. Many show periodicvariability, presumably due to rotation of a spotted star in most cases.For the nonperiodic stars, we have estimated an upper limit to theperiodic variability. We have also analyzed 26 stars suspected to haveperiods longer than about 10 days. Of these 26 stars, at least 14 haveperiods significantly longer than 10 days; their variability may be dueto processes in the disk, or the effect of a companion, since most (butnot all) T Tauri stars have rotational periods less than this value. Fora few of the rotational variables, namely AA Tau, DK Tau, DL Tau, DNTau, GK Tau, GM Aur, Rox 29, V1121 Oph, V410 Tau, and V649 Ori, we havefound long-term variability of the amplitude, presumably due to activitycycles; the time scales are 1500-4000 days.
| Candidate Coronagraphic Detections of Protoplanetary Disks Around Four Young Stars
We present potential detections of H-band scattered light emissionaround four young stars, selected from a total sample of 45 young starsobserved with the Coronagraphic Imager with Adaptive Optics coronagraphof the Subaru telescope. Two Classical T Tauri stars, CI Tau and DI Cep,and two weak-lined T Tauri stars, LkCa 14 and RXJ 0338.3+1020, weredetected. In all the four cases, the extended emission is within thearea of the residual point-spread function halo, and is revealed onlythrough careful data reduction. We compare the observed extendedemission with simulations of the scattered light emission to evaluatethe plausibility and nature of the detected emission.
| A search for Keplerian periods in the brightness variations of T Tauri and Ae Herbig stars
Long-term, uniform series of U BV R observations of T Tauri and AeHerbig stars obtained over 20 yr at the Maidanak Observatory as part ofthe ROTOR program are analyzed. We find a linear relationship betweenthe characteristic variability time scale and the bolometric luminosityof the star+disk system: the higher the luminosity, the slower thebrightness variations. This dependence is valid over a wide range ofmasses and luminosities, from T Tauri stars to Ae Herbig stars. Onaverage, the variability time scale is one-quarter the Keplerian periodat the dust-sublimation radius, which is known from interferometricobservations. Some T Tauri stars have periods from 25 to 120 days, whichare preserved over several observing seasons. These periods correspondto Keplerian orbits with semi-major axes from 0.14 to 0.52 AU. Theresults obtained provide indirect evidence for the existence ofprotoplanets in the gas-dust disks of stars in early stages of theirevolution toward the main sequence.
| A Search for long-period brightness variations of the T Tauri stars T Tau, DI Cep, and SU Aur
We have searched for protostellar or protoplanetary structures in thevicinity of young T Tauri stars using a technique that is able todistinguish long-period components of the observed light curves. Weperform a statistical spectral analysis of the one-year mean lightcurves of three selected stars (T Tau, DI Cep, and SU Aur) and plotsynthetic light curves for the most reliable period of each star. In afirst approximation, the results show a good coincidence between oursynthetic light curves and the original light curves, supporting thehypothesis that protostellar or protoplanetary structures are present inthe studied systems. An analysis of the 0.36-20 µm spectral energydistributions of the program stars also leads to the conclusion thatinfrared anomalies for young stars are most likely due to thermalradiation from not-yet-formed companions in the vicinity of the star.
| Near-Infrared Interferometric, Spectroscopic, and Photometric Monitoring of T Tauri Inner Disks
We present high angular resolution observations with the KeckInterferometer, high-dispersion spectroscopic observations withKeck/NIRSPEC, and near-IR photometric observations from PAIRITEL of asample of 11 solar-type T Tauri stars in nine systems. We use theseobservations to probe the circumstellar material within 1 AU of theseyoung stars, measuring the circumstellar-to-stellar flux ratios andangular size scales of the 2.2 μm emission. Our sample spans a rangeof stellar luminosities and mass accretion rates, allowing investigationof potential correlations between inner disk properties and stellar oraccretion properties. We suggest that the mechanism by which the dustyinner disk is truncated may depend on the accretion rate of the source;in objects with low accretion rates, the stellar magnetospheres maytruncate the disks, while sublimation may truncate dusty disks aroundsources with higher accretion rates. We have also included in our sampleobjects that are known to be highly variable (based on previousphotometric and spectroscopic observations), and for several sources, weobtained multiple epochs of spectroscopic and interferometric data,supplemented by near-IR photometric monitoring, to search for inner diskvariability. While time-variable veilings and accretion rates areobserved in some sources, no strong evidence for inner disk pulsation isfound.
| Synchronous UBVR photometry and spectroscopy of DI Cephei
We present our synchronous spectroscopy and photometry of DI Cep, aclassical T Tauri star. The equivalent widths and radial velocities ofthe individual components and H?, H?, D1 and D2 Na I, and HeI?5876 Å emission line profiles exhibit variability. We havefound a clear positive correlation between the brightness and equivalentwidth for the H? and H? emission lines. The photometric andspectroscopic data are satisfactorily described in phases of a 9-dayperiod. The expected magnetic field of the star has been estimated usingexisting magnetospheric models to be 655 1000 G. The star is suspectedto be a binary.
| Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
Context: .T Tauri stars exhibit variability on all timescales, whoseorigin is still debated. Aims: .We investigate the long termvariability of CTTs over up to 20 years, characterize it from a set ofstatistical parameters and discuss its origin. Methods: .Wepresent a unique, homogeneous database of photometric measurements forClassical T Tauri stars extending up to 20 years. The database containsmore than 21 000 UBVR observations of 72 CTTs. All the data werecollected within the framework of the ROTOR-program at Mount MaidanakObservatory (Uzbekistan) and together they constitute the longesthomogeneous, accurate record of TTS variability ever assembled. Wecharacterize the long term photometric variations of 49 CTTs withsufficient data to allow a robust statistical analysis and propose anempirical classification scheme. Results: .Several patterns oflong term photometric variability are identified. The most commonpattern, exhibited by a group of 15 stars which includes T Tau itself,consists of low level variability (Δ V≤0.4 mag) with nosignificant changes occurring from season to season over many years. Arelated subgroup of 22 stars exhibits a similar stable long termvariability pattern, though with larger amplitudes (up to ΔV≃1.6 mag). Besides these representative groups, we identify threesmaller groups of 3-5 stars each which have distinctive photometricproperties. Conclusions: .The long term variability of most CTTsis fairly stable and merely reflects shorter term variability due tocold and hot surface spots. Only a small fraction of CTTs undergosignificant brightness changes on the long term (months, years), whichprobably arise from slowly varying circumstellar extinction.
| Silicate dust in astrophysical environments.
Not Available
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| Calcium II infrared triplet line models in classical T Tauri stars
We study the formation of the calcium II infrared triplet lines 8498Å, 8542 Å and 8662 Å, in the accreting magnetosphericflows of Classical T Tauri stars (CTTS), and present a grid of modelsfor a large range of magnetospheric conditions. We apply our models tothe interpretation of multi epoch observations of the CTTS DI Cep. Wefind that these lines form in the magnetospheric infall and that thevariability of the CaII triplet lines in DI Cep can be explained in thecontext of changes in the mass accretion rate/temperature of theaccretion column gas.
| Active Motion of Matter in the Envelope of DI Cephei
Not Available
| The veiling spectrum of DI Cephei and its relationship to emission line profiles
High spectral resolution variability studies of classical T Tauri stars(CTTS) are an essential instrument for probing the physical conditionsand dynamics of their atmospheres and immediate vicinity. The shapes ofthe excess continuum emission and of the line profiles, theirvariability and relationship are all crucial tools to achieve that goal.We use high spectral resolution optical data of the CTTS DI Cep tosuggest a new diagnostic tool to investigate the relationship betweenthe line emission/absorption and the excess continuum emission. Bycorrelating the veiling continuum to the line flux in discrete velocitybins across the emission line we obtain a correlation profile, fromwhich one can discriminate between parts of the line that relatedifferently to the veiling. An earlier report of an unexpected humparound 5300 Å in the continuum excess emission spectrum of acouple of CTTS is not explained by current models of those stars. Weidentified a similar feature in the veiling spectrum of DI Cep anddiscuss, in this context, the relevance of the broad photosphericabsorption features present in the spectra of late-type stars. RegardingDI Cep, we find that its radial velocity seems to be variable but nosignificant periodicity could be derived, possibly due to inadequatetime sampling. We argue that this CTTS is most probably observed nearlyequator on. Accretion flows could not be identified directly in theemission lines, but their presence is inferred from the analysis of theveiling spectrum, which yields typical projected accretion rates around2.5×10-7 M_ȯ yr-1.
| Tycho-2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infraredemission due to the presence of circumstellar material. To identify suchstars, we have positionally correlated the infrared Mid-Course SpaceExperiment (MSX) Point Source Catalogue and the Tycho-2 opticalcatalogue. Near-mid-infrared colour criteria have been developed toselect infrared excess stars. The search yielded 1938 excess stars; overhalf (979) have never previously been detected by IRAS. The excess starswere found to be young objects such as Herbig Ae/Be and Be stars, andevolved objects such as OH/IR (infrared) and carbon stars. A number ofB-type excess stars were also discovered whose infrared colours couldnot be readily explained by known catalogued objects.
| 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 New Classification Scheme for T Tauri Light Curves
Based on many years of observational data from a photometric database onyoung stars, we propose a new classification scheme for the light curvesof classical T Tauri stars. Our analysis of master light-curve shapesfor 28 classical T Tauri stars is used to distinguish up to fivelight-curve types. The proposed scheme suggests a qualitativeinterpretation in terms of interaction of the central star with itscircumstellar accretion disk.
| Periodic Variability of the T Tauri Star DI Cephei
Original spectroscopic and photometric observations are used togetherwith data from the literature to compile a master light curve for DI Cepand to analyze individual data series for periodicity. The spectroscopicdata reveal the period P=9.24d. Data from different authors show thenine-day quasi-periodicity in the brightness variations fairly reliably,although the period cannot be derived from the entire master set ofphotometric data. The period P=18.28d is also derived from the Vbrightnesses. We find variations of the zero epoch of the nine-dayperiod, possibly due to the changing location of an accretion-disk hotspot.
| On the Possible 9-day Periodic Variability of DI Cep
We reveal a periodic component in the spectroscopic and photometricvariability of DI Cep, with a 9.24-day period.
| T Tauri Stars
The observable characteristics of young solar-mass stars (T Tauri stars)are reviewed. Modern theoretical concepts and models of these stars arediscussed briefly.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Galactic environment and the 10-μm silicate feature of young stellar objects
Disc and sphere dust models are used to fit 8-13 μm flux spectra of19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The13 non-photospheric low-mass YSOs in quiescent environments and the fiveHerbig AeBe stars have mean disc temperature indices of 0.4, indicatingthat the emission arises from optically thin layers above a flaredoptically thick disc; 10 out of 14 of the low-mass YSO and four out offive of the Herbig AeBe features contain an optically thin silicateemission component. The radius of the peak 10-μm emission for nineout of the 13 low-mass YSOs is 10-130 au, and three out of the fiveHerbig AeBe stars are 10-30 au in size. In contrast, the five YSOs fromdisrupted molecular clouds that have been shaped by expanding supernovaremnants have temperature indices of between 0.3 and 0.8; four out ofthe five are optically thick and three out of the five have radii <~2au. The photosphere-like continuum of Taurus-Elias 18 could be fittedonly with truncated optically thick models, implying the presence of avoid between the >500 K and cold (<~100 K) foreground dust.Silicates surrounding low-mass YSOs in quiescent molecular clouds aresimilar to those in the Trapezium region of the Orion Nebula except whenAV<~ 2 mag. In the low-AV case and in low-massYSOs in disrupted molecular clouds the silicates are similar tocircumstellar dust around the evolved star μ Cephei.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are 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/399/1083
| Near infrared hydrogen lines as diagnostic of accretion and winds in T Tauri stars
From a sample of 50 T Tauri stars, mostly from the Taurus-Aurigacomplex, Pa beta line profiles were obtained for 49 of the stars and Brgamma profiles for 37 of the stars. Emission at Pa beta was observed for42 stars and emission at Br gamma was found for 30 stars. The mostconspicuous features in the line profiles is the almost complete absenceof blueshifted absorption components and the relatively high frequencyof inverse P Cygni profiles (IPC). At Pa beta, 34% of the profiles areIPC while at Br gamma 20% are IPC. The redshifted absorption featuresindicate infall at velocities of about 200 km s-1, compatiblewith free fall from a few radii out. In general, line profiles are broadcentrally peaked with slightly blueshifted line peaks. Existing wind andaccretion models fail, in quantitative terms, to explain the shape ofthe observed profiles. Magnetospheric accretion models, being thecurrently preferred ones, produce lines too narrow (by ~ 100 kms-1 FWHM), wings extending to velocities too low (by at least~ 100 km s-1) and with maximum normalized intensities toohigh by factors of a few. A qualitative agreement between some of theaccretion model predicted profiles and some observations hint thatemission in these lines might, at least partially, arise from infallingmaterial. Definite claims regarding the origin of the emission in theselines cannot be made until models match observations much better thanthey currently do. Tables 4 and 5 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strastg.fr (130.79.128.5) orvia http://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/90 Tables 6 and 7are also available in electronic form at the CDS
| An IUE Atlas of Pre-Main-Sequence Stars. II. Far-Ultraviolet Accretion Diagnostics in T Tauri Stars
We use our ultraviolet (UV) atlas of pre-main-sequence stars constructedfrom all useful, short-wavelength, low-resolution spectra in theInternational Ultraviolet Explorer (IUE) satellite Final Archive toanalyze the short-wavelength UV properties of 49 T Tauri stars (TTSs).We compare the line and continuum fluxes in these TTSs with each otherand with previously published parameters of these systems, includingrotation rate, infrared excess, and mass accretion rate. Theshort-wavelength continuum in the classical TTSs (CTTSs) appears tooriginate in a ~10,000 K optically thick plasma, while in the naked TTSs(NTTSs-stars without dusty disks) the continuum appears to originate inthe stellar atmosphere. We show that all of the TTSs in our sample liein the regime of ``saturated'' magnetic activity due to their smallRossby numbers. However, while some of the TTSs show emission linesurface fluxes consistent with this saturation level, many CTTSs showsignificantly stronger emission than predicted by saturation. In thesestars, the emission line luminosity in the high ionization lines presentin the spectrum between 1200 and 2000 Å correlates well with themass accretion rate. Therefore, we conclude that the bulk of theshort-wavelength emission seen in CTTSs results from accretion relatedprocesses and not from dynamo-driven magnetic activity. Using CTTSs withknown mass accretion rates, we calibrate the relationship between M andLC IV to derive the mass accretion rate for some CTTSs whichfor various reasons have never had their mass accretion rates measured.Finally, several of the CTTSs show strong emission from molecularhydrogen. While emission from H2 cannot form in gas at atemperature of ~105 K, the strength of the molecular hydrogenemission is nevertheless well correlated with all the other emissionsdisplayed in the IUE short-wavelength bandpass. This suggests that theH2 emission is in fact fluorescent emission pumped by theemission (likely Lyα) from hotter gas.
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| The far-UV spectrum of T Tauri stars - I. The relevance of the IUE Newly Extracted Spectra
The far-UV spectrum of the T Tauri stars (TTSs) provides important cluesabout the structure of the stellar atmospheres, winds and accretionshocks. The IUE (International Ultraviolet Explorer) Final Archivecontains the most complete data base for such studies. A new extractionsystem, the IUE Newly Extracted Spectra (ines), has been developed toovercome the disadvantages of the extraction system used in the IUEFinal Archive, the Signal Weighted Extraction Technique (swet). We havecompared the ines spectra of the whole sample of TTSs in the far-UVrange (1200-2000Å) with the swet low-resolution spectra availablein the IUE Final Archive. Although in most of the cases there is a goodagreement between both samples, an important enhancement of the inesline fluxes with respect to the swet line fluxes is reported forparticular spectra. The line fluxes are enhanced by as much as a factorof ~2.5 in some objects, which is significant for variability studies ofTTSs because the variations of the UV lines are typically of this order.The emission-measure distributions built to study the atmospheres ofthese stars are based on the UV emission line fluxes, so the new systemis susceptible to introduce changes in these models. Moreover, thenon-linear enhancement of the ines line fluxes produces variations indiagnostic line ratios usually taken as temperature and density tracersin late-type stars. These line ratios can vary by as much as a factor of3 when the ines data are compared with the swet, with the subsequentvariation of the physical parameters derived from them.
| Flickering in FU Orionis
We analyze new and published optical photometric data of FU Orionis, aneruptive pre-main-sequence star. The outburst consists of a 5.5 mag riseat B with an e-folding timescale of ~50 days. The rates of decline at Band V are identical, 0.015+/-0.001 mag yr-1. Randomfluctuations superimposed on this decline have an amplitude of0.035+/-0.005 mag at V and occur on timescales of 1 day or less.Correlations between V and the color indices U-B, B-V, and V-R indicatethat the variable source has the optical colors of a G0 supergiant. Weassociate this behavior with small-amplitude flickering of the inneraccretion disk.
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| High veiling at near infrared wavelengths in classical T Tauri stars
The near infrared veiling (the ratio of any non-photospheric excess fluxto the photospheric flux) is studied for a sample of 50 mainly ClassicalT Tauri stars (CTTS), mostly from the Taurus-Auriga complex, based onhigh resolution spectra ({R} ~ 20,500) of wavelength regions in the Jand K wavebands (near Pa β - 1.28215 mu m - and near Br γ -2.16611 mu m respectively). The method used to compute the veiling atthese wavelengths is independent of any assumptions aboutreddening/extinction towards the studied objects. Photosphericabsorption lines are identified in the spectra of 73% of the CTTSobserved at J and in 71% at K. For these stars the veiling in the J(rJ) and K (rK) wavebands was determined. Averagevalues for all the stars are < rJ> = 0.57 and = 1.29. Considering only stars with veilingsdetermined to better than 3 sigma these means are < rJ>= 0.97 and < rK> = 1.76. For the remaining 27% of thestars observed at J and 29% at K, for which no photospheric lines wereidentified, lower limits were obtained for rJ andrK. Considering only those stars with lower limits the meanlower limits are < rJ> > 1.34 and > 2.4. Our results indicate high veiling of thesestars in the NIR. The NIR veilings deduced are considerably greater thanexpected from extrapolation of the veilings measured in the optical,assuming that they result from an accretion shock, and are also biggerthan the expected veiling from an accretion disk. Although the origin ofthis large NIR veiling is not clear one possibility deserving furtherstudy is that it partially arises within shocks at the base ofmagnetospheric accretion columns which cover a large fraction of thestellar surface. The possibility that the near infrared veiling ofphotospheric lines is not caused by a continuum, but is rather due tonon-photospheric line emission from the disk or from a shock at the baseof the accretion columns, should also be investigated by detailedmodelling including the Ti, Fe, Si & Mn lines as opacity sources.
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| קבוצת-כוכבים: | קפיאוס |
| התרוממות ימנית: | 22h56m11.52s |
| סירוב: | +58°40'01.7" |
| גודל גלוי: | 11.155 |
| תנועה נכונה: | -3.3 |
| תנועה נכונה: | -4.7 |
| B-T magnitude: | 12.229 |
| V-T magnitude: | 11.244 |
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