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Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| On the Transience of Circumstellar Shells about |B|>=10° OH/IR Stars. I. Basic Statistics
There are 62 OH/IR stars in the Arecibo sky with|b|>=10deg, |l|<90deg, and a 25 μm fluxS(25)>=2 Jy. These were identified as a result of a complete OHsurvey of color-selected IRAS sources, which also identified four O-richproto-planetary nebulae (PPNs). Since OH/IR stars evolve into PPNs, the~103 yr expansion age of one, 18095+2704, calibrates an ~1670 yr averagenet duration for 1612 MHz emission from high-latitude stars. This iscomparable with the 770 yr wind travel time from the photosphere out tothe 1612 MHz emission zone of WX Psc and the ~500 yr duration of theluminosity spike following after a thermal pulse. There is, however,only one PPN candidate without an OH maser to match to the sample's 118OH/IR-star color mimics (circumstellar shells with the IR colors ofOH/IR stars that do not exhibit 1612 MHz masers), where eight areimplied if mimics independently evolve through a PPN phase. Most mimicsare therefore likely to be the immediate precursors of OH/IR stars. Theaverage net duration of heavy mass loss in a superwind is then estimatedfrom the ratio between the number of IR sources and PPNs at ~3700 yr.However, the absence of any O-rich PPNs to match to OH/IR stars andmimics in the Galactic anticenter region (AGC), even though there aretwo carbon stars and four C-rich PPN with appropriate IR colors there,offers a strong hint that most OH/IR stars in the AGC exit theirasymptotic giant branch phase as C-rich PPNs. This carries the directimplication that the superwind phase in high-latitude stars is usually acyclical phase.
| 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.
| Galaxies with spectral energy distributions peaking near 60 micrometers. 1: Optical spectroscopy, infrared photometry, and radio continuum data
We present and discuss broadband infrared photometry in the 1-100micrometer wavelength range, optical spectroscopy, and radio continuumobservations of a sample of IRAS galaxies with unusual spectral energydistributions that peak near 60 micrometers. For inclusion in thissample of '60 PKs', the galaxies must have satisfied the followingcriteria: IRAS flux ratios f60/f100 greater than 1and 1 less than f60/f25 less than 4, and galacticlatitude absolute value of b greater than 10 deg. In this paper, whichis the first of a series, we show that the 60PKs are relatively scarceobjects that represent about 2% of the spatial density of 60micrometer-selected galaxies in the range L(60 micrometers) =109 to 1012 solar luminosity, but have afar-infrared luminosity function of nearly identical shape. They aredetected up to redshifts of 0.2. Besides having the usual highpercentage of active galaxies (approximately 30% H II-region like, 50%Seyfert 2, 10% Seyfert 1, 10% unknown) associated with a flat 25 to 60micrometer spectral index, the sample also includes most of the galaxiesthat have been found to have a dust-obscured broadline region. We showthat the additional f60/f100 greater than 1constraint selects galaxies with dust that is more centrallyconcentrated and exposed to a more intense radiation field than in mostother IRAS galaxies. In particular, the cirrus component of thefar-infrared radiation, which is typically the dominant contributor tothe 100 micrometer emission from spiral galaxies, is negligible ormissing. This is consistent with the fact that the most distinctiveoptical signature of 60PKs is the absence of spiral structure: they tendto be peculiar and/or amorphous objects.
| OH/IR star color mimics
IR color criteria are used to select potential OH/IR stars from the IRASPoint Source Catalog. These OH/IR star color mimics, despite oftenhaving thick and demonstrably O-rich dust shells to shield theirmolecules against interstellar UV, have no 1612 MHz masers. The mostlikely reason for this is that these stars have degenerate companions,which collect an accretion disk from the red giant wind, which in turnprovides them with a local source of UV to dissociate molecules fromwithin their dust shells. In some cases this self-generated UV issufficient to excise all molecules from a shell, as happens withsymbiotic novae; in some cases it merely reduces their number and theability of a shell to support a maser. It is suggested that D-typesymbiotic stars can be identified among sources with thick opaque dustshells by a persistent absence of appropriate masers: these are theOH/IR color mimics.
| New OH/IR stars from color-selected IRAS sources. II - an unbiased 1612 MHz survey
Results are reported from the Arecibo 1612-MHz survey of color-selectedIRAS sources. This paper examines 1294 sources, to detect 86, 79 of themnew detections, all with 25-micron fluxes greater than 2 Jy. The specialfeature of this work is its coverage of sources with high absolute valueof b(II), so the 1612-MHz characteristics of stars with small progenitormasses are determined. This provides direct observational evidence thatradiation pressure acting on dust grains influences mass loss from redgiants.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Orion |
| Ascensió Recta: | 05h24m08.05s |
| Declinació: | +02°27'46.9" |
| Magnitud Aparent: | 9.744 |
| Moviment propi RA: | 1.8 |
| Moviment propi Dec: | -0.4 |
| B-T magnitude: | 9.952 |
| V-T magnitude: | 9.762 |
Catàlegs i designacions:
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