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SuperWASP observations of the 2007 outburst of Comet 17P/Holmes
We present wide-field imaging of the 2007 outburst of Comet 17P/Holmesobtained serendipitously by SuperWASP-North on 17 nights over a 42-nightperiod beginning on the night (2007 October 22-23) immediately prior tothe outburst. Photometry of 17P's unresolved coma in SuperWASP datataken on the first night of the outburst is consistent with exponentialbrightening, suggesting that the rapid increase in the scatteringcross-section of the coma could be largely due to the progressivefragmentation of ejected material produced on a very short time-scale atthe time of the initial outburst, with fragmentation time-scalesdecreasing from tfrag ~ 2 × 103 to ~1× 103 s over our observing period. Analysis of theexpansion of 17P's coma reveals a velocity gradient suggesting that theouter coma was dominated by material ejected in an instantaneous,explosive manner. We find an expansion velocity at the edge of the dustcoma of vexp = 0.55 +/- 0.02 kms -1 and a likelyoutburst date of t0 = 2007 October 23.3 +/- 0.3, consistentwith our finding that the comet remained below SuperWASP's detectionlimit of mV ~ 15mag until at least 2007 October 23.3.Modelling of 17P's gas coma indicates that its outer edge, which wasobserved to extend past the outer dust coma, is best explained with asingle pulse of gas production, consistent with our conclusionsconcerning the production of the outer dust coma.
| Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue
Context: A new reduction of the astrometric data as produced by theHipparcos mission has been published, claiming that the accuracies fornearly all stars brighter than magnitude Hp = 8 are improved, by up to afactor 4, compared to the original catalogue. As correlations betweenthe underlying abscissa residuals have also been reduced by more than anorder of magnitude to an insignificant level, our ability to determinereliable parallaxes and proper motions for open clusters should beimproved. Aims: The new Hipparcos astrometric catalogue is usedto derive mean parallax and proper motion estimates for 20 openclusters. The HR-diagrams of the nearest clusters are compared andcombined to provide future input to sets of observational isochrones. Methods: Three different methods are applied, according to theproximity of the cluster, to compensate, where needed, for projectioneffects, spread in position along the line of sight, and the internalvelocity dispersion of the cluster stars. Results: The newparallaxes have accuracies between 2 and 2.5 times higher than what hadbeen derived from the original Hipparcos catalogue. At least two tothree groups of clusters, mostly of similar ages, are observed to eachoccupy their own specific space in the HR diagram. A significantdiscrepancy in distance moduli from those obtained with isochrone-basedmain-sequence fitting remains, in particular for one of these groups,containing the Pleiades, NGC 2516, and Blanco 1. The difference inabsolute magnitudes between this group and one containing the Hyades andPraesepe clusters appears to be correlated with systematic differencesin the Strömgren ? c0 index between those groups.The same dependency has been known to exist for a long time, and isagain confirmed by the Hipparcos data, in variations in absolutemagnitudes for field stars of the same effective temperature. Conclusions: The positions of the cluster HR diagrams are consistentwithin different groups of clusters shown for example by thenear-perfect alignment of the sequences for the Hyades and Praesepe, forComa Ber and UMa, and for the Pleiades, NGC 2516, and Blanco 1. Thegroups are mutually consistent when systematic differences in ?c0 are taken into account, where the effect of thesedifferences on the absolute magnitudes has been calibrated usingfield-star observations.
| Membership and binarity of solar-type dwarfs in the nearby open cluster Alpha Persei (Mel 20)
Context: The Alpha Persei open cluster (Melotte 20) is one of the nearbyclusters (d < 500 pc) for which a long-term systematicradial-velocity monitoring of the solar-type stars has not beenpublished so far. Aims: We undertook an observing program in 1982 tostudy the membership, duplicity, and rotation of the main-sequence starsand to compare the results with those of the other nearby clusters. Methods: Radial-velocity observations of 60 solar-type stars in Mel 20were obtained with CORAVEL, and of 44 fainter stars with ELODIEspectrographs, in the magnitude range 9.5 < V < 13.85 Results:The membership of 37 stars from the list compiled by Heckmann isconfirmed according to their proper motions, radial velocities, andphotometry. We discovered twelve spectroscopic binaries (3 double-lined,8 single-lined, 1 suspected) among the members and determined an orbitfor the double-lined binary He 848. The observed frequency ofspectroscopic binaries is therefore 32% (12/37). Four stars arephotometric binaries. In addition, the membership of 33 stars with APnumbers is supported by the present radial velocities. Two single-linedand three double-lined binaries were detected in this sample. Thecluster mean radial velocity is -1.39 ± 0.17 km s-1based on 18 stars within 2σ. We found a systematic difference of3.67 ± 1.48 km s-1 between observed- and astrometricradial velocities. Conclusions: The Alpha Persei cluster members definea tight main sequence in the colour-magnitude diagrams. The remainingscatter observed in the colour-magnitude diagrams and in radialvelocities may be due to the difficulty of separating the core AlphaPersei stars from the extended comoving stream.based on observations collected at the Haute-Provence Observatory(France). Table 5 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/488/409
| OB association members in the ACT and TRC catalogues
The Hipparcos Catalogue contains members of nearby OB associationsbrighter than 12th magnitude in V. However, membership lists arecomplete only to magnitude V=7.3. In this paper we discuss whetherproper motions listed in the `Astrographic Catalogue+Tycho' referencecatalogue (ACT) and the Tycho Reference Catalogue (TRC), which arecomplete to V~10.5mag, can be used to find additional associationmembers. Proper motions in the ACT/TRC have an average accuracy of~3masyr-1. We search for ACT/TRC stars which have propermotions consistent with the spatial velocity of the Hipparcos members ofthe nearby OB associations already identified by de Zeeuw et al. Thesestars are first selected using a convergent-point method, and thensubjected to further constraints on the proper-motion distribution,magnitude and colour to narrow down the final number of candidatemembers. Monte Carlo simulations show that the proper-motiondistribution, magnitude, and colour constraints remove ~97per cent ofthe field stars, while at the same time retain more than 90per cent ofthe cluster stars. The procedure has been applied to five nearbyassociations: the three subgroups of Sco OB2, plus Per OB3 and Cep OB6.In all cases except Cep OB6, we find evidence for new associationmembers fainter than the completeness limit of the Hipparcos Catalogue.However, narrow-band photometry and/or radial velocities are needed topinpoint the cluster members, and to study their physicalcharacteristics.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Membership of low-mass stars in the open cluster Alpha Persei
The results of a combined astrometric, photometric, and spectroscopicsearch for low-mass members in the intermediate-age open cluster AlphaPersei are presented. Over 130 low-mass new members have been identifiedto M(v) about 12.5, almost doubling the previous number of knownmembers. The new membership information suggests a slight upwardrevision of Alpha Per's age to about 8 x 10 exp 7 yr. Alpha Per isnoticeably spatially elongated in a direction parallel to the Galacticplane, most likely due to tidal deformation. Analysis of thedistribution of relative H-alpha emission strengths among Alpha Permembers confirms the view that the mean H-alpha strength decreases inincreasingly older systems. Allowing for survey incompleteness, onecannot reject the idea that the luminosity function is consistent withthe field luminosity function to M(v) about 9 or 10.
| The H-R diagram of the alf Per cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..242M&db_key=AST
| Photometry of the α Persei Cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1960ApJ...132...68M&db_key=AST
| Eigenbewegungen in der Umgebung von α Persei
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