\/ Oh/Ir Stars in the Galaxy

$\/ Oh/Ir Stars in the Galaxy$ \/ OH/IR STARS IN THE GALAXY B. BAUD OH/IR STARS IN THE GALAXY OH/IR STARS IN THE GALAXY PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE WISKUNDE EN NATUURWETENSCHAPPEN AAN DE RIJKSUNIVERSITEIT TE LEIDEN, OP GEZAG VAN DE RECTOR MAGNIFICUS DR. D.J. KUENEN, HOOGLERAAR IN DE FACULTEIT DER WISKUNDE EN NATUURWETENSCHAPPEN, VOLGENS BESLUIT VAN HET COLLEGE VAN DEKANEN TE VERDEDIGEN OP WOENSDAG 12 APRIL 1978 TE KLOKKE 15.15 UUR DOOR BOUDEWIJN BAUD GEBOREN TE DJAKARTA IN 1948 STERREWACHT LEIDEN 1978 elve/laborvlncit - lelden PROMOTOR: DR. HJ. HABING Aan mijn moeder Aan Alberl ine CONTENTS Summary CHAPTER I A SYSTEMATIC SEARCH AT 1612 MHZ FOR OH MASER SOURCES I SURVEYS NEAR THE GALACTIC CENTRE I Introduction 2 II Equipment and survey observations 2 III Analysis and confirmatory observations 5 IV The Type II OH/IR sources 6 V Selection effects 15 V.1 Velocity coverage and spectral resolution 15 V.2 Variability 16 CHAPTER II A SYSTEMATIC SEARCH AT 1612 MHZ FOR OH MASER SOURCES II A LARGE-SCALE SURVEY BETWEEN 10 i 15° and \b\ 17 I Introduction 17 II Equipment 17 11.1 The survey 17 11.2 Confirmation observations 20 III Results 20 111.1 Analysis of the survey observations 20 111.2 Confirmation of Type II OH/IR sources 21 111.3 The new Type II OH/IR sources 21 111.4 Known Type II OH/IR sources 28 111.5 Unclassified sources 33 IV Discussion 35 CHAPTER III GALACTIC DISTRIBUTION AND EMISSION PROPERTIES OF OH/IR SOURCES 38 I Introduction 38 II Observations and basic data 39 III Distribution of OH/IR sources in velocity,longitude and latitude 43 111.1 Velocity distribution 43 111.2 Longitude distribution 47 111.3 Latitude distribution 48 IV Luminosity function 49 IV.I Spectral resolution 49 IV.2 Mean peak flux density of OH/ItC sources 50 IV.3 Luminosity distribution 51 V Comparison with the identified Type II OH/IR sources 53 VI The galactic distribution of OH/IR sources 54 VI. 1 Distribution in log S"& ,. and I 54 VI.2 The radial velocity distribution 57 VII Discussion 59 VII.1 Stellar dynamics,age of the objects 59 VII.I.I Velocity dispersion perpendicular to the plane 6Ü VII.1.2 Association with stellar objects, age determination 60 VII.2 Density Distribution 62 VII.2.1 Comparison with the gas distribution 62 VII.2.2 OH/IR sources and planetary nebulae 62 VII.2.3 Spiral arm structure 63 VII.3 Luminosities 63 VII.3.1 Influence of the UV radiation field on the occurrence of the maser emission 64 VII.3.2 Luminosity function and AV - distribution 65 VII.4 Conclusions 68 CHAPTER IV OH/IR SOURCES IN THE NUCLEAR REGION OF THE GALAXY 71 I Introduction 71 'I II Observations and selection effects 71 11.1 Observations 71 11.2 Selection effects within 2° from the Galactic Centre 73 III Distribution of OH/IR sources on the sky 74 IV Radial velocities 75 IV. 1 The sources at II < 5 76 IV.2 Difference in the distribution as a function of AV 78 V Emission properties and luminosities 79 VI Discussion 80 Samenvatting 82 Studie overzicht 84 Dankbetuiging 85 SUMMARY Around 1970 strong radio emission lines of the OH molecule were detected in the direction of Mira stars and M supergiants. These cool stars, in a late evolutionary stage, appear to loose a lot of mass. Due to this mass loss they form an expanding circumstellar shell, in which the OH line is emitted. Because of the special physical conditions in the shell this line emission is strongly amplified. These socalled OH/1R sources are characterized by their double peaked OH emission profile at a wavelength of 18 cm and by their often strong IR infrared emission. They offer a new and unexpected opportunity to study the spatial distribution and kinematical properties of stellar objects throughout the Galaxy with radio astronomical methods, i.e. unhindered by the interstellar extinction. This extinction has always been a severe limitation for optical studies. The first systematic radio surveys, in Sweden and Australia, revealed the presence of several new OH/IR sources. Their strong concentration to the galactic plane confirmed the suggestion that these objects were detectable at large distances from the Sun. However, none of these new sources could be identified optically; as a result, their association with Miras or H supergiants was uncertain. Also, many were found with large radial velocities tind they appeared to be very '• I luminous at radio wavelengths as compared to the nearby, optically identified sources. This thesis describes the radio astronomical observations that have led to the discovery of 71 additional OH/lR sources; in addition, an analysis is presented of the population characteristics of a large sample of sources. In Chapter I the observations around the nuclear part of the Galaxy are presented. They were done with the Dwingeloo 25 m telescope, the Effelsberg 100 m telescope (West Germany) and the NRAO 43 m telescope (U.S.A.); 38 new sources were found. Chapter II contains a description of the large-scale survey that covers the major part of the Galaxy, visible on the northern hemisphere. These observations, done with the Dwingeloo telescope, revealed the existence of 33 new OH/IR sources. Several previously known sources were also observed. The completeness of the Dwingeloo survey is studied and an atlas of OH line profiles of nearly all discussed objects is presented. In Chapter III I analyse the distribution and the velocities of 114 previously known and new OH/IR sources, found inside the area covered by the Dwingeloo survey. The parameter AV (the velu- city separation between the two emission peaks of the 18 cm line profile) appears to be a good criterion for a population classification. Sources with large AV are young objects, with small peculiar motions; they are probably associated with M supergiants. The sources with small AV show large peculair motions similar to the Miras. Model calculations indicate that both kinematical groups have a maximum density at 5 kpc from the Galactic Centre; the density decreases steep- ly on either side of this maximum. This characteristic density distribution suggests, that OH/IR sources are not the precursors of planetary nebulae, as has been suggested before. The large-AV sources appear to be concentrated in spiral arms. The similarity in spatial distributions to that of the gas in the galactic plane ',ets a lower limit to the age of this gas distribution. The radio luminosity function is consistent with an increase in the number of OH/IR sources with decreasing luminosity. This indicates an increase in the mass loss rate of the star in the course of its lifetime as an OH/IR source. In Chapter IV the results of Chapter I are analysed. It is shown that the central part of the Galaxy contains very few OH/IR sources, in agreement with the conclusions in Chapter III. The radial velocity distribution of these stars within 5° longitude is similar to that of the planetary nebulae. The spatial distribution, however, is more like the gas distribution. A comparison with the radial velocity distribution of the gas clouds is hampered by various selection effects, which are discussed in the first part of this chapter. The AV-distribution of sources within 90 pc from the Centre appears to be different from the AV-distribution of sources found elsewhere in the Galaxy, possibly indicating a higher m^tal abundance. The results of the present study, in particular the discovery of the correlation between AV and population characteristics, clearly demonstrate that a further study of OH/IR "Sources can provide an important contribution to our knowledge of the dynamical properties and the evolu- tion of the Galaxy. ... Miras consist of a continuum of kinematic families with overiapping period-frequency distrubutions. Smak, 1966 -J CHAPTER I: A SYSTEMATIC SEARCH AT 1612 MHZ FOR OH MASER SOURCES I Surveys near the Galactic Centre I Introduction Recent surveys of 1612 MHz OH emission have revealed a large number of optically unidenti- fied Type II OH/IR roaser sources in the plane of the Galaxy (e.g. Caswell and Haynes, 1975; Johansson et al., 1977a). Such sources snow strong emission in the 1612 »Hz transition of the ground rotational state of OH and sometimes weak main line emission (Wilson and Barrett, 1972). The characteristic double-peakod emission profile with a velocity separation, Av, between the two peaks of ^ 30 km s"' allows them to be easily identified. Because of tlie high brightness temperature of the maser emission the stronger ones may be seen throughout a large part of the Galaxy. The strong concentration of Type II OH/IR sources to the galactic plane, as found in systematic surveys, suggests that many are situated at large distances. The association of some nearby maser sources with long period (Mira) variables, considered to be intermediate population or Population II objects, and Che general increase in the number of Type II OH/IR sources towards smaller longitudes, as found by .Johansson et al. (1977b), indicate that there may be a concentration of such sources in the central part of the Galaxy. We have searched for 1612 MHz emission around the Galactic Centre (G.C.) using both the Dwingeloo 25 m telescope and the 100 m telescope at Effelsberg. The Dwingeloo observations covered an area of 3° x 4° around the G.C. with low sensitivity. With the 100 m telescope we surveyed with three times better sensitivity a narrow strip between í = 358 and Í.

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