Are isolated planetary-mass objects really isolated?. A brown dwarf-exoplanet system candidate in the σ Orionis cluster
Context: .Free-floating planetary-mass objects have masses below thedeuterium burning mass limit at about 13 Jupiter masses, and have mostlybeen found in very young open clusters. Their origin and relationship tostars and brown dwarfs are still a mystery. Aims: .The recentdetection by direct imaging of three giant planets at wide separation(50-250 AU) from their primaries has raised the question about the true"isolation" of planetary-mass objects in clusters. Our goal was to testthe possibility that some free-floating planetary-mass objects could infact be part of wide planetary systems. Methods: .We searched inthe literature for stellar and brown-dwarf candidates members of theσ Orionis cluster (~3 Ma, ~360 pc) at small angular separationsfrom published candidate planetary-mass objects. We found one candidateplanetary system composed of an X-ray source, SE 70, and aplanetary-mass object, S Ori 68, separated by only 4.6 arcsec. In orderto assess the cluster membership of the X-ray source, we obtainedmid-resolution optical spectroscopy using ISIS on the William HerschelTelescope. We also compiled additional data on the target from availableastronomical catalogues. Results: .We have found that SE 70follows the spectrophotometric sequence of the cluster and displaysspectroscopic features of youth, such as lithium in absorption andchromospheric Hα emission. The radial velocity is consistent withcluster membership. Hence, SE 70 is very probably a member of theσ Orionis cluster. The projected physical separation between SE 70and S Ori 68 is 1700~± 300 AU at the distance of the cluster. Ifa common proper motion is confirmed in the near future, the system wouldbe composed of an M5-6 brown dwarf with an estimated mass of ~45 M_Jupand an L5 ± 2 giant planet with an estimated mass of ~5 M_Jup. Itwould be the widest and one of the lowest-mass planetary systems knownso far.
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XMM-Newton observations of the σ Orionis cluster. II. Spatial and spectral analysis of the full EPIC field
We present the results of an XMM-Newton observation of the young (2{-}4 Myr) cluster around the hot star σ Orionis. In a previouspaper we presented the analysis of the RGS spectrum of the central hotstar; here we discuss the results of the analysis of the full EPICfield. We have detected 175 X-ray sources, 88 of which have beenidentified with cluster members, including very low-mass stars down tothe substellar limit. We detected eleven new possible candidate membersfrom the 2MASS catalogue. We find that late-type stars have a median logL_X/L_bol -3.3, i.e. very close to the saturation limit. Wedetected significant variability in 40% of late-type members orcandidates, including 10 flaring sources; rotational modulation isdetected in one K-type star and possibly in another 3 or 4 stars.Spectral analysis of the brightest sources shows typical quiescenttemperatures in the range T_1 0.3{-}0.8 keV and T_2 1{-}3 keV,with subsolar abundances Z 0.1{-}0.3 Z_ȯ, similar to what isfound in other star-forming regions and associations. We find nosignificant difference in the spectral properties of classical andweak-lined T Tauri stars, although classical T Tauri stars tend to beless X-ray luminous than weak-lined T Tauri stars.
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Ultra low-mass star and substellar formation in σ Orionis
The nearby young σ Orionis cluster (360 pc, 3 Myr) isbecoming one of the most important regions for the study of ultralow-mass star formation and its extension down to the mass regimes ofthe brown dwarfs and planetary-mass objects. Here, I introduce theσ Orionis cluster and present three studies that the {JOVIAN}group is developing: a pilot programme of near-infrared adaptive-opticsimaging of stars of the cluster, intermediate-resolution opticalspectroscopy of a large sample of stars of the cluster and a study ofthe mass function down to the planetary-mass domain. This paper is asummary of the content of four posters that I presented in the Ultralow-mass star formation and evolution Workshop, as single author or onbehalf of different collaborations.
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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).
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The Ultraviolet Extinction Curve of Intraclump Dust in Taurus (TMC-1): Constraints on the 2175 Å Bump Absorber
Ultraviolet extinction curves for three early-type stars that sampledust in the Taurus dark cloud in the vicinity of TMC-1 are analyzed toexamine the contribution of internal (intraclump) dust to the totalline-of-sight extinction. One of our selected targets samples only dustin the ``diffuse-screen'' component of the cloud, where the dust grainslack ice mantles and have optical properties similar to average``diffuse-ISM'' dust. Comparison with data for more heavily reddenedstars that also sample dust within TMC-1 (where grains are mantled)allows us to determine the intraclump extinction. Our principal resultis that the 2175 Å ``extinction bump'' is extremely weak or absentinside the clump. The feature present in the observed extinction curvesappears to arise almost entirely in the diffuse screen. This dramaticreduction in bump amplitude in the high-density environment isconsistent with the prediction of the graphite model that the feature issuppressed by hydrogenation. Amplitude reduction is accompanied by amore subtle change in the profile of the bump, specifically, an increasein width. Comparison of Taurus and other clouds strongly suggests thatthe increase in width results from accumulation of surface coatings onthe carrier grains rather than from particle clustering.
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Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster
We report an optical photometric survey of 0.89 deg2 of theOrion OB1b association centered on σ Ori. This region includesmost of the σ Ori cluster, the highest-density region within OrionOB1b. We have developed a statistical procedure to identify the young,low-mass, pre-main-sequence population of the association. We estimatethat the cluster has ~160 members in the mass range (0.2<=M<=1.0Msolar). The cluster has a radius of ~3-5 pc and an estimatedage of 2.5+/-0.3 Myr. We estimate that the total mass of the cluster is225+/-30 Msolar. This mass is similar to the estimated massof the ~5×105 yr old cluster NGC 2024. NGC 2024 andσ Ori appear to be a well-matched pair of clusters, except for the~2 Myr difference in their ages.
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Photoelectric Photometry of Stars in the Orion Standard Region
Magnitudes and color indices in the Vilnius seven-color photometricsystem are given for 108 stars in the Orion standard region around theOrion Belt and the star lambda Orionis. New spectral and luminosityclasses, estimated from the photometric data, are given for some of thestars.
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Far-Ultraviolet (912--1900 Angstrom ) Energy Distribution in Early-Type Main-Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...449..280C&db_key=AST
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A new list of effective temperatures of chemically peculiar stars. II.
Not Available
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Incidence of X-ray sources among magnetic chemically peculiar stars
Cash & Snow (1982) and Golub et al. (1983) have detected X-rayemission from 3 out of 7 observed magnetic chemically peculiar (CP)stars. Although the incidence of X-ray sources apparently is very high,these authors concluded that such emission is not unquestionable becauseof the presence of a companion. To determine the incidence of X-raysources among hot CP stars, I have checked the list of B-type starsmeasured by Grillo et al. (1992) selecting 90 stars. Of the 4 magneticCP stars showing X-ray emission 3 are members of a binary system. Thusit appears that detectable X-ray emission from magnetic CP stars is notvery common and still questionable. To find out whether HD 37017presents X-ray emission (Drake et al. 1987) or not (Grillo et al. 1992),I have analyzed an HRI frame from the ROSAT satellite finding noevidence for X-ray emission at this star's position.
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Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)
Not Available
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An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
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Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
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Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)
Not Available
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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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The binary companion of the classical Cepheid AW Per
An analysis of IUE spectra of the companion of AW Per shows it to be anextreme BpSi/HeW star with a spectral type of B7 V to B8 V. The flux ofthe composite spectrum from 1200 A through V is found to be well matchedby F7 Ib and B8 V standard stars with Delta M(V) = 3.1 mag. The resultssuggest that the mass of the Cepheid must be greater than 4.7 solarmasses. The flux distribution of the star and the mass limits from theorbit are consistent with a companion that is itself a binary.
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Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
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The small scale structure of the interstellar medium in the Orion association: The flotsam of star formation
The small scale dynamics and structure of the interstellar medium of theOrion OB1b, c association was studied using IUE high dispersion SWP andLWR/LWP spectra. The area surveyed centers on the Belt and encompassesthe Orion Cloak. The dynamics and abundances of the ionized and warmneutral gas surrounding the central parts of the association aredetermined using 35 B stars. Results are compared with the picture ofthe Orion interstellar medium gained using COPERNICUS, especially forthe structure and dynamics of the Orion Cloak.
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Catalog of AP and AM stars in open clusters
The previous results of Raab (1922), Markarian (1951), and Collinder(1931) have been used to catalog Ap and Am stars that are in the fieldof open clusters. Tabular data are presented for the clusterdesignation, the HD or HDE number, the right ascension (1900), thedeclination (1900), and the magnitude. Also listed are the spectraltypes and, for certain stars, the probability of cluster membership.
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IUE observations of the broad continuum feature at 1400 A in the silicon and related stars
The upper main sequence chemically peculiar stars have previously beenstudied spectrophotometrically using low-resolution TD-1 spectra. Theseshow that the broad continuum feature at 1400 A is a useful indicator ofSi anomaly. In this paper the observations are extended to IUElow-resolution spectra and it is shown that this feature is indeed wellcorrelated with Si and also with optical chemical peculiarity indices.Its utility in selecting silicon stars is further demonstrated. Thisfeature is likely due to autoionization of Si II and is the bestavailable discriminator of the overabundance of a key element in the Apstars.
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On the Effective Temperatures of Chemically Peculiar Stars
Not Available
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Observational problems of investigation of stars with anomalous helium lines.
Not Available
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A survey of ultraviolet objects
An all-sky survey of ultraviolet objects is presented together with astatistical analysis that leads to the conclusion that there is asignificantly higher population of hot subdwarfs lying below themain-sequence than hitherto thought. The distribution of all ultravioletobjects, main sequence ultraviolet objects, and MK unclassifiedultraviolet objects are shown in galactic coordinates, and the absolutemagnitudes and color-color diagrams for these groups are presented.Scale heights are derived, giving values similar to planetary nebulaefor the hottest groups.
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Spectroscopic studies of stars in ORI OB1 /Belt/
MK spectral classifications are presented for 120 B- and A-type stars inthe Belt subgroup of the Ori OB1 association. Combined with otherpublished spectroscopic data, and the assumed absolute magnitudes givenby Schmidt-Kaler (1965) and Lesh (1979), a mean corrected distancemodulus of 8.56 + or - 0.11 is computed for the eastern region, whilefor the remainder of the Belt a modulus of 8.20 + or - 0.05 is derived.Several B9-A2 subgiants and A7-F0 main-sequence stars were observed inthe western and central regions of the Belt. The ages of these starsimply a spread in formation ages for these subgroups from about 1million yr to at least 10 million yr.
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A Photoelectric Search for Ap-Stars in the Orion Association
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1981A&A....94..134J&db_key=AST
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A photometric study of the Orion OB 1 association. III - Subgroup analyses
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...36..497W&db_key=AST
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A photometric study of the Orion OB 1 association. I - Observational data.
A catalog of observational data is presented for stars in the region ofthe young stellar association Orion OB 1. Photoelectric observationsobtained in the uvby-beta and UBV systems are compiled along withprevious photoelectric and spectroscopic data for all these stars aswell as for several bright members of the association with availablephotometric indices. Mean weighted values are computed for thephotometric data and summarized in tables expected to be reasonablycomplete for association members earlier than spectral type A0.Membership criteria are derived, and qualitative membershipprobabilities summarized, for the 526 stars in the final program. Theanalytical procedures are discussed for association stars of B,intermediate, and AF types. Effects of the nebular environment andvarious calibrations of Balmer-line and four-color indices areconsidered for the determination of absolute magnitudes for the B-typestars.
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Evidence of helium abundance differences between young groups of stars
Seven groups of B-type stars - Sco-Cen, NGC 6231, LacOBI,OriOBIb, h + chi Per, CepOBIII, and field stars - wereinvestigated photoelectrically to determine whether helium abundance isuniform in the solar neighborhood and in a region of the spiral arm ofthe Galaxy. Observations were performed using an echelle spectrometer inthe narrow band index of the I (4026) of the He I-4026 line. Amodel-atmosphere analysis of I (4026) and the beta index of the H betaline shows that in the spectral range B O-B 2, the relation between I(4026) and beta is insensitive to differences in the surface gravity butsensitive to differences in the helium to hydrogen ratio. It is foundthat stars in the two groups most distant from the sun - h + chi Per andCepOBIII - have a lower helium abundance than stars in the solarneighborhood.
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A search for He-weak stars in very young clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....19..482B&db_key=AST
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