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PHOTOMETRIC VARIABILITY in the HR-DIAGRAM L. Eyer, M. Grenon

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PHOTOMETRIC VARIABILITY in the HR-DIAGRAM L. Eyer, M. Grenon 467 PHOTOMETRIC VARIABILITY IN THE HR-DIAGRAM L. Eyer, M. Grenon Geneva Observatory, CH-1290 Sauverny, Switzerland the Atlas of Unsolved variables light curves which ABSTRACT contains a selection of well de ned curves. Many new variables were discovered and many others have a The Hipparcos satellite has detected systematically b etter de nition of their b ehaviour. variable stars when the amplitudes exceed a mag- nitude dep endent threshold, as small as a few mil- Here, we attempt a more global approach, since Hip- limag for bright stars. The published data ab out parcos allows a general description throughout the variable stars result from the work of two groups at HR-diagram of stellar stability and variability. The the Geneva Observatory and at the Royal Greenwich HIC Hipparcos Input Catalogue, Turon et al. 1992 Observatory. It contains information ab out p erio dic was built in order to b e a complete survey up to the variables extrema, p erio ds and epochs and ab out magnitude V < 7:9+ 1:1 sinb for stars earlier than `unsolved' variables amplitudes. Here, a more gen- G5 and V < 7:3+ 1:1 sinb for stars later than G5, eral description of the HR-diagram is made in terms where b is the galactic latitude. The fainter stars of stability, microvariability,variability, and variation in the program emerged as a selection of prop osed time scales. A particular emphasis is put on the lim- stars. Altogether, this gives a large sample on which its of stability areas and their signi cation. statistical b ehaviours can b e studied. The choice was made to represent the HR-Diagram in a non conventional way. That is to express it with Key words: Variable Stars, photometry, Hipparcos. the sp ectral classi cation. In the abscissa is displayed the sp ectral typ e and in the ordinate the luminosity class. The sp ectral typ es were taken from the HIP 1. INTRODUCTION catalogue 47 p er cent of the stars have a complete in- formation of sp ectral classi cation. This for several reasons. First, we didn't have the parallaxes. Second, Hipparcos furnished more than 13 million photomet- the sp ectral classi cation o ers a natural way to bin ric measurements for 118 204 stars, that is a mean stars. Third, it is reddening free. Fourth, intrinsi- of 110 Hp magnitudes per star over a time span of cally very luminous stars with reliable parallaxes are 3.3 years. However the number of these measure- rare. ments may vary strongly from one star to an other as well as the time sampling coverage over the ob- served p erio d. The Hp passband is wide, it extends from 320 nm up to 850 nm, with a mean wavelength 2. NON-VARIABLE STARS of 537 nm, close to that Johnson V 556 nm. Before Hipparcos, the systematic searches for vari- The precision on an individual Hp magnitude de- able stars were done photographically, with a mini- p ends strongly on the magnitude itself, although mum detectable amplitude of around 0.3 m . pg more observing time was allo cated for fainter stars. Thus the threshold to detect stars as variable is in- With the advance of technologies photo electric detec- creasing with the magnitude, mainly a result of pho- tors and CCD cameras b ecame available. The pre- ton statistics and sky background. Constant stars cision achieved reached few hundreds to few milli- are then de ned as stars not detected as variable at magnitudes. But there was no global survey of the a certain level, magnitude dep endent. whole sky and stars found photo electrically as vari- able were discovered accidentally or using sp ectral The detection threshold limit for stars with a mean criteria. However, some general studies were done on number of measurement N = 110 is given in Fig- large data collections, for example with the Geneva m ure 1. If in the shaded area, a star must b e consid- photometric data Grenon 1993. ered as of unknown status, constantorvariable with a p eak-to-p eak amplitude <A . If lo cated in the After the analysis of the main mission data, two cat- lim zone b etween the solid and dotted lines, it is de ned alogues and three atlases were pro duced: the Cat- as a susp ected variable and as a con rmed variable if alogues of Perio dic stars and of Unsolved stars, the ab ove the dotted line with 0:1 p er cent probability Atlas of Perio dic stars folded curves, the Atlas of to b e constant. Light Curves of stars in common with AAVSO, and 468 Six di erent tests were used to detect variables, with The applied pro cedure provides the main mo de, in di erent sensitivities to light-curve features. The case of pulsation, whereas many stars as Scuti mathematical expression for the limiting amplitude or SPB Slowly Pulsating B stars show a complex is given in Section 4. power sp ectrum of oscillation. The p eculiar time sampling by Hipparcos pro duces For constant stars, i.e. with A < 0:02 mag or so, quite easily spurious short p erio ds. In case of doubt, the standard error on the median magnitude may the p erio d was tested for its robustness. All solutions be extremely small less than 1 mmag. The set of pro duced by b oth teams were carefully compared. constant stars provides a high accuracy photometric reference system. Nevertheless the very high precision on Hp is de- med 3.1. Perio d Distribution in the HR Diagram graded when Hp magnitudes are transformed into Johnson V or Strmgren y magnitude b ecause of In order to investigate the intrinsic prop erties of sin- residuals on transformation equations, function of the gle stars, the con rmed or susp ected eclipsing bina- star temp erature, gravity, reddening or even metal- ries were removed from the sample. Sp ectroscopic licity. binaries may still contaminate the sample, namely in the area of G0-G5 I I I-IV stars. For all sp ectral typ es a broad p erio d luminosity re- lation exists see Figure 2, the high luminosity stars having the longest p erio ds. Outside the main stability areas, we observe three main instability zones. That of early typ e stars, shows the shortest p erio ds for Cepheids stars in the luminosity range class II-III to IV. Late B and early A main sequence stars show p erio ds around 1 day SPB, Bp. Around the classical instability strip, the dichotomy known for RR Lyrae stars, i.e. RRc and RRab is also found for p opulation I stars. Ftyp e subgiants < F1 showP<0.2 day and of ab out 1 day if near the red edge of the instability strip. Both G and early K dwarfs show p erio ds within 2 to 10 days, when the variability is rotationally induced. The p erio d increase with luminosity and with de- creasing temp erature is very conspicuous for late K and M giants and sup ergiants. Because of abundance e ect b oth on the luminosity Figure 1. Limits between constant, suspected variable, class and on the sp ectral typ e, p opulation I I stars as and variable stars for stars with a mean number of obser- RR Lyrae and RVTau are not considered. vations. Short p erio ds 2{5 days are present for hot sup er- The con rmation of the variability character from giants, these mo des are p ossibly sup erimp osed on Hipparcos time series requires careful examination. longer p erio d variations. Atypical case is HIP 67261 Outliers due to light contamination, image over- with an oscillation p erio ds of around 4 days. lapping, misp ointing may mimic ares or eclipses. Trends due to incorrect colour used in the reduction mimic secular changes as observed for some Be stars. 4. AMPLITUDE VARIABILITY LEVEL As a result of the data analysis, most spurious events are agged in the ep o ch photometry. When the metho ds fail to detect any p erio dic signal, the star is put in the Catalogue of Unsolved variables. 3. PERIODIC VARIABLES This do es not necessarily mean that the star is not p erio dic. We mighthave missed the p erio d e.g. HIP 29055 or some minima for eclipsing binaries due to When the ratio of the exp ected amplitude over the the p eculiar sampling, or the p erio dic b ehaviour is noise was ab ove a threshold tuned to leave less than to o complex to be describ ed with the present data 5 p er cent of spurious detections as variable Eyer et set e.g. multip erio dic variables. In most cases, the al. 1994, a search for p erio dicitywas done. Several variations are indeed ap erio dic as it can b e seen in the metho ds were used dep ending on the variabilitytyp e Atlas of light curves of Unsolved variables Grenon van Leeuwen 1997. Once the p erio d was found a et al. 1997. curve tting pro cess was applied to pro duce quanti- ties as Hp , Hp , the amplitude, the ep o chs and In order to characterize quantitatively the unsolved min max the errors on these parameters. variables, in addition to their light curves, we de ned 469 STELLAR VARIABILITY STELLAR VARIABILITY in the HR-Diagram in the HR-Diagram Luminosity class Luminosity class Ia Ia Ib Ib II II II-III II-III III III III-IV III-IV IV IV IV-V IV-V V V 0 0 F0 B0 A0 K0 F0 G0 B0 A0 K0 M0 G0 M0 Period in days : < 0.2 Spectral type Percentage of Amp > 0.05 : <2 Spectral type 0.2 - 0.5 2 - 4 0.5 - 2.0 4 - 6 2.0 - 5.0 6 - 8 5.0 - 10.0 8 - 15 10.0 - 20.0 15 - 20 20.0 - 50.0 20 - 40 50.0 - 100.0 40 - 80 > 100.0 >80 Figure4.
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