Rapid Changes in the Light Curve of the Active, Late-Type Subgiant CF Octantis
B and V photometry of CF Octantis, obtained in 2006 July-2007 March,showed the spot-wave underwent a rapid change near HJD 2454040. Theamplitude changed from 0.12 mag to less than half this. Such a rapidchange has not been reported previously for this active subgiant.
|
Archival light curves from the Bamberg Sky Patrol: CF Octantis, 1964-76
We use the archive of the Bamberg Sky Patrol to obtain light curves ofthe active K subgiant CF Octantis (HD 196818) for the interval 1964-76.Digitized images of the field near CF Oct were obtained with a flat-bedscanner. Aperture photometry was performed of photo-positives of theseimages. Using a transformation to second order in plate magnitude, andfirst order in B-V, for nine field stars for each plate, the Bmagnitudes of CF Oct were obtained for just over 350 plates. Theestimated precision of an individual determination of the B magnitude ofCF Oct is 0.05 mag. Analysis of the resulting data reveals the known 20d rotational variation of this star, and shows the evolution of thelight curves from year to year. We obtain light curves with good phasecoverage for 1964 to 1969 inclusive, partial light curves for 1970 and1976, and a few data points from 1971. The amplitude of variation rangesfrom ~0.2 to ~0.4 mag. There is evidence that the characteristicrotation period of the star in the 1960s was slightly less than thatmeasured from photoelectric photometry in the 1980s.
|
An Historical Light Curve of CF Octantis from Digitised Images of the Bamberg Plate Archive
Digital images were made of Bamberg Observatory Sky Patrol plates of thefield surrounding the active-chromosphere star CF Octantis (HD 196818).These images, taken with an inexpensive camera, were analysed usingstandard aperture photometry techniques. Good agreement was found withcatalogued photographic magnitudes for stars in the range mpg~8.5 to ~10.5. The root-mean-square deviations in themeasured differences for non-variable field stars was found to be oforder 0.10mag, although a small number of larger differences occurred.For CF Oct, a period search of data from 1966 recovered the known 20dvariation due to starspot rotational modulation, with a range ofvariation of order 0.5mag photographic. For active-chromosphere starswith moderately large photometric variations (>~0.3mag), and moderateto long rotation periods (>~1 week), careful analysis of similarlyobtained digital copies of archive plates may provide valuable insightsinto historical actvity.
|
From the Solar Corona to Clusters of Galaxies: The Radio Astronomy of Bruce Slee
Owen Bruce Slee is one of the pioneers of Australian radio astronomy.During World War II he independently discovered solar radio emission,and, after joining the CSIRO Division of Radiophysics, used a successionof increasingly more sophisticated radio telescopes to examine anamazing variety of celestial objects and phenomena. These ranged fromthe solar corona and other targets in our solar system, to differenttypes of stars and the ISM in our Galaxy, and beyond to distant galaxiesand clusters of galaxies. Although long retired, Slee continues to carryout research, with emphasis on active stars and clusters of galaxies. Aquiet and unassuming man, Slee has spent more than half a century makingan important, wide-ranging contribution to astronomy, and his workdeserves to be more widely known.
|
New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
|
The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
|
The Monash Spectrograph simulation program
The Monash University Physics Department is constructing a spectrograph,to be attached to a 0.46-metre Cassegrainian telescope. To help futureusers of the spectrograph determine the operational capabilities of thespectrograph a PC-based software package was created. This programallows the user to simulate the response of the spectrograph to variousstellar types under differing observational constraints. We haveestimated the precision of measurements of stellar radial velocitieswith the spectrograph. The estimates are reasonable but are yet to becompared with real data.
|
Observations of active-chromosphere stars - IV. Photometry and spectroscopy of CF Octantis (HD 196818)
We present photometry and spectroscopy of the single, K-type,active-chromosphere star CF Octantis (=HD 196818), from 1982 to 1986.The photometric data show the 20-d rotation period seen previously, andalso demonstrate that the light curve is variable on time-scales of afew months, which we ascribe to variations in the properties ofstarspots. We postulate that a temporary dimming of minimum light seenin mid-1983 was a result of the appearance and later disappearance of arelatively small spot or spot group, which had a lifetime between 10 and24 months. The main spot appeared relatively stable during the intervalof our observations, with a lifetime of at least 3 years. Our radialvelocity data are consistent with the results of other work in that novariations above observational scatter are seen. Both the Ca K andHα emission are variable, with tentative evidence for the latterthat a lower emission level occurred near the phase of photometricmaximum, although the spectroscopic and photometric data were notcontemporaneous. The lithium abundance is found to be logN(Li)=0.45+/-0.15. The star may have evolved from a rapidly rotating A-Fmain-sequence star.
|
Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
|
Optical positions of radiostars. III
Precise optical positions, relative to either the Perth 70 or the AGK3Rcatalog have been obtained for 44 radiostars proposed to the Hipparcosobserving list. Precision levels between 0.17 and 0.28 arcsec wereobtained.
|
An astrometric catalogue of radio stars
The first part is presented of a radio star catalog encompassing 186objects whose selection was guided by the priority criteria of theHipparchos Input Catalogue Consortium. Since these criteria are wellsuited to the need for linkage of ground-based optical systems to radioreference frames, this first selection is also considered a suitablebase for the catalog. Seventeen categories of stellar parameters arefurnished for each of the stars, including optical and radio positions,optical and radio parallax, radial velocity, type of variability, andoptical structure.
|
Accurate positions for variable and suspected variable stars south of -67 deg
Improved positions for 950 confirmed variable and 368 suspected variablestars located south of -67 deg have been determined by measuring thefirst-epoch plates of the Yale-San Juan southern proper-motion survey.The new positions, referred to the system of the SRS, have an averagestandard error of 0.7 arcsec in both R.A. and Dec. Some of thedifferences with the coordinates quoted in the GCVS are as large as 1deg.
|
CF Octantis, an RS CVn-type variable with a prominent activity cycle
Photoelectric photometry has been obtained for CF Octantis on 39 nights.The object is a member of the RS CVn class and has a highly activechromosphere. A 20.15 + or - 0.06 d period has been found from the B andV magnitudes which is interpreted as the rotational modulation of thelight from a nonuniformly spotted star. The amplitude of this spot waveis observed to vary slowly between Delta V of about 0.1 to 0.3 mag,which may be evidence for an activity cycle of 9 + or - 3 years.
|
Stellar radio luminosity and stellar rotation
Two measurements of the microwave surface brightness of 63 activechromospheric stars, one related to surface flux and one related to fluxratio, are correlated with rotational parameters. Although both methodsare found to provide similar results, the surface flux method accountsfor more of the variance in the regression. The consistency of theobserved surface brightnesses of the sun and three BY-Dra-type flarestars with the present rotational relationships is considered.
|
A relation between radio luminosity and rotation for late-type stars
A relation is found between peak radio luminosities measured at 8 GHzand the rotational velocity of 51 late-type F, G, and K stars (includingthe sun). The sample includes both single stars and active components ofclose binary systems, with equatorial surface velocities ranging from 1to 100 km/s. A gyrosynchrotron source model originally developed toexplain solar microwave bursts could explain the relation. The mainparameter depending on rotation rate is the filling factor, i.e., thefraction of the stellar surface and corona occupied by intense magneticfields. As the rotation speed increases, the scale size of the coronalstructures emitting microwave gyrosynchrotron radiation increases, andthere is a corresponding increase in the area of the surface covered byintense starspot magnetic fields. However, the peak magnetic field ofthe starspots probably does not increase significantly above observedsunspot values.
|
Microwave spectra and polarization of active stars
Almost-simultaneous observations of 24 active-chromosphere stars withthe VLA and the Parkes 64-m telescope at 4.9, 8.4 and 15 GHz over a 24-hinterval on September 14, 1985 are reported. In all, 14 stars weredetected. There is a striking difference in the power outputs andbrightness temperatures of the higher-optical-luminosity stars (RS CVn,Algol-like and CAII emitters) and the low-luminosity dMe stars; this canbe explained by differences in the surface areas and rotation velocitiesof the two groups of stars. Two three-point spectra show a broad peak inthe region near 8.4 GHz, indicating a transition from optically thick tooptically thin radio sources. For another three stars, two-pointspectral indices are negative, indicating that their spectra peakprobably well below 15 GHz; two stars with two-point positive spectralindices have higher-frequency turnovers, although perhaps below 15 GHz.
|
A microwave survey of southern active stars
The results of a survey of 153 active-chromosphere stars made with theParkes 64-m telescope at 5.0/8.4 GHz from 1981 to 1987 are reported.Microwave emission was detected from 70 stars on at least one occasion,with the highest detection rate of 68 percent from the RS CVn group. TheCa II stars and Algol-type binaries yielded detection rates of 44 and 30percent, respectively. The maximum powers emitted by the stars at5.0/8.4 GHz ranged over five orders of magnitude, with a median of 2.5 x10 to the 10th W/Hz. The maximum brightness temperatures had a smallerrange of three orders of magnitude, with a median of 3.6 x 10 to the 9thK.
|
UBV(RI)c photometry for CaII emission stars. II - Observations at Mt. John University Observatory and at Mt. Stromlo
UBV(RI)c data are given for 17 stars with strong Ca II H and K emission.Techniques of observation and reduction are described briefly.
|
UBV(RI)c photometry for CaII emission stars. I - Observations at Sutherland
Extensive photometry in the UBV(RI)c system is given for 55 stars withstrong Ca II H and K emission. The UBV observations were made relativeto the standard stars of Lloyd Evans et al. (1983) while the (RI)cobservations were made relative to E-region standards over greaterangular distances. The data are plotted in phase diagrams.
|
Radial velocities of calcium emission stars. II - Observations at Mt. Stromlo and Siding Spring Observatories and at Mt. John University Observatory
Radial velocities are presented for 17 bright late-type stars withstrong emission cores in the Ca II H and K lines. Most of the stars arespectroscopic binaries.
|
Radial velocities of calcium emission stars. I - Observations at Sutherland
Radial velocities are presented for 53 bright late-type stars withemission in the H and K lines of Ca II, which is indicative of a highlevel of chromospheric activity. Most of the stars are spectroscopicbinaries, for which orbital elements are presented.
|
Outstanding stellar microwave flares in 1986
Bright microwave events that were first detected with the Parkes 64-mtelescope at 8.4 or 22 GHz from six active-chromosphere stars aredescribed. The stars were in a wide variety of evolutionary states,ranging from a single pre-main-sequence star, two RS CVn binaries, anAlgol, and two apparently single K giants. Their high brightnesstemperatures, positive spectral indices, and low polarization areconsistent with optically thick gyrosynchrotron emission from mildlyrelativistic electrons with average energies 0.5 to 3 MeV gyrating ininhomogeneous magnetic fields of 5 to 100 G.
|
Observations of active stars at 843 MHz
The Molonglo Observatory Synthesis Telescope is being used in ancontinuing program of observation of known Southern active stars. By May1987, a total of 41 stars had been observed, and 843-MHz quiescentemission, presumably associated with the star, had been detected in nineinstances. The emission from five of these stars has shown markedvariation on a time scale over 1 day.
|
Results of a Southern Radio Survey of Active Chromosphere Stars
Not Available
|
Rapidly rotating single late-type giants: New FK Comae stars?
A group of rapidly rotating single late-type giants was found fromsurveys of chromospherically active stars. These stars have V sin I'sranging from 6 to 46 km/sec, modest ultraviolet emission line fluxes,and strong H alpha absorption lines. Although certainlychromospherically active, their characteristics are much less extremethan those of FK Com and one or two other similar systems. One possibleexplanation for the newly identified systems is that they have evolvedfrom stars similar to FK Com. The chromospheric activity and rotation ofsingle giant stars like FK Com would be expected to decrease with timeas they do in single dwarfs. Alternatively, this newly identified groupmay have evolved from single rapidly rotating A, or early F stars.
|
Radial velocities of CaII emission stars - Photographic data
Photographically determined radial velocities are given for eighteenstars selected from lists of G, K and M stars with strong CaII emissionreversals. The results of observations of radial velocity standards arealso given.
|
The 67th Name-List of Variable Stars
Not Available
|
Halpha Variability in Two Southern RS CVn Candidates
Not Available
|
B and V Photometry of the Southern RS CVn Candidate HD 196818
Not Available
|
Local photometric standards for CaII emission stars
UBV data are given for 108 stars which are suitable local standards for52 stars with strong Ca2 emissions. An additional eight stars wererejected as possible standards because of suspected variability.
|
