Active Dwarf Research Articles

A study of velocity fields in the transition region of ε Eri (K2 V)

Analyses of the widths and shifts of optically thin emission lines in the ultraviolet spectrum of the active dwarf Epsilon Eri (K2 V) are presented. The spectra were obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. The line widths are used to find the non-thermal energy density and its variation with temperature from the chromosphere to the upper transition region. The energy fluxes that could be carried by Alfven and acoustic waves are investigated, to test their possible roles in coronal heating. Acoustic waves do not appear to be a viable means of coronal heating. There is, in principle, ample flux in Alfven waves, but detailed calculations of wave propagation are required before definite conclusions can be drawn about their viability. The high sensitivity and spectral resolution of the above instruments have allowed two-component Gaussian fits to be made to the profiles of the stronger transition region lines. The broad and narrow components which result share some similarities with those observed in the Sun, but in Epsilon Eri the broad component is redshifted relative to the narrow component and contributes more to the total line flux. The possible origins of the two components and the energy fluxes implied are discussed. On balance our results support the conclusion of Wood, Linsky & Ayres, that the narrow component is related to Alfven waves reaching to the corona, but the origin of the broad component is not clear.

Spots and Active Regions on Emission Stars. III. LQ Hya

Quasisimultaneous photoelectric, polarimetric, and spectroscopic observations of the active single red dwarf LQ Hya are presented. The photometric variability of LQ Hya is fully described by a zone model. Spotted regions occupy up to 25% of the entire surface of the star with a temperature difference of 800 K between the spots and the calm photosphere. The spots are localized in the middle and low latitudes. A cyclical variation is observed in the total areas of the spotted regions and in the average latitude of the spots. The most spotted regions, local magnetic fields, and chromospherically active regions tend to concentrate in the same distinct active longitudes.

Magnetic activity in the young solar analog LQ Hydrae

We present the first evidence that a single active dwarf of solar type can show a long-lived, nonaxisymmetric spot distribution - active longitudes on opposite hemispheres, similar to evolved, rapidly rotating RS CVn-type binary stars. We analyse new as well as published photometric observations of the young active dwarf LQ Hya, spanning almost 20 years. We find that activity of the star has three activity cycles: a 5.2-yr flip-flop cycle, a 7.7-yr period in the amplitude modulation of the brightness and an approximately 15-yr period in variations of the mean brightness. The two shorter cycles are related to the alternating active longitudes and are similar to cycles observed in RS CVn-type stars. The 15-yr cycle reflects periodic changes of the mean spottedness of the star and resembles the solar 11-year cycle. The spot rotation period (about 1.6 days) changes during the 15-yr cycle, indicating the presence of small dierential rotation. The lengths of the three cycles are related as 3:2:1, with the repetition of the spot configuration after 15 years. We discuss the possibility that the observed spot cycles represent two dierent magnetic dynamo modes operating in LQ Hya: an axisymmetric mode, as in the Sun, and a nonaxisymmetric higher order mode with two cycles in spot patterns. Our results suggest that young stars exhibit their cycles in spot distribution, as seen in LQ Hya. This is in contrast to the conclusion based on the analysis of Caii H&K emission from plages. The results suggest also that the Vaughan-Preston gap represents a transition from a multiple-mode dynamo to a single-mode dynamo.

Compact Star Clusters in Nearby Dwarf Irregular Galaxies

Nearby dwarf irregular galaxies were searched for compact star clusters by using data from the Hubble Space Telescope archives. Eight of the 22 galaxies in our sample were found to host compact clusters of some type. Three of these have populous clusters, with MV < -9.5 at a fiducial age of 10 Myr, and the same three also have super–star clusters, with MV <-10.5 at 10 Myr. Four other dwarf galaxies, two of which contain populous and super–star clusters, are also considered by using data in the literature. The results suggest that galaxies fainter than MB = -16 or with star formation rates less than 0.003 M⊙ yr-1 kpc-2 do not form populous or super–star clusters and that even the brighter and more active dwarf galaxies rarely form them. Yet when they do form, the associated star formation activity is very high, with numerous compact clusters of similar age in the same complex and evidence for a galaxy-wide perturbation as the trigger. This tendency to concentrate star formation in localized regions of high column density is consistent with previous suggestions that self-gravity must be strong and the pressure must be high to allow a cool phase of gas to exist in equilibrium. Statistical considerations emphasize the peculiarity of super–star clusters in dwarf galaxies, which are too small to sample the cluster mass function to that extreme. We suggest that triggered large-scale flows and ambient gravitational instabilities in the absence of shear make the clouds that form super–star clusters in small galaxies. This is unlike the case in spiral galaxies in which density wave flows and scale-free compression from turbulence seem to dominate. Further comparisons with spiral galaxies give insight into Larsen & Richtler's relation between the star formation rate per unit area and the fraction of young stars in massive dense clusters. We suggest that this relation is the result of a physical connection between maximum cluster mass, interstellar pressure, interstellar column density, and star formation rate, combined with a size-of-sample effect.

Generalized concept of flares on late-type stars

Abstract Different kinds of stellar flares – very fast flares (0.1–10 s spikes), impulsive flares lasting 100–1000 s and flares with a long decay of the soft X-rays over hours–days – are described. Impulsive flares arises as a consequence of the energy input into the magnetically confined plasma. There is reason to suppose that these flares, which occur mainly on active red dwarfs (UV Cet-type), are a result of evolution of local magnetic fields, and they develop in low-lying loops. The response of the outer atmosphere of a star to impulsive heating is considered. It allows us to understand the origin of the X-ray and optical flare radiation in the framework of a uniform approach. In particular, optical radiation of sources formed during the gas-dynamic process considered does describe the flare observations quite well. Long-decay X-ray flares (LDF), which are more typical for components of RS CVn binaries, especially, for late-type subgiants, are a result of post-eruptive energy release. The duration of ...

Simultaneous ROSAT XRT and WFC observations of a sample of active dwarf stars

The X-ray observations of the ROSAT-PSPC All-Sky Survey have revealed bright and energetic coronae for a number of late-type main-sequence stars, many of them flare stars. We have detected 31 X-ray flares on 14 stars. A search for simultaneous X-ray and EUV (extreme ultraviolet) flares using ROSAT Wide Field Camera survey data revealed a large number of simultaneous flares. These results indicate that the heating mechanisms of the X-ray and EUV-emitting regions of the stellar coronae are similar. We find X-ray quiescent variability for nine of the 14 stars and simultaneous X-ray and EUV quiescent variability for seven of these nine stars. These results imply that the stellar coronae are in a continuous state of low-level activity. There are tight linear correlations of X-ray flare luminosity with the ‘quiescent’ X-ray as well as with the stellar bolometric luminosity. The similarity between the X-ray-to-EUV quiescent and flare luminosity ratios suggests that the two underlying spectra are also similar. Both are indeed consistent with the previously determined Einstein two-temperature models. We suggest that both the variability and spectral results could indicate that the quiescent emission is composed of a multitude of unresolved flares.

The Surface Temperatures of White Dwarf Accretors in Dwarf Novae: The Active Dwarf Nova CN Orionis during Quiescence

ABSTRACTWe have carried out a synthetic spectral analysis of five IUE NEWSIPS archival spectra of the peculiar, very active dwarf nova CN Orionis taken at the system’s lowest flux levels during quiescence. AAVSO data indicate the visual magnitude V∼14 for CN Ori at this time. On the assumption that the white dwarf contributes significantly to the far‐UV light of the system, we have computed a two‐parameter grid of synthetic, high‐gravity spectra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and carried out fits of these pure photospheric models to the far‐UV continuum and narrow absorption line spectra. We find that the far‐UV spectrum in quiescence is well represented by a hot (Teff = 30,000 K, log g = 7) white dwarf with probable subsolar silicon abundance and all other observed metal transitions at essentially their solar values. For comparison, we have fitted optically thick accretion disk models to the same spectra for Mwd = 0.80 M⊙ and disk inclination angle i = 60° for accretion rates M˙ = 10-10.5 and 10−9.5 M⊙ yr−1. We compare the Teff of CN Ori, one of a handful of dwarf novae above the period gap with a model photospheric analysis, to all other dwarf novae with white dwarfs of known Teff derived using white dwarf model atmospheres. We find evidence that the white dwarfs in dwarf novae above the period gap are hotter than the ones below the gap. This trend suggests more heated, younger degenerates above the gap and is consistent with the overall empirical evidence that mass transfer rates in dwarf novae above the period gap are higher than mass transfer rates in systems below the gap.

Magnetic Field Variations with a Rotational Period on Solar‐like Star ξ Bootis A

We examine the global con—guration of the magnetic —eld of the active dwarf m Boo A as a function of stellar rotation phase (P 6.1455 ^ 0.0003 days), using direct high-precision spectropolarimetric mea- surements. The magnetic —eld varies from ( 10 Gu p to)30 G. Integrated magnetic —ux on the surface changes polarity during a single rotation period on m Boo A, resulting in a —ux ratio of *' ' /*' ~ D 1.5. This is the —rst study of the general magnetic —eld as a function of rotation on a solar-like star other than the Sun. Subject headings: stars: individual (m Bootis A) ¨ stars: late-typestars: magnetic —elds

The SOHO–Stellar Connection

The ‘solar– stellar connection’ bridges the daytime and nighttime communities; an essential link between the singular, but detailed, views of our Sun, and the broad, but coarse, glimpses of the distant stars. One area in particular – magnetic activity – has profited greatly from the two way traffic in ideas. In that spirit, I present an evolutionary context for coronal activity, focusing on the very different circumstances of low-mass main-sequence stars like the Sun, compared with more massive stars. The former are active mainly very early in their lives, whereas the latter become coronal only near the end of theirs, during the brief incursion into the cool half of the Hertzsprung–Russell diagram as yellow, then red, giants. I describe tools at the disposal of the stellar astronomer; especially spectroscopy in the ultraviolet and X-ray bands where coronae leave their most obvious imprints. I compare HST STIS spectra of solar-type dwarfs – ζ Dor (F7 V), an active coronal source, and α Cen A (G2 V), near twin of the Sun – to the SOHO SUMER UV solar atlas. I also compare the STIS line profiles of the active coronal dwarf to the corresponding features in the mixed activity ‘hybrid chromosphere’ bright giant α TrA (K2 II) and the archetype ‘non-coronal’ red giant Arcturus (α Boo; K2 III). The latter shows dramatic evidence for a ‘cool absorber’ in its outer atmosphere that is extinguishing the ‘hot lines’ (like Si iv λ1393 and N v λ1238) below about 1500 A; the corona of the red giant seems to lie beneath its extended chromosphere, rather than outside as in the Sun. I present an early taste of the moderate resolution spectra we can expect from the recently launched Chandra X-ray Observatory (CXO), and contemporaneous STIS high resolution UV measurements of the CXO calibration star Capella (α Aur; G8 III + G1 III). Last, I describe preliminary results from a May 1999 observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer (IRIS). The purpose was to explore the dynamics of the quiet solar atmosphere through the key ‘magnetic transition zone’ that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar phenomena to properties of the average line shapes (widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying physical insights from the solar setting to the realm of the distant stars.

Time-resolved Spectroscopy of RS Canum Venaticorum Short-Period Systems. II. RT Andromedae, WY Cancri, and XY Ursae Majoris

We present the second part of the results of our spectroscopic study of orbital phase resolution of short-period active binaries. In this paper we present the observations of the binaries RT And, WY Cnc, and XY UMa, for which we obtained simultaneous low-resolution spectra in the Hα, Hβ, and Ca II infrared triplet (IRT; 8498 and 8542 A) lines, and higher resolution spectra in Hα for WY Cnc and XY UMa. Hα and Hβ emission excess was found in all the observed orbital phases of WY Cnc and XY UMa, while RT And showed Hα emission only during some orbital phases. We measure a clear Ca II (IRT) excess in all the spectra of the three binaries. The ratio of the flux excesses derived for these binaries, EHα/EHβ 1–3 and E8542/E8498 1–3, give low values typical of solar plage structures. In the higher resolution Hα spectra, we found emissions that can be associated with both stellar components of XY UMa and WY Cnc, indicating that the faint secondary stars in both binaries are active late-type dwarfs. Some Hα emission components, which could originate in extended structures between the stars, were detected in both systems.

X‐Ray Spectroscopy of Rapidly Rotating, Late‐Type Dwarf Stars

We present an X-ray spectroscopic analysis of two active dwarf stars with extremely short rotation periods, HD 197890 (Speedy Mic) and Gliese 890, using X-ray data acquired with the ASCA observatory. We analyze the X-ray spectrum of Speedy Mic in two separate time intervals, one corresponding to an apparent stellar flare, the other to quiescence or the nonflaring state. We also present a reanalysis of the ASCA spectrum of the M dwarf stellar binary YY Gem, during both quiescent and flaring states. We use the recently updated MEKAL plasma code to model these spectra and find that a minimum of two temperature components are required to obtain good fits for all three stars. The inferred emission measures of the hot components in YY Gem and Speedy Mic, and the temperature of the hot component in the case of YY Gem, increase significantly during the observed X-ray flares, in agreement with previous studies of similar stellar X-ray flares. In addition, metal abundances that are low compared to the solar photospheric values are preferred (but not required) for the quiescent coronae of Gliese 890 and Speedy Mic, while for the quiescent corona of YY Gem, subsolar metal abundances are required and solar abundance models can be ruled out with high confidence. We combine the results of this study with those of previous ASCA and ROSAT analyses of late-type dwarf stars with known rotation rates to obtain a sample of 17 G, K, and M dwarfs. We find a strong correlation of the X-ray to bolometric luminosity ratio, L X /L bol , both with the rotation period and the Rossby number (R o ), in agreement with previous studies. Using a homogeneous subset of 10 dwarfs for which we have purely ASCA-derived temperatures and elemental abundances, we find that the temperature of the hottest component present in their coronae is correlated with this luminosity ratio, as expected. Finally, we note that the Fe abundances for all active dwarf stars having normalized X-ray luminosities log (L X /L bol ) > −3.7 are significantly subsolar, whereas the Fe abundances in the less active stars are within a factor of 2 of the solar value.

The Resolved Stellar Population of the Poststarburst Galaxy NGC 1569

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) photometry of the resolved stellar population in the poststarburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains ~2800 stars with a photometric error of ≤0.2 mag down to mF439, mF555 26 and is complete for mF555 23. Adopting the literature-distance modulus and reddening, our CMD samples stars more massive than ~4 M☉, allowing us to study the star formation (SF) history over the last ~0.15 Gyr. The data are interpreted using theoretical simulations based on stellar evolutionary models. The synthetic diagrams include photometric errors and incompleteness factors. Testing various sets of tracks, we find that the ability of the models to reproduce the observed features in the CMD is strictly related to the shape of the blue loops of the sequences with masses around 5 M☉. The field of NGC 1569 experienced a global SF burst of 0.1 Gyr duration, ending ~5-10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than ~10 Myr. The level of the SF rate was very high; for a single-slope initial mass function (IMF) ranging from 0.1 to 120 M☉, we find values of 3, 1, and 0.5 M☉ yr-1 for α = 3, 2.6, and 2.35 (Salpeter), respectively. When scaled for the surveyed area, these rates are approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, though our best models indicate slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.

Influence of red dwarf component activity on accretion in close binary systems

Abstract Active late type stars lose mass as due to the effect of magnetic instabilities in their corona connected to flares. The phenomenon is similar to what happens in the Sun, but with greates intensity, with the stellar wind reaching M ∼ 10−11 Mθ yr−1. In later spectral types the effect is suspected to be more discontinuous and dominated by coronal mass ejections (CMEs): transient events of gas expulsion due to large-scale reconnection of the magnetic field. CMEs from active red dwarfs have been detected in the last few years. In these transient events up to 1016 g of gas are expelled on time-scales of some minutes. Some observations suggest that such events can support a mass loss rate from the active star M ∼ 10−11 Mθ yr−1, similar to the above case. We investigate the possibility that interaction between the expelled gas and the accretion pattern around the compact object in Cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) gives rise to observable energy release. We estimate that in...

EUV activity in late-type stars during the ROSAT WFC All-Sky Survey - II. Comparisons with other observations

The extreme ultraviolet (BUV) observations of the ROSAT Wide Field Camera AlI­ Sky Survey have revealed bright and energetic coronae for a sufficiently large number of main-sequence stars that detailed comparisons with other observables can be made. We have detected a large number of energetic BUV flares on stars of different spectral classes, from dF5 to dM6, including flare stars. The BUV emission is well-correlated to several stellar properties such as stellar colour, spectral type, bolometric luminosity, far-infrared photospheric emission, etc. These relations are stronger for the highly active stars like flare stars, and they become somewhat looser for the less active dwarfs. We find that there is a better correlation between the BUV luminosity and stellar rotation period for low-activity stars than for high-activity ones, and this also applies to the RS CVn stars. This is a strong indication of magnetic/active region saturation in these more active stars. We find that there is also a tendency of the L Evv to increase with magnetic field intensity, although there are very few stars with reliable magnetic field strength measurements. These last two correlations support the view that the BUV emission from these stars is regulated by magnetic processes. Our results indicate that the stellar coronae are continuously being heated by small-scale flare events.

Rotation of solar-like main sequence stars

The results of the photometric survey of 16 solar-type, active, field stars are presented. During our observations 9 stars showed appreciable light variability with amplitudes of a few hundredths of a magnitude but for three of them periods could not be determined. Most of the observed variable stars have periods shorter than about 10 days. It is suggested that, similarly as is observed in the Hyades cluster, small amplitude light variations are quite common among active field solar-type dwarfs with rotation periods around one week or less. A strong modulation of amplitude of some variable stars over the time scale of years is demonstrated. The amplitude may sometimes decrease even below the detectability threshold. A special case is HD 17576 – a visual binary consisting of a G0 dwarf and a much fainter hot subdwarf. It has the largest amplitude of all the stars observed, which suggests an intense spot activity, a very strong H and K line core emission and a very high X-ray emission flux, close to the saturation limit. Yet its variability period is equal to 18.74 days – almost an order of magnitude longer than expected for such an active dwarf.

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.

New Infrared Measurements of Magnetic Fields on Cool Stars

I present a preliminary analysis of IR spectra of five K and M dwarfs and two RS CVn variables. Evidence for significant magnetic flux is found on several stars, a number of which are detected for the first time. Field strengths (B) on the RS CVn variables are lower than in the active dwarfs, consistent with the concept of pressure balance limiting B in stellar photospheres. I compare the results with previous measurements.

Stellar Coronal Structures

AbstractLarge magnetic structures in the coronae of stars containing gas at a wide variety of temperatures are now being studied in X-rays, radio wavelengths, and Hα. Here I will summarize what we are learning about coronal structures in three types of stellar systems: the magnetic chemically peculiar stars, the RS CVn binary systems containing G- and K-type subgiants, and active solar-type dwarfs like V471 Tauri and AB Doradus.

Aspects of the Angular Momentum Loss Problem for Cool Close Binary Systems

We concentrate on the application of centimeter wavelength observational data to the coronae of rapidly rotating active dwarf stars. In particular we seek insight into coronal loop geometries, and their possible relevance to the magnetic braking mechanism, which must play a key role in binary evolution scenarios for such stars.