Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Excitation and visibility of slow modes in rotating B-type stars
We use the traditional approximation to describe oscillations withfrequencies comparable to the angular rotation rate. Validity of thisapproximation in application to main-sequence B stars is discussed.Numerical results regarding mode stability and visibility are presentedfor a model of the Be star HD 163868. For this object, Walker et al.detected a record number of mode frequencies using data from the smallspace telescope MOST. Our interpretation of these data differs from thatof Walker et al. In particular, we interpret peaks in the lowestfrequency range as retrograde g modes. We find instability in a largenumber of modes that remain undetectable because of unfavourable aspectand/or effect of cancellation. There is no clear preference toexcitation of prograde modes.
| MOST Detects g-Modes in the Late-Type Be Star β Canis Minoris (B8 Ve)
The Microvariability and Oscillations of Stars (MOST) satellite hasdetected low-amplitude light variations (Δm~1 mmag) in the Be starβ CMi (B8 Ve). The observations lasted 41 days and the variationshave typical periods ~0.3 days. We demonstrate that the dominantfrequencies are consistent with prograde high-order g-modes of m=-1excited by the Fe bump of opacity in an intermediate-mass (~3.5Msolar) star with a nearly critical rotation period of 0.38days. This is the first detection of nonradial g-mode pulsations in a Bestar later than B6 leading to the possibility that pulsations areexcited in all classical Be stars.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institute ofAerospace Studies, and the University of British Columbia with theassistance of the University of Vienna.
| MOST Detects g- and p-Modes in the B Supergiant HD 163899 (B2 Ib/II)
The Microvariability and Oscillations of Stars (MOST) satellite observedthe B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star anddetected 48 frequencies <~2.8 cycles day-1 with amplitudesof a few millimagnitudes (mmag) and less. The frequency range embracesg- and p-mode pulsations. It was generally thought that no g-modes areexcited in less luminous B supergiants because strong radiative dampingis expected in the core. Our theoretical models, however, show that suchg-modes are excited in massive post-main-sequence stars, in accordancewith these observations. The nonradial pulsations excited in modelsbetween 20 Msolar at logTeff~4.41 and 15Msolar at logTeff~4.36 are roughly consistent withthe observed frequency range. Excitation by the Fe bump in opacity ispossible because g-modes can be partially reflected at a convective zoneassociated with the hydrogen-burning shell, which significantly reducesradiative damping in the core. The MOST light curve of HD 163899 showsthat such a reflection of g-modes actually occurs and reveals theexistence of a previously unrecognized type of variable, slowlypulsating B supergiants (SPBsg) distinct from α Cyg variables.Such g-modes have great potential for asteroseismology.Based on data from the MOST satellite, a Canadian Space Agency mission,operated jointly by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| Discovery of the New Slowly Pulsating B Star HD 163830 (B5 II/III) from MOST Space-based Photometry
We report the discovery of a new slowly pulsating B star, with thelargest number of detected frequencies to date by more than a factor of3, based on 37 days of MOST (Microvariability and Oscillations of STars)satellite guide star photometry. The star HD 163830 (V=9.3, B5 II/III)varies in 20 detected frequencies in the range 0.035-1.06day-1 (0.4-12.3 μHz) with amplitudes from 0.7 to 7.6 mmag(with a signal-to-noise ratio from 4 to 41). Eighteen of thesefrequencies are consistent with low-degree, high-order nonradial g-modesof seismic models of an evolved 4.5 Msolar star. We areunable to identify one unique model due to lack of mode identifications.The lowest two frequencies may be associated with the rotation of HD163830, but firm proof of this must await future spectroscopic data.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| MOST Detects g-Modes in the Be Star HD 163868
We have extracted a 37 day light curve with a precision of 0.0012 magper point for the Microvariability and Oscillations of Stars (MOST)guide star, HD 163868 (B5 Ve). Its rich frequency spectrum resemblesthat of a slowly pulsating B (SPB) star but, being a rapid rotator, wedesignate it SPBe. The 60 most significant periods lie in three distinctgroups centered on 8 days and 14 and 7 hr. We demonstrate that the 14and 7 hr periods can be modeled by two swarms of high-order, progradesectorial g-modes (m=-1, -2), which are destabilized by the iron opacitybump. Our model also predicts a group of r-modes with periods near 2.3days, which correspond to frequencies observed in the tail of the 8 daygroup. The remaining periodicities, between 7 and 11 days, cannot beexplained by unstable modes in our model.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| New beta Cephei and SPB Stars Discovered in Hipparcos Photometry
We discuss 34 stars for which we detected new frequencies in HipparcosHp magnitudes. 13 of these stars are variables discovered in this paper.For 20 stars, we derive log T_eff and log g from Stromgren or Genevaphotometry.We classify one new beta Cep star, HIP 88352, two new beta Cep suspects,HIP 54753 and 88123, four new SPBs, HIP 1030, 39206, 46192 and 111147,and two SPB suspects, HIP 75787 and 98778. We find the last star to betriply-periodic and we show that the frequencies detected in Hpmagnitudes are present in the photoelectric observations of Hill etal(1976).Finally, we discover the hottest variable of the SPB type, namely, HIP1030.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| Short-Period Variable Be Stars Discovered or Confirmed through Self-Correlation Analysis of Hipparcos Epoch Photometry
We have surveyed 277 bright Be stars for short-period (0.2-2 day)photometric variability using self-correlation analysis of Hipparcosepoch photometry. This extends the work of Percy et al.; the methods areas described there. We report the discovery, timescale, and amplitude ofshort-period variability in: HD 7636 (V764 Cas), HD 11606 (V777 Cas), HD13661 (V549 Per), HD 34921, HD 36408, HD 40978, HD 58343 (FW CMa), HD63460 (o Pup), HD 88195 (17 Sex), HD 89353 (AG Ant, HR 4049), HD 129954(CO Cir), HD 158220 (V862 Ara), HD 173219 (V947 Sct), and HD 187567(V1339 Aql). We report the confirmation of short-period variabilityusing an independent form of analysis in HD 52918 (19 Mon), HD 105382(V863 Cen), HD 137387 (κ-1 Aps), HD 157832 (V750 Ara), and HD163868 (V3984 Sgr). These include a β Cephei star (19 Mon), aVega-type star (17 Sex), and a post-asymptotic giant branch B star (HR4049). We find that short-period variability is more prevalent inearly-B stars than in late-B stars. We have found no β Cephei typevariables in our sample of Be stars, other than 19 Mon, which isapparently not a Be star.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| The long-term behaviour of the Be star HD163868
In this paper we discuss the light variations of HD163868 on the basisof all available photometric data originating from photometricmonitoring during the last two decades. We suggest that one explanationof the on-and-off type variability can be a binary configuration with acompact companion and a period of the order of 850 days, though theobserved effects could also be seen in the context of positiveinterference of NRPs. The bright stages are associated with strongreddening consistent with an increase in circumstellar material. We drawthe attention to the similarity with cyclically-recurrent mild S Dorphases of LBVs. Based on observations obtained at the European SouthernObservatory at La Silla, Chile (observing proposals 55D-0317, 56D-0249and 57D-0133) and on data obtained with the Observatoire deGenève Swiss Telescope at La Silla
| WR 121 obscured by a dust cloud: the key to understanding occasional ``eclipses'' of ``dusty" Wolf-Rayet WC stars?
We observed the Wolf-Rayet star WR 121 (= AS 320, WC9) coming out of aminimum with a depth of 0fm8 to its normal brightness in a dozen days.The nature of this event was analyzed by means of Walraven five-colourphotometry. The colour changes are similar to those caused byinterstellar dust. Hot dust is known to be formed continuously aroundthis carbon-rich Wolf-Rayet star. Therefore, we suggest that the fadingof WR 121 was caused by the temporary condensation of an optically thickline-of-sight dust cloud, comparable to what occurs around R CoronaeBorealis stars. We suggest that occasional ``eclipses'' shown by other``dusty" Wolf Rayet stars (WR 113, WR 103) are also caused by suchtemporary condensing dust clouds. In addition, we present observationsof a new ``eclipse'' of WR 103. This brightness dip was more shallowthan the earlier ``eclipses'' and the star reddens during the descent.This also supports the model of a condensing dust cloud. >Frommodeling the shapes of the various ``eclipses'' we find that thecondensation takes place at radii ranging from 80 to 800Rsun, i.e. between the stellar surface and the permanent dustshell that is inferred from the infrared excess. >From the colourchanges we estimate the sizes of the particles in the clouds to be oforder 0.1 mu m and using the depth of the darkening we derive a dustmass condensation rate per column in the range of 3 -- 20 10(-10) kgm(-2) s(-1) . The dust mass flux per solid angle turns out to becomparable to that of the shell. Moreover, we find two possible trendswithin our small set of ``eclipses'': (1) the closer the condensationoccurs to the star, the larger the dust mass flux is, and (2) the closerthe condensation, the larger the particles are. These correlations arediscussed within the framework of the model. Based on observationscollected at the European Southern Observatory (ESO), La Silla, Chile
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| Far-ultraviolet stellar photometry: Fields in Sagittarius and Scorpius
Far-ultraviolet photometry for 741 objects in a field in Sagittariuscentered near M8 and 541 objects in a field centered near sigma Scorpiiis presented. These data were extracted from electographic imagesobtained with two cameras during a shuttle flight in 1991 April/May. Thecameras provided band passes with lambdaeff = 1375 A andlambdaeff = 1781 A. Synthetic colors show that these bandsare sensitive to effective temperature for hot stars. Our measurementswere placed on a quantitative far-ultraviolet magnitude scale byconvolving the spectra of stars observed by IUE with our cameras'spectral response functions. Fifty-eight percent of the ultravioletobjects were identified with visible stars using the SIMBAD databasewhile another 40% of the objects are blends of early type stars tooclose together to separate with our resolution. Our photometry iscompared with that from the TD-1, OAO 2, and ANS satellites and the S201(Apollo 16) far-ultraviolet camera and found to agree at the level of afew tenths of a magnitude. Unlike previous studies, almost half of theidentified visual counterparts to the ultraviolet objects are early Bstars. A plot of distance modulus against ultraviolet color excessreveals a significant population of stars with strong ultravioletexcess.
| Longterm Photometry of Variables at ESO - Part Two - the Second Data Catalogue 1986-1990
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102...79S&db_key=AST
| Distances, reddenings and distribution of emission B-stars in the galactic centre region /l/ not greater than 45 deg
The distribution of Be stars in the region surrounding the Galacticcenter and their correlation to the spiral structure of the Galaxy hasbeen studied. The results are discussed in terms of reddenings anddistances of these stars. Data are presented on the Galacticcoordinates, colors, interstellar color excesses, reddening-freemagnitudes and colors, adopted absolute magnitudes, distances in kpc,distances from the Galactic plane, and MK spectral type.
| Photometric observations of emission B-stars in the southern Milky Way
In order to study the distribution of Be stars and their correlation tothe local spiral structure of the Galaxy photoelectric UBV photometrywas carried out for a total of 488 Be stars located in the southernMilky Way between galactic longitudes 315 and 45 deg. UBV magnitudes arepresented for these stars.
| The curious case of the 'eclipsing' WC9 star HD 164270
The WC9 star HD 164270 faded by nearly 1 mag in June 1980 and September1909. The photoelectric observations of the 1980 event, however, showthat this remarkable occurrence was accompanied by no discernible changein the colors of the star. Null results of other Harvard patrol plateslimit the allowed periods of these events to 70.8 yr, 35.4 yr, or 17.7yr, and we give times of predicted eclipse in the hopes that others mayconfirm or eliminate one of these possibilities. If these events areeclipses, then any companion must be large and cool. The consideredlimitations of the physical parameters of this alleged companion arediscussed, and some alternate interpretations are considered.
| The S201 far-ultraviolet imaging survey. III - A field in Sagittarius
Far-ultraviolet imagery of a 20 deg diameter field in Sagittarius,centered near (1950) R.A. 18 h 34 m, decl. -30 deg 25 arcmin, wasobtained by the S201 far-ultraviolet camera during the Apollo 16mission. In a 10-minute exposure covering the 1250-1600 A wavelengthrange, 1034 star images are detectable, with a limiting ultravioletmagnitude of about 10. Most of these objects are identified withearly-type stars listed in the Smithsonian Astrophysical ObservatoryStar Catalog, the Catalog of Stellar Identifications, or both, but 203objects remain unidentified or are identified with late-type stars. Thephotometric measurements appear to be in reasonable agreement with thoseof the International Ultraviolet Explorer for stars in common, and withexpectations for A0 stars. A detailed photometric study was made of theMessier 8 region, and it is concluded that dust-scattered starlightcontributes about half of the total radiation observed from the centralregion of M8.
| UBV photometry for southern OB stars
New UBV photometry of 1227 OB stars in the southern Milky Way ispresented. For 1113 of these stars, MK spectral types have been reportedpreviously in a comprehensive survey to B = 10.0 mag.
| Erratum - Distant Blue Stars in the Southern Milky-Way
Not Available
| On the 4430 A interstellar band - A visual classification
A system of visual classification of the strength of the interstellarband 4430 A has been developed, based on spectra taken at spectralclassification dispersion. The intensity of the band was divided into 8main classes, defined by a group of selected standard stars. The systemwas applied to 1,111 southern OB stars contained in the catalog ofspectral types by Garrison, Hiltner, and Schild (1977). The mean errorin the classification was estimated to be plus or minus 1 class.Equivalent widths were measured for 100 stars; an excellent correlationwith the visual estimations was obtained. A calibration between thesetwo parameters is given.
| 63rd Name-List of Variable Stars
Not Available
| MK spectral classifications for southern OB stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977ApJS...35..111G&db_key=AST
| Observations of southern emission-line stars
A catalog of 1929 stars showing H-alpha emission on photographic platesis presented which covers the entire southern sky south of declination-25 deg to a red limiting magnitude of about 11.0. The catalog providesprevious designations of known emission-line stars equatorial (1900) andgalactic coordinates, visual and photographic magnitudes, H-alphaemission parameters, spectral types, and notes on unusual spectralfeatures. The objects listed include 16 M stars, 25 S stars, 37 carbonstars, 20 symbiotic stars, 40 confirmed or suspected T Tauri stars, 16novae, 14 planetary nebulae, 11 P Cygni stars, 9 Bep stars, 87 confirmedor suspected Wolf-Rayet stars, and 26 'peculiar' stars. Two new Tassociations are discovered, one in Lupus and one in Chamaeleon. Objectswith variations in continuum or H-alpha intensity are noted, and thedistribution by spectral type is analyzed. It is found that the skydistribution of these emission-line stars shows significantconcentrations in the region of the small Sagittarius cloud and in theCarina region.
| HD 163868, a New Bright Southern Variable
Not Available
| The ultra-violet sky-survey telescope in the TD-IA satellite.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973MNRAS.163..291B&db_key=AST
| Ultra-violet photometry of early type stars from TD I satellite observations.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973MNRAS.163....1H&db_key=AST
| Catalogue and Bibliography of Stars of Classes B and a whose Spectra have Bright Hydrogen Lines
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1933ApJ....78...87M&db_key=AST
| Not Available
Not Available
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Sagitario |
| Ascensión Recta: | 17h59m56.43s |
| Declinación: | -33°24'30.0" |
| Magnitud Aparente: | 7.35 |
| Distancia: | 1098.901 parsecs |
| Movimiento Propio en Ascensión Recta: | 6.1 |
| Movimiento Propio en Declinación: | -3.2 |
| B-T magnitude: | 7.303 |
| V-T magnitude: | 7.347 |
Catálogos y designaciones:
|
