Active Chromosphere Stars Research Articles

An Historical Light Curve of CF Octantis from Digitised Images of the Bamberg Plate Archive

AbstractDigital images were made of Bamberg Observatory Sky Patrol plates of the field surrounding the active-chromosphere star CF Octantis (HD 196818). These images, taken with an inexpensive camera, were analysed using standard aperture photometry techniques. Good agreement was found with catalogued photographic magnitudes for stars in the range mpg ∼8.5 to ∼10.5. The root-mean-square deviations in the measured differences for non-variable field stars was found to be of order 0.10 mag, although a small number of larger differences occurred. For CF Oct, a period search of data from 1966 recovered the known 20 d variation due to starspot rotational modulation, with a range of variation of order 0.5 mag photographic. For active-chromosphere stars with moderately large photometric variations (>∼0.3 mag), and moderate to long rotation periods (>∼1 week), careful analysis of similarly obtained digital copies of archive plates may provide valuable insights into historical actvity.

Library of Medium‐Resolution Fiber Optic Echelle Spectra of F, G, K, and M Field Dwarfs to Giant Stars

We present a library of Penn State Fiber Optic Echelle (FOE) observations of a sample of field stars with spectral types F to M and luminosity classes V to I. The spectral coverage is from 3800 to 10000 Å with a nominal resolving power of 12,000. These spectra include many of the spectral lines most widely used as optical and near-infrared indicators of chromospheric activity such as the Balmer lines (Hα to H), Ca II H & K, the Mg I b triplet, Na I D1, D2, He I D3, and Ca II IRT lines. There are also a large number of photospheric lines, which can also be affected by chromospheric activity, and temperature-sensitive photospheric features such as TiO bands. The spectra have been compiled with the goal of providing a set of standards observed at medium resolution. We have extensively used such data for the study of active chromosphere stars by applying a spectral subtraction technique. However, the data set presented here can also be utilized in a wide variety of ways ranging from radial velocity templates to study of variable stars and stellar population synthesis. This library can also be used for spectral classification purposes and determination of atmospheric parameters (Teff, log g, [Fe/H]). A digital version of all the fully reduced spectra is available via ftp and the World Wide Web (WWW) in FITS format.

Libraries of High and Mid-Resolution Spectra of F, G, K, and M Field Stars

I have compiled here the three libraries of high and mid-resolution optical spectra of late-type stars I have recently published. The libraries include F, G, K and M field stars, from dwarfs to giants. The spectral coverage is from 3800 to 1000 A, with spectral resolution ranging from 0.09 to 3.0 A. These spectra include many of the spectral lines most widely used as optical and near-infrared indicators of chromospheric activity. The spectra have been obtained with the aim of providing a library of high and mid-resolution spectra to be used in the study of active chromosphere stars by applying a spectral subtraction technique. However, the data set presented here can also be utilized in a wide variety of ways. A digital version of all the fully reduced spectra is available via FTP and the World Wide Web (WWW) in FITS format.

Optical identifications of southern rosat EUV and soft X-ray sources

This paper discusses the first all-sky surveys of cosmic extreme ultra-violet and soft X-ray sources, discovered by ROSAT. Details of the surveys are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification programs are described, and the major results summarized. We then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV observations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified as hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe and Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as “standard candles” in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. We finish with a discussion of the on-going ROSAT Galactic Plane Survey (RGPS) identification program.

TheROSAT survey of extreme UV sources: a new window on the universe

This paper discusses the first all-sky suvey of cosmic extreme ultra-violet sources, discovered by theROSAT Wide Field Camera. Details of the instrument and the survey are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification program is described, and the major results summarized. I then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV obserevations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified with hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe & Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as “standard candles” in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. I conclude the paper with a discussion of the new magnetic cataclysmic variables discovered at SAAO, which have been the subject of intensive follow-up observations.

Evidence that the active stars HD 102077 (V 838 Cen) and HD 139084 (V 343 Nor) are members of the Local Association

The space motions, rotational v sin i values and lithium abundances are determined for the two southern-hemisphere active-chromosphere stars HD 102077 and 139084. The results imply that both stars are likely members of the Pleiades group as defined by Eggen (V=−25 km s −1 ). This indicates an age of 7−10×10 7 yr, consistent with our derived lithium abundance. Photometric data are also presented and compared with previous observations, to provide evidence for changes in the spot-induced light curves for each star

Studies of active chromosphere stars at Monash University

Studies of active chromosphere stars at Monash University

Photometry of 50 suspected variable stars

Fifty stars have been chosen as suspected variable stars and analyzed for variability. A large portion of this sample are stars that are either proved active chromosphere stars or are candidates for such activity. The photometric data base consists of differential V measurements of the Vanderbilt 16 inch (41 cm) automatic photoelectric telescope and 25 observers at 26 observatories worldwide. Published photometric data have also been utilized, with proper adjustments made to ensure that all magnitudes are differential. Searches for photometric period, amplitudes, and times of minimum light showed 68 percent of the sample to be photometrically variable with periods found for 34. Two stars were deemed norvariable for the period of observation. Conclusive statements could not be made concerning the photometric variability of the 14 remaining stars.

Observations of active-chromosphere stars. III - Photometry of PZ Tel 1979-1985

Les observations photometriques de l'etoile a rotation rapide TZ Tel sont presentees. Les courbes de lumiere sont analysees. La relation entre les variations des courbes de lumiere et les taches stellaires est etudiee. La rotation diffferentielle de TZ Tel est etablie

Very active chromosphere stars and the age, chromosphere activity relation

The wide range of objects in stellar superclusters and groups near the sun is used in an attempt to define normal stellar chromospheric behavior, with which 'very active chromosphere' (VAC) stars can be isolated. Two chromosphere signatures, soft X-ray flux, and Mg II h and k line flux, are discussed. The X-ray flux is of limited use in this regard because of a wide range of variation with time, although the maximum values are well defined for stars of a given age. The VAC stars, which have an excessive X-ray flux, also have an excessive bolometric flux. The ratio of the Mg II h and k (and Ca II H and K) line emission and the total bolometric emission, R(hk) decreases with bolometric luminosity. The rate of decrease in log R(hk), 0.175 (35.00 - log L sub BOL), is not very dependent on stellar age, but the zero point at log L sub BOL = 35.00 decades varies from log R(hk) = -4.45 decades for main-sequence stars in the Pleiades supercluster (200-300 million yr) to - 4.91 decades for those in the HR 1614 group (8 billion yr). The age dependence of R(hk) is clearly demonstrated not only by themore » supercluster and group stars but by the 'Mount Wilson sample' of objects that have been observed in Ca II H and K for over 15 years (e.g., Noyes et al., 1984). Several determinations (e.g., Soderblom and Clements, 1987) have established a linear relation between R(hk) and R(HK). Some 50 stars in the Mount Wilson sample have well-established luminosities and space motions (e.g., Eggen, 1989) that allow the separation of young and old disk stars in velocity space (Eggen, 1989). The young disk stars all have values of R(hk) that identify them as having ages less than the youngest Hyades supercluster members (about 600 million yr). 46 refs.« less

Radio and Optical Observation of an Intense Flare on HD 32918

AbstractDuring routine monitoring of active-chromosphere stars in August 1987 an intense radio flare on the FK Comae star HD 32918 was detected by the Parkes radiotelescope. Other observatories were notified, enabling data to be collected at 843 MHz, 1.4 GHz, 8.4 GHz and optical frequencies.The flare at radio frequencies was largely similar to a previous event on this star, reaching a peak radio power of 5 < 1012 W Hz−1 at 8.4 GHz. During the flare the spectrum varied as ∼ v1.4 and later flattened as the flare progressed. Compared to the previous flare, circular polarization was higher.Ca II and Hα optical spectra taken at the AAO 3.9-metre telescope show the star to have a highly active chromosphere and a strong wind, with a terminal velocity of ≈230 km s−1. While much of this activity appears to be long lived, there is good evidence for an increase in the strength of the chromospheric lines during the radio flare event.

H-alpha spectroscopy of active-chromosphere stars. I - Six G-K giants

view Abstract Citations (27) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS H alpha Spectroscopy of Active Chromosphere Stars. I. Six G--K Giants Bopp, Bernard W. ; Dempsey, Robert C. ; Maniak, Steven Abstract Initial results from an ongoing program to monitor the long-term behavior of the H-alpha line in active-chromosphere stars are presented. The performance of a Reticon system used to obtain H-alpha spectra with 0.3 A resolution and 100:1 SNR is discussed, and results for six G-K giants are presented. The rapidly rotating G0 III stars 31 Com and 35 Cnc show no signs of activity at H-alpha and thus have normal H-alpha absorption profiles. 24 UMa and Delta CrB both show emission at H-alpha, which is more easily seen as a high residual intensity compared with standard stars of the same spectral type. The RS CVn binaries Sigma Gem and Lambda And show H-alpha features that are significantly affected by emission. Lambda And shows the weakest H-alpha absorption line of these six stars, thus the highest inferred activity level. Publication: The Astrophysical Journal Supplement Series Pub Date: December 1988 DOI: 10.1086/191304 Bibcode: 1988ApJS...68..803B Keywords: G Stars; Giant Stars; H Alpha Line; K Stars; Stellar Spectra; Absorption Spectra; Chromosphere; Line Spectra; Stellar Rotation; Astrophysics; LINE PROFILES; STARS: CHROMOSPHERES; STARS: EMISSION-LINE; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (13)

A microwave survey of southern active stars

We report the results of a survey of 153 active-chromosphere stars made with the Parkes 64-m telescope at 5.0/8.4 GHz from 1981 to 1987. Most of the stars were observed on at least 10 separate days in order to better establish flare rate and luminosity. Our sample included RS CVn and Algol-type binaries, Ca II emitters with as yet unknown stellar content and AM-Her-type cataclysmic variables. For some of the strongest stellar flares we measured the circular polarization at 8.4 GHz and obtained crude radio spectra involving frequencies in the range 0.843<ν<22 GHz. We detected microwave emission from 70 stars on at least one occasion, with the highest detection rate of 68 per cent from the RS CVn group. The Ca II stars and Algol-type binaries yielded detection rates of 44 and 30 per cent respectively. Two of the four observed AM-Her-type binaries were detected. The maximum powers emitted by the stars at 5.0/8.4 GHz ranged over five orders of magnitude, with a median of 2.5×1010W Hz−1. The power and distance distributions of the detected stars in the RS CVn, Ca II and Algol classes are not significantly different. The distance distributions of the detected and undetected stars are not significantly different for stars in the RS CVn/Ca II classes but are markedly different for the Algols. Thus RSCVn/Ca II stars are not detected because they emit less power or have a lower flare rate; the lower detection rate for Algols is probably due to their greater distances. The maximum brightness temperatures had a smaller range of three orders of magnitude, with a median of 3.6×109 K. The brightness temperature distributions of stars in the RS CVn/Ca II classes are not significantly different, but the Algols possess a markedly higher median brightness temperature of 1.1×1010 K. The AM Her binaries are still higher by at least one order of magnitude. Measurements of radio spectra during the brighter phases of flares on HD 22468, 32918, 36705 and 127535 yielded spectral indices in the range −0.1<α<1.2 (flux ∝ να), suggesting optically thick sources. We have measured circular polarization of 0 to 60 per cent in the 8.4-GHz emission from HD 22468,32918, 36705, 81410, 127535, 132742 and 196818. The moderate values of brightness temperature of most of the stars, combined with a positive spectral index and generally low polarization, suggest that optically thick gyro-synchrotron emission is the dominant mechanism. However, there are a number of stars, including most of the Algols and the AM Her binaries, with sufficiently high brightness temperatures (sometimes combined with high polarization) that may emit by a coherent process.

Outstanding Stellar Microwave Flares in 1986

AbstractWe describe bright microwave events that were first detected with the Parkes 64-m telescope at 8.4 or 22 GHz from six active-chromosphere stars. In some flares spectral data were obtained over a large frequency range from simultaneous measurements with the Parkes reflector (8.4 or 22 GHz), the Tidbinbilla interferometer (8.4 and 2.29 GHz), the Fleurs synthesis telescope (1.42 GHz) and the Molonglo Observatory synthesis telescope (0.843 GHz). Data on circular polarization were obtained from the Parkes observations at 8.4 GHz.The stars were in a wide variety of evolutionary states, ranging from a single pre-main-sequence star (HD 36705), two RS CVn binaries (HD 127535, HD 128171), an Algol (HD 132742) and two apparently single K giants (HD 32918 and HD 196818). Their high brightness temperatures, positive spectral indices and low polarization are consistent with optically thick gyrosynchrotron emission from mildly relativistic electrons with average energies 0.5 to 3 MeV gyrating in inhomogeneous magnetic fields of 5 to 100 G.

The unknown, remembered gate: Present and future observations of active chromosphere stars

The optical behavior of active chromosphere stars is reviewed, with particular reference to determination of the parameters (size, temperature, location, magnetic field strength) that characterize a stellar active region. Three areas of ground-based observational technique are highlighted for their contributions and future promise. Doppler Imaging, the reconstruction of the surface brightness distributions of spotted stars by analysis of distortions in spectral line profiles, promises to yield information on location and time evolution of active regions. Magnetic field measurements, especially those utilizing infrared FTS data, will in the future provide complementary data that will permit investigations of the field evolution of spots. Lastly, asteroseismology, utilizing either very high precision radial velocity or intensity measures, will provide input on internal structures of stars. Since parameters such as stellar convection zone depth are crucial input information for modeling the stellar dynamo, asteroseismology will aid the refinement of theoretical models of stellar activity.

UBV photometry of ten southern hemisphere active-chromosphere stars

High-precision UBV photometry of ten southern hemisphere active-chromosphere stars with strong Ca II H and K and/or H-alpha emission has been obtained. Eight of these stars showed variability during June 1985. Complete or partial light curves are presented for the stars, and these data, as well as mean V magnitudes and colors, are compared with the results of other investigators. In a number of cases, significant changes in photometric amplitude are found, which may serve to track the formation and evolution of active regions on these stars.

Variable star observations with automatic telescopes

Small automatic telescopes have been used for several years to make wide band, differential photometric observations of brighter variable stars. For example, a single automatic telescope located in Arizona has been used to study essentially the entire class of RS Canum Venaticorum binaries. These stars have changing spot structures that require once-a-night photometric observations - an ideal job for an automatic telescope located in the clear skies of Arizona. The operation of such automatic systems has become so routine that an “Automatic Photoelectric Telescope Service” now makes photometric observations for any institution so requesting. A list of stars is sent to the Arizona site of this service, and every three months the observational results are returned, with the cost typically being similar to the publication page charges. Currently the use of automatic telescopes is being extended to fainter stars and narrower bandwidths. For instance, a highly specialized one-meter telescope has been designed for automatic observations of Ca II K-line emissions from brighter active chromosphere stars. Further extensions of such automation seem likely.

X-ray observations of active chromosphere stars

HD3196 = 13 Cet α, HD30495 = 58 Eri and HD32918 have been detected as new late-type soft X-ray sources. HD32918 is a member of the FK Com class.

An Optlcal and Radio Investigation of the Active RS CVn Star HD 127535

AbstractWe present some preliminary results of an optical and radio study of the very active RS CVn binary HD 127535. Photometric measurements show the presence of a large amplitude wave which exhibits marked changes in shape and range on time scales as short as a few months. This photometric variation is almost certainly due to large cool starspots on the cooler, more luminous component. As part of a survey of southern active-chromosphere stars with the Parkes radio telescope, HD 127535 has been observed at 5, 8.4 and 22 GHz. No detection was made at 5 GHz, possibly because of confusion due to the angular proximity of the star to the galatic plane. However, it is one of the strongest sources detected in the 8.4 GHz survey, and is one of only two stars detected at 22 GHz. Photometry obtained two cycles before the 8.4 GHz observations suggest a possible correlation between the radio emission and the photometric wave, i.e. spot visibility, but more data are needed.

CA II Emission surface fluxes in active chromosphere stars

view Abstract Citations (47) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CaII emission surface fluxes in active chromosphere stars. Bopp, B. W. Abstract Ca II emission-line surface fluxes are derived for 14 stars from 17 Å mm-1 photographic spectra. Most of the stars observed are active chromosphere binaries; a few are known X-ray sources or have been observed by the IUE. The status of optical information on each of the objects is reviewed, and new information on v sin i and duplicity presented. Publication: The Astrophysical Journal Supplement Series Pub Date: March 1984 DOI: 10.1086/190935 Bibcode: 1984ApJS...54..387B Keywords: Binary Stars; Calcium; Chromosphere; Emission Spectra; Stellar Spectra; X Ray Sources; H Lines; K Lines; Astrophysics full text sources ADS | data products SIMBAD (13)