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Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| SCUBA observations of dust around Lindroos stars: evidence for a substantial submillimetre disc population
We have observed 22 young stars from the Lindroos sample at 850 μmwith SCUBA on the James Clerk Maxwell Telescope to search for evidenceof dust discs. Stars in this sample are the less massive companions ofB-type primaries and have well-defined ages that are 10- 170 Myr; i.e.they are about to, or have recently arrived on the main sequence. Dustwas detected around three of these stars (HD 112412, 74067 and 99803B).The emission around HD 74067 is centrally peaked and is approximatelysymmetrically distributed out to ~70 arcsec from the star. This emissionarises from either a two-component disc, one circumstellar and the othercircumbinary with dust masses of 0.3 and <27 M⊕,respectively, or an unrelated background object. The other twodetections we attribute to circumsecondary discs with masses of 0.04 and0.3 M⊕. We were also able to show that a circumprimarydisc is present around HD 112413 with a similar mass to that around thecompanion HD 112412. Cross-correlation of our sample with the IRAScatalogues only showed evidence for dust emission at 25 and 60 μmtoward one star (HD 1438); none of the submillimetre detections wereevident in the far-infrared data implying that these discs are cold(>40 K assuming β= 1). Our submillimetre detections are some ofthe first of dust discs surrounding evolved stars that were not detectedby IRAS or ISO and imply that 9-14 per cent of stars could harbourpreviously undetected dust discs that await discovery in unbiased sub-mmsurveys. If these discs are protoplanetary remnants, rather thansecondary debris discs, dust lifetime arguments show that they must bedevoid of small <0.1 mm grains. Thus it may be possible to determinethe origin of these discs from their spectral energy distributions. Thelow inferred dust masses for this sample support the picture thatprotoplanetary dust discs are depleted to the levels of the brightestdebris discs (~1 M⊕) within 10 Myr, although if theextended emission of HD 74067 is associated with the star, this wouldindicate that >10 M⊕ of circumbinary material canpersist until ~60 Myr and would also support the theory that T Tauridiscs in binary systems are replenished by circumbinary envelopes.
| Photographic observations of visual double stars
Photographic observations of 238 pairs of visual binaries obtained on770 plates using the 60-cm visual refractor of Bosscha Observatory(Lembang, Java) during 1976-1979 are reported, continuing the catalog ofvan Albada-van Dien (1983). The data-reduction procedures are brieflydescribed, and the data are presented in a table.
| A study of visual double stars with early type primaries. IV Astrophysical data
Astrophysical parameters (MK class, color excess, absolute magnitude,distance, effective temperature, mass, and age) are derived fromcalibrations of the uvby-beta indices for the members of 253 doublestars with O or B type primaries and faint secondaries. The photometricspectral classification is compared to the MK classes, and the agreementis very good. The derived data together with spectroscopic and JHKL dataare used for deciding which pairs are likely to be physical and whichare optical, and it is shown that 98 (34 percent) of the secondaries arelikely to be members of physical systems. For 90 percent of the physicalpairs the projected separation between the components is less than25,000 AU. A majority of the physical secondaries are late-type stars,and 50 percent of them are contracting towards the zero-agemain-sequence. Also presented are new uvby-beta data for 43 secondariesand a computer program for determining astrophysical parameters fromuvby-beta data.
| Close visual binaries. III - Parameters and evolutionary status
New Yale isochrones, which have been tested for accuracy by Corbally(1984), provide the means to investigate interesting visual binaries,especially those whose classifications and photometry do not match well.Various parameters are deduced for those binaries which fitted theisochrones (e.g. ages, metal abundances, luminosities of peculiarstars); various solutions are systematically developed for those whichdid not fit; and a likely status of evolution is proposed (e.g.,duplicity of the components, premain-sequence, blue straggler,horizontal branch, optical pair, data inaccuracies). Evolution aroundthe helium flash and diffusion theory are briefly considered. Theseparameters and statuses provide a wealth of new stellar data andsuggestions for further investigation.
| Search for Beta Cephei stars south of declination -20 deg. II - Photometric and spectrographic observations of early B giants and subgiants - Winter objects
Results of the second part of a program begun in 1975 (Jerzykiewicz andSterken, 1977) are presented. Out of 39 Beta Cephei candidates observedphotometrically, seven turned out to vary with the b ranges exceeding0.020 m, while 21 have been found constant to within 0.010 m or better.For the 56 comparison stars these numbers are 6 and 25, respectively.From spectrographic observations one of the candidates, HD 129557 = HR5488 (B2 III), iis found to be a Beta Cephei variable with a period of0.135 d and the 2 K amplitude of 18.4 km/s. A comparison of the resultsof this program with other recent photometric searches for Beta Cepheivariables shows good agreement in most cases. There are severaldiscrepant stars, however. These are discussed in some detail. Aconclusion emerges that in the low temperature extension of the BetaCephei region, if such an extension exists at all, on-and-off typepulsations occur.
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