COMMUNICATIONS MITTEILUNGEN
FROM THE DER
KONKOLY OBSERVATORY STERNWARTE
OF THE DER UNGARISCHEN AKADEMIE
HUNGARIAN ACADEMY OF SCIENCES DER WISSENSCHAFTEN
BUDAPEST-SVÁBHEGY

No. 101.

(Vol. 13, Part 1)

XZ Dra: observations spanning 70 years*

Szeidl B., Jurcsik J., Benkö J. M. and Bakos G. Á.

Konkoly Observatory of the Hungarian Academy of Sciences,
P.O. Box 67, H-1525 Budapest, Hungary
BUDAPEST, 2001

* The data are available in electronic form at http://www.konkoly.hu/Mitteilungen/Mitteilungen.html

ISBN 963 8361 395
HU ISSN 0238-2091
Felelös kiadó: Balázs Lajos

TABLE OF CONTENTS

INTRODUCTION

OBSERVATIONS

Visual Data

Photographic Material

Photoelectric Data

CCD Observations

Radial Velocity Measurements

TIMES AND MAGNITUDES OF LIGHT MAXIMA AND O-C VALUES

ACKNOWLEDGEMENTS

REFERENCES

XZ DRA: OBSERVATIONS SPANNING 70 YEARS

During the past seventy years a great number of observations of XZ Dra have been collected at Konkoly Observatory. Here we publish 344 visual observations made by L. Detre in the years 1932 and 1934, 4440 photographic magnitudes obtained in the years 1935-1957, 40 white light photoelectric data from 1957, and 6066 UBV(RI)C observations made between 1958-1988, and further CCD observations from 2001. All published observations are reviewed and the times of light maximum of XZ Dra are collected as well. New normal maxima have been determined in order to follow the star's long-term period changes.

Key words: Stars - variable: RR Lyrae; stars - individual: XZ Dra - Techniques: photometric - Techniques: radial velocities

INTRODUCTION

The variability of XZ Dra (AN 433.1928, BD+64o1332, GSC 04225-00305, HIP 94134, {alpha}2000=19h 9m 42. s6, {delta}2000=64o 51' 32'') was discovered by Schneller (1929) on Babelsberg plates. Soon after its discovery, Beyer (1934) made the first thorough investigation of the star's light variation. He confirmed the RR Lyrae type variability suspected by the discoverer (Schneller, 1931) and already took notice of the large scatter of observations around light maximum. Balázs & Detre (1941) based on their preliminary study demonstrated that the star had, indeed, strong light curve variation with a period of 76 days.

Since the early studies a great number of observations have been made on XZ Dra by different observers. In this paper we publish the visual, photographic, photoelectric and CCD observations obtained at Konkoly Observatory during the past 70 years and carry out a rigorous treatment and elaboration of all published observations.

Here we also publish radial velocity measurements taken at the Dominion Astrophysical Observatory (DAO, Canada) in 1971.

OBSERVATIONS

Visual Data

During the years 1931-1937 visual observations of a number of RR Lyrae stars were carried out by L. Detre at Konkoly Observatory. The 20cm Heyde-Refractor equipped with a Graff-type wedge photometer was used. A detailed description of the instrument and the observational procedure adopted is given in the paper of Detre & Lassovszky (1939).

One of Detre's program stars was XZ Dra, which was extensively observed in the years 1932-1934: 344 visual estimates were obtained during 20 nights. Detre's original visual observations are listed in Table 1. The comparison stars he used were BD+64o1331 (=TYC 4225_0931_1), TYC 4225_1323_1 and BD+64o1335 (=TYC 4225_0331_1) with visual magnitudes Vvis=8.61, 10.27 and 10.61, respectively.

Table 1. Visual observations of XZ Dra

101-t1.txt

Photographic Material

A new 16cm astrograph was installed and extensive photographic observational program was started at Konkoly Observatory in 1934 in order to study the period changes and the light curve variations of RR Lyrae stars. The large focal ratio (f/14) made the telescope very suitable to series of exposures, usually several dozens of exposures were taken on each plate (for detailed description of the instrument and the observational method see Balázs & Detre, 1938).

The photographic observations of XZ Dra was commenced by Júlia Balázs in 1936 and carried out by the staff of the observatory. Until 1945 (between JD 2428356 and 2431708) Eastman 40 photographic plates were used. Later, between 1951 and 1957 (JD 2433896 and 2436142), aged Agfa Astro and Guilleminot Superfulgur plates were only available, therefore these photographic observations had inferior quality.

Throughout the photographic observations of XZ Dra three minutes exposure time was applied. Altogether 4440 utilizable exposures were taken on 188 plates during 100 nights.

Table 2. Comparison stars for photographic measurements



Tycho number {alpha}2000 {delta}2000 BT Bpg

 h m s o ' '' [mag] [mag]





4224_0047_1 19 04 14.729 +65 12 19.26 9.271
4224_0467_1 19 06 02.748 +65 26 30.43 11.073
4224_0561_1 19 04 42.387 +64 53 17.32 10.404
4224_0631_1 19 04 19.948 +65 25 45.13 11.455
4224_0695_1 19 05 50.344 +65 36 10.57 10.322
4224_0849_1 19 04 07.779 +65 02 50.19 9.948
4224_1255_1 19 04 44.353 +65 01 45.92 9.335
4224_1781_1 19 05 38.254 +64 27 13.48 9.887
4224_2009_1 19 08 08.749 +64 22 56.27 9.564
4225_0056_1 19 12 34.496 +65 22 03.77 10.181 9.80
4225_0117_1 19 11 16.524 +65 19 48.15 11.062
4225_0141_1 19 16 57.381 +65 10 27.28 10.448
4225_0197_1 19 16 42.116 +65 33 09.75 10.190
4225_0404_1 19 16 22.759 +65 07 11.05 11.037
4225_0419_1 19 09 56.270 +65 23 06.02 10.132 9.63
4225_0622_1 19 10 53.788 +65 31 27.22 10.692 10.39
4225_0742_1 19 11 40.660 +64 52 33.00 11.125
4225_0931_1 19 09 14.637 +65 11 45.50 9.153 8.74
4225_1010_1 19 15 42.572 +65 07 49.02 9.895
4225_1029_1 19 09 33.376 +65 19 59.44 10.879 10.51
4225_1096_1 19 11 10.372 +65 14 44.35 11.361
4225_1323_1 19 10 50.282 +64 51 15.61 11.157 10.85
4229_0479_1 19 13 27.931 +65 41 40.94 10.421

The plates were digitized by an Umax PowerLook 3000 commercial scanner in 2001, making 1200 dpi 8 bits FITS format output images. This resolution yielded stellar images with 3-5 pixels full width at half maximum (FWHM).

Aperture photometry (IRAF*/DAOPHOT/PHOT) of 23 or less comparison stars and the variable were derived. The comparison stars, covering about 1.5 magnitude range, were chosen among the stars given in Table 2. All those were used which could be measured reliable. Typically at least 15 comparison stars determined the magnitudes of the variable during each observational run.

* IRAF IS DISTRIBUTED BY THE NOAO, OPERATED BY THE ASSOCIATION OF UNIVERSITIES FOR RESEARCH IN ASTRONOMY INC., UNDER CONTRACT WITH THE NSF.

Tycho BT magnitudes of the comparison stars were adopted (ESA, 1997, Høg et al., 2000) for transforming the intensities (the plate densities) to magnitudes by fitting third or fourth order polynomials. Six of these comparison stars were originally used to define a photographic sequence. Their photographic Bpg magnitudes were determined from two plates of the North Polar Sequence taken in 1936 and 1939. These Bpg values (also given in Table 2) define a 0. m35 brighter magnitude scale than the BT magnitudes (if TYC 4225_0419_1 with its discrepant 0. m5 magnitude difference is omitted). Thus the final magnitudes were shifted by 0. m35 in order to match the photographic B scale of the early photographic studies. The errors of the photographic measurements range between 0. m05 and 0. m20 depending mostly on the quality of the plate material and its development.

The photographic observations are given in Table 3.

Table 3. Photographic observations of XZ Dra

101-t3.txt

Photoelectric Data

Table 4. Telescope constants for photoelectric data.





Year {epsilon} µ {psi} r i Telescope*







1958 -0.12 0.91


NC
1961 -0.15 1.10


NC
1966 -0.13 1.17


NC
1969 -0.065 1.10 1.14

NC
1970 -0.10 1.11 1.03

NC
1971 -0.12 1.13


NC
1972 -0.105 1.115 1.065

NC
1973 -0.11 1.11 1.065

NC
1974 -0.16 1.07 1.07

NC
1974 0.08 0.93 1.21

C**
1975 -0.16 1.07 1.07

NC
1976 -0.17 1.08


NC
1978 -0.165 1.075


NC
1983 -0.06 1.20 0.86

C
1984 -0.06 1.20 0.86

C
1985 -0.128 1.187 1.106

C
1988 -0.029 0.973 1.109 1.112 1.094 RCC

*NC = 60cm Newton-Cassegrain telescope, Budapest,

C = 50cm Cassegrain telescope, Piszkéstetö Mountain Station,

RCC = 1m Ritchey-Chrétien-Coudé telescope, Piszkéstetö Mountain Station.

**On JD = 2442279.

The first photoelectric measurements of XZ Dra were made at Konkoly Observatory during the night April 27/28 1957 (JD=2435956). At the Newtonian focus of the 60cm Newton-Cassegrain telescope an RCA 1P21 photomultiplier was employed without any filter. During this night BD+64o1327 (=TYC 4224_0707_1) was used as a comparison star.

From 1958 the star was observed through the conventional filters of the UBV system. After the aluminization of the mirror of the 60cm telescope in 1963 an EMI 9052 B tube was used. Since 1972 photoelectric observations were also made close to the UBV system with the 50cm Cassegrain telescope at Konkoly Observatory's mountain station at Piszkéstetö. At this location we used an integrating photometer equipped with an unrefrigerated EMI 9058 QB photomultiplier.

In 1988 on three nights XZ Dra was also measured by the Konkoly Observatory's 1m RCC telescope equipped with a thermoelectrically cooled UBV(RI)C photon counting photometer furnished with an EMI 9659 QB tube.

Throughout our photoelectric photometry GSC 04225-01323 (=TYC 4225_1323_1) was chosen as a comparison star (except the first night, see above). The brightness and colours of this star were given by Sturch (1966) as:

V=10.493 B-V=0.572 U-B=0.041.
The photoelectric observations have been transformed into the UBV system in the traditional way (see e.g. Hardie, 1962). The actual transformation coefficients for different epochs and telescopes are shown in Table 4. The accuracy of the photoelectric observations depending on the sky conditions ranged typically between 0. m002-0. m010 in B, V, RC and 0. m01-0. m02 in U and IC. As the mirror has been aluminized only since 1963 the transformation of the early observations (before 1963) into the UBV system is rather uncertain.

Altogether 6106 photoelectric data were obtained. In Tables 5.a-f all these measurements are listed in the sense variable-comparison for unfiltered, U, B, V, RC and I C measurements, respectively.

Table 5.a Photoelectric differential observations of XZ Dra without filter

101-t5a.txt

Table 5.b Photoelectric differential U observations of XZ Dra

101-t5b.txt

Table 5.c Photoelectric differential B observations of XZ Dra

101-t5c.txt

Table 5.d Photoelectric differential V observations of XZ Dra

101-t5d.txt

Table 5.e Photoelectric differential RC observations of XZ Dra

101-t5e.txt

Table 5.f Photoelectric differential IC observations of XZ Dra

101-t5f.txt

CCD Observations

CCD observations were obtained on 5 nights in July-August 2001, with the 60/90/180cm Schmidt telescope at Piszkéstetö Mountain Station of the Konkoly Observatory. A Photometrics CCD camera with a thermoelectrically cooled Kodak KAF-1600 1024×1536 chip yielded a 19'×28' field of view, with 1. ''0/pixel resolution. Standard Johnson V filter was used and the total amount of frames were 405. The typical exposition time was between 30 and 15 sec. To the calibration process of the camera and other technical details see Bakos (2000).

Since there were no pattern in the bias images, we applied a simple zero offset value subtraction to the frames determined from their overscan area. Common flat field corrections were carried out using calibration images taken of the sky during twilight. Other corrections were unnecessary.

Finally, aperture photometry (IRAF/DAOPHOT/PHOT) was accomplished to obtain instrumental magnitudes. The relative magnitudes Var-Comp (=TYC 4225_1323_1, the same comparison star used for photoelectric observations) are shown in Table 6. We note here, that the colour system satisfyingly realizes the international one without any transformation (see Benkö et al., 2001). The accuracy of the individual points is estimated to be ±0. m002 from the IRAF noise statistics.

On 8th July 134 frames of the field of XZ Dra were taken with 60 sec exposition time yielding good enough signal-to-noise ratio to measure the fainter stars as well. This enabled us to check the constancy of the neighbouring stars. The ISIS-2.1 program package was applied as a realization of the Image Subtraction Method (Alard & Lupton 1998, Alard 2000) to find possible variable stars. Except XZ Dra, we could not detect variability of any of the stars shown in Fig 1. during this night. Based on the other nights' observations longer term variability of the brighter objects were not found either.


60 sec exposition V filter CCD frame taken with the Schmidt telescope. XZ Dra and the photoelectric/CCD comparison TYC 4225_1323_1 are marked. North is up, east is to the left.

On two nights in July, 2001 we observed XZ Dra with the second small Hungarian Automatic Telescope (HAT-2) during its test run phase. About the HAT project see Bakos (2001a,b) The fully automatized observatory was equipped with a 0.1m diameter, f/10 Maksutov MT0 lens, and an amateur Meade Pictor 416xte CCD camera (512×768 9µm pixels). The system yielded a resolution of 1. ''85 per pixel, and FOV of ~16'×24'. 47 frames of the field of XZ Dra were taken with I filter. After standard bias, dark and skyflat calibration, aperture photometry was performed. The relative instrumental i magnitudes are given in Table 7 (XZ Dra-TYC 4225_1323_1).

Table 6. Differential CCD V observations of XZ Dra

101-t6.txt

Table 7. Differential CCD i observations of XZ Dra

101-t7.txt

Radial Velocity Measurements

During eight nights in 1971 XZ Dra was observed spectroscopically with the 2131 spectrograph at the Cassegrain focus of the 72 inch telescope of the DAO (Dominion Astrophysical Observatory, Canada). 26 spectrograms were obtained in order to determine the radial velocity variation. The spectra, at the dispersion of 60 Å/mm, were taken on baked Eastman Kodak IIaO emulsion. The lines measured and the wavelengths accepted for them were chosen from those adopted at DAO. Standard-velocity stars were observed on some of the nights that spectrograms were obtained of XZ Dra. Velocities of these stars, measured in the adopted wavelength system, showed a satisfactory freedom from systematic errors.

Values of radial velocities of XZ Dra are presented in Table 8 (Col. 2,6). In the table the Julian date of the middle of exposures (Col. 1,5), the internal mean error of the velocities (Col. 3,7) and the number of lines measured (Col. 4,8) are also given.

Table 8. Radial velocities of XZ Dra

101-t8.txt

TIMES AND MAGNITUDES OF LIGHT MAXIMA AND O-C VALUES

The times and magnitudes of light maxima from the Konkoly photographic (Table 9.a), photoelectric B, V and CCD (Table 9.b) data were determined from polynomial fits to the data around maxima. The maximum magnitudes were obtain by adopting Struch's (1966) V=10.493, B=11.065 magnitudes for the comparison star.

Table 9.a Times and magnitudes of photographic maxima

Table 9.b Times and magnitudes of photoelectric maxima

101-t9.txt

In Table 10.a and 10.b individual and normal maxima and O-C values derived from all the available published measurements are listed. Both professional and amateur observations are regarded, but only those which have publicly available references. The GEOS RR Lyrae database http://www.upv.es/geos/ lists some further unpublished amateur (visual) observations which we have not included in our compilation. From multicolor observations B data were considered. The only published measurements of XZ Dra that is not included in either of the tables is that of Butler et al. (1982). These observations cover the linear part of the descending branch on two successive nights and no unambiguous normal maxima could have been extrapolated from these data.

In each cases when not only maxima timings but the original observations were also given, individual maxima times and normal maxima were independently determined. In Tab 10.a-b these values are given instead of the originally published data. This causes very small if any difference in the cases of individual maxima, but may lead to significantly different normal values.

Individual maxima were determined by polynomial fitting of the data around maximum values with the exception of Batyrev's (1955) measurements, where the times of the brightest magnitudes were taken as estimates of O-C values. Wenske (1982) gave the times of the mean magnitudes on the ascending branch instead of the maxima. These values were shifted by 0. d027 (the mean value of the time difference between the timings of the mean magnitude on the ascending branch and the maximum [{Delta}T = {overline}Tmax-Tmed] determined from the Budapest photoelectric B observations) to estimate the times of maxima. It has to be emphasized, however, that these transformed values are not true maxima times, since depending on the Blazhko phase, the actual value of {Delta}T can differ by ±0. d015 from 0. d027.

An 8th order Fourier fit to the Budapest photographic observations between HJD 2430433 and 2431708 were regarded as a basic normal light curve using P=0. d4764955 period. These data cover the whole pulsational period well, and in this interval no definite period change has been observed. The normal maximum of these data was determined to be at HJD=2431244.383, at the mid-time of the observations. This normal epoch corresponds thus to a calculated maximum and not to real observations. All the O-C values were accordingly calculated by using the ephemeris:

MaxHJD = 2431244.383 + 0.d4764955 ×E

In Table 10.a all the individual maxima timings (Col. 1,5) found in the literature and given in the present paper, O-C values (Col. 2,6), type of detector (Col. 3,7), and references (Col. 4,8) are given.

Normal maxima were determined from measurements of one or two observing season(s) but there were also some cases when only a few nights' (sometimes consecutive) observations defined the normal values. The most deviant points were omitted from each data set. The observations were folded by the period given in Eq. (1) and were fitted both vertically (magnitude-shifts to compensate the differences of the different photometric systems) and horizontally (phase-shifts which can be regarded as O-C value) to the basic light curve applying a standard least squares method. In this way we could eliminate the problem of the different magnitude scales, and the effect of the different shapes of the light curves in different wavelengths. The normal maximum times were then calculated at the mean Julian date of the observations by using these overall phase shift values and ephemeris Eq. (1).

In Table 10.b normal maxima timings (Col. 1), O-C values (Col. 2), the first and the last dates of the observations (Col. 3), number of measurements (Col. 4), type of detector (Col. 5), and references (Col. 6) are listed. The references of those data which were taken from the literature and not directly determined from the original observations (due to the lack of published data) are denoted by asterisks both in Table 10.a and 10.b. Maximum times which seems to give erroneous O-C values are in parentheses, independently whether or not they were published as uncertain values.

Table 10.a O-C values calculated from observed maxima

101-t10a.txt

Table 10.b O-C values calculated from normal points

101-t10b.txt

ACKNOWLEDGEMENTS

The extended observational material of XZ Dra has not been congregated without the inspiring interest of Prof. László Detre, the late director of the Konkoly Observatory about Blazhko type RR Lyrae stars. We would like to thank the present and former staff members of Konkoly Observatory especially to Drs. K. Oláh and L. Szabados for taking part in the observations. The continuous help of Oliver Rauch in operating the UMAX PowerLook 3000 scanner under Linux is also acknowledged. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and also that of the GEOS RR Lyrae Database. This work has been partly supported by OTKA grants T30954 and T30955.

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