|Perugia University Observatory|
Electronic version by Alessandro Nesci
Web review by Stefano Ciprini
In the 1950's, examining a suitable number of plates obtained with the 60 cm reflector at the Loiano Astronomical Station, with the 120 cm reflector and with the 50/40 Schmidt at the Asiago Observatory, one of us found that hypersensitized infrared emulsion was a powerfull tool for the discovery and the obervation of the Long Period Variable (LPV) stars.Using I-N emulsions hypersensitized with NH3 in conbination with a filter RG5, between 1957 and 1962, 9 Infrared Mira variables were discovered in three different fields, two of which of about only one square degree (Maffei 1963).
It was believed that the principal reasons of this finding could be the following:1) Most of these stars have the maximum emission in the red-infrared spectral region. 2) The large decrease of the interstellar absorption in this spectral region allows to reach more distant stars i.e. a deeper exploration. 3) The use of the infrared combination I-N + RG5 allows to reach all the spectral region between 6800 and 8800 Å. In such a way the observed spectral range is not restricted to the continum only, but includes also the molecular bands. The formation of these bands is very sensitive to small variations of the temperature wich, in these stars, is very close to the dissociation temperature of the molecular compounds present in the atmospheres. For this reason a small decrease of temperature, which causes an insignifiant variation of the emittence, can be cause of strong absorptions of the radiation emitted by a star in the spectral range we observe. This phaenomenon induce large variations of the magnitude, especially in the LPVs of the later spectral types, and favours the discovery of the redder and cooler variables of this type. Starting from 1959 red and infrared plates were collected more systematically with different instruments. These observations showed that the infrared light curves appeared more regular that these formerly obtained: visually or in blue light. Therefore were obtained for the LPVs the first phase light curves (Maffei,1966) which in most of the cases appeared very regular over some period.The number of the new infrared Mira type stars reached 15 and the quoted explication of the phaenomenon was given (Maffei,1967). After this preliminary but very suggesting result, a finalized reserch on LPVs in selected fields was started in the summer 1967, when the Schmidt telescope 90/67 cm of the Asiago Observatory begin to operate. In the 1970's some other astronomers started the infrared observation of the LPVs, e.g. Rosino et al. (1976 and 1978) in the Galaxy. The researches were also extended throught observations in the infrared "colours" J, H and K. Nevertheless the finalized research started in 1967 was different from those cited above, in: the method, the purposes, and the results obtained. The principal particular characteristics of these observations are the following: a) In order to obtain a good sensitiveness to the variability by means of the large change of the magnitude due to the presence and opacity of the absorbtion bands with small variations of the temperature, the whole spectral range from 6800 to 8800 Å, was covered. b) As a rule, together with every infrared plate a blue one was effected. This was decided with two purposes. The first was to estimate the increasing of the variable stars observed in the photographic infrared in comparison with a similar survey obtained with blue plates. The second one was to compare the behaviour of the light curves for the variables wich appeared in both colours. c) To check the constancy or not of the shape of the light curves and of the periods, the observations were repeated various time during the space of some 30 years: the longer period covered till now in the infrared. In the affirmative it was possible to fix with accuracy the value of the period; if not, it was examinated the type of the change and an explanation was attempted. d) With the aim to reach results of statistical and evolutionary interest, four fields were chosen, all near the galactic plane but at different longitudes. The fields selected were the following: one centered at (1900.0) R.A. = 18h 14m; DEC = -1450', including M16 and M17 (l = 16°) a second centered at (1950.0) R.A. = 2h 40m DEC = 60°30'(l = 135°) and two centered on the stars gamma Cygni (l = 78°) and gamma Cassiopeae (l = 124°) respectively. Of course the research conducted also to the discovery of variable stars different from the LPVs one. Three lists relative to the first, the second and the third field has been published by Maffei(1975),Gasperoni,Maffei,Tosti (1991) and Maffei (1977).On the whole 312 variables were numbered with the prefix M (Maffei), but some previsional number is lacking because the corresponding variable, at first accepted, was eliminated for various reasons, i.e.: serious doubts on the variability, missidentification of some known variables, etc. The present volume regards the variables in the field containing M16 - M17.It gives the updated characteristics of all the variables, in particular the coordinates and the finding charts. The Atlas shows the light curves of 176 LPVs observed in the field.They represent the majority of the infrared variables found in the three fields till now scanned.The blue and infrared plates examined are given in Tab.1 and Tab.2 respectively. Each one contain the dates and the reference to the names of the observers. The results obtained for all the variables are given in Tab.3. In Figures 1 we give the finding charts, reproduced from a good infrared plate,for all the variables reported in Maffei (1975) till now not published. The Atlas reports phase light curves for 176 LPVs given in Table 3. The representative points of each observational period are expressed with a different symbolism. When the variable magnitude was below the limiting one of the plate no representative point or other indication has been reported in the light curve.
OBSERVATIONSAll photographic plates examinated was discontinuously collected with the two Schmidt telescopes of the Asiago Observatory and with the Schmidt telescope of the Catania Astrophysical Observatory from the year 1961 to 1991. 103a-O without or with filter GG13 and hypersensitized with NH3 or pre-flashed I-N + RG5 were used. Date, JD, exposure time, emulsion/filter combination and the observer for each plate are listed in Table 1 and Table 2. The variable search was carried out with the blink microscope plate comparator of the Asiago Observatory. This operation was based mostly on four pairs of infrared plates selected from 50 plates. A comparable number of plate pairs obtained in blue ligth were also blinked. Sequences of blue comparison stars were obtained trasferring the B sequence in the cluster M16 (Walker 1961) with the Askania photometer of the Monte Porzio (Roma) Observatory. The infrared magnitudes were derived from the Walker's B and V sequence by means of the trasformation formulae given by Allen (1955).Therefore the value of infrared magnitudes must be considered as provisional.Sequences of infrared comparison stars were then obtained with the same procedure used for the blue magnitudes. From these sequences, visually estimated comparison stars were obtained around each variable and then used to estime their blue and infrared magnitudes on all the plates. The errors turned out to be 0m.1–0m.3 depending: from telescope, from seeing and from the magnitude of the variable.Possible uncertainties on the zero point can affect the absolute value of the magnitudes, but not the relative amplitude of luminosity variations, of interest here.
DATA REDUCTIONS AND RESULTSIn the first phase the attribution of the variability type was made by examining the light curve pieces. For all variables with incert variability type and to find or to improve the periods of all LPVs, spectral analysis of the light curve was performed with a computer program, reported by Press & Rybicki (1989), implementing the method developed by Scargle (1982). The most statistically significative periods present in the periodogram of each variable were then used to build the phase light curves. After a carefully discussion, the period minimizing the dispersions of the points in the phase light curve of the variable was adopted. All the variables, but five, were visible only on the infrared plates. Table 3 shows the results obtained for all variables in the field. Col.(1) and Col.(2) give the provisional name in Maffei (1975) and the name in the CGVS (General Catalog of Variable Stars, Fifth Edition) or NSVC (New Suspected Variable Stars Catalog), respectivelly; Col.(3) and Col.(4) give the positions; Col.(5) and Col.(6) give the observed infrared magnitude at the maximum and at the minimum; when blue estimations were possible the same data are reported below the infrared ones; Col.(7) gives the variability type; Col.(8) and Col.(9) give the period, in days, and the heliocentrical julian day Epoch referred to a maximum; Col.(10) gives the total number of the observations for each variable. The total number of observations in Col.(10) changes from a variable to another because the fields observed with the various telescopes show some differences according to the focal lenght of the instrument, the size of the plates and their orientation. Table 3 reports too improved data for the known LPVs discovered by Maffei (1963) and Impallomeni,Maffei (1965). For these variables the provisional numbers and the coordinates are not given; the number of observations starts with the year 1957.
THE ELECTRONIC ATLAS
List of Blue plates
List of Infrared plates
Principal elements of the new variables in the field of M16-M17
*M001-M020* M021-M040* M041-M060* M061-M080* M081-M100* M101-M120* M121-M140* M141-M160* M161-M180* M181-M200* M201-M208* Already known variables*
M001-M024* M025-M049* M050-M073* M074-M097* M098-M122* M123-M148* M149-M177* M178-M202* M203-M208*