Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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.
| Investigations of Stephenson's Hα stars
Hitherto unstudied objects from Stephenson's list of Hα emissionline objects at high galactic latitude were observed spectroscopicallyto prove their nature. 9 out of 11 objects show Hα in emission.Spectroscopy combined with photometric information indicates most ofthem being classical Be stars, while one object is a Post-AGB star andone a T-Tauri star. The classification of two objects, which are showingHα in emission, is unclear.
| Optical spectroscopic and 2MASS measurements of Stephenson Halpha stars
We present the results of spectroscopic observations for 52 objects fromthe list of Halpha emission stars of Stephenson(\cite{Ste86}). Out of six known T Tauri stars observed, five showedHalpha in emission and in one (StHa 40), Halpha changed from being in absorption to emission over a period of two years,accompanied by photometric and spectral type variability. We confirm theT Tauri nature of one Stephenson object (StHa 48) on the basis of thepresence of Halpha and Hbeta in emission, Li Ilambda6708 in absorption, infrared excess and X-ray emission. Among the52 objects observed, there were other emission line objects: 1 Ke star,1 BQ[ ] star, 2 galaxies and 2 Be stars. We present a higher-resolutionspectrum of StHa 62 showing permitted and forbidden lines in emissiontypical of BQ[ ] stars. Twenty five out of 30 newly observed objectsfailed to show Halpha in emission. We also present 2MASSobservations for 112 StHa objects. We suggest three Stephenson objects(StHa 52, 125 and 129) to be YSOs on the basis of 2MASS, IRAS and ROSATobservations. These and all other known YSOs amongst StHa stars arefound in regions of star-forming clouds in Taurus, Orion and Ophiuchus.YSOs at high galactic latitudes in other parts of the sky are thereforerare.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/963
| Double-periodic blue variables in the Magellanic Clouds
We report the discovery, based on an inspection of the OGLE-II database,of a group of blue variables in the Magellanic Clouds showingsimultaneously two kinds of photometric variability: a short-term cyclicvariability with typical amplitude Delta I ~ 0.05 mag and periodP1 between 4 and 16 days and a sinusoidal, long-term cyclicoscillation with much larger amplitude Delta I ~ 0.2 mag with periodP2 in the range of 150-1000 days. We find that both periodsseems to be coupled through the relationship P2 = 35.2 +/-0.8 P1. In general, the short term variability is reminiscentof those shown by Algol-type binaries. We propose that the long-termoscillation could arise in the precession of a elliptical disc fed by aRoche-lobe filling companion in a low mass ratio Algol system.
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 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/380/609
| Photometric orbital modulation in V 1080 Tauri
We present a photometric study of the variable star V 1080 Tauri. Allphotometric data, spanning in total about 9 years, can be folded withthe period of 8.8451741 days, yielding a single-wave course with thefull amplitude of 0.14 mag. The Hipparcos data show that the seculartrend in the brightness variations is not larger than 0.04 mag and thatthe modulation persists for the whole covered interval. We modeled thelight curve by ellipsoidal variations because the spectroscopic analysisof Martín (1993) proved V 1080 Tau to be a close binary. We arguethat the true orbital period is 17.69348 days, giving a double-wavelight curve which can be attributed to the proximity effects in thesemi-detached Algol binary. The absence of eclipses and the relativelylarge amplitude suggest the inclination angle within 55̂- 69̂. The deviations from the ellipsoidal modulation,mainly near the phases 0.25 and 0.5, are interpreted in terms ofinteraction in the binary.
| 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.
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| Emission-Line Stars in the Vilnius Photometric System
Various types of emission-line stars, observed in the Vilniusphotometric system, are analyzed. They include Be-type stars, HerbigAe/Be stars, T Tauri-type stars and K- and M-type dwarfs with activechromospheres. It is shown that all stars of these types, except forlate-type dwarfs, in their active stages can be identified by theirinterstellar reddening-free parameters. For emission-line stars of B andA types interstellar reddening determination is also possible.
| Photoelectric Photometry of Herbig Ae/be and Related Stars in the Vilnius Photometric System
A catalog of photoelectric photometry of 62 Herbig Ae/Be and relatedstars in the Vilnius system is presented. It contains stars down to V =12 mag located mainly in the northern hemisphere and observed in theperiod of 1994--1996.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| The 72nd Name-List of Variable Stars
Not Available
| A new catalogue of members and candidate members of the Herbig Ae/Be (HAEBE) stellar group
A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and relatedobjects is certainly needed, for both well-seasoned researchers and, inparticular, for new investigators starting to study the many interestingastrophysical properties of these very young objects. We present a briefdiscussion of the current observational characteristics that distinguishthis class from their main sequence counterparts. The HAEBE and relatedstars are listed in five tables, containing 287 objects. Table 1contains all Ae and Be stars which historically are recognized as trueHAEBE stars or potential candidate members. Table 2 gives the stars ofspectral type Fe, and emission line stars with very uncertain or unknownspectral type. In Table 3 are given all known Extreme Emission LineObjects (EELOs), of which most have not been identified to belong to anyspecific group. Table 4a and b list other Bep or B[e] stars with strongIR-excess and unknown spectral type. Table 5 contains the non-emissionline possible young objects. Furthermore, Table 6 contains 35 starsrejected from former published lists of HAEBE stars. In these tables weare including coordinates, spectral types, visual magnitudes, ranges inphotometric variability and references of several key publicationsrelated to each object. Relevant remarks, such as the presence of anebula in the vicinity of an object, are also given.
| Pre-main sequence lithium burning. 1: Weak T Tauri stars
We report high-resolution spectroscopic observations for a sample of 38T Tauri stars (TTS), complemented by UBVRI photometry of 13 TTS, and CCDVRI photometry for 2 visual binaries. Based on these observations anddata taken from the literature, we derive lithium abundances in 53 TTS,concentrating on weak-line TTS (WTTS). The sample spans the range inspectral types from KO-M3, approximately corresponding to masses between1.2 and 0.2 solar mass.
| COYOTES - Part One - Multisite UBVRI Photometry of 24 Pre Main Sequence Stars of the Taurus-Auriga Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&AS..101..485B
| Water Masers Associated with Herbig Be-Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...272..249P
| Coyotes-I - the Photometric Variability and Rotational Evolution of T-Tauri Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...272..176B
| The peculiar nature of BD + 24 deg 676
The H-alpha emission star BD + 24 deg 676, formerly classified as a TTauri star and more recently as a Herbig Ae star, has been observed atmoderately high resolution in the red spectral region. The main resultsare: the metallic lines over the wavelength range 6400-6750 A haveequivalent widths comparable to early G stars; the radial velocity isnot consistent with that of members of Taurus-Auriga; the lithiumabundance is much lower than expected for a T Tauri star; H-alpha has achanging complex profile. The data do not appear to be compatible withthe hypothesis that BD + 24 deg 676 is a premain-sequence star. Theproperties can be qualitatively understood if it is a close binarycomposed by a cool postmain-sequence subgiant and a hot main-sequencestar, although more data are needed to clarify its nature.
| BD + 24 deg 676 - an intermediate-mass, pre-main-sequence star
An analysis of the optical spectrum and UBVRIJHKL photometry of BD + 24deg 676 indicates that it is a heavily reddened emission-line star ofspectral type A3. The star has substantial near-IR and UV excesses. Acomparison with previous photometry suggests that this star may bevariable. It is suggested that BD + 24 deg 676 is an intermediate-mass,premain-sequence star in the Taurus-Auriga region.
| On the Spectrum of BD +24 676
Not Available
| A New Bright T Tauri Star in Taurus
Abstract image available at:http://adsabs.harvard.edu/abs/1988PASP..100.1443W
| New H-alpha emission stars found above 10 deg galactic latitude
Approximately 1300 5.2 x 5.2 deg red-sensitive objective prism plateshave been searched for H-alpha emission stars in order to complete thesearch of the entire sky. The survey here reported represents 59 percentof the whole sky, defined by the region north of declination -25 deg andmostly further than 10 deg from the Galactic plane; the omitted portionshad already been surveyed, in part at Warner and Swasey, in partelsewhere. Two hundred and six stars have been newly found, mostly ofunknown spectral type. The stars' latitude distribution suggests thatthey are mainly moderately high luminosity disk population stars. Thereare new finds especially in Orion and the Taurus dark clouds (limitingmag of about 13).
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Boga |
| Sað Açýklýk: | 04h40m32.65s |
| Yükselim: | +24°26'31.3" |
| Görünürdeki Parlaklýk: | 10.614 |
| özdevim Sað Açýklýk: | 3.1 |
| özdevim Yükselim: | -13.6 |
| B-T magnitude: | 11.71 |
| V-T magnitude: | 10.705 |
Kataloglar ve belirtme:
|
INHABITED SKY
