Lang-Term Photometrie Campaign at ESO and the New Eelipsing P Cygni Star R 81 in the LMC 2 3 4 B

Lang-Term Photometrie Campaign at ESO and the New Eelipsing P Cygni Star R 81 in the LMC 2 3 4 B

Lang-term Photometrie Campaign at ESO and the New Eelipsing P Cygni Star R 81 in the LMC 2 3 4 B. WOLF 1, C. STERKEN , 0. STAHL and J. MANFROIo 1Landessternwarte Heide/berg, 2Vrije Universiteit Brusse/s, 3ESO, 4 Universite de Liege 1. A Programme for Long-term ticipants who work in a tight team with approach is a very efficient and useful Photometry of Variable Stars the principal investigators. From the be­ one. Besides the direct yield of data, the ginning we aimed at obtaining several project also offers to young scientists At the ESO workshop The Most Mas­ months of observing time each observ­ (e. g. Ph. O. students) the possibility to sive Stars in November 1981 (Eds. ing season, and this for a time span of at get into contact with the fields of re­ O'Odorico, Baade and Kjär), C. Sterken least one decade. search of the other participants, wh ich presented results of his observations of A project of that size, including the substantially contributes to their obser­ HO 160529 during a time span of more interpretation of measurements ob­ vational experience. than 8 years. HO 160529 is an A2 hyper­ tained at different telescopes and by giant wh ich was found to be variable by different observers cannot work without 2. R 81, the Counterpart of P Cyg Wolf et al. (1974) in an irregular way with astriet agreement on observing proce­ in the LMC an amplitude of several hundredths of a dures and reduction techniques. Espe­ magnitude. Sterken continued the cially for what concerns the data reduc­ P Cygni is one of the most outstand­ monitoring of this star at every possible tion, high requirements are needed so ing stars of the Galaxy. It had a remark­ occasion in the period 1973-1981 and that the final data may be of the highest able outburst in 1600 and is particularly found evidence for the presence of obtainable accuracy. It was decided distinguished by its unusual spectrum. periodic light variation of about 100 that all reductions would be done cen­ The so-called P Cygni profiles in the days. From the phase diagram he con­ trally (and not by the individual obser­ visual range (wh ich have been disco­ cluded that in spite of the intensive ob­ vers) at the University of Liege by J. vered already in the 19th century) are servations by one person, there still re­ Manfroid. The reduction algorithm is a named after this prototype and indicate main long gaps in the phase diagram. If generalized method developed by Man­ that this star is losing mass at an ex­ sampling of observations happens in froid and Heck (1983) and is charac­ cessive rate. P Cygni belongs to the such an irregular way (gaps caused by terized by the use of practically every luminous (Mbol = -9 to -11) blue vari­ irregularly allocated short observing measurement of any non-variable star. ables (LBV's) which have been recog­ runs), long-term orbital effects may be In doing so, all measurements are trans­ nized during the past few years as key­ hidden by or ascribed to the super­ formed into a standard system and the objects for the understanding of the giant's irregular intrinsic fluctuations. It observations obtained during different evolution of the very massive (initial was therefore suggested that the obser­ seasons and with different instruments masses ~ 50 MCi)) stars. According to vations of e. g. variable supergiants be can be re-evaluated regularly so that the current evolutionary models (cf. e. g. done by a larger team of observers at a homogeneity of the data is assured. Humphreys 1986, Chiosi and Maeder dedicated small instrument. Such an Because of the availability of the suit­ 1986) these very massive stars do not approach would also be very useful for able photometers on the ESO 50 cm become red supergiants; instead a criti­ all differential photometry on a basis of and the Oanish 50 cm telescopes. and cal luminosity boundary exists (calIed one measurement per night or less: for the usefulness for astrophysical in­ Humphreys Davidson limit) beyond supergiants, Be stars, Ap stars ... terpretation of the data. we decided to which the most luminous stars do not Ouring the discussions following the carry out all measurements in the evolve. The LBV's are located close to talk, several colleagues expressed their Strämgren uvby system. Since we deal this boundary and are supposed to be interest in the idea, and by the end of the with variable stars, preference was gi­ the immediate progenitors of the Wolf­ Workshop, the project Long-term ven to differential measurements with Rayet stars. Photometry of Variables was born. In two comparison stars (although we have The physical properties of P Cygni February 1982 a meeting was held at also carried out absolute measure­ have been scrutinized during the past the University of Brussels, and the pro­ ments). few years at a wide wavelength range ject finally started with an application for In practice the participants may ex­ from the UV with IUE to the IR with observing time submitted in April 1982. pect to receive their data not later than IRAS! A drawback of P Cygni is that The participants (originally about 15) three to six weeks after termination of both its interstellar reddening and its merged the objects for wh ich photome­ the observing run. Since 1982 the distance are difficult to determine (see try on a long-time basis was needed into number of participants has doubled, e. g. Lamers, de Groot and Cassatella one object list of about 70 entries, and and this is also so for the number of 1983). For this reason we searched the then the file was split in seven sections stars. At least 30 stars, for which zoo of the luminous blue supergiants of according to the nature of the variable enough data were obtained, have been the Large Magellanic Cloud (LMC). As stars (see Sterken, 1983). Later on, an omitted from the object list. 26 observ­ outlined e. g. in our recent atlas of high eighth section (Peculiar Late Type Stars) ing runs with a duration of about one dispersion spectra of luminous LMC was added. For each section a principal month length each have already been stars (Stahl et al. 1985) the P Cygni type investigator and a co-investigator were allotted by ESO and a huge amount of stars form in fact a populous group appointed. These scientists decide ab­ data are already available. The results among the peculiar emission line stars. out which objects need monitoring, and have already led to more that 20 pub­ Already in 1981 Wolf et al. presented they carry out and/or supervise the lished scientific papers. a detailed spectroscopic and photome­ analysis of the data of the stars be/ong­ The nature of some discoveries (Iike trie study of R 81 of the LMC. R 81 ing to their sections. We agreed that the e. g. the binary nature of R 81 wh ich is turned out to be a particularly close observers would be volunteering par- described below) clearly proofs that this counterpart of the galactic star P Cygni. 10 bound dust envelope since Stahl et al. R 81 I UE, LWR (1987) identified R 81 as an IRAS point source. The dust formation could be a 1982, Feb. 17 !SI (Y) consequence of their slow and dense N ~ Q) N Q) u:> N le N N u:> winds. le le N ~ le le le ~ ~ .... One of the most paradoxical results ~ ~ ~ ~ ~ ~ ::: ::: ::: ::: ::: ::: ::: derived from the IUE spectrum of P .. Cl Cl Cl Cygni (Cassatella et al. 1979) is that this ..,>-.. u.. u.. u.. u.. I I I I hot star with its unusual P Cygni profiles (l) c in the visual range is right the other way ..,CD distinguished by a lack of P Cygni pro­ C ..... files in the ultraviolet (which are other­ CD > wise so typical for hot stars in this .., range). Just this behaviour is found in 0 the case of R 81 as weil. Still more in­ CD L triguing is the fact that e. g. the Fe II lines of both stars agree even in details. The (Y) lf) complex profiles show a multiple com­ ....Cl ­+ + ponent structure (for R 81 see Fig.1) ascribed to the ejection of discrete shells (cf. Lamers et al. 1985 and Stahl et al. 1987). 0. In addition to the major historical out­ 2612. 2622. A[J.J 2632. bursts of P Cygni in the 17th century, Figure 1: A seetion of the IUE high-dispersion spectrogram around 2600 A. This wavelength irregular photometrie variations of 0.1 to range is dominated by Fell absorption lines. Like in P Cyg, different components are present. 0.2 magnitude have been reported dur­ The velocities of these components are indicated for different lines. ing this century by many authors. No strict periodicity has yet been found (van Gent and Lamers 1986). Likewise in the Both stars are (of course) characterized of the Hertzsprung-Russell Diagram case of R 81, which until 1980 was re­ by strong P Cygni profiles of the Balmer (HRD) close to the Humphreys-David­ peatedly observed during various ran­ lines and by very similar stellar parame­ son limit. The winds of both stars are domly distributed photometrie observ­ 1 ters: Teff = 20,000 K, Mbol = -10, and R slow (vmax = 250 to 300 km 5- ) and very ing runs, variations of about this am­ 5 I = 70 R0 for R 81 and Teff = 19,300 K, massive (M = 3 .

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