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On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of lambda Bootis type stars
Generally, chemical peculiarity found for stars on the upper mainsequence excludes delta Scuti type pulsation (e.g. Ap and Am stars), butfor the group of lambda Bootis stars it is just the opposite. This makesthem very interesting for asteroseismological investigations. The groupof lambda Bootis type stars comprises late B- to early F-type,Population I objects which are basically metal weak, in particular theFe group elements, but with the clear exception of C, N, O and S. Thepresent work is a continuation of the studies by Paunzen et al.(\cite{Pau97}, \cite{Pau98}), who presented first results on thepulsational characteristics of the lambda Bootis stars. Since then, wehave observed 22 additional objects; we found eight new pulsators andconfirmed another one. Furthermore, new spectroscopic data (Paunzen\cite{Pau01}) allowed us to sort out misidentified candidates and to addtrue members to the group. From 67 members of this group, only two arenot photometrically investigated yet which makes our analysis highlyrepresentative. We have compared our results on the pulsationalbehaviour of the lambda Bootis stars with those of a sample of deltaScuti type objects. We find that at least 70% of all lambda Bootis typestars inside the classical instability strip pulsate, and they do sowith high overtone modes (Q < 0.020 d). Only a few stars, if any,pulsate in the fundamental mode. Our photometric results are inexcellent agreement with the spectroscopic work on high-degree nonradialpulsations by Bohlender et al. (\cite{Boh99}). Compared to the deltaScuti stars, the cool and hot borders of the instability strip of thelambda Bootis stars are shifted by about 25 mmag, towards smaller(b-y)_0. Using published abundances and the metallicity sensitiveindices of the Geneva 7-colour and Strömgren uvbybeta systems, wehave derived [Z] values which describe the surface abundance of theheavier elements for the group members. We find that thePeriod-Luminosity-Colour relation for the group of lambda Bootis starsis within the errors identical with that of the normal delta Scutistars. No clear evidence for a statistically significant metallicityterm was detected. Based on observations from the Austrian AutomaticPhotoelectric Telescope (Fairborn Observatory), SAAO and Siding SpringObservatory.
| Precise radial velocities of late-type stars in the open clusters M11 and M67
Precise radial-velocity measurements of 39 stars and 170 stars areobtained in the fields of the open clusters M11 and M67, respectively;by the vast majority of the stars observed are cluster members. Allstars observed are of spectral type later than F5; the data set includesessentially all of the giants in both clusters and all main-sequencestars in M67 brighter than V = 12.8, plus an assortment of nonmembersand fainter members. Single-measurement precisions range from 0.3 to 0.8km/s. Almost all stars have been observed repeatedly, with clustermembers typically having 5-25 velocity measurements. Observations wereobtained over a span of 16 yr, although not all stars were observedthroughout that period. The time distribution of the observations is acombination of low-frequency coverage (less than 1/yr) over the first 11yr, and higher-frequency coverage (greater than 2/yr) over the final 5yr.
| Investigation of proper motions in the field of the cluster M 67 II,
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Cancro |
| Ascensione retta: | 08h48m18.01s |
| Declinazione: | +11°35'02.5" |
| Magnitudine apparente: | 8.374 |
| Moto proprio RA: | -16.3 |
| Moto proprio Dec: | -8.9 |
| B-T magnitude: | 9.649 |
| V-T magnitude: | 8.48 |
Cataloghi e designazioni:
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